Oxygen-limited thermal tolerance in Antarctic fish investigated by MRI and 31P-MRS

The hypothesis of an oxygen-limited thermal tolerance was tested in the Antarctic teleost Pachycara brachycephalum. With the use of flow-through respirometry, in vivo P-31-NMR spectroscopy, and MRI, we studied energy metabolism, intracellular pH (pH(i)), blood flow, and oxygenation between 0 and 13degreesC under normoxia (PO2: 20.3 to 21.3 kPa) and hyperoxia (PO2: 45 kPa). Hyperoxia reduced the metabolic increment and the rise in arterial blood flow observed under normoxia. The normoxic increase of blood flow leveled off beyond 7degreesC, indicating a cardiovascular capacity limitation. Ventilatory effort displayed an exponential rise in both groups. In the liver, blood oxygenation increased, whereas in white muscle it remained unaltered (normoxia) or declined (hyperoxia). In both groups, the slope of pH(i) changes followed the alpha-stat pattern below 6degreesC, whereas it decreased above. In conclusion, aerobic scope declines around 6degreesC under normoxia, marking the pejus temperature. By reducing circulatory costs, hyperoxia improves aerobic scope but is unable to shift the breakpoint in pH regulation or lethal limits. Hyperoxia appears beneficial at sublethal temperatures, but no longer beyond when cellular or molecular functions become disturbed.