Acute exposure of rainbow trout to mild and deep hypoxia: O2 affinity and O2 capacitance of arterial blood.

Respiratory properties and pH of blood were followed during acute exposure of rainbow trout to three levels of environmental hypoxia at 15 degrees C. In a first stage, the blood oxygen affinity was preserved (mild hypoxia, PwO2 = 60 mm Hg) or slightly increased (deep hypoxia, PwO2 = 35 mm Hg), despite a simultaneous drop in arterial pH within the first 5-10 min. This is possibly caused by a catecholamine induced increase in red cell pH. The second stage showed for the mild hypoxia group a temporary increase in affinity followed by a recovery within 60 min, correlating with the changes in arterial pH. The deep hypoxia group, however, further increased the blood oxygen affinity, due to a rapid decrease in the ATP:Hb4 and GTP:Hb4 molar ratios within the following 1-2 h. This was associated with a complete pH recovery. Very deep hypoxia (PwO2 = 30 mm Hg) furthermore elicited a 20% increase in blood hemoglobin concentration within 20 min. This group showed a more pronounced drop in blood pH, without a complete recovery. Calculated values of the arterial blood oxygen capacitance, beta bO2, are discussed in the context of the very different responses of trouts acutely subjected to mild and deep hypoxia, respectively.