Oxygen-limited thermal tolerance in Antarctic fish

Mark, F. C., C. Bock, and H. O. Pörtner. Oxygen limited thermal tolerance in Antarctic fish investigated by MRI and 31P-MRS. Am J Physiol Regul Integr Comp Physiol 283: R1254–R1262, 2002. First published August 8, 2002; 10.1152/ajpregu.00167.2002.—The hypothesis of an oxygenlimited thermal tolerance was tested in the Antarctic teleost Pachycara brachycephalum. With the use of flow-through respirometry, in vivo 31P-NMR spectroscopy, and MRI, we studied energy metabolism, intracellular pH (pHi), blood flow, and oxygenation between 0 and 13°C 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 7°C, 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 pHi changes followed the alpha-stat pattern below 6°C, whereas it decreased above. In conclusion, aerobic scope declines around 6°C 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.