Effects of ghrelin and other neuropeptides (CART, MCH, orexin A and B, and GLP-1) on the release of insulin from isolated rat islets

Background and Aims: Ghrelin, a neuropeptide containing 28 amino acids, shows a reciprocal diurnal plasma fluctuation to that of plasma insulin. The aim of this study is to clarify the dose- and glucose-dependency of ghrelin on the insulin secretion and to compare its effect with that of other neuropeptides - GLP-1, CART (55-102), CART (55-76), CART (62-76), MCH, orexin A, and B. Materials and Methods: Rat islets were incubated with 1 pmol/l-1 mumol/l of ghrelin, CART fragments, MCH, orexin A or B, or GLP-1 ( n = 16 - 32) in the presence of 16.7 mmol/l glucose. Ghrelin ( 10 nmol/l) was added to islets at glucose concentrations of 3.3, 6.6, 16.7 and 25 mmol/l, respectively ( n = 28 - 32). Also, INS-1E cells were incubated with ghrelin ( 1 nmol/l) in the presence of glucose ( 3.3, 6.6, 16.7, and 25 mmol/l). In addition, we measured the mRNA expression of the ghrelin receptor using RT-PCR. Results: Ghrelin inhibited insulin secretion from islets and INS-1E cells in a dose- and glucose-dependent manner. Neither 10 pmol/l-1 mumol/l of CART fragments, MCH, orexin A, nor orexin B changed the insulin secretion at 16.7 mmol/l glucose, while GLP-1, as expected, stimulated the insulin release from rat islets. Interestingly, ghrelin receptors were expressed both in islets, INS-1E, MIN 6 and alpha cell Tca-9 lines. Conclusions: Ghrelin inhibits the insulin secretion in vitro in a dose- and glucose-dependent manner. Beta cells contain ghrelin receptors. CART fragments did not affect the insulin secretion. Ghrelin may play a physiological role for the regulation of insulin secretion.