Evidence for Na+/Ca2+ exchange in intact single skeletal muscle fibers from the mouse |
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Authors: | Balnave, Christopher D. Allen, David G. |
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Abstract: | The myoplasmic free Ca2+concentration([Ca2+]i)was measured in intact single fibers from mouse skeletal muscle withthe fluorescent Ca2+ indicatorindo 1. Some fibers were perfused in a solution in which theconcentration of Na+ was reducedfrom 145.4 to 0.4 mM (low-Na+solution) in an attempt to activate reverse-modeNa+/Ca2+exchange (Ca2+ entry in exchangefor Na+ leaving the cell). Undernormal resting conditions, application oflow-Na+ solution only increased[Ca2+]iby 5.8 ± 1.8 nM from a mean resting[Ca2+]iof 42 nM. In other fibers,[Ca2+]iwas elevated by stimulating sarcoplasmic reticulum (SR)Ca2+ release with caffeine (10 mM)and by inhibiting SR Ca2+ uptakewith2,5-di(tert-butyl)-1,4-benzohydroquinone(TBQ; 0.5 µM) in an attempt to activate forward-modeNa+/Ca2+exchange (Ca2+ removal from thecell in exchange for Na+ influx).These two agents caused a large increase in[Ca2+]i,which then declined to a plateau level approximately twice the baseline[Ca2+]iover 20 min. If the cell was allowed to recover between exposures tocaffeine and TBQ in a solution in whichCa2+ had been removed, theincrease in[Ca2+]iduring the second exposure was very low, suggesting thatCa2+ had left the cell during theinitial exposure. Application of caffeine and TBQ to a preparation inlow-Na+ solution produced a large,sustained increase in[Ca2+]iof ~1 µM. However, when cells were exposed to caffeine and TBQ in alow-Na+ solution in whichCa2+ had been removed, a sustainedincrease in[Ca2+]iwas not observed, although[Ca2+]iremained higher and declined slower than in normalNa+ solution. This suggests thatforward-modeNa+/Ca2+exchange contributed to the fall of[Ca2+]iin normal Na+ solution, but whenextracellular Na+ was low, aprolonged elevation of[Ca2+]icould activate reverse-modeNa+/Ca2+exchange. The results provide evidence that skeletal muscle fibers possess aNa+/Ca2+exchange mechanism that becomes active in its forward mode when [Ca2+]iis increased to levels similar to that obtained during contraction. |
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