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

Abstract

We examined the role of Ca²⁺ 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 Ca²⁺ from the incubation medium (measured Ca²⁺ concentration 5μm) decreased basal enzymic activity, and significantly (P2+ further decreased hormone-induced rises in ornithine decarboxylase activity. Despite the presence of 1.27mm concentrations of extracellular Ca²⁺, the administration of presumptive Ca²⁺ antagonists, believed to impair trans-membrane Ca²⁺ influx [verapamil (10–100μm), nifedipine (1–100μm) or CoCl₂ (1mm)] suppressed hormone-stimulated ornithine decarboxylase activity. The inhibitory effects of verapamil or of Ca²⁺ 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 Ca²⁺ ions in the modulation of hormone action and cellular function in normal ovarian cells.