The autonomic nervous control of heart rate in ducks during voluntary diving

Autonomic nervous control of heart rate was studied in voluntarily diving ducks (Aythya affinis). Ducks were injected with the muscarinic blocker atropine, the beta-adrenergic blocker nadolol, the beta-adrenergic agonist isoproterenol, and a combination of both atropine and nadolol. Saline injection was used as a control treatment. The reduction in heart rate (from the predive level) normally seen during a dive was abolished by atropine. Nadolol reduced heart rate during all phases of diving activity-predive, dive, and postdive-indicating that sympathetic output to the heart was not withdrawn during diving. Isoproterenol increased heart rate before, during, and after the dive, although the proportional increase in heart rate was not as high during the dive as compared with the increase in routine heart rate or heart rate during the predive or postdive phase. The parasympathetic system predominates in the control of heart rate during diving despite the maintenance of efferent sympathetic influences to the heart, perhaps due to accentuated antagonism between the two branches of the autonomic nervous system.