MITOCHONDRIA CONTRIBUTE TO CA2-MUSCLE CELLS( REMOVAL IN SMOOTH)

Recent evidence, from a variety of cell types, suggests that mitochond ria play an important role in shaping the change in intracellular calc ium concentration ([Ca2+](i)) that occurs during physiological stimula tion. In the present study, using a range of inhibitors of mitochondri al Ca2+ uptake, we have examined the contribution of mitochondria to C a2+ removal from the cytosol of smooth muscle cells following stimulat ion. In voltage-clamped single smooth muscle cells, we found that foll owing a 8-s train of depolarizing pulses, the rate of Ca2+ extrusion f rom the cytosol was reduced by more than 50% by inhibitors of cytochro me oxidase or exposure of cells to the protonophore carbonyl cyanide P -trifluoromethoxy-phenylhydrazone. Using the potential-sensitive indic ator tetramethyl rhodamine ethyl ester, we confirmed that the effect o f these agents was associated with depolarization of the mitochondrial membrane. Since, the primary function of the mitochondria is to provi de the cell's ATP, it could be argued that it is the ATP supply to the ion pumps which is limiting the rate of Ca2+ removal. However, experi ments carried out with the mitochondrial Ca2+ uniporter inhibitor ruth enium red produced similar results, while the ATP synthetase inhibitor oligomycin had no effect, suggesting that the effect was not due to A TP insufficiency. These results establish that mitochondria in smooth muscle cells play a significant role in removing Ca2+ from the cytosol following stimulation. The uptake of Ca2+ into mitochondria is propos ed to stimulate mitochondrial ATP production, thereby providing a mean s for matching increased energy demand, following the cell's rise in [ Ca2+](i), with increased cellular ATP production.