Function of mammalian- type and reptilian- type nephrons in kidney of desert quail

Transcription

1 AMERICAN.JOURNAL OF PHYSIOLOGI' VcJ. 222, No. 3, March Printfd r us A. Function of mammaian- type and reptiian- type nephrons in kidney of desert quai ELDON J. BRAUN AND WILLIAM H. DANTZLER Department of Physioogy, Coege of Medicine, University of Arizona, Tucson, Arizona WRAUN, ELDON J., ~%LLCAM 1% J)ANTZLER. Function of mamunaian-type and reptiian-type nephrons in kidney of desert quai. Am. J. Physio. 222(3): I Bird kidneys have mammaiantype (MT) as we as reptiian-type (RT) nephrons. Whoekidney and individua nephron functions were studied in the desert quai during a contro diuresis and during 6% NaC infusion. Gomeruar fitration rate (GFR) decreased after ca. 2 meq NaC/kg and fe to 2% of contro after cit. 4 meq NaC/ kg. Tubuar maximum for para-aminohippurate secretion decreased with GFR, suggesting that changes in GFR resuted from changes in number of functioning nephrons. Histoogica data showed 71 y. of RT proxima convouted tubue (PCT) umina open in hydrated birds and 84 /, umina cosed in sat-oaded birds, suggesting that it was RT nephrons which ceased fitering during sat oading. Singe-nephron fitration rates (SNGFR) during contro diuresis were 15.8, 1.9, and 6.4 n/min for arge mammaian-type (MT-L), sma mammaian-type (MT-S), and RT nephrons, respectivey. During sat oading, SNGFR vaues were 13.4 and 12.4 n/min for MT-L and MT-S. No RT nephrons were fitering during sat oading. Birds with imited concentrating abiity and without sat gands can respond to osmotic stress by reducing number of functioning RT nephrons. Possibe reguatory mechanisms for this differentia functioning of nephrons and the reationships between SNGFR, PCT ength, and gomeruar voume are discussed. BIRDS, LIKE MAMMALS, CAN PRCIDIJ~IZ A URNS hyperosmotic to the pasma. Athough one race of Savannah sparrow, which ives in sat marshes, can produce a urine about 5 times the osmoaity of the pasma (28), other birds studied cannot produce a urine much more than twice the osmoaity of the pasma (4, 23, 28, 38). Gambe s quai, Lophortyx gambeii, the common desert quai of the southwestern United States, can produce a urine about 2.5 times the osmoaity of the pasma (23). The anatomy of the avian kidney suggests that the concentrating abiity woud not be marked since the medua is ess we deveoped than in the mammaian kidney. In fact, the avian kidney contains a mixture of nephrons resembing reptiian and mammaian nephrons. Most nephrons resembe reptiian nephrons with simpe proxima and dista tubues without oops of Hene and empty at right anges into coecting ducts. However, some nephrons have oops of Hene which, together with 617 vasa recta and parae coecting ducts, form meduary cones ( 14, 2, 27, 4, 41). These structures appear to function as countercurrent mutipiers, as in mammaian kidneys, to aow the production of a concentrated urine (39). With ony a modest rena concentrating abiity, birds are imited in their capacity to reduce pasma osmoaity by the rena excretion of sat in excess of water. Athough some terrestria birds do have functiona sat gands (3, 33), these desert quai and most other terrestria birds do not. In what way then do these desert birds, which might be subjected to severe dehydration, respond to an acute osmotic oad? Previous work on domestic chickens (7) has indicated that a severe sat oad causes a fa in GFR. Moreover, studies of the tubuar maxima (Tm) for the secretion of para-aminohippurate (PAH) and the reabsorption of gucose have suggested that the decrease in GFR resuted from a decrease in the number of functioning nephrons. This apparent decrease in the number of functioning nephrons raises some intriguing questions concerning birds in genera and desert birds in particuar (9). First, In those birds which might be subject to severe dehydration, does fitration rate decrease more readiy than in domestic birds? Second, When a decrease in the number of functioning nephrons occurs, does this invove nephrons of both reptiian and mammaian types. 3 We suggested (9) that, if ony nephrons of the reptiian type ceased functioning, nephrons of the mammaian type coud continue functioning together, aowing maintenance of concentrating abiity. Moreover, norma fow through nephrons of the mammaian type woud reduce the possibiity of urate deposits within the oops of Hene in the meduary cones. To examine these questions more cosey, we subjected Gambe s quai to a sodium choride oad simiar to that used in domestic fow (7). To evauate individua nephron function we examined the reationship of TmPAn to GFR, made histoogica studies, and measured singe-nephron gomeruar fitration rates (SNGFR) by the modification of Hanssen s ( 17) technique deveoped by de Rouffignac, Diess, and Bonvaet ( 13). The resuts indicate that: I) the overa GFR of Gambe s quai may be sighty more responsive to an osmotic oad than that of the domestic fow, 2) the fitration rates of individua mammaian-type (MT) nephrons are more than twice those of reptiian-type nephrons during a contro diuresis, and, 3) when a decrease in overa GFR occurs, it resuts from a decrease in the number of fitering RT nephrons. Downoaded from by on Apri 13, 217

2 618 E. J. BRAUN AND W. H. DANTZLER METHODS Anima and operative procedures. Gambes quai, Lophortyx gambeii, were used as experimenta animas. The birds weighed from 14 to 16 g (mean wt: 156 g). The quai were maintained in an outside aviary exposed to natura environmenta conditions at Tucson, Arizona. The birds had free access to a cracked-grain mixture and green fodder and were aowed water ad ibitum. Animas were paced supine with outstretched wings on a speciay designed bird board. The wings were taped to atera extensions from the board and the feet to vertica extensions of the board. During experiments, the board was tited to eevate the head of the birds. Equi-thesin (Jensen-Sasbery, Kansas City, MO.) was administered as a genera anesthesia in a dose of 3. m/kg prior to the operative procedure and 2 % idocaine was used as a oca anesthetic during the operations. The eft brachia vein and artery were cannuated with PE-1 poyethyene tubing. Infusions were given through the brachia vein and bood sampes were coected from the brachia artery. In experiments in which individua nephron fitration rates were studied, the musces of the eft fank were separated by bunt dissection and a PE- 1 poyethyene cannua was paced in the sciatic artery. In birds, the rena arteries which suppy the arge posterior division of the kidney come off the sciatic artery. The poyethyene cannua inserted into the sciatic artery was advanced in a retrograde fashion to the eve of the rena arteries. Its exact position was checked by dissection foowing the experiments. The ureters were cannuated by the method of Munsick, Sawyer, and van Dyke (25). A sma incision was made dorsa to the coaca1 vent, the ureters were freed by bunt dissection, and a PE-5 poyethyene cannua was tied into each. Dead space in each cannua was 18-2 ~1. Cearance studies. Gomeruar fitration rates were estimated as inuin cearances using inuin-carboxy-14c. A priming injection of 1 PC was given and the bood eve was maintained by a constant infusion of.6 &kg per min. Before the first cearance period 4 min were aowed for equiibration. A contro diuresis (mean vaue r)z SE of 11 contro periods:.186 =I=.8 m/kg per min from each kidney, Tabe 1) was produced by a constant intravenous infusion of 2.5 % mannito soution at.4 m/kg per min. Cearances were determined separatey for each kidney. A coection periods were 1 min in ength. Bood sampes of.2-.3 m were coected at the midpoint of each period. For evauation of TmPAH,.2 g PAH was given as a priming injection and bood eves were maintained at a minimum of 8 mg/ 1 m by a constant infusion of 4 TABLE 1. Rena functions during contro periods Experiments with inuin-yj Urine fow.186 =t=.8 (11) GFR.882 =f=.36 (97) Experiments with Na ferrocyanidej4c Urine fow.121 =f=.4 (4) GFR.858 =t:.39 (4) Vaues are means =1= SE of miiiters per kiogram per minute. Numbers in parentheses are numbers of cearance periods. mg/kg per min through the brachia vein with the mannito and NaC infusions. The exact pasma PAH concentration at which Tm PAH was reached was not determined in these experiments. However, Dantzer (7) reported that the TmPAH in domestic chickens was reached at pasma PAH concentrations of o-2 mg/oo m. To study the effect of an osmotic oad on rena function, 6.25 meq/kg of NaC (as a 6 % soution) were given into the brachia vein as a priming injection and the contro infusion was repaced with 6 % NaC. The infusion rate of.4 m/kg per min was continued, and an appropriate interva was aowed for the NaC soution to repace mannito in the dead space of the infusion system before the first coection period during the sat oad was begun. At this rate of infusion, the animas received 4 meq/kg body wt of NaC during each ensuing cearance period. Histoogica studies. Histoogica sections were made of kidneys taken from birds under the conditions of a contro diuresis and of sat oading. The birds were prepared and maintained under anesthesia as described above. The contro diuresis was produced by the infusion of 2.5 % mannito and the sat oad by the infusion of 6% NaC at.4 m/kg per min. When the desired imits of the infusions were reached, the abdomina cavity of each bird was rapidy opened and the kidneys fooded in situ with 1 % ice-cod buffered Formain. The synsacrum of each bird, with the kidneys sti in pace, was removed and paced in a bath of ice-cod 1 % Formain in which the kidneys were carefuy freed from their bony concavity. After fixation in Formain, the tissue was embedded in paraffin and sections 6-y thick were cut from it. The sections were stained with hematoxyin and eosin. Injection and corrosion studies. Corrosion casts of quai kidneys were made by injecting the rena arteries, efferent rena veins, and ureters with Batson s no. 17 anatomica corrosion compound (Poyscience, Inc., Ryda, Pa.). The casts were aowed to harden overnight in distied water and were then paced in 1 % potassium hydroxide to aow corrosion to take pace. These studies were used to hep reconstruct the anatomy of the quai kidney. Measurement of individua-nephron gomeruar Jitration rates. Singe-nephron gomeruar fitration rates were determined with the modification of Hanssen s ( 17) technique deveoped by de Rouffignac, Diess, and Bonvaet ( 13). This technique permits the measurement of fitration rates in nephrons inaccessibe to micropuncture techniques and can be carried out in birds without a ong exposure of the kidney. In this technique, carbon 14-abeed sodium ferrocyanide is used as a marker to estimate gomeruar fitration rate. The anesthetized birds were prepared as described above. The 14C-abeed sodium ferrocyanide (Schwarz/Mann, Orangeburg, N.Y. ; SA: 1-32 mc/mmoe, depending on the ot used), in a concentration of 2 &m, was infused through a wing vein with the 2.5 % mannito soution or the 6 % NaC soution. A priming dose of 2 PC Na ferrocyanide- 14C was given before the start of the sustained infusion, and 4 min were aowed for equiibration before the start of the cearance periods. Preiminary experiments showed that, foowing equiibration, Na ferrocyanide-14c at pasma concentrations as ow as 2. X o+ M gave an estimate of gomeruar fitration rate for the whoe kidney identica Downoaded from by on Apri 13, 217

3 NEPHRON FUNCTION IN DESERT QUAIL 619 with that obtained simutaneousy using 3H-abeed inuin. The contro GFR estimated with inuin-3h was 1.46 f.22 (SE of four determinations) whie the GFR measured simutaneousy with Na ferrocyanidej4c was.968 f.2 (SE of four determinations). At a pasma concentration one-tenth the above vaue, the cearance of sodium ferrocyanide was ony about 75% of the simutaneous inuin cearance, indicating a sma saturabe reabsorptive process or possiby imited pasma protein binding of ferrocyanide. A simiar pattern has been found with rats (2). In the present study, the pasma concentration of sodium ferrocyanide was aways we above 2. X o- M. After the cearance periods necessary for a contro diuresis or a 4 meq/kg sat oad, an arteria bood sampe was coected, the abdomina cavity was opened, and the viscera were genty moved to one side to expose the eft kidney. A bous of 15 ~1 of a saturated soution of nonradioactive sodium ferrocyanide was given as a singe puse through the sciatic artery into the eft posterior rena artery. A few seconds were aowed for this nonradioactive sodium ferrocyanide to pass about 5% of the distance down the proxima tubue. Preiminary experiments indicated that this time interva was about 1 sec. At the end of this period, the bood fow to the kidney was stopped and the kidney was snap-frozen by fooding the entire region with iquid nitrogen. No attempt was made to stop the bood fow to the kidney by igating the rena arteries because they were too fragie. It was assumed that the iquid nitrogen stopped the bood fow to the kidney the moment it contacted the rena arteries. The period from the injection of the nonradioactive sodium ferrocyanide to the stopping of the rena bood fow and the freezing of the kidney was timed to the nearest.1 sec. The time from exposing the kidney by opening the abdomina cavity to freezing was about 35-4 sec. Observations of the birds and measurements of arteria bood pressure indicated that this brief period of exposure did not ead to significant deterioration of the animas. The mean arteria pressure measured through the sciatic artery with a P23GB Statham pressure transducer and recorded on a Brush recorder was 115 mm Hg and remained stabe when kidneys were exposed for as ong as 3 min. Immediatey after freezing with iquid nitrogen, the entire synsacrum with the kidney sti frozen in situ was removed and transferred to a dish of ethy acoho chied in Dry Ice. In this bath, the eft kidney was dissected free of the synsacrum, fragmented, and transferred to a soution of acohoic ferric choride (95 m ethy acoho, 5 m concentrated HC, and 3 g anhydrous ferric choride) at -2 C. The tissue was maintained in this soution at -2 C for 24 hr. During this period a freeze-substitution reaction occurred in which sodium ferrocyanide was converted to insoube ferric ferrocyanide (Prussian bue). The tissue was then digested for 2.25 hr in 2% HC at 37 C, washed with distied water, and refrigerated overnight in a.2% ferric choride- 1% acetic acid soution. It was then put in distied water for at east 4 hr, to soften, before separation of the individua nephrons was attempted. The individua nephrons were isoated using finey drawn gass needes. The nephrons were transferred to a drop of 5% gycerine on a gass side and their outines were drawn with the aid of FIQ. 1. A singe mammaian-type nephron with a band of bue ferric ferrocyanide precipitate in umen (arrows indicate bous of precipitate). X55. a drawing tube attached to a binocuar dissecting microscope. The nonradioactive sodium ferrocyanide coud be seen as a band of bright-bue precipitate in the proxima tubue (Fig. 1). The ength of the nephrons was determined by tracing the outine on the drawing with a map reader. A fiar micrometer on the dissecting microscope was used to measure the gomeruar dimensions and the diameter of the proxima tubues. The nephrons were broken at the front of the bright-bue band of nonradioactive precipitate, and the radioactivity in the nephron from the gomeruus to this point was determined. This radioactivity represented the amount of sodium ferrocyanide fitered from the time the nonradioactive sodium ferrocyanide was given to the time the kidney was frozen. The individua-nephron fitration rate was cacuated by dividing the radioactivity fitered during this time by the pasma radioactivity. Cacuation of voume of gomerui. The shape of the gomerui of the argest mammaian nephrons appeared to be that of an obate spheroid with one diameter consistenty much onger than the other. Therefore, the formua for the voume of an obate spheroid was used to cacuate the voume of these gomerui. The diameters of a other nephrons appeared to be equa in any direction and the assumption was made that these gomerui were spherica for the purposes of cacuating their voume. Determination of number of nephrons. The number of nephrons was determined in nonexperimenta kidneys after India ink injections through the rena arteries. Statistica counts of gomerui were made using the technique of Damadian, Shwayri, and Bricker (6). This was modified so that the fina voume of distied water containing the gomerui for counting aso contained 2.5% Formain to reduce gomeruar breakdown. Anaytica methods. The activities of inuin-carboxyj4c, inuin-3h, and Na ferrocyanidej4c were determined in a iquid scintiation spectrometer (Nucear-Chicago Corporation, Uniux II). The scintiation soution was the same as that used by Truniger and Schmidt-Niesen (43). To keep proteins in soution, the sampes were mixed with.5 m of Hyamine hydroxide prior to the addition of 15 m of scintiation fuid. This same system was used for counting the activity in the individua nephrons. The tiny tubuar fragments disappeared after mixing with the Hyamine. It was not possibe to determine whether these were in soution. Downoaded from by on Apri 13, 217

4 62 E. J. BRAUN AND W. H. DANTZLER However, counts of a contro precipitate of 14C-abeed ferric ferrocyanide (Prussian bue) showed no change in counting over a period of 24 hr or onger, indicating that there was no change in the physica status of the precipitate in the Hyamine-scintiation soution mixture. Quenching was checked by the interna standard method for a tubue and pasma sampes containing Na ferrocyanide-14c and was found to be ess than 2 % in a cases. Consequenty, since the cacuation of individua-nephron fitration rates invoved the ratio of the tubue counts to the pasma counts, quenching coud be ignored. A tubue sampes were counted to a minimum of 1, counts above background. SHORTLOOP MHUALIAN TYPE NfPHRON The iquid scintiation system was chosen over the gas-fow counting system used by de Rouffignac, Diess, and Bonvaet ( 13) when preiminary experiments showed that it gave far LONG LWP NAYNALIAN IYPf NfPNRON greater counting efhciency than the gas-fow system. Pcm-aminohippurate concentrations were determined by the method of Friedman, Poey, and Friedman (16) adapted for sma sampes using Beckman/Spinco equipment (1). Tota osmoaity of pasma and urine sampes was determined with a Fiske osmometer on.2-m sampes. mm Pasma and urine sodium concentrations were determined FIG. 2. A three-dimensiona drawing of a section of avian kidney with a Baird-Atomic KY-3 fame photometer with interna showing types of nephrons present, their reative positions in kidney, and their reationship to other rena structures. ithium standard. RESULTS Anatomy of quai kidney. The avian kidney is an eongate, fattened organ fitted cosey into the bony concavity formed by the synsacrum. The kidney of Gambe s quai is divided into three gross divisions-a arge posterior division, a very sma midde division, and an anterior division somewhat arger than the midde division. The types of nephrons, their reative positions in the kidney, and their reationship to other rena structures are shown in Fig. 2. The reptiian-type nephrons ( 18, 36) are ocated at the surface of the kidney arranged around a core formed by the centra (efferent) vein. The cyinders formed by the RT nephrons and centra vein form repeating units grouped in radiating patterns from centra points over the entire surface of the kidney (Fig. 2). The majority of the RT nephrons are very simpe tubues foded upon themseves 4 times as indicated in Figs. 2 and 3. The structure of the nephrons graduay becomes more compex as the center of each radia group of cyinders is approached on its ventra aspect. The nephrons begin forming more convouted proxima and dista tubues and intermediate segments resembing oops of Hene. The RT nephrons drain at right anges into coecting ducts which ie at the periphery of each cyinder (Fig. 2). At east two arteries enter each cyindrica structure and give rise to the afferent arterioes. The capiary tufts within the capsue of the RT nephrons are very simpe. The afferent arterioe may enter the capsue, bifurcate, and reunite to form the efferent arterioe (36). Thus, there may be ony two capiary oops in the gomeruus of an RT nephron. The efferent arterioe emerges from the capsue and enters a capiary pexus which surrounds the RT nephrons. Afferent veins which have their origins from within the rena porta system ie near the periphery of each cyinder. CfNRALVfN~ff~fRtf~ Sma right-ange branches from these afferent veins enter the capiary networks which surround the RT nephrons (Fig. 2). The onger, more compex nephrons within the bird kidney have the same distinguishabe segments as the nephrons in mammaian kidneys. They have highy convouted proxima tubues, oops of Hene with thick and thin imbs, and dista convouted tubues (Figs. 2 and 3). The transition from the simpe reptiian-type nephrons to the mammaian-type nephrons is not abrupt but gradua as indicated in Fig. 2. The MT nephrons are never found near the surface of the kidney but are situated deep to the shorter RT nephrons (Fig. 2). The arteries which suppy the RT nephrons aso suppy the MT nephrons. The Bowman s capsue: of the MT nephrons are arger than those of the RT nephrons (Fig. 3) and the gomuar tufts are more compex (36). The efferent arterioes, on emerging from capsues, descend to form vasa recta about the oops of Hene of the MT nephrons (Fig. 2) (36). The oops of Hene from the MT nephrons, the vasa recta, and the coecting ducts which drain the RT and MT nephrons from each radia group of cyinders are bound by a connective tissue sheath into a tapering structure referred to as a meduary cone (Fig. 2). The meduary cone is tapered because of the varying engths of the oops of Hene and the successive fusion of the coecting ducts. The coecting ducts continue to fuse unti ony one duct remains at the tip of the meduary cone (Fig. 2). This one duct is termed a uretera branch and is continuous with the ureter. Unike the mammaian kidney where a tip of papia fits into the rena pevis or cayx, there is no break between the ureter and the meduary cones. The gross shape of the meduary cone may differ from the simpe curve shown in Fig. 2. The cones may be twisted into more compex shapes resembing the etter c or s (2). Downoaded from by on Apri 13, 217

5 NEPHRON FUNCTION IN DESERT QUAIL 621 CONTROL DIURESIS SNGFR (n/min) MEAN ALL MAMMALIAN TYPE SA,LT LOAD SNGFR (n/min) MEAN AL L MAMMALIAN TYPE LOOP OF HENLE LENGTH (mmi MEAN ALL M AMMALIAN TYPE PROX. CON. TU 8. LENGTH ( mm) MEAN ALL MAMMALIAN TYPE MAMMALIAN TYPE MAMMALIAN TYPE REPTILIAN TYPE LONG LOOP SHORT LOOP 15.8~.81 ( f 1.24 (7) ( 27) 13.4f.9 (26) 12.4f.66 (44) (7) (46) 1.5+,.5 (51) 2.+,.? (97) 3. (42) 3.4kO.12 (52) 3.5kO.8 (94) (41). (15) 1.6+,.9 (4) GLOMERULAR VOL. (n) ).237 f.255 (5) (29) MEAN ALL MAMMALIAN TYPE.245f:O.O184 (86) FIG. 3. Three representative nephrons of types that can be found in avian kidney, Nephrons were a photographed at same magnification and enarged to same degree. Data are means & SE with sampe numbers given in parentheses. Downoaded from by on Apri 13, 217 Efects of 6% NaC 2n. f usion on pasma osmoaity and sodium concentration. The sodium choride infusion produced an increase in pasma osmoaity of about 12 mosm/iter for each 4 meq/kg infused. The contro vaue for the pasma osmoaity was 378 & 2.6 mosm,/iter (mean & SE of 14 determinations). The pasma osmoaity reached a high of 5 mosm/iter on one occasion after 5 meq/kg of NaC had been infused, an increase of 122 mosm/iter over the contro vaue (Fig. 4). The increase in pasma osmoaity coud be accounted for by the increase in the pasma sodium concentration (Fig. 4). The contro vaue for pasma sodium concentration was 178 Jo 4.2 (mean =t SE of 21 determinations) meq/iter. The pasma sodium concentration rose as high as 286 meq/iter, an increase of 18 meq/iter from the contro eve, when 5 meq/kg NaC had -been infused. Efect of 6% NaC 2n f usion on urine fow and gomeruar and tubuar functions. The urine fow remained very near the contro eve (.186 =t:.8 m/kg per min; mean & SE of 11 determinations; Tabe 1) during the eary phases of the saine infusion. After 23 meq NaC/kg had been infused, the urine fow began to fa and fe to about 2 % of the contro vaue when 4 meq NaC/kg had been given (Fig. 5). The gomeruar fitration rate foowed a pattern simiar to that of the urine fow (Fig. 5). The mean contro GFR was.882 &.36 m/kg per min (SE of 97 determinations Tabe 1). The fitration rate remained at, or sighty beow, the contro vaue unti 23 meq NaC/kg had been given at which point the fitration rate began to fa. The fa in GFR was not as rapid initiay as the fa in urine fow. However, at the point where they both began to fa (foowing infusion of 23 meq NaC,ikg), the mean urine fow was sighty above the contro vaue whie the mean GFR was

6 622 E. J. BRAUN AND W. H. DANTZLER 2 t PLASMA OSMOLALTY o URINE OSMOLALITY No C INFUSED meq/kg BODY WEIGHT FIG. 4. Pasma osmoaity, urine osmoaity, and pasma sodium concentration during sodium choride infusion. Each point for pasma osmoaity and pasma sodium, except where no standard error (SE) is given, represents a mean of at east 5 vaues. Each point for urine osmoaity, except where no SE is given, represents a mean of at east 6 vaues. No SE is given for any point that had ess than 4 vaues. Vertica ines above and beow each point = AZ SE. 16. URINE FLOW 14 GFR I P -- I --- T GFR in the present study. i --- I i ---! ir w-e ( E 1 Ii ;r 7 II No CL INFUSED meq/kg BODY WEIGHT FIG. 5. Urine fow and gomeruar fitration rate during sodium choride infusion. Vaues are expressed as percents of average vaue of 4 contro periods in same anima before start of sodium choride infusion. For both urine fow and fitration rate, except where no standard error (SE) is given, each point represents mean of at east 6 vaues. No SE is given for any point that had ess than 4 vaues. Vertica ines above and beow each point = 3~ SE. sighty beow the contro vaue (Fig. 5). Like the urine fow, the GFR fe to about 2 % of the contro vaue after 4 meq NaC/kg had been infused. With the sighty increased urine fow and sighty decreased GFR during the eary phases of the NaC infusion, the inuin urine-to-pasma (U/P) ratio decreased moderatey from the contro of 4.7 rf:.12 (SE of 98 determinations) to a ow of 3.2 &.14 (SE of 11 determinations) after the infusion of 23 meq NaC/kg (Fig. 6). As both urine fow and GFR fe with continued hyperosmotic NaC infusion, the inuin U/P ratio started to increase and eventuay exceeded the contro vaue. The tota osmoaity of the urine increased but remained beow that of the pasma in the face of the marked osmotic diuresis induced by the mannito and NaC infusion (Fig. 4). The urine-to-pasma osmoaity ratio remained quite constant unti meq NaC/kg had been infused. At this point, when urine fow and GFR were quite ow, the tubuar reabsorption of water increased in spite of the severe osmotic effect of the sat oad. This is indicated by urine osmoaities equa to or exceeding the corresponding pasma osmoaities (Fig. 4). As might be expected, the percent of fitered sodium reabsorbed by the rena tubues decreased during the infusion of the sat oad. As can be seen in Fig. 6, the mean sodium cearance increased from about 7.5 % of that fitered to about 23 %. Individua nephron function based on Tm,,,. It was first suggested by Ranges et a. (29) that the Tm for gucose coud be studied to determine whether changes in GFR resuted from changes in fitration by each gomeruus or from changes in the number of functioning gomerui. If changes in GFR resuted from changes in the amount fitered by each gomeruus, but a continued to function, the Tm for gucose woud not be expected to change. If, however, changes in GFR resuted from changes in the number of functioning gomerui, the Tm woud be expected to vary directy with GFR. Studies on frogs ( 15, 35) and chickens (7) showed that the Tm for PAH, a substance secreted by the rena tubues, varied with GFR in the same fashion as the Tm for gucose, a substance reabsorbed by the tubues. Athough there is now some controversy over the use of TrnPAH as a measure of functiona tubuar mass ( 12, 42), we have examined the reationship between TmpAH and The tubuar maximum for PAH decreased with decreasing GFR, suggesting, according to the cassica concept, that gomerui function intermittenty. This reationship is shown for a representative anima in Fig. 7. The mean coefficient of correation for TmPAH vs. GFR is.912 &.19 for 11 experiments. Individua nephron function based on histoogica data. If individua gomerui stopped fitering when GFR decreased, the proxima tubues of these nephrons woud be expected to coapse as the remaining fitrate was reabsorbed (34). Therefore, cosed proxima tubue umina woud indicate gomerui that had ceased functioning. An increase in the number of these cosed umina with a decrease in fitration rate woud support the concept that changes in overa GFR resut from changes in the number of individua functioning gomerui. To examine this possibiity, we made histoogica sections of kidneys from a bird in a contro N&IN X 2- =xr 15 - IO- == 5-8 I: = I: INULIN 6 U/P 4 O o L 2 O T L, n OO I I I NaC INFUSED meq/kg BODY WEIGHT FIG. 6. Inuin urine-to-pasma (U/P) ratios and sodium cearances during sodium choride infusion. Sodium cearance is shown as percent of inuin cearance. Standard errors for inuin U/P ratios were too sma to be shown on figure. However, each point is mean of at east 6 vaues except ast one, which is a mean of 2 vaues. For sodium cearance each point, except where no standard error (SE) is shown, represents mean of at east 6 vaues. No SE is given for any point with ess than 4 vaues. Downoaded from by on Apri 13, 217

7 NEPHRON FUNCTION IN DESERT QUAIL F 2 \ e z E Correotion Coefficient / (Mean Vaue for A Studies) / / 912+ o.op /,/. 1 / /. / I /,/.,A I /,/. I GFR m/kg/min FIO. 7. Reationship between Tmpm and GFR for a representative anima. diuresis and from a bird subjected to an intravenous sodium choride infusion. The bird in the contro diuresis had received 9 min of a 2.5% mannito infusion at.4 m/kg per min before the kidneys were taken for histoogy. The anima subjected to a sat oad had received 43 meq NaC/kg infused as a 6 % NaC soution before its kidneys were taken for histoogy. At this time, the fitration rate woud have been about 2% of the contro eve (Fig. 5). Since it appeared most ikey that RT nephrons woud cease to fiter when GFR decreased, sections from superficia areas where ony RT nephrons are ocated were studied. Sections of kidneys from the sat-oaded anima were compared with sections from the same areas of the kidneys from the contro anima. The proxima convouted tubue can be identified easiy in cross section in birds, as in mammas, by the presence of a microvius border. The sections were examined for the presence of tubues with open and cosed umina. The number of each were counted by two independent observers, one of whom did not know whether the sections came from contro or satoaded birds, and the resuts were pooed. The resuts were expressed as the percentage of proxima tubues with open umina in any one microscopic fied (examined at a magnification of X45, ca. 32 tubues in each fied). On the eft side of Fig. 8 is a representative section showing RT proxima convouted tubues from a kidney of a bird in a contro marmito diuresis. As can be seen in Fig. 8, most of the proxima tubue umina are open. In the sections examined from the contro kidneys, an average of 71 f 3.1% (SE of 5 determinations) of the proxima tubue umina were fuy open. On the right of Fig. 8 is a representative section showing RT proxima convouted tubues from a kidney of a sat-oaded bird. As can be seen, most of the tubue umina are cosed. In the sections examined, ony an average of 16 f 2.2% (SE of 58 determinations) of the proxima tubue umina were fuy open. The arge number of coapsed tubues in kidneys from sat-oaded birds supports the idea that the observed decrease in GFR resuts from a decrease in the number of fitering RT nephron;. Sections through the kidneys cutting across MT proxima convouted tubues aso incuded RT proxima convouted tubues. Since it was not possibe to differentiate with certainty between RT and MT proxima convouted tubues in these sections, we compared sections through meduary cones from kidneys from contro and sat-oaded birds. As noted above, the meduary cones contain the oops of Hene from the MT nephrons, the coecting ducts, and the vasa recta. Figure 9 shows cross sections of meduary cones from the same kidneys as those shown in Fig. 8. A oops of Hene in these and simiar sections from contro and satoaded animas had open umina. These observations suggest that MT nephrons continue fitering when GFR decreases. Individua nephron function based on measureme& of SNGFR. Since the Tmr, data and the histoogica data suggested that some nephrons stopped fitering when the tota GFR decreased during the administration of a sat oad, we measured the fitration rates of individua nephrons using the modification of Hanssen s (17) technique deveoped by de Roufhgnac, Diess, and Bonvaet ( 13). This technique was chosen for severa reasons. First, the avian kidney is tighty fitted into the bony synsacrum from which it cannot be freed in the iving anima without damage. This, aone, woud make an approach by micropuncture techniques extremey difficut. Second, the speciaized nature of the avian respiratory system makes it difficut to keep the body cavity open for ong periods of time. As noted above (see METHODS), this technique requires the opening of the body cavity for ony a brief period. Third, a direct micropuncture approach woud have permitted study of ony the superficia RT nephrons. The present technique permitted identification and study of nephrons from both the reptiian and mammaian popuations. The mean vaues for SNGFRs for reptiian-type and mammaian-type nephrons during the contro diuresis and PIO. 8. Photomicrographs comparing proxima convouted tubues of reptiian-type nepgo6 from a hy&&ed bird (right) and a satoaded bird (Itft). Note coapsed nature of tubues on right, X34, Downoaded from by on Apri 13, 217

8 624 E. J. BRAUN AND W. H. DANTZLER / CONTROL OIUAESIS SALT LOAD FIG. 9. Photomicrographs comparing sections from meduary cones taken from a hydrated kidney (right) and a sat oaded kidney (eft). Magnification is X34 in both cases, but section on eft is taken nearer to tip of cone than section on right. during a sat oad are shown in Fig. 3. Since a reptiiantype nephrons have a simiar structure (Figs. 2 and 3), they were considered as a singe group for purposes of presentation. However, for the purpose of summarizing.the data, the mammaian-type nephrons were arbitrariy divided into two groups based on the ength of the oop of Hene (Fig. 3). The separation was made by isting the engths of the oops of,hene in rank order. The break was made between engths of 2.5 and 2.2 mm since no oops had engths between these vaues. The mean ength of the oop of Hene for the mammaian-type nephrons with ong oops (MT-L) was 2.7 mm (Fig. 3). That for the mammaian-type nephrons with short oops (MT-S) was 1.5 mm (Fig. 3). The difference between these engths is significant at the.1 eve of probabiity. The oops of Hene for a mammaian-type nephrons studied ranged in ength from.7 to 3.7 mm. The mean vaues for the engths of the proxima convouted tubue and the gomeruar voumes for these two groups of mammaian-type nephrons were not significanty different. During the contro diuresis, the mean SNGFR for MT-L nephrons was 15.8 n/min and that for MT-S nephrons was 1.9 n/min (Fig. 3). The difference between these mean SNGFRs is significant at the.1 eve of probabiity. The mean SNGFR for the RT nephrons was 6.4 n/min (Fig. 3). This is significanty ess than that for either the MT-L or MT-S nephrons (P <.1). Thus, during a contro diuresis, a definite gradient existed in the SNGFRs from the MT nephrons with the ongest oops to the short RT nephrons. The mean SNGFR for a mammaian-type nephrons combined (14.6 n/min) (Fig. 3) was aso significanty greater than that for the RT nephrons (P <.1). Foowing a sat oad of 4 meq/kg, the mean SNGFR for the MT-L nephrons decreased and that for the MT-S nephrons increased compared with the vaues during a contro diuresis (Fig. 3). Athough these changes were not statisticay significant, the SNGFR for a mammaian-type nephrons foowing a sat oad ( 12.7 n/min) was significanty ower (P <.5) than that for a mammaian-type nephrons during the contro diuresis (Fig. 3). Foowing the sat oad, no nonradioactive bue ferric ferrocyanide precipitate was found within the umen or gomeruus of any of the reptiian-type nephrons, and it appeared that the nephrons were not fitering (Fig. 3). Instead, the bue precipitate was found in fine strands ying around the repitiian-type nephrons (Fig. 1). These fine strands, which were not seen when the nephrons were dissected from kidneys during a contro diuresis, appeared to represent the peritubuar capiaries. Thus, athough the nonradioactive puse of sodium ferrocyanide did not appear to be fitered by the RT nephrons, it did appear to reach the capiary network surrounding them. These data, indicating that the nonradioactive sodium ferrocyanide reached the RT nephrons foowing a sat oad but was not fitered, support the concept that the observed decrease in GFR in sat-oaded birds resuted primariy from a decrease in the number of functioning RT nephrons. ReationshiP between SNGFR and proxima convouted tubue ength. The reationship between SNGFR and proxima convouted tubue (PCT) ength for a nephrons dissected from the kidneys of animas in a contro diuresis is shown in Fig. 11. When both RT and MT nephrons were considered, there was a significant positive correation between SNGFR and PCT ength (r =.7895 for 64 nephrons). A simiar correation was found for the RT nephrons aone (Y =.7326 for 39 nephrons). However, the correation between SNGFR and PCT ength for the MT nephrons aone was not neary as marked (r =.3313 for 25 nephrons). In Fig. 12 is shown the reationship between SNGFR and PCT ength for nephrons isoated from the kidneys of animas that had been subjected to a hyperosmotic saine infusion. Since no SNGFRs coud be measured for nephrons of the repitiian type after a sat oad, a the data shown are for nephrons of the mammaian type. Again there was a significant positive correation between SNGFR and PCT ength when a nephrons were considered (I =.6247 for pro. 1. Isoated reptiian-type nephrons showing peritubuar capiaries fied with ferric ferrocyanide precipitate (arrows indicate precipitate). X 9. I Downoaded from by on Apri 13, 217

9 : NEPHRON FUNCTION IN DESERT QUAIL RT MT 8 O I I I I I I I PROXIMAL TUBULE LENGTH (mm) FIG. 11. Reationship between SNGFR and proxima convouted tubue ength for nephrons isoated from kidneys in contro diuresis PROXIMAL TUBULE LENGTH (mm) FIG Reationship between SNGFR and proxima convouted tubue ength for nephrons isoated from kidneys foowing a sat oad. A nephrons are mammaian type. 7 1 nephrons). However, when this same reationship for MT nephrons from each individua sat-oading experiment was examined, the correation did not hod consistenty. For one experiment, the correation between SNGFR and PCT ength was significanty positive (r =.6363 for 24 nephrons). For two other sat-oading experiments, however, the correation coefficients were (for 24 nephrons) and (for 23 nephrons). Reationship between SNGFR and gomeruar voume. The reationship between SNGFR and gomeruar voume for a nephrons dissected from the kidneys of animas in a contro diuresis is shown in Fig. 13. When both RT and MT nephrons were considered, there was a significant positive correation between SNGFR and gomeruar voume (r =.7323 for 35 nephrons). A simiar strong positive correation was found when this reationship was considered for the RT nephrons aone (r =.9473 for 16 nephrons). However, this correation between SNGFR and gomeruar voume did not hod for the MT nephrons aone (r =.759 for 19 nephrons). The SNGFR was potted against the gomeruar voume for a nephrons isoated from the kidneys of animas that had been subjected to a hyperosmotic sat oad in Fig. 14. Since no RT nephrons that were fitering were dissected from these kidneys, a the nephrons considered were of the mammaian type. When a these MT nephrons from satoaded animas were considered (Fig. 14), there was a significant positive correation between SNGFR and gomeruar voume (r =.6763 for 64 nephrons). When this same reationship for the individua MT nephrons was considered separatey for each sat-oading experiment, however, the correation was not so marked. The correation coefficients for the individua experiments were.2382 (for 21 nephrons),.1134 (for 24 nephrons), and.6289 (for 19 nephrons). Gomeruar counts and reationship of SNGFR to tota-kidney fitration rate. Using the technique of Damadian, Shwayri, and Bricker (6), we found 46,778 =t 4,955 (mean rt SE of 9 determinations) gomerui per quai kidney. The very arge gomerui from the arge MT nephrons coud be differentiated easiy from the much smaer gomerui of the RT nephrons (see data in Fig. 3 for mean gomeruar voumes; aso note that, because of the way the gomeruus of the RT nephron in Fig. 3 is foded upon the proxima convouted tubue, it appears to be about twice its actua size). However, the- range of gomeruar voumes for MT nephrons (Figs. 13 and 14) made it impossibe to differentiate the gomerui of the smaest MT nephrons from those of RT nephrons. Consistenty, 1% of the tota gomerui counted were of the very arge size from MT nephrons. Using the mean gomeruar count and the mean SNGFR vaue for different nephron- types, we cacuated the totakidney fitration rate. For animas in a contro diuresis, we assumed that 1 % (4,678) of the nephrons were of the mammaian type and that the mean SNGFR for these nephrons was 14.6 n/min (Fig. 3). From these figures (4,678 X 14.6), we cacuated that the fitration rate for a the MT nephrons during a contro diuresis was.68 m/ min. Assuming that the remainder of the nephrons (42,1) m8 O* e e e m RT I1 11 L L u I I.I4.I GLOMERULAR VOLUME (r-d) FIG. 13. Reationship between SNGFR and gomeruar voume for nephrons isoated from kidneys in contro diuresis. MT 625 Downoaded from by on Apri 13, 217

10 k I I1 h I, I I I I I,,,, I I,,,,,, (,, 626 E. J. BRAUN AND W. H. DANTZLhK 2% - = 2- < c E (3 z IO- ee e FIG. 14. Reationship between SNGFR and gomeruar voume for nephrons isoated from kidneys foowing a sat oad. A nephrons are mammaian type. 5-.2,6.I.I4.I8.22, ,62 GLOMERULAR VOLUME (n) were of the reptiian type and 71% of these were fitering during a contro diuresis (see histoogica data and Fig. 8) at an average rate of 6.4 n/min (Fig. 3), we cacuated (29,891 X 6.4) that they woud contribute.191 m/min to the overa fitration rate of the kidney. Thus, the overa GFR for the kidney during a contro diuresis cacuated from the SNGFRs for the MT and RT nephrons was.259 m/ min. The mean vaue for the GFR of the kidneys from which the nephrons were taken for determination of the SNGFRs, determined from the contro cearance periods before the kidney was frozen, was.29 &.5 m/mm (SE of four determinations). For animas that had received 4 meq NaC/kg, we assumed that the 4,678 MT nephrons were fitering at the rate of 12.7 n/min each (Fig. 3). From these figures (4,678 X 12.7) we cacuated that the MT nephrons woud have contributed.59 m/min to the overa GFR of the kidney. Despite the fact that no bue precipitate was seen in any individua RT nephrons examined, we assumed from the histoogica data (Fig. 8) that 16 % of the RT nephrons (6,73 7) were fitering at an average rate of 6.4 n/min each (Fig. 3). From these figures (6,736 X 6.4)) we cacuated that the RT nephrons woud contribute.43 m/min to the overa GFR of the kidney. Thus, the overa GFR for the sat-oaded kidney, cacuated from the SNGFRs for the MT and RT nephrons was.12 m/min. The mean GFR for the three sat-oaded kidneys from which these nephrons were taken, determined from the ast cearance period just before each kidney was frozen, was.126 m/min. DISCUSSION Effects of NaC infusion on GFR for whoe kidney. In the present study, a hyperosmotic sodium choride infusion produced a marked decrease in gomeruar fitration rate in Gambe s quai. This began eary during the infusion and tended to precede sighty the decrease in urine fow rate. The decrease in GFR with a constant infusion of a hyperosmotic saine soution was simiar to that observed under simiar conditions with domestic chickens (7). In both that study and the present one the decrease in GFR began after the infusion of about 2 meq NaC/kg. However, there were some differences between the two studies. In the present study on desert quai, the GFR decreased by about 8% from a contro vaue of about.9 m/kg per min after the infusion of 43 meq/kg. At this point, the pasma osmoaity had increased about 1 mosm. In the previous study on domestic chickens, the GFR decreased by about 6 % from a contro vaue of about 1.2 m/kg per min after the infusion of 45 meq/kg. At this point, the pasma osmoaity of the chicken had increased about 15 mosm. These data woud suggest that the fitration rate of the desert quai is more sensitive to increases in pasma osmoaity than that of the domestic chicken. However, athough both the desert quai and the domestic chickens had adequate access to water before the experiments and their weights were stabe, the quai may have maintained a naturay smaer extraceuar fuid voume compared with the chickens. This is suggested by the observation that, at the start of the experiments, the pasma osmoaity of the quai was about 5 mosm higher than that of the chickens. Maintenance of a naturay smaer extraceuar fuid voume and higher pasma osmoaity by the quai may have accounted for their ower initia GFR and greater sensitivity to further increases in pasma osmoaity. Such differences in hydration might aso have accounted for the observation that the same sodium choride infusion caused a marked increase in urine fow initiay in chickens (7) but very itte change in desert quai. It shoud aso be noted, however, that the quai in the present study were anesthetized whie the chickens in the earier study were not. This might aso expain some differences in sensitivity of fitration rate to an increase in pasma osmoaity. With sodium choride infusions of ess than 2 meq/kg no effect on GFR was observed. These observations agree with those of Skadhauge and Schmidt-Niesen (38) on domestic fow. As suggested by these authors, the increase in pasma osmoaity produced by this infusion, which tends to depress GFR, may have been counterbaanced by an 1 increase in extraceuar fuid voume, which tends to increase GFR. Effects of NaC infusion on individua nephron function. The observed decreases in GFR for the whoe kidney might have been the resut of decreases in the number of functioning gomerui or the resut of decreases in the fitration rates of a the nephrons. The reationship between TmPAH and GFR suggests that decreases in GFR in Gambe s quai resut from decreases in the number of functioning gomerui. This correation between TmPAH and GFR is simiar to that observed earier between TmPAn and Tmo and GFR in domestic chickens. However, the studies of Deetien and Downoaded from by on Apri 13, 217

11 NEPHRON FUNCTION IN DESERT QUAIL 627 Sonnenberg ( 12) have indicated that TmpAH may be determined by the intraumina concentration of PAH and the fow through the tubues. Thus, TmPAH may not be an adequate indication of functioning tubuar mass or, in this case, of the number of functioning gomerui. The findings of Deetjen and Sonnenberg ( 12) have been chaenged recenty by the studies of Tanner and Isenberg (42) which do indicate a reationship between TmPAH and the mass of functioning tissue. Aso, the data of Deetjen and Boyan (11) and Rhode and Deetjen (3 1) suggest that gucose reabsorption is dependent on mass of tubuar tissue as we as deivery to tubuar sites and possiby eectroyte reabsorption. Thus, the reationship of Tmo to GFR in the earier study on chickens (7) may give a better indication of 4 omeruar intermittency in the avian kidney. Even if the reationship between TmPAH and TmG and GFR does indicate gomeruar intermittency, it sti gives no direct indication of which nephrons are ceasing to function. The histoogica studies support the concept of gomeruar intermittency and indicate that ony the RT nephrons cease functioning. It coud be argued that the coapsed proxima tubue umina of RT nephrons seen in histoogica sections from sat-oaded animas were the resut of postmortem changes. However, the kidneys from the sat-oaded animas and the animas in a contro diuresis were treated in exacty the same manner in preparation for the histoogy and any postmortem changes shoud have been present in sections from contro as we as sat-oaded kidneys. Moreover, the sections through the meduary cones from both contro and sat-oaded kidneys showed no coapsed tubues or other evidence of postmortem changes. Since the meduary cones are deep to the superficia RT nephrons (Fig. Z), they woud have been more ikey than the superficia nephrons to undergo postmortem changes during the time between the fooding of the kidneys with ice-cod Formain and their pacement in a bath of ice-cod Formain. Thus, it seems highy unikey that postmortem changes coud have accounted for the histoogica differences between contro and sat-oaded animas. As noted earier (see RESULTS), it is not possibe to make sections across MT proxima tubues without cutting across RT proxima tubues (Fig. 2). Since the proxima tubues of the two types of nephrons cannot be easiy differentiated in histoogica cross section, ony sections through oops of Hene in the meduary cones can be used to indicate if MT nephronc are functioning. Since it is possibe that oops of Hene woud not coapse even when fitration ceased, open oops of Hene may not be an adequate indication of functioning MT nephrons. The use of the modification of Hanssen s ( 17) technique deveoped by de Rouffignac, Diess, and Bonvaet ( 13) has made it possibe to determine directy the gomeruar fitration rates of individua nephrons of both mammaian and reptiian types. Using this technique, we found that even during a contro diuresis the SNGFR of the MT nephrons was more than twice that of the fitering RT nephrons. Foowing a severe sat oad, there was a barey significant decrease in the SNGFRs of MT nephrons, but the RT nephrons examined appeared to have ceased fitering. This is strongy suggested by the fact that no nonradioactive ferric ferrocyanide (Prussian bue) precipitate was found in the tubuar umina or gomerui of any RT nephrons examined. That the nonradioactive ferrocyanide reached these nephrons in the time from the injection of the puse to the freezing of the kidneys is indicated by the finding of the nonradioactive precipitate in the peritubuar capiary network surrounding the RT nephrons. If this nonradioactive ferrocyanide had passed through the gomeruar capiaries to reach this peritubuar network, at east some of it shoud have been fitered and found in the gomerui or tubuar umina. Any nonradioactive ferrocyanide fitered by the RT nephrons shoud sti have been in the tubuar umina at the time of freezing. It woud not have passed through the nephrons into the coecting ducts in the time between injection and freezing since precipitate was sti in the MT nephrons and, even under contro conditions, the fitration rate of the RT nephrons was much ess than that of the MT nephrons. Moreover, no precipitate of nonradioactive ferrocyanide was found in the coecting ducts. Thus, it appears that these RT gomerui were not fitering and that bood bypassed them into the peritubuar capiary network. An anatomica basis for such a bypass can be found in the studies of Sier and Hinde (36) on domestic fow. They studied the vascuar anatomy of the fow kidney by various injections, corrosion, and radiographic techniques. They found occasiona straight vesses, as we as the afferent arterioes to the gomerui, arising from the main arteries within the cyindrica units. These straight vesses bypassed the gomerui and appeared to enter the capiary network in the periphery of the cyindrica units. The authors referred to these as gomeruar bypass vesses but coud attach no functiona significance to them. From the resuts of the present study it now appears ikey that, when fitration rate fas, the afferent arterioes to the RT gomerui constrict, reducing the number of functioning nephrons, and the bood in the main arteria vesses is shunted through the straight bypass vesses into the peritubuar capiary network. The factors responsibe for reguating this reduction in the number of RT gomerui in response to rising pasma osmoaity are unknown. Arginine vasotocin ( AVT), the naturay occurring antidiuretic principe in a birds, repties, and amphibians examined (24, 25, 32) appears to reguate the number of functioning nephrons in repties (8) and amphibians ( 19, 44). Earier studies (7, 37) showed no effect of even massive doses of AVT on avian gomeruar fitration rate. However, more recent studies (E. Skadhauge, persona communication) have suggested that AVT may reduce avian GFR. Thus, AVT may act to constrict the afferent arterioe to RT nephrons, reducing the number of functioning gomerui and eading to a redistribution of rena bood fow. Other humora factors such as epinephrine, which heps to reguate the tota porta bood fow to the avian kidney (3), may aso pay a roe in reguating the number of functioning RT gomerui and the intrarena distribution of the arteria bood suppy. Reationship of SNGFR to tota-kidney fitration rate. The fitration rate for the whoe kidney during a contro diuresis cacuated from the SNGFRs was about 25 % higher than the measured cearance for the same kidneys. From these cacuations, it might be concuded that the vaues obtained Downoaded from by on Apri 13, 217