Richard A. Meiss

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

SummaryWhen 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

SummaryThe 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.

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.

An Analysis of Length-Dependent Active Stiffness in Smooth Muscle Strips

The measured stiffness of contracting smooth muscle is strongly dependent on the level of developed force. This force-dependent stiffness is a consequence of contractile activity, and it is possible that a portion of it represents the stiffness of the population of attached crossbridges. The relationship between force and stiffness is sensitive to the particular stage of the contraction-and-relaxation cycle (Meiss, 1978), to specific external mechanical constraints imposed on the muscle (Meiss, 1987), and to the length of the muscle when the stiffness is measured (Meiss, 1978; Meiss, 1990). The character of the length-dependent stiffness relationship depends on the mechanical mode of contraction, and interpretation of these effects rests on assumptions regarding how the process of stiffness measurement interacts with changing tissue dimensions. The purpose of this paper is to characterize the difference between the length-dependent stiffness measured in isotonic and isometric contractions. Possible reasons for the differences will be considered, and a tentative model to account for the isotonic length-dependence of stiffness will be proposed.

Physiological state, contractile properties of heart and lateral muscles of fishes from different depths

Abstract 1. 1. Fishes taken from depths of 2000m (200 atm) were moribund. Those with swimbladders were distended and showed tissue disruption. Those lacking swimbladders were also moribund. 2. 2. Hearts were inactive, and pressurization with or without cooling failed to revive them. Muscle potassium was low and sodium concentration high. Blood was much hemolyzed. 3. 3. Hearts of fish and frogs from one atmosphere showed enhanced contractions, unchanged electrocardiograms at 200 atm, irregular beats and depression at 300 atm. 4. 4. It is unlikely that death was due to high temperature or to gas expansion; it is suggested that decompression may result in leakiness of cell membranes. Decompression of deep-water fish may also remove a positive tonic action of pressure on cardiac muscle.

Smooth Muscle Tissue Response to Applied Vibration Following Extreme Isotonic Shortening

The objectives of the present study are to investigate the response of a tracheal smooth muscle tissue to an applied longitudinal vibration following isotonic shortening, and, using experimental data, to simulate the mechanical response through a non-linear finite element analysis. The response of an activated smooth muscle tissue to forced length oscillations at 33Hz for 1 second was obtained. The response in terms of stiffness change and hysteresis was estimated from the experimental data. A finite element simulation was carried out to simulate the vibratory response under experimental conditions. The results obtained indicate that the approach and the vibratory response obtained may be useful for describing the cross-bridge deattachements within the cells as well as connective tissue connections characteristics of tracheal smooth muscle tissue.Copyright

Myosin Heavy Chain Isoform Patterns Do Not Correlate with Force-Velocity Relationships in Pulmonary Arterial Compared with Systemic Arterial Smooth Muscle

Velocity of shortening is dependent on the myosin heavy chain (MHC) isoform pattern in both skeletal and cardiac muscle (Pagani and Julian, 1984). Furthermore, it has been reported that a shift in MHC isoform ratio occurs with certain physiological or pathophysiological changes such as hypertrophy and/or hyperplasia of striated muscles (Litten et al., 1974). Such shifts in MHC isoform proportions accompany concomitant changes in shortening velocity and ATPase activity (Alpert et al., 1979; Alpert and Mulieri, 1980). At least two different MHC isoforms have been reported to exist in various different smooth muscles (Sparrow et al., 1987). The 200 kDa form and the 204 kDa form have been designated MHC1 and MHC2, respectively. The ratio of MHC1:MHC2 has been shown to vary dependent on smooth muscle type, animal species, stage of development, and under certain different physiological or pathophysiological conditions for the same muscle type (Sparrow et al., 1987; Mohammed and Sparrow, 1988). However, no functional correlation has yet been made between MHC isoform ratio and shortening velocity for smooth muscle. Therefore, the purpose of this study was to compare force-velocity (F-V) relationships and MHC isoform ratios from two different arterial muscles (pulmonary versus caudal) from the same species (rat).

A viscoelastic material model to represent smooth muscle shortening

The mechanical properties of a contracting smooth muscle can be changed by changing its length. A viscoelastic material model was developed to predict the length-dependent stiffness changes when a constrained muscle is allowed to shorten under a constant external force. Three-dimensional finite element simulations were carried out to estimate the stiffness changes and compared to available experimental data. A good agreement was found indicating that the viscoelastic material model developed gives a valid representation of the length dependent stiffness changes of a smooth muscle. Sensitivity analysis was carried out to determine the relative effects of material constants in the model on the length dependent stiffness.

Prediction of peak forces for a shortening smooth muscle tissue subjected to vibration.

The objective of the present study is to investigate the peak forces for a tracheal smooth muscle tissue subjected to an applied longitudinal vibration following isotonic shortening. A non-linear finite element analysis was carried out to simulate the vibratory response under experimental conditions that corresponds to forced length oscillations at 33 Hz for 1 second. The stiffness change and hysteresis estimated from the experimental data was used in the analysis. The finite element results of peak forces are compared to the experimental data obtained. The comparison of results indicate that the approach and the vibratory response obtained may be useful for describing the cross-bridge de-attachments within the cells as well as connective tissue connections characteristic of tracheal smooth muscle tissue.

Effect of Off-Axis Cell Orientation in Smooth Muscle Tissue

The cell alignment in a smooth muscle tissue plays a significant role in determining its mechanical properties. In addition to shortening strain, the off-axis cell orientation θ also modify the shear stress relationship significantly. A simulation model based on finite element analysis is developed to study the effect of stresses of tracheal smooth muscle tissue when its cells are orientated off-axially. Results obtained indicate that the maximum shear stress values of tracheal smooth muscle tissue at 45% strain are 2.5 times the values at 20% strain for all three off-axis orientation values θ = 15°, 30° and 45°.Copyright