INSULIN RELEASE FROM ISOLATED MOUSE ISLETS INVITRO - NO EFFECT OF PHYSIOLOGICAL LEVELS OF MELATONIN OR ARGININE VASOTOCIN

Some data in the literature suggest that heightened activity of the pineal gland may be diabetogenic. The onset of insulin-dependent diabetes mellitus is highest during the winter months and at puberty when melatonin levels are also greatest. To study the direct effects of pineal hormones on insulin release, hand-dissected ob/ob-mouse islets of Langerhans were incubated in vitro with melatonin (1 nmol/l to 100-mu-mol/l) or arginine vasotocin (1 pmol/l to 10-mu-mol/l) and D-glucose (3 or 20 mmol/l for 1 hr. Melatonin did not affect basal or glucose-stimulated insulin release. Arginine vasotocin (AVT) did not affect basal insulin release, but at presumably pharmacological levels (1 and 10-mu-mol/l) the peptide significantly increased glucose-stimulated insulin release. We conclude that melatonin and AVT at physiological concentrations have no direct effect on islet insulin release, and that any diabetogenic effect of the pineal gland must occur via suppression of insulin action or via production of a metabolite or hormone that suppresses insulin release.