Extracellular sodium dependence of GnRH-stimulated growth hormone release in goldfish pituitary cells.

Abstract

In goldfish, gonadotropin-releasing hormone (GnRH) stimulation of growth hormone (GH) release has been shown to involve extracellular Ca2+ entry through voltage-sensitive Ca2+ channels and the activation of protein kinase C (PKC). In this study, the possible involvement of extracellular Na+ in mediating the GH response to GnRH was examined using dispersed pituitary cells. Perifusion with Na(+)-depleted medium reversibly reduced the acute GH response to 5-min pulses of either 10 nM salmon (s)GnRH or 10 nM chicken (c)GnRH-II. Similarly, replacement of normal medium with Na(+)-depleted medium attenuated the long-term GH release response to sGnRH and cGnRH-II under static incubation conditions. These results suggest that GnRH-induced GH release requires the presence of extracellular Na+. Treatment with 5-min pulses of the Na(+)-channel agonist veratridine (10 microM) increased GH release in an extracellular Ca(2+)-dependent manner, presumably due to activation of voltage-sensitive Ca2+ channels resulting from the depolarizing effect of increased Na+ influx. On the other hand, Na+ entry through tetrodotoxin (TTX)-sensitive, voltage-dependent Na+ channels is not involved in GnRH-induced GH release. Application of 250 nM TTX, which abolished the voltage-sensitive Na+ currents in identified goldfish somatotropes, did not affect the acute GH responses to 5-min pulses of sGnRH and cGnRH-II. The possible participation of Na+/H+ antiport in mediating the extracellular Na(+)-dependent GnRH action on GH release was then examined. In static incubation experiments, sGnRH- and cGnRH-II-induced GH secretion were reduced by inhibitors of the Na+/H+ antiport, amiloride and dimethylamiloride (DMA). Likewise, the GH response to the PKC activator, tetradecanoyl phorbol acetate, was attenuated by treatment with Na(+)-depleted medium, amiloride, and DMA. The inhibitory actions of amiloride and DMA were selective as these drugs did not affect the GH response elicited by the Ca2+ ionophore ionomycin and the voltage-sensitive Ca2+ channel agonist, Bay K 8644. Taken together, these results indicate that extracellular Na+ and the Na+/H+ exchanger are involved in the mediation of GnRH-stimulated GH release in goldfish. Furthermore, this dependence on Na+ and Na+/H+ antiport probably occurs distal to the activation of PKC by GnRH.