The development of thermotolerance protects blowfly flight muscle mitochondrial function from heat damage

The LD50 of 10-day-old adult blowflies was determined to be 38.12±0.07 °C. A transitory increase in heat resistance occurred following the exposure of adult blowflies to a sublethal heat shock at 36 °C. This thermotolerance was apparent 1 h after the application of the shock, was maximal 2­3 h later and had disappeared 6 h after exposure. Oxidative phosphorylation by flight muscle mitochondria from control flies was impaired by an LD50 dose in vivo using both pyruvate+proline (P+P) and glycerol 3-phosphate (G3P) as substrates. Acceptor control (state III respiration/state IV respiration) was lost with G3P as substrate and so ADP:O ratios were not measurable. The effect of experimental temperature in vitro on respiratory performance of mitochondria isolated from control and thermotolerant flies was also determined between 19 and 39 °C. State III respiration was markedly temperature-dependent in mitochondria from control flies with both substrates; it was maximal at 24­29 °C and fell progressively at higher measuring temperatures. In mitochondria from thermotolerant flies, state III respiration was less temperature-dependent with both substrates but this was most marked for G3P. The effect of experimental temperature on state IV respiration was similar in mitochondria from control and thermotolerant flies with each substrate, but differed between the two substrates. With G3P as substrate, respiration rate rose with temperature with a Q10 of approximately 1.5; however, with P+P as substrate, the trend was for respiration rate to fall as experimental temperature rose. Using G3P as substrate, acceptor control was demonstrable at 34 °C in some preparations of mitochondria from thermotolerant flies but not in those from control flies at that temperature. With P+P as substrates, acceptor control was demonstrable in mitochondria from both control and thermotolerant flies at all experimental temperatures.