Auxotonic contractile responses of rat tracheal and bronchial airway smooth muscle

The purposes of the present study were to directly compare: (1) the degree of trachealis muscle shortening and changes in tracheal dimensions and (2) ACh-mediated auxotonic contraction of trachea and intraparenchymal bronchi. The auxotonic contractile properties of tracheal and bronchial airway smooth muscle were assessed from 1-2 mm thick tracheal sections and similar to1 mm thick lung slices using videomicrometry in vitro at 37degreesC. Acetylcholine resulted in reductions in luminal area, perimeter, mean radius, length, and breadth (22.0, 10.0, 11.9, 10.7 and 12.0%, respectively). Trachealis muscle shortening reached a maximum of 39.8+/-4.3%. The K I channel blocker 4-aminopyridine significantly augmented the ACh-mediated reductions in tracheal luminal dimensions. In response to ACh (10(-3) M), reductions in bronchial dimensions were significantly greater than those of the trachea for luminal area, perimeter and mean radius (44.6 vs. 18.6, 32.0 vs. 8.0 and 28.9 vs. 9.9%, respectively). These data indicate that auxotonic contractile responses of rat tracheal smooth muscle differ from those previously reported in the dog and guinea pig, that ACh-mediated auxotonic contraction of tracheal smooth muscle is augmented by 4-aminopyridine, and that proportionate reductions in luminal dimensions in response to ACh are considerably greater for bronchial than tracheal airways. (C) 2001 Academic Press.