High-fat diet feeding elevates skeletal muscle uncoupling protein 3 levelsbut not its activity in rats

Objective: The objective of this study is to test the impact of high-fat di et (HFD) feeding on skeletal muscle (SM) uncoupling protein 3 (UCP3) expres sion and its association with mitochondrial ion permeability and whole-body energy homeostasis. Research Methods and Procedures: Sprague-Dawley rats were fed ad libitum ei ther a HFD (60% of energy from fat, n = 6) or a low-fat diet (12% of energy from fat, n = 6) for 4 weeks. Twenty-four-hour energy expenditure was meas ured by indirect calorimetry in the last week of the dietary treatment. Blo od samples were collected for plasma leptin and free fatty acid assays, and mitochondria were isolated from hindlimb SM for subsequent determinations of UCP3 levels and mitochondrial ion permeability. Results: Plasma leptin levels were higher in rats fed the HFD despite the s ame body weight in two groups. The same dietary treatment also rendered a 2 -fold increase in plasma free fatty acid and SM UCP3 protein levels (Wester n blot) compared with the group fed the low-fat diet. However, the elevated UCP3 protein levels did not correlate with mitochondrial swelling rates, a measure of mitochondrial chloride, and proton permeability, or with 24-hou r energy expenditure. Discussion: The high correlation between the levels of plasma free fatty ac id levels and SM UCP3 suggests that circulating free fatty acid may play an important role in UCP3 expression during the HFD feeding. However, the dis sociation between the UCP3 protein levels and 24-hour energy expenditure as well as mitochondrial ion permeability suggests that mitochondrial proton leak mediated by muscle UCP3 may not be a major contributor in energy balan ce in HFD feeding, and other regulatory mechanisms independent of gene regu lation may be responsible for the control of UCP3-mediated uncoupling activ ity.