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Articles in PresS, published online ahead of print November 13, 2001
Am J Physiol Endocrinol Metab, 10.1152/ajpendo.00303.2001
Submitted on July 9, 2001
Accepted on November 8, 2001
1 Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada
2 Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
* To whom correspondence should be addressed. E-mail: ddyck{at}uoguelph.ca.
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 wks with leptin (LEPT; 0.5mg/kg/d) using subcutaneously implanted mini-osmotic pumps. Control (AD-S) and pairfed (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, using 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, CS) and FA (hydroxy-acyl-CoA dehydrogenase, ß-HAD) 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. CPT I regulation) or electron transport uncoupling (i.e. UCP3 expression).
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