The study shows that the peptide GHRP‑6 makes rat pituitary cells take up more sodium inside the cell, which then depolarizes the cell, opens calcium channels, and triggers growth‑hormone release. This sodium effect needs external sodium and is blocked by a generic cation‑channel blocker, but not by typical sodium‑channel or calcium‑handling drugs.

The objective of the present study was to further investigate the ionic mechanism of the action of GHRP-6 on male rat pituitary cells in culture. A synthetic hexapeptide, GHRP-6 stimulates the secretion of growth hormone both in vivo and in vitro. It is generally accepted that Ca2+ and protein kinase C but not cAMP are involved in the signal transduction pathway of the action of GHRP-6. Ca2+-influx through voltage-gated Ca2+ channels and mobilization of internal stored Ca2+ are thought to be responsible for an increase in cytosolic Ca2+ concentration. For activation of the voltage-gated Ca2+ channels, however, it is not determined whether the membrane Na+ permeability plays a role. To answer this question, we measured intracellular Na+ concentration of the pituitary cells with ion imaging technique. We found that GHRP-6 increased [Na+]i; the Na+ response depended on the presence of extracellular Na+ and was blocked by Gd3+, known as a blocker of nonselective cation channels but not by tetrodotoxin, a blocker of the voltage-gated Na+ channel; thapsigargin, an inhibitor of endoplasmic reticulum Ca2+ ATPase, had no effect on the response; Ca2+ chelating agent, BAPTA had no inhibitory effect on the response; ouabain, an inhibitor of Na+-K+ ATPase, did not block the rise in [Na+]i induced by GHRP-6; somatostatin, which hyperpolarizes the cells by activating K+ channels, suppressed the response. These data clearly showed that GHRP-6 increased [Na+]i in the rat pituitary cells including somatotrophs. The rise in [Na+]i is likely to be due to an increase in the membrane Na+ permeability which should depolarize the cells, thereby activating the voltage-gated Ca2+ channels. This process leads to an influx of Ca2+ and subsequent increase in [Ca2+]i which results in an exocytotic release of GH.