Mechanical properties of resting taenia coli smooth muscle

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Abstract
Stress-relaxation characteristics and stress-strain relationship of resting [guinea pig] taenia coli (TC) were analyzed. To suppress spontaneous activity in this muscle, Ca-free ethylene glycol-bis (.beta.-aminoethyl ether)N,N''-tetraacetic acid (EGTA) solution, temperature lowering and epinephrine were used. The stress response to a step increase in strain was measured in spontaneous TC and the in same specimen after spontaneous contraction were arrested. Stress response depends on the method of arresting the spontaneous contractions. There is no unique parallel element in the Hill model of muscle if it is applied to taenia coli. This suggests that Hill''s is, in general, inappropriate for smooth muscle. If Hill''s model is used in an ad hoc manner to evaluate the active contractile property, then the epinephrine method preferred. With epinephrine the normalized stress-relaxation function G(t) [stress response to step stretch] was independent of the degree of stretch and equal to or lower than that in the latent period of the spontaneous muscle, whereas other methods seem to inhibit relaxation to various extents. A mathematical expression for G(t) is presented that has a continuous spectrum of relaxation constants, corresponding to a mechanical model of viscoelasticity consisting of springs and dashpots and defined by 3 constants. The nonlinear stress-strain relationship of the resting muscle is presented in the form T = .beta..epsilon..alpha. [elastic response as a function of magnitude of stress], where T is the tensile stress and .epsilon. is the strain. The normalized relaxation function of resting TC is similar to those of nonspontaneously contracting tissues. The stress-strain relationship is similar to that of the aorta, but differs from that of the papillary muscle, mesentery and ureter.

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