Sarcoplasmic reticulum (SR) Ca2+ uptake and Ca2+-Mg2+-ATPase activity were examined in muscle homogenates and the purified SR fraction of the superficial and deep fibers of the gastrocnemius and vastus muscles of the rat after treadmill runs of 20 or 45 min or to exhaustion (avg time to exhaustion 140 min). Vesicle intactness and cross-contamination of isolated SR were estimated using a calcium ionophore and mitochondrial and sarcolemmal marker enzymes, respectively. Present findings confirm previously reported fiber-type specific depression in the initial rate and maximum capacity of Ca2+ uptake and altered ATPase activity after exercise. Depression of the Ca2+-stimulated ATPase activity of the enzyme was evident after greater than or equal to 20 min of exercise in SR isolated from the deep fibers of these muscles. The lowered ATPase activity was followed by a depression in the initial rate of Ca2+ uptake in both muscle homogenates and isolated SR fractions after greater than or equal to 45 min of exercise. Maximum Ca2+ uptake capacity was lower in isolated SR only after exhaustive exercise. Ca2+ uptake and Ca2+-sensitive ATPase activity were not affected at any duration of exercise in SR isolated from superficial fibers of these muscles; however, the Mg2+-dependent ATPase activity was increased after 45 min and exhaustive exercise bouts. The alterations in SR function could not be attributed to disrupted vesicles or differential contamination in the SR from exercise groups and were reinforced by similar changes in Ca2+ uptake in crude muscle homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of a long-term, high intensity training program upon histochemically assessed myofibrillar actomyosin ATPase, myosin composition, peptide pattern of sarcoplasmic reticulum (SR), and parvalbumin content were analysed in muscles from the same rats which were used in a previous study (Green et al. 1983). Following 15 weeks of extreme training, an increase in type I and type II A fibres and a decrease in type II B fibres occurred both in plantaris and extensor digitorum longus (EDL) muscles. In the deep portion of vastus lateralis (VLD), there was a pronounced increase from 10 +/- 5% to 27 +/- 11% in type I fibres. No type I fibres were detected in the superficial portion of vastus lateralis (VLS) both in control and trained animals. An increase in slow type myosin light chains accompanied the histochemically observed fibre type transition in VLD. Changes in the peptide pattern of SR occurred both in VLS and VLD and suggested a complete transition from type II B to II A in VLS and from type II A to I in VLD. A complete type II A to I transition in the VLD was also suggested by the failure to detect parvalbumin in this muscle after 15 weeks of training. Changes in parvalbumin content and SR tended to precede the transitions in the myosin light chains. Obviously, high intensity endurance training is capable of transforming specific characteristics of muscle fibres beyond the commonly observed changes in the enzyme activity pattern of energy metabolism. The time courses of the various changes which are similar to those in chronic nerve stimulation experiments, indicate that various functional systems of the muscle fibre do not change simultaneously.

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Skeletal muscle fatigue is often associated with diminished athletic performance and work productivity as well as increased susceptibility to injury. The exact cause of muscle fatigue probably involves a number of factors which influence force production in a manner dependent on muscle fiber type and activation pattern. However, a growing body of evidence implicates alterations in intracellular Ca2+ exchange as a major role in the fatigue process. These changes are thought to occur secondary to reductions in the rates of Ca2+ uptake and release by the sarcoplasmic reticulum (SR). This hypothesis is based on the finding that peak myoplasmic Ca2+ concentration ([Ca2+]i) is reduced as force declines during fatigue. In addition, direct measurements of Ca2+ uptake and release show that fatiguing activity causes intrinsic alterations in the functional properties of the SR. We also propose that fatigue-induced alterations in Ca2+ exchange may be beneficial, reducing the rate of energy utilization by the muscle fiber and preventing irreversible damage to the cell.