INTERACTION BETWEEN ACTIVIN-A AND CAMP IN THE INDUCTION OF FSH RECEPTOR IN CULTURED RAT GRANULOSA-CELLS

The acquisition of FSH receptor during preantral folliculogenesis is believed to be a key step in the subsequent development of follicles. We examined the interaction between activin and cAMP in FSH receptor induction in rat granulosa cells by measuring I-125-FSH binding and FSH receptor mRNA. In the I-125-FSH binding study, 0.2 mM 8-Br-cAMP and 1 mu M forskolin were maximally effective in FSH receptor induction (169 and 220% respectively of control), while higher concentrations gave attenuated responses. It appears that cAMP has ambivalent effects on FSH receptor induction depending on the concentration and length of exposure. Activin alone dramatically increased the number of FSH receptors (314% of control). Moreover, synergistic effects of activin and 8-Br-cAMP or forskolin were observed on FSH receptor induction: a combination of activin (80 ng/ml) and low doses of 8-Br-cAMP (0.2 mM) or forskolin (1 mu M) was most effective (160 or 140% of that induced by activin alone) and receptor levels reached a maximum at 24 h. These levels then markedly decreased after 72 h of incubation. Northern blot analysis revealed that the combination of activin (80 ng/ml) and 8-Br-cAMP (0.2 mM) or forskolin (1 mu M) increased FSH receptor mRNA to about 140% of that induced by activin alone. These results indicate that activin and cAMP induced FSH receptor synergistically. However, activin did not enhance the production of cAMP induced by forskolin. In addition, a protein kinase A inhibitor (H89) (2 mu M), which inhibited the effects of forskolin, had no effect on the action of activin. Taken together, the present findings suggest that the action of activin is not via a cAMP pathway, and that activin works co-operatively with cAMP on folliculogenesis.