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-s ensitive receptors and UA CO2 reflexly affects breathing, UA dilator muscle activity and UA resistance. These effects may function in the control of U A patency, There is evidence that some UA reflexes are stronger in young th an in adult animals, but it is not known whether CO2-sensitive receptors ar e present in the UA of young animals, and the effects of UA CO2 on UA resis tance and on UA dilator muscle activity have not been investigated in young animals. The responses of ventilation, UA resistance and geniohyoid muscle electromy ographic activity to warm air containing 10% CO2 applied to the isolated UA were measured in anaesthetized, vagotomized young guinea-pigs breathing sp ontaneously through a low-cervical tracheostomy, Upper airway carbon dioxide caused an increase in ventilation (46.7+/-16.3 to 49.9+/-16.8 mL.min(-1). 100 g body weight(-1)) and upper airway resistan ce (56.8+/-14.8 to 63.7+/-17.7 cmH(2)OL(-1).s(-1). kg body weight(-1)). Sim ilar effects were obtained following vagotomy, Geniohyoid activity became a pparent following vagotomy and this activity was reduced by upper airway ca rbon dioxide. These responses were abolished by topical anaesthesia of the upper airway, This suggests that the reflexes seen are due to carbon dioxid e-sensitive receptors in the upper airway.