Ultraturrax- and Nagarse-released populations of mitochondria were characterized with respect to their Ca2+-uptake activities (i) by means of the indirect polarographic technique and (ii) directly by the 45Ca Ruthenium Red-quench method of Reed & Bygrave [(1974) Biochem. J. 140, 143-155]. The denervated-muscle subsarcolemmal and intermyofibrillar mitochondrial fractions displayed markedly decreased rates and capacities for Ca2+ uptake compared with their respective controls. The implications of these findings with respect to the process of cell necrosis are discussed.
