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PubMed1974

Analysis of a simple prototypal muscle model near to and far from equilibrium.

Accurate calculations of force generated and of ATP flux, in steady isotonic contractions, are made on a simple, two-state muscle model of the sliding filament type. The objective is to illustrate the proper formulation and use of a complete and self-consistent molecular model of muscle. Otherwise, the model is not meant to be realistic. Calculations were made near equilibrium, including linear and quadratic terms in a power series expansion, and arbitrarily far from equilibrium by direct numerical solution of the appropriate differential equation in the probability of cross-bridge attachment. The results obtained from the two different methods agree where they overlap near equilibrium. The efficiency of free energy conversion is emphasized, and the relation to linear irreversible thermodynamics is pointed out.

Adenosine Triphosphate↗
PubMed1974

Further analysis of a simple prototypal muscle model near to and far from equilibrium.

The same prototypal model used in a previous paper to illustrate proper construction of a muscle model is modified here with the much more realistic choice e(Deltap) = 10(8) rather than e(Deltap) = 100, where e(Deltap) is the ratio of physiological ATP activity to equilibrium ATP activity. For steady isotonic contractions, the range 1 </= e(Delta) </= 10(4) can be approximated quite well by use of linear terms only in expansions of F (force) and J (ATP flux) in powers of e(Delta) - 1 and v (velocity). This will presumably also be true in most cases of much more complicated models. However, this region is of theoretical interest only (irreversible thermodynamics, etc.) because F and J are very small. In addition, numerical calculations of F and J were made in the region 10(4) </= e(Delta) </= 10(8). The optimal efficiency eta(*) is larger under physiological conditions (about 1%) than at equilibrium by a factor of 2.1 x 10(4). The rate of entropy production is discussed in this connection.

Adenosine Triphosphate↗
PubMed1980

Examination of the conventional assumption of internal equilibrium within the states of a biochemical cycle.

If k is a representative "internal" rate constant between substates of a given biochemical state, and if alpha is representative of the rate constants of the biochemical cycle to which the state belongs, then cyclical activity at steady state pulls the substates out of internal equilibrium with each other by a factor of order 1 +/- O(alpha/k). For transients or steady isotonic contractions in muscle, the departures from internal equilibrium can be larger than this. The simplifying assumption that internal equilibrium is always maintained between the substates is justified at steady state, as a good approximation, if k/alpha greater than or equal to 100. In muscle contraction at maximum velocity, something like k/alpha greater than or equal to 500 is required. This problem is superficially similar to the question, in Eyring's rate theory, of the extent to which activated complex leads to products pulls the activated complex out of the assumed equilibrium with reactants.

Actins↗
PubMedDate not supplied

Influence of time of day and partial sleep loss on muscle strength in eumenorrheic females.

Disrupted sleep is the most common form of sleep deprivation in travellers, shift workers, athletes the night before important competitions and among parents of infants. The influence of partial sleep loss on muscle strength might differ according to the time of testing on the following day. This study was therefore designed to assess the interaction between the effects of partial sleep loss and time of day on muscle strength in females. Eight sedentary eumenorrheic females (mean +/- SD; age 30 +/- 6 years, height 1.62 +/- 0.06 m and body mass 67 +/- 5.0 kg) took part in the study, in a counterbalanced design. Measurements of muscle strength were carried out at 06:00 and 18:00 hours after the one control night (no sleep loss) and the one night of partial sleep loss, during menses. Muscle strength measures included isokinetic (at 1.05, 3.14 rad s(-1); 90 degrees range of motion) and isometric peak torque (at 60 degrees of knee flexion) of knee extensors and flexors (dominant leg). In addition, isometric force of knee extensors with super-imposed electrical twitches (50 Hz, 250 V, 200 mus pulse width) was measured using the same procedure in order to control for motivation. Rectal temperature was measured during the 30 min before muscle strength measurements. Partial sleep loss consisted of allowing 2.5 h sleep (between 03:00 and 05:30 h), whilst in the control condition (no sleep loss) subjects retired between 22:30 and 23:30 h, rising at 05:30 hours. All measurements were conducted at just one phase of the menstrual cycle (menses) to prevent any masking effect due to different phases of the menstrual cycle. In both conditions (with and without partial sleep loss) a diurnal variation was observed in peak torque of knee flexors at 1.05 (F(1,7) = 5.5, p < 0.05) and 3.14 rad s(-1) (F(1,7) = 8.0, p < 0.05); values at 18:00 hours were 4.5 and 5.9% higher than at 06:00 hours, respectively. No significant diurnal variation was observed for the other muscle strength measures. No significant effect of partial sleep loss or interaction effect (sleep x time of day) was observed for muscle strength measures. However, the performance rhythms were in phase with the circadian rhythm in rectal temperature. Partial sleep deprivation over one night did not have any adverse effect on maximal muscle strength, nor on diurnal variations of muscle strength indices. As the effect of time of day was observed with some of the muscle strength measures, it is suggested that, in designing future studies using females, the control of time of day is essential.

Adult↗
PubMed1998

Evaluation of the ability of power to predict low frequency lifting capacity.

An experiment was conducted to examine the role that maximal lifting power has in predicting maximum acceptable weight of lift (MAWL) for a frequency of one lift per 8 h. The secondary aim of the study was to compare the ability of power to predict MAWL to previously used measures of capacity including two measures of isometric strength, five measures of isokinetic strength, and isoinertial capacity on an incremental lifting test. Twenty-five male subjects volunteered to participate in the experiment. The isometric tests involved maximum voluntary contractions for composite lifting strength at vertical heights of 15 and 75 cm. Peak isokinetic strength was measured at velocities of 0.1, 0.2, 0.4, 0.6 and 0.8 m s-1 using a modified CYBEX II isokinetic dynamometer. Isoinertial lifting capacity was measured on the X-factor incremental lifting machine and peak power was measured on the incremental lifting machine by having subjects lift a 25 kg load as quickly as possible. The results indicate that peak isoinertial power is significantly correlated with MAWL, and this correlation was higher than any of the correlations between the other predictor variables and MAWL. The relationships between the isokinetic strength measures and MAWL were stronger than the relationships between the isometric measures and MAWL. Overall, the results suggest that tests used to predict MAWL should be dynamic rather than static.

Adult↗
PubMed1995

Plasticity in canine airway smooth muscle.

The large volume changes of some hollow viscera require a greater length range for the smooth muscle of their walls than can be accommodated by a fixed array of sliding filaments. A possible explanation is that smooth muscles adapt to length changes by forming variable numbers of contractile units in series. To test for such plasticity we examined the muscle length dependence of shortening velocity and compliance, both of which will vary directly with the number of thick filaments in series. Dog tracheal smooth muscle was studied because its cells are arrayed in long, straight, parallel bundles that span the length of the preparation. In experiments where muscle length was changed, both compliance and velocity showed a strong dependence on muscle length, varying by 1.7-fold and 2.2-fold, respectively, over a threefold range of length. The variation in isometric force was substantially less, ranging from a 1.2- to 1.3-fold in two series of experiments where length was varied by twofold to an insignificant 4% variation in a third series where a threefold length range was studied. Tetanic force was below its steady level after both stretches and releases, and increased to a steady level with 5-6 tetani at 5 min intervals. These results suggest strongly that the number of contractile units in series varies directly with the adapted muscle length. Temporary force depression after a length change would occur if the change transiently moved the filaments from their optimum overlap. The relative length independence of the adapted force is explained by the reforming of the filament lattice to produce optimum force development, with commensurate changes of velocity and compliance.

Adaptation, Physiological↗
PubMed1961

The importance of calcium in poststimulation potentiation.

Isotonic contractions recorded both before and during poststimulation potentiation in the toad isolated ventricle (Bufo marinus) revealed that the phenomenon of poststimulation potentiation was not altered by the presence or absence of the catechol amines, or by the specific amine antagonist, DCI. Similarly the inhibitors, sodium fluoride and sodium iodoacetate, were without effect. Changes in [Ca(++)], [Mg(++)], and [Na(+)] affected the degree of potentiation. High [Ca(++)] as well as the cardiac glycosides abolished it, low [Na(+)] and the absence of Mg(++) depressed it. It has been shown that the percentage potentiation depends to some extent upon the total number of contractions occurring during the rapid stimulation phase. The amplitude of the contractions during this stage did not influence the degree of potentiation. These results are discussed in terms of Ca(++) accumulation or redistribution associated with an early phase of the membrane depolarization.

Animals↗
PubMed1977

Tension generation by threads of contractile proteins.

Threads of contractile proteins were formed via extrusion and their isometric tensions and isotonic contraction velocities were measured. We obtained reproducible data by using a new and sensitive tensiometer. The force-velocity curves of actomyosin threads were similar to those of muscle, with isometric tensions of the order of 10g/cm2 and maximum contraction velocites of the order of 10(-2) lengths/s. The data could be fitted by Hill's equation. Addition of tropomyosin and troponin to the threads increased isometric tension and maximum contraction velocity. Threads which contained troponin and tropomyosin required Ca++ for contraction and the dependence of their isometric tension on the level of free Ca++ was like that of muscle. The dependence of tension or of contraction velocity upon temperature or upon ionic strength is similar for actomyosin threads and muscle fibers. In contrast, the dependence of most parameters which are characteristic of the actomyosin interaction in solution (or suspension) upon these variables is not similar to the dependence of the muscle fiber parameters. The conclusion we have drawn from these results is that the mechanism of tension generation in the threads is similar to the mechanism that exists in muscle. Because the protein composition of the thread system can be manipulated readily and because the tensions and velocities of the threads can be related directly to the physiological parameters of muscle fibers, the threads provide a powerful method for studying contractile proteins.

Actins↗
PubMed2001

Alpha-smooth muscle actin expression upregulates fibroblast contractile activity.

To evaluate whether alpha-smooth muscle actin (alpha-SMA) plays a role in fibroblast contractility, we first compared the contractile activity of rat subcutaneous fibroblasts (SCFs), expressing low levels of alpha-SMA, with that of lung fibroblasts (LFs), expressing high levels of alpha-SMA, with the use of silicone substrates of different stiffness degrees. On medium stiffness substrates the percentage of cells producing wrinkles was similar to that of alpha-SMA-positive cells in each fibroblast population. On high stiffness substrates, wrinkle production was limited to a subpopulation of LFs very positive for alpha-SMA. In a second approach, we measured the isotonic contraction of SCF- and LF-populated attached collagen lattices. SCFs exhibited 41% diameter reduction compared with 63% by LFs. TGFbeta1 increased alpha-SMA expression and lattice contraction by SCFs to the levels of LFs; TGFbeta-antagonizing agents reduced alpha-SMA expression and lattice contraction by LFs to the level of SCFs. Finally, 3T3 fibroblasts transiently or permanently transfected with alpha-SMA cDNA exhibited a significantly higher lattice contraction compared with wild-type 3T3 fibroblasts or to fibroblasts transfected with alpha-cardiac and beta- or gamma-cytoplasmic actin. This took place in the absence of any change in smooth muscle or nonmuscle myosin heavy-chain expression. Our results indicate that an increased alpha-SMA expression is sufficient to enhance fibroblast contractile activity.

3T3 Cells↗
PubMed1976

Myocardial contractility in aged rats.

The elastic and mechanical behaviour of isolated papillar muscle was studied in aged and in young rats. The spontaneous fall in tension (stress relaxation phenomenon) and the duration of the active state of the contractile element during the isometric and isotonic contraction were greater in aged than in young rats. The elastic properties of the elements in parallel and in series and the intensity of the active state in the contractile elements, on the contrary, did not exhibit any significant difference between aged and young rats.

Aging↗
PubMed2000

Abnormal transmitter release at neuromuscular junctions of mice carrying the tottering alpha(1A) Ca(2+) channel mutation.

Neurotransmitter release at many synapses is regulated by P/Q-type Ca(2+) channels containing the alpha(1A) pore-forming subunit. Mutations in alpha(1A) cause cerebral disorders including familial hemiplegic migraine (FHM) and ataxia in humans. Tottering (tg) alpha(1A) mutant mice display ataxia and epilepsy. It is not known whether alpha(1A) mutations induce impairment of synaptic function, which could underlie the symptoms of these cerebral disorders. To assess whether alpha(1A) mutations influence neurotransmitter release, we studied P-type Ca(2+) channel-mediated acetylcholine (ACh) release at tg neuromuscular junctions (NMJs) with micro-electrode measurements of synaptic potentials. We found a Ca(2+)-, Mg(2+)- and K(+)-dependent increase of spontaneous ACh release at both homo- and heterozygote tg NMJs. Furthermore, there was increased run-down of high-rate evoked release at homozygous tg NMJs. In isotonic contraction experiments this led to block of synaptic transmission at lower concentrations of the ACh antagonist tubocurarine than were needed in wild-type muscles. Our results suggest that in tg motor nerve terminals there is increased influx of Ca(2+) under resting conditions. This study shows that functional consequences of alpha(1A) mutations causing cerebral disorders can be characterized at the NMJ.

Acetylcholine↗
PubMed1985

Age-related differences in knee muscle strength in normal women.

This study provides normal baselines of knee muscle strength for women up to 86 years of age. Knee muscle strength (torque) was measured during maximum isometric and dynamic (isokinetic) contractions in 72 normal healthy women in three age groups between 20 and 86 years. Strength of the oldest group ranged from 56 to 78% of that in the youngest group, depending on knee joint position. Strength values were greater for isometric contractions than for isokinetic contractions (performed at 36 degrees per second) and were approximately twice as great for the extensor as for the flexor muscles. For both muscle groups, the torque generated with the knee in 30 degrees of flexion was usually lower than that generated with the knee at the 45- or 60-degree position. Despite the significant deficit found in the strength of the oldest group, it is likely that this relative weakness would not be detected on routine clinical examination using manual muscle testing.

Adult↗
PubMed1992

Obtaining reliable measurements of knee extensor torque produced during maximal voluntary contractions: an experimental investigation.

The purpose of this study was to develop and test a protocol that could be used to obtain reliable measurements of knee extensor torque produced during maximal voluntary contractions. On each of 3 days, 10 subjects performed six consecutive maximal voluntary contractions, in the same randomized order, for each of the following 10 conditions: concentric isokinetic contractions at velocities of 30 degrees, 90 degrees, 120 degrees, and 180 degrees/s; eccentric isokinetic contractions at velocities of 30 degrees, 90 degrees, 120 degrees, and 180 degrees/s; and isometric contractions at 40 and 60 degrees of knee flexion. The peak torques produced were examined to determine on which day and during which contraction subjects produced the greatest torques for each condition. This information was used to develop a practice protocol. Fifteen different subjects were tested following this protocol. Subjects participated in two practice sessions, a test session, and a retest session. Intraclass correlation coefficients (ICCs) were calculated to determine the degree of agreement between torques for the test and retest sessions. The ICCs ranged from .87 to .98. The protocol developed appears to be useful for obtaining reliable measurements of knee extensor torque.

Acceleration↗
PubMed1993

Injury to skeletal muscle fibers during contractions: conditions of occurrence and prevention.

Contraction-induced injury results in the degeneration and regeneration of muscle fibers. Of the three types of contractions--shortening (concentric), isometric, and lengthening (eccentric)--injury is most likely to occur and the severity of the injury is greatest during lengthening contractions. The magnitude of the injury to muscle fibers may be assessed by direct measures of cellular and ultrastructural damage; by indirect measures of changes in enzyme efflux, calcium influx, ratio of oxidized to reduced glutathione, and force development; and, in human beings, by reports of muscle soreness. The sequence of events includes an initial injury that is primarily mechanical and a secondary metabolic, or biochemical, injury that peaks 1 to 3 days after the injurious contractions. The recovery from contraction-induced injury is usually complete within 30 days. Repeated exposures to protocols of lengthening contractions result in "trained" muscles that are not injured by the protocol that previously caused injury.

Animals↗
PubMed1983

Influence of castration of the neonatal rat on the contractile effects of barium chloride and adrenaline in isolated vas deferens: the role of calcium.

Time-response curves to maximal concentrations of barium chloride (BaCl2) (3 X 10(-2) M) and adrenaline (10(-4) M) were studied in vasa deferentia from 3-month-old rats castrated at birth. Either barium or adrenaline was left in the organ baths for 5-min periods, at intervals of about 30 min, and the corresponding isotonic contractions recorded. Two types of effects were measured: the fade response (Jurkiewicz et al., 1977) and the rate at which responses were reduced after Ca2+ withdrawal from nutrient solution. The fade response for BaCl2 was strikingly greater than that in controls. When calcium was removed from the nutrient solution, an almost complete loss of the response to BaCl2 was achieved in less than 3 min for preparations of 3-day castrates, in about 40 min for the organs of 15-day castrates, and in more than 140 min for normal preparations. Treatment with testosterone, 1 week before the experiments, abolished the fade response to BaCl2 and antagonized the loss of responsiveness observed for this substance in a calcium-deficient solution. These data suggest that the production of testosterone by the testis during the critical period of neonatal differentiation is important for the translocation of calcium ions in the isolated vas deferens of the adult rat.

Animals↗
PubMed1986

Effects of hypothyroidism and hypoparathyroidism on rat myocardium: mechanical and electrical alterations.

The mechanical and electrical effects of hypoparathyroidism (Px), hypothyroidism (Tx), and hypothyroidism combined with hypoparathyroidism (TPx) were investigated by comparing simultaneously recorded transmembrane action potentials and isometric and isotonic contractions recorded from the myocardium. Left ventricular papillary muscles from male Wistar rats were studied electrically and mechanically in a muscle bath at 30 degree C, stimulated at 0.1 Hz and external calcium = 2.4 mM. No significant difference was found between control (C), Px, Tx, and TPx preparations with regard to resting tension and developed tension. However, time to peak tension, time to one half relaxation and time to peak shortening were significantly increased in preparations from animals that were Px, Tx, and TPx as compared with age-matched controls. Maximum velocities of shortening (Vs) and relengthening (Vr) at all relative loads studied were significantly depressed in Px preparations when compared with those of C muscles. A greater depression was found in the Tx muscle and still greater depression of these indices was noted in TPx muscles. No significant difference was found between C, Px, Tx, and TPx action potential with regard to resting membrane potential (RMP), action potential amplitude (AMP), overshoot (OS), or maximum rate of rise of the upstroke (Vmax).(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1975

Fluroxene and isolated heart muscle.

The direct myocardial effects of fluroxene were examined in isometrically and isotonically contracting isolated rat heart muscle. MAC, the minimum anesthetic concentration needed to prevent movement in response to tail clamping, was found to be 5.0 vol per cent fluroxene in the rat. At 4.6 vol per cent fluroxene, peak developed isometric tension and maximum rate of tension development were decreased 29 and 24 per cent, respectively. At 11 vol per cent, the depressions were 39 and 33 per cent. At 26.4 vol per cent, the depressions were around 60 per cent. Vmax (the maximum shortening velocity of unloaded muscle) of the force-velocity relation was unaltered by fluroxene concentrations of 0.8, 4.6, and 11 vol per cent. Even at 26.4 vol per cent, the depression in Vmax was only 25 per cent. Po (the maximum force at zero velocity), work, and power were lowered much more, with reductions ranging from 15 to 27 per cent at 4.6 vol per cent, from 40 to 42 per cent at 11.0 vol per cent, and from 65 to 69 per cent at the 26.4 vol per cent. Series elastic extension was unchanged at 0.8 and 4.6 vol per cent fluroxene, but was decreased 16 per cent at 11.0 vol per cent and 46 per cent at 26.4 vol per cent fluroxene. The data indicate the fluroxene has a direct negative inotropic effect that is associated with increased series elastic stiffness, but does not involve Vmax of the force-velocity relation until quite high anesthestic concentrations are reached. Comparative studies were also carried out with halothane. MAC for halothane in the rat was 1.0 vol per cent. The relative potency of halothane compared with fluroxene in depression of Vmax was 13.2, and its relative potency in depression of Po, 4.0.

Anesthesia, Inhalation↗
PubMedDate not supplied

Coefficient of variation in maximal and feigned static and dynamic grip efforts.

The purpose of this study was to investigate the validity of the coefficient of variation as an identifier of feigned grip effort. Seventeen healthy female aged 20 to 25 yr participated in the study. Maximal and feigned efforts were measured isometrically and isokinetically (concentric and eccentric) using the Jamar and KinCom dynamometers, respectively. Findings indicated that, in all situations, the coefficient of variation derived from the maximal effort was significantly (P < 0.0001) lower than that derived from the feigned effort. However, the extent of overlapping between the two was sufficiently large to render the test sensitivities very low. Consequently, regardless of the measurement method, the coefficient of variation is not a valid tool for identifying feigned grip effort in healthy subjects.

Adult↗