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 (cGnRH-II) on GTH release from goldfish pituitary cells were compared. In cell column perifusion experiments, treatment with inhibitors of phospholipase A2 (50 microM quinacrine or 50 microM bromophenacylbromide) or the lipoxygenase enzyme (50 microM nordihydroguaiaretic acid) reduced the GTH response to 100 nM sGnRH, but not the response to 100 nM cGnRH-II. These results suggest that AA mobilization through phospholipase A2 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, perfusion with 1 microM 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 cGnRH-II administration was also investigated by monitoring the levels of [3H]InsPs in [3H]inositol-prelabeled goldfish pituitary cells. Incubation with 100 nM sGnRH increased [3H]InsP1 by 5 min and [3H]InsP2, [3H]InsP3, and other higher [3H]InsPs by 10 min. In contrast, following 10 min of stimulation by 100 nM cGnRH-II, only [3H]InsP2 levels were elevated, suggesting that cGnRH-II may activate a different set of enzymes in the phosphoinositide metabolic pathways compared to sGnRH. The lack of an InsP3 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.