Sodium/calcium exchange in amphibian skeletal muscle fibers and isolated transverse tubules

Abstract

The Na⁺/Ca²⁺exchanger participates in Ca²⁺homeostasis in a variety of cells and has a key role in cardiac muscle physiology. We studied in this work the exchanger of amphibian skeletal muscle, using both isolated inside-out transverse tubule vesicles and single muscle fibers. In vesicles, increasing extravesicular (intracellular) Na⁺concentration cooperatively stimulated Ca²⁺efflux (reverse mode), with the Hill number equal to 2.8. In contrast to the stimulation of the cardiac exchanger, increasing extravesicular (cytoplasmic) Ca²⁺concentration ([Ca²⁺]) inhibited this reverse activity with an IC₅₀of 91 nM. Exchanger-mediated currents were measured at 15°C in single fibers voltage clamped at −90 mV. Photolysis of a cytoplasmic caged Ca²⁺compound activated an inward current (forward mode) of 23 ± 10 nA ( n = 3), with an average current density of 0.6 μA/μF. External Na⁺withdrawal generated an outward current (reverse mode) with an average current density of 0.36 ± 0.17 μA/μF ( n = 6) but produced a minimal increase in cytosolic [Ca²⁺]. These results suggest that, in skeletal muscle, the main function of the exchanger is to remove Ca²⁺from the cells after stimulation.