The importance of CO₂ for Ca²⁺ uptake by some mitochondria

Abstract

METABOLISING mammalian mitochondria normally will take up a considerable (30–80 nmol per mg protein) quantity of Ca²⁺ when no anion, such as phosphate or acetate, has been added to move in and balance the charges on the Ca²⁺ accumulated^1,2. This has been attributed to reaction of the Ca²⁺ with membrane components. Recently^3,4 the importance of adventitious phosphate, always present in mitochondrial suspensions³, has been recognised, and it has been shown that inhibition of phosphate movement lessens^3,5, or can abolish, Ca²⁺ uptake⁴. Carbon dioxide is a further source of anion to accumulate with Ca²⁺ in mitochondria possessing carbonic anhydrase activity⁶. The importance of this process, which readily proceeds using the dissolved CO₂ already present in the system, is now illustrated using a sensitive indicator of Ca²⁺ movement. Also, the production of 2H⁺ per Ca²⁺ accumulated as CaCO₃ can account for the increase in the ratio H⁺: Ca²⁺, which is observed when phosphate uptake is inhibited.