The distribution of a ryanodine‐sensitive Calcium pump in skeletal muscle fractions

Rabbit skeletal muscle homogenates were fractionated by differential centrifugation over the range 2,000−35,000 × G, and the calcium pumping activity of each fraction was assayed, together with the sensitivity of this activity to ryanodine. The greatest ryanodine sensitivity was found in material sedimenting between 2,000−4,000 × G, with decreasing sensitivities seen in the successively lighter fractions. Muscle mitochondria accumulated calcium slowly, the process being essentially insensitive to ryanodine but greatly inhibited by azide. With oxalate present in the incubation medium the ryanodine‐sensitive fractions showed no inhibition by azide or by dinitrophenol; when oxalate was omitted the pumping activity decreased greatly but now was inhibited by azide and little affected by ryanodine. Although the cellular origin of the sensitive calcium pumping element is not yet known, it appears that it is derived from the sarcoplasmic reticulum rather than from mitochondria, and may represent a substructure of the reticulum possessing specialized pharmacological properties.