The role of the autonomic nervous system in control of cardiac and air-breathing responses to sustained aerobic exercise in the African sharptooth catfish Clarias gariepinus

Abstract

Clarias gariepinus is a facultative air-breathing catfish that exhibits changes in heart rate (ƒ_H) associated with air-breaths (AB). A transient bradycardia prior to the AB is followed by sustained tachycardia during breath-hold. This study evaluated air-breathing and cardiac responses to sustained aerobic exercise in juveniles (total length~20cm), and how exercise influenced variations in f_H associated with AB. In particular, it investigated the role of adrenergic and cholinergic control in cardiac responses, and effects of pharmacological abolition of this control on air-breathing responses. Sustained exercise at 15, 30 and 45cms^-1 in a swim tunnel caused significant increases in f_AB and f_H, from approximately 5breathsh^-1 and 60heartbeatsmin^-1 at the lowest speed, to over 60breathsh^-1 and 100beatsmin^-1 at the highest, respectively. There was a progressive decline in the degree of variation in f_H, around each AB, as f_AB increased with exercise intensity. Total autonomic blockade abolished all variation in f_H during exercise, and around each AB, but f_AB responses were the same as in untreated animals. Cardiac responses were exclusively due to modulation of inhibitory cholinergic tone, which varied from >100% at the lowest speed to <10% at the highest. Cholinergic blockade had no effect on f_AB compared to untreated fish. Excitatory β-adrenergic tone was approximately 20% and did not vary with swimming speed, but its blockade increased f_AB at all speeds, compared to untreated animals. This reveals complex effects of autonomic control on air-breathing during exercise in C. gariepinus, which deserve further investigation.

Keywords: Adrenergic tone; Cholinergic tone; Heart rate; Swimming.