Gonadotropin-Releasing Hormone Induced Ca Influx in Nonsecreting Pituitary Adenoma Cells: Role of Voltage-Dependent Ca Channels and Protein Kinase C

The action mechanism of gonadotropin-releasing hormone (GnRH) on the cytosolic free calcium concentration (Ca²⁺]_i) and high-threshold voltage-dependent Ca²⁺ channel activity was studied in human nonsecreting (NS) pituitary adenoma cells. Ca²⁺]_i was monitored in individual cells by dual emission microspectrofluorimetry using indo1 as intracellular fluorescent Ca²⁺ probe. The whole-cell recording patch-clamp technique was used to study Ca²⁺ channels. A short application of GnRH (1 to 100 nM) induced an increase in Ca²⁺]_i due to Ca²⁺ entry through plasma membrane voltage-sensitive L-type Ca²⁺ channels. Protein kinase C (PKC) depletion induced by a pretreatment with 1 μM PMA for 24 h abolished spontaneous Ca²⁺ transients and the action of GnRH on Ca²⁺]_i and Ca²⁺ channels. Phloretin (250 μM and staurosporine (20 nM), two protein kinase C inhibitors, inhibited Ca²⁺ channel activity, thereby suppressing the effect of GnRH. On the other hand, activation of PKC by a short application of phorbol myristate acetate (10 nM) stimulated Ca²⁺ influx through Ca²⁺ channels. These findings demonstrate that, in human NS adenoma cells, GnRH (1 to 100 nM) induces an increase in Ca²⁺]_i, principally due to Ca²⁺ entry through L-type voltage-activated Ca²⁺ channels. PKC regulates this mechanism as well as basal ion channel activity, thus exerting both positive and negative control of Ca²⁺]_i in stimulated and unstimulated NS adenoma cells.