The effects of hydrogen ions on the kinetics of calcium transport by rat kidney mitochondria

The influence of pH on the kinetics of the initial rate of calcium uptake by isolated kidney mitochondria was studied using the ruthenium red-ethylene glycol bis(β-aminoethyl ether) N,N-tetraacetic acid quench method (K. Reed and F. Bygrave, 1975, Eur. J. Biochem.55, 497–504). In the absence of phosphate, the K_m is increased 50% and the V is decreased 57% when the pH is decreased from pH 7.4 to 7.0. Conversely, when the pH is increased to 7.8, the K_m is decreased 25% while the V is not affected. The presence of 0.1 or 0.4 mm phosphate in the incubation medium abolishes the change in K_m at a low pH while the V remains depressed by 36 and 25%, respectively. The presence of phosphate does not affect the decrease in the K_m seen with an increased medium pH. Mitochondria incubated in steady-state conditions with a medium free calcium of 0.7 μm also show significant changes in calcium exchange and distribution with pH. Two kinetic calcium pools are found in isolated mitochondria. Decreasing pH from 7.4 to 7.1 decreases mitochondria total calcium 32%, decreases the rapidly exchanging pool 28%, and depresses both the mitochondrial membrane and an intramitochondrial calcium exchange by 54 and 22%, respectively. Raising the pH to 7.7 increases both exchangeable pools (63 and 46%), and increases the mitochondrial membrane calcium exchange 44%. These results are consistent with previous studies on the influence of intracellular pH on calcium metabolism of kidney cells in which the mitochondrial pool was markedly affected by pH (R. Studer, and A. Borle, 1979, J. Membrane Biol.48, 325–341). Alterations in cellular pH may modify mitochondrial calcium transport and cellular calcium metabolism and thus affect cell functions which are calcium dependent.