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Control of heart rate during diving in ducks Furilla, Robert Alan


Forced submergence of redhead ducks (Aythya americana) caused heart rate to fall from 100 ± 3 beats⋅min⁻ ¹ (mean ± s.e.m., N = 12) to a stable rate of 35 ± 4 beats⋅min⁻ ¹ (N = 12) within 5 seconds after submergence. Bradycardia was unaffected by breathing oxygen before the dive or by denervation of the baroreceptors, but was virtually eliminated by local anaesthesia of the narial region. When freely diving on a man-made pond, heart rate of redhead ducks and lesser scaup (A. affinis) two seconds after submergence was positively correlated with the pre-dive rate (r² = 0.71). Breathing oxygen before the dive and denervation of baroreceptors had little effect on this relationship. Chasing to induce submergence caused a slight enhancement of bradycardia, heart rate during the dive being about 10% lower than after a voluntary dive. Local anaesthesia of the narial region inhibited voluntary diving, but heart rates in chase-induced dives after nasal blockade were significantly higher (10-30%) than those obtained from untreated ducks in chase-induced dives. Dive heart rate, at 2-5 seconds submergence, was linearly related to the logarithm of the pre-dive rate for all voluntary and forced dives as well as dabbles. Even the heart rate which occurred 2-5 seconds after being trapped under water as a function of the rate immediately before trapping fitted this relationship. The function was described by the equation Y = -451 + 246 LOG X, where Y = dive (or trapped) and X = pre-dive (or pre-trap) heart rate (r² = 0.98). The relationship was unaltered by β-blockade with propranolol. Data from stimulation of the cut distal ends of vagal and cardiac sympathetic nerves suggest that a similar increase in vagal activity occurs on submergence in all of these dives. The first cardiac interval in voluntary dives represents a lower heart rate, indicating a higher level of vagal activity. When dabbling ducks (Anas platyrhynchos) dabble, heart rate at two seconds submergence is little changed from the pre-dabble rate. When these birds dive, however, heart rate at two seconds submergence is about 250 beats⋅min⁻ ¹, regardless of the pre-dive rate. Bilateral denervation of arterial baroreceptors significantly altered the dive:pre-dive relationship . These results have shown that nasal receptors are responsible for bradycardia in diving ducks when forcibly submerged, but that nasal receptors contribute little to the change in heart rate when ducks dive voluntarily. The results also suggest that there is a psychogenic modulation of the heart rate in voluntary dives which influences the pre-dive rather than the dive heart rate. Finally, dabbling ducks differ from diving ducks in their response to forced and voluntary diving. Chemoreceptors are responsible for the majority of the response in forced dives, and baroreceptors provide primary control in voluntary dives.

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