Ghrelin increases growth hormone production and functional expression of Na sub(V)1.1 and Na sub(V)1.2 channels in pituitary somatotropes

A variety of ion channels are expressed in the plasma membrane of somatotropes within the anterior pituitary gland. Modification of these channels is linked to intracellular Ca super(2+) levels and therefore to hormone secretion. Previous investigations have shown that the gut-derived orexigenic peptide hormone ghrelin and synthetic GH-releasing peptides (GHRPs) stimulate release of growth hormone (GH) and increase the number of functional voltage-gated Ca super(2+) and Na super(+) channels in the membrane of clonal GC somatotropes. Here, we reveal that chronic treatment with ghrelin and its synthetic analog GHRP-6 also increases GH release from bovine pituitary somatotropes in culture, and that this action is associated with a significant increase in Na super(+) macroscopic current. Consistent with this, Na super(+) current blockade with tetrodotoxin (TTX) abolished the ghrelin- and GHRP-6-induced increase in GH release. Furthermore, semi-quantitative and real-time RT-PCR analysis revealed an upregulation in the transcript levels of GH, as well as of Na sub(V)1.1 and Na sub(V)1.2, two isoforms of TTX-sensitive Na super(+) channels expressed in somatotropes, after treatment with ghrelin or GHRP-6. These findings improve our knowledge on (i) the cellular mechanisms involved in the control of GH secretion, (ii) the molecular diversity of Na super(+) channels in pituitary somatotropes, and (iii) the regulation of GH and Na super(+) channel gene expression by ghrelin and GHRPs.