Gonadotropin-Releasing Hormone-Induced Rise in Cytosolic Free Ca2+ Levels: Mobilization of Cellular and Extracellular Ca2+ Pools and Relationship to Gonadotropin Secretion

Abstract

Addition of GnRH to pituitary gonadotrophs preloaded with Quin 2 resulted in a rapid (~8 s) mobilization of an ionomycin-sensitive intracellular Ca2+ pool. A second component of Ca2+ entry via voltage dependent channels contributed about 45% of the peak cytosolic free Ca2+ concentration ([Ca2+],). Thereafter, influx of Ca2+ via voltage-sensitive and -insensitive channels is responsible for maintenance of elevated [Ca2+], during the second phase of GnRH action. Addition of inositol 1, 4, 5-trisphosphate (IP3) to permeabilized pituitary cells resulted in a Ca2+ transient, released from a nonmitochondrial pool, which maintained ambient free Ca2+ concentration around 170 mu in an ATP-dependent mechanism. Successive stimulations of the cells with IP3 produced an attenuated response. Elevation of the gonadotroph [Ca2+]i by ionomycin, to levels equivalent to that induced by GnRH, resulted in LH release amounting to only 45% of the response to the neurohormone. Activation of the voltage-dependent Ca2+ channels by the dihydropyridine Ca2+- agonist [methyl 1, 4-dihydro-2, 6-dimethyl-3-nitro-4-(2-tri-fluoromethylphenyl)-pyridine-5-carboxylate (BAYK8644)] stimulated LH release, 36% of the GnRH (100 nm) response being reached by 10-8 m of the drug, both [Ca2+], elevation and GnRH-induced LH released were inhibited similarly (40-50%) by the dihydropyridine Ca2+- antagonist nifedipine. The results indicate that peak [Ca2+]i induced by GnRH in pituitary gonadotrophs is derived mainly from ionomycin-sensitive cellular stores most likely via IP3 formation. L type voltage-sensitive Ca2+ channels contribute partially to the peak [Ca2+]i, and may participate with other types of Ca2+ channels (i.e. transient T channels and voltageinsensitive channels) in the secondary elevation of [Ca2+]i and gonadotropin secretion. Influx of Ca2+ is necessary but not sufficient to mediate GnRH-stimulation of gonadotropin secretion. © 1988 by The Endocrine Society.