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 μm. The response was inhibited by heparin but not ryanodine. Perfusion of the cells with Ca2+ buffers in excess of 1 μm leads to large increases in mitochondrial Ca2+ that are much greater than the perfused Ca2+. These increases, which average around 10 μm, 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+.