Diel and seasonal rhythms of the heart rate in the common eel (Anguilla anguilla L.): role of cardiac innervation.

Using a computerized system, ECG recordings of eels were made during long-term experiments, to show circadian and circannual variations in the heart rate. The circadian variations synchronize with the photoperiod. The maximum frequency is observed during the dark phase, the peak values occurring just after sunset and before sunrise. By using specific blockers--atropine, propranolol, desmethylimipramine--the respective roles of neurotransmitters (acetylcholine and catecholamines) were evaluated. The results point to the existence of a dual nervous regulation of the cardiac function involving a permanent sympathetic tone, even in the resting eel.