Stiffness of active smooth muscle during forced elongation

Effects of a FCBP gene polymorphism, location, and sex on Young's modulus of the tenth primary feather in racing pigeons

Young's modulus (E) is a measure for stiffness of a material and a higher E means a higher stiffness. The respective polymorphism of the feather corneous beta-protein gene causes the replacement of glycine by cysteine. We looked for possible effects of the three FCBP genotypes on E in the 10th primaries of racing pigeons. However, we did not find a statistically significant difference of E between the genotypes, even within the sexes and/or within different locations under our test conditions. Our findings do not preclude the possibility that under other conditions (temperature, moisture) an influence of the glycine/cysteine polymorphism on E may exist. Compared to the more proximal locations of the rachis (base and middle) we observed lower values for E in the distal region (tip). The 10th primary constitutes the leading edge of the pigeon wing and this special function may require higher stiffness in the proximal parts of the shaft. We observed significantly higher values of E in females than in males, which result only from statistically significantly higher values in the middle region. The higher stiffness of female primaries may also contribute to the better results of hens compared to cocks in pigeon races.

Biphasic force response to iso-velocity stretch in airway smooth muscle

Airway smooth muscle (ASM) in vivo is constantly subjected to oscillatory strain due to tidal breathing and deep inspirations. ASM contractility is known to be adversely affected by strains, especially those of large amplitudes. Based on the cross-bridge model of contraction, it is likely that strain impairs force generation by disrupting actomyosin cross-bridge interaction. There is also evidence that strain modulates muscle stiffness and force through induction of cytoskeletal remodeling. However, the molecular mechanism by which strain alters smooth muscle function is not entirely clear. Here, we examine the response of ASM to iso-velocity stretches to probe the components within the muscle preparation that give rise to different features in the force response. We found in ASM that force response to a ramp stretch showed a biphasic feature, with the initial phase associated with greater muscle stiffness compared with that in the later phase, and that the transition between the phases occurred at a critical strain of ∼3.3%. Only strains with amplitudes greater than the critical strain could lead to reduction in force and stiffness of the muscle in the subsequent stretches. The initial-phase stiffness was found to be linearly related to the degree of muscle activation, suggesting that the stiffness stems mainly from attached cross bridges. Both phases were affected by the degree of muscle activation and by inhibitors of myosin light-chain kinase, PKC, and Rho-kinase. Different responses due to different interventions suggest that cross-bridge and cytoskeletal stiffness is regulated differently by the kinases.

Short-range mechanical properties of skeletal and cardiac muscles

Striated muscles are disproportionately stiff for small movements. This facet of their behavior can be demonstrated by measuring the force produced when the muscle is stretched more than about 1% of its initial length. When this is done, it can be seen that force rises rapidly during the initial phases of the movement and much less rapidly during the latter stages of the stretch. Experiments performed using chemically permeabilized skeletal and cardiac muscles show that the initial stiffness of the preparations increases in proportion with isometric force as the free Ca²(+) concentration in the bathing solution is raised from a minimal to a saturating value. This is strong evidence that the short-range mechanical properties of activated muscle result from stretching myosin cross-bridges that are attached between the thick and thin filaments. Relaxed intact muscles also exhibit short-range mechanical properties but the molecular mechanisms underlying this behavior are less clear. This chapter summarizes some of the interesting features of short-range mechanical properties in different types of muscle preparation, describes some of the likely underlying mechanisms and discusses the potential physiological significance of the behavior.

Influence of intercellular tissue connections on airway muscle mechanics

Contraction of smooth muscle in visceral organs is modified by structures external to the muscle. Within muscle tissue itself, connective tissue plays an important role in force transference among the contractile cells. Connections arranged radially can affect contractile mechanics by limiting tissue expansion at short lengths. Previous work suggests that increased stiffness at extreme shortening is due to such radial constraints. Two approaches to further study of these effects are reported. To increase radial constraints, very thin Silastic bands were placed loosely about strips of canine trachealis muscle at rest length. The strips were allowed to shorten under light afterloads, expanding until restrained by the bands. Subsequent removal of the bands allowed increased shortening, with less increase in stiffness at short lengths. Related isometric effects were observed. To reduce constraints, muscle strips were partially digested with collagenase. Compared with control conditions, this treatment permitted further shortening, with less increase in stiffness at short lengths. These results emphasize the role of extracellular structures in determining mechanical function of smooth muscle.

Mechanisms for the mechanical response of airway smooth muscle to length oscillation

Airway smooth muscle tone in vitro is profoundly affected by oscillations in muscle length, suggesting that the effects of lung volume changes on airway tone result from direct effects of stretch on the airway smooth muscle. We analyzed the effect of length oscillation on active force and length-force hysteresis in canine tracheal smooth muscle at different oscillation rates and amplitudes during contraction with acetylcholine. During the shortening phase of the length oscillation cycle, the active force generated by the smooth muscle decreased markedly below the isometric force but returned to isometric force as the muscle was lengthened. Results indicate that at rates comparable to those during tidal breathing, active shortening and yielding of contractile elements contributes to the modulation of force during length oscillation; however, the depression of force during shortening cannot be accounted for by cross-bridge properties, shortening-induced cross-bridge deactivation, or active relaxation. We conclude that the depression of contractility may be a function of the plasticity of the cellular organization of contractile filaments, which enables contractile element length to be reset in relation to smooth muscle cell length as a result of smooth muscle stretch.

Contractility and myosin heavy chain isoform patterns in developing tracheal muscle

Changes in airway smooth muscle reactivity with development may be caused by either modification of the excitation-contraction coupling system or alteration of the contractile apparatus. The mechanism responsible for the reported changes in reactivity was addressed in this study by examining airway smooth muscle contractility and myosin heavy chain isoform patterns as a function of post-neonatal development. Changes in length and force, in response to supramaximal electrical stimulation, were recorded simultaneously as functions of time for tracheal smooth muscle (TSM) strips from 8-week-old and 25-week-old male rabbits. Both the passive and active length-tension (L-T) curves as well as the force-velocity (F-V) curves for the two age groups of rabbit TSM were not significantly different indicating no changes in contractility during post-neonatal development in rabbits. This conclusion is surprising in light of reports of myosin heavy chain (MHC) isoform shifts in porcine trachealis during comparable periods of development. Therefore, MHC isoform ratios were compared by sodium dodecyl sulfate-polyacrylimide gel electrophoresis for tracheal smooth muscle from male rabbits of 8 and 25 weeks of age. Unlike the reported MHC isoform shifts in the pig tracheal muscle, the rabbit trachealis showed no difference in MHC isoform ratios between the two age groups compared in this study. In conclusion, no changes occur in contractility or MHC isoform patterns during post-neonatal development of rabbit tracheal smooth muscle. Therefore, reported changes in airway muscle reactivity are likely due to changes in receptors or in second messenger systems rather than to changes in the contractile apparatus.

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

When isometrically-contracting strips of ovarian ligament smooth muscle were suddenly shortened by 10-20% of their length, force fell rapidly and then redeveloped along an exponential time course. The amount of force recovered fell short of that expected in an isometric contraction at the new length, and this force deficit was proportional to the magnitude of the length step (approximately 80% of force was recovered after a 10% shortening). A sudden imposed decrease in length was more effective in reducing subsequent force than was isotonic shortening. Early in the recovery phase the stiffness of the muscle was decreased to less than its expected value; stiffness recovered to expected levels on an exponential time course approximately three to four times faster than force recovery itself. Force-velocity curves made during the redevelopment phase showed a reduced maximal force (Fmax) and an increased maximal shortening velocity (Vmax) when compared with control contractions matched in force, time and length. The curves crossed at approximately 10% of Fmax. During isometric relaxation the muscles showed an increase in their expected stiffness; prior imposed shortening (as above) reduced the relaxation stiffness increase in proportion to the prior force deficit. The persistent effects of early events on the later phases of the contraction, as well as the increase in shortening velocity with very light loads, are consistent with the hypothesis that the sudden shortening detaches crossbridges and that same fail to reattach during force recovery. During isotonic shortening of unperturbed muscle some slowly-cycling crossbridges may act as an internal load and reduce shortening velocity.

Limits to shortening in smooth muscle tissues

The extent of shortening in smooth muscle tissues is limited by a number of internal and external factors. In this study, continuous measurements of the stiffness of active muscle were made to characterize the mechanical forces acting to limit shortening. Rabbit ovarian ligament and mesotubarium superius muscles were allowed to shorten as far as possible under light afterloads; under these conditions a stiffness increase was observed that was closely related to the instantaneous muscle length and that was unaffected by other factors influencing the degree of shortening (afterload, time and intensity of activation, temperature, etc.). The results are considered in terms of a hypothesis relating the tissue-based constraints on radial expansion at short lengths to an additional load on the contractile apparatus, an internal force that is externally manifested as an increase in axial stiffness. Changing the cellular volume by varying the tonicity of the bathing medium provided tentative confirmation of the hypothesis.