Unique calcium dependencies of the activating mechanism of the early and late aldosterone biosynthetic pathways in the rat.

This study compared the extracellular calcium dependency and the enzymatic locus of that dependency for N6 O2'-dibutyryl cyclic AMP (dbcAMP)-, angiotensin II- and potassium-stimulated aldosterone secretion in dispersed rat glomerulosa cells. The need for extracellular calcium, calcium influx, and specifically for calcium influx through the calcium channel was examined. dbcAMP, angiotensin II and potassium, in the presence of calcium (3.5 mmol/l), significantly (P less than 0.01) increased aldosterone output by at least 1.5-fold. Yet in the absence of extracellular calcium or in the presence of lanthanum (an inhibitor of calcium influx by most mechanisms) all three stimuli failed to increase aldosterone secretion. Nifedipine, a dihydropyridine calcium channel antagonist, significantly (P less than 0.01) reduced angiotensin II- and potassium-stimulated aldosterone secretion, but had no effect on dbcAMP-stimulated aldosterone secretion (100 +/- 14 vs 105 +/- 19 pmol/10(6) cells). Likewise nitrendipine failed to inhibit ACTH-stimulated aldosterone secretion. Angiotension II and potassium activation of both the early aldosterone biosynthetic pathway (as reflected by pregnenolone production in the presence of cyanoketone) and also its late pathway (as reflected by the conversion of exogenous corticosterone to aldosterone in the presence of cyanoketone) were significantly (P less than 0.01) inhibited by lanthanum, nifedipine and by reducing the extracellular calcium concentration. However, with dbcAMP stimulation, none of these manipulations modified pregnenolone production. Late pathway activation by dbcAMP was inhibited by lanthanum and a reduction in extracellular calcium, but not by nifedipine. These observations suggest that: the extracellular calcium dependency of dbcAMP-, angiotensin II- and potassium-stimulated aldosterone secretion reflects a need for calcium influx; with dbcAMP stimulation, activation of the late pathway is dependent on calcium influx by a calcium channel-independent mechanism, whereas activation of the early pathway is not dependent on extracellular calcium or calcium influx and activation of both the early and late pathway by angiotensin II and potassium is dependent on calcium influx by a calcium channel-dependent mechanism. Therefore, we conclude that the mechanism of activation of the early aldosterone biosynthetic pathway by dbcAMP is different from angiotensin II or potassium and early pathway activation is distinct from that of late pathway activation with dbcAMP stimulation.