AREGU May 47/5

Willis, W. T., M. R. Jackman, M. E. Bizeau, M. J. Pagliassotti, and J. R. Hazel. Hyperthermia impairs liver mitochondrial function in vitro. Am J Physiol Regulatory Integrative Comp Physiol 278: R1240–R1246, 2000.—The effects of temperature on the relationships among the rates of pyruvate carboxylation, O2 uptake (Jo), oxidative phosphorylation (Jp), and the free energy of ATP hydrolysis (Gp) were studied in liver mitochondria isolated from 250-g female rats. Pyruvate carboxylation was evaluated at 37, 40, and 43°C. In disrupted mitochondria, pyruvate carboxylase maximal reaction velocity increased from 37 to 43°C with an apparent Q10 of 2.25. A reduction in ATP/ADP ratio decreased enzyme activity at all three temperatures. In contrast, in intact mitochondria, increasing temperature failed to increase pyruvate carboxylation (malate 1 citrate accumulation) but did result in increased Jo and decreased extramitochondrial Gp. Jp was studied in respiring mitochondria at 37 and 43°C at various fractions of state 3 respiration, elicited with a glucose 1 hexokinaseADP-regenerating system. The relationship between Jo and Gp was similar at both temperatures. However, hyperthermia (43°C) reduced the Jp/Jo ratio, resulting in lower Gp for a given Jp. Fluorescent measurements of membrane phospholipid polarization revealed a transition in membrane order between 40 and 43°C, a finding consistent with increased membrane proton conductance. It is concluded that hyperthermia augments nonspecific proton leaking across the inner mitochondrial membrane, and the resultant degraded energy state offsets temperature stimulation of pyruvate carboxylase. As a consequence, at high temperatures approaching 43°C, the pyruvate carboxylation rate of intact liver mitochondria may fail to exhibit a Q10 effect.