Effect of divalent ions on basal and hormone-activated renal adenylate cyclase/cyclic AMP system

To elucidate the effect of Ca++ on adenylate cyclase (AC)/cyclic AMP system and its response to parathyroid hormone (PTH), experiments with parathyroidectomized rats' renal cortical tissue were performed in vitro. Increasing Ca++ from 0 to 5 mM reduced basal AC from 5.4 +/- (SE) 0.5 to 2.5 +/- 0.2 pmol cyclic AMP/mg protein/min (p less than 0.001). Raising Mg++ from 5 mM (basal concentration) to 20 mM restored AC to 5.7 +/- 0.55 (p less than 0.001). Adding 1 mM Mn++ to the incubation medium without Ca++ raised basal AC activity from 5.8 +/- 0.4 to 10.8 +/- 1.35 (p less than 0.01) and reversed the Ca++-induced inhibition. NaF-stimulated AC was reduced from 77.8 +/- 5.4 to 41.0 +/- 3.8 by 5 mM Ca++ (p less than 0.001), while adding Mg++ abolished this effect. PTH- and calcitonin-induced increase in AC was relatively resistant to Ca++ inhibition. Increasing Mn++ concentration from 0 to 5 mM (without Mg++) completely abolished PTH-induced increase in AC. The apparent Km of basal AC for Mg++ was 2.4, while in the presence of PTH the Km of AC for Mg++ was reduced to 1.1. The relative resistance of the PTH-induced increase in AC to Ca++ inhibition as opposed to the response of the basal activity to Ca++ may be explained by PTH-increased affinity of AC for Mg++.