HYPOTHESIS - EXCESSIVE BRONCHOCONSTRICTION IN ASTHMA IS DUE TO DECREASED AIRWAY ELASTANCE

Based on the strikingly different mechanical properties of airway smooth muscle preparations of different species, we hypothesized that a decrease in the elastance of nonmuscle elements within airway walls of asthmatics reduces the load limiting smooth muscle shortening, thereby allowing excessive smooth muscle shortening and bronchoconstriction. Full thickness, circumferentially cut, lobar bronchial preparations were obtained from one asthmatic and six nonasthmatic lobectomy subjects. Passive tension of the asthmatic preparation was less than that for any nonasthmatic preparation at all lengths below that for optimal force generation (Lmax). Maximal isometric force generation was greater in the asthmatic specimen (2.32 g) than in the nonasthmatic specimens (0.90+/-0.15 g), with stress threefold higher for the asthmatic tissue. Isotonic shortening of the asthmatic preparation was strikingly greater at starting lengths less than or equal to Lmax, with maximal fractional shortening being 31% versus 11+/-2% for nonasthmatic preparations. Morphometric analysis revealed no differences in cross-sectional areas of smooth muscle for asthmatic versus nonasthmatic preparations. We conclude that the reduced tissue elastance may account for the greater muscle shortening by placing a lesser load upon the smooth muscle. Airway inflammation in asthma may alter connective tissue matrix elements within airway walls leading to this decreased elastance and excessive smooth muscle shortening.