Role of skeletal muscle mitochondrial density on exercise-stimulated lipid oxidation

Abstract

Reduced skeletal muscle mitochondrial density is proposed to lead to impaired muscle lipid oxidation and increased lipid accumulation in sedentary individuals. We assessed exercise-stimulated lipid oxidation by imposing a prolonged moderate-intensity exercise in men with variable skeletal muscle mitochondrial density as measured by citrate synthase (CS) activity. After a 2-day isoenergetic high-fat diet, lipid oxidation was measured before and during exercise (650 kcal at 50% VO(2)max) in 20 healthy men with either high (HI-CS = 24 ± 1; mean ± s.e.) or low (LO-CS = 17 ± 1 nmol/min/mg protein) muscle CS activity. Vastus lateralis muscle biopsies were obtained before and immediately after exercise. Respiratory exchange data and blood samples were collected at rest and throughout the exercise. HI-CS subjects had higher VO(2)max (50 ± 1 vs. 44 ± 2 ml/kg fat free mass/min; P = 0.01), lower fasting respiratory quotient (RQ) (0.81 ± 0.01 vs. 0.85 ± 0.01; P = 0.04) and higher ex vivo muscle palmitate oxidation (866 ± 168 vs. 482 ± 78 nmol/h/mg muscle; P = 0.05) compared to LO-CS individuals. However, whole-body exercise-stimulated lipid oxidation (20 ± 2 g vs. 19 ± 1 g; P = 0.65) and plasma glucose, lactate, insulin, and catecholamine responses were similar between the two groups. In conclusion, in response to the same energy demand during a moderate prolonged exercise bout, reliance on lipid oxidation was similar in individuals with high and low skeletal muscle mitochondrial density. This data suggests that decreased muscle mitochondrial density may not necessarily impair reliance on lipid oxidation over the course of the day since it was normal under a high-lipid oxidative demand condition. Twenty-four-hour lipid oxidation and its relationship with mitochondrial density need to be assessed.

Figure 1

Muscle maximal citrate synthase activity, (a) maximal aerobic capacity (per kg fat-free mass (FFM)), (b) in vitro skeletal muscle palmitate oxidation rate (c) and type I skeletal muscle fiber content (d) in subjects with high and low skeletal muscle citrate synthase activity (Means ± s.e.). Maximal aerobic capacity per kg of body weight in HI-CS vs. LO-CS were: 42 ± 1 vs. 37 ± 2 ml O /kg BW/min (P = 0.01). HI-CS, high skeletal muscle citrate synthase activity; LO-CS, low skeletal muscle citrate synthase activity.

Figure 2

Distribution of skeletal muscle mitochondrial DNA and mitochondrial protein complexes I to V in individuals with high and low skeletal muscle citrate synthase (CS) activity.

Figure 3

Plasma glucose, lactate, free-fatty acids, insulin, epinephrine, and norepinephrine concentrations before and during exercise (Means ± s.e.) in subjects with high (black line) or low (gray line) skeletal muscle citrate synthase activity.

Figure 4

Whole-body respiratory quotient (a) and cumulative lipid oxidation (b) before and during exercise (Means ± s.e.) in subjects with high or low skeletal muscle citrate synthase activity. ^aP < 0.05.

Figure 5

Association between cumulative lipid oxidation and exercise-induced muscle glycogen change (a) and plasma free-fatty acid (FFA) area under the curve (b). HI-CS, high skeletal muscle citrate synthase activity; LO-CS, low skeletal muscle citrate synthase activity.