It has been reported that the luteal function may be regulated by the intracellular Ca++ level which may be adjusted partially by the high affinity Ca++-ATPase in luteal cell membranes. Then, one may expect that luteotropic and/or luteolytic agents, such as gonadotropins, prostaglandin F2α (PGF2α) and ouabain, affect the intracellular Ca++ level. In this present study, therefore, we examined the effects of luteinizing hormone (LH, or human chorionic gonadotropin, hCG), PGF2α and ouabain on the kinetic properties of the high affinity Ca++-ATPase in light membrane, heavy membrane, and microsomal fractions from the highly luteinized ovary. LH (or hCG) increased the affinity and the Vmax for Ca++ both in light membrane and heavy membrane. PGF2α increased the Vmax in light membrane and decreased the Km in heavy membrane for Ca++ at low concentration (5 μg/ml). At higher concentration, however, PGF2α oppositly affected on kinetic properties, that shown at low concentration. Ouabain, a potent inhibitor of Na+-K+-ATPase, increased the Km at high concentration (10-4 M), however, decreased the Vmax for Ca++ in light membrane at low concentration (10-6 M). Also, ouabain increased the Km for Ca++ in heavy membrane without changes in the Vmax at both concentrations. It seems that LH and low dose of PGF2α increase the intracellular Ca++ level and cause in activation of Ca++-ATPase, however, higher dose of PGF2α and ouabain inhibit directly Ca++-ATPase activity and result in increase in intracellular Ca++ level. According to the above results, we suggest that luteotropic and/or luteolytic agents regulate the luteal progesterone (P4) production through two different pathways; one is cyclic adenosine monophosphate (cAMP)-dependent and another is Ca++-dependent. Intracellula. Ca++ level regulated by the high affinity Ca++-ATPase may affect both pathways in a time-dependent fashion. LH (or hCG) acts on the luteal P4 production via both pathways. The initial step is Ca++ dependent, and the late step is cAMP dependent. PGF2α and ouabain increase the intracellular Ca++ concentration so that basal luteal P4 production is increased and LH-stimulated P4 production is inhibited by the inhibiting LH-dependent adenylate cyclase activity.
