Role of calcium in the modulation of ornithine decarboxylase activity in isolated pig granulosa cells in vitro

We examined the role of Ca(2+) in the control of basal and hormone-stimulated ornithine decarboxylase activity in isolated pig granulosa cells maintained under chemically defined conditions in vitro. Omission of Ca(2+) from the incubation medium (measured Ca(2+) concentration 5mum) decreased basal enzymic activity, and significantly (P<0.01) impaired the response to maximally stimulating doses of either lutropin or follitropin. No significant alteration occurred in the concentration of either gonadotropin required to elicit half-maximal effects. The addition of EGTA (1.27-2.0mm) to chelate residual extracellular Ca(2+) further decreased hormone-induced rises in ornithine decarboxylase activity. Despite the presence of 1.27mm concentrations of extracellular Ca(2+), the administration of presumptive Ca(2+) antagonists, believed to impair trans-membrane Ca(2+) influx [verapamil (10-100mum), nifedipine (1-100mum) or CoCl(2) (1mm)] suppressed hormone-stimulated ornithine decarboxylase activity. The inhibitory effects of verapamil or of Ca(2+) omission from the medium were not overcome by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.25mm), or by cholera toxin, or by an exogenously supplied cyclic AMP analogue, 8-bromo cyclic AMP. Conversely, micromolar concentrations of a putative bivalent-cation ionophore, A23187, increased significantly the stimulation of ornithine decarboxylase activity by saturating concentrations of lutropin or 8-bromo cyclic AMP. Thus the present observations implicate Ca(2+) ions in the modulation of hormone action and cellular function in normal ovarian cells.