Upregulation of voltage-gated Na channels by long-term activation of the ghrelin-growth hormone secretagogue receptor in clonal GC somatotropes

A central question in adenohypophyseal cell physiology concerns the role of transmembrane ionic fluxes in the initiation of the hormone secretion process. In the current report, we investigated the effects of the growth hormone (GH) secretagogues ghrelin and GH-releasing peptide-6 (GHRP-6) on the regulation of the functional expression of voltage-gated Na+ channels using the tumoral somatotrope GC cell line as a model. Cells were cultured under control conditions or in presence of the GH secretagogues (GHS) for 96 h, and Na+ currents (INa) were characterized in whole cell patch-clamp experiments. GHS treatment significantly increased INa density in a dose-dependent manner. The effects of GHRP-6 were accompanied by an augment in conductance without changes in the kinetics and the voltage dependence of the currents, suggesting an increase in the number of channels in the cell membrane. Sustained inhibition of L-type Ca2+ channel activity decreased INa density and prevented the effects of the GHS, whereas lo...