PULMONARY RECEPTOR CHEMOSENSITIVITY AND THE VENTILATORY RESPONSE TO INHALED CO2 IN THE TURTLE

Ventilatory responses of unanesthetized turtles to changes in the intrapulmonary CO2 content of a vascularly isolated lung and an intact lung were measured during spontaneous breathing. The hyperpnea associated with inhalation of CO2 by the vascularly isolated lung was 19% of that associated with inhalation of CO2 by the intact lung. Transection of the vagus nerve supplying the isolated lung abolished this response. Both inhibition of pulmonary stretch receptor discharge with increasing levels of FICO2 [fraction of O2 in inspired gas] and a functional increase in central inspiratory volume threshold during hypercapnea apparently contribute to tidal volume increases following CO2 inhalation in normal animals. The major component of the ventilatory response of intact turtles to increasing levels of FICO2, was an increase in respiratory frequency. When CO2 was inspired by only the vascularly isolated lung the increase in respiratory frequency was only 21% of that recorded when the same levels of CO2 were inspired by the intact lung. The ventilatory response of turtles to increasing levels of FICO2 is primarily dependent upon concomitant hypercapnea.