Laurence Bresson-Bepoldin1, L. Dufy-Barbe
1Laboratoire de Neurophysiologie, Universite de Bordeaux, France.
Tóm tắt
GHRP6 is a synthetic hexapeptide which stimulates growth hormone (GH) secretion from the pituitary in vivo and in vitro. We have previously shown that in identified somatotrophs, GHRP6 induces a biphasic increase in cytosolic Ca2+ concentration ([Ca2+]i) consisting of an abrupt increase (first phase) followed by a sustained plateau of elevated [Ca2+]i (second phase). The first phase corresponds to mobilization of intracellular Ca2+ pools and the second phase to influx of extracellular Ca2+ ions through voltage‐sensitive Ca2+ channels. In these experiments, we investigated the specific role of each of these two phases in the hormone response to GHRP6. We found that inhibition by thapsigargin of the intracellular Ca2+ mobilization phase significantly inhibited the hormone response to the peptide during 30 min incubations. Inhibition of the extracellular Ca2+ influx phase by nifedipine, a blocker of voltage‐sensitive Ca2+ channels, resulted in a 53% reduction of the secretory response to 10−5 M GHRP6. Antagonism of PKC by phloretin, a flavonoid which prevents PKC activation, and PKC depletion induced by a 24 h treatment with 10−6 M PMA, completely inhibited the response to GHRP6. Somatostatin, which also inhibits the second phase of the Ca2+ response, suppressed the secretory response to GHRP6. We conclude that, Ca2+ is the main second messenger and both Ca2+ mobilization and Ca2+ entry play a role in the response to GHRP6. However, experiments with PKC depletion and SRIF suggest that other messengers are implicated in GHRP6 signalling in somatotrophs.
