Modulation of mitochondrial Ca2+ by nitric oxide in cultured bovine vascular endothelial cells

Modulation of mitochondrial Ca2+ by nitric oxide in cultured bovine vascular endothelial cells. Am J Physiol Cell Physiol 289: C836-C845, 2005. First published May 18, 2005; doi:10.1152/ajpcell.00011.2005. - In the present study, we used laser scanning confocal microscopy in combination with fluorescent indicator dyes to investigate the effects of nitric oxide ( NO) produced endogenously by stimulation of the mitochondria-specific NO synthase (mtNOS) or applied exogenously through a NO donor, on mitochondrial Ca2+ uptake, membrane potential, and gating of mitochondrial permeability transition pore (PTP) in permeabilized cultured calf pulmonary artery endothelial (CPAE) cells. Higher concentrations (100-500 mu M) of the NO donor spermine NONOate (Sper/NO) significantly reduced mitochondrial Ca2+ uptake and Ca2+ extrusion rates, whereas low concentrations of Sper/NO (< 100 mu M) had no effect on mitochondrial Ca2+ levels ([Ca2+](mt)). Stimulation of mitochondrial NO production by incubating cells with 1 mM L-arginine also decreased mitochondrial Ca2+ uptake, whereas inhibition of mtNOS with 10 mu M L-N-5-(1-iminoethyl) ornithine resulted in a significant increase of [Ca2+] mt. Sper/NO application caused a dose-dependent sustained mitochondrial depolarization as revealed with the voltage-sensitive dye tetramethylrhodamine ethyl ester ( TMRE). Blocking mtNOS hyperpolarized basal mitochondrial membrane potential and partially prevented Ca2+-induced decrease in TMRE fluorescence. Higher concentrations of Sper/NO (100-500 mu M) induced PTP opening, whereas lower concentrations (< 100 mu M) had no effect. The data demonstrate that in calf pulmonary artery endothelial cells, stimulation of mitochondrial Ca2+ uptake can activate NO production in mitochondria that in turn can modulate mitochondrial Ca2+ uptake and efflux, demonstrating a negative feedback regulation. This mechanism may be particularly important to protect against mitochondrial Ca2+ overload under pathological conditions where cellular [ NO] can reach very high levels.