Pathological accumulation of calcium by mitochondria: modulation by magnesium.

Magnesium causes a marked decrease in the initial rates of respiration-supported calcium uptake by isolated heart mitochondria. Differential responses of mitochondrial respiration and cytochrome b redox states in the presence and absence of magnesium indicate that with magnesium present, heart mitochondria retain the ability to phosphorylate ADP after calcium uptake. Electron microscopy of the isolated mitochondria after calcium accumulation revealed markedly different crystal structures within the matrix space depending on the presence or absence of magnesium. The results suggest a "protective" effect of magnesium on the phosphorylating mechanism of mitochondria during active calcium uptake. Further, magnesium appears to determine the type of crystal structure formation within the intramitochondrial compartment.