INHIBITION BY NITRENDIPINE OF RB-86(+) FLUXES IN SUBCONFLUENT MDCK CELLS

Part of the natriuretic mechanism of dihydropyridine Ca2+ channel antagonists involves the inhibition of renal tubular sodium reabsorption. To identify the membrane ion transport system involved in this natriuretic action, we tested nitrendipine on unidirectional Rb-86(+) fluxes in Madin-Darby canine kidney (MDCK) cells. To dissect between direct and indirect effects (via cytosolic Ca2+) of nitrendipine, the compound was re-examined on ion fluxes in human erythrocytes. In MDCK cells, external Ca2+ (3 mM), adrenalin (100 mu M) and the Ca2+ ionophore A23187 (20 mu M) strongly and transiently stimulated Rb-86(+) efflux. All these stimulatory actions were fully inhibited by quinine (1 mM) suggesting that they reflect the opening of Ca2+-sensitive K+ channels. Nitrendipine was able to inhibit these Ca2+-sensitive K+ channels, but this inhibitory action required high concentrations of the compound (approximate to 100 mu M). Regarding Rb-86(+) influx, the most significant result with nitrendipine was a partial inhibition of bumetanide-sensitive Rb-86(+) influx. This effect represented a maximal flux inhibition of about 70% and required very low nitrendipine concentrations (IC50 approximate to 1 nM). The Ca2+ ionophore A 23187 strongly stimulated bumetanide-sensitive Rb-86(+) influx in MDCK cells. Conversely, a very important reduction (approximate to 79%) of this influx component was found in Ca2+ depleted cells. In human red blood cells, Na+, K+, Cl- cotransport fluxes were resistant to nitrendipine, even at high concentrations of the compound (100-500 mu M). Conversely, Ca2+-sensitive K+ channels were inhibited by nitrendipine with IC50 = 6 +/- 3 mu M (mean +/- S.E.M., n = 3). In conclusion, nitrendipine is an inhibitor of Ca2+-sensitive K+ channels and a partial inhibitor of the Na+, K+, Cl- cotransport system in MDCK cells. The cotransport inhibitory action, but not that on K+ channels, was observed at pharmacological concentrations of nitrendipine. This cotransport inhibition is probably mediated via a decrease in cytosolic free Ca2+ contents. The physiological relevance of these findings and the link with the natriuretic effect of nitrendipine deserves further investigation.