THE EFFECTS OF ELEVATION AND DEPLETION OF INTRACELLULAR FREE CALCIUM ON PROGESTERONE AND PROSTAGLANDIN PRODUCTION BY THE PRIMATE CORPUS-LUTEUM

The role of the phosphatidylinositol second messenger system in luteal regulation has not been extensively studied, particularly in the primate. The objectives of this study were (1) to further characterize the response of the primate CL to the calcium ionophore A23187, in terms of intracellular free calcium concentrations ([Ca2+]i) and progesterone (P) production; and (2) to assess the effects of depleting, as well as elevating, available calcium on luteal P and prostaglandin (PG) production. The response to A23187, in terms of [Ca2+]i was measured by fura-2 fluorescence microscopy of single small and large luteal cells. A23187 significantly increased [Ca2+]i in both cell types (p < 0.01). P production (basal and hCG-stimulated) by dispersed primate luteal cells incubated for various times (1-8 h) with and without A23187 was measured. Treatment with A23187 rapidly (within 1-2 h) attenuated (p < 0.05) the time-dependent increase in basal and hCG-stimulated P production. Luteal P and PG production following treatment with the calcium ionophore, ionomycin, alone or in combination with additional CaCl2, was also monitored. Treatment with ionomycin (p < 0.01) and CaCl2(p < 0.01) inhibited luteal P production. In contrast, treatment with ionomycin stimulated (p < 0.01) luteal PG production. To determine the effects of Ca2+ depletion on luteal function, P and PG production by cells incubated for 2 and 8 h in the absence and presence of the Ca2+-chelator EGTA was measured. Luteal production of both P and PG was inhibited by 8-h treatment with EGTA. In conclusion, elevations in [Ca2+]i differentially regulate PG and P production in the primate CL, leading to elevated PG and suppressed P production in CL collected in the mid-luteal phase. Since this pattern of luteal PG and P production is also evident late in the luteal phase, the phosphatidylinositol pathway may be involved in regression of the primate CL. In contrast to the selective alterations in luteal function following increases in [Ca2+]i, depletion of available calcium leads to suppression of both PG and P production. Thus, an optimal level of [Ca2+]i may be necessary for maximal luteal function.