Growth hormone-releasing hexapeptide elevates intracellular calcium in rat somatotropes by two mechanisms

The actions of GH-releasing hexapeptide (His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 or GHRP-6) on single rat somatotropes were studied using whole cell patch clamp electrophysiology and indo-1 Ca2+ photometry. GHRP-6 elevated intracellular free Ca2+ ([Ca2+]i) in two phases: a rapid transient phase, followed by a persistent phase. Based on its insensitivity to treatments that block Ca2+ entry [removal of external Ca2+, addition of the dihydropyridine Ca2+ channel blocker nitrendipine (1 microM), and the hyperpolarizing action of zero external Na+ or 100 nM somatostatin], the transient elevation is the result of release of Ca2+ from intracellular stores. The half-maximal concentration for the peak [Ca2+]i rise during Ca2+ release was 49 nM GHRP-6. Prior treatment of cells with caffeine (10 mM) or ryanodine (50 microM) abolished or partially occluded GHRP-6-induced Ca2+ release. Simultaneous measurement of [Ca2+]i and membrane current or potential revealed that the transient release of Ca2+ by GHRP-6 activates a voltage-independent Ca(2+)-activated K+ conductance, which transiently hyperpolarizes the somatotrope. The GHRP-6-induced persistent [Ca2+]i elevation is abolished by removal of external Ca2+ or external Na+ or the addition of 1 microM nitrendipine or 100 nM somatostatin, consistent with Ca2+ entry through voltage-dependent Ca2+ channels. In nondialyzed cells (perforated patch recording), we have identified a long-lasting GHRP-6-induced depolarization which may be responsible for the persistent [Ca2+]i elevation.