Calcium Transport in Mitochondria

Abstract

The energy-linked uptake of Ca2+ by mitochondria is supported by the membrane potential created either by respiration or by the hydrolysis of ATP; in the former case, it is abolished by respiratory inhibitors, in the latter by oligomycin; in both cases it is abolished by uncouplers (Lehninger et al., 1967; Carafoli and Crompton, 1976; Bygrave, 1977). The uptake of Ca2+ is inhibited by La3+ (Mela, 1968) and ruthenium red (Moore, 1971) which are effective at very low concentrations, e.g., 50% inhibition is obtained with 0.03 nmol La3+, and 0.07 nmol ruthenium red per mg of liver mitochondrial protein, respectively. In the absence of added co-penetrating anions (membrane loading), the process normally leads to a maximal accumulation of between 100 and 150 nmol of Ca2+ per mg of mitochondrial protein. In the presence of an anion that permeates electroneutrally by co-transport with protons, e.g., phosphate, or of a permeant weak acid, e.g., actetate, the maximal level may be increased, in particular if the solubility product of the calcium salt accumulated is low (e.g., calcium phosphate). In the latter case, most of the Ca2+ is precipitated in the matrix in the form of insoluble deposits, visible in the electronmicroscope (matrix loading). These basic features are valid for most mitochondrial types studied. However, each type of mitochondria has also specific features, for example, different responses to compounds such as magnesium (Crompton et al., 1976) and sodium (Crompton et al., 1977).