Cardioventilatory responses to hypoxia and NaCN in the neotenous axolotl.

Ventilatory and cardiac responses to hypoxia, sodium cyanide (NaCN), and intra-arterial injection of atropine, noradrenaline and DL-propranolol were investigated in the neotenous axolotl (Ambystoma mexicanum). Hypoxia elicited increased gill and lung ventilation and a tachycardia. Gill ventilation and air-breathing were stimulated by NaCN infused either into the ventilatory water stream or into the bloodstream. Cardiac responses to NaCN were complex, with an initial bradycardia followed by a tachycardia. In all animals, the tachycardia developed subsequent to lung ventilation. Atropine raised resting heart rate and abolished the bradycardia elicited by NaCN. Noradrenaline stimulated gill ventilation and heart rate but not air-breathing. These responses were not abolished by DL-propranolol, and propranolol had no effect on responses to NaCN. The results indicate that the axolotl possesses O2 chemo-sensitivity to both external and internal milieu, shows similar O2 chemo-reflexes to those of larval anurans and air-breathing fish, but does not exhibit the beta-adrenergic effects on ventilation observed in water-breathing fish.