Stimulatory Effect of Ghrelin on Isolated Porcine Somatotropes

Abstract

Research on the mechanism for growth hormone secretagogue (GHS) induction of growth hormone secretion led to the discovery of the GHS receptor (GHS-R) and later to ghrelin, an endogenous ligand for GHS-R. The ability of ghrelin to induce an increase in the intracellular Ca²⁺ concentration – [Ca²⁺]_i – in somatotropes was examined in dispersed porcine pituitary cells using a calcium imaging system. Somatotropes were functionally identified by application of human growth hormone releasing hormone. Ghrelin increased the [Ca²⁺]_i in a dose-dependent manner in 98% of the cells that responded to human growth hormone releasing hormone. In the presence of ( D-Lys³)-GHRP-6, a specific receptor antagonist of GHS-R, the increase in [Ca²⁺]_i evoked by ghrelin was decreased. Pretreatment of cultures with somatostatin or neuropeptide Y reduced the ghrelin-induced increase of [Ca²⁺]_i. The stimulatory effect of ghrelin on somatotropes was greatly attentuated in low-calcium saline and blocked by nifedipine, an L-type calcium channel blocker, suggesting involvement of calcium channels. In a zero Na⁺ solution, the stimulatory effect of ghrelin on somatotropes was decreased, suggesting that besides calcium channels, sodium channels are also involved in ghrelin-induced calcium transients. Either SQ-22536, an adenylyl cyclase inhibitor, or U73122, a phospholipase C inhibitor, decreased the stimulatory effects of ghrelin on [Ca²⁺]_i transiently, indicating the involvement of adenylyl cyclase-cyclic adenosine monophosphate and phospholipase C inositol 1,4,5-trisphosphate pathways. The nonpeptidyl GHS, L-692,585 (L-585), induced changes in [Ca²⁺]_i similar to those observed with ghrelin. Application of L-585 after ghrelin did not have additive effects on [Ca²⁺]_i. Preapplication of L-585 blocked the stimulatory effect of ghrelin on somatotropes. Simultaneous application of ghrelin and L-585 did not cause an additive increase in [Ca²⁺]_i. Our results suggest that the actions of ghrelin and synthetic GHS closely parallel each other, in a manner that is consistent with an increase of hormone secretion.