Evidence for a bicarbonate leak in the proximal tubule of the rat kidney

The effects of respiratory alkalosis and acidosis on net bicarbonate flux along the rat loop of Henle in vivo

We have studied the effects of acute respiratory alkalosis (ARALK, hyperventilation) and acidosis (ARA, 8% CO2), chronic respiratory acidosis (CRA; 10% CO2 for 7-10 days), and subsequent recovery from CRA breathing air on loop of Henle (LOH) net bicarbonate flux (JHCO3) by in vivo tubule microperfusion in anesthetized rats. In ARALK blood, pH increased to 7.6, and blood bicarbonate concentration ([HCO3-]) decreased from 29 to 22 mM. Fractional urinary bicarbonate excretion (FEHCO3) increased threefold, but LOH JHCO3 was unchanged. In ARA, blood pH fell to 7.2, and blood [HCO3-] rose from 28 to 34 mM; FEHCO3 was reduced to < 0.1%, but LOH JHCO3 was unaltered. In CRA, blood pH fell to 7.2, and blood [HCO3-] increased to > 50 mM, whereas FEHCO3 decreased to < 0.1%. JHCO3 was reduced by approximately 30%. Bicarbonaturia occurred when CRA rats breathed air, yet LOH JHCO3 increased (by 30%) to normal. These results suggest that LOH JHCO3 is affected by the blood-to-tubule lumen [HCO3-] gradient and HCO3- backflux. When the usual perfusing solution at 20 nl/min was made HCO3- free, mean JHCO3 was -34.5 +/- 4.4 pmol/min compared with 210 +/- 28.1 pmol/min plus HCO3-. When a low-NaCl perfusate (to minimize net fluid absorption) containing mannitol and acetazolamide (2 x 10(-4) M, to abolish H(+)-dependent JHCO3) was used, JHCO3 was -112.8 +/- 5.6 pmol/min. Comparable values for JHCO3 at 10 nl/min were -35.9 +/- 5.8 and -72.5 +/- 8.8 pmol/min, respectively. These data indicate significant backflux of HCO3-along the LOH, which depends on the blood-to-lumen [HCO3-] gradient; in addition to any underlying changes in active acid-base transport mechanisms, HCO3- permeability and backflux are important determinants of LOH JHCO3 in vivo.

The effect of acute metabolic alkalosis on bicarbonate transport along the loop of Henle. The role of active transport processes and passive paracellular backflux

The loop of Henle (LOH) reabsorbs approximately 15% of filtered HCO3- via a luminal Na(+)-H+ exchanger and H+ATPase. During acute metabolic alkalosis (AMA) induced by i.v. HCO3- infusion, we have observed previously inhibition of LOH net HCO3- reabsorption (JHCO3-), which contributes to urinary elimination of the HCO3- load and correction of the systemic alkalosis. To determine whether the activities of the Na(+)-H+ exchanger and/or H(+)-ATPase are reduced during AMA, two inhibitors believed to be sufficiently specific for each transporter were delivered by in vivo LOH microperfusion during AMA. AMA reduced LOH JHCO3- from 205.0 +/- 10.8 to 96.2 +/- 11.8 pmol.min-1 (P < 0.001). Luminal perfusion with bafilomycin A1 (10(-4) mol.l-1) caused a further reduction in JHCO3- by 83% and ethylisopropylamiloride (EIPA; 5.10(-4) mol.l-1) completely abolished net HCO3- reabsorption. The combination of bafilomycin A1 and EIPA in the luminal perfusate was additive, resulting in net HCO3- secretion (-66.6 +/- 20.8 pmol.min-1; P < 0.001) and abolished net fluid reabsorption (from 5.0 +/- 0.6 during AMA to 0.2 +/- 1.1 nl.min-1; P < 0.001). To establish whether HCO3- secretion via luminal stilbene-sensitive transport mechanism participates in LOH adaptation to AMA, we added diisothiocyanato-2,2'-stilbenedisulphonate (DIDS; 10(-4) mol.l-1) to the perfusate. No effect was found.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of medium tonicity on transepithelial H(+)-HCO3-fluxes in rat proximal tubule

1. The effect of luminal and capillary perfusion with hypotonic or hypertonic solutions containing 25 mM NaHCO3 or NaH2PO4 plus NaCl, K+, Ca2+, Mg2+ and acetate at an osmolality of 100 or 500 mosmol kg-1 on rat proximal H+ secretion was estimated by monitoring luminal pH with Sb microelectrodes. The results were compared to perfusions with the same ionic concentration in which tonicity was adjusted to 300 mosmol kg-1 with raffinose. 2. The kinetics of acidification of luminally injected bicarbonate buffer permits evaluations of H(+)-HCO3-fluxes as well as stationary pH gradients; the kinetics of alkalinization of luminally injected acid phosphate buffer indicates H(+)-HCO3-backfluxes from blood to lumen. 3. In alkalinization experiments, luminal perfusion with hypotonic solution during presence of blood in capillaries or hypotonic capillary perfusion leads to a decrease of stationary pH, an increase of alkalinization half-time and consequently a decrease of passive H(+)-HCO3-backflux. 4. In alkalinization experiments, during luminal and/or capillary perfusions with hypertonic solutions, no significant differences in the stationary pH, alkalinization half-time and H(+)-HCO3-backflux were found. 5. During acidification experiments, with both hypo- and hypertonic perfusions, no significant differences in stationary pH, acidification half-time and H(+)-HCO3-flux were observed. 6. Luminal perfusion with hypotonic solution increases specific epithelial resistance in the presence of blood in capillaries. Luminal perfusion with hypertonic solution does not change this parameter. 7. Volume changes, measured by the split-drop method, are slow during the first 30 s and do not explain the increased alkalinization half-time during luminal perfusion with hypotonic solution, since this is the period of fastest pH change. 8. Luminal perfusion with hypotonic solution decreases apparent H+ permeability in the presence of blood or hypotonic solution in capillaries. Hypertonic solutions in all experimental conditions had no significant effect on this parameter. 9. The data indicate that decrease of tonicity of fluids in contact with proximal tubule epithelium affects passive H(+)-HCO3-backflux, which proceeds in part through the shunt path, while acidification (H+ secretion), which is transcellular, is not affected by extracellular tonicity.

Bicarbonate transport along the loop of Henle. I. Microperfusion studies of load and inhibitor sensitivity

We microperfused the loop of Henle (LOH) to assess its contribution to urine acidification in vivo. Under control conditions (Na HCO3- = 13 mM, perfusion rate approximately 17 nl/min-1) net bicarbonate transport (JHCO3-) was unsaturated, flow- and concentration-dependent, and increased linearly until a bicarbonate load of 1,400 pmol.min-1 was reached. Methazolamide (2 x 10(-4) M) reduced JHCO3 by 70%; the amiloride analogue ethylisopropylamiloride (EIPA) (2 x 10(-4) M) reduced JHCO3 by 40%; neither methazolamide nor EIPA affected net water flux (Jv). The H(+)-ATPase inhibitor bafilomycin A1 (10(-5) M) reduced JHCO3 by 20%; the Cl- channel inhibitor 5-nitro-2'-(3-phenylpropylamino)-benzoate (2 x 10(-4) M) and the Cl(-)-base exchange inhibitor diisothiocyanato-2,2'-stilbenedisulfonate (5 x 10(-5) M), had no effect on fractional bicarbonate reabsorption. Bumetanide (10(-6) M) stimulated bicarbonate transport (net and fractional JHCO3-) by 20%, whereas furosemide (10(-4) M) had no effect on bicarbonate reabsorption; both diuretics reduced Jv. In summary: (a) the LOH contributes significantly to urine acidification. It normally reabsorbs an amount equivalent to 15% of filtered bicarbonate; (b) bicarbonate reabsorption is not saturated; (c) Na(+)-H+ exchange and an ATP-dependent proton pump are largely responsible for the bulk of LOH bicarbonate transport.

pH-stat experiments in proximal renal tubules

The pH-stat technique has been used to measure H+ fluxes in gastric mucosa and urinary bladder "in vitro" while keeping mucosal pH constant. We now report application of this method in renal tubules. We perfused proximal tubules with double-barreled micropipettes, blocked luminal fluid columns with oil and used a double-barreled Sb/reference microelectrode to measure pH, and Sb or 1 N HC1-filled microelectrodes to inject OH- or H+ ions into the tubule lumen. By varying current injection, pH was kept constant at adjustable levels by an electronic clamping circuit. We could thus obtain ratios of current (nA) to pH change (apparent H(+)-ion conductance). These ratios were reduced after luminal 10(-4) M acetazolamide, during injection of OH-, but they increased during injection of H+. The point-like injection source causes pH to fall off with distance from the injecting electrode tip even in oil-blocked segments. Therefore, a method analogous to cable analysis was used to obtain H+ fluxes per cm2 epithelium. The relation between JH+ and pH gradient showed saturation kinetics of H fluxes, both during OH- and H+ injection. This kinetic behavior is compatible with inhibition of JH by luminal H+. It is also compatible with dependence on Na+ and H+ gradients of a saturable Na/H exchanger. H(+)-ion back-flux into the tubule lumen also showed saturation kinetics. This suggests that H+ flow is mediated by a membrane component, most likely the Na(+)-H+ exchanger.

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.

Relationship between energy requirements for Na+ reabsorption and other renal functions

In the mammalian kidney, the use of the ratio, delta net T-Na+/delta Q-O2, provides an overestimate of the energy requirements for unidirectional active Na+ transport. In the proximal tubule, the overestimate of the energy cost for T-Na+ is due to these phenomena: (1) The "leaky" characteristics of the proximal tubule does not permit an accurate estimate to be made of the active fraction of the unidirectional flux of Na+. Thus, the net Na+ or net HCO3- reabsorption rate alone cannot be used to determine the stoichiometry for unidirectional extrusion of Na+ (with HCO3-) by the Na,K-ATPase, since backflux of HCO3- into the lumen occurs. (2) There is a moiety of active Na+ with Cl- along the pars recta. Whether this reabsorptive rate is altered and O2 uptake also changed when GFR or NaHCO3 reabsorption is varied is not yet known. (3) The occurrence of energy-requiring synthetic functions (substrate-interconversions) in the proximal tubule, coupled, in part, to the rate of Na+ entry into the proximal tubule cells, results in changes in renal O2 uptake proportional to some (undetermined) fraction of the change in Na+ reabsorption. The utilization of a portion of these reabsorbed substrates in endergonic syntheses must account for a portion of the Na+-stimulated suprabasal O2 uptake rate. Hence, the presence of synthetic functions in the proximal tubule also contributes to the overestimation of the energy value of net Na+ reabsorption when the ratio, delta net TNa-+/delta Q-O2, is used.(ABSTRACT TRUNCATED AT 250 WORDS)

Dinitrophenol effect on proximal tubular acidification in the rat

The effect of 10(-3) M-dinitrophenol (DNP) on renal tubular acidification was studied in proximal tubules of rat kidneys perfused with mammalian Ringer solution. Alkaline (pH 7.8) or acid (pH 5.8) phosphate-buffered solutions were injected into the lumen, and pH changes recorded with antimony micro-electrodes. Luminal perfusion with DNP caused complex acidification or alkalinization curves, an initial rapid shift toward a higher than control pH being followed by a slower acidification. Acidification half-times of the initial phase (t1/2 = 1.6 s) were markedly shorter than controls (6.2 s). This response was probably due to transient action of DNP, since keeping constant peritubular DNP levels by capillary perfusion caused simple exponential pH curves. In such experiments luminal pH increased from pH 6.6-6.8 to 7.1-7.2, while acidification and alkalinization t1/2 decreased from about 7 s to 3-5 s. Secretory H-ion fluxes increased transiently and then fell below controls after a few minutes of perfusion, while H-ion efflux from the lumen increased progressively. These data suggest that, besides its known effect on cell metabolism, DNP acts directly on proximal tubular cell membranes, increasing the rate of passive H-ion equilibration, both mechanisms impairing the tubular capacity to maintain normal proximal pH gradients and fluxes.

Proximal tubular HCO3-, H+ and fluid transport during maleate-induced acidification defect

The mechanism of tubular acidification was studied in proximal tubular acidification defect induced in rats by acute parenteral infusion of maleate (200 mg/kg), which causes diuresis and bicarbonaturia. Proximal tubular bicarbonate reabsorption and H+ ion secretion were determined by stopped-flow microperfusion and measurement of luminal pH by Sb microelectrodes. Stationary pH increased in proximal tubule from 6.78 to 7.25 and bicarbonate reabsorption decreased from 1.32 to 0.51 nmol/cm2 X s. In these segments, mean cell PD fell from -66.6 to -20.2 mV, while Jv as estimated by the Gertz technique fell to 15% of controls. A similar impairment of acidification was observed during luminal and capillary perfusion with phosphate Ringer's. Since H+-ion efflux from the lumen was not significantly increased and both acidification and alkalinization half-times (t/2) were increased, no evidence for an increase in passive permeability for H+/HCO3- was obtained. The increased t/2 found during luminal perfusion with acid phosphate indicates, according to an electrical analog model, a reduction in pump series conductance. These results show that maleate affects both proximal Na+ and H+ transport; this effect may be ascribed to impairment of sodium-dependent transport systems in the brush-border membrane.

Factors affecting proximal tubular acidification of non-bicarbonate buffer in the rat

The effect of peritubular PCO2 and pH changes within the physiological range on proximal tubular acidification of non-bicarbonate (phosphate) buffer was evaluated with and without carbonic anhydrase inhibition by stopped-flow microperfusion and Sb micro-electrode techniques. Luminal steady-state pH was reduced from 6.69 to 6.37 and H ion fluxes (JH+) increased from 0.63 to 1.57 nmol cm-2 s-1 by increasing capillary CO2 from 0 to 9.6% at pH 7.2. After acetazolamide a marked, although attenuated, effect of CO2 on acidification was still observed; JH+ increased from 0.088 nmol cm-2 s-1 at 0% CO2 to 0.78 at 9.6% CO2. Most of this effect can be explained by titration of luminal buffer by CO2, uncatalysed CO2 hydration and H2CO3 recirculation. An increase in capillary CO2 reduced acidification half-times (t/2), which, according to an analogue circuit model, may be due to increased H ion access to the pump. Peritubular pH changes at 0% CO2 also modified tubular acidification, increasing JH+ from 0.73 nmol cm-2 s-1 at pH 7.6 to 0.99 at pH 7.0. After acetazolamide, JH+ still increased from 0.11 nmol cm-2 s-1 at pH 7.6 to 0.57 at pH 7.0. In conclusion, both peritubular CO2 changes at constant pH and pH changes at 0% CO2 were effective to modify JH+, in the presence and absence of carbonic anhydrase activity. In the studied range, capillary CO2 induced larger changes in JH+ than pH. The data show substrate (H ion) is a limiting factor for tubular H ion secretion.

Carbonic anhydrase independent bicarbonate reabsorption

The present study was designed to define the prerequisites of carbonic anhydrase independent bicarbonate reabsorption. In free flow experiments during systemic application of carbonic anhydrase inhibitor benzolamide (50 mg/kg B. W.) bicarbonate recovery in % of filtered load was found to be 74 +/- 8% in late proximal convoluted tubules, 39 +/- 6% in distal convoluted tubules and 32 +/- 4% in urine, indicating that most of carbonic anhydrase independent bicarbonate reabsorption occurs in tubule segments prior to distal convoluted tubules. In vivo continuous microperfusion experiments in proximal convoluted tubules demonstrated that luminal benzolamide (0.5 mmol/l) virtually abolishes net bicarbonate fluxes, when bicarbonate concentration in the luminal perfusate (25 mmol/l) is close to peritubular plasma concentration (24.4 mmol/l). In contrast, a significant downhill reabsorptive flux occurs, when perfusate bicarbonate concentration is 75 mmol/l and a significant downhill secretory flux is observed, when the perfusate is initially free of bicarbonate. The corresponding apparent permeabilities are 1.0 +/- 0.1 X 10(-6) cm2/s for influx and 1.6 +/- 0.4 X 10(-6) cm2/s for efflux of bicarbonate. Clearance studies reveal that carbonic anhydrase dependent and independent bicarbonate reabsorption are not saturable but depend on the rate of volume reabsorption in the kidney. In conclusion, passive movements of bicarbonate do occur in proximal convoluted tubules and most likely contribute to carbonic anhydrase independent bicarbonate reabsorption.

Effect of luminal and peritubular HCO3(-) concentrations and PCO2 on HCO3(-) reabsorption in rabbit proximal convoluted tubules perfused in vitro

The effect of luminal and peritubular HCO3(-) concentrations and PCO2 on HCO3(-) reabsorption was examined in rabbit proximal convoluted tubules perfused in vitro. Increasing luminal HCO3(-) concentration from 25 to 40 mM without changing either peritubular HCO3(-) concentration or PCO2, stimulated HCO3(-) reabsorption by 41%. When luminal HCO3(-) concentration was constant at 40 mM and peritubular HCO3(-) concentration was increased from 25 to 40 mM without changing peritubular PCO2, a 45% reduction in HCO3(-) reabsorption was observed. This inhibitory effect of increasing peritubular HCO3(-) concentration was reversed when peritubular pH was normalized by increasing PCO2. Passive permeability for HCO3(-) was also measured and found to be 1.09 +/- 0.17 X 10(-7) cm2 s-1. Using this value, the passive flux of HCO3(-) could be calculated. Only a small portion (less than 23%) of the observed changes in net HCO3(-) reabsorption can be explained by the passive HCO3(-) flux. We conclude that luminal and peritubular HCO3(-) concentrations after HCO3(-) reabsorption by changing the active H+ secretion rate. Analysis of these data suggest that both luminal and peritubular pH are major determinants of HCO3(-) reabsorption.

H+ ion secretion in proximal tubule of low-Co2/HCO-3 perfused isolated rat kidney

Acidification in proximal tubule of the isolated rat kidney, perfused in vitro, was studied by stopped-flow microperfusion techniques, using Sb microelectrodes to measure luminal pH. The kidney was perfused with mammalian Ringer's solution at pH 7.4 buffered by 20 mmol/l phosphate and containing 7.5 g/100 ml bovine albumin, equilibrated with air. Final urine pH was 6.88 +/- 0.5. Steady-state pH in proximal segments was 6.81 +/- 0.03 (n = 80), and acidification half-time (t/2) 7.25 +/- 0.33 (80) s, giving a net secretory H+ ion flux of 0.51 +/- 0.05 nmol . cm-2 . s-1. This flux was about 70% of "in vivo" (blood perfused kidneys). During luminal perfusion with solutions at pH 6.2, back-flux of H+ was 0.82 +/- 0.08 nmol . cm-2 . s-1, with an alkalinization t/2 of 6.33 +/- 0.34 (34) s. The difference between acidification and alkalization t/2 was not significant. This is compatible with a pump-leak system of H+ transport. This is compatible with a pump-leak system of H+ transport. The back flux of H from the lumen was markedly reduced in low Na+ perfused kidneys in the presence of 10(-4) mol/l amiloride in the lumen, indicating that this process is mediated by the luminal Na/H exchanger. Observations in the presence of high K levels suggest that it may have also a charged component. 10(-4) mol/l acetazolamide added to the kidney perfusate reduced acidification to 0.5% of control, and 10(-6) mol/l SITS to 25% of control. Thus, despite the low pCO2 (0.1-0.4 kPa, or 1-3 mm Hg), the CO2/HCO-3 buffer system still plays an important role in tubular acidification in this preparation.

Effect of temperature on proximal tubular acidification

The effect of temperature on proximal tubular acidification was studied in isolated rat kidney, perfused with 20 mM phosphate Ringer's containing 7.5 g/100 ml bovine albumin, equilibrated with air. Tubular pH was measured with Sb microelectrodes during stopped-flow microperfusion. The temperature of the kidney was varied between 10 and 46 degrees C. At 10 degrees C the proximal tubule was still able to maintain pH gradients of about 0.7 pH units. However, half-times (t/2) of both acidification and alkalinization were markedly increased, from 6-7 s at 37 degrees C to 27-30 s at 10 degrees C. In consequence, net H+-ion flux into the tubule was reduced to 26% of that at 37 degrees C. In this system, in the absence of exogenous HCO-3 and CO2, t/2 of acidification and alkalinization were very similar at 37 degrees C and below. Above 37 degrees C alkalinization t/2 fell markedly to 1.43 +/- 0.09 (11) s at 46 degrees C, while acidification t/2 stayed at about 7 s. H+-ion back-fluxes increased progressively from 10-46 degrees C, while secretory JH reached a maximal value at 37 degrees C and fell at higher temperatures. Apparent activation energies calculated from rate coefficients were 8.48 kcal . mol-1 for acidification, and 9.30 for alkalinization, and those calculated from JH were 6.30 and 9.55 respectively. These data indicate that both H-ion secretion and back-flux are carrier-mediated, probably flowing through the Na/H exchanger in the luminal membrane, since their activation energies are of the same order of magnitude and markedly higher than those for protons in solution.