ARACHIDONIC-ACID INHIBITS HCG-STIMULATED PROGESTERONE PRODUCTION BY CORPORA-LUTEA OF PRIMATES - POTENTIAL MECHANISM OF ACTION

Arachidonic acid (AA) is a precursor of metabolites known to affect the corpus luteum (CL) in many species, including primates. We have shown that some of these products (prostaglandins F-2 alpha and E(2) inhibit pro-gesterone (P-4) production and activate the phosphatidylinositol (PI) pathway in CL of rhesus monkeys. A direct role of AA in luteal function has also been suggested. The current experiments were designed to investigate the effect of AA on P-4 synthesis and to examine the ability of AA to activate the PI pathway in CL of rhesus monkeys. Basal and hCG-stimulated P-4 production by luteal cells collected during the midluteal phase was measured after treatment with AA (1, 5, and 10 mu M) or linoleic acid (1, 5, and 10 mu M). Dispersed cells (50,000/tube) were incubated at 37 degrees C for 2 h. AA elicited a dose-dependent decrease in hCG-stimulated, but not in basal, P-4 production, hCG-stimulated P-4 production was reduced (P < 0.01) at AA doses of 5 mu M (12.1 +/- 2.6 ng/mL) and 10 mu M (8.6 +/- 2.8 ng/mL compared to hCG alone (18.6 +/- 1.6 ng/mL). There was no significant effect of 1 mu M AA (15.2 +/- 1.6). Response to linoleic acid was dissimilar and was not dose-dependent. Viability of cells was not affected by any treatment. Indomethacin, a prostaglandin synthesis inhibitor, and nordihydroguaiaretic acid, an inhibitor of lipoxygenase, did not interfere with the inhibitory effect of AA. Activation of the PI pathway was assessed by monitoring the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2) to inositol phosphates and by monitoring increases in intracellular free calcium concentrations ([Ca2+](i)) in individual cells. Moreover, the ability of AA to activate protein kinase C (PKC) in luteal cells was measured using a [H-3]phorbol dibutyrate (PDBu) binding assay. AA did not alter PIP2 hydrolysis or [CA(2+)](i), however, AA (10 mu M ) increased specific binding of [H-3]PDBu to luteal cells (P < 0.05). We conclude that AA inhibits hCG-stimulated P-4 production by primate luteal cells. AA exerts this action without being converted to prostaglandins or leukotrienes. This inhibition may be mediated through the activation of PKC. These results suggest 4 possible role for AA in the regulation of luteal function in primates, and that PKC-activation by AA may promote its effects.