Elevation of mitochondrial calcium by ryanodine-sensitive calcium-induced calcium release

Calcium is an important regulator of mitochondrial function. Since there can be tight coupling between inositol 1,4,5-trisphosphate-sensitive Ca2+ release and elevation of mitochondrial calcium concentration, we have investigated whether a similar relationship exists between the release of Ca2+ from the ryanodine receptor and the elevation of mitochondrial Ca2+. Perfusion of permeabilized A10 cells with inositol 1,4,5-trisphosphate resulted in a large transient elevation of mitochondrial Ca2+ to about 8 mu M. The response was inhibited by heparin but not ryanodine. Perfusion of the cells with Ca2+ buffers in excess of 1 mu M leads to large increases in mitochondrial Ca2+ that are much greater than the perfused Ca2+. These increases, which average around 10 mu M, are enhanced by caffeine and inhibited by ryanodine and depletion of the intracellular stores with either orthovanadate or thapsigargin. We conclude that Ca2+-induced Ca2+ release at the ryanodine receptor generates microdomains of elevated Ca2+ that are sensed by adjacent mitochondria. In addition to ryanodine sensitive stores acting as a source of Ca2+, Ca2+-induced Ca2+ release is required to generate efficient elevation of mitochondrial Ca2+.