Ghrelin reduces voltage-gated potassium currents in GH3 cells via cyclic GMP pathways

Ghrelin is an endogeneous growth hormone secretagogue (GHS) causing release of GH from pituitary somatotropes through the GHS receptor. Secretion of GH is linked directly to intracellular free Ca2+ concentration ([Ca2+]i), which is determined by Ca2+ influx and release from intracellular Ca2+ storage sites. Ca2+ influx is via voltage-gated Ca2+ channels, which are activated by cell depolarization. Membrane potential is mainly determined by transmembrane K+ channels. The present study investigates the in vitro effect of ghrelin on membrane voltage-gated K+ channels in the GH3 rat somatotrope cell line. Nystatin-perforated patch clamp recording was used to record K+ currents under voltage-clamp conditions. In the presence of Co2+ (1 mM, Ca2+ channel blocker) and tetrodotoxin (1 µM, Na+ channel blocker) in the bath solution, two types of voltage-gated K+ currents were characterized on the basis of their biophysical kinetics and pharmacological properties. We observed that transient K+ current (I A) represented a significant proportion of total K+ currents in some cells, whereas delayed rectifier K+ current (I K) existed in all cells. The application of ghrelin (10 nM) reversibly and significantly decreased the amplitude of both I A and I K currents to 48% and 64% of control, respectively. Application of apamin (1 µM, SK channel blocker) or charybdotoxin (1 µM, BK channel blocker) did not alter the K+ current or the response to ghrelin. The ghrelin-induced reduction in K+ currents was not affected by PKC and PKA inhibitors. KT5823, a specific PKG inhibitor, totally abolished the K+ current response to ghrelin. These results suggest that ghrelininduced reduction of voltage-gated K+ currents in GH3 cells is mediated through a PKG-dependent pathway. A decrease in voltage-gated K+ currents may increase the frequency, duration, and amplitude of action potentials and contribute to GH secretion from somatotropes.