Influence of muscle length on series elasticity in arterial smooth muscle

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Abstract
The applicability of Hill''s 3 element muscle models to canine arterial smooth muscle (SM) was tested by determining the dependence of iliac and carotid artery series elasticity on muscle length. Measurements of pressure and diameter were made on intact arterial segments in vitro. Series elasticity (SE) was determined from pressure and diameter responses to repetitive small pressure releases during the time course of isometric contractions to maximal norepinephrine. The releases were 5-25 mmHg in amplitude, 150 ms in duration and performed at a rate of 10-25 min-1. Measurements were made at several different initial passive pressure levels, i.e., muscle lengths. Incremental stiffness-stress curves were found to be nonlinear, especially at low values of muscle length and wall stress. These stiffness-stress curves were shifted to the right with increasing muscle length and only showed a tendency to converge at higher values of the muscle length. SE stress-extension curves computed on the basis of a Maxwell model were shifted to the left increasing muscle length, i.e., they appeared stiffer. Neither the Voigt nor Maxwell type of model is adequate for describing the functional properties of iliac and carotid artery SM unless the model parameters are made dependent on muscle length.