Present address and address for reprint requests: Istituto di Scienze Farmacologiche, via Balzaretti 9, Università di Milano, Milano 20133, Italy.

The role of sensory afferent information from the gills of Amia calva in cardiovascular and ventilatory control was investigated by bilateral branchial denervation and pseudobranch ablation.

Aquatic hypoxia or 1 mg of sodium cyanide (NaCN) in the water flowing over the gills stimulated bradycardia, and gill and air ventilation in sham-operated fish. Sodium cyanide, noradrenaline (NA) and adrenaline (A) infusion into the dorsal aorta increased gill ventilation, and NA and A infusion also stimulated tachycardia and an increase in blood pressure.

Following denervation and pseudobranch ablation, O2 consumption (V·OO2), airbreathing frequency (fAB) and arterial O2 tension (PaOO2) declined, and circulating NA levels increased, as compared with sham-operated fish. Cardiovascular and air-breathing responses to hypoxia were abolished and gill ventilatory responses attenuated. All ventilatory and cardiovascular responses to NaCN were abolished and gill ventilatory responses to NA and A were attenuated in animals following denervation and pseudobranch ablation.

These results demonstrate that O2-sensitive chemoreceptors in the gills and pseudobranch control reflex bradycardia and air-breathing responses in Amia, but that gill ventilatory responses to hypoxia, NA and A are partially mediated by extrabranchial mechanisms. Plasma NA levels increased during hypoxia in shamoperated and denervated animals, indicating that circulating NA may have mediated gill ventilatory responses in denervated animals.

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