THE EFFECTS OF ACCLIMATION TEMPERATURE ON THE DYNAMICS OF CATECHOLAMINE RELEASE DURING ACUTE HYPOXIA IN THE RAINBOW TROUT ONCORHYNCHUS MYKISS

The response of cannulated rainbow trout (Oncorhynchus mykiss) to acute hypoxia was studied in fish acclimated to two temperatures (5 and 15 °C). Blood/water respiratory variables and plasma catecholamine levels were measured before and 15 min after exposure to hypoxic water varying between 4.0 and 10.7 kPa (30­80 mmHg) oxygen partial pressure (PwO2). Arterial blood PO2 (PaO2) and oxygen content (CaO2) fell during hypoxia in a similar manner at both temperatures, although the changes in CaO2 were often more pronounced in the fish acclimated to 15 °C. Regardless of acclimation temperature, plasma catecholamine levels were consistently elevated at PwO2 values below 8.0 kPa (60 mmHg); the largest increases in plasma catecholamine levels occurred below PwO2=5.3 kPa (40 mmHg). Adrenaline was the predominant catecholamine released into the circulation. Adrenaline was released at PwO2 values of 8.0 kPa or below, whereas noradrenaline was released at PwO2 values of 6.7 kPa or below. The construction of in vivo oxygen dissociation curves demonstrated an obvious effect of acclimation temperature on haemoglobin (Hb) oxygen-affinity; the P50 values at 15 °C and 5 °C were 3.6 kPa (26.7 mmHg) and 1.9 kPa (14.0 mmHg), respectively. At 15 °C, catecholamines were released into the circulation abruptly at a PaO2 threshold of 4.6 kPa (34.5 mmHg) while at 5 °C the catecholamine release threshold was lowered to 3.3 kPa (24.5 mmHg). The difference in the PaO2 catecholamine release thresholds was roughly equivalent to the difference in the P50 values at the two distinct temperatures. Catecholamine release thresholds, calculated on the basis of arterial blood oxygen-saturation (expressed as CaO2/[Hb]), were similar at both temperatures and were approximately equal to 53­55 % Hb O2-saturation. The results support the contention that the lowering of blood oxygen content/saturation rather than PO2 per se is the proximate stimulus/signal causing catecholamine release in rainbow trout during acute hypoxia.