RECOVERY FROM DIVING BRADYCARDIA IN BULLFROGS

During forced submergence, bullfrogs, Rana catesbeiana, gradually develop bradycardia while immediate elevation of cardiac rate takes place upon emergence. Injection of N2 into the lungs of submerged bullfrogs does not affect the bradycardia while injections with air or O2 cause heart rate elevation following a 15–20 s latency. It is concluded that stimulation of pulmonary stretch receptors and elimination of hypoxia are not important to the instantaneous heart rate increase upon emergence. Emerging bullfrogs into an atmosphere of air or low O2 ((Formula presented.)=10–35 mm Hg) results in identical patterns of immediate and sustained heart rate recovery. Restoration of normal blood oxygen thus does not seem necessary for recovery of heart rate during at least the initial 6 min of emergence. Simultaneously monitoring respiratory and cardiac responses during emergence suggests that initial recovery from diving bradycardia in bullfrogs is dependent on initiation of respiratory movements. © 1979, Springer-Verlag. All rights reserved.