Growth hormone-releasing factor induces a biphasic modulation of K+ permeability in perfused cultured rat somatotrophs.

Growth hormone-releasing factor (GRF) induced a biphasic response in 86Rb outflow in rat somatotrophs. An early inhibition was observed at all GRF concentrations tested, I pM-10 nM, and this was followed by a later increase. The increase in 86Rb outflow rate came earlier at high GRF concentrations and GRF had a dose-related effect on the magnitude of the increase. The effects of GRF on 86Rb outflow were observed also in the absence of extracellular calcium. Peak growth hormone release preceded peak 86Rb outflow rate. The timing of the effects of GRF on potassium outflow is consistent with the hypothesis that reduced potassium outflow is involved in somatotroph depolarization leading to increased exocytosis, followed by a later increase in potassium outflow leading to repolarization.