THE EFFECTS OF CATECHOLAMINES ON VENTILATION IN RAINBOW-TROUT DURING HYPOXIA OR HYPERCAPNIA

This study assessed the effect of experimentally elevated plasma catecholamine levels on gill ventilation in rainbow trout (Oncorhyncus mykiss) exposed to various external ventilatory stimulants. Trout were exposed to hypoxia (water P(O2) (PW(O2)) = 90 Torr) or hypercapnia (water P(CO2) (PW(CO2)) = 4.5 Torr) for 30 min. These conditions caused gill ventilation volume (Vw) to increase by 2.3- and 1.5-fold, respectively, but did not stimulate release of catecholamines into the blood. While the stimulus (hypoxia or hypercapnia) was maintained, fish were given a bolus injection (0.3 ml), followed by intra-arterial infusion (0.6 ml.h-1), of a catecholamine mixture (2 x 10(-5) mol.1-1 adrenaline + 5 x 10(-6) mol.l-1 noradrenaline) to mimic the physiological concentrations and ratios of these catecholamines observed under severe hypoxic or hypercapnic conditions. In hypoxic fish, this treatment caused a significant, but transient (5 min) depression of ventilation while during hypercapnia, the administration of exogenous catecholamines caused a more prolonged hypoventilatory response. These hypoventilatory responses occurred despite a catecholamine-induced blood acidosis (a potential ventilatory stimulant). To assess the importance of initial Vw and/or blood respiratory status on catecholamine-mediated hypoventilation, these experiments were repeated under hyperoxic (PW(O2) = 640 Torr) hyperoxic hypercapnic (PW(O2) = 510 Torr, PW(CO2) = 4.8 Torr) or normoxic (PW(O2) = 151 Torr) conditions in which Vw was either depressed (3.9-fold during hyperoxia) or unaffected. Intra-arterial infusion of catecholamines did not affect Vw under either of these experimental conditions. These results demonstrate that during a respiratory challenge, such as hypoxia or hypercapnia, physiologically relevant levels of circulating catecholamines can depress Vw and therefore do not support a stimulatory role for circulating catecholamines in the control of ventilation in fish.