The role of catecholamines in regulating arterial oxygen content during acute hypercapnic acidosis in rainbow trout (Salmo gairdneri).

The involvement of catecholamines in regulating arterial oxygen content (CaO2) during hypercapnic acidosis in rainbow trout (Salmo gairdneri) was investigated by comparing physiological responses during acute normoxic hypercapnia (a condition in which plasma epinephrine is elevated) and acute hyperoxic hypercapnia (a condition in which plasma epinephrine is not elevated). Red blood cell pH (rbc pH) was maintained significantly higher in the normoxic hypercapnic fish despite similar reductions in whole blood pH (pHe) in both groups. Elevation of rbc pH in the normoxic hypercapnic fish was abolished by pre-treatment with the beta-adrenoceptor antagonist, propranolol, whereas injection of epinephrine into hyperoxic hypercapnic fish significantly raised rbc pH. Arterial blood oxygen carrying capacity increased only in the normoxic hypercapnic fish due to significant increases in blood haemoglobin (Hb) levels. The ability of trout to elevate blood Hb was abolished by pre-treatment with the alpha-adrenoceptor antagonist, phentolamine. Injections of epinephrine into normocapnic fish caused increases in blood Hb and concomitant decreases in spleen wet weight and Hb content. Adrenergic elevation of blood Hb was not observed in splenectomized fish. CaO2, although depressed during normoxic hypercapnia, was indeed regulated when compared to CaO2 values predicted from the in vitro Root effect. Pre-treatment with phentolamine, but not propranolol, abolished the ability of trout to regulate CaO2.