Summary

1. 1. Carp, when subjected to air stress in vivo, developed a hypoxaemia associated with an acidosis due to elevation of PCO2 and arterial lactate content. Adrenaline and noradrenaline levels rose markedly, and the mean corpuscular haemoglobin concentration (MCHC in mmol Hb l-1 red blood cells) decreased, indicating swelling of the erythrocytes. 2. No effect of adrenaline could be observed in vitro after equilibration at normal pH and oxygenation level. 3. Simulation of air stress in vitro, by using hypercapnic hypoxia together with adrenaline, caused appreciable swelling of the red cells. The effect of adrenaline could be blocked by propranolol and was reversible when the blood gases were readjusted to normal values. Swelling due to hypercapnic hypoxia was not affected by propranolol. 4. At normal PO2, lowering pH by increasing PCO2 or adding HCl to the blood at PCO2 = 3.5 mmHg (1 mmHg = 133.3 Pa) both led to swelling of red cells; further swelling occurred when adrenaline was added. 5. At normal or even elevated intracellular pH, hypoxia led to red cell swelling; further swelling occurred when adrenaline was added. 6. The swelling of red cells under hypoxic and/or acidotic conditions (induced by either added HCl or increased PCO2) was associated with an increase in the intracellular K+, Na+ and Cl- levels, together with an increase in the Donnan distribution ratio of Cl-, rCl. delta K+:delta Na+ was 4:1. The swelling associated with the addition of adrenaline was also accompanied by an increase in K+, Na+, Cl- and rCl. In this case the ratio delta K+:delta Na+ was 1:2.4. 7. There was a correlation between the Donnan ratio, rCl, and the change in MCHC, even in those cases where extracellular and intracellular pH were above normal. In addition to pH and oxygen saturation, rCl should therefore be considered as a possible triggering factor for the action of adrenaline. 8. In addition to the increased catecholamine levels, stress during sampling may be inferred if low values of MCHC and high values of intraerythrocytic chloride and a high chloride distribution ratio, rCl, are found in whole blood.