Calcium Mobilization in the Pituitary Gonadotrope: Relative Roles of intra- and Extracellular Sources*

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Abstract
GnRH [gonadotropin releasing hormone] stimulates pituitary gonadotropin release by a Ca2+-dependent mechanism. While it is clear that Ca2+ fulfills the requirements of a 2nd messenger, the relative roles of Ca2+ mobilized from intra- and extracellular sources have never been distinguished. The requirements for intra- and extracellular Ca2+ were examined by 3 different means. In static cultures a specific Ca2+ ion channel blocker, methoxyverapamil (D600) was used to block entry of extracellular Ca2+ into pituitary cell cultures to determine if brief elevation of intracellular Ca2+ (whether derived from external or internal sources) could support continued gonadotropin release. Studies over a wide range of GnRH concentrations indicated that blockade of Ca2+ entry into the gonadotrope (in the presence of continued occupancy of the GnRH receptor by the releasing hormone) resulted in termination of LH [luteinizing hormone] release. Compounds that stabilize intracellular Ca2+ (preventing its mobilization), such as 8-(N,N-dimethylamino)octyl 3,4,5-trimethoxybenzoate-HCl (TMB-8) and dantrolene (Dantrium), did not alter the potency or efficacy of GnRH in stimulating LH release. A system of perifused cells was used 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 Ca2+ in the extracellular compartment. LH release in response to GnRH is primarily dependent on Ca2+ mobilized from extracellular sources. Termination of accessibility to this Ca2+ 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 Ca2+ ion channel; continued access to the extracellular Ca2+ pool is required for continued LH release.