Effects of ghrelin on the insulin and glycemic responses to glucose, arginine, or free fatty acids load in humans
Ghrelin possesses central and peripheral endocrine actions including influence on the endocrine pancreatic function. To clarify this latter ghrelin action, in seven normal young subjects [age (mean ± SEM), 28.3 ± 3.1 yr; body mass index, 21.9 ± 0.9 kg/m2), we studied insulin and glucose levels after acute ghrelin administration (1.0 μg/kg iv) alone or combined with glucose [oral glucose tolerance test (OGTT), 100 g orally], arginine (ARG, 0.5 g/kg iv) or free fatty acid (FFA, Intralipid 10%, 250 ml). Ghrelin inhibited (P < 0.05) insulin and increased (P < 0.05) glucose levels. OGTT increased (P < 0.01) glucose and insulin levels. FFA increased (P < 0.05) glucose but did not modify insulin levels. ARG increased (P < 0.05) both insulin and glucose levels. Ghrelin did not modify both glucose and insulin responses to OGTT as well as the FFA-induced increase in glucose levels; however, ghrelin administration was followed by transient insulin decrease also during FFA. Ghrelin blunted (P < 0.05) the insulin response to ARG and enhanced (P < 0.05) the ARG-induced increase in glucose levels. In all, ghrelin induces transient decrease of spontaneous insulin secretion and selectively blunts the insulin response to ARG but not to oral glucose load. On the other hand, ghrelin raises basal glucose levels and enhances the hyperglycemic effect of ARG but not that of OGTT. These findings support the hypothesis that ghrelin exerts modulatory action of insulin secretion and glucose metabolism in humans.