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The Effect of GHRP‐6 on the Intracellular Na + Concentration of Rat Pituitary Cells in Primary Culture
Author(s) -
Masakatsu Kato,
Yasuo Sakuma
Publication year - 1999
Publication title -
journal of neuroendocrinology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.062
H-Index - 116
eISSN - 1365-2826
pISSN - 0953-8194
DOI - 10.1046/j.1365-2826.1999.00394.x
Subject(s) - thapsigargin , medicine , endocrinology , chemistry , bapta , extracellular , channel blocker , ouabain , intracellular , biophysics , biology , biochemistry , sodium , calcium , organic chemistry
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 Ca 2+ and protein kinase C but not cAMP are involved in the signal transduction pathway of the action of GHRP‐6. Ca 2+ ‐influx through voltage‐gated Ca 2+ channels and mobilization of internal stored Ca 2+ are thought to be responsible for an increase in cytosolic Ca 2+ concentration. For activation of the voltage‐gated Ca 2+ 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 Gd 3+ , 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 Ca 2+ ATPase, had no effect on the response; Ca 2+ 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 Ca 2+ channels. This process leads to an influx of Ca 2+ and subsequent increase in [Ca 2+ ] i which results in an exocytotic release of GH.