Influence of Calcium on NADH and Succinate Oxidation by Rat Heart Submitochondrial Particles

Physiological increases in matrix calcium are known to stimulate three mitochondrial dehydrogenases. In mitochondria isolated from rat heart, calcium stimulates rates of State 3 respiration during oxidation of succinate and of several NAD-linked substrates. In this study, we investigated the effects of calcium on NADH dehydrogenase and succinate dehydrogenase activities since the mechanism of these effects is unresolved. The respiratory activities of intact mitochondria and submitochondrial particles (SMP) were compared during incubation in media containing either ethylene glycol bis(β-aminoethyl ether)-N,N′-tetraacetic acid (EGTA) or a Ca2+/EGTA buffer (∼1 μM free Ca2+). In intact mitochondria oxidizing 20 mM glutamate plus 2 mM malate, the membrane potential (ΔΨ) and matrix NAD(P)H were maintained at higher levels, and the maximal rate of ADP-stimulated respiration (State 3) was increased twofold by the presence of calcium. With succinate as substrate, calcium stimulated State 3 respiration but it did not influence the pyridine nucleotides redox state or membrane potential. Stimulation of succinate-supported respiration by addition of 6–10 μM ADP in the presence of hexokinase caused a sudden decrease in NAD(P)H and collapse of ΔΨ. This effect was not caused by inhibition of succinate dehydrogenase or by opening of the nonspecific pore. Calcium did not influence the oxidation of succinate by SMP containing either activated or nonactivated succinate dehydrogenase. In addition, calcium did not alter the kinetics of succinate dehydrogenase activation. Calcium and magnesium, in the concentration range of 0.02 to 5 mM, did not influence the NADH dehydrogenase activity of SMP. Energization of SMP by oligomycin addition, however, dramatically influenced the kinetic properties of NADH dehydrogenase, It is proposed that in heart mitochondria, calcium does not affect directly the components of electron transport but it may influence the activity of NADH dehydrogenase indirectly by increasing ΔΨ.