Airway smooth muscle mechanics: reduced activation and relaxation

Reduced activation of skeletal muscle fibers at below-optimal lengths has been reported before. We studied tracheal smooth muscle (TSM) as a model of airway smooth muscle to see whether such a phenomenon existed in smooth muscle also; we found that active length-tension curves at normal and raised Ca2+ concentrations were significantly different. This indicated reduced activation of the TSM at below-optimal lengths. This reduction was in addition to that arising from the length-tension effect. We also studied the mechanics of relaxation in TSM and noted that in afterloaded isotonic tension records the time course of TSM relaxation appeared to closely follow that of the isometric contraction at all loads. In this it differed markedly from relaxation in skeletal muscle where the time courses of relaxation at different loads differed from each other and from that of the isometric muscle.