Thermal sensitivity of mitochondrial function in the Antarctic Notothenioid, Lepidonotothen nudifrons

The thermal sensitivity of mitochondrial function was investigated in the stenothermal Antarctic fish Lepidonotothen nudifrons. State 3 respiration increases with increasing temperature between 0 °C and 18 °C with a Q 10 of 2.43–2.63. State 4 respiration in the presence of oligomycin, an inhibitor of mitochondrial ATP synthase, quantifies the leakage of protons through the inner mitochondrial membrane, which causes oxygen consumption without concomitant ATP production. This parameter shows an unusually high Q 10 of 4.21 ± 0.42 (0–18 °C), which indicates that proton leakage does not depend merely on ion diffusion but is an enzyme-catalysed process. The differential thermal sensitivity of oxidative phosphorylation (=state 3) and proton leakage (=state 4 in the presence of oligomycin) leads to progressive uncoupling of the mitochondria and decreased efficiency of oxidative phosphorylation under in vivo conditions if the body temperature of L. nudifrons increases

Mitochondrial function and critical temperature in the Antarctic bivalve, Laternula elliptica

Thermal sensitivities of maximum respiration and proton leakage were compared in gill mitochondria of the Antarctic bivalve Laternula elliptica for an assessment of the contribution of mitochondrial mechanisms to limiting temperature tolerance. Proton leakage was measured as the oxygen consumption rate during blockage of oxidative phosphorylation (state IV respiration + oligomycin). The maximum capacity of NADP dependent mitochondrial isocitrate dehydrogenase (IDH) was investigated as part of a proposed mitochondrial substrate cycle provoking proton leakage by the action of transhydrogenase. State III and IV + respiration rose exponentially with temperature. Thermal sensitivities of proton leakage and IDH were unusually high, in accordance with the hypothesis that H(+) leakage is an enzyme catalysed process with IDH being involved. In contrast to proton leakage, state III respiration exhibited an Arrhenius break temperature at 9 degrees C, visible as a drop in thermal sensitivity close to, but still above the critical temperature of the species (3-6 degrees C). Progressive uncoupling of mitochondria led to a drop in RCR values and P/O ratios at high temperature. The same discontinuity as for state III respiration was found for the activity of mitochondrial IDH suggesting that this enzyme may influence the thermal control of mitochondrial respiration. In general, the high thermal sensitivity of proton leakage may cause an excessive rise in mitochondrial oxygen demand and a decreased efficiency of oxidative phosphorylation. This may exceed the whole animal capacity of oxygen uptake and distribution by ventilation and circulation and set a thermal limit, characterized by the transition to anaerobic metabolism. (C) 1999 Elsevier Science Inc. All rights reserve

Temperature-dependent shift of pHi in fish white muscle: contributions of passive and active processes

This study was designed to determine the mechanisms causing temperature-induced pH shifts in the white muscle of the marine teleost Zoarces viviparus. The white musculature undergoes an intracellular acidification with increasing body temperature at a slope of the pH-temperature relationship equal to -0.016 +/- 0.003 U/degree C. This is in good accordance with the overall relationship between the change in pK and the change in temperature of the intracellular proteins, which was determined to be -0.013 +/- 0.001 U/degree C. Thus the dissociation state of muscle proteins is kept fairly constant in white muscle of Zoarces viviparus. The passive component of the observed pH shift, which is due to the physicochemical response of the intracellular buffers to temperature change, accounts for only 35% of the pH transition. Ventilatory adjustment of intracellular PCO2 does not contribute to the temperature-induced shift of intracellular pH (pHi) in Zoarces viviparus. Therefore, the remaining 65% of pH adjustment must be ascribed to ion exchange mechanisms. The nonbicarbonate buffer value amounted to 34.4 +/- 2.3 meq.pH-1 kg cell water-1 at 12 degrees C and decreased slightly but not significantly with temperature. On the basis of our data we calculated that a removal of 0.52 mmol base equivalents.kg cell water-1.degree C-1 was necessary to shift pHi to its new steady state. </jats:p