Role of luminal anion and pH in distal tubule potassium secretion.

Potassium secretory flux (J(K)) by the distal nephron is regulated by systemic and luminal factors. In the present investigation, J(K) was measured with a double-barreled K(+) electrode during paired microperfusion of superficial segments of the rat distal nephron. We used control solutions (100 mM NaCl, pH 7.0) and experimental solutions in which Cl(-) had been replaced with a less permeant anion and/or pH had been increased to 8.0. J(K) increased when Cl(-) was replaced by either acetate ( approximately 37%), sulfate ( approximately 32%), or bicarbonate ( approximately 62%), and also when the pH of the control perfusate was increased ( approximately 26%). The majority (80%) of acetate-stimulated J(K) was Ba(2+) sensitive, but furosemide (1 mM) further reduced secretion ( approximately 10% of total), suggesting that K(+)-Cl(-) cotransport was operative. Progressive reduction in luminal Cl(-) concentration from 100 to 20 to 2 mM caused increments in J(K) that were abolished by inhibitors of K(+)-Cl(-) cortransport, i.e., furosemide and [(dihydroindenyl)oxy]alkanoic acid. Increasing the pH of the luminal perfusion fluid also increased J(K) even in the presence of Ba(2+), suggesting that this effect cannot be accounted for only by K(+) channel modulation of K(+) secretion in the distal nephron of the rat. Collectively, these data suggest a role for K(+)-Cl(-) cotransport in distal nephron K(+) secretion.

Cisplatinum-induced lesion of proximal tubule acidification in the rat.

Rats were given a 4- to 6-mg/kg body weight intraperitoneal injection of the antitumor drug, Cisplatin, 5-7 days prior to experiments to study tubule acidification by clearance and stationary microperfusion techniques. Cisplatin reduced the glomerular filtration rate markedly and caused a moderate degree of metabolic acidosis, but urine acidification (pH) was well maintained. Proximal tubule stationary pH and bicarbonate concentrations, as measured by pH microelectrodes, were significantly increased. The defect of proximal H+ secretion is reflected by increased acidification half-times (from 4.44 to 10.2 s) and reduced bicarbonate reabsorption to 37% of control values. H-ion back flux, measured during tubule and capillary perfusions with Ringer's bicarbonate- and CO2-free phosphate solutions, was reduced to 68% of control values. The apparent H-ion permeability was lowered from 0.79 to 0.54 cm/s. These results indicate that proximal acidification is reduced by impairment of H+ transport and not by increased transepithelial H+ shunting. Blunted acidification is compatible with a reduction in the number of Na/H exchangers in the proximal brush border and/or a decrease in the apical sodium gradient, the driving force for proximal H-ion secretion. Cortical distal tubule acidification, measured by double-barreled ion-exchange resin/PD microelectrodes, was not significantly affected by Cisplatin. This accounts for the observation that, in spite of the impaired proximal acidification, urine pH is kept within the normal range.

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.

Filtered load of buffer and renal H-ion secretion: mechanism of proximal tubule load dependence.

When the filtered load of buffers like bicarbonate or phosphate is increased by elevating GFR or buffer concentration in plasma, the overall renal reabsorption of bicarbonate or the formation of titratable acidity are markedly increased. The same happens when buffer concentration or flow rate are varied during proximal microperfusion. We have recently studied the mechanisms of this functional dependence. We have observed that the rate of bicarbonate reabsorption is always proportional to luminal buffer concentration when a stationary fluid column is injected into the proximal lumen. H-ion secretion is also proportional to luminal levels of non-bicarbonate buffers. Using a pH-stat technique adapted to renal tubules, we have shown that H-ion secretion is dependent on proximal pH independently of the used buffer species. A kinetic analysis of these data shows a non-linear relationship between luminal H+ and H+ secretion, compatible with carrier mediated transport.

Filtered load of buffer and renal H-ion secretion: mechanism of proximal tubule load dependence.

When the filtered load of buffers like bicarbonate or phosphate is increased by elevating GFR or buffer concentration in plasma, the overall renal reabsorption of bicarbonate or the formation of titratable acidity are markedly increased. The same happens when buffer concentration or flow rate are varied during proximal microperfusion. We have recently studied the mechanisms of this functional dependence. We have observed that the rate of bicarbonate reabsorption is always proportional to luminal buffer concentration when a stationary fluid column is injected into the proximal lumen. H-ion secretion is also proportional to luminal levels of non-bicarbonate buffers. Using a pH-stat technique adapted to renal tubules, we have shown that H-ion secretion is dependent on proximal pH independently of the used buffer species. A kinetic analysis of these data shows a non-linear relationship between luminal H+ and H+ secretion, compatible with carrier mediated transport.