Abstract
Clarias gariepinus is a facultative air-breathing catfish that exhibits changes in heart rate (ƒH) 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 fH 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 fAB and fH, 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 fH, around each AB, as fAB increased with exercise intensity. Total autonomic blockade abolished all variation in fH during exercise, and around each AB, but fAB 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 fAB compared to untreated fish. Excitatory β-adrenergic tone was approximately 20% and did not vary with swimming speed, but its blockade increased fAB 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.