Desensitization of gonadotropin-releasing hormone action in alphaT3-1 cells due to uncoupling of inositol 1,4,5-trisphosphate generation and Ca2+ mobilization.
Gonadotropin-releasing hormone (GnRH) acts via a G-protein coupled receptor on gonadotropes to increase cytosolic Ca2+ and stimulate gonadotropin secretion. Sustained exposure causes desensitization of these effects, but the GnRH receptor has no C-terminal tail and does not undergo rapid (<5 min) desensitization. Nevertheless, pretreatment of alphaT3-1 cells with GnRH reduced the spike Ca2+ response to GnRH and decreased the GnRH effect on inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) by 30-50%. Ca2+-free medium with or without thapsigargin also decreased GnRH-stimulated Ins(1,4,5)P3 generation, implying that attenuation of the Ca2+ response underlies the Ins(1,4,5)P3 reduction rather than vice versa. Intracellular Ca2+ pool depletion cannot explain desensitization of the Ca2+ response because pool depletion and repletion were faster (half-times, <1 min) than the onset of and recovery from desensitization (half-times 10-20 min and 4-6 h). Moreover, 1-h GnRH pre-treatment attenuated the spike Ca2+ response to GnRH but not that to ionomycin, and brief GnRH exposure in Ca2+-free medium reduced the response to ionomycin more effectively in controls than in desensitized cells. GnRH pretreatment also attenuated the Ca2+ response to PACAP38. This novel form of desensitization does not reflect uncoupling of GnRH receptors from their immediate effector system but rather a reduced efficiency of mobilization by Ins(1,4,5)P3 of Ca2+ from an intact intracellular pool.
McArdle
C A
CA
Department of Medicine, Bristol University, Bristol, BS2 8HW, United Kingdom.
Willars
G B
GB
Fowkes
R C
RC
Nahorski
S R
SR
Davidson
J S
JS
Forrest-Owen
W
W
D002118:Calcium / Q000502:physiology*; D002460:Cell Line; D003593:Cytoplasm / Q000378:metabolism; D015536:Down-Regulation; D007987:Gonadotropin-Releasing Hormone / Q000502:physiology*; D015544:Inositol 1,4,5-Trisphosphate / Q000502:physiology*; D015759:Ionomycin / Q000494:pharmacology; D009479:Neuropeptides / Q000494:pharmacology; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D010903:Pituitary Gland, Anterior / Q000502:physiology*; D011966:Receptors, LHRH / Q000502:physiology*; D015398:Signal Transduction; D010738:Type C Phospholipases / Q000502:physiology
D009479:Neuropeptides; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D011966:Receptors, LHRH; D007987:Gonadotropin-Releasing Hormone; D015759:Ionomycin; D015544:Inositol 1,4,5-Trisphosphate; D010738:Type C Phospholipases; D002118:Calcium