Concurrent measurement of smooth muscle shortening, lumen narrowing and flow to acetylcholine in large and small porcine bronchi

Models of airway function indicate that responsiveness (Bow reduction) to b ronchoconstrictor provocation depends on airway smooth muscle shortening an d airway wall morphology, The contribution of these factors to the responsi veness of central and peripheral bronchi was assessed, Lumen flow was recorded in porcine perfused small (2 min i.d.) and large br onchial segments (6 mm i.d.). Lumen diameter was recorded in the same airwa ys after inserting an endoscope. Smooth muscle shortening, relative wall ar ea (WAr), smooth muscle and cartilage thickness and mucosal folds were meas ured morphometrically, The effect of acetylcholine (ACh; 10(-6)-10(-1) M) o n functional measurements was determined by curve fitting. Maximum muscle shortening was 30% in small and 19% in large bronchi (p<0.01 ) and lumen narrowing was 49% and 39%, respectively, High doses of ACh stop ped flow in small bronchi, but produced a plateau in large bronchi, Small a irways were 250-times more sensitive to ACh than large airways, for all mea surements, Smooth muscle and cartilage thickness and numbers of mucosal fol ds were greater in large than in small bronchi (p less than or equal to 0.0 1). Lumen narrowing and Bow reduction were greater than predicted on the ba sis of muscle shortening and WAr (p<0.05). The structure of airways from the two groups was qualitatively similar, but responses were markedly different. Greater narrowing and Bow responses of small bronchi were directly associated with smooth muscle responsiveness in situ. The results suggest that in vivo changes in airway wall shape or dim ensions, or luminal secretions, exert a significant effect on airway flow, particularly in the small airways.