Effects of intracellular calcium on proximal bicarbonate absorption.

Using in vivo microperfusion in the proximal convoluted tubule (PCT) of the Munich-Wistar rat, we assessed the impact of raising intracellular calcium concentration on bicarbonate transport and its regulation by angiotensin II. Luminal perfusion with the calcium ionophore A23187 caused a dose-dependent increase in S1 PCT bicarbonate absorption, with a maximal change of 60 peq.mm-1.min-1. Subsequent addition of phorbol ester (PMA) after A23187 further increased transport but to a level no higher than previously shown with PMA alone, and A23187 was nonstimulatory when added after PMA. These results suggest that intracellular calcium affects the same pathway as protein kinase C in stimulating proximal acidification. Pretreatment with A23187 (+/- PMA) attenuated by approximately one-third the bicarbonate absorptive response in the S1 PCT usually observed after angiotensin II administration. A23187 had similar actions, but of smaller magnitude, in the S2 PCT. In conclusion, a rise in intracellular calcium increases bicarbonate absorption in the S1 and S2 PCT in vivo.