Dependence of gonadotropin-releasing hormone-stimulated luteinizing hormone release upon intracellular Ca2+ pools is revealed by desensitization and thapsigargin blockade.
Sustained GnRH-stimulated LH release requires extracellular Ca2+, but GnRH transiently increases LH release in Ca(2+)-free medium. Here we have tested the dependence of the transient effect on intracellular Ca2+ pools. In superfused pituitary cells three Ca(2+)-mobilizing stimuli (GnRH, A23187, and endothelin-1) all caused sustained increases in LH release in normal medium (plateau responses), but only transient increases in Ca(2+)-free medium (spike responses). In Ca(2+)-free medium, GnRH (10(-10) or 10(-9) M) increased LH release transiently and desensitized the cells to the LH-releasing effect of subsequent stimulation with 10(-7) M GnRH. This desensitization was reversed by brief exposure to Ca(2+)-containing medium between the two GnRH stimulation periods. Heterologous desensitization between GnRH and A23187 and between GnRH and endothelin-1 also occurred in Ca(2+)-free medium. Thapsigargin, which inhibits the endoplasmic reticulum Ca(2+)-ATPase and thereby elevates cytosolic Ca2+, stimulated LH release (EC50, approximately 20 microM) in static culture, an effect which, unlike those of GnRH and A23187, was not markedly reduced in Ca(2+)-free medium. Low doses of thapsigargin, which had no effect on LH release alone, inhibited both sustained GnRH-stimulated LH release from static cultures in normal medium and transient GnRH-stimulated LH release from cells superfused in Ca(2+)-free medium. These data suggest that the spike phase of GnRH-stimulated LH release is not only associated with but is also dependent upon the mobilization of a GnRH- and thapsigargin-sensitive intracellular Ca2+ pool and that the Ca2+ pool mediating this GnRH effect is identical to or substantially interchangeable with A23187- and endothelin-1-mobilizable intracellular Ca2+ pools. Inhibition of sustained GnRH-stimulated LH release by thapsigargin also suggests the involvement of an intracellular Ca2+ pool in this phase of GnRH action.
McArdle
C A
CA
Institute for Hormone and Fertility Research, Hamburg, Germany.
Poch
A
A
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors*; D002478:Cells, Cultured; D016232:Endothelins / Q000494:pharmacology; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007700:Kinetics; D007986:Luteinizing Hormone / Q000378:metabolism*; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D013729:Terpenes / Q000494:pharmacology*; D019284:Thapsigargin; D013997:Time Factors
D016232:Endothelins; D013729:Terpenes; D007987:Gonadotropin-Releasing Hormone; D000001:Calcimycin; D019284:Thapsigargin; D007986:Luteinizing Hormone; D000252:Calcium-Transporting ATPases; D002118:Calcium