INFLUENCE OF HYPOXIA AND ADRENALINE ADMINISTRATION ON CORONARY BLOOD FLOW AND CARDIAC PERFORMANCE IN SEAWATER RAINBOW TROUT (ONCORHYNCHUS MYKISS)

To investigate the relationship between cardiac performance and coronary perfusion, cardiovascular variables (Q(dot), Vs, fh, Pda) and coronary blood flow (q·cor) were measured in rainbow trout (Oncorhynchus mykiss) (1.2­1.6 kg) before and after adrenergic stimulation (1.0 µg kg-1 adrenaline) under conditions of (1) normoxia, (2) hypoxia (approximate PwO2 12 kPa) and (3) 2.5 h after returning to normoxia. q·cor for resting fish under normoxic conditions was 0.14±0.02 ml min-1 kg-1 (approximately 0.85 % of Q(dot)). When exposed to hypoxia, although both resting Q(dot) and q·cor increased, q·cor increased to a greater degree (Q(dot) by 17 % and q·cor by 36 %). During hypoxia, maximum adrenaline-stimulated Q(dot) was comparable to that observed for normoxic fish. However, because Q(dot) was elevated in resting hypoxic fish, the capacity of hypoxic fish to increase Q(dot) above resting levels was 50 % lower than that measured in normoxic fish. Although maximum q·cor in adrenaline-injected hypoxic trout was greater than that measured in normoxic trout, post-injection increases in q·cor (above resting levels) were not different between the two groups. Two and a half hours after hypoxic exposure, resting Q(dot) was still elevated (11 %) above normoxic levels, and the ability to increase Q(dot) when adrenergically stimulated was not fully restored. These results suggest (1) that resting q·cor in salmonids is approximately 1 % of Q(dot), (2) that increases in q·cor may be important in maintaining cardiovascular performance during hypoxic conditions, (3) that interactions between alpha-adrenergic constriction and metabolically related vasodilation of the coronary vasculature are important in determining q·cor in fish, (4) that exposure of fish to moderate environmental hypoxia reduces the scope for adrenergically mediated increases in Q(dot), and (5) that periods of recovery in excess of several hours are required before cardiovascular performance returns to pre-hypoxic levels.