We have compared the effect of moderate heating (42 and 45 °C) on the respiration and NAD(P)H fluorescence in isolated rat heart mitochondria incubated at two different Ca²⁺ concentrations (1 and 10 μM). The main effect of temperature elevation above the physiological level is an increased permeability of the inner mitochondrial membrane. This effect is pronounced at 42 °C and is followed by the secondary consequences – partial uncoupling, deenergizing, decrease in NAD(P)H and in the efficiency of oxidative phosphorylation. Although less effectively, mitochondria are capable to perform energy transformation at 42 °C. This ability is almost completely lost when the temperature is raised from 42 to 45 °C. At this temperature, mitochondria are Almos completely uncoupled, they do not maintain the membrane potential and NAD(P)H/NAD(P)+ equilibrium, hardly phosphorylate, and their respiratory chain is inhibited (the latter effect is more pronounced for glutamate + malate, but also evident for pyruvate + malate oxidation). Moderate heating significantly and progressively reduced the impairment of mitochondrial function by Ca²⁺ overload. In comparison to the high energy state at 37 °C, Ca²⁺ effect on respiration is much smaller in partially deenergized mitochondria at 42 °C and minimized at 45 °C when oxidative phosphorylation is almost completely uncoupled. Temperature effects are smoothened in Ca²⁺ overload (10 μM Ca²⁺) inhibited mitochondria in comparison to mitochondria respiring at the optimal 1 μM Ca²⁺ concentration.

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