Autonomic regulation of the heart during digestion and aerobic swimming in the European sea bass (Dicentrarchus labrax)

The autonomic regulation of the heart was studied in European sea bass (Dicentrarchus labrax) during digestion and aerobic exercise by measuring cardiac output (Q), heart rate (fH), stroke volume (Vs) and oxygen consumption (MO2) before and after pharmacological blockade by intraperitoneal injections of atropine and propranolol. The significant rise in MO2 (134 ± 14 to 174 ± 14 mg kg−1 h−1) 6 h after feeding (3% body mass) caused a significant tachycardia (47.7 ± 10.9 to 72.6 ± 7.2 beats min−1), but only a small elevation of Q. MO2 of fasting fish increased progressively with swimming speed (0.7–2.1 BL s−1) causing a significant tachycardia (43 ± 6 to 61 ± 4 mL min−1 kg−1) and increased Q but Vs did not change. Inactive fish were characterized by a high vagal tone (98.3 ± 21.7%), and the tachycardia during digestion and exercise was exclusively due to a reduction of vagal tone, while the adrenergic tone remained low during all conditions. Intrinsic fH, revealed after double autonomic blockade, was not affected by digestion (71 ± 4 and 70 ± 6 min−1, respectively), indicating that non-adrenergic, non-cholinergic (NANC) factors do not contribute to the tachycardia during digestion in sea bass.