Relationship between cytosolic activities of calcium and pH in forg proximal tubules.

Abstract

To examine the effect of acid-base changes on the cytosolic calcium activity, (Ca)i, we used ion-selective microelectrodes in doubly perfused preparations of bullfrog kidney proximal tubule. For analyzing the time course of changes in (Ca)i and cytosolic pH (pHi) in response to peritubular acid or alkali perfusion and high K+, low Na+, or low Ca2+ perfusion single-barreled PVC-resin Ca2(+)-selective microelectrodes and double-barreled pH-sensitive microelectrodes were inserted into the cells. Control values (mean +/- S.E., number of observations) of (Ca)i and pHi averaged 17.2 +/- 1.0 nM (n = 25) and 7.39 +/- 0.01 (n = 25), respectively. Peritubular perfusion with a low pH perfusate (low HCO3-, pH 6.7) was found to reduce (Ca)i to about 4 nM in association with a moderate degree of cell acidification (to pH 7.09) and depolarization of the peritubular membrane potential (control -60 mV to experimental approximately -40 mV). Further, peritubular alkalinization (high HCO3- 30 mM, pH 8.0) induced a transient elevation of (Ca)i and hyperpolarization. In contrast, the peritubular perfusion of high K+ solution induced a rise of (Ca)i and pHi with membrane depolarization, while low Na+ perfusion decreased (Ca)i and pHi. These results support the view that 1) the experimentally induced changes in the membrane potential may be ascribed in large part of alterations of pH-sensitive conductance across the peritubular membrane, and 2) the cell pH and extracellular Ca2+ affect the cytosolic Ca2+ of the proximal tubule.