Dynamic stiffness of rabbit mesotubarium smooth muscle: effect of isometric length

Abstract

The dynamic stiffness of mesotubarium smooth muscle from nonpregnant adult rabbits was measured continuously during isometric contraction by applying small (0.5 percent of the muscle length) sinusoidal length perturbations and measuring the amplitude and phase of the resulting tension perturbations. Stiffness during contraction was directly proportional to muscle tension; during relaxation stiffness at all tensions was significantly increased as compared to the values encountered during the rise of tension. Peak isometric tension and dynamic stiffness (determined at a common tension level) both decreased at shorter muscle lengths; the relative falloff in stiffness was significantly less than the tension decrease. Varying levels of muscle activation (obtained by changing stimulus strength and by applying quick releases to active muscle) had little effect on the measured elastic modulus. Comparisons of these results with published data on single-cell contractile properties imply a cellular locus for a portion of the measured stiffness.

Persistent mechanical effects of decreasing length during isometric contraction of ovarian ligament smooth muscle

“…This observation has been made for skeletal muscle (e.g. Edman, 1980), cardiac muscle (Taylor & Rudell, 1970;Jewell, 1977), and smooth muscle (Stephens & van Niekerk, 1977;Meiss, 1978;Gunst, 19815). In general, the changes in the force capability of a muscle following a length change made while the muscle is active differ from the length-tension characteristics measured after muscle length is changed in the interval between isometric contractions.…”

Mechanics of Vascular Smooth Muscle

“…It is fortunate, then, that smooth muscle tissue CC analyses appear to partially reflect cellular mechanics because the anatomically in-series cells are coupled forcetransmitting structures, at least at muscle lengths near the optimum length for muscle contraction (108,191,479). Tissue mechanical analyses allow for inclusion of the properties of noncellular materials and the mechanical interplay between VSM cell and ECM (312,314,397,418,484), and do not disrupt cell-to-cell and cell-to-ECM contacts that serve the critical function not only of mechanical transmission from the molecular motor to the environment but also as outside-in signals (mechanotransduction) that modify cell signaling involved in regulation of contraction (216,279,304). Further reductionist experiments using various methodologies to evaluate mechanical properties of isolated VSM cells (121,515) [reviewed nicely in (306)] of separate arterial components (195,269,392,397,398,413), of actomyosin bundles (265), and of single molecules (65,127,270,441) have added considerable insight.…”

“…Ultimately, coarse-grain (macroscale) reconstruction from fine-grain (nanoscale) analyses are needed to gain a more complete understanding of the mechanics of the whole system (vascular tissues) based on mechanics of, and mechanical interactions between, each part (62). Notably, passive force falls in tissues permeabilized by Triton X-100 (484), and the stiffness of a smooth muscle strip is less than that of a single cell, indicating that tissue architecture matters (312). Endothelial cells produce a number of notable chemical mediators that affect VSM tone and appear also to directly participate in small vessel mechanics (90,169,426).…”

“…Instantaneous stiffness can be measured by applying a constant velocity ramp stretch at a rate ∼10-fold > the maximum velocity of active muscle shortening (423,424). A lever or other device such as an audio speaker, piezoelectric element or the driving arm of a galvanometer-type pen motor that can induce rapid, low amplitude length changes can be used to continuously measure muscle stiffness concomitantly with force at the onset of stimulation and throughout contraction (153,256,312,439). To study isobaric muscle shortening and measure mechanics in a more physiological arrangement, arterial cylinders can be isolated and placed between two micropipettes within a pressure myograph (that houses pressure transducers and enables manual or computercontrolled arterial pressure clamping) attached to a microscope housing a charge-coupled device camera for diameter visualization and measurement (109,490).…”

“…A ramp or sinusoidal loading and unloading protocol provides this information because stretching and releasing are performed over time. Sinusoidal strain changes can provide additional information including a dynamic measurement of stiffness and the ability to separate elastic and viscous components (139,312). Thus, an analogous experimental protocol to the strain-step/strain-clamp can be accomplished using a device to induce a time-controlled ramp-or sinusoidal-stretch and release (load-unload) cycle.…”