Intrapulmonary receptors in the bullfrog: Sensitivity to CO2

Abstract

Single-unit vagal afferent recordings were made on 55 intrapulmonary receptors in 15 anesthetized or decerebrate bullfrogs. Intrapulmonary CO2 concentration and intrapulmonary pressure were controlled independently by unidirectionally ventilating the lungs. No CO2 receptors (insensitive to stretch of the lung) of the kind reported in birds and reptiles were found; all 55 receptors were mechano-sensitive. Of these mechanoreceptors, 39 adapted slowly to inflation of the lung and 16 adapted rapidly. Thirtythree of the slowly-adapting receptors and 15 of the rapidly adapting receptors decreased their discharge frequency as intrapulmonary CO2 concentration was increased. Inflating the lung enhanced CO2 sensitivity. The results indicate that the frog possesses CO2-sensitive pulmonary mechanoreceptors similar to those of mammals and reptiles.