Relative Roles of Calcium Derived from Intra- and Extracellular Sources in Dynamic Luteinizing Hormone Release from Perifused Pituitary Cells

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Abstract
GnRH releases LH from pituitary gonadotropes by a calcium-dependent mechanism. Previous studies in static cell cultures have not revealed a role for intracellular-derived calcium during GnRH-stimulated LH release. In the present study we have reexamined this possibility using a perifusion system, which permits a more dynamic assessment of early cellular events. Chelation of extracellular calcium by EGTA and calcium channel blockade by methoxyverapamil prevented sustained LH release. A component of early LH release occurred independently of extracellular calcium mobilization. This previously unrecognized aspect of LH release was shown to be dependent upon intracellular calcium. The molecular mechanism by which this calcium-dependent signal is translated into a cellular response does not appear to be mediated by calmodulin or protein kinase C, whereas sustained LH release appears mediated by calmodulin. While calcium derived from extracellular sources is still viewed as the major extracellular sources is still viewed as the major messenger for sustained LH release, these experiments provide evidence for the involvement of intracellular-derived calcium during early GnRH-stimulated LH release.