Differences in the Acute Actions of sGnRH and cGnRH-II on Gonadotropin Release in Goldfish Pituitary Cells

Abstract

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 A 2 (50 μ M quinacrine or 50 μ M bromophenacylbromide) or the lipoxygenase enzyme (50 μ M nordihydroguaiaretic acid) reduced the GTH response to 100 n M sGnRH, but not the response to 100 n M cGnRH-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 μ M verapamil, a voltage-sensitive calcium channel inhibitor, reduced both 100 n M sGnRH- and 100 n M 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 n M sGnRH increased [ 3H]InsP 1 by 5 min and [ 3H]InsP 2, [ 3H]InsP 3, and other higher [ 3H]InsPs by 10 min. In contrast, following 10 min of stimulation by 100 n M cGnRH-II, only [ 3H]InsP 2 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 InsP 3 response may also reflet 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.