Autonomic cardiovascular control during submergence and emergence in bullfrogs

Unanesthetized bullfrogs were involuntarily submerged for 25 min in air-saturated water at 21 degrees C. Significant bradycardia was observed while systemic blood pressure was maintained or slightly elevated. Upon emergence, heart rates immediately returned to presubmergence levels or higher. Similar responses were observed in frogs allowed to make voluntary dives in an experimental tank. Heart rates of vagal-blocked (atropine) frogs did not change during submergence or emergence. beta-Adrenergic blockade (propranolol) had little effect on the magnitude of heart rate decrease during submergence or its increase upon emergence. After alpha-adrenergic blockade (phentolamine), frogs developed diving bradycardia while undergoing a fall in systemic blood pressure. It is concluded that, in bullfrogs, 1) bradycardia during submergence is entirely due to increased vagal activity, 2) the immediate cardiac rate increase upon emergence apparently results from a decrease in vagal tone; and 3) there appears to be no substantial reciprocal sympathetic influence on heart rate during alterations in vagal tone.