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1 Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1; and 2 Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Leptin acutely
increases fatty acid (FA) oxidation and triacylglycerol (TG) hydrolysis
and decreases TG esterification in oxidative rodent muscle. However,
the effects of chronic leptin administration on FA metabolism in
skeletal muscle have not been examined. We hypothesized that chronic
leptin treatment would enhance TG hydrolysis as well as the capacity to
oxidize FA in soleus (SOL) muscle. Female Sprague-Dawley rats were
infused for 2 wk with leptin (LEPT; 0.5 mg · kg
1 · day1) by use of
subcutaneously implanted miniosmotic pumps. Control (AD-S) and pair-fed
(PF-S) animals received saline-filled implants. Subsequently, FA
metabolism was monitored for 45 min in isolated, resting, and
contracting (20 tetani/min) SOL muscles by means of pulse-chase
procedures. Food intake (
33 ± 2%, P < 0.01)
and body mass (12.5 ± 4%, P = 0.01) were
reduced in both LEPT and PF-S animals. Leptin levels were elevated
(+418 ± 7%, P < 0.001) in treated animals
but reduced in PF-S animals (73 ± 8%, P < 0.05) relative to controls. At rest, TG hydrolysis was
increased in leptin-treated rats (1.8 ± 2.2, AD-S vs. 23.5 ± 8.1 nmol/g wet wt, LEPT; P < 0.001). In contracting
SOL muscles, TG hydrolysis (1.5 ± 0.6, AD-S vs. 3.6 ± 1.0 µmol/g wet wt, LEPT; P = 0.02) and palmitate
oxidation (18.3 ± 6.7, AD-S vs. 45.7 ± 9.9 nmol/g wet wt,
LEPT; P < 0.05) were both significantly increased by
leptin treatment. Chronic leptin treatment had no effect on TG
esterification either at rest or during contraction. Markers of overall
(citrate synthase) and FA (hydroxyacyl-CoA dehydrogenase) oxidative
capacity were unchanged with leptin treatment. Protein expression of
hormone-sensitive lipase (HSL) was also unaltered following leptin
treatment. Thus leptin-induced increases in lipolysis are likely due to
HSL activation (i.e., phosphorylation). Increased FA oxidation
secondary to chronic leptin treatment is not due to an enhanced
oxidative capacity and may be a result of enhanced flux into the
mitochondrion (i.e., carnitine palmitoyltransferase I regulation) or
electron transport uncoupling (i.e., uncoupling protein-3 expression).
pulse chase; hormone-sensitive lipase; citrate synthase; 
-hydroxyacyl-coenzyme A dehydrogenase
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