Smooth muscle in human bronchi is disposed to resist airway distension

• The physiological function of airway smooth muscle still eludes us.
• Studying isolated human bronchi in a dynamic setting that closely simulates the in vivo conditions within which airway smooth muscle normally operates may provide important insights.
• Our results indicate that, during length oscillations that simulate the swings in airway caliber imparted by lung volume excursions, the tension ascribed to the activation with a spasmogen: 1- increases as the length of airway smooth muscle increases; and 2- is markedly greater during the lengthening than during the shortening of airway smooth muscle at any given length.
• We conclude that airway smooth muscle is better disposed to resist airway dilation during inflationary maneuvers than to narrow the airway lumen.

Studying airway smooth muscle (ASM) in conditions that emulate the in vivo environment within which the bronchi normally operate may provide important clues regarding its elusive physiological function. The present study examines the effect of lengthening and shortening of ASM on tension development in human bronchial segments. ASM from each bronchial segment was set at a length approximating in situ length (Linsitu). Bronchial tension was then measured during a slow cyclical strain (0.004 Hz, from 0.7 Linsitu to 1.3 Linsitu) in the relaxed state and at graded levels of activation by methacholine. In all cases, tension was greater at longer ASM lengths, and greater during lengthening than shortening. The threshold of methacholine concentration that was required for ASM to account for bronchial tension across the entire range of ASM lengths tested was on average smaller by 2.8 logs during lengthening than during shortening. The length-dependency of ASM tension, together with this lower threshold of methacholine concentration during lengthening versus shortening, suggest that ASM has a greater ability to resist airway dilation during lung inflation than to narrow the airways during lung deflation. More than serving to narrow the airway, as has long been thought, these data suggest that the main function of ASM contraction is to limit airway wall distension during lung inflation.