Effect of diameter on force generation and responsiveness of bronchial segments and rings

In this study, isovolumic bronchial segments and bronchial rings were used to investigate the influence of airway diameter on smooth muscle force generation and acetylcholine responsiveness. Segments with internal diameters ranging from 1.0-6.0 mm were obtained from the mainstem bronchus of eight pigs. Responses to increasing acetylcholine concentrations were quantified in segments by intralumenal pressure (cmH2O), and in rings by tension (g.cm-1). The negative log of the concentration producing half the maximal effect (EC50) (i.e. pD2) to acetylcholine was calculated for each segment and ring. Ring tension was used to calculate a theoretical lumen pressure for each ring, and this, along with the pD2, was compared with values obtained from segments of the same diameter. Intermediate-sized segments produced significantly greater intralumenal pressures than did large or small segments. Small segments were 160 times more sensitive to acetylcholine than large segments. In contrast to the segments, bronchial rings showed no effect of size on acetylcholine sensitivity. Theoretical ring lumen pressures matched those measured for large and intermediate segments, but not for small segments. The different behaviour of bronchial segments and rings obtained from the same sized airway suggests that the three-dimensional architecture of the airway is an important factor in determining behaviour, particularly in small airways.