Arachidonic acid and its metabolites increase cytosolic free calcium in bovine luteal cells

We studied the effects of arachidonic acid and its metabolites on intracellular free calcium concentrations ([Ca2+]i) in highly purified bovine luteal cell preparations. Corpora lutea were collected from Holstein heifers between days 10 and 12 of the estrous cycle. The cells were dispersed and small and large cells were separated by unit gravity sedimentation and flow cytometry. The [Ca2+]i was determined by spectrofluorometry in luteal cells loaded with the fluorescent Ca2+ probe, Fura-2. Arachidonic acid elicited a dose-dependent increase in [Ca2+]i in both small and large luteal cells, having an effect at concentrations as low as 5 microM; and was maximally effective at 50 microM. Several other fatty acids failed to exert a similar response. Addition of nordihydroguaiaretic acid (NDGA) or indomethacin failed to suppress the effects of arachidonic acid. In fact, the presence of both inhibitors resulted in increases of [Ca2+]i, with NDGA exerting a greater stimulation of [Ca2+]i than indomethacin. Prostaglandin F2 alpha (PGF2 alpha) as well as prostaglandin E2 (PGE2) increased [Ca2+]i in the small luteal cells. These results support the idea that arachidonic acid exerts a direct action in mobilizing [Ca2+]i, in the luteal cells. Furthermore, they demonstrate that the cyclooxygenase (PGF2 alpha and PGE2) and lipoxygenase products of arachidonic acid metabolism also play a role in increasing [Ca2+]i in bovine luteal cells. Since the bovine corpus luteum contains large quantities of arachidonic acid, these findings suggest that this compound may regulate calcium-dependent functions of the corpus luteum, including steroid and peptide hormone production and secretion.