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

Abstract

1 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.