The temperature dependence of oxidative phosphorylation and of the activity of various enzyme systems in liver mitochondria from cold- and warm-blooded animals

Top-down control analysis of the effect of temperature on ectotherm oxidative phosphorylation

Top-down control and elasticity analysis was conducted on mitochondria isolated from the midgut of the tobacco hornworm ( Manduca sexta) to assess how temperature affects oxidative phosphorylation in a eurythermic ectotherm. Oxygen consumption and protonmotive force (measured as membrane potential in the presence of nigericin) were monitored at 15, 25, and 35°C. State 4 respiration displayed a Q10of 2.4–2.7 when measured over two temperature ranges (15–25°C and 25–35°C). In state 3, the Q10s for respiration were 2.0 and 1.7 for the lower and higher temperature ranges, respectively. The kinetic responses (oxygen consumption) of the substrate oxidation system, proton leak, and phosphorylation system increased as temperature rose, although the proton leak and substrate oxidation system showed the greatest thermal sensitivity. Whereas there were temperature-induced changes in the activities of the oxidative phosphorylation subsystems, there was no change in the state 4 membrane potential and little change in the state 3 membrane potential. Top-down control analysis revealed that control over respiration did not change with temperature. In state 4, control of respiration was shared nearly equally by the proton leak and the substrate oxidation system, whereas in state 3 the substrate oxidation system exerted over 90% of the control over respiration. The proton leak and phosphorylation system account for <10% of the temperature-induced change in the state 3 respiration rate. Therefore, when the temperature is changed, the state 3 respiration rate is altered primarily because of temperature's effect on the substrate oxidation system.

Dantrolene and the neuromuscular junction: evidence for intracellular calcium stores

Dantrolene sodium (DaNa) markedly depressed the frequency of spontaneous miniature potentials at the frog neuromuscular junction. Its action is still observed at low [Ca2+]o and is little affected by temperature; its effect is not readily reversible. Theophylline antagonises the action of DaNa. DaNa prevents the stimulatory effect of the ionophore A23187 at 25 degrees C, but is ineffective in antagonising the action of X537A. DaNa and theophylline do not affect the functioning and integrity of mitochondria in vitro and it is suggested that DaNa acts at intracellular Ca2+ stores (other than the mitochondria) in the presynaptic terminals.