Mechanisms of Action of Gonadotropin Releasing Hormone on Rat Granulosa Cells

Abstract

To elucidate the mechanisms of stimulatory actions of GnRH on rat granulosa cells (GC), we have compared the actions of a GnRH agonist with those of a tumor-promoting phorbol ester, 12–0-tetra-decanoylphorbol 13-acetate (TPA) and Ca⁺² ionophore, A23187. GC were obtained from immature (28–29 days old) rats 48 h after injection of 20 IU PMSG. Following prelabeling with ³[H]arachidonic acid (AA), the cells were incubated with the test substances for 10 min and AA release determined. A GnRH agonist, [D-A]a⁶, des-Gly-NH₂10] GnRH ethylamide (GnRHa; 10 ng/ml) increased AA release 175% compared to the control value. AA release in the presence of GnRHa was larger than that due to 1 μM A23187 or 40 nM TPA alone. A23187 or TPA increased GnRHa-stimulated AA release further. GC were incubated with the test substances for longer time periods, i.e., up to 5 h. GnRHa caused a 4-fold increase in prostaglandin (PG) synthase activity at 5 h. GnRHa increased PGE accumulation to the same extent as TPA, but only increased PG synthase activity about half as much. In combination with TPA, GnRHa had no influence on TPA-stimulated PG synthase activity, but increased PGE accumulation to levels comparable to those with A23187 plus TPA. GnRHa caused a 2.5 fold increase in progesterone (P) accumulation, which was the same as TPA. P accumulation in the presence of GnRHa was affected by neither A23187 nor TPA. These data indicate that the combination of TPA and A23187 can substitute for GnRH action on PGE and P accumulation in rat GC.