Activin modulates the intracellular signaling system activated by gonadotropin-releasing hormone: dual effect on calcium messenger system and protein kinase-C pathway.

This study was performed to assess the effects of activin on intracellular mechanisms involved in GnRH action. When rat pituitary cell cultures were pretreated with activin-A (5-80 ng/ml) for 3 days, subsequent FSH and LH release (percentage of total cellular FSH and LH released during 4 h) in response to GnRH (10(-10)-10(-6) M) was not significantly different from that in cells pretreated with medium alone. In contrast, activin pretreatment increased the potency of both A23187 (Ca2+ ionophore) and phorbol 12-myristate 13-acetate [a protein kinase-C (PKC) activator] as secretagogues for FSH and LH release. FSH or LH release in response to another Ca(2+)-mobilizing secretagogue, maitotoxin (an activator of the GnRH receptor-associated Ca2+ channel), was not increased by activin. Although PKC is capable of influencing the actions of Ca2+, which is believed to be the second messenger for GnRH action, neither GnRH- nor maitotoxin-stimulated gonadotropin release was increased by activin even when the influence of activin on PKC was eliminated by the addition of a PKC inhibitor (staurosporine; 100 nM) during the final 30 min of the 3-day pretreatment period. These results indicate that although activin does not influence GnRH action with regard to gonadotropin release, it increases the sensitivity of the system regulating gonadotropin release to increases in cytosolic Ca2+ concentrations and PKC activation. Furthermore, activin appears to exhibit an inhibitory effect(s) at some point(s) in GnRH action in a PKC-independent manner, which could be responsible for opposing the increased sensitivity of the gonadotrope to Ca2+. The differential effects of activin on gonadotropin release in response to Ca(2+)-mobilizing secretagogues (ionophore and maitotoxin) raise the possibility that the activity of the GnRH receptor-associated Ca2+ channel may be suppressed by activin.