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 Ca2+-free medium. Here we have tested the dependence of the transient effect on intracellular Ca2+ pools. In superfused pituitary cells three Ca2+-mobilizing stimuli (GnRH, A23187, and endothelin-1) all caused sustained increases in LH release in normal medium (plateau responses), but only transient increases in Ca2+-free medium (spike responses). In Ca2+-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 Ca2+-containing medium between the two GnRH stimulation periods. Heterologous desensitization between GnRH and A23187 and between GnRH and endothelin-1 also occurred in Ca2+-free medium. Thapsigargin, which inhibits the endoplasmic reticulum Ca2+-ATPase and thereby elevates cytosolic Ca2+, stimulated LH release (EC50, approximately 20-mu-M) in static culture, an effect which, unlike those of GnRH and A23187, was not markedly reduced in Ca2+-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 Ca2+-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.