HC03 Reabsorption in the Newborn Dog1

Transcription

1 003 I -3998/90/ $02.00/0 PEDIATRIC RESEARCH Copyright O 1990 Interntionl Pcditric Reserch Foundtion, Inc Vol. 27, No. 6, 1990 Prinled in U. S. A. Effects of Euvolemic nd Hypervolemic Hyperbicrbonemi on Segmentl Nephron HC03 Rebsorption in the Newborn Dog1 JOHN M. LORENZ AND LEONARD I. KLEINMAN Deprtment of Peditrics, School ofmedicine, Stte University ofnew Yorlc t Stony Brook, Stony Brook, NEW York ABSTRACT. Studies were undertken to determine the effect of elevted plsm bicrbonte concentrtion (PHC03) with nd without volume expnsion on segmentl nephron HC03 rebsorption in newborn nd dult dogs using the technique of distl blockde. Rebsorption of bicrbonte per ml glomerulr filtrte (RHC03lGFR) in the proximl nephron ws more suppressed in euvolemic newborns thn euvolemic dults s PHC03 ws incresed from bseline to mm. Insmuch s totl nephron rebsorption hs been shown to be essentilly complete under these conditions in both newborns nd dults, distl nephron HC03 delivery nd the frction of the distl HC03 lod rebsorbed must hve been greter in euvolemic newborns thn dults when PHC03 ws elevted. Totl nephron RHC03/GFR ws less suppressed by NHC03 volume expnsion in the newborn thn it ws in the dult. However, proximl nephron RHC03/GFR ws similrly suppressed by NHC03 volume expnsion in newborns nd dults. Thus, the NHC03-expnded newborn must hve rebsorbed greter proportion of the incresed distl HC03 lod thn did the NHC03-expnded dult. Proximl nephron HC03 rebsorption is blnce between rebsorption effected by ctive proton secretion nd pssive HC03 bck lek. Euvolemic increse in peritubulr HCO3 concentrtion hs been shown to suppress proximl tubulr proton secretion; volume expnsion increses proximl tubule HC03 permebility nd bck lek. Thus, we interpret these results to indicte tht: 1) the limited cpcity of the immture proximl tubule for proton secretion is most likely responsible for the greter suppression of proximl RHC03/GFR by incresed PHC03 in euvolemic newborn dogs compred with dults; 2) the increse in HC03 permebility of the immture proximl nephron with volume expnsion is no greter thn tht which occurs in the mture proximl nephron with volume expnsion; nd 3) the immture distl nephron displys greter propensity for HCO3 rebsorption s PHC03 is incresed thn does the mture distl nephron, with or without volume expnsion. (Peditv Res 27: , 1990) Abbrevitions FRHC03, frctionl rebsorption of the filtered bicrbonte lod GFR, glomerulr filtrtion rte PHC03, plsm bicrbonte concentrtion Received My 22, 1989; ccepted Jnury 12, Correspondence nd reprint requests: John M. Lorenz, M.D., Deprtment of Peditrics, School of Medicine, Stte University of New York t Stony Brook, Stony Brook, NY ' Presented in prt t the nnul meeting of the Society for Peditric Reserch, My RHC03/@FR, rebsorption of bicrbonte per ml glomerulr filtrte U:PN, urine sodium to plsm sodium rtio U:POsm, urine osmol to plsm osmol rtio (U-P)Pco2, difference in Pcoz between urine nd plsm For mny yers it hd been climed tht newborn infnts hd lower plsm threshold for bicrbonte excretion thn did dults (1-3). However, the infusion of bicrbonte in these studies probbly expnded extrcellulr volume. When PHC03 ws progressively elevted in newborn dogs without producing volume expnsion by exchnge trnsfusion with high PHC03 nd low plsm chloride concentrtion whole blood, there ws no tubulr mximum for bicrbonte nd renl tubulr bicrbonte rebsorption ws essentilly complete over rnge of PHC03 from 10 to 50 mm (4). Thus, it seems tht suppression of bicrbonte rebsorption by extrcellulr volume expnsion, rther thn limited resorptive cpcity, my well hve ccounted for the lower plsm threshold observed in newborn infnts. On the other hnd, volume expnsion ntriuresis is ttenuted in the newborn compred with the dult (5-8). This ttenuted ntriuresis is due to enhnced frctionl rebsorption of sodium by the immture distl nephron (5, 7, 9, 10). Volume expnsion ctully suppresses frctionl sodium rebsorption by the proximl nephron more in the newborn thn in the dult (7). In our study, the effect of elevted PHC03 without volume expnsion on proximl nephron bicrbonte rebsorption nd the effect of elevted PHC03 with volume expnsion on segmentl bicrbonte rebsorption were investigted in the newborn nd dult dog using the technique of distl nephron blockde. The purpose ws to compre how volume expnsion modifies the segmentl response of the immture nd mture nephrons to n incresed filtered lod of bicrbonte. MATERIALS AND METHODS Studies were performed on newborn (4-22 d) nd dult mongrel dogs of either sex. The protocol conformed to the NIH Guide for the Cre nd Use of Lbortory Animls (I 1) nd ws pproved by the Lbortory Animl Users Committee of the Stte University of New York t Stony Brook. Animls were nesthetized with mg/kg of i.v. pentobrbitl. Additionl pentobrbitl ws given s necessry during surgery to mintin n dequte level of nesthesi. Rectl temperture ws mintined t 36.9"C in the newborn dogs (12) on wrmed operting tble with servo-controlled het lmp. Rectl temperture ws mintined between 38 nd 40 C in dult dogs (1 3) with n overhed rdint wrmer. After cnnu-

2 ltion of the trche, ll nimls were routinely plced on pproprite size volume-controlled ventiltors (Hrvrd Apprtus Co., Inc., S. Ntick, MA) with supplementl oxygen. Polyethylene ctheters were plced in femorl vein for infusion of 3Hinulin in mintennce solution with or without diuretics; in n externl jugulr vein for infusion of the urine replcement solution; in one or both femorl rteries for blood smpling nd hert rte nd blood pressure monitoring; nd in ech ureter for timed urine collection. In ddition. the second femorl vein ws ctheterized in nimls undergoing volume expnsion for infusion of isotonic sodium bicrbonte solution. All incisions were surgiclly closed. After surgery, priming injection of 3H-inulin ws given followed by constnt infusion of lbeled inulin in glucose, multielectrolyte mintennce solution t rte of mlmin-'. kg-'. The respirtor ws djusted so tht the prtil pressure of crbon dioxide in rteril blood ws mm Hg. After 1-h recovery nd equiliztion period, the experimentl protocol ws begun. Protocol I. Euvolemic hyperbicrbonemi with distl blockde. The purpose of this protocol ws to compre the effect of elevted PHC03 on proximl nephron bicrbonte rebsorption in the newborn nd the dult in the bsence of volume expnsion. After the I-h recovery nd equilibrtion period, priming injection of 2.5 mg/kg of ethcrynic cid nd 2.0 mg/kg miloride ws given, followed by constnt infusion of these diuretics in the inulinlmintennce solution t rtes of 1 mg. h-'. kg-' nd 2.4 mg. h-'. kg-', respectively. As diuresis begn, urine output ws replced volume for volume with low bicrbonte solution (140 mm NCl + 5 mm KHCO,). When U:POsm ws 0.95 to 1.05, control renl clernce of 30 to 120 s ws obtined. During this nd ll subsequent clernce periods, rteril blood ws smpled for totl plsm protein concentrtion, plsm sodium concentrtion, plsm potssium concentrtion, rteril ph, nd Pco2. All blood smpled ws replced volume for volume with whole blood. Immeditely fter obtining this control clernce, high bicrbonte replcement solution (1 50 mm NHC mm KHC03) ws substituted for the low bicrbonte replcement solution. The respirtor ws subsequently djusted to prevent rpid rte of rise in Pco~. AS PHCO, rose, four to seven dditionl clernces were obtined until PHC03 reched 60 to 70 mm, t which point the experiment ws ended. Dt were included in the nlysis only from clernce periods during which the urine to plsm sodium concentrtion rtio (U:PN), s well s the U:POsm, ws confirmed to be 0.95 to Protocol II. NHC03 volume expnsion followed by distl blockde. The purpose of this protocol ws to compre the effects of volume expnsion on the reltionships between PHC03 nd segmentl bicrbonte rebsorption in the newborn nd the dult. After surgery, protocol I1 nimls were prlyzed with pncuronium bromide nd the respirtor djusted so tht rteril Pco2 ws 35 to 45 mm Hg. Prlysis ws mintined throughout the experiment nd no subsequent respirtor djustments were mde. No further surgery ws performed fter the dministrtion of pncuronium bromide. After 1-h period for recovery nd equilibrtion, control clernce of 30 to 60 min durtion ws obtined. An rteril blood smple ws obtined t the midpoint of this nd subsequent clernce periods for determintion of totl plsm protein concentrtion, plsm sodium concentrtion, plsm potssium concentrtion, ph, nd Pco~. All blood smpled ws replced volume for volume with whole blood. After this initil control clernce, isotonic NHC03 (1 50 mm) ws infused t 2.0 ml.min-'.kg-' for 15 min nd then t 0.5 ml min-'.kg-' for the reminder of the experiment. After 45 min t the slower infusion rte, two consecutive clernces (of 30 min durtion in the newborn nd 15 min durtion in the dult) were obtined. After these clernce collections, priming injections of 1.25 mg/kg of ethcrynic cid nd 1.0 mglkg miloride were given, followed by the constnt infusion of the diuretics in the inulinlrnintennce solution t I mg. h-', kg-' HCO, REABSORPTION IN N EWBORN AND ADULT DOG 605 nd 2.4 mg.h-'.kg-', respectively. Urine output during the diuretic infusion ws replced isovolumetriclly with solution contining 140 mm sodium, 5 mm potssium, 140 mm chloride, nd 5 mm bicrbonte. After 1 h, three 5- to 15-min consecutive clernces were obtined. Only clernce periods with U:POsm nd U:PN rtios of 0.95 to 1.05 during distl blockde were used. Smple Anlysis. The tritium ctivity of plsm nd urine smples ws determined by liquid scintilltion counting. Bicrbonte concentrtions were clculted from the ph nd Pcoz of rteril blood nd urine smples obtined with commercil blood gs nlyzer using the Henderson-Hsselblch eqution. A pk' (negtive logrithm of the pprent dissocition constnt) of 6.1 ws used for the clcultion of PHC03. Urine bicrbonte concentrtion ws clculted using pk' estimted from the eqution b, where b is the sum of sodium nd potssium concentrtions in the urine in mol/l, to ccount for ionic strength (14). Plsm nd urine sodium concentrtions were determined by flme emission spectrophotometry. Plsm nd urine potssium concentrtions were determined by tomic bsorption spectrophotometry. Osmolrity ws determined by vpor pressure osmometry. Plsm protein concentrtion ws mesured by refrctometry. Clcultions. GFR ws equted with inulin clernce. Bicrbonte rebsorption ws clculted s the difference between filtered bicrbonte lod (GFR x PHC03) nd bicrbonte excretion (urine flow rte x urine bicrbonte concentrtion). FRHC03 ws clculted s 1 - (bicrbonte clernce/gfr x 100) nd it thus expressed s percent. Renl tubulr bicrbonte rebsorption is expressed s pmol of bicrbonte rebsorbed per ml of glomerulr filtrte to correct for differences in filtered bicrbonte lod due to differences in GFR between the newborn nd the dult. RHC03/GFR before infusion of ethcrynic cid nd miloride represents totl nephron RHCOdGFR. Bicrbonte rebsorption during diuretic infusion (ll clernces in protocol I nd the lst set of clernces in protocol 11) represents proximl nephron rebsorption. In protocol 11, bicrbonte delivery to nd rebsorption in the distl segment of the nephron (loop of Henle, distl convoluted tubule, nd collecting duct) during NHC03 expnsion could be clculted. Distl bicrbonte lod ws clculted s the difference between filtered bicrbonte lod during NHC03 expnsion without diuretics nd proximl nephron RHCO,. Distl RHC03/ GFR ws clculted s the difference between RHC03/GFR during NHC03 expnsion without diuretics nd proximl RHCOJGFR. In both of these clcultions, proximl RHC03/ GFR ws clculted s the product of the filtered bicrbonte lod during NHC03 expnsion without diuretics nd FRHC03 during NHC03 expnsion with diuretics. The frction of the distl bicrbonte lod tht ws rebsorbed in the distl nephron segments ws clculted s the quotient of distl nephron bicrbonte rebsorption nd distl bicrbonte lod, multiplied by 100. The percentge increse in extrcellulr volume during N- HC03 expnsion in protocol I1 ws clculted s [(plsm protein concentrtion during the control clernce/plsm protein concentrtion during volume expnsion) - I] x 100. This clcultion ssumes tht totl intrvsculr protein content remins constnt during the experiment. Dt Anlysis. All dt re presented s men 5 SEM. Comprisons of single mesure dt between newborn nd dults within ech protocol were mde using the unpired t test if the dt were normlly distributed or the Wilcoxon rnk sum test if they were not. When single-mesure dt ws normlly distributed, but the F test for equlity of vrinces indicted tht group vrinces were significntly different, the unequl vrince t test (15) ws used to mke sttisticl comprisons between newborns nd dults. In protocol I, both the observed PHC03 vlues nd the number of clernce periods vried within nd between the ge groups s

3 606 LORENZ AND KLEINMAN result of the nture of the experimentl design. Therefore, filtered), p = 0.04 for interction effect. FRHCO, by the immliner regression nlysis ws used to obtin n eqution tht ture proximl nephron tended to be lower thn tht rebsorbed best relted the PHC03 nd the corresponding RHCO JGFR by the mture proximl nephron under both control conditions ctully observed in ech niml. The unpired t test ws then ( versus %) nd t the highest PHCO, used to test whether the slopes of these regression equtions were observed (73.4? 4.1 versus 85.1 f 2.4%), p = 0.07 for min different between ge groups. Composite regression equtions effect of ge). were obtined for ech group by clculting the men + SEM of Proximl nephron RHC03/GFR ws linerly relted to RHC03/GFR vlues predicted by the individul regression equ- PHCO, in six of seven newborn (r = , p = ) tion t ech of severl PHC03 rnging from 20 to 60 mm. A nd six of six dult dogs (r = , p = ). The composite regression line nd 95% confidence intervl ws then individul regression lines for ll nimls re shown in Figure 1. generted for ech ge group describing the effect of rising There ws no correltion of the regression line slopes with ge in PHCO, without volume expnsion on RHC03/GFR. In ddi- the newborn group (r = 0.05, p = 0.92). The composite regression tion, between- nd within-ge groups comprisons of the ob- line nd 95% confidence intervls re shown in Figure 2 for the served dt under control conditions nd t the highest PHC03 newborn group (left pnel) nd for the dult group (right pnel). were mde using two-fctor nlysis of vrince with repeted The eqution for these composite regression lines re RHC0,/ mesures of one fctor. GFR = PHCO, in the newborn nd RHC03/GFR Between- nd within-ge groups comprisons under control = PHC in the dults. The slope of the dult nd NHCO, volume-expnded conditions were mde using composite regression line is significntly steeper thn tht of the two-fctor nlysis of vrince with repeted mesures of one newborn, p = 0.05 by unpired t test. fctor. When the F vlue for interction effect exceeded the Protocol II. Studies were performed in seven newborn (men criticl F t the 0.05 level of significnce, further comprisons ge 8? 2 d, rnge d) nd six dult dogs. Arteril blood gs were mde using the lest significnt difference test. Comprisons nd plsm nd urine electrolyte dt for control nd NHC03 of proximl nd distl nephron function between ge groups expnsion clernce periods in protocol I1 re presented in Tble were mde using the unpired t test. 2. Control PHC03 were similr in newborn nd dults nd similr to those in the protocol I nimls. Renl clernce dt under control- nd NHC0,-expnded RESULTS conditions in the bsence of distl blockde is shown in Figure Protocol I. Studies were performed in seven newborn (men 3. As expected, rebsorption of the filtered lod under control ge 15 f 2 d, rnge d) nd six dult dogs. Arteril blood conditions ws essentilly complete t control PHC03 vlues in gs nd plsm nd urine electrolyte dt for the control cler- the bsence of volume expnsion in both groups. FRHC03 ws nce period nd the clernce period with the highest PHCO, in % in newborns nd % in dults. protocol I re presented in Tble 1. PHC03 ws similr in Isotonic NHC03 infusion incresed PHC03 nd the filtered newborns nd dults under control conditions. PHCO, ws lod of bicrbonte per GFR (to nd 31.8 f 2.2 rised to similr vlues in both groups by replcing urine output pmol/ml filtered) nd extrcellulr volume (by 24 k 2 nd 21 with the high bicrbonte solution. These increses in PHCO, + 4%) similrly in the newborn nd dult. Inhibition of bicrwere not ccompnied by extrcellulr volume expnsion in bonte rebsorption ws significntly greter in the dult thn in either ge group. the newborn under these conditions. FRHC03 in the volume- Proximl nephron RHCO,/GFR ws similr in newborns nd expnded stte ws 94.4? 1.9% in newborns nd only 77.0 f dults under control conditions. Proximl nephron RHC03/ 1.9% in dults ( p = ). GFR incresed with PHCO, in both groups ( p < 0.00 I), but not Segmentl renl function during NHC0, volume expnsion s much in the newborn (from to 40? 3.9 pmol/ml is shown in Figure 4. Bicrbonte rebsorption in the presence of distl nephron trnsport blockde with ethcrynic cid nd filtered) s in the dult ( to pmol/ml Tble 1. Arteril blood gs nd plsm nd urine electrolyte dt for control clernce period nd clernce period with highest PHCO, in rotocol I * - Control Highest PHCO? PH t Pco~, mmhgt,s PHCO,, mmt POsm, mmt, UOsm, mmt, U:POsm PN, mm UN, mm U:PNt PK, mmt, UK, mm Newborn Adult Newborn Adult TP. ddl6 -, " * POsm, plsm osmolrity; UOsm, urine osmolrity; PN, plsm sodium concentrtion; UN, urine sodium concentrtion; PK, plsm potssium concentrtion; UK, urine potssium concentrtion; nd TP, totl plsm protein concentrtion. t p for bicrbonte effect (control vs highest PHCO,) by nlysis of vrince. 8 p for interction effect by nlysis of vrince. 5 p for ge effect by nlysis of vrince.

4 u E E \ 50 HCO, REABSORPTION IN NEWBORN AND ADULT DOG PROXIMAL RHC03 VS. PHC03 IN EUVOLEMIC DOGS (PROTOCOL 1) 0-2 IC $ I Z g 20- I W Z J X 0 F - NEWBORNS O - ' " ' " ' ~ - ~ " ' ~ ~ ~ ~ I PHCO (mm) Fig. 1. Proximl RHCO,/GFR vs PHCO, in euvolemic dogs (protocol I). Individul best-fit lines of regression of proximl nephron RHCO,/ GFR on PHC03 in seven euvolemic newborn dogs (leff pnel) nd in six euvolemic dult dogs (right pnel). - EFFECT OF VOLUME EXPANSION ON PROXIMAL RHC03 NEWBORNS ADULTS n S r 20 Z 0 r W Z - lo- Fig. 2. Effect of volume expnsion on proximl RHC03/GFR. Left pnel: Proximl nephron RHCO,/GFR vs PHCO, for ech of the NHCO, volume-expnded newborn dogs (protocol 11) re depicted by open circles. The open circle with SEM brs depicts the men for these seven nimls. The composite regression line (solid line) nd 95% confidence intervl (dshed lines) relting RHCO,/GFR to PHC03 in euvolemic (protocol I) newborn dogs is shown for comprison. Right pnel: Proximl nephron RHCO,/GFR vs PHC03 for ech of the NHC0, volume-expnded dult dogs (protocol 11) re depicted by filled circles. The filled circles with SEA4 brs depicts the men for these six nimls. The composite regression line (solid line) nd 95% confidence intervl (dshed line) relting RHC03/GFR to PHC03 in euvolemic (protocol I) dults is shown for comprison.

5 608 LORENZ AND KLEINMAN Tble 2. Arteril blood gs nd plsm nd urine electrolyte dt for control nd NHC03-expnded clernce periods in protocol - IT* - Control NHC0,-expnded Newborn Adult Newborn Adult PH t k 0.02 Pco~, mmhg PHCO,, mmt rt 2.2 POsm, mmt rt rt 2 UOsm, mmt,$, t k 67 PN, mm UN, mmq: PK, mmt, UK, mmt TP, g/dlt, 4.4 & * POsm, plsm osmolrity; UOsm, urine osmolrity; PN, plsm sodium concentrtion; UN, urine sodium concentrtion; PK, plsm potssium concentrtion; UK, urine potssium concentrtion; nd TP, totl plsm protein concentrtion t p for bicrbonte effect (control vs NHC0,-expnded) by nlysis of vrince. $ p for interction effect by nlysis of vrince. 5 p for ge effect by nlysis of vrince. miloride is shown on the left of ech pnel of br grphs. This is proximl nephron bicrbonte rebsorption. Distl nephron bicrbonte rebsorption during volume expnsion is shown on the right of ech pnel in Figure 4. U:POsm nd U:PN during distl blockde were 1.OO nd 1.OO k 0.01, respectively, in the newborn nd nd 1.OO f 0.01 in the dult. There ws no significnt difference in proximl nephron RHCOJGFR between the newborn nd the dult t PHC03 vlues of nd 27.4 k 2.0. However, proximl nephron RHC03/GFR for three of the seven newborns ws within the 95% confidence intervl of the line describing the reltionship of PHC03 nd proximl RHC03/GFR without volume expnsion nd the group's men proximl RHC03/GFR ws t the lower limit of the 95% confidence intervl (Fig. 2, left pnel). On the other hnd, none of the dults hd proximl nephron RHCOJGFR tht fell within the 95% confidence intervl of the line in dults without volume expnsion nd thus neither did men proximl RHC03/GFR for the dult group (Fig. 2, right pnel). Becuse there ws no difference in filtered bicrbonte lod per GFR or proximl RHC03/GFR between the newborn nd the dult, distl bicrbonte delivery per GFR ws similr (top right pnel, Fig. 4). However, distl RHC03/GFR ws greter in the newborn thn the dult (8.5 f 1.3 versus pmol/ ml filtered, p = 0.06) nd, therefore, so ws the percent of the distl lod tht ws rebsorbed (83 + 6% versus 28 + lo%, p < 0.001). DISCUSSION The newborn kidney is frequently chrcterized s bicrbonte wsting. There ws no evidence of this in previous study from this lbortory in which PHCO, ws progressively elevted in newborn dogs without ccompnying volume expnsion. Tubulr rebsorption of filtered lods s high s 50 pmol/ml of glomerulr filtrte ws essentilly complete (4). This study demonstrtes tht tubulr bicrbonte rebsorption ws suppressed in the newborn nd the dult when the increse in filtered lod ws ccompnied by extrcellulr volume expnsion. However, the newborn rebsorbed significntly greter frction of the filtered lod thn did the dult. Thus, the immture kidney conserves bicrbonte when filtered bicrbonte is incresed whether or not there is n ccompnying increse in extrcellulr volume. Discussion of protocol I results nd of the segmentl loci of bicrbonte rebsorption when extrcellulr volume is expnded nd/or PHC03 elevted is dependent upon the vlidity of the distl blockde technique. This technique is bsed on the rtionle tht urine excreted during the dministrtion of ethcrynic cid nd miloride t pproprite dose rtes resembles tubulr fluid t the end of the proximl stright tubules. Two ssumptions must be met for this to be the cse. First, ethcrynic cid nd miloride must hve no effect on the proximl tubule electrolyte rebsorption. In the doses used in this study nd in the bsence of secondry contrction of extrcellulr volume, this is likely the cse (16-20). The high concentrtion of miloride required to block N+-H' exchnge in the proximl tubule mkes it unlikely tht this ction is physiologiclly relevnt (2 1). Second, distl electrolyte rebsorption nd secretion must be completely blocked. To ssure this, clernces during distl blockde were included in the dt only if urine to plsm sodium concentrtion nd osmolrity rtios were The men urine to plsm chloride concentrtion rtio decresed from to in dults in protocol I s PHC03 rose nd proximl bicrbonte rebsorption decresed, nd ws 1.20 during NHC03 volume expnsion in protocol 11. These re the rtios expected if distl blockde is complete nd urine resembles fluid t the end of the proximl tubule (22-25). However, continued isotonic rebsorption during distl blockde is not excluded by these rtios. The dose of miloride used in this study bolishes nerly ll potssium secretion (17, 26-28) nd inhibits bicrbonte rebsorption in the collecting tubule (29,30). Moreover, proximl frctionl bicrbonte rebsorption in protocol I under control conditions in the dult dog (95.6%) ws very close to the FRHC03 of 94% before the distl tubule in hydropenic dogs by micropuncture (3 1). Thus, urine flow rte nd composition during ethcrynic cid nd miloride dministrtion resonbly pproximtes tubulr fluid flow rte nd composition t the end of the proximl stright tubule. The clcultion of distl nephron trnsport from sequentil clernces without nd with diuretics in protocol I1 requires third ssumption: tht the chnge in trnsport fter diuretic dministrtion nd reequilibrtion cn be principlly ttributed to inhibition of distl nephron function nd not due to n effect of time. This ssumption is supported by previously reported results of NHC03 expnsion in time control experiments (32). RHC03/GFR by the proximl nephron ws linerly sup-

6 HCO~ REABSORPTION IN NEWBORN AND ADULT DOG 609 TOTAL NEPHRON NHC03 VOLUME EXPANSION 40 - CONTROL N HC03 VE 40 - PROXIMAL DISTAL - NEPHRON NEPHRON - 40 * "'73 mi5 1;; O W J W 0 ti I.- g % = 2 z 0- WE 20- [L w 2 I- 0 w 2: J E io- 10- Q e E NEWBORN ADULT Fig. 3. Totl nephron bicrbonte hndling under control conditions nd during NHC03 expnsion (protocol 11). Right pnel: Filtered HCO3 lod per GFR, totl nephron RHCO,/GFR nd FRHC03 under control conditions (left) nd during NHCO3 volume expnsion (right) in newborn (open brs) nd dult (solid brs) dogs. Br heights depict ge group mens nd error brs depict SEM. pressed in both the newborn nd dult s PHC03 ws progressively elevted from control vlues to mm when extrcellulr volume ws mintined constnt. Although RHC03/GFR ws similr t PHC03 ner control vlues, suppression of proximl nephron RHC03/GFR ws more pronounced in the newborn thn in the dult s PHC03 rose. Volume expnsion with isotonic NHC03 significntly suppressed proximl RHC03/ GFR more thn elevtion of PHC03 lone in both ge groups t comprble PHC03. In contrst to hyperbicrbonemi lone, however, there ws no difference between the ge groups in the degree of suppression produced by hyperbicrbonemic volume expnsion. If nything, the decrement in proximl nephron RHC03/GFR with volume expnsion over tht with comprble elevtion of PHCO, lone ws less in the newborn thn the dult (Figs. 2 nd 3). We hve observed the sme tendency with NCl loding (32). Furthermore, this is opposite of the effect tht volume expnsion hs on proximl nephron sodium rebsorption (5, 7, 10, 32). The former suggests tht the decrement in NEWBORN ADULT Fig. 4. Segmentl nephron RHCO,/GFR during NHC03 expnsion (protocol 11). Proximl (left) nd distl (right) nephron bicrbonte lods per GFR, RHCO,/GFR, nd FRHCO, in NHC03 volume-expnded newborn nd dult dogs. Proximl vlues re those obtined during volume expnsion with ethcrynic cid nd miloride blockde of distl nephron function. Distl vlues re clculted from vlues obtined during volume expnsion with nd without distl blockde. There ws min effect of ge on proximl nephron RHC03 (F = 4.57, p = 0.05). proximl FRHC03/GFR seen in protocol I1 over tht in protocol I is the result of volume expnsion per se; the ltter suggests tht suppression of proximl nephron RHC03 by volume expnsion is not directly linked to n effect on proximl sodium rebsorption. Alpern et l. (33) hve demonstrted tht bicrbonte bsorption in the proximl nephron cn be described by two prllel components, both sensitive to luminl bicrbonte concentrtion: I) sturble proton secretion, nd 2) pssive bicrbonte lek. Active proton secretion is suppressed by increses in PHC03 but is unffected by volume expnsion. Pssive bicrbonte lek is determined by the proximl tubule bicrbonte permebility, which increses with volume expnsion, nd the trnstubulr concentrtion grdient (34). The more pronounced suppression of proximl RHC03/GFR observed in the newborn by euvolemic hyperbicrbonemi, then, could be the result of lesser cpcity for proton secretion or greter bck lek of bicrbonte in the lte proximl tubule s luminl bicrbonte concentrtion flls long the tubule. However, the similr (or perhps less pronounced) suppression of proximl RHC03/GFR by volume expnsion in newborns t comprble PHCO, suggests tht the

7 6 10 LORENZ AND increse in HC03 permebility with volume expnsion is similr (or perhps even less) in the newborn thn the dult. Therefore, the limited cpcity of the immture proximl nephron for proton secretion seems to be the most likely explntion for the differences in RHC03/GFR observed between the ge groups in protocol I. Clcultion of distl nephron (which by this method includes the loop of Henle, the distl convoluted tubule, nd the collecting tubule) lod nd rebsorption in protocol I1 reveled greter RHC03/GFR of similr HC03 lod/gfr in the newborn versus the dult. The design of protocol I did not llow totl nephron rebsorption to be determined in these nimls. However, previous studies performed in our lbortory hve shown tht bicrbonte rebsorption is essentilly complete in newborn nd dult dogs up to PHC03 of 50 mm in the euvolemic stte (4). Thus, rebsorption of bicrbonte in the distl nephron must increse commensurtely with the delivered lod, so tht it is essentilly complete over the rnge of PHC03 observed in protocol I. Moreover, the frction of the filtered lod tht must be rebsorbed distlly increses more in the euvolemic newborn thn in the euvolemic dult s PHC03 rises. As PHCO3 rises from control vlues to the vlues observed during NHC03 expnsion to 60 mm, the frction of the filtered lod rebsorbed distlly increses from 9 to 19 to 30% in the euvolemic newborn nd 7 to 10 to 15% in the euvolemic dult. Thus, the immture distl nephron displys greter propensity for bicrbonte rebsorption thn does the mture distl nephron with elevtion of PHC03 whether or not there is concomitnt volume expnsion. As discussed bove, these conclusions require the ssumption tht distl rebsorption of bicrbonte is completely (or t lest similrly) inhibited in the newborn nd dult kidney by ethcrynic cid nd miloride. If diuretic dministrtion inhibited distl nephron bicrbonte rebsorption less completely in the dult thn it did in the newborn, then distl bicrbonte delivery nd rebsorption would ctully be greter in the dult thn wht we concluded them to be. If this were the cse, then delivery of bicrbonte per GFR to the distl nephron nd distl nephron RHC03/GFR could ctully be similr in the newborn nd the dult s PHC03 is elevted. To investigte this possibility, we exmined the (U-P)Pcoz (35). The (U-P)Pco2 is due lrgely to proton secretion in the presence of tubulr bicrbonte in the distl nephron. Incresed delivery of bicrbonte to the distl nephron lso increses (U-P)Pco2. The (U-P)Pco2 ws greter in the newborn thn in the dult during diuretic dministrtion in both protocol 1 ( versus 8? 7 mm Hg, p = 0.03) nd protocol I1 ( versus mm Hg, p = 0.03). Thus, these dt re inconsistent both with greter distl bicrbonte delivery per GFR nd with greter distl RHC03/GFR in the dult thn the newborn. Although (U-P)Pco2 lso vries with medullry countercurrent trpping of C02 (35), it is unlikely tht this fctor ccounts for these differences in (U-P)Pco2. Shorter loops of Henle (36) nd reltively greter medullry blood flow (37) in the immture kidney would tend to decrese medullry C02 trpping nd the (U-P)Pco2 in the newborn. If nything, then, these (U-P)Pcoz dt suggest tht inhibition of distl nephron bicrbonte rebsorption my hve been more, not less, complete in the dult thn in the newborn. In this cse, segmentl nephron differences in renl tubulr bicrbonte rebsorption per GFR between the newborn nd the dult would be even greter thn we hve estimted them to be. This greter vidity of the distl nephron when distl delivery is incresed is lso observed when distl sodium nd chloride delivery re incresed (7, 32) nd thus is probbly common to more thn one segment of the distl nephron. The fctors responsible for this greter propensity for electrolyte rebsorption when delivery is incresed cnnot be determined from these studies. It is difficult to identify single fctor tht could effect the rebsorption of these electrolytes in different segments. It should be noted tht rebsorption of sodium, chloride, nd bicrbonte per g kidney re much lower in the newborn thn in the dult under ll of these experimentl conditions, even though the frctions of the distl lods rebsorbed re higher, becuse the distl lods re much lower s result of lower GFR per g kidney in the newborn (7, 32). Thus, even if the rebsorptive cpcities per g kidney of these segments of the immture distl nephron were less thn those ofthe mture distl nephron, it is possible tht these cpcities might be exceeded in the dult but not in the newborn. Enhnced electrolyte rebsorption by the immture distl nephron when delivery is incresed could, then, be the result of the lower GFR nd the resultingly lower distl electrolyte delivery per g kidney in the newborn. In summry, s PHC03 nd filtered HC03 lod increse there is less suppression of RHC03/GFR in the newborn thn in the dult dog whether or not these increses re ccompnied by extrcellulr volume expnsion. Suppression of proximl nephron RHC03/GFR is more pronounced in the euvolemic newborn thn in the euvolemic dult. Volume expnsion, on the other hnd, produces no greter inhibition of proximl RHC03 in the newborn thn it does in the dult. We conclude tht the immture proximl tubule hs lower cpcity for ctive proton secretion, but similr increse in bicrbonte permebility in response to volume expnsion, thn the mture proximl tubule. 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