Effects of melatonin on fuel utilization in exercised rats: role of nitricoxide and growth hormone

We have previously reported that melatonin modifies carbohydrate and lipid utilization in exercised rats, maintaining glycemia 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 growth hormone (GH) secretion. Male Wistar rats received melatonin i.p. at a dose of 0.5 mg/kg body weight 30 min befo re 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 i ncrease in plasma urea, free fatty acid, P-hydroxybutyrate, and nitrite ind uced by exercise. Melatonin-treated exercised rats had significantly elevat ed liver glycogen content and hepatic tissue showed a lowered expression of both inducible and constitutive NO synthase (iNOS and cNOS). Administratio n of the NO inhibitor N-G-nitro-L-arginine (L-NAME) to exercised rats cause d a significant reduction in plasma nitrite, but liver glycogen and biochem ical parameters in blood did not significantly differ from untreated exerci sed animals, indicating the absence of a direct association between melaton in effects on fuel metabolism and NO levels. Although results of treatment with pyridostigmine, a cholinergic agonist drug that stimulates GH release, partially differed from that of melatonin, modulation of GH secretion coul d play a role in the metabolic actions of the hormone because effects of me latonin on exercised rats were almost completely blocked by simultaneous ad ministration of L-NAME.