Branchial chemoreceptors mediate ventilatory responses to hypercapnic acidosis in channel catfish

The effects of hyperoxic hypercapnia on cardiovascular and ventilatory variables and blood gas and acid/base parameters were examined in conscious and anesthetized spontaneously breathing (ASB) channel catfish, Ictalurus punctatus. These separate experiments were designed to determine: (1) if channel catfish show a ventilatory response to hypercapnic acidosis when blood O(2) content is maintained in conscious animals; and (2) whether branchial receptors innervated by cranial nerves IX and X mediate this response. The combination of high O(2) and CO(2) tensions allowed the cardioventilatory effects of hypercapnic acidosis to be assessed independently of Root or Bohr mediated changes in blood O(2) content. In the absence of significant changes in dorsal or ventral aorta O(2) content, hyperoxic hypercapnia significantly stimulated ventilation, relative to hyperoxic exposure. Hypercapnic acidosis, however, had no significant effects on blood pressure or heart rate. Branchial denervation in ASB fish abolished the ventilatory response to hypercapnic acidosis. The results indicate that hypercapnic acidosis independently stimulates ventilation in channel catfish. This response is mediated by CO(2)/pH-sensitive branchial receptors innervated by cranial nerves IX and X.