This study investigated the possible role of catecholamines in the ventilatory response of rainbow trout (Oncorhynchus mykiss) to acute external hypercapnia. The ventilatory response to hypercapnia [partial pressure of CO2 in water (PwCO2=0.76 kPa)] of fish pre-treated with the selective beta-adrenoceptor antagonist, d,l-propranolol, was compared with that of d-propranolol (an isomer with minimal beta-antagonistic activity) and saline pre-treated fish (sham). A sustained 3.6- fold increase in gill ventilation volume (V(dot)w) was observed in the sham and d-propranolol-treated groups during the 30 min interval of hypercapnia. Fish pre-treated with d,l-propranolol displayed a blunted hyperventilatory response to hypercapnia (1.9-fold increase at 30 min). These results indicate that the beta-component of an adrenergic response is involved in the usual hyperventilatory response to external hypercapnia. It is suggested that the impaired hyperventilatory response of the d,l- propranolol-treated group reflects an inhibition of central adrenergic mechanism(s) involved in the hyperventilatory reflex to respiratory acidosis.

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