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 mediating the acute gonadotropin (GtH) response to salmon gonadotropin-releasing hormone (sGnRH) and chicken gonadotropin-releasing hormone-II (cGnRH-II) in goldfish was examined using dispersed pituitary cells in perifusion. Perifusion with Ca(2+)-deficient medium reduced the GtH responses to 5-min pulses of either GnRH, indicating the participation of [Ca2+]o in acute GnRH action. Using a 10-min GnRH pulse application protocol, the dependence of the acute GtH responses to the two GnRHs on [Ca2+]o entry through voltage-sensitive Ca2+ channels (VSCC) was examined using the dihydropyridine VSCC blocker nifedipine and the cation Co2+. Treatment with nifedipine consistently reduced the acute GtH response to either sGnRH or cGnRH-II. Similarly, perifusion with CoCl2 reduced the sGnRH-induced GtH release. In contrast to its effects on sGnRH, CoCl2 abolished the cGnRH-II-induced GtH release. These results indicate 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 mediating GnRH action was also examined. Treatment with a CaM antagonist, calmidazolium, or with a Ca2+/CaM-dependent protein kinase II inhibitor, KN62, reduced GtH responses to sGnRH and cGnRH-II in 2-hr static incubation, but not in perifusion studies with dispersed goldfish pituitary cells using 5- or 10-min GnRH pulses. These results suggest that CaM-dependent mechanisms participate in mediating the long-term, but not the acute, GtH response to GnRH. Compared to sGnRH, cGnRH-II-induced 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 native GnRH peptides on GtH release in goldfish.
Jobin
R M
RM
Department of Biological Sciences, University of Alberta, Edmonton, Canada.
Neumann
C M
CM
Chang
J P
JP
D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine / Q000031:analogs & derivatives*; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D002147:Calmodulin / Q000037:antagonists & inhibitors / Q000502:physiology*; D002478:Cells, Cultured; D003035:Cobalt / Q000494:pharmacology; D003470:Culture Media; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006054:Goldfish / Q000502:physiology*; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D006062:Gonadotropins / Q000378:metabolism*; D007093:Imidazoles / Q000494:pharmacology; D007546:Isoquinolines / Q000494:pharmacology; D008297:Male; D009543:Nifedipine / Q000494:pharmacology; D010879:Piperazines / Q000494:pharmacology; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*
D002121:Calcium Channel Blockers; D002147:Calmodulin; D003470:Culture Media; D004791:Enzyme Inhibitors; D006062:Gonadotropins; D007093:Imidazoles; D007546:Isoquinolines; D010879:Piperazines; D007987:Gonadotropin-Releasing Hormone; D003035:Cobalt; C031938:calmidazolium; C063302:KN 62; D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D017871:Calcium-Calmodulin-Dependent Protein Kinases; C018021:cobaltous chloride; D009543:Nifedipine; D002118:Calcium