Sodium-calcium ion exchange in skeletal muscle sarcolemmal vesicles

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Abstract
Summary The Ca2+ permeability of rabbit skeletal muscle sarcolemmal vesicles was investigated by means of radioisotope flux measurements. A membrane vesicle fraction highly enriched in sarcolemma, as revealed by enzymatic markers, was obtained from the 22–27% region of sucrose gradients after isopycnic centrifugation. The ability of sarcolemmal vesicles to exchange Na+ for Ca2+ was investigated by measuring Ca2+ influx into and efflux from sarcolemmal vesicles in the presence and absence of a Na+ gradient. It was found that Ca2+ movements were enhanced in the direction of the higher Na+ concentration. When intra- and extravesicular Na+ concentrations were high, Na+−Na+ exchange predominated and Na+−Ca2+ exchange was low or absent. The presence of the Ca2+ ionophore A23187 in the dilution medium resulted in the rapid release of Ca2+ and the elimination of the Na+-enhanced efflux of Ca2+, suggesting that internal rather than bound external Ca2+ was exchanged with Na+. La3+ abolished Na+−Ca2+ exchange and decreased overall membrane permeability. Na+−Ca2+ exchange was not due to sarcoplasmic reticulum or mitochondrial contaminants. This investigation suggests that skeletal muscle, like cardiac muscle and neurons, is capable of a transmembranous Na+−Ca2+ exchange.