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

Abstract Respiratory properties and pH of blood were followed during acute exposure of rainbow trout to three levels of environmental hypoxia at 15°C. In a first stage, the blood oxygen affinity was preserved (mild hypoxia, P W O 2 = 60 mm Hg ) or slightly increased (deep hypoxia, P W O 2 = 35 mm Hg ), despite a simultaneous drop in arterial pH within the first 5–10 min. This is possibly cause 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: Hb 4 and GTP: Hb 4 molar ratios within the following 1–2 h. This was associated with a complete pH recovery. Very deep hypoxia ( P W O 2 = 30 mm Hg ) furthermore elicited at 20% increase in blood hemoglobin concentration within 20 min. This group showed a more pronounced drop in blood pH, without a complete. Calculated values of the arterial blood oxygen capacitance, βb O2 , are discussed in the context of the very different responses of trouts acutely subjected to mild and deep hypoxia, respectively.