Growth hormone-releasing hormone decreases voltage-gated potassium currents in GH4C1 cells

The electrophysiological properties of anterior pituitary somatotropes integrally involve the function of voltage-gated K⁺ currents. In this study, we have used GH4C1 cell lines to investigate the effect of human GHRH on voltage-gated K⁺ currents. Because of a clear 'rundown' of the K⁺ current with classic whole cell recording (WCR) without ATP in pipette solution, nystatin-perforated WCR was the major recording configuration used. Using a low Ca²⁺ (0.5 mM) bath solution containing Co²⁺ (1 mM) and TTX (1 μM), GH4 cells predominantly exhibited an outward delayed rectifier K⁺ current (I(K)). Local application of growth hormone releasing hormone (GHRH) (100 nM) reversibly reduced the amplitude of the K⁺ currents (to 83% of control). There was no effect of GHRH on the activation curve of the K⁺ current and no difference observed using 2.5 mM Ca²⁺ or low Ca²⁺ (0.5 mM Ca²⁺ + 1 mM Co²⁺) bath solutions. Under the condition of low Ca²⁺ bath solution, the application of apamin (1 μM) or charybdotoxin (1 μM), two specific blockers of the Ca²⁺-activated K⁺ current, did not alter the K⁺ current or the response to GHRH. This reduction in the K⁺ current by GHRH was also observed with classic WCR with a pipette solution containing ATP (2 mM). The GHRH-induced reduction in the K⁺ current was completely abolished by the presence of GDP-β-s (500 μM) in the pipette solution or by addition of PKC inhibitors, calphostin C (100 nM) and chelerythrine (1 μM), in bath solution. Inhibitor for cAMP-PKA system (Rp-cAMP and H₈₉) did not affect the K⁺ current response to GHRH. These results suggest that GHRH reduces the voltage-gated K⁺ current in GH4C1 cells, a response that is mediated by G-proteins and PKC system but not by cAMP-PKA system. The reduction in the K⁺ current may partially contribute to the GHRH-stimulated growth hormone (GH) secretion.