Calcium influx through L-type channels is required for selective activation of extracellular signal-regulated kinase by gonadotropin-releasing hormone

The hypothalamic decapeptide gonadotropin-releasing hormone stimulates mobi lization of two discrete pools of calcium in clonal (alpha T3-1) and primar y pituitary gonadotropes, A multidisciplinary approach was implemented to i nvestigate the effects of discrete calcium fluctuations on the signaling pa thways linking the gonadotropin-releasing hormone receptor to activation of mitogen-activated protein kinases and immediate early genes. Blockade of c alcium influx through nifedipine-sensitive voltage-gated calcium channels r educed buserelin-induced activation of extracellular signal-regulated kinas e (ERK) and c-Fos while activation of c-Jun N-terminal kinase and c-Jun was unaffected, Inhibition of buserelin-stimulated ERK activity by nifedipine was also observed in rat pituitary cells in primary culture. Direct activat ion of alpha T3-1 cell L-type calcium channels with the agonist Bay-K 8644 resulted in phosphorylation of ERK. and induction of c-Fos. However, simple voltage-induced channel activation did not produce a sufficient calcium si gnal, since depolarization with 35 mM KCl failed to induce activation of ER R, Depletion of intracellular calcium stores with thapsigargin did not affe ct buserelin-induced ERR activation. An inhibitor of protein kinase C decre ased calcium influx through nifedipine-sensitive calcium channels and phosp horylation of ERK induced by buserelin. pharmacological inhibition of prote in kinase C did not block Bay-K 8644-induced ERK activation. These observat ions suggest that calcium influx through L-type channels is required for Gn RH-induced activation of ERK and c-Fos and that the influence of calcium li es downstream of protein kinase C.