Sodium fluoride provokes gonadotrope desensitization to gonadotropin-releasing hormone (GnRH) and gonadotrope sensitization to A23187: evidence for multiple G proteins in GnRH action.
Pretreatment of pituitary cell cultures with GnRH causes altered gonadotrope responsiveness to LH secretagogues. The precise mechanism by which this occurs is not understood. Because a G protein appears to be activated after GnRH stimulation of the gonadotrope, a role for this moiety in GnRH-stimulated alterations in gonadotrope responsiveness was assessed. We show that 3 h pretreatment of pituitary cell cultures with 10 mM NaF (a G protein activator), resulted in decreased gonadotrope responsiveness to subsequent GnRH treatment (3 h, 100 nM; 34.4 +/- 1.6% vs. 23.4 +/- 1.5% of total cellular LH). NaF-provoked gonadotrope desensitization to GnRH also occurred in the presence of 3 mM EGTA and in cells which had been depleted of protein kinase C. Desensitization to GnRH did not occur in response to pretreatment with (Bu)2cAMP (8 h, 1 mM). In addition, neither GnRH nor NaF stimulated inositol phosphate production above basal levels after the NaF pretreatment. GnRH receptor binding also decreased by 30% with NaF pretreatment. In contrast, 3 h NaF (10 mM) pretreatment enhanced responsiveness of the gonadotrope to the Ca2+ ionophore A23187 in a protein kinase C- and cAMP-dependent manner. Responsiveness to the phorbol ester, phorbol 12-myristate 13-acetate, was also increased, whereas responsiveness to the Ca2+ channel activator maitotoxin was unchanged. These data suggest that G protein activation by NaF provokes gonadotrope desensitization to GnRH stimulation by both decreasing receptor numbers and by uncoupling of the receptors from inositol phosphate production. In addition, a distinct G protein action appears to be involved in sensitizing the gonadotrope to A23187 and phorbol 12-myristate 13-acetate.
Hawes
B E
BE
Department of Pharmacology, University of Iowa College of Medicine, Iowa City 52242-1109.
Marzen
J E
JE
Waters
S B
SB
Conn
P M
PM
D000818:Animals; D001386:Azides / Q000494:pharmacology; D003994:Bucladesine / Q000494:pharmacology; D000001:Calcimycin / Q000494:pharmacology*; D002478:Cells, Cultured; D004533:Egtazic Acid / Q000494:pharmacology; D005260:Female; D019204:GTP-Binding Proteins / Q000378:metabolism*; D007987:Gonadotropin-Releasing Hormone / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D007295:Inositol Phosphates / Q000378:metabolism; D007700:Kinetics; D007986:Luteinizing Hormone / Q000378:metabolism*; D008954:Models, Biological; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism*; D011493:Protein Kinase C / Q000037:antagonists & inhibitors; D051381:Rats; D011919:Rats, Inbred Strains; D011966:Receptors, LHRH / Q000187:drug effects / Q000378:metabolism; D019810:Sodium Azide; D012966:Sodium Cyanide / Q000494:pharmacology; D012969:Sodium Fluoride / Q000494:pharmacology*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D001386:Azides; D007295:Inositol Phosphates; D011966:Receptors, LHRH; D007987:Gonadotropin-Releasing Hormone; D000001:Calcimycin; D004533:Egtazic Acid; D003994:Bucladesine; D012969:Sodium Fluoride; D007986:Luteinizing Hormone; D019810:Sodium Azide; D011493:Protein Kinase C; D019204:GTP-Binding Proteins; D013755:Tetradecanoylphorbol Acetate; D012966:Sodium Cyanide