The effect of GHRP-6 on the intracellular Na+ concentration of rat pituitary cells in primary culture.
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.
Kato
M
M
Department of Physiology and Nippon Medical School, Tokyo, Japan. mkato@nms.ac.jp
Sakuma
Y
Y
D000818:Animals; D000893:Anti-Inflammatory Agents / Q000494:pharmacology; D002478:Cells, Cultured; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism; D005682:Gadolinium / Q000494:pharmacology; D006728:Hormones / Q000494:pharmacology*; D007424:Intracellular Fluid / Q000378:metabolism; D008297:Male; D009842:Oligopeptides / Q000494:pharmacology*; D010042:Ouabain / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism*; D013004:Somatostatin / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology
D000893:Anti-Inflammatory Agents; D002614:Chelating Agents; D004791:Enzyme Inhibitors; D006728:Hormones; D009842:Oligopeptides; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D013779:Tetrodotoxin; C041048:growth hormone releasing hexapeptide; D013004:Somatostatin; D004533:Egtazic Acid; D010042:Ouabain; D019284:Thapsigargin; D012964:Sodium; D005682:Gadolinium; C038958:gadolinium chloride