ROLES OF CALCIUM AND CALMODULIN IN THE MEDIATION OF ACUTE AND SUSTAINED GNRH-STIMULATED GONADOTROPIN-SECRETION FROM DISPERSED GOLDFISH PITUITARY-CELLS

The possible involvement of extracellular Ca2+ ([Ca2+](o)) in mediatin g the acute gonadotropin (GtH) response to salmon gonadotropin-releasi ng hormone (sGnRH) and chicken gonadotropin-releasing hormone-II (cGnR H-II) in goldfish was examined using dispersed pituitary cells in peri fusion. Perifusion with Ca2+-deficient medium reduced the GtH response s to 5-min pulses of either GnRH, indicating the participation of [Ca2 +](o) in acute GnRH action. Using a 10-min GnRH pulse application prot ocol, the dependence of the acute GtH responses to the two GnRHs on [C a2+](o) entry through voltage-sensitive Ca2+ channels (VSCC) was exami ned using the dihydropyridine VSCC blocker nifedipine and the cation C o2+. Treatment with nifedipine consistently reduced the acute GtH resp onse to either sGnRH or cGnRH-II. Similarly, perifusion with CoCl2, re duced the sGnRH-induced GtH release. In contrast to its effects on sGn RH, CoCl2 abolished the cGnRH-II-induced GtH release. These results in dicate that [Ca2+](o) entry through VSCC participates in the acute GtH response to both native GnRHs; however, the cGnRH-II-stimulated acute release is relatively more dependent on [Ca2+](o) and VSCC functions than sGnRH-induced secretion. The involvement of calmodulin (CaM) in m ediating GnRH action was also examined. Treatment with a CaM antagonis t, calmidazolium, or with a Ca2+/CaM-dependent protein kinase II inhib itor, KN62, reduced GtH responses to sGnRH and cGnRH-II in 2-hr static incubation, but not in perifusion studies with dispersed goldfish pit uitary cells using 5- or 10-min GnRH pulses. These results suggest tha t CaM-dependent mechanisms participate in mediating the long-term, but not the acute, GtH response to GnRH. Compared to sGnRH, cGnRH-II-indu ced GtH release was more sensitive to inhibition by KN62, indicating a higher degree of dependence of cGnRH-II action on CaM. These results extend our understanding of the differential involvement of [Ca2+](o) and CaM in mediating the short-term and long-term actions of the two n ative GnRH peptides on GtH release in goldfish, (C) 1996 Academic Pres s, Inc.