Abstract
We have have previously reported that melatonin modifies carbohydrate and lipid utilisation in exercised rats, maintaining glycaemia and reducing plasma and liver lactate and plasma beta-hydroxybutyrate. This study was undertaken to determine whether effects on fuel metabolism were related to changes in nitric oxide (NO) production or GH secretion. Male Wistar rats received melatonin i.p. at a dose of 0.5 mg/kg body weight 30 min before being exercised to exhaustion on a treadmill at a speed of 24 m/min and a 12% slope. Melatonin ameliorated the decrease in plasma glucose and the increase in plasma urea, free fatty acid, b-hydroxybutyrate and nitrite induced by exercise. Melatonin-treated exercised rats had significantly elevated liver glycogen content and hepatic tissue showed a lowered iNOS expression. Administration of the NO inhibitor L-NAME to exercised rats caused a significant reduction in plasma nitrite and iNOS expression, but liver glycogen and biochemical parameters in blood did not significantly differ from untreated exercised animals, indicating the absence of a direct association between melatonin effects on fuel metabolism and NO levels. Although results of treatment with pyridostigmine, a cholinergic agonist drug that stimulates GH release, differed from that of melatonin, modulation of GH secretion could play a role in the metabolic actions of the hormone because effects of melatonin on exercised rats were almost completely blocked by simultaneous administration of L-NAME.

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