Calcium mobilization in the pituitary gonadotrope: relative roles of intra- and extracellular sources.

GnRH stimulates pituitary gonadotropin release by a Ca+2-dependent mechanism. Indeed, while it is clear that Ca+2 fulfills the requirements of a second messenger, the relative roles of Ca+2 mobilized from intra- and extracellular sources have never been distinguished. In the present study we examined the requirements for intra- and extracellular Ca+2 by three different means. First, in static cultures we used a specific Ca+2 ion channel blocker, methoxyverapamil (D600), to block entry of extracellular Ca+2 into pituitary cell cultures to determine if brief elevation of intracellular Ca+2 (whether derived from external or internal sources) could support continued gonadotropin release. Studies over a wide range of GnRH concentrations indicated that blockade of Ca+2 entry into the gonadotrope (in the presence of continued occupancy of the GnRH receptor by the releasing hormone) resulted in termination of LH release. Second, compounds that stabilize intracellular Ca+2 (preventing its mobilization), such as 8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate-HCl (TMB-8) and dantrolene (Dantrium), were shown not to alter the potency or efficacy of GnRH in stimulating LH release. Third, we used a system of perifused cells to measure the actions of D600, EGTA, or the removal of GnRH on stimulated LH release to correlate precisely the release process with access to Ca+2 in the extracellular compartment. The results of these studies suggest that LH release in response to GnRH is primarily dependent on Ca+2 mobilized from extracellular sources. Termination of accessibility to this Ca+2 pool also results in termination of release. The data are consistent with a model in which GnRH occupancy of its receptor regulates a plasma membrane Ca+2 ion channel; continued access to the extracellular Ca+2 pool is required for continued LH release.