Differences in the acute actions of sGnRH and cGnRH-II on gonadotropin release in goldfish pituitary cells

The signal transduction mechanisms mediating the acute actions of salmon gonadotropin (GTH)-releasing hormone (sGnRH) and chicken GnRH-II (cCnRH-II) on GTH release from goldfish pituitary cells were compared. In cell column perifusion experiments, treatment with inhibitors of phospholipase A(2) (50 mu M quinacrine or 50 mu M bromophenacylbromide) or the lipoxygenase enzyme (50 IJ-M nordihydroguaiaretic acid) reduced the GTH response to 100 nM sGnRH, bur not the response to 100 nM cCnRH-II. These results suggest that AA mobilization through phospholipase A(2) and the subsequent metabolism of AA through the lipoxygenase pathway are involved in rapid sGnRH-induced GTH secretion, but not in acute cGnRH-II action. Consistent with the idea that calcium entry through voltage-sensitive calcium channels is involved in acute GnRH action, perifusion with 1 mu M verapamil, a voltage-sensitive calcium channel inhibitor, reduced both 100 nM sGnRH- and 100 nM cGnRH-II-induced GTH secretion. However, the response to cGnRH-II was decreased to a greater extent compared to sGnRH-elicited release, suggesting a greater dependence on extracellular calcium entry for acute cGnRH-II-stimulated GTH secretion. The metabolism of inositol phosphates (InsPs) following acute sGnRH and cCnRH-II administration was also investigated by monitoring the levels of [H-3]InsPs in [H-3]inositol-prelabeled goldfish pituitary cells. Incubation with 100 nM sGnRH increased [H-3]InsP(1) by 5 min and [H-3]InsP(3), [H-3]InsP(3), and other higher [H-3]InsPs by 10 min. In contrast, following 10 min of stimulation by 100 nM cGnRH-II, only [H-3]InsP(3) levels were elevated, suggesting that cGnRH-II may activate a different set of enzymes in the phosphoinositide metabolic pathways compared to sCnRH. The lack of an InsP(3) response may also reflect the relative ineffectiveness of cGnRH-II to mobilize calcium from intracellular stores. Taken together, these results strongly indicate that the mechanisms mediating rapid sGnRH-induced and cGnRH-II-elicited GTH responses are different as in the case for prolonged GnRH action. (C) 1995 Academic Press, Inc.