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Calcium mobilization is a prerequisite for the expression of phorbol ester-stimulated luteinizing hormone secretion from pituitaries of male and acutely ovariectomized rats

Das S, Fahmy NW, Bourne GA. Calcium mobilization is a prerequisite for the expression of phorbol esterstimulated luteinizing hormone secretion from pituitaries of male and acutely ovariectomized rats. Eur J Endocrinol 1994;130:151–8. ISSN 0804–4643

Ionomycin, which mobilizes Ca²⁺, and phorbol 12-myristate 13-acetate (PMA) which activates protein kinase C (PKC), were used to compare the effects/interactions of Ca²⁺ and PKC on luteinizing hormone (LH) secretion from pituitaries of intact male and acutely ovariectomized (72 h) rats. Quartered pituitaries from donor animals were perifused at 0.25 ml/min and sequential effluent fractions were collected every 10 min. Continuous administration (4 h) of 1 nmol of gonadotropin-releasing hormone (GnRH) resulted in an increase in LH secretion. Cycloheximide (5 µmol) dissociated the GnRH-stimulated LH responses into protein synthesis-independent and -dependent components. While ionomycin (10 µmol) stimulated LH secretion from pituitaries of both sexes by protein synthesis-independent mechanisms, PMA (1 µmol) and the inactive phorbol ester, 4-phorbol 12,13-didecanoate (PDD), were ineffective secretagogues. However, PMA (but not PDD) interacted synergistically with ionomycin and GnRH to augment LH secretion by protein synthesis-dependent mechanisms. These results suggest a similarity in the effects/interactions of Ca^{2 +} and PKC in gonadotropes from male and ovariectomized rats. If the effects of PMA can be attributed to PKC activation, then it also appears that Ca²⁺ mobilization is necessary for the manifestation of PKC as a mediator of LH secretion from these gonadotropes. While PKC activity can be divorced from the protein synthesis-independent component of LH release (this component appears to be mediated by Ca²⁺ mobilization), the enzyme might be involved in amplifying the response to Ca²⁺ mobilization through synergistic, protein synthesis-dependent mechanisms.

Gregory Bourne, Department of Physiology, College of Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OWO