Possible role for mitochondrial calcium in angiotensin II- and potassium-stimulated steroidogenesis in bovine adrenal glomerulosa cells

In adrenal zona glomerulosa cells, the action of angiotensin II (Ang II) and of potassium (K+) on aldosterone synthesis is mediated by the Ca2+ messenger system. The major part of the steroidogenic pathway takes place inside the mitochondrial and Ca2+ must enter the mitochondrial matrix to stimulate the steroidogenic cascade. To examine how changes in the cytosolic free calcium concentration ([Ca2+](c)) induced by Ang II and K+ are relayed into the mitochondrial matrix, we transfected bovine adrenal zona glomerulosa cells in primary culture with a chimeric complementary DNA encoding for the signal presequence targeting human cytochrome c oxidase subunit VIII to the matrix, linked to a complementary DNA coding for the Ca2+- sensitive photoprotein aequorin. Resting mitochondrial free calcium concentration ([Ca2+](m)) amounted to 0.41 +/- 0.18 mu M (n = 40). Ang II induced a concentration-dependent (EC(50) = 11.3 +/- 6.0 nM), biphasic rise of [Ca2+](m). After a large transient initial peak (5.13 +/- 0.89 mu M, n = 28), [Ca2+](m) decreased to a plateau that remained higher than basal [Ca2+](m) for several minutes in the presence of the hormone. By contrast, studies in cells transfected with cytosolic aequorin indicated that the rise of [Ca2+](c) triggered by Ang II was confined to 1.34 +/- 0.26 mu M (n = 17). In Ca2+-free medium, a reduced peak [Ca2+](m) response to Ang II occurred without a secondary plateau. On readdition of extracellular Ca2+, in the presence of the hormone, the resulting Ca2+ influx was accompanied by small rise of [Ca2+](m). The mitochondrial uncoupler, carbonyl cyanide p-(trifluoro-methoxy)phenyl-hydrazone, prevented the Ang II-induced [Ca2+](m) rise but not the [Ca2+](c) response, thus demonstrating the mitochondrial location of transfected aequorin. In contrast to Ang II, K+ (13 mM) induced a sustained [Ca2+](c) response, which was relayed without amplification into the mitochondrial matrix as a plateau of [Ca2+](m). This plateau of [Ca2+](m) was suppressed by the addition of the dihydropyridine, nifedipine (200 nM). The inhibitor of the mitochondrial Na+/Ca2+ exchanger, CGP37157, reduced significantly the rate of decrease of [Ca2+](m) following the peak induced by Ang II. In cells whose [Ca2+](c) was clamped at various levels (0.05-0.860 mu M) with ionomycin, a concentration-dependent stimulation of pregnenolone output was induced by Ca2+. Under these conditions, the output of pregnenolone - the early product of steroidogenesis - was markedly potentiated by CGP37157. These results suggest the existence of microdomains of high [Ca2+](c) elicited by Ang II in the proximity of mitochondria. Moreover, our observations are consistent with a mitochondrial site of action for calcium in the activation of the steroidogenic cascade.