Mechanisms by which Calcium Ions Regulate the Steroidogenic Actions of Luteinizing Hormone in Isolated Ovarian Cells in Vitro*

Abstract

Incubation of swine granulosa cells in chemically defined medium selectively deficient in calcium ions markedly impaired progesterone production in response to submaximal and maximally stimulating concentrations of LH. Accumulation of progesterone in response to LH was reduced significantly in both cells and medium, without a discernible shift in the time-course of progestin production. The reduction in progesterone accumulation could not be accounted by increased formation of the catabolite 20 alpha-hydroxypregn-4-en-3-one. In addition, progesterone secretion basally or in response to exogenously supplied pregnenolone was not altered in calcium-deficient incubations. Administration of verapamil or diltiazem, organic inhibitors of net transmembrane calcium uptake, also suppressed LH-stimulated progesterone production. Conversely, micromolar concentrations of the divalent cation ionophore A23187 significantly enhanced the stimulatory effects of LH. The mechanisms of calcium action were examined further in relation to the cAMP effector system. Calcium deprivation significantly suppressed the dose-dependent accumulation of cAMP in granulosa cells treated with LH but had no effect on basal levels. Omission of calcium ions from the extracellular medium also markedly impaired production of progesterone in response to 8-bromo-cAMP, cholera toxin, or 3-isobutyl-l-methylxanthine. The present studies suggest that calcium ions significantly modulate LH-stimulated progesterone biosynthesis in isolated ovarian cells in vitro. Specific regulatory actions of calcium ions in granulosa cells may be exerted at several levels, including LH-stimulated cAMP accumulation and at intracellular loci distal to the actual generation of cAMP.