Obestatin is a bioactive peptide encoded by the same gene that encodes ghrelin. Our aim was to investigate the effect of obestatin on insulin secretion. We evaluated the effects of obestatin on insulin secretion from rat islet cells which had been incubated overnight in the presence of 8.3, 11.1, and 22.2 mmol/l of glucose. In vivo, the serum levels of glucose and insulin were measured 0, 1, 5, 10, 20, 40, and 60 min after the intravenous administration of saline or glucose (1 g/kg), with or without obestatin, and the area under the 60 min curve of insulin concentration (AUC(insulin)) Was calculated. Obestatin (0.01-100 nmol/l) inhibited insulin secretion from rat islets in a dose-dependent fashion. In vivo, when administered intravenously to rats together with glucose, obestatin (10, 50, and 250 nmol/kg) inhibited both the rapid I min insulin response and the AUCinsulin in a dose-dependent fashion. Our data demonstrate that under glucose-stimulated conditions, exogenous obestatin acts as a potent inhibitor of insulin secretion in anaesthetized rats in vivo as well as in cultured islets in vitro. (c) 2008 Elsevier Inc. All rights reserved.