The effects of zinc and lanthanum on calcium uptake by mitochondria and fragmented sarcoplasmic reticulum of frog skeletal muscle.

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AbstractCalcium uptake by mitochondria and fragmented sarcoplasmic reticulum (FSR) isolated from frog skeletal muscle was studied. These fractions were characterized by electron microscopy, succinic dehydrogenase assay and by using mitochondrial inhibitors. With high (100 μM) Ca in the medium, the Ca accumulating capacity of the two fractions was similar. Zinc in concentrations of 5–10 μM in the medium had no effect on Ca uptake by either fraction whereas higher concentration of Zn (25 μM) reduced Ca uptake in both fractions. Five micromolar lanthanum lowered Ca uptake by 70% in mitochondria but had no effect on Ca uptake by FSR. With 10 and 25 μM La, Ca uptake by FSR decreased by 12 and 20% respectively. Addition of La (5 μM) to Ca‐loaded mitochondria had no effect indicating that La could only interfere with the Ca binding step and was unable to release Ca that was already stored. In the medium that originally contained low (10 μM) Ca FSR was able to reduce the Ca concentration below 0.1 μM. In contrast mitochondria, although possessing an equal capacity for Ca uptake were unable to accumulate Ca from the medium when Ca was lowered to approximately 4 μM. Presence of 5–10 μM La in the low Ca medium had no effect on the total amount of Ca taken up by FSR in two minutes but reduced the rate of Ca uptake significantly. The relation of the effects of Zn and La on the isolated fractions to their reported effects on the contractile response of skeletal muscle is discussed.