Effect of cations on the temperature sensitivity of Ca2+ transport in rat-liver mitochondria and safranine uptake by liposomes

The activation energy of mitochondrial Ca²⁺ transport has been studied in various conditions by Arrhenius plots in the temperature range 6–20°C. In the presence of Mg²⁺ the activation energy is decreased to 18 kJ/mole from that of 40 kJ/mole found in a sucrose medium. In the presence of the polyamine spermine the activation energy is practically 0 kJ/mole. A lanthanide Eu³⁺, which is a potent inhibitor of Ca²⁺ transport, has no significant effect on the activation energy. In a KCl medium the activation energy is increased to 70 kJ/mole. When both K⁺ and Mg⁺ are present the activation energy is nonlinear between 11 and 18°C. In the presence of K⁺ and spermine it is about 0 kJ/mole between 6 and 13°C and at higher temperatures 68 kJ/mole. Neither Mg²⁺ nor spermine affect the slope of the Arrhenius plot for state 4 respiration. Spermine decreases slightly the activation energy of Ca²⁺-stimulated respiration. Spermine also decreases the activation energy of valinomycin- or gramicidin-induced safranine uptake by liposomes from 68 to almost 0 kJ/mole between 17 and 30°C. The results indicate that Ca²⁺ binding to the polar head groups of the phospholipids at the membrane surface is the rate-limiting step of mitochondrial Ca²⁺ transport, because agents that inhibit Ca²⁺ binding to these sites (Mg²⁺, spermine, K⁺) have the most marked effect, whereas Eu³⁺, which, because of the small concentration used, ought to interact mainly with the mitochondrial Ca²⁺ transport system, has no significant effect on the temperature sensitivity of mitochondrial Ca²⁺ transport.