Airway ammonia negates the normal ventilatory response to airway CO2 in garter snakes

Abstract

Earlier studies from this lab showed that unidirectionally ventilated snakes, maintained on high airway [CO₂], breathe slowly with a large tidal volume. If airway [CO₂] is abruptly reduced during inspiration, inspiratory duration (ti) and tidal volume increase. On the other hand, in an animal normally receiving fresh air (no CO₂ in the inspired air) on each inspiration, if fresh air is withheld for one breath, ti and tidal volume decrease. To test the effect of producing an alkaline environment in the presence of CO₂ during these two maneuvers, six unidirectionally ventilated snakes weighing 32±16 g were maintained on 4% CO₂ in air flowing at 300 ml/min even during inspiration. NH₃ (4%) was introduced into the lung during one inspiration. During that breath, ti increased significantly, as if CO₂ had been removed from the lung. In another study, six different snakes weighing 21±6 g received fresh air at each inspiration. At the end of inspiration [CO₂] was again raised to 4%. However, for one breath, [CO₂] was maintained high during inspiration and airway NH₃ (3–4%) was simultaneously introduced into the airstream. During this breath, ti was expected to decrease, owing to the elevated [CO₂]; however, there was no significant difference in the ti of the test breath compared with the previous breath, indicating that ammonia interfered with the expected CO₂ response. This study suggests that pH (probably intracellular) rather than P_CO2 produces this ventilatory reflex.