Branchial chemoreceptors mediate ventilatory responses to hypercapnic acidosis in channel catfish

The effects df hyperoxic hypercapnia on cardiovascular and ventilatory vari ables and blood gas and acid/base parameters were examined in conscious and anesthetized spontaneously breathing (ASB) channel catfish, Ictalurus punc tatus. These separate experiments were designed to determine: (1) if channe l catfish show a ventilatory response to hypercapnic acidosis when blood O- 2 content is maintained in conscious animals; and (2) whether branchial rec eptors innervated by cranial nerves IX and X mediate this response. The com bination of high O-2 and CO2 tensions allowed the cardioventilatory effects of hypercapnic acidosis to be assessed independently of Root or Bohr media ted changes in blood O-2 content. In the absence of significant changes in dorsal or ventral aorta O-2 content, hyperoxic hypercapnia significantly st imulated ventilation, relative to hyperoxic exposure. Hypercapnic acidosis, however, had no significant effects on blood pressure or heart rate. Branc hial denervation in ASB fish abolished the ventilatory response to hypercap nic acidosis. The results indicate that hypercapnic acidosis independently stimulates ventilation in channel catfish. This response is mediated by CO2 /pH-sensitive branchial receptors innervated by cranial nerves IX and X. (C ) 2000 Elsevier Science Inc. All rights reserved.