Increased uncoupling protein 3 content does not affect mitochondrial function in human skeletal muscle in vivo

Abstract

Phosphocreatine (PCr) resynthesis rate following intense anoxic contraction can be used as a sensitive index of in vivo mitochondrial function. We examined the effect of a diet-induced increase in uncoupling protein 3 (UCP3) expression on postexercise PCr resynthesis in skeletal muscle. Nine healthy male volunteers undertook 20 one-legged maximal voluntary contractions with limb blood flow occluded to deplete muscle PCr stores. Exercise was performed following 7 days consumption of low-fat (LF) or high-fat (HF) diets. Immediately following exercise, blood flow was reinstated, and muscle was sampled after 20, 60, and 120 seconds of recovery. Mitochondrial coupling was assessed by determining the rate of PCr resynthesis during recovery. The HF diet increased UCP3 protein content by approximately 44% compared with the LF diet. However, this HF diet-induced increase in UCP3 expression was not associated with any changes in the rate of muscle PCr resynthesis during conditions of maximal flux through oxidative phosphorylation. Muscle acetylcarnitine, free-creatine, and lactate concentrations during recovery were unaffected by the HF diet. Taken together, our findings demonstrate that increasing muscle UCP3 expression does not diminish the rate of PCr resynthesis, allowing us to conclude that the primary role of UCP3 in humans is not uncoupling.

Figure 1

(a) Representative Western blot of human lung (1), human liver (2), human white adipose tissue (3), and two human vastus lateralis muscle samples (4 and 5) after loading similar amounts of protein. (b) Western blot of medial gastrocnemius muscle from a transgenic mouse overexpressing human UCP3 (1) and a wild-type littermate (2). Note the lack of cross-reactivity in tissues known to express UCP2 but not UCP3 (a) and the lack of cross-reactivity with endogenous mouse UCP3 (b).

Figure 2

Muscle torque production recorded during 20 successive knee extensions. The solid line represents the LF trial, and the dashed line represents the HF trial. AU, arbitrary units.

Figure 3

Muscle UCP3 protein content assessed by Western blotting for the LF and HF diets, expressed in arbitrary units (AU) after scanning optical density. *P < 0.05 as compared with the LF diet.

Figure 4

Muscle PCr concentrations during recovery from 20 maximal knee extensions with limb blood flow occluded. Immediately after exercise, limb blood flow was reinstated, and needle muscle biopsies were taken 20, 60, and 120 seconds after the cessation of exercise. Values represent means ± SD. The solid lines represent the LF diet, and the dashed lines represent the HF diet. dw, dry weight.

Figure 5

Relationship between UCP3 protein content (arbitrary units) and PCr resynthesis rates (mmol/kg dw/s). The filled diamonds represent the LF diet, and the open squares represent the HF diet. Note the lack of relation between PCr resynthesis rate and UCP3 content.