Heart-Rate Response to Forced and Voluntary Diving in Swamp Rabbits Sylvilagus aquaticus

Biotelemetry transmitters were surgically implanted in the abdominal cavity of 12 mature swamp rabbits, Sylvilagus aquaticus, to monitor the heart-rate response to forced and voluntary diving. Atropine and propranolol were used to investigate autonomic control of heart rate. Each untreated rabbit showed a marked (- 79%) bradycardia when forcefully submerged for 10 s. Mean heart-rate values during predive, dive, and recovery were 213 ± 12.6, 44 ± 7.5, and 206 ± 14.9 beats/min (X̅ ± SE), respectively. Onset of bradycardia occurred rapidly at a rate of - 169 (beats/min)/s. The bradycardia was fully established during the first second of the dive. A marked sinus arrhythmia often occurred during forced diving. Recovery occurred at a rate of 206 (beats/min)/s during the first second, then dropped to 15.9 (beats/min)/s. Propranolol reduced bradycardia to - 65%, although dive values were not significantly different from control diving. Mean values were 146 ± 11.2, 51 ± 9.5, and 130 ± 11.9 beats/min, respectively. In propanolol experiments, onset was rapid (95 beats/min)/s, but recovery rate was reduced to 35 (beats/min)/s during the first second and to 7.4 (beats/min)/s thereafter. Atropine abolished diving bradycardia. Diving increased parasympathetic tone from 29.8% to 75.5% and reduced sympathetic tone from 46.2% to 35.1%. Together, these data suggest that both portions of the autonomic nervous system are important for forced diving bradycardia. Parasympathetic activity initiates and maintains the response. Removal of the sympathetic tone enhances the response, as evidenced by the propranolol experiments. Sympathetic activity is necessary for rapid recovery. Four rabbits were trained to voluntarily dive for a food reward. Voluntary dives lasted 4 s. Mean heart-rate values were 197 ± 10.1, 211 ± 19.3, and 243 ± 15.9 beats/min. No bradycardia was observed during voluntary diving, suggesting that the bradycardia associated with forced diving is a response to fear and not submergence.