Sodium/calcium exchange in amphibian skeletal muscle fibers and isolated transverse tubules.
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 degrees 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 muA/muF. External Na(+) withdrawal generated an outward current (reverse mode) with an average current density of 0.36 +/- 0.17 muA/muF (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.
Cifuentes
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F
Instituto de Ciencias Biomédicas, Facultad de Medicina, Universidad de Chile, Santiago 7, Chile.
Vergara
J
J
Hidalgo
C
C
D000085:Acetates / Q000378:metabolism / Q000528:radiation effects; D000818:Animals; D001001:Anura / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D005029:Ethylenediamines / Q000378:metabolism / Q000528:radiation effects; D066298:In Vitro Techniques; D007425:Intracellular Membranes / Q000378:metabolism; D018485:Muscle Fibers, Skeletal / Q000378:metabolism; D018482:Muscle, Skeletal / Q000378:metabolism*; D009994:Osmolar Concentration; D010782:Photolysis; D012964:Sodium / Q000502:physiology; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*
D000085:Acetates; D005029:Ethylenediamines; D019831:Sodium-Calcium Exchanger; C057014:DM-nitrophen; D012964:Sodium; D002118:Calcium