The Effect of Arterial-Blood PO2, PCO2, and pH on Diving Bradycardia in the Bullfrog Rana catesbeiana

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Abstract
Bullfrogs were submerged for 25 min in water at 25 C saturated with either O₂, air, or N₂. The heart rates of all three groups were not significantly different prior to submergence. The heart rate of the O₂ group did not change during submergence while the air and N₂ groups experienced significant bradycardia. In paired comparisons, only the O₂ and N₂ groups exhibited significantly different degrees of bradycardia. The extent of bradycardia, therefore, was correlated inversely with the O₂ saturation of the water. Upon emergence, heart rates were elevated above the initial rates in all three groups. All three groups showed predive arterial PO₂ ( ), PCO₂ ( ), and pH values that were not significantly different. All three groups underwent significant changes in all three blood parameters during submergence. In paired comparisons, the O₂ and air groups and the O₂ and N₂ groups showed values during submergence that were different and which related directly to levels of O₂ in the water. No differences were observed among the three groups in either or arterial pH levels during the dive period. Bullfrogs breathing N₂ exhibited an increase in heart rate, but levels were not different from the group submerged in air at middive. It is concluded that a decrease in arterial PO₂ coupled with inhibition of lung ventilation is correlated with development of diving bradycardia in the bullfrog.

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