Relaxation of tetanized canine tracheal smooth muscle.

As is the case for striated muscle, relaxation in smooth has been little studied and is less understood. We report studies of load bearing capacity during relaxation of airway smooth muscle. The model employed was the canine tracheal smooth muscle (TSM). The effect of load on the time course of relaxation was analyzed either by comparing afterloaded contractions against various loads or by imposing abrupt alterations in load (load clamps). Unlike mammalian cardiac muscle in which relaxation was reported sensitive to loading conditions, relaxation in TSM was largely independent of loading conditions. In this it resembled frog heart muscle and mammalian cardiac muscle cells without functioning calcium sequestering systems. This type of relaxation which is not influenced by manipulation of loading conditions, has been termed "inactivation-dependent' relaxation. It appears to operate in muscle tissue in which the calcium sequestering apparatus is poorly developed and the dissipation of activation (removal of activating calcium, detachment of force generating sites, etc.) appears to be the rate limiting step during relaxation.