Acetylcholine in isolated airways of rat, guinea pig, and human: species differences in role of airway mucosa

Stored endogenous acetylcholine (ACh) and in vitro synthesis of [3H]ACh were measured in isolated, mucosa-intact and mucosa-denuded airways of rat, guinea pig, and humans. In addition, choline acetyltransferase (ChAT) activity and ACh content were measured in freshly isolated airway mucosa as well as in cultured epithelial cells of rat tracheas. Rat tracheas stored 25 nmol/g ACh, whereas guinea pig tracheas and human bronchi contained only 2-3 nmol/g ACh. When incubated with [3H]choline, the isolated airways of rat, guinea pig, and human synthesized significant amounts of [3H]ACh. In guinea pig and human airways, removal of the mucosa affected neither stored ACh nor in vitro synthesis of [3H]ACh. In rat tracheas, however, removal of the mucosa resulted in a 50% reduction of stored ACh. Freshly isolated mucosa wiped off from the luminal surface of rat tracheas contained large amounts of ACh (6.5 nmol/g airway), whereas in human mucosa (central bronchi) only small amounts of ACh were found. In enzymatically isolated mucosal cells of rat tracheas, a considerable ChAT activity (21 nmol.mg protein 1.h-1) was detected, blockable by bromoacetylcholine. Enzymatically isolated human mucosa contained a rather low ChAT-like activity (0.5 nmol.mg protein 1.h-1), not sensitive to bromoacetylcholine. In cultured epithelial cells of rat tracheas (4th-6th passage), neither ChAT activity nor ACh was detected. The large airways of rat, guinea pig, and humans contain considerable amounts of ACh, supporting histological evidence of a dense cholinergic innervation, particularly of rat tracheas. The mucosa of rat tracheas synthesizes and stores large amounts of ACh, whereas the low ChAT activity in human mucosa argues against the presence of cholinergic neurons able to synthesize and store ACh.