The time course of reflex changes in gill ventilation variables caused by a step-wise change in PO2 of inspired water and by the introduction of NaCN into inspired water was measured in two populations of unanesthetized larval bullfrogs (Rana catesbeiana) ­ one with intact gill arches and a second with bilateral ablation of the first gill arch. Developmental stages TK V­VII, IX­XI and XVII­XIX were examined. Inspiring hypoxic water or a pulse of NaCN significantly increased gill ventilation within 7 s in control larvae. Ablation of the first gill arches in larval R. catesbeiana eliminated the initial, quick response of gill ventilation to changes in the PO2 of inspired water and to the presence of NaCN in all three developmental groups. These data suggest that O2-sensitive chemoreceptors are located on the first gill arch and are responsible for the initial rapid ventilatory response. However, a slow response (>15 s) to changes in PO2 of inspired water persisted even after removal of the first gill arch. This response is far slower than the minimum blood circulation time (approximately 5­8 s as measured in all three groups), indicating that a second population of receptors is not directly monitoring arterial or venous blood. Instead, this second population of receptors is likely to be located 'behind' a significant diffusion barrier, possibly monitoring cerebrospinal fluid.

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