Estradiol modulation of PMA--and ionomycin-stimulated LH secretion from pituitaries of castrated rats

Quartered pituitaries from castrated (72 h) +/- estradiol (E2)-treated (24 h) rats were used in a perifusion system to investigate estradiol modulation of ionomycin and ionomycin + PMA stimulated LH secretion. Estradiol enhanced the LH responses to GnRH (1 nM) and ionomycin (10 microM), and was necessary for the manifestation of PMA-stimulated (1 microM) LH secretion. Cycloheximide (5 microM) inhibited the E2-enhanced responses to GnRH, ionomycin and PMA. The protein synthesis inhibitor also partially suppressed the GnRH response from pituitaries of castrates, but was totally ineffective against the ionomycin-induced LH secretion. Protein synthesis-dependent, synergistic interactions between PMA and ionomycin were evident from pituitaries of castrates (even though PMA alone was an ineffective secretagogue). Synergistic interactions were not apparent from pituitaries of castrated + E2-treated rats. These results indicate that: (i) estradiol enhances the responsiveness of male gonadotropes to ionomycin and PMA by protein synthesis-dependent mechanisms which appear to mask their synergistic interactions; and (ii) increases in cytoplasmic Ca2+ might be a prerequisite for an expression of the involvement of PKC as a mediator of LH secretion in the absence of high concentrations of estradiol.