Gill and lung ventilatory responses to steady-stae aquatic hypoxia and hyperoxia in the bullfrog tadpole

Abstract

Gill ventilation frequency (f g) , the pressure amplutude (PBC) and stroke volume (Vs) of buccal ventilation cycles, the frequency of air breaths (f l), water flow over the gills (V̇ w), gill oxygen uptake (Ṁ g O 2 ), oxygen utilization (U), and hear frequency (f h) have been measured in unanaesthetized, air breathing Rana catesbeiana tadpoles (stage XVI-XIX). The animals were unrestrained except for ECG leads or cannulae, and were able to surface voluntarily for air breathing. They were subjected to aquatic normoxia, hyperoxia, and three levels of aquatic hypoxia, and their respiratory responses recorded in the steady state. The experiments were performed at 20 ± 0.5°C. In hyperoxia there was an absence of air breathing, and f g, PBC and V̇ w fell from the normoxic values, while U increased, resulting in no significant change in Ṁ g O 2 . Animals in normoxia showed a very low f l which increased in progressively more hypoxic states. V̇ w increased from the normoxic value in mild hypoxia (P O 2 = 96 ± 2 mm Hg), but fell, associated with a reduction in PBC, in moderate (P O 2 = 41 ± 1 mm Hg) and severe (P O 2 = 21 ± 3 mm Hg) hypoxia in the presence of lung ventilation. Gill Ṁ g O 2 was not significantly different from teh normoxic value in mild hypoxia but fell in moderate hypoxia, while in severe hypoxia oxygen was lost to the ventilating water from the blood perfusing the gills. There was no significant change in f h from teh normoxic value in either hypoxia or hyperoxia. These data indicate, that in the bimodally breathing bullfrog tadpole, aquatic P O 2 exerts a strong control over both gill and lung ventilation. Furthermore, there is an interaction between gill and lung ventilation such that the onset of a high frequency of lung ventilation in moderate and severe hypoxia promotes a suppression of gill ventilation cycles.