The role of mitochondria in the contraction-relaxation cycle and other Ca2+-dependent activities of heart cells.

The problem of whether mitochondria participate in the contraction and relaxation of heart by binding Ca2+ reversibly has been discussed actively in recent times. The role of mitochondria has not yet been conclusively established, but is suggested by several lines of evidence, particularly in amphibian hearts. Apart from contraction and relaxation, the Ca2+ transport system of mitochondria may influence many Ca2+-sensitive cell reactions. Some are mitochondrial (succinate oxidase, alpha-glycerophosphate dehydrogenase, pyruvate dehydrogenase phosphate phosphatase) and may control the operations of the respiratory chain and of the trichloroacetic acid cycle. Among the extramitochondrial Ca2+-sensitive reactions are phosphorylase b kinase, phospholipases, and pyruvate kinase. The latter activity has been studied in detail, and it has indeed been found that it can be modulated by mitochondria. It appears possible that the mitochondrial system for binding and transporting Ca2+ is an efficient means to regulate various phases of cell metabolism.