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

Abstract

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 (I(Na)) were characterized in whole cell patch-clamp experiments. GHS treatment significantly increased I(Na) 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 Ca(2+) channel activity decreased I(Na) density and prevented the effects of the GHS, whereas long-term exposure to an L-channel agonist increased I(Na) density and enhanced the actions of GHRP-6, indicating that Ca(2+) entry through these channels plays a role in the regulation of Na(+) channel expression. Likewise, GHRP-6 failed to enhance Na(+) channel expression in the presence of membrane-permeable inhibitors of protein kinases A and C, as well as the Ca(2+)/calmodulin-dependent kinase II. Conversely, treatment with a cAMP analog or a protein kinase C activator enhanced both basal and GHS-induced secretion of GH measured by enzyme-linked immunoassay, suggesting that GHRP-6 acting through the ghrelin receptor and different signaling pathways enhances Na(+) channel membrane expression, which favors hormone release from GC somatotropes.