Increased CO2 stimulates K/Rb reabsorption mediated by H-K-ATPase in CCD of potassium-restricted rabbit

Apical H-K-ATPase in the cortical collecting duct (CCD) plays an important role in urinary acidification and K reabsorption. Our previous studies demo nstrated that an H-K-ATPase mediates, in part, Rb reabsorption in rabbit CC D (Zhou X and Wingo CS. Am J Physiol Renal Fluid Electrolyte Physiol 263: F 1134-F1141, 1992). The purpose of these experiments was to examine using in vitro microperfused CCD from K-restricted rabbits 1) whether an acute incr ease in PCO2 and, presumably, intracellular acidosis stimulate K absorptive flux; and 2) whether this stimulation was dependent on the presence of a f unctional H-K-ATPase. Rb reabsorption was significantly increased after exp osure to 10% CO2 in CCD, and this effect was persistent for the entire 10% CO2 period, whereas 10 muM SCH-28080 in the perfusate totally abolished the stimulation of Rb reabsorption by 10% CO2. After stimulation of Rb reabsor ption by 10% CO2, subsequent addition of 0.1 mM methazolamide, an inhibitor of carbonic anhydrase, failed to affect Rb reabsorption. However, simultan eous exposure to 10% CO2 and methazolamide prevented the stimulation of Rb reabsorption. Treatment with the intracellular calcium chelator MAPTAM (0.5 muM) inhibited the stimulation of Rb reabsorption by 10% CO2. Similar inhi bition was also observed in the presence of either a calmodulin inhibitor, W-7 (0.5 muM), or colchicine (0.5 mM), an inhibitor of tubulin polymerizati on. In time control studies, the perfusion time did not significantly affec t Rb reabsorption. We conclude the following: 1) stimulation of Rb reabsorp tion on exposure to 10% CO2 is dependent on the presence of a functional H- K-ATPase and appears to be regulated in part by the insertion of this enzym e into the apical plasma membrane by exocytosis; 2) insertion of H-K-ATPase requires changes in intracellular pH and needs a basal level of intracellu lar calcium concentration; and 3) H-K-ATPase insertion occurs by a microtub ule-dependent process.