Acute Heat Acclimation and Kidney Function in Broilers 1

Transcription

1 Acute Heat Acclimatin and Kidney Functin in Brilers 1 ROBERT F. WIDEMA 2 and BOIE. FORD Department f Pultry Science, The Pennsylvania State University, University Park, Pennsylvania JAMES D. MAY and BERRY D. LOTT USDA, Agricultural Research Service, Suth entral Pultry Research Labratry, Mississippi State, Mississippi ABSTRAT Brilers previusly expsed t high envirnmental temperatures (heat-acclimated) are mre resistant t heat stress and cnsume mre water during heat stress than nnacclimated cntrls. Tw experiments were cnducted t determine whether heat-acclimated brilers cnserve bdy water by reducing urine and slute (a) excretin. In the first experiment, renal functin studies were cnducted at an ambient temperature (T a ) f apprximately 21 using anesthetized 7-wk-ld male brilers. ntrl birds reared at a cnstant T a f (Grup : nncycled T a ) were cmpared with birds that had been heat-acclimated by expsure fr 3 t 6 d t a daily sinusidal cycle f t 35 t (Grup : cycled T a ). In the secnd experiment, renal functin studies were cnducted n anesthetized 5-wk-ld cntrl and heat-acclimated male brilers while they were expsed t a T a f 21 (Ambient T a : Grups A, A), r t a T a f 32 (High T a : Grups H, H). When high intravenus infusin rates (.37 ml/kg bdy mass per min) were used t simulate the vlume expansin caused by thermgenic plydipsia, urine flw rates were significantly lwer in Grups and A than in Grups and A, smlal clearances were lwer in Grups A and H than in Grups A and H, and all heatacclimated grups in bth experiments (Grups, A, H) had significantly lwer glmerular filtratin rates (GFR), filtered lads f a, and tubular a reabsrptin rates than the respective cntrl grups (Grups, A, H). These changes in kidney functin ptentially wuld minimize urinary fluid and slute lss when heat-acclimated brilers cnsume large quantities f water t supprt evaprative cling. Reductins in GFR, filtered lads f a, and tubular a reabsrptin rates als may help heat-acclimated brilers reduce the metablic heat lad assciated with active (energy requiring) recvery f slute (a) frm the glmerular ultrafiltrate. (Key wrds: briler, heat acclimatin, urine flw, sdium excretin, fluid balance) Received fr publicatin June 10, Accepted fr publicatin September 1, Trade names in this article are used slely t prvide specific infrmatin. Use f trade names des nt cnstitute a guarantee r warranty by USDA and des nt signify that the prduct is apprved t the exclusin f ther cmparable prducts. 2 T whm crrespndence shuld be addressed: E207 Animal Sciences Bldg., Department f Pultry Science, University f Arkansas, Fayetteville, AR Pultry Science 73:75-88 ITRODUTIO Dmestic fwl acclimated t a high, cyclic ambient temperature (T a ) are mre resistant t heat stress than cntrl birds that have experienced a mderate, cnstant T a. Strains f dmestic fwl genetically selected fr fast grwth and meat prductin (brilers) attain cnsiderable resistance t heat stress after nly 3 d expsure t a t 35 t daily

2 76 WIDEMA ET AL. acclimatin cycle (Reece et al, 1972; May et al, 1987; Ltt, 1991). In additin t cardivascular, respiratry, and metablic adaptatins (Hutchinsn and Sykes, 1953; Sturkie, 1967; Weiss et al, 1963; hwalibg, 1990), acute and chrnic heat acclimatin invlve specific adaptatins that attenuate dehydratin and hypvlemia during heat stress. These adaptatins help maximize the bdy water reserves essential fr sustaining evaprative cling and serve t maintain bld vlume at levels sufficient t supprt an increased cardiac utput when thermally induced peripheral vasdilatin enhances cnvective heat dissipatin (Rmjin and Lkhrst, 1961; Whittw et al, 1964; Siegel, 1969; Whittw, 1976; Van Kampen, 1981; Darre and Harrisn, 1987). Fr example, water cnsumptin increases t match r exceed increased respiratry water lss during heat stress (Fx, 1951; Van Kampen, 1981; Brantn et al, 1986; Sykes and Fataftah, 1986; Smith and Teeter, 1987; Belay and Teeter, 1993; Belay et al, 1993), and acutely heat-acclimated birds cnsume mre water than nnacclimated cntrls during heat stress (Ltt, 1991). mplex renal and endcrine respnses influence water balance when pultry are expsed t a high T a. Fr example, heat stress stimulates the secretin f avian antidiuretic hrmne (arginine vastcin: AVT) in nnacclimated brilers (Wang et al, 1989), and nnacclimated laying hens restricted t fixed-rate intravenus infusins as their sle surce f water "intake" exhibit an antidiuretic respnse (reduced urine flw rates, increased urine smlalities) as T a increases frm 20 t 32 (Azahan and Sykes, 1980). Increasing T a t 40 in the same hens caused a significant increase in cre bdy temperatures, triggering a diuresis (increased urine flw, decreased urine smlality) that presumably reflects the renal respnse t stressinduced catechlamine secretin (Edens and Siegel, 1975; Azahan and Sykes, 1980). In a separate study, clstmized hens permitted ad libitum water cnsumptin exhibited a psitive crrelatin between drinking water intake and urine flw ver a T a range f 5 t 35 (Van Kampen, 1981). This bservatin indicates that the cmmnly bserved increase in excreta misture during heat stress represents a secndary cnsequence f increased urine flw, rather than direct passage f unabsrbed drinking water thrugh the intestinal tract (Van Kampen, 1981). Increased water cnsumptin, increased urine flw, and decreased urine smlality als were bserved in clstmized brilers during heat stress (T a = 32 t 35 ), and this diuresis persisted when water availability was restricted, supprting previus evidence that stress-induced influences n kidney functin may substantially alter water balance when brilers are expsed t a high T a (Belay and Teeter, 1993; Belay et al, 1993). These bservatins indicate that the renal respnses f dmestic fwl during heat expsure depend n whether a negative (dehydrated) r psitive (vlume expanded) water balance has been established and n the degree f acute stress that has been induced (Azahan and Sykes, 1980; Van Kampen, 1981). An AVTinitiated antidiuresis clearly wuld be advantageus if limited water intake and increased evaprative water lss cause a negative water balance t develp (Wang et al, 1989), whereas a diuresis triggered by stress wuld be disadvantageus under the same circumstances (Azahan and Sykes, 1980). If water is freely available, increased excretin f dilute urine may prevent excessive extracellular fluid vlume expansin and hemdilutin caused by thermgenic plydipsia (Van Kampen, 1981). Increased thrughput f drinking water, warmed t bdy temperature and excreted as dilute urine, als may cntribute mdestly t heat dissipatin as lng as cl drinking water is prvided (Fx, 1951; Van Kampen, 1981; Pardue et al, 1985; Belay et al, 1993). Hwever, bdy water reserves are valuable during heat stress, and renal electrlyte excretin tends t increase when diuresis is induced by vlume expansin (Gregg and Wideman, 1990) r by heat stress (Azahan and Sykes, 1980). Additinal electrlyte may be lst in tine frm f catins (a+, K+) accmpanying increased urinary bicarbnate excretin during heat stress (Deetz and Ringrse, 1976; Azahan and Sykes, 1980; Bttje and Harrisn, 1985). There-

3 HEAT ALIMATIO AD KIDEY FUTIO 77 fre, electrlyte acquisitin and retentin may be essential fr maintenance f extracellular fluid istnicity during thermgenic plydipsia. In this cntext, lwer rates f water cnsumptin by nnacclimated birds during heat stress (Ltt, 1991) may at least partly arise frm the need t minimize urinary electrlyte excretin. Electrlyte additin is mre effective in stimulating water cnsumptin in nnacclimated man in heat-acclimated brilers (Ltt, 1991; Ltt and May, unpublished data), and electrlyte additin increases water cnsumptin and reduces mrtality during heat stress (Brantn et ah, 1986). These cnsideratins suggest the prcess f heat acclimatin ptentially may include adaptatins in kidney functin that facilitate water and electrlyte retentin. The influence f heat acclimatin n kidney functin has nt been previusly investigated in dmestic fwl; therefre the present study was designed t evaluate renal functin in nnacclimated and heat-acclimated brilers. Renal functin studies were cnducted at nrmal and high T a, using intravenus infusins t nrmalize the states f hydratin f birds in different experimental grups. MATERIALS AD METHODS Experiment 1 Prtcl Avian x Avian male chicks btained frm a cmmercial hatchery were placed n wd shavings litter in an envirnmentally cntrlled huse. Incandescent lighting was n cntinuusly, and feed and water were available fr ad libitum cnsumptin. rnsybean meal diets were frmulated t meet r exceed atinal Research uncil (1984) requirements. The T a was maintained at 29 fr the 1st wk and was reduced 2.7 /wk until 21 was reached. At 31 d f age, the birds were hused in tw envirnmental chambers. The T a in the cntrl chamber remained at a cnstant (Grup : nncycled T a ). The T a in the experimental chamber initially was set at until the birds reached 43 d f age, then 3Sigma hemical., St. Luis, MO the T a was changed t a daily sinusidal cycle f t 35 t (Grup : cycled T a ). The dewpint in bth chambers was 18 thrughut the study. Between 47 and 50 d f age, nine birds frm Grup ( kg bdy mass) and eight birds frm Grup ( kg bdy mass) were remved frm the envirnmental chambers and anesthetized with intramuscular injectins f allbarbital (5,5- Diallyl-Barbituric Acid; 3 25 mg/ml, 3 ml/ kg bdy mass). A surgical plane f anesthesia was maintained thrughut the renal functin studies, which were cnducted at an ambient labratry temperature f apprximately 21. A slutin cntaining 25 g mannitl/l was infused intravenusly at.1 ml/kg bdy mass per min (LOW infusin rate) thrugh a cannula inserted in the brachial vein. All birds were preequilibrated at this LOW infusin rate prir t the renal functin studies t nrmalize any preexisting differences in their relative states f hydratin. A cannula was inserted in the cartid artery fr bld sample cllectins. The intestine was ligated just prximal t the claca t prevent fecal cntaminatin f the urine, the claca was swabbed t remve residual material, and a plyethylene cannula was inserted in the claca fr urine sample cllectin as described previusly (Laverty and Wideman, 1989). After surgical preparatins were cmplete, a slutin cntaining 25 g mannitl/ L, 1.5 g para-aminhippuric acid (PAH)/L, and 1.5 g inulin/l was infused intravenusly at the LOW infusin rate fr 30 min t btain stable plasma inulin and PAH cncentratins. Fllwing this equilibratin perid, three cnsecutive urine samples were cllected (LOW infusin prtcl: Samples 1 thrugh 3). ext, t evaluate kidney functin during vlume expansin, the infusin rate was increased t.37 ml/ kg bdy mass per min and, after a 20-min re-equilibratin perid, six cnsecutive urine samples were cllected (HIGH infusin prtcl: Samples 4 thrugh 9). Finally, saline (.5M al) was added t the infusin slutin cntaining mannitl, inulin, and PAH, and six cnsecutive urine samples were cllected withut allwing additinal time fr re-equilibratin (SA LIE infusin prtcl: Samples 10

4 78 WIDEMA ET AL. thrugh 15). The intravenus infusin f hypertnic saline at the HIGH rate was designed t prvide additinal slute fr urinary excretin and t trigger endgenus AVT secretin (Dantzler, 1966). Each urine cllectin perid lasted 10 min. Arterial bld samples (1.5 ml each) were cllected during urine sampling intervals 2, 5, 8, 11, and 14. Experiment 2 Prtcl One year fllwing the first experiment, a secnd experiment was cnducted t evaluate the reprducibility f intergrup differences previusly bserved during vlume expansin at the HIGH infusin rate and t evaluate the pssible impact f the T a at which the renal functin studies were cnducted. Avian x Avian male chicks were btained frm a cmmercial hatchery and placed n wd shavings litter in envirnmental chambers. Temperature prtcls fr grups used in Experiment 2 are summarized in Table 1. The temperature was 29 the 1st wk, 27 the 2nd wk, and thereafter except fr during acclimatin r renal functin studies. The cntrl (Grup : nncycled T a ) brilers were maintained at. The acclimated (Grup : cycled T a ) brilers were placed in a chamber with a daily t 35 t cycle when they were 27 t 29 d ld and were expsed t the cyclic T a fr 6 r 7 d. The dewpint in all chambers was 18 thrughut the experiment. TABLE 1. Temperature prtcls fr nncycled () and cycled () grups in Experiment 2 expsed t ambient (A: 21 ) r high (H: 32 ) temperatures during renal functin studies Age r time Days 0 t 7 Days 8 t 14 Days 15 t 27 7 t 8 d prir t renal functin study 8 t 30 h prir t renal functin study During the renal functin study Grup A t 35 t (cycled) (nncycled) 21 Brilers were prepared fr renal functin studies as in Experiment 1. Fifteen nncycled brilers kg bdy mass) and 15 cycled brilers ( kg bdy mass) were used. Surgical preparatins and renal functin studies were cnducted either at an ambient (A) labratry T a f apprximately 21 fr six nncycled (Grup A) and six cycled (Grup A) brilers r in an envirnmental chamber at a high (H) T a f 32 fr nine nncycled (Grup H) and nine cycled (Grup H) brilers. Grup A was mved frm the cyclic temperature regimen t a cnstant chamber t 30 h befre renal functin determinatin. Grups H and H were mved t a cnstant 32 chamber 8 t 15 h befre the renal functin determinatin (Table 1). Thrughut the surgical preparatins and renal functin studies, all birds were infused intravenusly at the HIGH infusin rate (.37 ml/kg bdy mass per min) with a slutin cntaining 25 g mannitl/l, 1.5 g para-aminhippuric acid (PAH)/L, and 1.5 g inulin/l. A 60-min equilibratin perid was allwed fr vlume expansin prir t cllecting urine fr 1 h. A single arterial bld sample (1.5 ml) was cllected at the midpint f urine sample cllectin. All birds were killed with an verdse f urethane at the end f the experiment. Sample Analysis Ammnium heparin (200 units/ml) was added t each bld sample t prevent Grup H t 35 t (cycled) 32 (nncycled) 32 Temperature -( ^ ) Grup A (nncycled) (nncycled) 21 Grup H (nncycled) 32 (nncycled) 32

5 HEAT ALIMATIO AD KIDEY FUTIO 79 cltting. The plasma was separated by centrifugatin and stred frzen until analysis. Freshly cllected urine and plasma were used fr measurements f smlality (Wescr 5500 Vapr Pressure Osmmeter 4 ). A prtin f each urine sample was mixed with an equal vlume f.5m LiOH t disslve uric acid precipitates, then the diluted urine samples were stred frzen until analysis. lrimetric methds were used t measure inulin (Waugh, 1977) and PAH (Brun, 1957). Flame phtmetry was used t measure a and K, and a was measured by atmic absrptin spectrphtmetry. Statistical Analysis Timed urine samples were cllected in preweighed tubes fr gravimetric determinatin f urine vlume and urine flw rate (UFR). The glmerular filtratin rate (GFR) was calculated as the clearance f inulin: GFR =, = (UFR x [ln]j/[ln] p, where [ln] u and [ln] p are the urine and plasma inulin cncentratins, respectively. The effective renal plasma flw rate was calculated as the clearance f PAH: PAH = (UFR x [PAH] U )/ [PAH] p, where [PAH] U and [PAH] p are the urine and plasma PAH cncentratins, respectively. The fractin f filtered water excreted as urine was calculated as: FpwE UFR/GFR, where UFR equals the excreted prtin f the glmerular ultrafiltrate and GFR equals the rate at which water enters the tubules by glmerular filtratin. The rate at which urinary excretin cleared smlal-equivalent slutes frm the plasma was calculated as the smlal clearance rate: ^ = (UFR x Osm u )/Osm p/ where Osm,, and Osrrtp are the urine and plasma smlalities, respectively. The rate at which the kidneys excreted (psitive values = "dilute" urine) r recvered (negative values = "cncentrated" urine) slute-free water was calculated as the free water clearance: H Q = (UFR - sj. The rate at which a entered the tubules thrugh glmerular filtratin was calculated as the filtered lad f a: FL a = (GFR x [a] p ), where [a] p is the plasma a cncentratin. 4 Wescr, Inc., Lgan, UT The abslute rate f urinary a excretin was calculated as: E a = (UFR x [a]j, where [a] u is the urine a cncentratin. The rate at which the tubules reabsrbed a frm the glmerular ultrafiltrate (tubular a reabsrptin rate) was calculated as the difference between the rate f filtratin and the rate f excretin: T R a = (FL^ - E a ). The fractin f the filtered lad f a excreted in the urine was calculated as: FE a = (IWFLJ, r FE^ = [([a] u /[a] p )/ ([ln] u /[ln] p )]. Renal functin data were nrmalized fr differences in bdy mass t cmpensate fr the effects f bdy mass n GFR and urine flw rate, as described previusly (Wideman et at, 1992). Data frm Experiment 1 were analyzed by AOVA in a 2 x 15 factrial arrangement using the SAS sftware (SAS Institute, 1982) General Linear Mdels prcedure, with treatment grup (2 = and ) and urine sample perids (15) as the main effects. Average values als were cmputed fr individual birds within the LOW (Samples 1 thrugh 3 pled), HIGH (Samples 4 thrugh 9 pled), and SALIE (Samples 10 thrugh 15 pled) infusin prtcls, and means calculated frm these averages were analyzed by AOVA in a 2 x 3 factrial arrangement cmparing treatment grup (2 = and ) and infusin prtcl (3 = LOW, HIGH, and SALIE) as the main effects. In Experiment 2, values were analyzed by AOVA. Differences between least squares means were cnsidered significant at P <.05. Experiment 1 RESULTS There were n significant grup- ( vs ) r time-related (Sample 1 vs 3) differences fr any f the variables cmpared during the LOW infusin prtcl (Figures 1 and 2, Table 2). Vlume expansin during the HIGH and SALIE infusin prtcls resulted in significantly higher GFR values fr bth grups when cmpared with the respective GFR values during the LOWinfusin prtcl (Figure 1). Glmerular nitratin rate was significantly higher in Grup than in Grup during sample Perids 6,10,13, and 15 (Figure 1), and the

6 80 WIDEMA ET AL. TABLE 2. Average plasma and renal functin values during Experiment 1 fr nncycled () and cycled () grups during the LOW (urine Samples 1 thrugh 3), HIGH (urine Samples 4 thrugh 9), and SALIE (urine Samples 10 thrugh 15) infusin prtcls, x SEM Variable Plasma smlality, mosm/kg H 2 0 Effective renal plasma flw rate, ml/kg per min Fractin f filtered water excreted as urine, % H + cncentratin, equivalents/l x 10~* alcium excretin rate, lim/kg per min Ptassium excretin rate, jtm/kg per min Grup LOW 312 2" b c ' 15 2» 15 3 a * b HIGH 308 2b b b b 12 1* 11 1* a * SALIE 319 7* 3 3* » * 7 lb 8 l b * * "^Means within a variable with n cmmn superscript differ significantly (P.05). average GFR fr Grup during the SALIE infusin prtcl ( ml/ kg per min) was significantly higher than the crrespnding average value fr Grup ( ml/kg per min). When cmpared with respective values during the LOW infusin prtcl, the HIGH infusin prtcl triggered significant increases in the urine flw rate, smlal clearance rate, and free water clearance rate fr Grup (sample Perids 5 t 7), and in the smlal clearance rate fr Grup (Figure 1). Values fr urine flw rate, smlal clearance rate, and free water clearance rate were significantly higher in Grup than in Grup during urine cllectin Perid 6 (Figure 1). The time- and grup-related changes in urine flw rate, smlal clearance rate, and free water clearance rate during the transitin frm the LOW t the HIGH infusin prtcls appeared t be crrelated qualitatively with the cntempraneus changes in GFR fr the respective grups (Figure 1). This apparent dminant rle f GFR in determining urine flw rate was supprted by the absence f significant intergrup differences in the fractin f filtered water excreted as urine during the HIGH infusin prtcl (Table 2). Fr example, 11 t 12% f the glmerular ultrafiltrate was being excreted as urine by bth grups during the HIGH infusin prtcl; cnsequently, the differences in urine flw rates during this perid were determined by crrespnding differences in GFR. There were n significant intergrup differences in the fractin f filtered water excreted as urine fr any f the individual sample perids during the HIGH infusin prtcl (P =.369 fr Grup vs Grup during sample Perid 6). Infusing.5 M al triggered a tubular antidiuresis, as reflected by significant reductins in the fractin f filtered water excreted as urine (SALIE vs LOW infusin prtcl cmparisn, Table 2), reductins in the urine flw rate (SALIE vs HIGH infusin prtcl cmparisns, Figure 1), and a transitin frm excreting hyptnic t excreting hypertnic urine (negative free water clearance rates) but smlal clearance rates remained cnstant (SALIE vs HIGH infusin prtcl cmparisns, Figure 1). This tubular antidiuresis enabled birds f bth grups t significantly reduce their urinary water lss in spite f the fact that, due t the nging high rate f intravenus infusin, GFR and thus the filtered lad f water remained elevated at vlume expanded levels thrughut the SALIE prtcl (Figure 1). Infusing.5 M al at the high infusin rate (SALIE prtcl) significantly increased plasma a cncentratins in bth grups (Figure 2), and increased plasma smlality in Grup (Table 2). Presumably it was the increase in plasma a during the SALIE infusin prtcl that triggered the cincident tubular antidiuresis.

7 HEAT ALIMATIO AD KIDEY FUTIO %2 c t.2«- U- b) St t g Or x si d> c t <Q c 2>1 O x O " 5 2 d> E P Urine Sample umber (10 min each) FIGURE 1. Renal functin values fr nncycled (: ) and cycled (: O) grups during the LOW (Samples 1 thrugh 3), HIGH (Samples 4 thrugh 9), and SALIE (Samples 10 thrugh 15) infusin prtcls (5c SEM) fr Experiment 1. Within a urine sample perid, values with different letters differed significantly (P.05). Within a grup, ft designate the earliest value t differ significantly frm the respective values during sample Perids 2 r 3, and * designate the earliest value t differ significantly frm the respective values during sample Perids 8 r 9.

8 WIDEMA ET AL. (OS r- fc «r E Z 1fiOj ; D ( 0 ^ 3 b D E x n> il * X «? Z * (0 A.E re c r * + T n en Z * v. x.25 3 a A ' 3 si 4-* fl> L. X UJ + n z m 7 re c SS u. X n> X X p -1 "5" z UJ u. ** c *>!» X Ul , Lw.04 -M-lnfusin Rate-> \ Urine Sample umber (10 min each) FIGURE 2. Plasma a, filtered a, reabsrbed a, abslute a excretin, and fractinal a excretin fr nncycled (: ) and cycled (: ) grups during the LOW (Samples 1 thrugh 3), HIGH (Samples 4 thrugh 9), and SALIE (Samples 10 thrugh 15) infusin prtcls (x" SEM) fr Experiment 1. Within a urine sample perid, values with different letters differed significantly (P.05). Within a grup, -sir designate the earliest value t differ significantly frm the respective values during sample Perids 2 r 3, and * designate the earliest value t differ significantly frm the respective values during sample Perids 8 r 9.

9 HEAT ALIMATIO AD KIDEY FUTIO 83 Glmerular filtratin rate strngly influenced the renal handling f a in bth grups (Figure 2). There were n cntempraneus intergrup differences in plasma a cncentratins; cnsequently, specific intergrup differences in the filtered lad f a during urine Samples 6, 10, 13, and 15 (Figure 2) were directly related t cntempraneus intergrup differences in GFR (Figure 1). Similarly, the verall increase in the filtered lad f a during the HIGH infusin prtcl (Figure 2) matched the crrespnding increase in GFR attributed t vlume expansin (Figure 1). Bth grups maintained almst perfect glmerulartubular balance. That is, whenever increases in GFR caused the filtered lad f a t increase (urine Samples 6,10,13, and 15), the mechanism(s) respnsible fr glmerular-tubular balance "autmatically" increased the tubular a reabsrptin rate, thereby minimizing intergrup (urine Samples 6,10,13, and 15) and timerelated (LOW vs HIGH infusin prtcls) increases in the abslute a excretin rate and in fractinal a excretin. Fr example, the average filtered lad f a fr Grup increased significantly frm /tm/kg per min during the LOW infusin prtcl, t jtm/kg per min during the HIGH infusin prtcl. In the face f this 146 /xm/kg per min net increase in the filtered lad f a, the tubular a reabsrptin rate acutely increased by an almst precisely equal magnitude, thereby hlding the abslute a excretin rate t a nnsignificant increase f frm jtm/kg per min during the LOW infusin prtcl t /xm/kg per min during the HIGH infusin prtcl. Glmerular-tubular balance was nt as precise when the increase in plasma a cncentratins during SALIE infusin caused a crrespnding increase in the filtered lad f a. In Grup fr example, the filtered lad f a increased significantly frm jtm/kg per min during the HIGH infusin prtcl t \MI kg per min during the SALIE infusin prtcl, which significantly increased the abslute a excretin rate frm an average f t /im/kg per min ver the same perid f time (Figure 2). Effective renal plasma flw increased significantly in bth grups during the HIGH infusin prtcl, and an additinal significant increase in the effective renal plasma flw rate ccurred in bth grups during the SALIE prtcl (Table 2). There were n cntempraneus intergrup r time-related changes in urine H + cncentratins r in a r K excretin rates (Table 2). Experiment 2 As shwn in Table 3, the secnd experiment qualitatively replicated the differences previusly bserved between heatacclimated and nnacclimated grups during the HIGH-infusin prtcl. Acutely heat-acclimated brilers (cyclic T a : Grups A, H) had significantly lwer GFR and smlal clearance values and similar fractin f filtered water excreted values when cmpared with the respective nnacclimated brilers (cnstant T a : Grups A, H). The T a during surgical preparatin, pre-equilibratin and urine sample cllectin had n significant impact n these variables (A vs H r A vs H). Urine flw rate was lwer fr grup A than fr grup A, and expsure t 32 during the renal functin studies caused a significant tubular antidiuresis, as reflected by significantly lwer values fr urine flw rate, fractin f filtered water excreted as urine, and free water clearance (Table 3; A vs H r A vs H). As a cnsequence f having higher GFR, the filtered lad f a was higher in nnacclimated brilers than in heatacclimated brilers. Glmerular-tubular balance effectively prevented the nnacclimated brilers frm excreting significantly mre a than acclimated brilers at (A vs A) r 32 (H vs H) by increasing the tubular a reabsrptin rate in the nnacclimated birds. Within the heatacclimated grup, bth abslute and fractinal a excretin were higher at 32 than at 21, reflecting a slight but significant heat-induced increase in urinary electrlyte lss (Grup H vs A, Table 3). Plasma a was significantly higher fr nnacclimated birds evaluated at 21 (Grup A) than at 32 (Grup H), hwever plasma a did nt differ significantly when nnacclimated and heat-acclimated grups were cmpared within a temperature prtcl (A

10 84 WIDEMA ET AL. TABLE 3. Average plasma and renal functin values during Experiment 2 fr nncycled () and cycled () grups expsed t ambient (21 ) r high (32 ) temperatures during renal functin studies, SEM Variable Plasma sdium, mm Plasma smlality, mosm/kg H 2 0 Glmerular filtratin rate, ml/kg per min Urine flw rate, ml/kg per min Fractin f filtered water excreted as urine, % Osmlal clearance rate, ml/kg per min Free water clearance rate, ml/kg per min Filtered lad f a, /*M/kg per min Tubular a reabsrptin rate, /xm/kg per min Abslute a excretin rate, nm/kg per min Fractinal a excretin, (n units) Grup Ambient temperature " 0.9 ab 2" 2b.11".09b.02".02b 1" 1" 17" 13 b 17" 13" ".01 b.02".02".8*.3 b.001b.001b High temperature b 0.8"b 2b 2b.08".13b.01 b c.01<: lb lb 10" 20 b 10" 20b "-"Means within a variable with n cmmn superscript differed significantly (P.05). vs A, H vs H). Plasma smlality was significantly higher in Grup A than in Grups A, H, and H (Table 3). DISUSSIO Several different experimental techniques have been used t evaluate avian renal functin during heat stress, each f which ffers specific advantages and limitatins. Fr example, chrnic clstmy r ureteral exteririzatin prcedures facilitate lng-term studies in which the bjective is t quantify nrmal urine flw and slute excretin rates as a measure f input-utput balance. These techniques prevent pstureteral reabsrptin f water and slutes by the lwer intestinal tract, which ptentially can disrupt water balance by causing prly hydrated r saltdepleted birds t lse up t 10 t 15% f the water and 66% f the al in ureteral urine (Hart and Essex, 1942; Brwn et al, 1958; Scheiber and Dziuk, 1969; Dicker and Haslam, 1972; Skadhauge, 1976)..01".01b.01b.01b.7".6".002" b.002" Unanesthetized intact birds can be used t avid the chrnic renal and endcrine adaptatins caused by diverting the urine away frm the claca. Hwever, a catechlamine-mediated diuresis can be triggered when unanesthetized birds are handled r when urine cllectin devices are inserted int the ureters r claca (Sharpe, 1912; Hester et al, 1940; Hart and Essex, 1942; Brwn et al, 1958; Sturkie and Jiner, 1959; Dicker and Haslam, 1966; Azahan and Sykes, 1980; Palmre et al, 1981). Anesthesia and intravenus infusins, as emplyed in the present study, als can perturb fluid balance and dissipate preexisting intergrup differences in states f hydratin. Hwever, anesthesia successfully reduces the variability assciated with repeated shrt-term sampling f unanesthetized birds, and renal clearance markers (inulin, PAH) must be infused cntinuusly if accurate physilgical evaluatins f kidney functin are t be

11 HEAT ALIMATIO AD KIDEY FUTIO 85 accmplished. Standardizing the states f hydratin f different experimental grups is an advantage when the bjective is t discriminate between primary alteratins in renal physilgy and transient respnses t differences in water balance. The strategy f using intravenus infusins t directly cmpare avian renal respnses befre, during and after the transitin frm LOW t HIGH infusin rates has prven successful fr expsing fundamental changes in the kidney functin f dmestic fwl subjected t a variety f experimental regimens (Wideman and Satnick, 1989; Wideman et at, 1989; Gregg and Wideman, 1990; Vena et ah, 1990; Davisn and Wideman, 1992). In Experiment 1, differences in kidney functin between nnacclimated and heatacclimated brilers were revealed nly when a HIGH intravenus infusin rate was used t simulate the vlume expansin caused by thermgenic plydipsia. Acute heat acclimatin attenuated the extent t which vlume expansin caused GFR t increase and, cincident with the effects n GFR, heat-acclimated brilers exhibited lwer increments in urine flw rate, smlal clearance rate, and free water clearance rate during vlume expansin than nnacclimated cntrls. Qualitatively similar influences f heat-acclimatin n GFR, urine flw rate, and smlal clearance rate were bserved when the same HIGH intravenus infusin rate was used in the secnd experiment. In bth experiments, cmparisns f the fractin f filtered water excreted as urine revealed that GFR was the principal renal variable affected by heat acclimatin, with the crrelated effect n urine flw rate ccurring secndary t the primary effect n GFR. This relative glmerular antidiuresis during vlume expansin ptentially shuld enable heat-acclimated brilers t retain mre f the extra water they cnsume during heat stress (Ltt, 1991). The water thus cnserved shuld help maintain bld vlume and cardivascular functin during heat stress, but a crrespnding minr reductin in the amunt f bdy heat dissipated by expelling urine als wuld ccur. The latter cnsideratin has questinable physilgical relevance because, as summarized recently by Belay et al. (1993), nly.001 kcal/ml per degree elsius can be dissipated when drinking water is warmed t bdy temperature and expelled as urine, whereas the latent heat f vaprizatin fr respiratry water lss at 41 is.57 kcal/ ml. Heat-acclimated brilers apparently adapt their kidneys t maximize the efficiency f heat dissipatin per milliliter f water cnsumed by channeling that water thrugh respiratry evapratin instead f urine prductin. Attenuating the increase in GFR during vlume expansin als served t reduce the rate at which plasma a was filtered int the kidney tubules f heat-acclimated brilers. Reducing the filtered lad f a theretically culd serve as a mechanism fr reducing urinary a excretin during heat stress, thereby minimizing the extent f extracellular fluid vlume cntractin when a negative water balance develps, r helping t prevent extracellular fluid hyptnicity if thermgenic plydipsia causes a psitive water balance t develp. Heat-acclimated brilers did have lwer ttal urinary slute excretin rates during the HIGH infusin prtcl, as reflected by their significantly lwer smlal clearance rates. Hwever, significant differences in the filtered lad f a were nt crrelated with differences in abslute r fractinal a excretin, as lng as the differences in the filtered lad f a were assciated with changes in GFR during vlume expansin (Experiment 1: LOW vs HIGH infusin prtcls), r with intergrup differences in GFR (Experiment 1: urine Samples 6, 10, 13, 15; Experiment 2: Grups A and H vs Grups A and H). This apparent dissciatin f abslute and fractinal a excretin frm the filtered lad f a is knwn as glmerular-tubular balance, which reflects the capacity f kidney tubules t autmatically adjust their a reabsrptin rate t recver virtually all f any increment in the filtered lad f a caused by an increase in GFR (Wilcx and Bayliss, 1985). Glmerular-tubular balance fr briler kidneys in the present study was nly marginally less effective when the increment in the filtered lad f a was caused

12 86 WIDEMA ET AL. by an increase in plasma a cncentratins (SALIE infusin prtcl) rather than by an increase in GFR (HIGH infusin prtcl), as has been reprted fr mammals (Wilcx and Bayliss, 1985). Because glmerular-tubular balance was highly effective, the higher values fr GFR and filtered lad f a attained by nnacclimated brilers during vlume expansin had n significant impact n a excretin, suggesting that the renal changes assciated with heat acclimatin functin primarily fr cnserving water (glmerular antidiuresis), rather than fr cnserving a. In agreement with this interpretatin, GFR and urine flw rate remained attenuated but abslute and fractinal a excretin increased during the tubular antidiuresis triggered in heatacclimated brilers evaluated at 32 instead f 21 (Table 3). Because changes in the filtered lad f a cause, thrugh glmerular-tubular balance, crrespnding changes in the tubular reabsrptin rate f a, the phenmenn f glmerular-tubular balance may cntribute t reducing the metablic heat lad f heat-acclimated brilers. Tubular a reabsrptin requires the hydrlysis f ATP by sdium- and ptassium-activated adensine triphsphatase (a-k-atpase) lcated alng the baslateral membranes f kidney tubules (Katz, 1982). The ATP thus cnsumed must be replenished by xidative metablism, and heat is liberated as a by-prduct f ATP hydrlysis and replenishment (hen and Barac- iet, 1973). Abve a small basal rate f renal O z cnsumptin related t cell maintenance phenmena, it is tubular a reabsrptin that requires the vast bulk f the O s cnsumed by the kidney, and a psitive linear crrelatin exists between increments in tubular a reabsrptin and increments in renal 0 2 cnsumptin (Knx et al, 1966; Balban and Mandel, 1980). Frm measurements indicating that the kidneys reabsrb apprximately 30 meq a+/mm f O z cnsumed, and assuming 6 mm f ATP are replenished/mm f 0 2 cnsumed r allwing calric equivalents ranging frm 112 cal/mm 0 2 cnsumed t 10 cal/mm ATP hydrlyzed, the maximal energy requirement fr a reabsrptin by dg kidneys has been estimated as 2 t 4 cal/meq f a + reabsrbed (hen and Barac-iet, 1973). When membrane transprt energetics, the different quantities f a actively reabsrbed by separate nephrn segments, and the different transprt parameters f separate nephrn segments are incrprated in apprpriate physical-chemical equatins, the resulting calculatins yield estimates f minimal energy requirements fr a reabsrptin ranging between 4.6 cal/meq fr a + transprted by the distal nephrn, and.16 cal/meq fr a+ transprted by the prximal tubule (hen and Barac-iet, 1973; Klahr and Hammerman, 1985). Althugh these calculatins cannt cnfidently be applied quantitatively t avian kidneys, they d suggest in the cntext f the glmerular-tubular balance bserved in the present study that, by attenuating the increment in GFR caused by vlume expansin, heatacclimated brilers may reduce the renal cmpnent f 0 2 cnsumptin and metablic heat prductin by reducing their filtered lad f a and thus reducing their tubular a reabsrptin rate. Mammalian kidneys cmprise nly.5% f the bdy mass while cnsuming apprximately 10% f the resting whle bdy 0 2 requirement, and nly the mammalian heart exceeds the mass-specific rate f renal xygen uptake (hen and Barac-iet, 1973; Klahr and Hammerman, 1985). Therefre, substantial prprtins f the basal metablic 0 2 cnsumptin and heat prductin in dmestic fwl may be related t tubular a reabsrptin. Reduced rates f 0 2 cnsumptin and reduced basal metablic rates have cnsistently been crrelated with the prcess f heat acclimatin in dmestic fwl (Sykes and Fataftah, 1986; Arieli, 1987; hwalibg, 1990). The present study des little t define the precise endcrine r adaptive mechanisms respnsible fr the changes in kidney functin bserved in heat-acclimated brilers; hwever, tw independent experiments clearly demnstrated that heatacclimated birds have lwer increments in their GFR under high infusin rates and saline lading. This primary change in kidney functin appears t subserve water cnservatin as a cnsequence f reducing urine flw rate (glmerular antidiuresis),

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