Release of Ca2+ from the sarcoplasmic reticulum increases mitochondrial Ca2+ in rat pulmonary artery smooth muscle cells

The Ca2+-sensitive fluorescent indicator rhod-2 was used to measure mitochondrial Ca2+ (Ca2+m) in single smooth muscle cells from the rat pulmonary artery, while simultaneously monitoring cytosolic Ca2+ (Ca2+i) with fura-2. 2. Application of caffeine produced an increase in Ca2+i and also increased Ca2+m. The increase in Ca2+m occurred after the increase in Ca2+i, and remained elevated for a considerable time after Ca2+i had returned to resting values. 3. The protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), which causes the mitochondrial membrane potential to collapse, markedly attenuated the increase in Ca2+m following caffeine application and also increased the half-time for recovery of Ca2+i to resting values. 4. Activation of purinoceptors with ATP also produced increases in both Ca2+i and Ca2+m in these smooth muscle cells. In some cells, oscillations in Ca2+i were observed during ATP application, which produced corresponding oscillations in Ca2+m and membrane currents. 5. This study provides direct evidence that Ca2+ release from the sarcoplasmic reticulum, either through ryanodine or inositol 1,4, 5-trisphosphate (InsP3) receptors, increases both cytosolic and mitochondrial Ca2+ in smooth muscle cells. These results have potential implications both for the role of mitochondria in Ca2+ regulation in smooth muscle, and for understanding how cellular metabolism is regulated.