Relation of Tissue Extensibility to Smooth Muscle Tone

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Abstract
Isolated specimens of rabbit gut or bladder were subjected to fixed loads, length changes being recorded kymographically. The elongation curves showed two essential phases. First there was a rapid visco-elastic extension whose amount was directly related to load. Second, there was a sustained creep whose slope was less clearly dependent upon load. This creep appeared not to develop until a critical load value was exceeded. A stretch reduced the viscosity, as reflected in the initial extension of a succeeding stretch. This change could be reversed with long recovery intervals allowed after load removal. Load removal was followed by a brief viscoelastic recoil, and then a long term length retraction which had the same slope regardless of the amount of prior extension. The recoil was always less in amount than the previous visco-elastic extension. The recovery of the initial viscosity, with time, could not be related to the recovery in length. While acetylcholine or epinephrine could change the tissue length, they had no clear effect upon the amount or rate of initial extension, upon the late creep, or upon the late length retraction upon load release. The contractile elements of the muscle would seem to be in series with visco-elastic elements, and the elongation pattern of the latter dominantly controls the over-all tissue extensibility. Only in a few cases can the tissue extensibility be related to the amount of muscle contraction.

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