Cytoplasmic free concentrations of Ca2+ and Mg2+ in skeletal muscle fibers at rest and during contraction

Abstract

This review summarizes estimates for cytoplasmic-free concentrations of Ca2+ ([Ca2+](i)) and Mg2+ ([Mg2+](i)) at rest and during contraction of skeletal muscles, from which substantial quantitative information about them has been accumulated. Although the estimates of resting [Ca2+](i) in the literature widely differ, which is because of the variety of difficulties related to different methodologies used, recent studies suggest that estimates of resting [Ca2+](i) of approximately 0.05-0.1 μM are likely to be correct. Following action potential propagation, the Ca2+ release from the sarcoplasmic reticulum causes a transient rise of [Ca2+](i) (Ca2+ transient). The large peak amplitude and brief time course of the Ca2+ transients have been established only recently by studies with low-affinity Ca2+ indicators developed in the past decade. These technical improvements in [Ca2+](i) measurements have made it possible to study relationships between [Ca2+](i) and force in intact muscle fibers. In the second part of this review, various estimates of [Mg2+](i) in the resting muscle are discussed. Relatively recent estimates of the [Mg2+](i) level appear to be about 1.0 mM. Using the current knowledge of concentrations and reaction properties of intracellular Ca2+-Mg2+ binding sites, we constructed a model for dynamic Mg2+ movement following Ca2+ transients. The model predicts that with a train of action potentials, the sustained rise of [Ca2+](i) produces an elevation of [Mg2+](i) of about 200 μM.