Sodium/calcium exchange in amphibian skeletal muscle fibers and isolated transverse tubules |
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Authors: | Cifuentes F Vergara J Hidalgo C |
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Affiliation: | Instituto de Ciencias Biomédicas, Facultad de Medicina, Universidad de Chile, Santiago 7, Chile. |
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Abstract: | TheNa+/Ca2+ exchanger participates inCa2+ homeostasis in a variety of cells and has a key rolein cardiac muscle physiology. We studied in this work the exchanger ofamphibian skeletal muscle, using both isolated inside-out transversetubule vesicles and single muscle fibers. In vesicles, increasingextravesicular (intracellular) Na+ concentrationcooperatively stimulated Ca2+ efflux (reverse mode), withthe Hill number equal to 2.8. In contrast to the stimulation of thecardiac exchanger, increasing extravesicular (cytoplasmic)Ca2+ concentration ([Ca2+]) inhibited thisreverse activity with an IC50 of 91 nM. Exchanger-mediated currents were measured at 15°C in single fibers voltage clamped at 90 mV. Photolysis of a cytoplasmic caged Ca2+ compoundactivated 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 outwardcurrent (reverse mode) with an average current density of 0.36 ± 0.17 µA/µF (n = 6) but produced a minimal increasein cytosolic [Ca2+]. These results suggest that, inskeletal muscle, the main function of the exchanger is to removeCa2+ from the cells after stimulation. |
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