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

1 Instituto de Ciencias Biomédicas, Facultad de Medicina, Universidad de Chile, Santiago 7; 3 Centro de Estudios Científicos, Valdivia, Chile; and 2 Department of Physiology, University of California Los Angeles, Los Angeles, California 90024 The Na + /Ca 2+ exchanger participates in Ca 2+ 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 Ca 2+ efflux (reverse mode), with the Hill number equal to 2.8. In contrast to the stimulation of the cardiac exchanger, increasing extravesicular (cytoplasmic) Ca 2+ concentration ([Ca 2+ ]) inhibited this reverse activity with an IC 50 of 91 nM. Exchanger-mediated currents were measured at 15°C in single fibers voltage clamped at 90 mV. Photolysis of a cytoplasmic caged Ca 2+ 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 [Ca 2+ ]. These results suggest that, in skeletal muscle, the main function of the exchanger is to remove Ca 2+ from the cells after stimulation. intracellular calcium regulation; electrogenic ion transport; calcium fluxes; calcium permeability; plasma membrane transporters