Distal tubule bicarbonate accumulation in vivo. Effect of flow and transtubular bicarbonate gradients

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

Metabolic Alkalosis, Bedside and Bench

Although significant contributions to the understanding of metabolic alkalosis have been made recently, much of our knowledge rests on data from clearance studies performed in humans and animals many years ago. This article reviews the contributions of these studies, as well as more recent work relating to the control of renal acid-base transport by mineralocorticoid hormones, angiotensin, endothelin, nitric oxide, and potassium balance. Finally, clinical aspects of metabolic alkalosis are considered.

A numerical model of acid-base transport in rat distal tubule

The purpose of this study is to develop a numerical model that simulates acid-base transport in rat distal tubule. We have previously reported a model that deals with transport of Na(+), K(+), Cl(-), and water in this nephron segment (Chang H and Fujita T. Am J Physiol Renal Physiol 276: F931-F951, 1999). In this study, we extend our previous model by incorporating buffer systems, new cell types, and new transport mechanisms. Specifically, the model incorporates bicarbonate, ammonium, and phosphate buffer systems; has cell types corresponding to intercalated cells; and includes the Na/H exchanger, H-ATPase, and anion exchanger. Incorporation of buffer systems has required the following modifications of model equations: new model equations are introduced to represent chemical equilibria of buffer partners [e.g., pH = pK(a) + log(10) (NH(3)/NH(4))], and the formulation of mass conservation is extended to take into account interconversion of buffer partners. Furthermore, finite rates of H(2)CO(3)-CO(2) interconversion (i.e., H(2)CO(3) &rlharr; CO(2) + H(2)O) are taken into account in modeling the bicarbonate buffer system. Owing to this treatment, the model can simulate the development of disequilibrium pH in the distal tubular fluid. For each new transporter, a state diagram has been constructed to simulate its transport kinetics. With appropriate assignment of maximal transport rates for individual transporters, the model predictions are in agreement with free-flow micropuncture experiments in terms of HCO reabsorption rate in the normal state as well as under the high bicarbonate load. Although the model cannot simulate all of the microperfusion experiments, especially those that showed a flow-dependent increase in HCO reabsorption, the model is consistent with those microperfusion experiments that showed HCO reabsorption rates similar to those in the free-flow micropuncture experiments. We conclude that it is possible to develop a numerical model of the rat distal tubule that simulates acid-base transport, as well as basic solute and water transport, on the basis of tubular geometry, physical principles, and transporter kinetics. Such a model would provide a useful means of integrating detailed kinetic properties of transporters and predicting macroscopic transport characteristics of this nephron segment under physiological and pathophysiological settings.

Endogenous endothelins mediate increased distal tubule acidification induced by dietary acid in rats

We examined if endogenous endothelins mediate the decreased HCO3 secretion and increased H+ secretion in in vivo-perfused distal tubules of rats fed dietary acid as (NH4)2SO4. Animals given (NH4)2SO4 drinking solution had higher endothelin-1 addition to renal interstitial fluid than those given distilled H2O (480+/-51 vs. 293+/-32 fmol g kidney wt(-1) min(-1), respectively, P < 0.03). (NH4)2SO4-ingesting animals infused with bosentan (10 mg/kg) to inhibit A- and B-type endothelin receptors had higher HCO3 secretion than baseline (NH4)2SO4 animals (-4.7+/-0.4 vs. -2.4+/-0.3 pmol mm(-1) min(-1), P < 0.01), but (NH4)2SO4 animals given a specific inhibitor of A-type endothelin receptors (BQ-123) did not (-2.0+/-0.2 pmol mm(-1) min(-1), P = NS vs. baseline). H+ secretion was lower in bosentan-infused compared with baseline (NH4)2SO4 animals (27.7+/-2.5 vs. 43.9+/-4.0 pmol mm(-1) min(-1), P < 0.03), but that for BQ-123-infused (NH4)2SO4 animals was not (42.9+/-4.2 pmol mm(-1) min(-1), P = NS vs. baseline). Bosentan had no effect on distal tubule HCO3 or H+ secretion in control animals. The data show that dietary acid increases endothelin-1 addition to renal interstitial fluid and that inhibition of B- but not A-type endothelin receptors blunts the decreased HCO3 secretion and increased H+ secretion in the distal tubule of animals given dietary acid. The data are consistent with endogenous endothelins as mediators of increased distal tubule acidification induced by dietary acid.

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.

Secretion of HCO3-/OH- in cortical distal tubule of the rat

Secretion of bicarbonate has been described for distal nephron epithelium and attributed to apical Cl-/HCO3- exchange in beta-intercalated cells. We investigated the presence of this mechanism in cortical distal tubules by perfusing these segments with acid (pH 6) 10 mM phosphate Ringer. The kinetics of luminal alkalinization was studied in stationary microperfusion experiments by double-barreled pH (ion-exchange resin)/1 M KCl reference microelectrodes. Luminal alkalinization may be due to influx (into the lumen) of HCO3- or OH-, or efflux of H+. The magnitude of the Cl-/HCO3- exchange component was measured by perfusing the lumen with solutions with or without chloride, which was substituted by gluconate. This component was not different from zero in control and alkalotic (chronic plus acute) Wistar rats. Homozygous Brattleboro rats (BRB), genetically devoid of antidiuretic hormone, were used since this hormone has been shown to stimulate H+ secretion, which could mask bicarbonate secretion. In these rats, no evidence for Cl-/HCO3- exchange was found in control BRB and in early distal segments of alkalotic animals, but in late distal tubule a significant component of 0.14 +/- 0.033 nmol/cm2.sec was observed, which, however, is small when compared to the reabsorptive flow found in control Wistar rats, of 0.95 +/- 0.10 nmol/cm2.sec. In addition, 5 x 10(-4) M SITS had no effect on distal bicarbonate reabsorption in controls as well as on secretion in alkalotic Wistar and Brattleboro rats, which is compatible with the absence of effect of this drug on the apical Cl-/HCO3- exchange in other tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal bicarbonate reabsorption in the rat. IV. Bicarbonate transport mechanisms in the early and late distal tubule

Bicarbonate transport was studied in vivo by separate microperfusion experiments of early and late distal tubules. Total CO2 was measured by microcalorimetry and fluid absorption by 3H-inulin. Significant bicarbonate absorption was observed in all experimental conditions. Bicarbonate transport was load-dependent upon increasing the luminal bicarbonate concentration from 15 to 50 mM in both early and late distal tubule segments and remained constant at higher concentrations at a maximum rate of 100-110 pmol/min per mm. At low lumen bicarbonate concentrations (15 mM), higher rates of bicarbonate absorption were observed in early (32.9 +/- 4.57 pmol/min per mm) as compared to late distal tubules (10.7 +/- 3.1 pmol/min per mm). Amiloride and ethyl-isopropylamiloride both inhibited early but not late distal tubule bicarbonate absorption whereas acetazolamide blocked bicarbonate transport in both tubule segments. Fluid absorption was significantly reduced in both tubule segments by amiloride but only in early distal tubules by ethyl-isopropylamiloride. Substitution of lumen chloride by gluconate increased bicarbonate absorption in late but not in early distal tubules. Bafilomycin A1, an inhibitor of H-ATPase, inhibited late and also early distal tubule bicarbonate absorption, the latter at higher concentration. After 8 d on a low K diet, bicarbonate absorption increased significantly in both early and late distal tubules. Schering compound 28080, a potent H-K ATPase inhibitor, completely blocked this increment of bicarbonate absorption in late but not in early distal tubule. The data suggest bicarbonate absorption via Na(+)-H+ exchange and H-ATPase in early, but only by amiloride-insensitive H+ secretion (H-ATPase) in late distal tubules. The study also provides evidence for activation of K(+)-H+ exchange in late distal tubules of K depleted rats. Indirect evidence implies a component of chloride-dependent bicarbonate secretion in late distal tubules and suggests that net bicarbonate transport at this site results from bidirectional bicarbonate movement.

Luminal chloride modulates rat distal tubule bidirectional bicarbonate flux in vivo

The effects of replacing luminal chloride with gluconate on distal tubule bicarbonate transport were studied in vivo in normally fed rats, overnight-fasted rats, and rats made mildly alkalotic by administration of desoxycorticosterone acetate (DOCA). In paired microperfusions of the same tubule with 0 or 55 mM Cl at 25 nl/min, net secretion of bicarbonate by distal tubules of fed rats was inhibited by chloride replacement. Zero chloride perfusion in DOCA rats also resulted in an inhibition of net bicarbonate secretion at 25 nl/min. In contrast, replacement of 45 mM chloride also perfused at 25 nl/min in fasted rats caused an increase in net bicarbonate reabsorption. To further characterize the effects of changes in luminal chloride, experiments were undertaken in fasted rats with 0, 45, and 100 mM chloride-containing solutions perfused at 8 and 25 nl/min. Perfusion with zero Cl resulted in net bicarbonate reabsorption at 8 nl/min that increased markedly with high flow, whereas bicarbonate reabsorption did not change significantly during perfusion at high flow with a 45-mM Cl perfusate. In marked contrast, perfusion with a 100-mM Cl solution resulted in only minimal bicarbonate reabsorption at 8 nl/min with significant secretion observed at high flow. Thus, chloride-free perfusates inhibit bicarbonate secretion and enhance bicarbonate reabsorption, while high chloride perfusates elicit net bicarbonate secretion in usually reabsorbing distal tubules.

Renal bicarbonate reabsorption in the rat. III. Distal tubule perfusion study of load dependence and bicarbonate permeability

Using continuous microperfusion techniques, we studied the load dependence of bicarbonate reabsorption along cortical distal tubules of the rat kidney and their bicarbonate permeability. Net bicarbonate transport was evaluated from changes in tracer inulin concentrations and total CO2 measurements by microcalorimetry. Bicarbonate permeability was estimated from the flux of total CO2 along known electrochemical gradients into bicarbonate-and chloride-free perfusion solution containing 10(-4) M acetazolamide. Transepithelial potential differences were measured with conventional glass microelectrodes. Significant net bicarbonate reabsorption occurred at luminal bicarbonate levels from 5 to 25 mM, and at perfusion rates from 5 to 30 nl/min. Bicarbonate reabsorption increased in a load-dependent manner, both during increments in luminal bicarbonate concentration or perfusion rate, reaching saturation at a load of 250 pmol/min with a maximal reabsorption rate of approximately 75 pmol/min.mm. Rate of bicarbonate reabsorption was flow dependent at luminal concentrations of 10 but not at 25 mM. During chronic metabolic alkalosis, maximal rates of reabsorption were significantly reduced to 33 pmol/min.mm. The bicarbonate permeability was 2.32 +/- 0.13 x 10(-5) cm/s in control rats, and 2.65 +/- 0.26 x 10(-5) cm/s in volume-expanded rats. Our data indicate that at physiological bicarbonate concentrations in the distal tubule passive bicarbonate fluxes account for only 16-21% of net fluxes. At high luminal bicarbonate concentrations, passive bicarbonate reabsorption contributes moderately to net reabsorption of this anion.

Secretion of bicarbonate by rat distal tubules in vivo. Modulation by overnight fasting

We have performed microperfusion studies on distal tubule bicarbonate reabsorption (JtCO2) of fed and fasted rats to extend our previous observations of in vivo bicarbonate secretion and to resolve certain discrepancies between free-flow and microperfusion data. When rats are fasted overnight, as in previous free-flow studies, distal tubule microperfusion with a 28-mM tCO2 solution results in significant JtCO2 (53 +/- 6 pmol.min-1.mm-1) at normal flow and increases briskly (91 +/- 16 pmol.min-1.mm-1) with bicarbonate load. This response is not influenced by the addition of other normal tubular fluid constituents. However, when normally fed rats are used, as in our previous microperfusion studies, distal tubule JtCO2 is not different from zero when a 28-mM tCO2 solution is perfused at normal flow rates but becomes negative (-54 +/- 13 pmol.min-1.mm-1) at high flow rates, which indicates the existence of bicarbonate secretion against a concentration gradient. Alkali loading of fasted rats also elicits bicarbonate secretion at high flow. These results demonstrate for the first time that normal feeding or alkali loading can induce bicarbonate secretion in a mammalian nephron segment in vivo, and resolves previous discrepancies between free-flow and microperfusion data.