Diverse intracellular signalling systems used by growth hormone-releasing hormone in regulating voltage-gated Ca2+ or K+ channels in pituitary somatotropes

Influx of Ca2+ via Ca2+ channels is the major step triggering exocytosis of pituitary somatotropes to release growth hormone (GH). Voltage-gated Ca2and K+ channels, the primary determinants of the influx of Ca2+, are regula ted by GH-releasing hormone (GHRH) through G-protein-coupled intracellular signalling systems. Using whole-cell patch-clamp techniques, the changes of the Ca2+ and K+ currents in primary cultured ovine and human somatotropes were recorded. Growth hormone-releasing hormone (10 nmol/L) increased both L- and T-type voltage-gated Ca2+ currents. Inhibition of the cAMP/protein k inase A (PKA) pathway by either Rp-cAMP or H-89 blocked this increase in bo th L- and T-type Ca2+ currents. Growth hormone-releasing hormone also decre ased voltage-gated transient (I-A) and delayed rectified (I-K) K+ currents. Protein kinase C (PKC) inhibitors, such as calphostin C, chelerythrine or downregulation of PKC, blocked the effect of GHRH on K+ currents, whereas a n acute activation of PKC by phorbol 12,13-dibutyrate (1 mu mol/L) mimicked the effect of GHRH. Intracellular dialysis of a specific PKC inhibitor (PK C19-36) also prevented the reduction in K+ currents by GHRH. It is therefor e concluded that GHRH increases voltage-gated Ca2+ currents via cAMP/PKA, b ut decreases voltage-gated K+ currents via the PKC signalling system. The G HRH-induced alteration of Ca2+ and K+ currents augments the influx of Ca2+, leading to an increase in [Ca2+]i and the GH secretion.