EXTRACELLULAR-SODIUM DEPENDENCE OF GNRH-STIMULATED GROWTH-HORMONE RELEASE IN GOLDFISH PITUITARY-CELLS

In goldfish, gonadotropin-releasing hormone (GnRH) stimulation of grow th hormone (GH) release has been shown to involve extracellular Ca2+ e ntry through voltage-sensitive Ca2+ channels and the activation of pro tein kinase G (PKC). In this study, the possible involvement of extrac ellular Na+ in mediating the GH response to GnRH was examined using di spersed pituitary cells. Perifusion with Na+-depleted medium reversibl y 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 r esponse to sGnRH and cGnRH-II under static incubation conditions. Thes e results suggest that GnRH-induced GH release requires the presence o f extracellular Na+. Treatment with 5-min pulses of the Na+-channel ag onist veratridine (10 mu M) increased GH release in an extracellular C a2+-dependent manner, presumably due to activation of voltage-sensitiv e Ca2+ channels resulting from the depolarizing effect of increased Na + influx. On the other hand, Na+ entry through tetrodotoxin (TTX)-sens itive, voltage-dependent Na+ channels is not involved in GnRH-induced GH release. Application of 250 nM TTX, which abolished the voltage-sen sitive Na+ currents in identified goldfish somatotropes, did not affec t the acute GH responses to 5-min pulses of sGnRH and cGnRH-II. The po ssible 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 dimethylam iloride (DMA). Likewise, the GH response to the PKC activator, tetrade canoyl phorbol acetate, was attenuated by treatment with Na+-depleted medium, amiloride, and DMA. The inhibitory actions of amiloride and DM A were selective as these drugs did not affect the GH response elicite d by the Ca2+ ionophore ionomycin and the voltage-sensitive Ca2+ chann el agonist, Bay K 8644. Taken together, these results indicate that ex tracellular Na+ and the Na+/H+ exchanger are involved in the mediation of GnRH-stimulated GH release in goldfish. Furthermore, this dependen ce on Na+ and Na+/H+ antiport probably occurs distal to the activation of PKC by GnRH.