Caffeine and Ca2+ stimulate mitochondrial oxidative phosphorylation in saponin-skinned human skeletal muscle fibers due to activation of actomyosin ATPase.

The rate of mitochondrial oxidative phosphorylation of saponin-skinned human muscle fibers from m. vastus lateralis in the presence of glutamate, malate and ATP is reported to be sensitive to caffeine and to changes of free calcium ion concentration. An approximately twofold increase in respiration was observed by the addition of 15 mM caffeine, because of the efflux of calcium from sarcoplasmic reticulum. Direct addition of a Ca2+/CaEGTA buffer, containing 1.5 microM free calcium ions had a similar effect. The ATP-splitting activity of skinned fibers was also stimulated by caffeine or calcium. These observations can be explained exclusively by the calcium-induced activation of actomyosin ATPase. (i) Thapsigargin, an inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, had no influence. (ii) In myosin-extracted 'ghost' fibers containing intact mitochondria and an intact sarcoplasmic reticulum caffeine had a negligible effect on oxidative phosphorylation. (iii) The caffeine-induced increase in rate of fiber respiration was concomitant with a decrease in mitochondrial membrane potential and a decrease in the redox state of the mitochondrial NAD system. (iv) The calcium ionophore A 23187 caused a stimulation of respiration and ATP-splitting activity, similar to caffeine. (v) The calcium dependencies of respiration and ATP splitting activity of saponin-skinned human muscle fibers were in experimental error identical. Therefore it is concluded that calcium efflux from sarcoplasmic reticulum affects oxidative phosphorylation in skeletal muscle mostly via the stimulation of actomyosin ATPase.