SUMMARY: Changes in the properties of mitochondria from oxidative muscle of rainbow trout Oncorhynchus mykiss were examined during warm (5°C to 15°C) acclimation. Trout were studied shortly after the initial thermal change and after 8 weeks acclimation to 15°C. To identify potential mechanisms by which oxidative capacities change, the modifications of phospholipid composition, membrane proteins and functional capacities of red muscle mitochondria were examined. Marked functional changes of isolated muscle mitochondria during warm acclimation of rainbow trout were reflected by a host of modifications in phospholipid composition, but by few shifts in protein components. Shortly after transfer of trout from 5°C to 15°C,the maximal oxidative capacity of mitochondria measured at 15°C increased slightly, but rates at both assay temperatures (5°C and 15°C)decreased markedly after warm acclimation. The increase in capacity in short-term warm exposed trout was most pronounced when rates at 15°C were expressed relative to cytochrome a and c1 levels. Non-phosphorylating (State 4) rates of oxygen uptake increased with short-term warm exposure before returning to initial levels after warm acclimation. Cytochrome c oxidase (CCO) activity in the mitochondrial preparations decreased with warm acclimation. The thermal sensitivity of the ADP affinity was markedly modified during short-term warm exposure, when the ADP/O ratio increased, but warm acclimation returned these values to those observed initially. ADP affinity increased after warm acclimation. Changes in the mitochondrial content of cytochromes and adenine nucleotide translocase (ANT)could not explain these patterns. On the other hand, changes in the proportions of the lipid classes and in the acyl chain composition of certain phospholipid classes mirror the modifications in functional properties. Short-term exposure to 15°C decreased the ratio of diacylphosphatidylethanolamine/diacylphosphatidylcholine (diacylPE/diacylPC),whereas warm acclimation led to restructuring of fatty acids (FA) and to increases of plasmalogen forms of PE and PC. Modification of overall membrane unsaturation did not appear to be the primary aim of restructuring membrane FA during warm acclimation, as total mitochondrial phospholipids and the major phospholipid classes only showed slight shifts of their acyl composition with warm acclimation. On the other hand, natural lysophosphatidylcholine (LysoPC)showed dramatic changes in FA content, as 16:0 and 18:1n-9 doubled whereas 22:6n-3 decreased from around 50% to 32% in warm acclimated trout. Similarly,in cardiolipin (CL), the levels of 16:0 and 18:1n-7 halved while 18:2n-6 increased to over 20% of the FA with warm acclimation. Given the central role of CL in modulating the activity of CCO, F0F1-ATPase and ANT, these changes suggest that specific compositional changes in CL are important modulators of mitochondrial capacities. The many structural changes in membrane lipids contrast with the limited modifications of the membrane protein components examined and support the concept of lipid structure modulating mitochondrial capacities.

Summary: As oxygen concentrations in marine coastal habitats can fluctuate rapidly and drastically, sessile marine organisms like the oyster Crassostrea gigas can experience marked and rapid oxygen variations. In this study, we investigated the responses of oyster gill mitochondria to short-term hypoxia (3 h and 12 h, at 1.7 mg O2 L-1) and subsequent re-oxygenation. Mitochondrial respiratory rates (state 3 and 4 stimulated by glutamate), phosphorylation efficiency (RCR ratio and ADP/O) were measured. Cytochrome c oxidase activity (CCO) and cytochrome concentrations (a, b, c1, c) were measured to investigate the rearrangements of respiratory chain subunits. The potential implication of an alternative oxidase (AOX) was investigated using an inhibitor of the respiratory chain (antimycin A) and through gene expression analysis in gills and digestive gland. Results indicate a down regulation of mitochondrial capacity, with 60% inhibition of respiratory rates after 12 h of hypoxia. RCR ratio remained stable, while ADP/O increased after 12 h of hypoxia and 1 h of re-oxygenation, suggesting increased phosphorylation efficiency. CCO showed a fast and remarkable increase of its catalytic activity only after 3 h of hypoxia. AOX mRNA levels showed similar patterns in gill and digestive gland, and were up regulated after 12 and 24 h of hypoxia and during re-oxygenation. Results suggest a set of controls in regulating mitochondrial functions in response to oxygen fluctuations and demonstrate the fast and extreme plasticity of oyster mitochondria in response to oxygen variations.