ABSTRACT
The time course of reflex changes in gill ventilation frequency, buccal pressure, branchial stroke volume and minute ventilation caused by a step-wise change in of inspired water and by the introduction of NaCN into the inspired water was measured in unanesthetized larval bullfrogs (Rana catesbeiana) at developmental stages TK V–VII, IX–XI and XVII–XIX. The inspiration of hypoxic or hyperoxic water caused significant increases in gill ventilation within 7 s, while hyperoxic water had the opposite effect. Introduction of NaCN into the inspired water stream caused nearly instantaneous respiratory responses (<3 s). Early and middle stage larvae showed a significantly faster and more pronounced reflex response than the older group in all experiments. The rapid changes in gill ventilation indicate the presence of a population of branchial receptors monitoring inspired water O2 levels or possibly blood O2 levels in the efferent branchial circulation. The progressive decrease in both the amplitude and rapidity of onset of the responses with development are consistent with the progressive degeneration of the gills and a reduction in the importance of branchial gas exchange in late larval stages.
