Relaxation of tetanised smooth muscle of canine saphenous vein.

Since vascular filling depends on the cross-sectional area of the venous bed and since this is determined by the state of relaxation of the mural smooth muscle, we have studied load bearing capacity during relaxation of the smooth muscle of canine saphenous vein. The effect of load on the time course of relaxation was analysed 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 the smooth muscle of the saphenous vein was largely independent of loading conditions. In this it resembled frog heart muscle. 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 sequestration or some process downstream to it appears to be the rate limiting step during relaxation.