The Effect of Sodium and Potassium on Calcium Uptake by Frog Skeletal Muscle Mitochondria and Vesicles

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Abstract
Calcium uptake by mitochondria of frog skeletal muscle increased when K replaced Na or sucrose in the medium. There was no difference in passive binding of Ca and the amount of ATP split when K replaced Na. Ca uptake was increased by substituting K for Na in the medium and was maximal in a medium containing 100 mM K and no Na. Azide and dinitrophenol (DNP) inhibited K-stimulated Ca uptake completely. Inhibition by these agents in Na medium was relatively small. Ca uptake by vesicles was little affected by changing from Na to K medium, by azide, or by DNP. Ageing reduced Ca uptake in both fractions and stimulation by K of mitochondrial Ca uptake nearly disappeared after 4 h of storage (at 4 °C). Since Ca was taken up to the same extent in Na or sucrose medium, it is concluded that Ca uptake by mitochondria is stimulated by K rather than inhibited by Na.

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