Calcium content and respiratory control index of skeletal muscle mitochondria during exercise and recovery KLAVS MADSEN, PER ERTBJERG, MOGENS STIG DJURHUUS, AND PREBEN K. PEDERSEN Department of Physical Education, Odense University, and Department of Clinical Chemistry, Odense University Hospital, DK-5230 Odense, Denmark Madsen, Klavs, Per Ertbjerg, Mogens Stig Djurhuus, and Preben K. Pedersen. Calcium content and respiratory control index of skeletal muscle mitochondria during exercise and recovery. Am. J. Physiol. 271 (Endocrinol. Metab. 34): E1044-E1050, 1996.-The purpose of this study was to evaluate the relationship between mitochondrial Ca2+ concen- tration and the respiratory control index (RCI; state III/state IV) in isolated mitochondria before and after exhaustive exercise at 75% of maximal O2 consumption. Muscle biopsies of loo-150 mg from 12 moderately trained men were sampled at rest, immediately after exercise, and 30 or 60 min after exercise. The mitochondrial Ca2+ content after exhaustive exercise was significantly higher than the preexercise level [Xl (range 39.4) vs. 11.6 (range 6.5) nmol/mg protein, respectively; P < 0.051, and RCI increased from 11.6 (range 14.4) at rest to 13.7 (range 15.0) at exhaustion (P < 0.05). After 60 min of recovery, the mitochondrial Ca2+ content was still high [l&8 (range 29.9) nmol/mg protein], but the RCI value was significantly depressed because of the increased state IV value and, in fact, was lower than the preexercise value [8.6 (range 5.1); P < 0.051. Our results show that the mitochondrial Ca2+ content is increased in human skeletal muscle after prolonged exhaustive exercise and that this is followed by an elevated RCI value, with slightly increased state 111 and decreased state Wrespiration. The restoration of the elevated mitochondrial Ca2+ level is slow and could be related to an increased state IV respiration, which together indicate uncoupled Ca2+ respiration during recovery. metabolic drift; muscle magnesium; plasma electrolytes MUSCULAR CONTRACTION and relaxation are highly depen- dent on the regulation of Ca2+ movement between different muscle compartments. When a muscle is stimulated, Ca2+ is released from the sarcoplasmic reticulum (SR) into the cytoplasm, leading to activation of the contractile filaments. Ca2+ is transported back into the SR by Ca2+ -adenosinetriphosphatase (ATPase), but Ca2+ may also leave cytoplasm via plasma mem- brane and mitochondrial membrane. Prolonged, acute exercise gradually reduces the activ- ity of the Ca2+-ATPase of the SR (6) and increases the cytoplasmic Ca2+ content (29). The cytoplasmic excess of Ca2+ may induce mitochondrial Ca2+ accumulation and lead to increased respiration from Ca2+ uncoupling (10, 20, 31). Ca2+ uncoupled respiration could be an important factor for fatigue, and it could be an explana- tion for the observed metabolic drift during prolonged exercise (28). The amount of mitochondrial sequestration of Ca2+ and its role during and after exercise remain, however, unexplored at least in mammalian and especially hu- man muscles because of inherent limitations in the techniques. One major problem is that small increases in mitochondrial Ca2+ levels serve to stimulate respira- tion, but accumulation of more Ca2+ depresses mitochon- drial function (20,32). Reports from Tate et al. (26) and Duan et al. (10) showed resting Ca2+ values in the mitochondria that are tenfold higher than those re- ported by McCormack et al. (20), and the isolation procedure of the mitochondria has likely interfered with the original Ca2+ content. Therefore, the mitochon- drial Ca2+ content after exhaustive exercise and the possible effect on the respiratory capacity of the mito- chondria are unknown. We have earlier reported a method for isolation of mitochondria with the purpose of determining the Ca2+ content and the oxidative phosphorylation capacity from human biopsies ob- tained with the Bergstrom technique (18). With this technique, it is possible to make studies on exercising humans and the effects on mitochondrial Ca2+ content and oxidative phosphorylation capacity. Authors have proposed that accumulation of intracel- lular Ca2+ or loss of Ca2+ homeostasis in the muscle cell leads to increased protease activity. This could explain the subsequent myofilament damage seen with exhaus- tion from prolonged exercise (for recent review, see Ref. 3). In a sense, magnesium (Mg2+) may be considered nature’s physiological Ca 2+ blocker, and several studies suggest that Mg2+ may serve to protect against cell injury by its competitive effect with respect to Ca2+. Elevated muscle Mg2+ concentration inhibits SR Ca2+ release (17, 22) and mitochondrial Ca2+ transport (25). We earlier reported an increase in muscle Mg2+ concen- tration in the late phase of prolonged exhaustive exer- cise (19), and it is possible that this increase inhibits excessive mitochondrial Ca2+ accumulation. The present study was designed to compare mitochon- drial Ca2+ content and oxidative phosphorylation capac- ity before and immediately after prolonged exhaustive exercise and after 1 h of recovery. Our purpose was to determine whether prolonged exhaustive exercise in- creases skeletal muscle mitochondrial Ca2+ content by a magnitude that would lead to uncoupled Ca2+ respira- tion from phosphorylation. METHODS Subjects Twelve moderately trained males took part in this study. Mean values of age, height, and body mass were 25.0 t 0.5 (&SE) yr, 183 5 2 cm, and 77.2 t 1.7 kg, respectively. They were all active in sports, and their mean maximal 02 consump- tion 07%max> was 4.65 t 0.10 Urnin, or 59 t 1 ml=min+kg-l. All subjects were fully informed of the risks and stresses El044 0193-1849/96 $5.00 Copyright o 1996 the American Physiological Society