Mechanisms Subserving Hormone Action in the Ovary: Role of Calcium Ions as Assessed by Steady State Calcium Exchange in Cultured Swine Granulosa Cells*

Abstract

We have appraised the properties of steady state calcium exchange in cultured swine granulosa cells prelabeled to isotopic equilibrium with 45Ca2+. Subsequent steady state efflux parameters were assessed by nonlinear least-squares curve-fitting to a two-compartment model. The resultant mean rate constants were 0.27 +/- 0.07 min-1 and 0.030 +/- 0.0015 min-1, which correspond to respective half-lives of calcium exchange of 2.6 +/- 0.67 min and 23 +/- 1.1 min (means +/- SD, n = 18 experiments). The estimated sizes of the rapidly and slowly exchangeable calcium compartments in ovarian cells were 1.8 +/- 0.51 and 4.47 +/- 0.15 mumol calcium/mg protein, respectively. Equilibrium exchange of calcium was temperature dependent, and accelerated markedly by putative stimulators of calcium influx, such as maitotoxin, A23187, and BAY K 8644. Conversely, presumptive inhibitors of the calcium-magnesium dependent ATPase, such as trifluoperazine and orthovanadate, significantly diminished the size of the slowly exchangeable pool and its corresponding rate constant. A similar profound reduction in the rate of slow calcium exchange was induced by a phorbol ester and by an inhibitor of intracellular calcium mobilization, dantrolene. Physiological effector hormones also altered equilibrium calcium exchange in swine granulosa cells. Thus, FSH and prostaglandin E2 increased the rapid and slowly exchanging rate constants by factors of approximately 2.2 and 1.6, whereas prostaglandin F2 alpha increased the sizes of the slowly and rapidly exchangeable calcium compartments. In summary, the present work delineates properties of physiological steady state calcium exchange in swine granulosa cells. Physiological effector hormones, such as FSH, prostaglandin E2, and prostaglandin F2 alpha, have distinct effects on steady state calcium exchange in ovarian cells.