Muscle mitochondrial morphology, body composition, and energy expenditure in sedentary individuals.

To investigate whether differences in metabolic rate are related to differences in muscle mitochondrial morphology and/or to differences in in vitro muscle respiration, we studied 17 healthy Caucasians, covering a wide range of body weight and composition [9 males, 8 females; body wt 96 +/- 37 (SD) kg; body fat = 28 +/- 10%]. Central and peripheral mitochondrial volume density (Vmit c and Vmit p, respectively) and the ratio of mitochondrial outer surface to volume of mitochondria (SVmit c in center and SVmit p at periphery) were determined by stereological analyses of transmission electron micrographs from samples of the vastus lateralis. There was no relationship between mitochondrial morphology or muscle respiration and 24-h energy expenditure, basal metabolic rate, or sleeping energy expenditure adjusted for differences in fat-free mass, fat mass, age, and sex. Although total body fat was not associated with muscle cell morphology, central distribution of body fat [waist-to-thigh circumference ratio (W/T)] correlated negatively with Vmit c (r = -0.58, P = 0.01), SVmit c (r = -0.59, P = 0.01), and SVmit p (r = -0.48, P = 0.05). W/T was also negatively related to muscle respiration (r = -0.59, P = 0.01). Despite the lack of relationship between metabolic rate and muscle mitochondrial morphology, central distribution of body fat is associated with lower mitochondrial density and larger mitochondria in skeletal muscle and is associated with a decreased oxidative capacity of muscle.

Effects of endurance training on a mitochondrial reticulum in limb skeletal muscle.

High voltage electron microscopy at 1500 kV, was used to examine the effects of endurance training on mitochondrial morphology in rat skeletal muscle. The soleus, deep portions of the vastus lateralis, and superficial portions of the vastus lateralis muscles were examined to represent slow-twitch-oxidative, fast-twitch-oxidative-glycolytic, and fast-twitch-glycolytic skeletal muscle fiber types, respectively. Muscle samples were removed from endurance trained and untrained control female Wistar rats (n = 6, each group). Tissues were fixed using standard electron microscopic techniques and sectioned transversely with respect to muscle fiber orientation to approximately, 0.5 micron thickness. The sections were stained on grids with uranyl acetate and Reynolds' lead citrate. Results confirmed the presence of a mitochondrial reticulum in all three skeletal muscle fiber types of both groups. Stereologic analyses indicated volume densities of intermyofibrillar mitochondria increased significantly (P less than 0.05) with endurance training in the three skeletal muscle fiber types. Surface-to-volume ratio of mitochondria was significantly decreased (P less than 0.05) after training only in the deep portion of the vastus lateralis muscle. It was concluded that the mitochondria in mammalian limb skeletal muscle are a reticulum which adapts to endurance training by proliferating.

Mitochondrial reticulum in limb skeletal muscle.

High-voltage electron microscopy at 1,500 kV was used to examine mitochondrial morphology in three skeletal muscles of the rat. The soleus, deep portion of the vastus lateralis, and superficial portion of the vastus lateralis muscles were examined to represent slow-twitch oxidative, fast-twitch oxidative, glycolytic, and fast-twitch glycolytic skeletal muscle fiber types, respectively. Muscle samples were removed from six female Wistar rats. The tissues were fixed using standard electron microscopic techniques and were sectioned transversely with respect to muscle fiber orientation to approximately 0.5-micron thickness. The sections were stained on grids with uranyl acetate and Reynolds' lead citrate. Results revealed a mitochondrial reticulum in all three skeletal muscle fiber types. Stereological analyses of the electron micrographs were performed to measure volume densities and surface-to-volume ratios of mitochondria in the muscle samples. Cross-sectional volume densities of mitochondria in the soleus (15.5 +/- 1%) and deep portion of the vastus lateralis (16.1 +/- 2%) were significantly greater (P less than 0.05) than in the superficial portion of the vastus lateralis (8.7 +/- 1%). Surface-to-volume ratios of mitochondria were not significantly different between fiber types. It was concluded that the mitochondria in mammalian limb skeletal muscle are a reticulum, or network.