Metabolic alkalosis in the rat. Evidence that reduced glomerular filtration rather than enhanced tubular bicarbonate reabsorption is responsible for maintaining the alkalotic state

Serum bicarbonate and mortality in stage 3 and stage 4 chronic kidney disease

BACKGROUND AND OBJECTIVES

The incidence and prevalence of metabolic acidosis increase with declining kidney function. We studied the associations of both low and high serum bicarbonate levels with all-cause mortality among stage 3 and 4 chronic kidney disease (CKD) patients.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We examined factors associated with low (<23 mmol/L) and high (>32 mmol/L) serum bicarbonate levels using logistic regression models and associations between bicarbonate and all-cause mortality using Cox-proportional hazard models, Kaplan-Meier survival curves, and time-dependent analysis.

RESULTS

Out of 41,749 patients, 13.9% (n = 5796) had low and 1.6% (n = 652) had high serum bicarbonate levels. After adjusting for relevant covariates, there was a significant association between low serum bicarbonate and all-cause mortality (hazard ratio [HR] 1.23, 95% CI 1.16, 1.31). This association was not statistically significant among patients with stage 4 CKD and diabetes. The time-dependent analysis demonstrated a significant mortality risk associated with a decline from normal to low bicarbonate level (HR 1.59, 95% CI 1.49, 1.69). High serum bicarbonate levels were associated with death irrespective of the level of kidney function (HR 1.74, 95% CI 1.52, 2.00). When serum bicarbonate was examined as a continuous variable, a J-shaped relationship was noted between serum bicarbonate and mortality.

CONCLUSIONS

Low serum bicarbonate levels are associated with increased mortality among stage 3 CKD patients and patients without diabetes. High serum bicarbonate levels are associated with mortality in both stage 3 and stage 4 CKD patients.

Mechanosensory function of microvilli of the kidney proximal tubule

Normal variations in glomerular filtration induce proportional changes in proximal tubule Na+ reabsorption. This "glomerulotubular balance" derives from flow dependence of Na+ uptake across luminal cell membranes; however, the underlying physical mechanism is unknown. Our hypothesis is that flow-dependent reabsorption is an autoregulatory mechanism that is independent of neural and hormonal systems. It is signaled by the hydrodynamic torque (bending moment) on epithelial microvilli. Such signals need to be transmitted to the terminal web to modulate Na+-H+-exchange activity. To investigate this hypothesis, we examined Na+ transport and tubular diameter in response to different flow rates during the microperfusion of isolated S2 proximal tubules from mouse kidneys. The data were analyzed by using a mathematical model to estimate the microvillous torque as function of flow. In this model, increases in luminal diameter have the effect of blunting the impact of flow velocity on microvillous shear stress and, thus, microvillous torque. We found that variations in microvillous torque produce nearly identical fractional changes in Na+ reabsorption. Furthermore, the flow-dependent Na+ transport is increased by increasing luminal fluid viscosity, diminished in Na+-H+ exchanger isoform 3 knockout mice, and abolished by nontoxic disruption of the actin cytoskeleton. These data support our hypothesis that the "brush-border" microvilli serve a mechanosensory function in which fluid dynamic torque is transmitted to the actin cytoskeleton and modulates Na+ absorption in kidney proximal tubules.

Prenatal dexamethasone programs hypertension and renal injury in the rat

Dexamethasone is frequently administered to the developing fetus to accelerate pulmonary development. The purpose of the present study was to determine if prenatal dexamethasone programmed a progressive increase in blood pressure and renal injury in rats. Pregnant rats were given either vehicle or 2 daily intraperitoneal injections of dexamethasone (0.2 mg/kg body weight) on gestational days 11 and 12, 13 and 14, 15 and 16, 17 and 18, or 19 and 20. Offspring of rats administered dexamethasone on days 15 and 16 gestation had a 20% reduction in glomerular number compared with control at 6 to 9 months of age (22 527+/-509 versus 28 050+/-561, P<0.05), which was comparable to the percent reduction in glomeruli measured at 3 weeks of age. Six- to 9-month old rats receiving prenatal dexamethasone on days 17 and 18 of gestation had a 17% reduction in glomeruli (23 380+/-587) compared with control rats (P<0.05). Male rats that received prenatal dexamethasone on days 15 and 16, 17 and 18, and 13 and 14 of gestation had elevated blood pressures at 6 months of age; the latter group did not have a reduction in glomerular number. Adult rats given dexamethasone on days 15 and 16 of gestation had more glomeruli with glomerulosclerosis than control rats. This study shows that prenatal dexamethasone in rats results in a reduction in glomerular number, glomerulosclerosis, and hypertension when administered at specific points during gestation. Hypertension was observed in animals that had a reduction in glomeruli as well as in a group that did not have a reduction in glomerular number, suggesting that a reduction in glomerular number is not the sole cause for the development of hypertension.

Renal nerve stimulation augments effect of intraluminal angiotensin II on proximal tubule transport

The proximal tubule synthesizes and secretes angiotensin II into the lumen, where it regulates transport. Renal denervation abolishes the effect of angiotensin II on proximal tubule transport. Using in vivo microperfusion, we examined whether renal nerve stimulation modulates the effect of angiotensin II on transport. The effect of angiotensin II was assessed by measuring the decrease in volume reabsorption with the addition of 10(-4) M luminal enalaprilat. Luminal enalaprilat did not alter volume reabsorption (2.80 +/- 0.18 vs. 2.34 +/- 0.14 nl x mm(-1) x min(-1)). However, with renal nerve stimulation, enalaprilat decreased volume reabsorption (3.45 +/- 0.22 vs. 1.67 +/- 0.20 nl x mm(-1) x min(-1), P < 0.0005). The absolute and percent decrements in volume reabsorption with luminal enalaprilat were higher with renal nerve stimulation than with native innervation (1.78 +/- 0.19 vs. 0.46 +/- 0.23 nl x mm(-1) x min(-1), P < 0.02, and 51.8 +/- 5.0 vs. 14.6 +/- 7.4%, P < 0.05, respectively). Renal nerve stimulation did not alter the glomerular filtration rate or renal blood flow. Renal nerve stimulation augments the stimulatory effect of intraluminal angiotensin II. The sympathetic renal nerves modulate the proximal tubule renin-angiotensin system and thereby regulate proximal tubule transport.

Rat proximal NHE3 adapts to chronic acid-base disorders but not to chronic changes in dietary NaCl intake

In the proximal tubule, the apical Na(+)/H(+) exchanger identified as NHE3 mediates most NaCl and NaHCO(3) absorption. The purpose of this study was to analyze the long-term regulation of NHE3 during alkalosis induced by dietary NaHCO(3) loading and changes in NaCl intake. Sprague-Dawley rats exposed to a low-NaCl, high-NaCl, or NaHCO(3) diet for 6 days were studied. Renal cortical apical membrane vesicles (AMV) were prepared from treated and normal rats. Na(+)/H(+) exchange was assayed as the initial rate of (22)Na(+) uptake in the presence of an outward H(+) gradient. (22)Na(+) uptake measured in the presence of high-dose 5-(N-ethyl-N-isopropyl) amiloride was not different among models. Changes in NaCl intake did not affect NHE3 activity, whereas NaHCO(3) loading inhibited (22)Na(+) uptake by 30%. AMV NHE3 protein abundance assessed by Western blot analysis was unaffected during changes in NaCl intake. During NaHCO(3) loading, NHE3 protein abundance was decreased by 65%. We conclude that proximal NHE3 adapts to chronic metabolic acid-base disorders but not to changes in dietary NaCl intake.

Effect of prenatal dexamethasone on rat renal development

BACKGROUND

Prenatal insults can program the developing fetus to develop diseases that manifest in later life. Dexamethasone is often administered to the developing fetus to accelerate pulmonary development. The purpose of the present study was to determine whether prenatal dexamethasone adversely affects renal development and predisposes rats to develop renal disease and hypertension in later life.

METHODS

Pregnant rats were given either vehicle or two daily intraperitoneal injections of dexamethasone (0.2 mg/kg body weight) on gestational days: 11 and 12, 13 and 14, 15 and 16, 17 and 18, 19 and 20, or 20 and 21. Tail cuff blood pressure, glomerular number, and inulin clearance were measured in control and prenatal dexamethasone-treated rats when the rats were 60 to 90 days of age.

RESULTS

Prenatal dexamethasone did not affect the length of gestation, the number of animals per litter, or the total body weight or kidney weight measured at one day of age. Offspring of rats administered dexamethasone on days 15 and 16 gestation had a 30% reduction in glomerular number compared with control at 60 to 70 days of age (24,236 +/- 441 vs. 30,453 +/- 579, P < 0.01). Rats receiving prenatal dexamethasone on days 17 and 18 had an approximate 20% reduction in glomeruli compared with control (P < 0.01). Offspring of rats receiving dexamethasone on days 15 and 16 gestation had systolic blood pressures at 60 to 90 days of age that were higher than any other group (P < 0.05). The glomerular filtration rate was comparable in all of the groups.

CONCLUSIONS

This study shows that two daily doses of prenatal dexamethasone (0.2 mg/kg body weight) in rats do not produce intrauterine growth retardation. Adult offspring of rats that received prenatal dexamethasone during specific times of gestation have a reduced number of nephrons and hypertension.

Effect of acute increases in filtered HCO3- on renal hydrogen transporters: II. H(+)-ATPase

Adaptive increases in renal bicarbonate reabsorption occur in response to acute increases in filtered bicarbonate (FLHCO3). In a previous study, we showed that an increase in FLHCO3 induced by plasma volume expansion increased the Vmax for Na+/H+ exchange activity in renal cortical brush border membrane vesicles (BBMV), providing a potential mechanism for the adaptive increase in HCO3- reabsorption. The present studies were undertaken to determine whether the increase in FLHCO3 induced by plasma expansion also stimulates the other major H+ transporter in cortical BBMV, the H(+)-ATPase. H(+)-ATPase activity was assessed in BBMV obtained from hydropenic and plasma expanded Munich-Wistar rats, using a NADH-linked ATPase assay. H(+)-ATPase activity was measured as the ouabain and oligomycin-insensitive, bafilomycin A1-sensitive component of total ATPase activity. Acute plasma expansion doubled single nephron FLHCO3, and this change was associated with a 64% increase in the Vmax for H(+)-ATPase activity, with no change in apparent Km. The Vmax for H(+)-ATPase activity correlated directly with whole kidney GFR and FLHCO3 (r = 0.68 and 0.72, respectively), and with single nephron GFR and FLHCO3 (r = 0.76 and 0.80, respectively). Thus, the mechanism for the adaptive increase in proximal tubular HCO3- reabsorption that occurs in response to acute increases in FLHCO3 appears to be related to increased activity of both H(+)-ATPase and Na+/H+ exchange in the apical membrane of the proximal tubule epithelium.

Acid-base disorders in medicine

The practice of internal medicine involves daily exposure to abnormalities of acid-base balance. A wide variety of disease states either predispose patients to develop these conditions or lead to the use of medications that alter renal, gastrointestinal, or pulmonary function and secondarily alter acid-base balance. In addition, primary acid-base disease follows specific forms of renal tubular dysfunction (renal tubular acidosis). We review the acid-base physiologic functions of the kidney and gastrointestinal tract and the current understanding of acid-base pathophysiologic conditions. This includes a review of whole animal and renal tubular physiologic characteristics and a discussion of the current knowledge of the molecular biology of acid-base transport. We stress an approach to diagnosis that relies on knowledge of acid-base physiologic function, and we include discussion of the appropriate treatment of each disorder considered. Finally, we include a discussion of the effects of acidosis and alkalosis on human physiologic functions.

A kinetically defined Na+/H+ antiporter within a mathematical model of the rat proximal tubule

The luminal membrane antiporter of the proximal tubule has been represented using the kinetic formulation of E. Heinz (1978. Mechanics and Engergetics of Biological Transport. Springer-Verlag, Berlin) with the assumption of equilibrium binding and 1:1 stoichiometry. Competitive binding and transport of NH+4 is included within this model. Ion affinities and permeation velocities were selected in a least-squares fit to the kinetic parameters determined experimentally in renal membrane vesicles (Aronson, P.S., M.A. Suhm, and J. Nee. 1983. Journal of Biological Chemistry. 258:6767-6771). The modifier role of internal H+ to enhance transport beyond the expected kinetics (Aronson, P.S., J. Nee, and M. A. Suhm. 1982. Nature. 299:161-163) is represented as a velocity effect of H+ binding to a single site. This kinetic formulation of the Na+/H+ antiporter was incorporated within a model of the rat proximal tubule (Weinstein, A. M. 1994. American Journal of Physiology. 267:F237-F248) as a replacement for the representation by linear nonequilibrium thermodynamics (NET). The membrane density of the antiporter was selected to yield agreement with the rate of tubular Na+ reabsorption. Simulation of 0.5 cm of tubule predicts that the activity of the Na+/H+ antiporter is the most important force for active secretion of ammonia. Model calculations of metabolic acid-base disturbances are performed and comparison is made among antiporter representations (kinetic model, kinetic model without internal modifier, and NET formulation). It is found that the ability to sharply turn off Na+/H+ exchange in cellular alkalosis substantially eliminates the cell volume increase associated with high HCO3- conditions. In the tubule model, diminished Na+/H+ exchange in alkalosis blunts the axial decrease in luminal HCO3- and thus diminishes paracellular reabsorption of Cl-. In this way, the kinetics of the Na+/H+ antiporter could act to enhance distal delivery of Na+, Cl-, and HCO3- in acute metabolic alkalosis.

Potassium and sodium transport along the loop of Henle: effects of altered dietary potassium intake

We assessed the effects of changes in potassium (K+) balance on the function of the loop of Henle by a combination of renal clearance and microperfusion experiments. Rat superficial cortical nephrons were perfused in vivo at 20 nl.min-1 from late proximal to early distal tubule with an artificial end-proximal solution containing either 3.8 or 1.8 mM potassium. Rats were fed a control diet, a low-potassium diet for at least three weeks, or a high-potassium diet for 10 to 14 days. When compared with the appropriate end-proximal potassium concentration in the perfusion fluid, potassium absorption along the loop of Henle (JK) increased in potassium-depletion whereas sodium (JNa) and fluid (Jv) absorption decreased. In rats fed a high-potassium diet, absorption of potassium, sodium and fluid was depressed. We propose that changes of external potassium balance affect the transport of electrolytes and fluid along the loop of Henle in vivo by modulating the transport of potassium and sodium primarily in the thick ascending limb. Changes in potassium reabsorption may also be affected by alterations of potassium-recycling.

Application of multivariate autoregressive modelling for analysing chloride/potassium/bicarbonate relationship in the body

The authors repeatedly analysed course data of acid-base disturbances accompanying hypochloraemia and/or hypokalaemia by means of multivariate autoregressive modelling. It was found that the regulatory relationship between chloride and bicarbonate is inverse between the following two hypochloraemic hyperbicarbonataemic states: the one induced by chloride depletion and the other induced by CO2 retention. Also, the study revealed an independent role of potassium deficiency in the development of metabolic alkalosis, especially in cases with mineralocorticoid-induced alkalosis. The present approach enabled the authors to solve a long-standing problem, i.e. to differentiate between the roles of chloride and potassium in the development of metabolic alkalosis.

Chronic neutral phosphate supplementation induces sustained, renal metabolic alkalosis

The aim of the present study was to test whether intravenous neutral phosphate supplementation, recently shown in our laboratory to acutely stimulate proton secretion in the distal nephron, was able to induce a sustained metabolic alkalosis. Neutral Na and K phosphate supplementation for seven days, with equivalent reduction in chloride supply and unchanged intake of sodium and potassium, in ADX rats receiving fixed physiological doses of aldosterone and dexamethasone (group 1, N = 7), was responsible for a severe metabolic alkalosis (MA; delta [HCO3] 11 +/- 1.3 mM, and delta pH 0.11 +/- 0.06 unit). Metabolic alkalosis was at least in part of renal origin, since net acid excretion (NAE) transiently increased, principally due to an increment in titratable acid excretion rate. Balances were equilibrated for sodium and negative for chloride and potassium, which may have contributed to the severity of the MA. Chronic i.v. neutral Na phosphate, without change in potassium and chloride supply, in ADX rats receiving the same doses of steroids (group 2, N = 5), was responsible for a less severe MA (delta [HCO3] 7.5 +/- 0.9 mM, and delta pH 0.07 +/- 0.01 unit), also of renal origin. In this group, balances were positive for chloride and sodium and equilibrated for potassium. Finally, neutral Na and K phosphate supplementation with reduction in chloride supply in intact rats (group 3, N = 4) was also able to induce a MA (delta [HCO3] 5.5 +/- 1.8 mM, and delta pH 0.06 +/- 0.01 unit) of renal origin, with balances negative for chloride and equilibrated for potassium and sodium. In all groups, the generation and maintenance of MA probably resulted from stimulated proton secretion in the distal nephron, as suggested by the observed increase of PCO2 over HCO3 concentration ratio in the urine and a fall in urine pH despite augmented urinary buffer content throughout the phosphate infusion period. Glomerular filtration rate did not significantly vary in any group. In conclusion, chronic supplementation of neutral phosphate appears to stimulate per se proton secretion in the distal nephron, independently of sodium, chloride, and potassium balances, and adrenal steroid secretion. Thus neutral phosphate supplementation should be added to the previously known factors able to induce MA.

Potassium depletion increases luminal Na+/H+ exchange and basolateral Na+:CO3=:HCO3- cotransport in rat renal cortex

Most HCO3- reabsorption in proximal tubules occurs via electroneutral Na+/H+ exchange in brush border membranes (BBMS) and electrogenic Na+:CO3=:HCO3- cotransport in basolateral membranes (BLMS). Since potassium depletion (KD) increases HCO3- reabsorption in proximal tubules, we evaluated these transport systems using BBM and BLM vesicles, respectively, from control (C) and KD rats. Feeding rats a potassium deficient diet for 3-4 wk resulted in lower plasma [K+] (2.94 mEq/liter, KD vs. 4.47 C), and higher arterial pH (7.51 KD vs. 7.39 C). KD rats gained less weight than C but had higher renal cortical weight. Influx of 1 mM 22Na+ at 5 s (pHo 7.5, pHi 6.0, 10% CO2, 90% N2) into BLM vesicles was 44% higher in the KD group compared to C with no difference in equilibrium uptake. The increment in Na+ influx in the KD group was DIDS sensitive, suggesting that Na+:CO3=:HCO3- cotransport accounted for the observed differences. Kinetic analysis of Na+ influx showed a Km of 8.2 mM in KD vs. 7.6 mM in C and Vmax of 278 nmol/min/mg protein in KD vs. 177 nmol/min/mg protein in C. Influx of 1 mM 22Na+ at 5 s (pHo 7.5, pHi 6.0) into BBM vesicles was 34% higher in the KD group compared to C with no difference in equilibrium uptake. The increment in Na+ influx in the KD group was amiloride sensitive, suggesting that Na+/H+ exchange was responsible for the observed differences. Kinetic analysis of Na+ influx showed a Km of 6.2 mM in KD vs. 7.1 mM in C and Vmax of 209 nmol/min/mg protein in KD vs. 144 nmol/min/mg protein in C. Uptakes of Na(+)-dependent [3H]glucose into BBM and [14C]succinate into BLM vesicles were not different in KD and C groups, suggesting that the Na+/H+ exchanger and Na+:CO3=:HCO3- cotransporter activities were specifically altered in KD. We conclude that adaptive increases in basolateral Na+:CO3=:HCO3- cotransport and luminal Na+H+ exchange are likely responsible for increased HCO3- reabsorption in proximal tubules of KD animals.

Renal regulation of electrolyte and acid-base balance in ruminants

The kidney maintains volume, electrolyte, and acid-base homeostasis. These functions are examined in the ruminant in response to differing dietary intakes and disease states. The consequences of renal disease for these homeostatic processes and the interpretation of urinary excretion data are reviewed.

Augmented bicarbonate reabsorption by both the proximal and distal nephron maintains chloride-deplete metabolic alkalosis in rats

Whether augmented bicarbonate reabsorption by renal tubular epithelium contributes to the maintenance of chloride-deplete metabolic alkalosis is not clear. This study used free-flow micropuncture to investigate bicarbonate reabsorption by surface nephron segments in a rat model of diuretic-induced alkalosis compared to control. The proximal and distal nephron of the alkalotic animals had higher values for both delivered load to and absolute reabsorption from these segments. The proximal tubules of alkalotic and control animals had similar values for the slopes of the linear regression of delivered load vs. reabsorption and for the bicarbonate tubular fluid to plasma (TF/P) ratio at the late proximal tubule. By contrast, the corresponding analysis for the distal segment of alkalotic animals revealed a greater slope (0.98 vs. 0.81, P less than 0.003) and a smaller bicarbonate TF/P ratio at the late distal tubule (0.10 vs. 0.16, P less than 0.006). The data indicate that augmented bicarbonate reabsorption by both the proximal and distal nephron contributes to maintaining the alkalosis of this model. The data suggest primary stimulation of bicarbonate reabsorption in the distal nephron and load-dependent reabsorption in the proximal tubule.

Regulation of Na/H exchange in renal microvillus vesicles in chronic hypercapnia

Several disturbances of acid-base balance, including chronic metabolic and respiratory acidoses and metabolic alkalosis, are associated with enhanced proximal tubule bicarbonate reabsorption. To determine whether augmented brush border Na/H exchange might mediate enhanced proximal tubule bicarbonate reabsorption in these disorders, we measured Na/H exchange activity in cortical brush border membrane vesicles (BBMV) prepared from rats and rabbits adapted to hypercapnia and other chronic acid-base disturbances. BBMV prepared from control animals and animals with chronic acid-base disturbances were similar as judged by marker enzymes, alkaline phosphatase, and ouabain-sensitive phosphatase. Despite profound respiratory acidosis, no increase in Na/H exchange activity could be detected in vesicles prepared from rats adapted to chronic (8 to 10 days) or subacute (24 hr) respiratory acidosis. In addition, vesicles prepared from rabbits exposed to chronic hypercapnia did not show increased Na/H exchange when compared with contemporaneous controls. By contrast, in agreement with previously published results, amiloride-sensitive sodium uptake was increased by 30% in vesicles derived from animals with ammonium chloride-induced acidosis compared with contemporaneous controls. Two models of chronic metabolic alkalosis were also studied; vesicles from alkalotic rats did not show any alteration in Na/H exchange. We conclude that metabolic acidosis, but not respiratory acidosis or metabolic alkalosis, leads to enhanced activity of the luminal Na/H exchanger.

On the mechanism by which chloride corrects metabolic alkalosis in man

To determine whether administration of chloride corrects chloride-depletion metabolic alkalosis (CDA) by correction of plasma volume contraction and restoration of glomerular filtration rate or by an independent effect of chloride repletion, CDA was produced in normal men by the administration of furosemide and maintained by restriction of dietary sodium chloride intake. Negative sodium balance (-112 +/- 16 meq) and reduced plasma volume (2.53 versus 2.93 liters, p less than 0.05) developed. The cumulative chloride deficit of 271 +/- 16 meq was then repleted by oral potassium chloride (267 +/- 19 meq) over 36 hours with continued serial measurements of glomerular filtration rate, effective renal plasma flow, plasma volume, body weight, and plasma renin and aldosterone levels. CDA was corrected, even though body weight, plasma volume, glomerular filtration rate, and renal plasma flow all remained reduced and plasma aldosterone was elevated; urinary bicarbonate excretion increased during correction. Administration of an identical potassium chloride load to similarly sodium-depleted but not chloride-depleted normal subjects produced no change in acid-base status. It is concluded that chloride repletion can correct CDA by a renal mechanism without restoring plasma volume or glomerular filtration rate or by altering sodium avidity.

Renal response to metabolic alkalosis induced by isovolemic hemofiltration in the dog

We describe a new model of chloride-depletion alkalosis (CDMA), in which the method of induction of alkalosis does not itself cause a direct alteration in sodium and fluid balance. We have used this model, which is based on hemofiltration techniques in the dog, to study the immediate response of the kidney to the induction of CDMA. Normal dogs maintained with a NaCl-free diet for several days underwent hemofiltration of 50 ml/kg over a 35 minute period. The hemofiltrate was replaced ml for ml with a solution containing sodium and potassium in the same concentrations as found in each animal's plasma water. In control animals, the replacement solution contained chloride and bicarbonate in the same ratio as in the plasma; in the experimental (CDMA) animals the replacement solution contained bicarbonate as the only anion. In the control group, the procedure of hemofiltration coupled with isovolemic replacement caused no appreciable changes in plasma composition, urinary excretion rates, GFR, or tubular handling of bicarbonate. In the CDMA group, 106 +/- 8.4 mEq of chloride were removed in exchange for bicarbonate. A marked metabolic alkalosis resulted, plasma bicarbonate concentration increasing from 21.9 +/- 0.6 to 33.3 +/- 0.6 mEq/liter. The hemofiltration procedure itself, by design, did not alter sodium or fluid balance. Nevertheless, cumulative urinary sodium excretion increased over 2.5 hours by 23.0 +/- 6.4 mEq. A natriuresis of this magnitude is equivalent to a loss of ECF volume of approximately 200 ml. GFR did not change significantly. The rate of tubular reabsorption of bicarbonate increased significantly from 1209 +/- 82 to 1559 +/- 148 mu Eq/min in CDMA animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Parallel adaptation of the rabbit renal cortical sodium/proton antiporter and sodium/bicarbonate cotransporter in metabolic acidosis and alkalosis

Recent studies have shown that the bicarbonate reabsorptive capacity of the proximal tubule is increased in metabolic acidosis. For net bicarbonate reabsorption to be regulated, there may be changes in the rate of apical H+ secretion as well as in the basolateral base exit step. The present studies examined the rate of Na+/H+ exchange (acridine orange method) and Na+/HCO3 cotransport (22Na uptake) in apical and basolateral membranes prepared from the rabbit renal cortex by sucrose density gradient centrifugation. NH4Cl loading was used to produce acidosis (arterial pH, 7.27 +/- 0.03), and Cl-deficient diet with furosemide was used to produce alkalosis (arterial pH, 7.51 +/- 0.02). Maximal transport rate (Vmax) of Na+/H+ antiporter and Na+/HCO3 cotransporter were inversely related with plasma bicarbonate concentration from 6 to 39 mM. Furthermore, the maximal transport rates of both systems varied in parallel; when Vmax for the Na+/HCO3 cotransporter was plotted against Vmax for the Na+/H+ antiporter for each of the 24 groups of rabbits, the regression coefficient (r) was 0.648 (P less than 0.001). There was no effect of acidosis or alkalosis on affinity for Na+ of either transporter. We conclude that both apical and basolateral H+/HCO3 transporters adapt during acid-base disturbances, and that the maximal transport rates of both systems vary in parallel during such acid-base perturbations.

Effects of chloride and extracellular fluid volume on bicarbonate reabsorption along the nephron in metabolic alkalosis in the rat. Reassessment of the classical hypothesis of the pathogenesis of metabolic alkalosis

Volume expansion has been considered essential for the correction of chloride-depletion metabolic alkalosis (CDA). To examine the predictions of this hypothesis, rats dialyzed against 0.15 M NaHCO3 to produce CDA and controls, CON, dialyzed against Ringer-HCO3 were infused with either 6% albumin (VE) or 80 mM non-sodium chloride salts (CC) added to 5% dextrose (DX) and studied by micropuncture. CDA was maintained in rats infused with DX. VE expanded plasma volume (25%), maintained glomerular filtration rate (GFR), but did not correct CDA despite increased fractional delivery of total CO2 (tCO2) out of the proximal tubule (36 +/- 2%) as compared with VE/CON (24 +/- 4%; P less than 0.05). In contrast, CC corrected CDA despite volume contraction (-16%) and lower GFR than CC/CON; proximal tCO2 delivery in CC/CDA (29 +/- 4%) did not differ from VE/CDA. CC was associated with an increment in tCO2 excretion. The data strongly suggest that maintenance and correction of CDA are primarily dependent upon total body chloride and its influences on intrarenal mechanisms and not on the demands of sodium or fluid homeostasis.

Dietary NaCl determines severity of potassium depletion-induced metabolic alkalosis

It is uncertain whether, in humans, potassium depletion can cause or sustain metabolic alkalosis of clinically important degree in the absence of coexisting known alkalosis-producing conditions. Previously we found, in normal humans ingesting abundant NaCl, that dietary K+ depletion alone can induce and sustain a small decrease in blood acidity and increase in plasma bicarbonate concentration; we hypothesized that more severe alkalosis was prevented by mitigating mechanisms initiated by renal retention of dietary NaCl that was induced by K+ depletion. To ascertain the acid-base response to dietary K+ depletion under conditions in which the availability of NaCl for retention is greatly limited, in the present study of six normal men we restricted dietary K+ as in the previous study except that intake of NaCl was maintained low (2 to 7 mEq/day, Low NaCl Group) instead of high (126 mEq/day, High NaCl Group). Plasma acid-base composition and renal net-acid excretion (NAE) did not differ significantly between groups during the control period. In the steady state of K+ depletion (days 11 to 15 of K+ restriction), neither plasma K+ concentration (2.9 +/- 0.9 mEq/liter vs. 3.0 +/- 0.1 mEq/liter) nor cumulative K+ deficit (399 +/- 59 mEq vs. 466 +/- 48 mEq) differed significantly between groups. During K+ restriction, persisting metabolic alkalosis developed in both groups, which was more severe in the Low NaCl Group: increment in [HCO3-]p, 7.5 +/- 1.0 mEq/liter versus 2.0 +/- 0.3 mEq/liter, P less than 0.001; decrement in [H+]p, 5.5 +/- 0.6 nEq/liter versus 2.9 +/- 0.4 nEq/liter, P less than 0.003. A significantly more severe alkalosis in the Low NaCl Group was evident at all degrees of K+ deficiency achieved during the course of the 15 days of K+ restriction, and the severity of alkalosis in the Low NaCl Group correlated with the degree of K+ deficiency. During the generation of alkalosis (days 1 to 7 of K+ restriction), NAE increased in the Low NaCl Group whereas it decreased in the High NaCl Group. During the maintenance of alkalosis (days 11 to 15), NAE stabilized in both groups after it returned to values approximating the control values. In both groups, urine Cl- excretion decreased during K+ restriction even though Cl- intake had not been changed, with the result that body Cl- content increased negligibly in the Low NaCl Group (28 +/- 6 mEq) and substantially in the High NaCl Group (355 +/- 64 mEq).(ABSTRACT TRUNCATED AT 400 WORDS)

Comparison of the natriuresis and chloruresis associated with glomerular hyperfiltration induced by atrial natriuretic factor or glucagon

The impact on renal sodium chloride reabsorption of an acute increase in glomerular filtration rate (GFR) induced by atrial natriuretic factor (ANF) or glucagon was examined in the conscious rat. These hormones have no direct effect on proximal solute transport and have opposite effects on distal transport. ANF and glucagon increased GFR to a comparable extent (2.0 +/- 0.2 to 3.5 +/- 0.4 ml/min, p less than 0.01, and 1.9 +/- 0.1 to 3.3 +/- 0.1 ml/min, p less than 0.001, respectively). While most (95-97%) of the increment in filtered sodium chloride was reabsorbed, a small portion (3-5%) escaped tubular reabsorption. Absolute sodium and chloride urinary excretion rates increased similarly in response to each hormone, by two- to three-fold. Slightly imperfect load-dependent sodium chloride reabsorptive response by the nephron, despite opposite direct effects on distal nephron transport, may account for the observed natriuresis and chloruresis associated with the acute glomerular hyperfiltration induced by ANF or glucagon administration.

Axial heterogeneity of bicarbonate, chloride, and water transport in the rat proximal convoluted tubule. Effects of change in luminal flow rate and of alkalemia

These studies examined regulation of superficial proximal convoluted tubule (PCT) transport as a function of length. When single nephron glomerular filtration rate (SNGFR) increased from 28.7 +/- 0.7 nl/min in hydropenia to 41.5 +/- 0.4 nl/min in euvolemia, bicarbonate, chloride, and water reabsorption in the early (1st mm) PCT increased proportionally: from 354 +/- 21 peq/mm X min, 206 +/- 55 peq/mm X min, and 5.9 +/- 0.4 nl/mm X min to 520 +/- 12 peq/mm X min, 585 +/- 21 peq/mm X min, and 10.1 +/- 0.4 nl/mm X min, respectively. These high transport rates did not increase further, however, when SNGFR went to 51.2 +/- 0.7 or 50.7 +/- 0.6 nl/min after atrial natriuretic factor or glucagon administration. Anion and water transport rates in the late PCT were lower and exhibited less flow dependence. During chronic metabolic alkalosis, acidification was inhibited in the late but not early PCT. In conclusion, the early PCT is distinguished from the late PCT by having high-capacity, flow-responsive but saturable, anion- and water-reabsorptive processes relatively unaffected by alkalemia.

Renal bicarbonate reabsorption in the rat. I. Effects of hypokalemia and carbonic anhydrase

Free-flow micropuncture studies were carried out on superficial rat proximal and distal tubules to assess the participation of different nephron segments in bicarbonate transport. Particular emphasis was placed on the role of the distal tubule, and micro-calorimetric methods used to quantitate bicarbonate reabsorption. Experiments were carried out in control conditions, during dietary potassium withdrawal, and after acute intravenous infusions of carbonic anhydrase. We observed highly significant net bicarbonate reabsorption in normal acid-base conditions as evidenced by the maintenance of significant bicarbonate concentration gradients in the presence of vigorous fluid absorption. Distal bicarbonate reabsorption persisted in hypokalemic alkalosis and even steeper transepithelial concentration gradients of bicarbonate were maintained. Enhancement of net bicarbonate reabsorption followed the acute intravenous administration of carbonic anhydrase but was limited to the nephron segments between the late proximal and early distal tubule. The latter observation is consistent with a disequilibrium pH along the proximal straight tubule (S3 segment), the thick ascending limb of Henle, and/or the early distal tubule.

Axial heterogeneity and filtered-load dependence of proximal bicarbonate reabsorption

A theoretical model was developed to examine the role of physical and chemical factors in the control of bicarbonate reabsorption in the renal proximal tubule. Included in the model were axial and radial variations in the concentrations of HCO3-, CO2 and related chemical species in the tubule lumen and epithelial cells. Relations between these concentrations and the solute fluxes across the brush border and basolateral membranes were also included, as were reaction rate and equilibrium expressions to describe the various buffering processes in the lumen and cells. The two most critical membrane parameters, the rate constant for H+ secretion at the brush border and the effective permeability of HCO3- at the basolateral membrane, were evaluated by comparing model predictions with available free-flow micropuncture data in the rat. It was found that the experimental observations could be explained only by decreasing one or both of these membrane parameters with axial position, suggesting a progressive decrease in HCO3- reabsorptive capacity along the tubule. For single nephron filtered loads of HCO3- up to about 1,400 pmol/min, absolute bicarbonate reabsorption was predicted to increase nearly in proportion to filtered load, whereas it was calculated to be relatively constant at higher filtered loads, irrespective of how filtered load was assumed to be varied. These predictions are in excellent agreement with most of the available micropuncture data in rats, as is the prediction that HCO3- reabsorption should change in parallel with CO2 partial pressure in the filtrate, at a given filtered load of HCO3-. Certain discrepancies between the model predictions and experimental observations are evident at very high filtered loads, and the implications of these are discussed in terms of possible adaptive responses of the tubule.

Load dependence of proximal tubular bicarbonate reabsorption in chronic metabolic alkalosis in the rat

Studies were undertaken in Munich-Wistar rats to determine whether maintenance of chronic metabolic alkalosis (CMA) is associated with an increase in proximal HCO3- reabsorption, or whether a reduction in glomerular filtration rate (GFR) is required to sustain the elevated plasma HCO3- concentration. Superficial single nephron glomerular filtration rate (SNGFR), and absolute proximal HCO-3 (APRHCO3) and water (APRH2O) reabsorption were measured 20 +/- 3 d after the induction of CMA in eight rats and the results compared with seven age-matched control animals. Plasma [HCO3-] was 39.1 +/- 1.8 mM in CMA rats compared with 26.0 +/- 0.4 mM in controls (P less than 0.001). In the CMA rats, SNGFR was 44.8 +/- 1.1 vs. 38.2 +/- 2.1 nl/min in controls (P less than 0.025). As a result, the single nephron filtered load of HCO3- (FLHCO3) increased from 1,147 +/- 61 pmol/min in control to 2,040 +/- 108 pmol/min in CMA (P less than 0.001). APRHCO3 increased by greater than 65%, from 970 +/- 65 pmol/min in control to 1,624 +/- 86 pmol/min in CMA (P less than 0.001). APRH2O increased from 18.4 +/- 1.6 nl/min in control to 24.0 +/- 0.8 nl/min in CMA (P less than 0.005). Tubular hypertrophy resulted in an increase in the length of the proximal convoluted tubule from 5.6 +/- 0.2 to 6.5 +/- 0.2 mm (P less than 0.005). The pattern of HCO3- reabsorption along the length of the proximal convoluted tubule in CMA was indistinguishable from that found in normal rats in which FLHCO3 was varied acutely by altering SNGFR. The increase in tubular length accounted for only 30% of the increase in APRH2O and 15% of the increase in APRHCO3. We conclude that a sustained reduction in GFR is not required for maintenance of CMA in the rat. If GFR is chronically restored to normal levels, the alkalosis is maintained by an increase in APRHCO3. The increase in reabsorption is accounted for by tubular hypertrophy, a chronic adaptive response, and a load-dependent response that is indistinguishable from that seen in normal rats when FLHCO3 is increased acutely by increasing SNGFR.

Effect of acute hypercapnia on renal and proximal tubular total carbon dioxide reabsorption in the acetazolamide-treated rat

The present study evaluates the effect of acute hypercapnia on renal total CO2 (tCO2) reabsorption after inhibition of renal carbonic anhydrase. Simultaneous renal clearance studies and free-flow micropuncture studies of the superficial proximal tubule were performed on plasma-repleted Sprague-Dawley rats treated with acetazolamide, 50 mg/kg body weight. Acute hypercapnia (arterial PCO2, 120 mmHg; blood pH, 7.02) was induced by ventilation with a 10% CO2-90% O2 gas mixture. Control rats (PCO2, 49.5 mmHg, pH 7.34) were ventilated with room air. The renal fractional excretion of tCO2 was approximately 20% lower in the hypercapnic group compared with the rats given acetazolamide alone. Acute hypercapnia reduced the fractional delivery of tCO2 to the late proximal tubule by a comparable amount. The absolute proximal reabsorption of tCO2 was increased by hypercapnia to 410 +/- 47 vs. 170 +/- 74 pmol X min-1, P less than 0.05. The single nephron glomerular filtration rate was 32.6 +/- 0.7 nl X min-1 in the hypercapnic group and 43.8 +/- 1.7 nl X min-1 in the rats given acetazolamide only, P less than 0.01. Acute hypercapnia enhances renal sympathetic nerve activity. To eliminate this effect, additional experiments were performed in which the experimental kidney was denervated before study. Denervation prevented the change in the single nephron filtration rate during acute hypercapnia, but absolute and fractional proximal tCO2 reabsorption remained elevated in comparison to denervated controls. The concentration of H2CO3 in the late proximal tubule, calculated from the measured luminal pH and bicarbonate concentration and the estimated cortical PCO2, was higher in the hypercapnic group, which was a finding compatible with H2CO3 cycling from lumen into proximal tubular cell, which provided a source of hydrogen ions for secretion.

Atrial natriuretic factor ameliorates chronic metabolic alkalosis by increasing glomerular filtration

The kidney maintains the elevated plasma concentration of bicarbonate that occurs in chronic metabolic alkalosis. A reduction in the glomerular filtration rate (GFR) can maintain the filtered bicarbonate load at a normal level so that a normal rate of bicarbonate reabsorption suffices to prevent urinary excretion of this anion. It is also possible that bicarbonate reabsorption might increase so as to maintain the alkalosis if GFR were not reduced. To examine this latter possibility, atrial natriuretic factor was used in alkalotic rats to restore a more normal GFR and to increase the amount of bicarbonate filtered by the glomerulus. Proximal bicarbonate reabsorption remained relatively static. Higher than normal amounts of bicarbonate were then delivered out of the proximal tubule, bicarbonate appeared in the urine, and the plasma concentration of bicarbonate fell. A reduction in GFR is thus necessary for the maintenance of chronic metabolic alkalosis. Normalizing GFR induces bicarbonaturia and initiates repair of the alkalosis.

Pathophysiology of chronic tubulo-interstitial disease in rats. Interactions of dietary acid load, ammonia, and complement component C3

The human end-stage kidney and its experimental analogue, the remnant kidney in the rat, exhibit widespread tubulo-interstitial disease. We investigated whether the pathogenesis of such tubulo-interstitial injury is dependent upon adaptive changes in tubular function and, in particular, in ammonia production when renal mass is reduced. Dietary acid load was reduced in 1 3/4-nephrectomized rats by dietary supplementation with sodium bicarbonate (NaHCO3), while control rats, paired for serum creatinine after 1 3/4 nephrectomy, were supplemented with equimolar sodium chloride. After 4-6 wk, NaHCO3-supplemented rats demonstrated less impairment of tubular function as measured by urinary excretory rates for total protein and low molecular weight protein and higher transport maximum for para-aminohippurate per unit glomerular filtration rate, less histologic evidence of tubulo-interstitial damage, less deposition of complement components C3 and C5b-9, and a lower renal vein total ammonia concentration. Such differences in tubular function could not be accounted for simply on the basis of systemic alkalinization, and differences in tubular injury could not be ascribed to differences in glomerular function. Because nitrogen nucleophiles such as ammonia react with C3 to form a convertase for the alternative complement pathway, and because increased tissue levels of ammonia are associated with increased tubulo-interstitial injury, we propose that augmented intrarenal levels of ammonia are injurious because of activation of the alternative complement pathway. Chemotactic and cytolytic complement components are thereby generated, leading to tubulo-interstitial inflammation. Thus, alkali supplementation reduces chronic tubulo-interstitial disease in the remnant kidney of the rat, and we propose that this results, at least in part, from reduction in cortical ammonia and its interaction with the alternative complement pathway.

Effects of urinary pH on renal interactions between probenecid and cefsulodin in rabbits

The effect of urinary pH on renal interaction of cefsulodin and probenecid was tested in rabbits. Probenecid was reabsorbed in acidic urine (fractional excretion [FE] = 8 +/- 4%) and secreted in alkaline urine (FE = 492 +/- 258%). Renal excretion of cefsulodin alone was not affected by the urinary pH (FE = ca. 100%). In acidic urine, probenecid significantly reduced tubular secretion of cefsulodin (FE = 74 +/- 8%). An inverse pattern was observed in alkaline urine (FE = 122 +/- 18%).

Volume-independent reductions in glomerular filtration rate in acute chloride-depletion alkalosis in the rat. Evidence for mediation by tubuloglomerular feedback

We have recently described reduced superficial nephron glomerular filtration rate (SNGFR) in chloride-depletion alkalosis (CDA) without volume depletion. To elucidate the mechanism of this phenomenon, we studied three degrees of increasing severity of CDA (groups CDA-1, 2, and 3) produced by one or two peritoneal dialyses against 0.15 M NaHCO3 and electrolyte infusions of different Cl and HCO3 content in Sprague-Dawley rats; control rats (CON) were dialyzed against and infused with Ringers-HCO3. Extracellular fluid (ECF) volume was assessed by blood pressure, hematocrit, plasma protein concentration, and 125I-albumin space; none of these variables differed among the four groups. Micropuncture of the latest proximal and earliest distal convolutions was carried out. As CDA intensified from CON to CDA-3 (plasma tCO2 25 +/- 1 to 43 +/- 1 meq/L; P less than 0.01), distally determined SNGFR declined progressively (40.9 +/- 1.7 to 28.3 +/- 1.8 nl/min; P less than 0.01), while in early distal tubule fluid, flow rate (8.6 +/- 0.7 to 3.4 +/- 0.6 nl/min) and Cl concentration (36 +/- 2 to 19 +/- 3 meq/L) decreased and osmolality (110 +/- 5 to 208 +/- 12 mosmol/kg) increased (P less than 0.01), and, in the loop segment, Cl reabsorption decreased progressively (2,009 +/- 112 to 765 +/- 128 peq/min; P less than 0.01). In early distal tubule fluid, Cl concentration correlated positively and osmolality negatively with distally determined SNGFR (P less than 0.05). Proximally determined SNGFRs did not differ among the four groups. Proximal tubule stop-flow pressure responses to increasing rates of orthograde perfusion of the loop segment from 0 to 40 nl/min did not differ between groups CON and CDA-2. We interpret these data to show that reductions in SNGFR in CDA in the rat can occur by tubuloglomerular feedback (TGF) in the absence of differences in ECF volume or of alterations in TGF sensitivity during metabolic alkalosis. Of the proposed signals for TGF sensed by the macula densa, distal tubule fluid osmolality or some related variable is the signal most compatible with our data.

Chronic hypercapnia stimulates proximal bicarbonate reabsorption in the rat

The hyperbicarbonatemia of chronic respiratory acidosis might be maintained by a reduction in filtration rate or an enhancement of tubular bicarbonate reabsorption. To investigate this question, 12 Munich-Wistar rats were exposed to a 10% CO2 atmosphere for 6-8 d. Chronic respiratory acidosis developed, with arterial pH 7.30 +/- 0.01, partial pressure of CO2 (pCO2) 80 +/- 2 mmHg, and total CO2 concentration 45 +/- 1 mM. Single nephron glomerular filtration rate was normal (42 +/- 1 nl/min). Chronic hypercapnia caused absolute proximal reabsorption to be significantly stimulated (1,449 +/- 26 pmol/min) as compared with reabsorption previously observed in normal animals (1,075 +/- 74 pmol/min) or in animals subjected to acute hypercapnia (1,200 +/- 59 pmol/min). This is the first demonstration that proximal bicarbonate reabsorption can be stimulated above normal euvolemic values. When eight animals were subsequently allowed to return toward a normocapnic state (arterial pCO2 46 +/- 1 mmHg) over the course of 1-1.5 h, bicarbonate reabsorption was still significantly higher (1,211 +/- 34 pmol/min) than in similarly alkalotic, normocapnic control groups (994 +/- 45 pmol/min). In conclusion, chronic, but not acute, hypercapnia stimulates absolute proximal bicarbonate reabsorption to exceed the level found in normal euvolemic rats.

Reduced glomerular filtration and enhanced bicarbonate reabsorption maintain metabolic alkalosis in humans

The mechanism that sustains chloride-depletion metabolic alkalosis is presumed to be a stimulation of renal acidification, so that the elevated filtered bicarbonate load that attends hyperbicarbonatemia is completely reabsorbed. However, such enhancement of renal bicarbonate reabsorption is not necessary to maintain hyperbicarbonatemia if the filtered bicarbonate load is not increased owing to a concomitant reduction in glomerular filtration rate (GFR). To assess the relative contributions of enhanced renal bicarbonate reabsorption and reduced GFR in the maintenance of chloride-depletion alkalosis in humans, selective hydrochloric acid depletion was induced in five normal subjects. Plasma bicarbonate concentration increased by 27% (25.3 +/- 0.1 to 32.1 +/- 0.3 mEq/liter, P less than 0.005), whereas the rate of renal bicarbonate reabsorption increased by only 17% (2.7 +/- 0.1 to 3.2 +/- 0.2 mEq/min, P less than 0.05) owing to a 10% reduction in GFR (93.2 +/- 4.4 to 84.3 +/- 4.1 ml/min, P less than 0.01). Thus, in chloride-depletion metabolic alkalosis in humans, the increase in plasma bicarbonate concentration is not attended by a commensurate increase in filtered bicarbonate and rate of renal bicarbonate reabsorption. Both a reduction in GFR and an enhancement of renal bicarbonate reabsorption contribute to maintenance of the alkalotic state.

Proximal tubular bicarbonate reabsorption and PCO2 in chronic metabolic alkalosis in the rat

Studies were undertaken to define the pattern of proximal tubular bicarbonate reabsorption and its relation to tubular and capillary PCO2 in rats with chronic metabolic alkalosis (CMA). CMA was induced by administering furosemide to rats ingesting a low electrolyte diet supplemented with NaHCO3 and KHCO3. Proximal tubular bicarbonate reabsorption and PCO2 were measured in CMA rats either 4-7 or 11-14 d after furosemide injection, in order to study a wide range of filtered bicarbonate loads. A group of nine age-matched control animals, fed the same diet but not given furosemide, was studied for comparison. In a third group of controls, the filtered load of bicarbonate was varied over the same range as in the CMA rats by plasma infusion and aortic constriction. The CMA rats had significant alkalemia and hypokalemia (4-7 d: pH 7.58, HCO3 38.3 meq/liter, K+ 2.1 meq/liter; 11-14 d: pH 7.54, HCO3 38.1 meq/liter, K+ 2.5 meq/liter). Nonetheless, proximal bicarbonate reabsorption was not significantly different from that seen in control rats at any given load of filtered bicarbonate (from 250 to 1,300 pmol/min). In both control and CMA rats, 83-85% of the filtered bicarbonate was reabsorbed by the end of the accessible proximal tubule. These observations indicate that proximal bicarbonate reabsorption is determined primarily by the filtered load in chronic metabolic alkalosis. When single nephron glomerular filtration rate (SNGFR) is reduced by volume depletion in the early postfurosemide period, the filtered load and the rate of proximal bicarbonate reabsorption remain at or below control levels, maintaining metabolic alkalosis. In the late postfurosemide period, however, SNGFR returned to control levels in some instances. In these animals, both the filtered load and rate of proximal reabsorption were increased above the highest levels seen in control animals. The PCO2 gradient between the peritubular capillaries and arterial blood (Pc-Art) was significantly higher in CMA than in control, even though the rate of proximal bicarbonate reabsorption did not differ. Thus, proximal bicarbonate reabsorption did not appear to be the primary determinant of Pc-Art PCO2. PCO2 in the early proximal (EP) tubule was significantly higher than in either the late proximal (LP) tubule or peritubular capillaries in both control and CMA rats. The EP-LP PCO2 gradient correlated directly with proximal bicarbonate reabsorption (P less than 0.05). The small elevation in PCO2 in EP may be related to CO2 generated at this site in the process of bicarbonate reabsorption.