Response of Avian Intrapulmonary Smooth Muscle to Changes in Carbon Dioxide Concentration ,

Abstract

How smooth muscle, at the openings of paleopulmonic parabronchi into the mediodorsal secondary bronchi, responds to changes in intrapulmonary CO2 concentrations was studied in spontaneously breathing and in paralyzed, unidirectionally ventilated ducks and geese. In the study, photographs of the parabronchi (taken through a small hole in the intercostal muscles and mediodorsal secondary bronchial wall) were analyzed. Also measured were changes in resistance to airflow through part of the lung in response to various CO2 concentrations in the ventilatory gas. There were no consistent changes in the cross-sectional area of these parabronchi to changes in concentrations of CO2 entering the lung; however, the parabronchial smooth muscle frequently contracted spontaneously. Contractions could also be readily induced by stimulating the smooth muscle mechanically or the vagus nerve electrically. None of the various bronchodilator drugs tested successfully inhibited spontaneous or mechanically induced contractions, but atropine sulfate blocked the contraction caused by vagal stimulation. Although the smooth muscle in this part of the lung can contract rapidly, it probably does not do so in response to changes in intrapulmonary CO2 concentration during eupneic breathing.