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PubMed2001

A novel method to study contraction characteristics of a single cardiac myocyte using carbon fibers.

To facilitate cardiac muscle research, we developed a novel method by which the force and length of a single ventricular myocyte can be recorded with a pair of carbon graphite fibers attached firmly to both ends. One fiber was stiff, whereas the other fiber was compliant to allow the recording of force and shortening during twitch contractions. The image of the compliant carbon fiber was projected onto a pair of photodiodes, and their output was fed to a piezoelectric transducer after variable amplifications to alter the effective compliance of the carbon fiber. Thus contraction of the myocyte was induced under virtually isometric conditions as well as under auxotonic conditions. We obtained a bell-shaped relation between the compliance under an auxotonic load and the work output of the myocyte, which was directly related to myocyte performance in the heart. Because it is easy to attach myocytes to the experimental apparatus, the present method would allow us to study cardiac muscle mechanics at the cellular and molecular levels.

Animals↗
PubMedDate not supplied

Complementary roles of central command and muscular reflex in the regulation of heart rate during submaximal isometric contraction.

During submaximal isometric contraction, the heart rate (HR) and the electromyographic activity (EMG) increase continuously. Although activation of the muscle and the cardiovascular center is placed partly under the common control of the central command, the nature of the relationship that may exist between HR and the integrated electromyogram (iEMG) is seldom studied. Seventeen healthy men, 22.4 +/- 0.5 years of age (M +/- SE), performed isometric contractions with the right elbow flexors. Forces of 25, 40, 50 and 65% of the maximum voluntary contraction (MVC) were used, and the contractions were sustained until (isotonic isometric contraction: IIC) and beyond exhaustion (anisotonic isometric contraction: AIC). During IIC, a linear relationship exists between HR and iEMG; the slope of this relationship is independent of the relative force developed, which is in favor of a predominant role played by the central command in HR increase. The increase in the ratio iEMG/HR at the approach of local muscular exhaustion would indicate that at the end of IIC there is an increase in the relative part furnished by the information of peripheral origin in HR regulation. During AIC, the force (F) decreases in an exponential manner and stabilizes at around 25% MVC from tAIC = 70 s on. The iEMG and HR change independently: iEMG decreases like F such that iEMG/F remains constant; HR continues to increase in the first phase corresponding to the rapid decrease in F and iEMG, then in a second phase, it decreases linearly with respect to time. Our results suggest that the action of the central command is dominant during stage 1 of AIC, while during stage 2 the relative part furnished by the muscle reflexes increases. Beyond tAIC = 70 s, there seems to be a certain degree of central fatigue.

Adolescent↗
PubMed2004

Active and passive components in the length-dependent stiffness of tracheal smooth muscle during isotonic shortening.

Contraction of smooth muscle tissue involves interactions between active and passive structures within the cells and in the extracellular matrix. This study focused on a defined mechanical behavior (shortening-dependent stiffness) of canine tracheal smooth muscle tissues to evaluate active and passive contributions to tissue behavior. Two approaches were used. In one, mechanical measurements were made over a range of temperatures to identify those functions whose temperature sensitivity (Q(10)) identified them as either active or passive. Isotonic shortening velocity and rate of isometric force development had high Q(10) values (2.54 and 2.13, respectively); isometric stiffness showed Q(10) values near unity. The shape of the curve relating stiffness to isotonic shortening lengths was unchanged by temperature. In the other approach, muscle contractility was reduced by applying a sudden shortening step during the rise of isometric tension. Control contractions began with the muscle at the stepped length so that properties were measured over comparable length ranges. Under isometric conditions, redeveloped isometric force was reduced, but the ratio between force and stiffness did not change. Under isotonic conditions beginning during force redevelopment at the stepped length, initial shortening velocity and the extent of shortening were reduced, whereas the rate of relaxation was increased. The shape of the curve relating stiffness to isotonic shortening lengths was unchanged, despite the step-induced changes in muscle contractility. Both sets of findings were analyzed in the context of a quasi-structural model describing the shortening-dependent stiffness of lightly loaded tracheal muscle strips.

Animals↗
PubMed2001

Activity of jaw-opening and jaw-closing muscles and their influence on dentofacial morphological features in normal adults.

The purpose of this study was to determine the relationship between the dentofacial morphology and the function of masticatory muscles estimated by kinesiological measurements during isotonic jaw-opening (O) and closing (C) muscle contractions. We examined 26 male adult subjects who had no history of masticatory dysfunction. Three kinesiological parameters, the maximum force (F(max)), maximum velocity (V(max)) and maximum power (P(max)) were measured in both O and C muscles. Correlation analysis was performed between these parameters and dentofacial morphological features determined on lateral roentgenographic cephalograms. The F(max) (C), V(max) (O), log O/C of V(max), and P(max) (O) significantly correlated with anterior overbite. P(max) (C) significantly and negatively correlated with SN-ArG. Furthermore, log O/C of P(max) significantly and negatively correlated with SNA, SNB and ArG/GMe, and positively with SN-ArG. These results, combined with our previous findings that electromyographic muscle activity during isometric contraction correlated significantly with mandibular morphology, indicate that the dentofacial morphology is influenced by the function of both O and C muscles.

Adult↗
PubMed1994

Mechanics of human quadriceps muscle.

Mechanics of human quadriceps muscle strips (vastus lateralis; n = 10) were investigated over the whole load continuum. Mechanical experiments were performed at 29 degrees C and in both twitch and tetanus modes. For a given level of isotonic total load (P) and over a large part of the contraction phase, instantaneous velocity (V) was shown to be a unique function of instantaneous length (L), regardless of time and initial length. By considering this time- and initial length-independent mechanical property between instantaneous L and instantaneous V over the whole P continuum, a three-dimensional P-V-L relationship was constructed. Any variations in stimulation conditions modified the time-independent P-V-L diagram. Such modifications in the P-V-L relationship were characteristics of changes in contractile performance. Moreover, characteristics of the P-V relationship were investigated in both twitch and tetanus modes. The curvature of the P-V hyperbola was significantly higher in tetanus at 30 Hz than in twitch mode (P < 0.001). In conclusion, our study indicates that, in human quadriceps muscles, contractility can be defined as the time- and initial length-invariant part of a three-dimensional P-V-L relationship. Moreover, our data are consistent with an increase in economy of force generation in tetanus contractions compared with that in twitches.

Adult↗
PubMed1982

Factors modulating the sensitivity of the relaxation to the loading conditions in rat cardiac muscle.

The load sensitivity of the relaxation phase was studied in rat papillary muscle, with isotonic afterloaded contractions and stretches applied after the peak of isometric twitches. The tension decay occurred earlier in isotonic than in isometric contractions. When a central region of the preparation was marked with small stainless steel pins, a lengthening of this region could be shown during relaxation of isometric (fixed end) contractions. This lengthening was earlier and faster in isotonic afterloaded contractions. Therefore the sensitivity of relaxation to load or length changes could be described in the context of the general mechanism of relaxation which takes into account the non uniform behaviour of the muscle and the internal movement during contractions. Interventions which decelerate the activation decay rate had different effects on the load dependence of relaxation. Caffeine addition and substitution of strontium for calcium abolished the load sensitivity while a temperature reduction had no influence on it.

Animals↗
PubMed2006

Natural neural output that produces highly variable locomotory movements.

We recorded fast extensor tibiae motor neuron activity during single-legged treadmill walking in the stick insect, Carausius morosus. We used this activity to stimulate the extensor muscle motor nerve, observed the resulting extensor muscle contractions under isotonic conditions, and quantified these contractions with a variety of measures. Extensor contractions induced in this manner were highly variable, with contraction measures having SDs of 12 to 51%, and ranges of 82 to 275%, when expressed as percentages of the means, an unexpectedly wide range for a locomotory pattern. Searches for correlations among the contraction measures showed that, in general, this high variability is not reduced by contraction measure covariation. Comparing responses (to identical input) across animals showed that extensor muscles from different animals generally significantly differed from one another. However, correlation analyses on these data suggested that these differences do not indicate that multiple extensor muscle subtypes exist. Extensor muscles instead appear to belong to a single class, albeit one with high animal to animal variability. These data thus provide another well-quantified example (along with Aplysia feeding) of a repetitive but highly variable motor pattern (in contrast to the high rhythmicity and stereotypy present in most other well-quantified repetitive motor patterns). We suggest this high variability could be an adaptive combination of locomotion, active sensing, and crypsis arising from the relatively low demand for locomotion in Carausius behavior, the highly fragmented environment the animal inhabits, and its need to avoid predatory attention.

Action Potentials↗
PubMed2000

Mechanical signals and mechanosensitive modulation of intracellular [Ca(2+)] in smooth muscle.

We tested the hypothesis that strain is the primary mechanical signal in the mechanosensitive modulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in airway smooth muscle. We found that [Ca(2+)](i) was significantly correlated with muscle length during isotonic shortening against 20% isometric force (F(iso)). When the isotonic load was changed to 50% F(iso), data points from the 20 and 50% F(iso) experiments overlapped in the length-[Ca(2+)](i) relationship. Similarly, data points from the 80% F(iso) experiments clustered near those from the 50% F(iso) experiments. Therefore, despite 2.5- and 4-fold differences in external load, [Ca(2+)](i) did not deviate much from the length-[Ca(2+)](i) relation that fitted the 20% F(iso) data. Maximal inhibition of sarcoplasmic reticular (SR) Ca(2+) uptake by 10 microM cyclopiazonic acid (CPA) did not significantly change [Ca(2+)](i) in carbachol-induced isometric contractions and isotonic shortening. CPA also did not significantly change myosin light-chain phosphorylation or force redevelopment when carbachol-activated muscle strips were quickly released from optimal length (L(o)) to 0.5 L(o). These results are consistent with the hypothesis and suggest that SR Ca(2+) uptake is not the underlying mechanism.

Animals↗
PubMed1994

Pressure generation in a contracting myocyte.

The central hypothesis of this investigation is that a shortening myocyte generates a time-varying transmural pressure, or intracellular pressure. A mathematical model was formulated for a single myocyte, consisting of a fluid-filled cylindrical shell with axially arranged contractile filaments, to quantitate the fiber-fluid interaction. In this model, the intracellular pressure mediates the interaction between myofilament force, cell shortening, and the mechanical properties of the sarcolemma. Shortening of myofibrils, which are embedded in the fluid-filled myocytes, deforms the myocyte, thereby altering its transmural fluid pressure. This increase in transmural pressure counteracts fiber shortening, hence constituting an internal load to shortening. The shortening of the myocyte is accompanied by thickening, due to the incompressible nature of its contents. Consequently, the overall contractile performance of the cell is integrally linked to the generation of intracellular pressure. The model manifests a positive transmural pressure during shortening, but not without shortening. The pressure in the myocyte, therefore, is not a direct function of the force generated, but rather of shortening. Intracellular pressure was measured through a fluid-filled glass micropipette (5 mu ID) employing a servo-nulling pressure transducer in a standard micropuncture technique. Measured intracellular pressure in a contracting isolated skeletal myocyte of the giant barnacle is observed to be dynamically related to shortening, but not to tension without shortening. The relation between the force of contraction, cell shortening, and intracellular pressure was assessed during both isotonic and isometric contractions. The results support the prediction that isometric, or nondeforming, contractions will not develop intracellular pressure and identify a reason for relengthening of the myocytes during relaxation.

Animals↗
PubMed1999

Polycythemia decreases fatigue in tetanic contractions of canine skeletal muscle.

PURPOSE: The effects of an acute polycythemia on muscle fatigue development were investigated in the self-perfused canine gastrocnemius in situ. METHODS: Following isolation of the gastrocnemius, dogs (N = 5) were made polycythemic through a bolus injection of packed erythrocytes (hematocrit (Hct) = 90-92%) to raise systemic Hct to 63.5 +/- 0.5%. Subsequently, the gastrocnemius was stimulated, through the sciatic nerve, to perform 20 min of isotonic tetanic contractions (60 x min(-1), 200 ms, 50Hz). Control (normocythemic) animals (N = 5) underwent an identical contraction regimen. RESULTS: Although blood flow to the gastrocnemius was not different at any time, oxygen delivery was significantly increased during polycythemia (peak = 33.7 +/- 2.2 mL x 100 g(-1) x min-1) over control (peak = 25.1 +/- 2.1 mL x 100 g(-1)x min(-1)) at all times during contraction. Oxygen uptake by the gastrocnemius, although consistently increased, was not significantly different between the normocythemic and polycythemic conditions at any time. The rate of fatigue was significantly decreased over the first 6 min of contraction in polycythemic animals (3.5 +/- 0.6% x min(-1)) when compared with controls (5.8 +/- 0.7% x min(-1)). Subsequent fatigue development was not different between groups. As a result of the early rate differences in fatigue, however, the work production in polycythemic animals was significantly greater than in normocythemic dogs for the duration of the contraction period. CONCLUSION: We conclude that during high metabolic rate isotonic tetanic contractions, muscle fatigue development is diminished by polycythemia, but the ergogenic effect appears to be transient.

Analysis of Variance↗
PubMed1976

Cytoplasmic actomyosin fibrils in tissue culture cells: direct proof of contractility by visualization of ATP-induced contraction in fibrils isolated by laser micro-beam dissection.

A special cell line derived from a rat mammary adenocarcinoma (RMCD cells) displays a distinct pattern of actomyosin fibrils (AM fibrils) visible with phase contrast, Nomarski interference and polarized light optics. It was shown that the cytoplasmic AM fibrils are arranged as bundles of highly parallel F-actin filaments. The chimical nature of the filaments was identified by incubation with heavy meromyosin from rabbit skeletal muscle. These cytoplasmic actomyosin fibrils actively contract under isotonic conditions. This was shown by contraction experiments under polarized light optics, by cinematographic analysis and by direct proof of the contractility of AM fibrils isolated by laser micro-dissection. Thus, cytoplasmic AM fibrils can be assumed to represent structures essential for motive force generation in contraction processes in non-muscle cells.

Actomyosin↗
PubMed1991

Attenuation of contraction of isolated canine coronary arteries by enflurane and halothane.

Contraction of vascular smooth muscle such as that existing in coronary arteries is regulated in part by Ca++ entry into cells via Ca++ channels. Volatile anaesthetics are known to attenuate agonist-induced coronary artery constriction. The purpose of this experiment was to determine if 1.5 MAC concentrations of halothane or enflurane attenuated contractions evoked by activation of one type of Ca++ channel--the potential operator channel. In the current experiment, potential operator channels were activated by depolarizing isolated canine coronary artery rings with high concentration of K+, causing Ca++ entry and vessel contraction. Rings without endothelium were suspended for isometric force measurement in organ chambers containing aerated Krebs-Ringer solution. Maximum response to Ca++ in rings depolarized with K+ was 120 +/- 5 per cent in untreated versus 101 +/- 3 per cent in rings treated with enflurane (P less than 0.01). The maximum response was 123 +/- 6 per cent in untreated versus 111 +/- 5 per cent during halothane administration (P less than 0.05). In contrast, nifedipine 10(-9) M depressed maximum contractions from 114 +/- 5 per cent to 37 +/- 4 per cent (P less than 0.01) and nifedipine 10(-8) M depressed contractions to 30 +/- 4 per cent (P less than 0.01). In a further series of experiments, sustained contractions were depressed by continued administration of the anaesthetics, indicating no loss of anaesthetic effect with time. The results indicate that 1.5 MAC halothane and enflurane attenuate contractions of canine coronary arteries evoked by depolarization and Ca++ entry through potential operated channels. However, neither halothane nor enflurane exhibited the marked depressant effect exerted by nifedipine.

Animals↗
PubMed2006

New insights into the passive force enhancement in skeletal muscles.

The steady-state isometric force following active stretching of a muscle is always greater than the steady-state isometric force obtained in a purely isometric contraction at the same length. This phenomenon has been termed "residual force enhancement" and it is associated with an active and a passive component. The origin of these components remains a matter of scientific debate. The purpose of this work was to test the hypothesis that the passive component of the residual force enhancement is caused by a passive structural element. In order to achieve this purpose, single fibers (n=6) from the lumbrical muscles of frog (Rana pipiens) were isolated and attached to a force transducer and a motor that could produce computer-controlled length changes. The passive force enhancement was assessed for three experimental conditions: in a normal Ringer's solution, and after the addition of 5 and 15mM 2,3-butanedione monoxime (BDM) which inhibits force production in a dose-dependent manner. If our hypothesis was correct, one would expect the passive force enhancement to be unaffected following BDM application. However, we found that increasing concentrations of BDM decreased the isometric forces, increased the normalized residual force enhancement, and most importantly for this study, increased the passive force enhancement. Furthermore, BDM decreased the rate of force relaxation after deactivation following active stretching of fibers, passive stretching in the Ringer's and BDM conditions produced the same passive force-sarcomere length relationship, and passive force enhancement required activation and force production. These results led to the conclusion that the passive force enhancement cannot be caused by a structural component exclusively as had been assumed up to date, but must be associated, directly or indirectly, with cross-bridge attachments upon activation and the associated active force.

Animals↗
PubMed1991

Effect of osmotic compression on the force-velocity properties of glycerinated rabbit skeletal muscle cells.

The force-velocity relations of single glycerinated rabbit psoas muscle fibers at 5 degrees C were studied at maximum and half-maximum activation in the presence of 0 (control) and 39-145 g/liter dextran T-70. Resting fiber diameter decreased progressively to approximately 70% of the nondextran control as the dextran concentration was increased. Isometric force at full activation increased to a maximum of 136% of control at 111 g/liter dextran and then fell to 80% of control in 145 g/liter dextran. Maximum velocity, which fell to 49% of the control value in the highest concentration of dextran, was nearly constant at approximately 65% control over the range of 58-111 g/liter dextran. Relative maximum power, which gives an estimate of changes in intermediate velocity, was not significantly reduced by dextran concentrations up to 76 g/liter, but then fell progressively to 62% of control in the highest concentration of dextran. At half-maximum activation, maximum velocity and relative maximum power were not significantly different from the values at full activation. The results obtained at partial activation indicate that the decline of velocity seen in the presence of dextran is not due to a passive internal load and that the dextran does not cause a viscous resistance to shortening. The increased velocity in the absence of dextran can be explained by the reduced ability of cross-bridges to resist shortening, as proposed by Goldman (1987. Biophys. J. 51:57).

Animals↗
PubMed1986

Characteristics of myocardial contractions activated under depolarizing conditions.

Guinea pig papillary muscles were used to study the activation of myocardial contractions under depolarizing conditions. Depolarization promoted by TKBa (normal Tyrode solution containing KCl and BaCl2) and TKAdr (normal Tyrode solution containing KCl and adrenaline) inactivates the fast Na current and under these conditions only slow responses are available to activate contractions. Since the slow response is sensitive to changes in rate and rhythm, we searched for mechanical correlates using isometrically contracting preparations to study the force-frequency relationship as well as rest potentiation. We also investigated if contractions are dependent only on the slow response or if the complete action potential plays a special role in the activation of contractions. We conclude that the slow response is the main mechanism for the activation of myocardial contraction, because complete mechanical activation was observed under depolarizing conditions. However, the electrical behavior of the slow response is reflected in the mechanical behavior of the depolarized preparations. At high rates, disturbances of excitation occur in depolarized cells and contractions appear to be bigeminal or with 2:1 block. Furthermore, after long pauses, post-rest contractions are depressed and increase progressively with repetitive stimulation, probably due to changes in the latency and threshold of the slow response. The complete action potential also plays a role in the activation of myocardial contractions which is necessary for the resting potentiation phenomena and also to avoid the larger depression of the rest contractions that can be seen in depolarized muscles when the pause is prolonged. This behavior seems to be related to the decrease of intracellular Na concentration produced by the inhibition of the fast inward Na-current.

Action Potentials↗