Cholinergic responses in different sections of rat airways.

Effect of cholinergic agonists on rat airway smooth muscle were studied, 4 different preparations were used: the trachea and the bronchus representing the extrapulmonary airways, the bronchiole and the parenchymal strip representing the intrapulmonary airways. The dose-dependent isometric contractile responses of these 4 preparations to acetylcholine (ACh) and carbacholine (CCh) were determined. A significant difference in sensitivity to both ACh and CCh was observed between the extrapulmonary and intrapulmonary airways. When pD2-values of these preparations were compared, the trachea and the bronchus showed to be 10--100 times more sensitive to these drugs than the bronchiole and the parenchymal strip. A significant difference between the trachea and the bronchus was also found in the pD2-values to ACh, the bronchus being more sensitive than the trachea. This difference in sensitivity was probably due to a greater activity of cholinesterase (ChE) in the tracheal tissue, for the difference was not seen in response to ACh with physostigmine or to CCh. It is suggested that there are more cholinergic receptors in the tracheal and bronchial smooth muscles than in the muscles of the bronchioles and the parenchymal tissue.