RELATIONSHIP BETWEEN BLOOD O2 CONTENT AND CATECHOLAMINE LEVELS DURING HYPOXIA IN RAINBOW-TROUT AND AMERICAN EEL

Plasma catecholamine levels and arterial blood respiratory variables were monitored in rainbow trout (Oncorhynchus mykiss) and American eel (Anguilla rostrata) acutely exposed (30 min) to graded levels of external hypoxia [water PO2 (PW(O2)) 20-90 Torr]. The experiments were designed to evaluate the factors controlling catecholamine mobilization in hypoxic fish and to elucidate the basis of marked interspecific differences. In trout, plasma catecholamine levels were unchanged when PW(O2) remained above 50 Torr but increased markedly when PW(O2) was lowered below this value; the predominant catecholamine released into the circulation was epinephrine. In eel, there was no such obvious PW(O2) threshold for catecholamine release although plasma levels were consistently elevated above baseline only at PW(O2) <35 Torr. The magnitude of the catecholamine release in eel was approximately an order of magnitude less than in trout. Unlike in trout, there was no increase in the plasma epinephrine-to-norepinephrine concentration ratio. During hypoxia, the relationship between arterial blood PO2 (Pa(O2)) and PW(O2) was similar in both species and thus could not explain the differences in the PW(O2) thresholds for catecholamine release. In trout, the calculated Pa(O2) thresholds for catecholamine release were 25.3 (epinephrine) and 20.5 Torr (norepinephrine) whereas in eel the corresponding values were 12.5 and 11.6 Torr, respectively. These Pa(O2) thresholds were in good agreement with the in vivo values for Pa(O2) at half-maximal hemoglobin (Hb)-O2 saturation (P50) for trout and eel blood of 22.9 and 11.1 Torr, respectively. Thus both species displayed essentially equivalent catecholamine release thresholds when expressed in terms of arterial blood O2 content corresponding to approximately 45-60% Hb-O2 saturation.