Involvement of extracellular sodium in agonist-induced gonadotropin release from goldfish (Carassius auratus) gonadotrophs

In goldfish, gonadotropin (GTH-II) responses to the two endogenous GnRHs, salmon-GnRH and chicken-GnRH-II, are mediated by activation of protein kinase C (PKC) and voltage-sensitive Ca2+ channels. In this study, we investigated the role of extracellular Na+, voltage-dependent Na+ channels, and the plasma membrane Na+/H+ exchanger in mediating GnRH-stimulated GTH-II release from dispersed goldfish pituitary cells. Perifusion with Na+-depleted medium reduced the GTH-II response to both GnRHs and the response to the protein kinase C activator, phorbol 12-myristate 13-acetate. Conversely, increasing Na+ influx with veratridine (100 mu M) stimulated GTH-II release in the presence and in the absence of extracellular Ca2+. However, the voltage-sensitive Na+ channel blocker, tetrodotoxin (1 mu M), did not affect GnRH-stimulated GTH-II release, and the GnRHs did not affect voltage-sensitive Na+ currents. In contrast, the Na+/H+ antiport inhibitors, amiloride or its analog, DMA, reduced GTH-II responses to the GnRHs and phorbol la-myristate 13-acetate. The Na+/H+ antiport inhibitors did not affect voltage-sensitive Ca2+ or Na+ currents or the GTH-II release response to the Ca2+ ionophore, ionomycin. These findings indicate that extracellular Na+ and the Na+/H+ exchanger are involved in the mediation of GnRH-stimulated GTH-II release. In addition, Na+ entry may modulate GTH-II release independent of extracellular Ca2+.