Lack of melatonin effects on insulin action in normal rats.

Despite a large number of studies, the role of melatonin on glucose metabolism is still controversial. The aim of the present work was to further characterize the effect of melatonin on insulin action during: i) intravenous insulin tolerance test performed at different times of the day using melatonin, a melatonin agonist (S-20304), a melatonin antagonist (S-20928) or in pinealectomized rats. ii) euglycemic-hyperinsulinemic clamp performed in melatonin agonist-treated as well as in pinealectomized rats. The fall in glycemia after the insulin injection was not significantly affected by melatonin and melatonin agonist (S-20304) at ZT6, nor by the melatonin antagonist (S-20928) at ZT13 nor in pinealectomized animals at ZT6 in comparison to their respective control. Acute treatment with S-20304 or chronic suppression of melatonin by pinealectomy did not significantly alter basal plasma glucose and insulin levels or hepatic glucose production and whole body or individual tissue glucose utilization. These data do not give support to a crucial role of melatonin on insulin action in normal rats.