NOVEL ADAPTATION OF A METHOD TO ASSESS RESPONSIVENESS OF BRONCHIAL SEGMENTS IN-VITRO

In vivo agonist delivery to the lungs is characterized by absorption t hrough the mucosal layer followed by access to the smooth muscle, When agonists are applied to perfused bronchial segments, a difference in potency to agonists applied to the serosal (outside) or mucosal (insid e) surface can be demonstrated. In order to elucidate this effect in c anine branchial segments, we adapted a method to assess responsiveness of agonists applied to the inside or outside surface of canine bronch ial segments, A 2 cm ''fluid-tight'' length of bronchus was cannulated and mounted in a perfusion chamber. Auxotonic contraction of the bron chus displaced fluid inside the segment up a column and the change in height of fluid within the column (afterload) was measured as a change in hydrostatic pressure (volumetric). We assessed the optimal conditi ons for measuring bronchial responsiveness to acetylcholine, Neither s tretching segments length ways from 100 to 140% of resting length, nor altering the transmural pressure from 3 to 21 cmH(2)O had a significa nt effect on the potency of acetylcholine applied to the outside surfa ce, Both acetylcholine (n=7) and methacholine (n=4) were approximately 10-fold more potent when applied to the outside surface than the insi de surface of bronchial segments (p<0.0001). Furthermore, mechanical r emoval of the epithelium (n=6) led to a 67-fold increase in potency of acetylcholine applied to the inside surface compared with segments wi th epithelium. The advantages that this system has over previously rep orted methods include: 1) agonists can be injected at a low Bow rate s o as to minimize epithelial stress; 2) changes in luminal volume are m easured under conditions of no flow; 3) after-load can be varied such that contraction is either practically isotonic or auxotonic; and 4) a ''barrier effect'' of the epithelium of canine bronchial segments can be demonstrated. This new adapted method will provide us with the mea ns to assess the relationship between in vitro and in vivo responsiven ess in human bronchi.