DIFFERENCES IN EXTRACELLULAR CALCIUM INVOLVEMENT MEDIATING THE SECRETION OF GONADOTROPIN AND GROWTH-HORMONE STIMULATED BY 2 CLOSELY RELATED ENDOGENOUS GNRH PEPTIDES IN GOLDFISH PITUITARY-CELLS

Two endogenous gonadotropin-releasing hormone (GnRH) peptides, salmon GnRH (sGnRH) and chicken GnRH II (cGnRH II), stimulate gonadotropin (GtH) and growth hormone (GH) secretion in the goldfish. The extracellular calcium (e-Ca2+) dependence of the GtH and GH response to the two GnRH peptides were compared using static incubations of dispersed goldfish pituitary cells. Incubation with Ca2+-depleted medium (without the addition of Ca2+ salts and in the presence of EGTA) did not alter basal GtH secretion, but reduced the GtH response to sGnRH, and abolished the cGnRH II-induced GtH release. Blockade of e-Ca2+ entry by low concentrations of CoCl2 had no effect on basal GtH secretion but reduced cGnRH II and sGnRH stimulated GtH release when applied at 0.1 and 0.5 mM concentrations, respectively. In general, treatments with voltage-sensitive Ca2+ channel (VSCC) antagonists, verapamil, nifedipine and nicardipine, did not alter basal GtH release but attenuated GnRH-stimulated GtH responses. cGnRH II-induced GtH release was decreased by 10 nM verapamil and 1 nM nifedipine, whereas the reduction of GtH responses to sGnRH required 100 times higher concentrations of these VSCC antagonists. cGnRH II but not sGnRH stimulation of GtH secretion was also abolished by 10-mu-M nicardipine. In contrast to GtH release, exposure to Ca2+-depleted medium reduced basal GH release and abolished the GH responses to both GnRH peptides. sGnRH and cGnRH II-stimulated GH responses were both abolished by 0.1 mM CoCl2, decreased by 1 nM verapamil, and reduced by 10 nM nicardipine. Addition of 0.1 and 10-mu-M nifedipine inhibited the GH responses to sGnRH and cGnRH II, respectively. Basal GH release was not affected by the VSCC antagonists tested. Results from this study indicate that entry of e-Ca2+, in part through VSCC, is involved in GnRH stimulation of GtH and GH release from gold-fish gonadotropes and somatotropes; however, the e-Ca2+ dependence of the GtH and GH responses to the two endogenous GnRHs differ. The stimulatory effects of cGnRH II on GtH secretion is more dependent on and sensitive to e-Ca2+ than sGnRH. Whereas the sensitivity of GH responses to manipulations of e-Ca2+ availability is, in most instances, similar for both GnRH peptides. These results further suggest that basal secretion of GH is more sensitive to e-Ca2+ than basal GtH release; however, VSCC are not involved in the maintenance of basal release of either hormone.