Characterization of intrapulmonary, rapidly adapting receptors of guinea pigs

The activity of 33 afferent vagal fibers arising from rapidly adapting pulmonary stretch receptors in the lungs of 21 guinea pigs was studied. These receptors were identified by their response to constant pressure hyperinflation and to negative pressure deflation of the lungs. The response was irregular bursts of action potentials with an adaptation index greater than 70% in 2s. Little or no activity was noted in these receptors during eupneic breathing. The conduction velocity of the afferent fibers from these receptors ranged from 3.1 to 57.1 m/s (mean = 20.2 ± 5.1 SD). Receptors were stimulated by intravenous injection of histamine and also by vapors of ether, alcohol, acetone, ammonia, and cigarette smoke. Stimulation by the inhalation of the vapors usually occurred in the absence of any change in tracheal pressure or even when the tracheal pressure became slightly lower than before vapor challenge, thus suggesting in direct action. If an increase in tracheal pressure occurred, the discharge of the receptor began well in advance. However, stimulation of the receptor by histamine was always accompanied by significant increases in tracheal pressure. Isoproterenol prevented the increase in tracheal pressures produced by histamine challenge. In addition, isoproterenol blocked the response of rapidly adapting receptors to histamine as well. Isoproterenol had no effect on receptor response to the various vapors used in this study. Therefore, stimulation of rapidly adapting receptors by histamine seems to be dependent on an increase in tracheal pressure, while a direct action is indicated by several 'irritant' gases. The characteristics of the rapidly adapting receptors in guinea pig lungs are more closely to the irritant receptor of the rabbit and the cat than to the rapidly adapting receptors in lungs of dogs.