Oxytocin and progesterone release from bovine corpus luteal cells in culture: effects of insulin-like growth factor I, insulin, and prostaglandins.

The ruminant corpus luteum synthesizes and secretes oxytocin, but little is known of the regulation of these processes in the ovary. In the present work we describe a method for the preparation of cells from the early bovine corpus luteum (1-5 days postovulation) and their maintenance in serum-free culture. The release of oxytocin and progesterone from these cells was increased by the addition of insulin or insulin-like growth factor I (IGF-I), but not by IGF-II. Hormone release (measured between 60 and 84 h of culture) was increased approximately 5-fold (oxytocin) and 2.5-fold (progesterone) by maximally effective concentrations of IGF-I (EC50, 0.27 nM) and insulin (EC50, 1.94 nM). Sustained exposure (0-84 h) to prostaglandins (PGs) caused a dose-dependent reduction in oxytocin release in the presence of IGF-I (PGF2 alpha EC50, 31 nM; rank order of potency, PGF2 alpha greater than PGE2 greater than PGE1), but did not markedly reduce progesterone release. The inhibitory effect of PG on oxytocin production was mimicked by sustained exposure to a protein kinase-C activator (phorbol 12,13-dibutyrate), supporting the proposed role for this enzyme as a mediator of PG action. These data provide the first demonstration that oxytocin release from early bovine corpus luteal cell cultures can be regulated by insulin, IGF-I, and PGs. Since granulosa and/or luteal cells produce and respond to IGF-I and PGF2 alpha, our data indicate functional interaction of these compounds in the regulation of luteal cell activity.