S. Sanchez-campos et al., Effects of melatonin on fuel utilization in exercised rats: role of nitricoxide and growth hormone, J PINEAL R, 31(2), 2001, pp. 159-166
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.