Stimulation of luteinizing hormone release by sodium fluoride is independent of protein kinase-C activity and unaffected by desensitization to gonadotropin-releasing hormone.
GnRH stimulates secretion of pituitary LH by increasing intracellular calcium. Increased calcium may result from activation of phospholipase-C, since there is an increase in inositol phosphates and diacylglycerol, and a redistribution of protein kinase-C (PKC) from cytosolic to a particulate cell fraction in GnRH-stimulated pituitary cultures. A GTP-binding protein (G-protein) may mediate GnRH actions, since GTP stimulates LH release in permeabilized gonadotropes and decreases receptor affinity for a GnRH analog. In the present study we have used sodium fluoride, an exogenous activator of G-proteins, to investigate the possibility of a G-protein link between GnRH receptor activation, phospholipase-C activity, and LH release. Treatment of primary pituitary cell cultures from immature female rats with sodium fluoride stimulated the release of 20% total cellular LH and increased inositol phosphate accumulation. Sodium fluoride-stimulated LH release was insensitive to cholera toxin and pertussis toxin. Sodium fluoride-stimulated LH release was additive with a maximally effective concentration of phorbol 12-myristate 13-acetate and was not inhibited by depletion of cellular PKC, suggesting that PKC does not mediate sodium fluoride effects. Treatment of cultures with 3 mM EGTA and 10 nM GnRH for 5 and 16 h reduced pituitary responsiveness to subsequent treatment with GnRH, but had no effect on sodium fluoride-stimulated LH release. Although the precise mechanism of sodium fluoride-stimulated LH release remains to be described, our results support a role for a G-protein in regulation of LH release by the releasing hormone.
Waters
S B
SB
Department of Pharmacology, University of Iowa College of Medicine, Iowa City 52242-1109.
Hawes
B E
BE
Conn
P M
PM
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002478:Cells, Cultured; D004533:Egtazic Acid / Q000494:pharmacology; D005260:Female; D019204:GTP-Binding Proteins / Q000502:physiology*; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007295:Inositol Phosphates / Q000378:metabolism; D007986:Luteinizing Hormone / Q000378:metabolism*; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D011966:Receptors, LHRH / Q000378:metabolism; D012969:Sodium Fluoride / Q000494:pharmacology*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D010738:Type C Phospholipases / Q000378:metabolism
D002121:Calcium Channel Blockers; D007295:Inositol Phosphates; D011966:Receptors, LHRH; D007987:Gonadotropin-Releasing Hormone; D004533:Egtazic Acid; D012969:Sodium Fluoride; D007986:Luteinizing Hormone; D011493:Protein Kinase C; D010738:Type C Phospholipases; D019204:GTP-Binding Proteins; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium