Effects of upper airway carbon dioxide on upper airway resistance and muscle activity in young guinea-pigs.

The upper airway (UA) of adult animals is known to contain carbon dioxide-sensitive receptors and UA CO2 reflexly affects breathing, UA dilator muscle activity and UA resistance. These effects may function in the control of UA patency. There is evidence that some UA reflexes are stronger in young than in adult animals, but it is not known whether CO2-sensitive receptors are present in the UA of young animals, and the effects of UA CO2 on UA resistance and on UA dilator muscle activity have not been investigated in young animals. The responses of ventilation, UA resistance and geniohyoid muscle electromyographic activity to warm air containing 10% CO2 applied to the isolated UA were measured in anaesthetized, vagotomized young guinea-pigs breathing spontaneously through a low-cervical tracheostomy. Upper airway carbon dioxide caused an increase in ventilation (46.7+/-16.3 to 49.9+/-16.8 mL x min(-1) x 100 g body weight(-1)) and upper airway resistance (56.8+/-14.8 to 63.7+/-17.7 cmH2O x L(-1) x s(-1) x kg body weight(-1)). Similar effects were obtained following vagotomy. Geniohyoid activity became apparent following vagotomy and this activity was reduced by upper airway carbon dioxide. These responses were abolished by topical anaesthesia of the upper airway. This suggests that the reflexes seen are due to carbon dioxide-sensitive receptors in the upper airway.