Muscle sound frequencies of the frog are modulated by skeletal muscle tension.

The purpose of this study was to determine the relationships among muscle sound frequencies, muscle tension, and stiffness. Time-frequency transformations of nonstationary acoustic signals provided measures of resonant frequency during isometric contractions of frog (Rana pipiens) semitendinosus and gastrocnemius muscles. A mathematical expression for muscle transverse resonant frequency, elastic modulus and tension, based on elastic beam theory, was formulated by the Rayleigh method adapted for muscles. For thin muscles, the elastic modulus was found to have negligible influence on transverse muscle resonant frequency. Changes in muscle tension were the major determinants of changes in transverse resonant frequency. Consequently, for thin muscles, the time course of muscle tension, but not elastic modulus, can be monitored acoustically during the early phase of contraction when muscles give rise to sounds. Muscles were found to be anisotropic with a modulus of elasticity, EL, measured via length perturbations near 0.1% muscle length peak-to-peak, that was much larger than the modulus of elasticity, Eb, that resists the lateral bending that causes sound production. The elastic and resonant behavior of a thin muscle is similar to a tensioned fibrous cable with distributed mass.     

Time dependence of series elasticity in tracheal smooth muscle

The stress-strain curve for the series elastic component (SEC) of tracheal smooth muscle was obtained by quick releasing the muscle from isometric tension to various afterloads and measuring the elastic recoils (SEC lengths) at a specific time after stimulation. A family of such curves was obtained by releasing the muscle at different points in time during contraction. Stiffnesses of the SEC (slopes of the stress-strain curves) at a specific stress level calculated from these curves (constant-stress stiffness) showed significant difference from one another. The same difference can also be characterized by the slope of the linear stiffness-stress curve, the constant A. The constant A during a 10-s isometric contraction was maximal at 2 s. It then decreased with time. This stiffness behavior is only seen when the effect of stress is held constant or eliminated. If stress is allowed to increase with time as it does during a tetanus then stiffness appears to increase monotonically. The SEC stiffness during active contraction was found to vary within the boundaries of the stiffness of muscle in rigor (upper limit) and that at resting state (lower limit).     

Dynamic properties of cat tenuissimus muscle

Some of the factors which influence the development of tension in cat tenuissimus muscle were studied quantitatively. Under isometric conditions, it was shown that the dynamic properties of the relationship between the tension of the muscle and its electrical stimulation depend on the mean rate of stimulation. This non-linear effect cannot be explained on the basis of the dependence of muscle tension on instantaneous rate of stimulation since the tension due to a stimulus following closely a previous stimulus is augmented, but the time course of the twitch response is unaltered. The interaction between the tension due to active contraction and that due to the viscoelastic properties of the muscle was investigated by independently varying muscle length and the rate of stimulation. Within the limits of resolution of the data, it was concluded that these two components of tension are additive and that muscle stiffness is related to the instantaneous tension of the muscle.     

Sensitivity of cultured and skinned chick myotube to calcium, strontium, and barium ions examined by recording isometric contractions.

Sensitivity of cultured chick myotubes to alkaline earth metal ions was investigated by recording contractile isometric tension through a semiconductor transducer. The myotubes were obtained by culturing myoblasts of chick embryo breast muscles, and skinned chemically before physiological experiments. Contractions developed in response to Ca2+ in a bathing medium higher than 3 x 10(-7) M and reached maximum at 1 x 10(-5) M. Sr2+ was less effective than Ca2+; the threshold concentration was 1 x 10(-5) M and the tension reached maximum at 1 x 10(-3) M. Ba2+ was the least effective among the three alkaline earth metal ions; only one fifth of the Ca(2+)-induced maximum tension was attained at 1 x 10(-3) M. The sensitivity was similar to that of the mature pectoral muscle fiber, a fast twitch muscle fiber, rather than that of the anterior latissimus dorsi, a slow tonic muscle fiber. The sensitivity was shown to be dependent on its troponin C by replacing it with troponin C from the mature pectoral or cardiac muscle. This indicates that TnC of a fast-muscle type is expressed in the cultured chick myotube as in the mature pectoral muscle. The contractile apparatus was thus shown to be well developed in the cultured myotube with characteristics similar to the mature fast twitch muscle fiber.     

Earliest mechanical evidence of cross-bridge activity after stimulation of single skeletal muscle fibers.

The stiffness of single fibers from frog skeletal muscle was measured by the application of small 2-kHz sinusoidal length oscillations during twitch and tetanic contractions at a range of initial sarcomere lengths. The earliest mechanical signs of activation were a fall in tension (latency relaxation) and a rise in stiffness. The earliest stiffness increase and the earliest tension fall occurred simultaneously at all sarcomere lengths. This suggests a cross-bridge origin for the latency relaxation. The lead of stiffness over tension seen during the rise of tension was substantially established during the latent period. Reducing the size of the twitch by reducing calcium release with D-600 (methoxyverapamil) reduced the latency relaxation and the stiffness development during latency much less than it reduced the twitch tension. For very small twitches the peak of the stiffness response occurred during the latent period and the times of onset of both latency relaxation and stiffness rise were delayed, but remained coincident. This suggests a strong connection between the latency relaxation and the rise of stiffness during the latent period, whereas the connection between these events and positive tension generation appears to be less strong.     

The stiffness of slow-twitch muscle lags behind twitch tension: implications for contractile mechanisms and behavior

Small, square stretches were applied during contractions of soleus and plantaris muscles in the cat to measure muscle stiffness. The stiffness of the slow-twitch soleus muscle (but not of the fast plantaris muscle) reaches a maximum after the peak in twitch tension. Since the number of active bonds should be maximum before the peak in tension, we suggest that many bonds are in the rigor state during the falling phase of the twitch. The stiffness of the bonds in this state may be useful for prolonging the twitch in slow-twitch muscles and for maintaining a posture.     

Isometric contractile properties and instantaneous stiffness of amphibian skeletal muscle in the temperature range from 0 to 20 degrees C

The isometric contractile properties of frog (Rana pipiens) and toad (Bufo bufo) sartorii have been studied over the temperature range from 0 to 20 degrees C. The isometric twitch tension was found to vary considerably between these two species and between muscles in the same species. Between 0 and 4 degrees C there was very little change in maximum isometric twitch tension. Between 4 and 12 degrees C several muscles from frog or toad showed a potentiation of twitch tension whereas others showed a decline. Over this temperature range the toad sartorii consistently demonstrated a greater potentiation. By 12 degrees C a steady decline in twitch tension in both muscles was seen as the temperature range the toad sartorii consistently demonstrated a greater potentiation. By 12 degrees C a steady decline in twitch tension in both muscles was seen as the temperature approached 20 degrees C. The maximum isometric tetanic tension recorded between 18 and 20 degrees C increased fractionally to an average of 1.504 +/- 0.029 (n = 4) for frog sartorii and to 1.377 +/- 0.008 (n = 5) for toad sartorii. The time to peak twitch tension and the half-relaxation time decreased markedly with an increase in temperature. Moreover, the half-relaxation time was reduced by a greater proportion than the time to peak twitch tension. Measurements of instantaneous stiffness by controlled velocity releases from the plateau of isometric tetani revealed that the large increase in isometric tetanus tension as the muscle was warmed was not accompanied by a corresponding increase in the total number of active cross-bridges. The possibility that a decreased availability of intracellular Ca2+ ions at the contractile sites contributing to the fall of isometric twitch tension at elevated temperatures is discussed. The possibility exists that at elevated temperatures a change inthe intrinsic contractile ability of the muscle occurs which produces an increased tension per cross-bridge.     

Characterization of cross-bridge elasticity and kinetics of cross-bridge cycling during force development in single smooth muscle cells

Force development in smooth muscle, as in skeletal muscle, is believed to reflect recruitment of force-generating myosin cross-bridges. However, little is known about the events underlying cross-bridge recruitment as the muscle cell approaches peak isometric force and then enters a period of tension maintenance. In the present studies on single smooth muscle cells isolated from the toad (Bufo marinus) stomach muscularis, active muscle stiffness, calculated from the force response to small sinusoidal length changes (0.5% cell length, 250 Hz), was utilized to estimate the relative number of attached cross-bridges. By comparing stiffness during initial force development to stiffness during force redevelopment immediately after a quick release imposed at peak force, we propose that the instantaneous active stiffness of the cell reflects both a linearly elastic cross-bridge element having 1.5 times the compliance of the cross-bridge in frog skeletal muscle and a series elastic component having an exponential length-force relationship. At the onset of force development, the ratio of stiffness to force was 2.5 times greater than at peak isometric force. These data suggest that, upon activation, cross-bridges attach in at least two states (i.e., low-force-producing and high-force-producing) and redistribute to a steady state distribution at peak isometric force. The possibility that the cross-bridge cycling rate was modulated with time was also investigated by analyzing the time course of tension recovery to small, rapid step length changes (0.5% cell length in 2.5 ms) imposed during initial force development, at peak force, and after 15 s of tension maintenance. The rate of tension recovery slowed continuously throughout force development following activation and slowed further as force was maintained. Our results suggest that the kinetics of force production in smooth muscle may involve a redistribution of cross-bridge populations between two attached states and that the average cycling rate of these cross-bridges becomes slower with time during contraction.     

Series elasticity in frog sartorius muscle during release and stretch

When a stretch is applied to an isolated muscle during tetanic stimulation, the force developed is higher than the maximal isometric tension (Po). This force puts the series elastic component (SEC) under tension and in a domain which is not well defined in terms of tension-extension curve. In the present work, an attempt was made to determine the stiffness of the SEC for tensions greater than Po, using the sartorius muscle of the frog. For this purpose, rapid releases and stretches of different amplitudes were given during maximal isometric contractions. Plotting normalized tension (P/Po) against normalized length changes (negative or positive extensions, delta L/Lo.10(2] produced a tension-extension curve. The slopes of the linear part of each relationship on both sides of Po indicated an increase in SEC stiffness when the muscle was rapidly stretched. Furthermore, the transient character of the increase in stiffness was studied by measuring SEC stiffness during rapid releases applied at various time intervals after stretches: the muscle was found to be stiffer as the time interval was shorter. The results are discussed in terms of (i) non-linear behaviour of the passive and active parts of the SEC, (ii) enhancement of storage and release of potential energy.     

Effects of series compliance on twitches superimposed on voluntary contractions.

The activation of skeletal muscle during voluntary isometric contraction has been assessed by measuring the increase in force caused by a superimposed maximal shock to the motor nerve (the twitch-interpolation technique). When the muscle is held isometric, the increase in force with stimulation (superimposed twitch force) decreases with increasing voluntary force, and a line fit through the data can be extrapolated to maximal voluntary force at the zero twitch force axis. In a previous paper we questioned the applicability of this technique in situations where a high series compliance allows the muscle to shorten during the superimposed twitch. To explore effects of series compliance, we measured force of the adductor pollicis during voluntary isometric contractions with noncompliant and compliant loading devices. With the compliant loading device, superimposed twitch force was systematically less than with the noncompliant device, and the plot of superimposed twitch force vs. voluntary force was often concave upward, preventing easy extrapolation to maximal voluntary force. These findings are consistent with force-velocity characteristics of muscle and suggest that twitch-interpolation data must be interpreted with caution when the muscle is not held isometric during the superimposed twitch.     

The direct depressant effect of LCB29 (idrocilamide) on mechanical tension of rat soleus muscle fibers

The effect of LCB29 was tested on twitch characteristics, tetanic tension, and K+ and voltage-clamp contractures of rat soleus muscle fibers. In concentrations ranging from 10(-6) to 5 x 10(-4) M, LCB29 simultaneously inhibited the twitch amplitude, the maximum rate of tension development, and the maximum rate of relaxation. In concentrations ranging from 10(-5) to 10(-4) M, tetanic tension (100 Hz, 1 s) was inhibited by the same amount. The effect of 5 x 10(-5) M LCB29 was studied on K+ contractures and contractures induced, under voltage-clamp conditions, by long-lasting depolarizations. Its effect was significantly stronger than those on twitch and tetanic tension. In addition, LCB29 had a dual effect on strength--duration curves for mechanical threshold. It increased both the rheobasic potential and the steepness of the curve. It is concluded that LCB29 exerts a direct myorelaxant effect on rat soleus muscle; two sites of action are probably involved.     

Tension and instantaneous stiffness of single muscle fibers immersed in Ringer solution of decreased tonicity.

Isometric tension and instantaneous stiffness were measured in frog semitendinosus single muscle fibers in both isotonic and hypotonic Ringer solution. In 0.7 and 0.5 x normal Ringer tension increased 17 and 20%, respectively. There was no corresponding increase in the measured stiffness. The increase in tension in hypotonic Ringer could be reversed by the addition of an osmotic equivalent of sucrose to the bathing solution. These findings suggest that the potentiated tension observed in hypotonic Ringer is due to an increased tension per cross-bridge and not to an increase in the number of attached cross-bridges.     

The effect of muscle fatigue on the isometric contractile characteristics of skeletal muscle in the cat

The rise time of an isometric twitch, the tetanic tension, the twitch tetanus ratio, the frequency-tension relationship, and the height of the MUAP (motor unit action potential) were measured in fast twitch (medial gastrocnemius) and slow twitch (soleus) muscles of the cat immediately before, in the middle, and immediately after fatiguing isometric contractions at tensions of 30, 50 and 80% of each muscle's initial strength (tetanic tension recorded from the unfatigued muscle). Although the twitch-tetanus ratio was always less for the soleus than for the medial gastrocnemius muscles, the twitch-tetanus ratio for any one muscle was constant throughout the duration of fatiguing isometric contractions at any of the tensions examined. In contrast, the twitch tension and tetanic tension of the muscles were both less after the contractions, the largest reduction occurring for both muscles during contractions sustained at the lowest isometric tensions. The time to peak tension of an isometric twitch was prolonged for both muscles following the contractions. This was associated with a corresponding shift in the frequency tension relationship such that at the point of muscular fatigue, the muscles tetanized at lower frequencies of stimulation than did the unfatigued muscle. In contrast, the amplitude of the MUAP showed only a modest reduction throughout the duration of the fatiguing contractions.     

Local movement in stimulated frog sartorius muscle

Local movement was recorded in tetanically contracting frog sartorius muscle to estimate the nonuniformity in the distribution of compliance in the muscle preparation and the compliance that resides in the attachments of the preparation to the measuring apparatus. The stimulated muscle was also subjected to rapid length changes, and the local movements and tension responses were recorded. The results indicate that during tension development at resting length the central region of the muscle shortens at the expense of the ends. After stimulation the "shoulder" in the tension, which divided the relaxation into a slow decline and a subsequent, rather exponential decay toward zero, was accompanied by an abrupt increase in local movement. We also examined the temperature sensitivity of the two phases of relaxation. The results are consistent with the view that the decrease in tension during relaxation depends on mechanical conditions. The local movement brought about by the imposed length changes indicates that the peak value of the relative length change of the uniformly acting part was approximately 20% less than the relative length change of the whole preparation. From these observations, corrections were obtained for the compliance data derived from the tension responses. These corrections allow a comparison with data in the literature obtained from single fiber preparations. The implications for the stiffness measured during the tension responses are discussed.     

Twitch and tetanic tension during culture of mature Xenopus laevis single muscle fibres

Investigation of the mechanisms of muscle adaptation requires independent control of the regulating factors. The aim of the present study was to develop a serum-free medium to culture mature single muscle fibres of Xenopus laevis. As an example, we used the culture system to study adaptation of twitch and tetanic force characteristics, number of sarcomeres in series and fibre cross-section. Fibres dissected from m. iliofibularis (n = 10) were kept in culture at a fibre mean sarcomere length of 2.3 microm in a culture medium without serum. Twitch and tetanic tension were determined daily. Before and after culture the number of sarcomeres was determined by laser diffraction and fibre cross-sectional area (CSA) was determined by microscopy. For five fibres twitch tension increased during culture and tetanic tension was stable for periods varying from 8 to 14 days ('stable fibres'), after which fibres were removed from culture for analysis. Fibre CSA and the number of sarcomeres in series remained constant during culture. Five other fibres showed a substantial reduction in twitch and tetanic tension within the first five days of culture ('unstable fibres'). After 7-9 days of culture, three of these fibres died. For two of the unstable fibres, after the substantial force reduction, twitch and tetanic tension increased again. Finally at day 14 and 18 of culture, respectively, the tensions attained values higher than their original values. For stable fibres, twitch contraction time, twitch half-relaxation time and tetanus 10%-relaxation time increased during culture. For unstable fibres these parameters fluctuated. For all fibres the stimulus threshold fluctuated during the first two days, and then remained constant, even for the fibres that were cultured for at least two weeks. It is concluded that the present culture system for mature muscle fibres allows long-term studies within a well-defined medium. Unfortunately, initial tetanic and twitch force are poor predictors of the long-term behaviour of the fibres.     

Functional morphology of the M. gastrocnemius medialis of the rat during growth

Length-force relations, both active and passive, and twitch contraction characteristics were quantified for left medial gastrocnemius muscles of four young, four adult, and four old male Wistar rats. Muscle and bundle optimum length and muscle weight were also determined and subsequently used for calculation of a number of morphological characteristics of the muscles. Fiber optimum length was derived from muscle bundle optimum length. Generally, physiological characteristics remained constant during growth. There was no change either in active tension at muscle optimum length or in active working range relative to fiber optimum length, relative passive fiber stiffness, active force relative to passive force at optimum length, twitch contraction time and twitch half relaxation time at optimum length. A number of morphological changes, however, did take place in the medial gastrocnemius muscle during growth. Fiber optimum length increased but only by about 2 mm from youth to old age, whereas muscle optimum length increased by approximately 14 mm, presumably owing to extensive hypertrophy of the muscle fibers during growth. The priority for force of the medial gastrocnemius muscle (defined as the quotient of physiological cross-sectional area of a muscle and the cubed root of its volume, a measure independent of architecture and dimensions of muscles) increased during growth. This increase indicates that during growth the muscle shifts relatively more towards force generation than towards excursion generation. These findings are discussed in view of existing scaling theories.     

The effects of isoproterenol, UDCG 115, and caffeine on the heart related to excitation-contraction coupling in heart muscle

A myothermal technique was used to measure initial heat and tension independent heat from isometrically contracting papillary muscles taken from the right ventricle of rabbits. Tension independent heat produced by the muscle at Lo was isolated with a 2,3-butanedione monoxime (diacetyl monoxime)--hyperosmotic Krebs solution. The effects of the inotropic drugs isoproterenol (1 X 10(-7) M), UDCG 115 (2 X 10(-4) M), and caffeine (2 X 10(-3) M) on heat and mechanical output were measured. We tested the hypothesis that these drugs alter peak twitch tension by increasing the total amount of Ca2+ cycled during the twitch, assuming that net tension independent heat is proportional to total Ca2+ cycled. The hypothesis was rejected for each drug as the positive inotropic effects of isoproterenol and UDCG 115 on twitch tension were not accompanied by increases in net tension independent heat. Net tension independent heat was actually depressed by UDCG 115. The negative inotropic effect of caffeine on twitch tension was accompanied by an increase in tension independent heat at times between the end of mechanical relaxation and the next stimulus. Possible mechanisms to account for these results are discussed.     

Effects of elastic loads on the contractions of cat muscles

The nerves to plantaris and soleus muscles in the cat were stimulated with maximal single shocks and with random stimulus trains which produced partially fused contractions. In order to obtain information on the mechanism of muscular contraction, the effects of allowing the muscles to shorten against various elastic loads were studied in the time domain and in the frequency domain. When springs of increasing stiffness were placed in series with the muscle, the twitch tension increased greatly. The gain of the frequency response curve was also much greater with stiffer springs. The shape of the frequency response curve for plantaris muscle could usually be described by that expected for a second-order system with two real time constants or rate constants. The rate constants changed in qualitatively similar ways in response to increased stiffness of an elastic load, increased muscle length and increased mean rate of nerve stimulation. These results are in agreement with the hypothesis that the linear responses of muscles working against elastic loads are determined by the values of two rate constants. Thus, of the many processes associated with contraction, only two are rate-limiting: one associated with the viscoelastic properties of muscle and the second associated with the reuptake of Ca into the sarcoplasmic reticulum. Non-linear aspects of muscular contraction are also discussed. These are more prominent in soleus muscle than in plantaris muscle.Graduate student of the Medical Research Council of Canada.Formerly a Post-doctoral Fellow of the Muscular Dystrophy Association of Canada.     

Twitch potentiation and caffeine contractures in isolated rat soleus muscle

1. Electrically-evoked twitch and tetanic tension were measured in isolated rat soleus muscle after exposure to caffeine. 2. Between 0.01 and 2.5 mM caffeine twitch tension was potentiated, reaching a peak of 150% of Resting Tension at 0.5 mM. 3. Biphasic Tension development with relaxation was observed at 2.5 mM caffeine with maximal contractures (110% tetanic tension) occurring at 20 mM. 4. Creatine phosphate and ATP stores were maintained throughout the period of tension development and relaxation. 5. In contrast with amphibian muscle, the isolated soleus is very sensitive to low doses of caffeine and produces biphasic caffeine contractures which relax in the presence of caffeine.     

A single order-disorder transition generates tension during the Huxley-Simmons phase 2 in muscle.

Increasing temperature was used to progressively interconvert non-force-generating into force-generating states in skinned rabbit psoas muscle fibers contracting isometrically. Laser temperature-jump and length-jump experiments were used to characterize tension generation in the time domain of the Huxley-Simmons phase 2. In our experiments, phase 2 is subdivisible into two kinetic steps each with quite different physical properties. The fast kinetic component has rate constant of 950 s-1 at 1 degrees C and a Q10 of approximately 1.2. Its rate is tension insensitive and its normalized amplitude declines with rising temperature--behavior that closely parallels the instantaneous stiffness of the cross-bridge. It is likely that this kinetic step is a manifestation of a damped elastic element/s in the fiber. The slow component of phase 2 is temperature-dependent with a Q10 of approximately 3.0. Its rate is sensitive to tension. Unlike the fast component, its amplitude remains in fixed proportion to isometric tension at different temperatures indicating direct participation in tension generation. Similar T-jump studies on frog fibers are also included. The combined results (frog and rabbit) suggest that tension generation occurs in a single endothermic (entropy driven) step in phase 2.