Abstract
The effect of extracellular calcium (Ca 2+) on the cellular action of forskolin was studied using a Na +, K +-ATPase inhibitor ouabain in rat renal papillary collecting tubule cells in culture. Forskolin-induced cAMP production was enhanced by the pretreatment of cells with ouabain, providing that a dose-dependent curve with forskolin shifted to the left. The enhancement by ouabain of cellular cAMP production in response to forskolin was totally blunted by cotreatment with cobalt, verapamil, or Ca 2+-free medium containing 1 mM EGTA. In addition, two dissimilar antagonists of calmodulin, namely trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W - 7), attenuated the ouabain's effect on cAMP production in response to forskolin. These results therefore indicate that ouabain enhances the activation of adenylate cyclase by forskolin, mediated through cellular free Ca 2+, in renal papillary collecting tubule cells, and that extracellular Ca 2+ is an important source for cellular Ca 2+ mobilization by ouabain.
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