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PubMedDate not supplied

On the evaluation of muscle fiber conduction velocity considering waveform properties of an electromyogram in M. biceps brachii during voluntary isometric contraction.

The surface myoelectric signal during 20% maximum voluntary contraction was measured in m. biceps brachii using array electrodes for ten subjects in order to evaluate the distribution of muscle fiber conduction velocity (MFCV) in a whole muscle. MFCV was estimated by two calculating methods of the peak maximum method and the cross-correlation method from the myoelectric signals which were processed by techniques of the averaging and the non-averaging. It was found that the values of MFCV depended on the location irrespective of the kind of calculating method used and the kind of processing technique of myoelectric signal. In both the motor end-plate zone and the tendon zone, the values of MFCV showed more than 7.0 m/s. In the regions other than the motor end-plate zone and the tendon zone, the values of MFCV showed about 3.90 m/s which were almost constant. The statistical differences of the values of MFCV were in the same locations measured not recognized between the two calculating methods nor between the two processing techniques. In the cross-correlation method, the relation between MFCV and the electrode location was evaluated by both the maximum correlation coefficient and the amplitude ratio between the different neighboring channels to evaluate the conductive waveform properties of action potentials. The changes of parameters (i.e., MFCV, maximum correlation coefficient and amplitude ratio) depended on the electrode location. The values of MFCV significantly increased in the regions of the motor end-plate zone and the tendon zone, where the maximum correlation coefficient and amplitude ratio significantly decreased. The values of the coefficient of variance (CV) of three parameters in those regions were larger than those in other regions, i.e., the regions other than the motor end-plate zone and the tendon zone. A high maximum correlation coefficient and a high amplitude ratio were necessary for a reliable measurement of the MFCV.

Adult↗

PubMed1993

The frequency content of common synaptic inputs to motoneurones studied during voluntary isometric contraction in man.

1. The discharges of pairs of individual motor units were recorded from intrinsic hand muscles in man. Single motor unit recordings were obtained either when both members of the motor unit pair were within first dorsal interosseous muscle (1DI:1DI recordings) or where one motor unit was within 1DI and the other in second dorsal interosseous muscle (1DI:2DI recordings). The pairs of motor unit spike trains were cross-correlated in the time domain and the results compared to those of coherence analysis performed on the same spike train data. Central peaks were present in the cross-intensity functions, indicating the presence of common synaptic input to the motoneurone pair. Coherence analysis of these data indicated significant association between motor unit firing in the frequency ranges 1-12 and 16-32 Hz. 2. Analysis of sequential non-overlapping segments of data recorded from individual motor unit pairs, demonstrated that both the central cross-intensity peak and coherence in the frequency bands 1-12 and 16-32 Hz were consistent features throughout a long recording. In these sequential recordings, the size of the central cross-intensity peak and the maximal value of coherence in the frequency band 16-32 Hz covaried from segment to segment. Analysis of the entire population of motor unit pairs confirmed a positive relationship between the magnitude of peak coherence and the size of the central cross-intensity peak. 3. Voluntary sinusoidal co-modulation of the firing rates of pairs of individual motor units recorded from within 1DI was found to produce significant values of coherence corresponding to the frequency of the common modulation. However, firing rate co-modulation was not found to affect either the size of the central cross-intensity peak or the maximum value of coherence in the frequency band 16-32 Hz. 4. Pairs of single motor units were recorded from within 1DI and biceps brachii muscles of healthy subjects. The number and size of the central cross-intensity peaks and coherence peaks detected were compared for the two muscles. The incidence and size of central cross-intensity peaks and the incidence and magnitude of 16-32 Hz coherence peaks were both found to be greater for 1DI recordings when compared to biceps brachii recordings. 5. Single motor unit recordings were made from the intrinsic hand muscles of a patient with severe peripheral deafferentation. Time- and frequency-domain analysis of these recordings revealed cross-intensity peaks and frequency bands of coherence similar to those seen in healthy subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1983

Energetics of isometric contractions as a function of muscle temperature.

The energetic cost of generating isometric force in isolated frog muscle was examined at 10, 20, and 30 degrees C. Recovery O2 consumption (delta O2) and recovery lactate production (delta Lact) were measured under conditions in which O2 was not limiting metabolism. Both increased linearly with the force-time integral (integral of Fdt) generated by the muscle. The slopes of the regression equations for both delta O2 and delta Lact as a function of integral of Fdt increased with increasing temperature with a temperature coefficient (Q10) near 3. Total high-energy phosphate resynthesis from recovery metabolism was calculated by scaling the delta O2 regression equation and the delta Lact regression equation into equivalent ATP units and summing them. This total recovery metabolism was modeled as the sum of two components, a "cost of maintaining force" (slope of the equation) and a saturable "start up cost" (intercept of the equation). The cost of maintaining force increased with temperature with a Q10 near 3 over the whole temperature range, whereas the start up cost was nearly independent of temperature between 0 and 20 degrees C and fell to near zero at 30 degrees C. Delta O2 measurements from a series of tetani given in rapid succession showed that for contractions subsequent to the first, no start up cost was incurred and that the "cost of generating force" for these contractions was equal to the slope of the regression line for single tetani. The practical consequence of these facts is that, in series of tetani, the cost of generating force increases with a Q10 of 3.

Animals↗

PubMed2004

Experimental muscle pain increases trapezius muscle activity during sustained isometric contractions of arm muscles.

OBJECTIVE: In the present study, the influence of experimental muscle pain on muscle co-ordination and fatigue development during sustained isometric elbow flexion was investigated. METHODS: Conventional surface electromyography (EMG) was recorded from the biceps brachii, brachioradialis, deltoideus and trapezius muscle during isometric elbow flexion at 40% maximum force. Single motor unit (MU) conduction velocity in the biceps brachii was assessed using a high spatial resolution surface EMG technique. Measurements were performed on 15 healthy subjects before, during and after (1) injection of hypertonic (pain condition) and (2) isotonic saline (control) into the biceps brachii. The pain intensity was assessed on a 10 cm visual analogue scale. RESULTS: The experimental results showed in both experimental sessions a fatigue-related increase of the root mean square value of EMG (222+/-164% of the baseline), and a decrease of the median frequency (118+/-16% of the baseline) in all investigated muscles. A maximum pain level of in average 3.2 cm on the visual analogue scale was reached after injection of hypertonic saline during contraction. Differences between painful and control condition were seen in an increased trapezius activity (230+/-141%) during pain. The global EMG activity of the brachioradialis and biceps brachii was unaffected by experimental muscle pain in line with unaffected single MU conduction velocity in the biceps brachii. Differences in endurance time (mean 89.3 and 102.3 s, pain and control, respectively) were not significant. CONCLUSIONS/SIGNIFICANCE: The findings suggest that upper extremity pain could be a possible source for overloading the trapezius muscle and as such is an important factor in occupational settings.

Adult↗

PubMed2007

Postactivation potentiation in a human muscle: effect on the rate of torque development of tetanic and voluntary isometric contractions.

Postactivation potentiation (PAP), a mechanism by which the torque of a muscle twitch is increased following a conditioning contraction, is well documented in muscular physiology, but little is known about its effect on the maximal rate of torque development and functional significance during voluntary movements. The objective of this study was to investigate the PAP effect on the rate of isometric torque development of electrically induced and voluntary contractions. To that purpose, the electromechanical responses of the thumb adductor muscles to a single electrical stimulus (twitch), a train of 15 pulses at 250 Hz (HFT(250)), and during ballistic (i.e., rapid torque development) voluntary contractions at torque levels ranging from 10 to 75% of maximal voluntary contraction (MVC) were recorded before and after a conditioning 6-s MVC. The results showed that the rate of torque development was significantly (P < 0.001) increased after the conditioning MVC, but the effect was greater for the twitch ( approximately 200%) compared with the HFT(250) ( approximately 17%) or ballistic contractions (range: 9-24%). Although twitch potentiation was maximal immediately after the conditioning MVC, maximal potentiation for HFT(250) and ballistic contractions was delayed to 1 min after the 6-s MVC. Furthermore, the similar degree of potentiation for the rate of isometric torque development between tetanic and voluntary ballistic contractions indicates that PAP is not related to the modality of muscle activation. These observations suggest that PAP may be considered as a mechanism that can influence our contractions during daily tasks and can be utilized to improve muscle performance in explosive sports.

Adaptation, Physiological↗

PubMed2005

Assessment of skeletal muscle fatigue in men with coronary artery disease using surface electromyography during isometric contraction of quadriceps muscles.

OBJECTIVE: To evaluate whether using surface electromyography to assess skeletal muscle fatigue during an isometric exercise has the potential to be clinically useful in patients with coronary artery disease (CAD). DESIGN: Double sample comparative study. SETTING: Cardiac rehabilitation service in France. PARTICIPANTS: Sixteen men with documented CAD and 9 age-matched healthy men. INTERVENTIONS: Assessment of quadriceps skeletal muscle fatigue on an isokinetic apparatus with surface electromyography measurements and a symptom-limited exercise test in a laboratory. MAIN OUTCOME MEASURES: The maximal voluntary isometric force (MVIF) of the quadriceps was quantified as a measure of muscle strength and isometric endurance was defined as the time required to sustain a contraction at 50% of MVIF until exhaustion. Surface electromyography signals were recorded from the vastus lateralis, rectus femoris, and vastus medialis during isometric endurance. The root mean square (RMS) and the median frequency (MF) were directly calculated on a computer and then normalized (as a percentage of the initial value). RESULTS: Muscle strength did not differ significantly between the patients with CAD and the healthy subjects (229+/-21N/m vs 228+/-52N/m), but isometric endurance was reduced (64+/-17s vs 90+/-7s, P <.01). The RMS values showed a significantly higher increase in the healthy subjects versus the patients with CAD for the vastus lateralis and vastus medialis ( P <.001). The MF values were significantly lower for the vastus lateralis, rectus femoris ( P <.01), and vastus medialis ( P <.05) in patients with CAD compared with the healthy subjects. CONCLUSIONS: Skeletal muscle fatigue occurs sooner in men with CAD relative to matched healthy men, despite similar muscle strength. This finding may be the result of an abnormality of skeletal muscle function and may play an important role in measuring functional capacity. In addition, it may be a useful tool to assess the efficacy of cardiac rehabilitation interventions.

Coronary Artery Disease↗

PubMed2006

Gender influence on fatigability of back muscles during intermittent isometric contractions: a study of neuromuscular activation patterns.

BACKGROUND: Gender difference in the fatigability of muscles can be attributed to muscle mass (or strength) and associated level of vascular occlusion, substrate utilization, muscle composition, and neuromuscular activation patterns. The purpose of this study was to assess the role of neuromuscular activation patterns to explain gender differences in back muscle fatigability during intermittent isometric tasks. METHODS: Sixteen males and 15 females performed maximal voluntary contractions (Strength) and a fatigue test to exhaustion (fatigue criterion=time to exhaustion), while standing in a static dynamometer measuring L5/S1 extension moment. The fatigue test consisted of repetitions of an 8-s cycle (1.5 s ramp to reach 40% of maximal voluntary contraction +5s plateau at 40% of maximal voluntary contraction +1.5s rest). Surface electromyography signals were collected bilaterally from 4 back muscles (multifidus at the L5 level, iliocostalis lumborum at L3, and longissimus at L1 and T10). FINDINGS: Males were stronger (P<0.05) than females (316, SD 82>196, SD 25 Nm) but showed significantly shorter time-to-exhaustion values (7.1, SD 5.2<13.0, SD 6.1 min.), the latter result being corroborated by electromyographic indices of fatigue. However, the gender effect on time to exhaustion disappeared when accounting for Strength, thus supporting the muscle mass hypothesis. Among the various electromyographic indices computed to assess neuromuscular activation patterns, the amount of alternating activity between homolateral and between contralateral muscles showed a gender effect (females>males). INTERPRETATION: These results support the muscle mass hypothesis as well as the neuromuscular activation hypothesis to explain gender differences in back muscle fatigability.

Adult↗

PubMed1989

The feeding with unsaturated fatty acids modifies the course of the isometric contraction of rabbit papillary muscle in hypoxia.

Alterations of cardiac membrane functions can be induced by lipid rich diets. As to this the influence of the admixture of cholesterol (group Ch) and rape oil (group R) to standard food during 12 weeks on the course of the isometric twitch of rabbit papillary muscles in normoxia and hypoxia was studied. In group R the rate of contraction and relaxation is enhanced. This points to alterations in the role of the sarcoplasmic reticulum (SR), most probably due to the incorporation of unsaturated fatty acids. During hypoxia at 4 mM Ca concentration the force is more decreased in group R than in the control group C, but the rate of the contraction remains higher. The relaxation is more impaired especially in the early phase. The load-length relation is shifted to the left. The influence of hypoxia in group Ch was similar to group C. The results are hints for the Ca overload of the cells, preferentially in group R. The SR function seems to be more damaged, f.i. by radicals reacting with unsaturated fatty acids and by Ca activated lipolyses.

Animals↗

PubMedDate not supplied

Muscle activation and force production during bilateral and unilateral concentric and isometric contractions of the knee extensors in men and women at different ages.

In experiment I ten young men (29 +/- 3 yrs; M30), 12 middle-aged men (50 +/- 4 yrs; M50) and 12 women (48 +/- 5 yrs; W50), 12 elderly men (67 +/- 4 yrs; M70) and 12 women (68 +/- 4 yrs; W70) volunteered for subjects for examination of maximal 1 RM strength and electromyographic activity of the knee extensor muscles during the bilateral and unilateral concentric contraction on a variable resistance knee extension dynamometer. In experiment II 10 young (Y) men (29 +/- 5 yrs) and 10 older (O) men (61 +/- 4 yrs) were examined for their maximal voluntary isometric force and force-time curves and electromyographic activity of the knee extensor muscles during the bilateral and unilateral contractions. The bilateral 1 RM of 165.5 +/- 25.5 kg in M30 was greater (p < 0.01) than that of 127.7 +/- 24.5 kg recorded for M50 the latter being also greater (p < 0.05) than that of 109.0 +/- 17.7 kg recorded for M70. The bilateral value of 87.4 +/- 13.4 kg in W50 was greater (p < 0.05) than that of 69.9 +/- 15.0 kg recorded for W70. The bilateral 1 RM values were slightly greater than the summed unilateral 1 RM values in all groups M50, W50 and W70 showing a significant (p < 0.05) difference. All groups showed slightly (ns.) greater mean maximal integrated EMG values during the bilateral conditions in comparison to that of the corresponding unilateral condition. The maximal isometric forces in Y men were 25% greater (p < 0.001) than in O men. In both groups the bilateral forces were somewhat greater (p < 0.05) than the summed unilateral forces and the bilateral IEMG values slightly (ns.) greater than the corresponding unilateral IEMG values. The early forces on the force-time curve were much greater (p < 0.05-0.001) in Y than O men in both conditions. The present findings suggest that both maximal voluntary isometric and concentric force, and especially explosive strength of the knee extensors decrease greatly with increasing age probably due to selective muscle atrophy and/or possible decreases in the amount or rate of voluntary activation of the muscles. However, no bilateral deficit could be found indicating that the central nervous system in a simple single joint isometric and maximal 1 RM concentric force production of the knee extensors was capable of activation of the two bilateral muscle groups simultaneously independent of age and sex of the subject. To which extent the activation and force production of the muscles would be different in terms of the bilateral deficit during various multijoint exercises utilizing isometric and higher velocity concentric, eccentric and various stretch shortening cycle exercise needs to be examined in the future.

Adult↗

PubMed2002

Prediction of endurance capacity of quadriceps muscles in humans using surface electromyogram spectrum analysis during submaximal voluntary isometric contractions.

The measurement of endurance time ( t(lim)) is the procedure commonly used to quantify the ability of a muscle to maintain force. The relationship between surface electromyographic (sEMG) manifestations of localised muscle fatigue and t(lim) during an effort at 50% of maximal voluntary isometric torque of the knee extensors (vastus lateralis and vastus medialis) until exhaustion was studied in 14 healthy volunteers. It was carried out to test whether changes in sEMG computed over shorter periods than expected t(lim) could be used to predict t(lim). Changes in mean muscle fibre conduction velocity, mean power frequency, median frequency, root mean square ), in the relative power in the 6-30 Hz and 30-60 Hz frequency bands were monitored using linear slope and area ratio index as statistical indicators. These indicators were computed over fixed periods shorter than t(lim). The subjects were able to maintain the required force level for [mean (SD)] 78.8 (9.5) s. During the fatigue trial, it was the greatest of the increases in the 6-30 Hz frequency band, recorded for either of the two muscles investigated, that was the only variable which correlated with t(lim). Significant relationships between t(lim) and changes in this low frequency band were observed as early as the first 15-30 s of the contraction. These results suggest that sEMG frequency banding may predict mechanical endurance without the need to maintain the contraction until exhaustion. From a clinical perspective, this could be an advantage for patients who might not be able to tolerate contractions to exhaustion.

Adult↗

PubMed1981

Blood flow and metabolism during isometric contractions in cat skeletal muscle.

The muscle blood flow, oxygen uptake, carbon dioxide production, muscle and blood lactate, muscle ATP, creatinine phosphate, glycogen, and venous pH were measured in the soleus (a slow-twitch muscle) and the medial gastrocnemius (a fast-twitch muscle) of the cat during fatiguing isometric exercise. Five tensions were examined: 10, 25, 50, 75, and 100% of the initial strength of the muscles (tetanic tension of the unfatigued muscle). Contractions were either sustained to fatigue or, for tensions of 10 and 25% initial strength of the soleus muscle, were sustained for 3 min. Analysis of the blood flow and metabolites from these muscles showed that the soleus was heavily dependent on its blood supply, using aerobic metabolism as the predominant pathway, whereas the medial gastrocnemius muscle seemed to use anaerobic metabolism even at low isometric tensions.

Adenosine Triphosphate↗

PubMed1984

Muscle stiffness changes during isometric contraction in frog skeletal muscle as studied by the use of ultrasonic waves.

In order to measure muscle stiffness changes with a high time resolution and with minimal disturbance to the contractile mechanism per se, we constructed an apparatus with which the propagation velocity of ultrasonic waves (MHz region) in the longitudinal or transverse direction was measured to serve as a measure of muscle stiffness. The longitudinal muscle stiffness started to increase on stimulation before the onset of isometric force, and reached a maximum before the peak twitch force. Analysis of experimental data indicated that, during an isometric tetanus, the increment of muscle longitudinal stiffness was about 6 X 10(7)N/m2, a value similar to those obtained by Truong (1974) and Ford et al. (1981) with sinusoidal vibrations (3 kHz) and length steps respectively. This suggests that the increment of muscle longitudinal stiffness during the activation of the contractile system results from the recruitment of an almost non-dispersive elastic component. In the case of transverse muscle stiffness, on the other hand, it started to decrease on stimulation before the onset of isometric force, and reached a minimum before the peak twitch force. Possible causes of this unexpected result is discussed in relation to the molecular mechanism of muscle contraction.

Animals↗

PubMed1968

The evolution of fatigue associated with isometric contraction in toad sartorius.

1. The development of fatigue in isolated toad sartorius undergoing short, periodic, isometric tetani of controlled duration, was studied. Stimulation was direct and curare was used.2. In the great majority of cases extratension decreased very regularly towards a steady value, following a simple exponential equation.3. The final steady tension (expressed as percentage of the maximum) and the constant of exponential decay varied as simple functions of the mean frequency of stimulation and their product was a constant.

Animals↗

PubMed1993

Two-dimensional time-resolved X-ray diffraction studies of live isometrically contracting frog sartorius muscle.

Results were obtained from contracting frog muscles by collecting high quality time-resolved, two-dimensional, X-ray diffraction patterns at the British Synchrotron Radiation Source (SERC, Daresbury, Laboratory). The structural transitions associated with isometric tension generation were recorded under conditions in which the three-dimensional order characteristic of the rest state is either present or absent. In both cases, new layer lines appear during tension generation, subsequent to changes from activation events in the thin filaments. Compared with the 'decorated' actin layer lines of the rigor state, the spacings of the new layer lines are similar whereas their intensities differ substantially. We conclude that in contracting muscle an actomyosin complex is formed whose structure is not like that in rigor, although it is possible that the interacting sites are the same. Transition from rest to plateau of tension is accompanied by approximately 1.6% increase in the axial spacing of the myosin layer lines. This is explained as arising from axial disposition of the interacting myosin heads in the actomyosin complex. Model calculations are presented which support this view. We argue that in a situation where an actomyosin complex is formed during contraction, one cannot describe the diffraction features as being either thick or thin filament based. Accordingly, the layer lines seen during tension generation are referred to as actomyosin layer lines. It is shown that these layer lines can be indexed as submultiples of a minimum axial repeat of approximately 218.7 nm. After lattice disorder effects are taken into account, the intensity increases on the 15th and 21st AM layer lines at spacings of approximately 14.58 and 10.4 nm respectively, show the same time course as tension rise. However, the time course of the intensity increase of the other actomyosin layer lines and of the spacing change (which is the same for both phenomena) shows a substantial lead over tension rise. These findings suggest that the actomyosin complex formed prior to tension rise is a non-tension-generating state and that this is followed by a transition of the complex to a tension-generating state. The intensity increase in the 15th actomyosin layer line, which parallels tension rise, can be accounted for assuming that in the tension-generating state the attached heads adopt (axially) a more perpendicular orientation with respect to the muscle axis than is seen at rest or in the non-tension-generating state. This suggests the existence of at least two structurally distinct interacting myosin head conformations.(ABSTRACT TRUNCATED AT 400 WORDS)

Actomyosin↗

PubMed1977

The aerobic metabolism of porcine carotid artery and its relationship to isometric force. Energy cost of isometric contraction.

1. The O2 consumption and aerobic lactate production of porcine carotid artery have been studied in relation to active isometric tension maintenance in order to estimate the tension cost, i.e., ATP hydrolysis rate per unit force maintained. 2. The relationship between total metabolism and isometric force was measured at several muscle lengths in order to distinguish tension-dependent and -independent components of ATP utilization. 3. At the maximum isometric tension observed, 2 kg wt/wt/cm2, 1.3 mumole ATP/M in per gram wet weight out of a total metabolism of 2 mumole/min is tension de pardent. 4. The absolute levels of force, O2 consumption, lactate production and the ratio of oxidative to glycolytic metabolism varied considerably with the stimulant; however, the tension-dependent ATP utilization was invariant with respect to the mode of stimulation.

Adenosine Triphosphatases↗

PubMed2006

Progressive suppression of intracortical inhibition during graded isometric contraction of a hand muscle is not influenced by hand preference.

GABAergic intracortical inhibition (ICI) in human motor cortex (M1) assists fractionated activation of muscles, and it has been suggested that hemispheric differences in ICI may contribute to hand preference. Previous studies of this issue have all been conducted at rest, with conflicting results. Testing during voluntary activation may reveal functionally relevant differences. In normal subjects, we assessed (1) operation of ICI circuits during selective activation of an intrinsic hand muscle at different forces, and (2) whether this differs between right and left hemispheres. Surface EMG was recorded bilaterally from abductor pollicis brevis (APB), first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles in eleven right-handed subjects. A circular coil applied paired transcranial magnetic stimulation (TMS) with posteriorly directed current in the brain. Conditioning intensity was 0.8 x active threshold and interstimulus interval was 3 ms. TMS was applied to right or left M1 while subjects were at rest or performing isometric thumb abduction at different forces (0.5, 1, 2, 3, 5 and 10 N) with the contralateral hand. Conditioning TMS was less effective at suppressing the muscle evoked potential in APB during 2-10 N thumb abduction (P < 0.0001) versus rest, but not with lower target forces (0.5, 1 N). Conditioning TMS was less effective for FDI and ADM only during 10 N thumb abduction. We conclude that differential modulation of ICI in M1 during selective muscle activation is a function of target isometric force level. At low forces (<5% MVC), ICI is not modulated for the corticospinal neurons controlling the active or inactive muscles. There is a progressive reduction of ICI effects on corticospinal neurons at higher forces, which is largely restricted to corticospinal neurons controlling the muscle targeted for activation over the range of forces tested (up to approximately 25% MVC). The pattern of ICI modulation with selective voluntary muscle contraction was similar in left and right hemispheres during this relatively simple static task. If hemispheric differences in operation of M1 ICI circuits contribute to hand preference, a more challenging finger movement protocol may be needed to demonstrate this asymmetry.

Adult↗

PubMed1989

Agonist-specific myosin phosphorylation and intracellular calcium during isometric contractions of arterial smooth muscle.

The relationship between phosphorylation of the 20-kDa myosin light chain, intracellular calcium levels ([Ca2+]i), and isometric force was studied during prolonged activation of arterial smooth muscle. Aequorin, preloaded into ferret aortic strips, was used as a [Ca2+]i indicator. Two dimensional polyacrylamide gel electrophoresis was used to determine the phosphorylation levels of the 20-kDa myosin light chain (LC20). During the 30-min depolarization of arterial smooth muscle by K+ (21 mM), both LC20 phosphorylation and [Ca2+]i increased significantly at all time points examined as did the steady state stress. A transient rise in LC20 phosphorylation and [Ca2+]i occurred within 30 s, followed by suprabasal levels through the 10-min period during a sustained alpha 1-mediated activation by 10(-5) M phenylephrine whereas a higher force was developed at a shorter time compared to K+. An active phorbol ester 12-deoxyphorbol 13-isobutyrate 20-acetate (DPBA, 10(-6) M) induced a slow contraction of similar magnitude to that induced by K+ without significantly changing either [Ca2+]i or LC20 phosphorylation over a 90-min period. These results demonstrate that the amount of LC20 phosphorylation correlates with the [Ca2+]i in all three types of activation. The initial levels of [Ca2+]i and LC20 phosphorylation correlate with the onset of force development but not the magnitude of steady state stress, suggesting a role for [Ca2+]i and LC20 phosphorylation in regulating the cross bridge cycling rate during tension development. The lack of a detectable increase in [Ca2+]i and LC20 phosphorylation during DPBA activation suggests that sites other than LC20, phosphorylated by protein kinase C, may be involved in regulating smooth muscle contraction.

Animals↗

PubMedDate not supplied

Analytical approach to mathematical modelling of the isometric contraction of the hypertrophied myocardium.

It is attempted on the basis of the Hill two-element model to simulate mathematically the rising phase of the isometric mechanogram in the pressure-hypertrophied rat myocardium (Goldblatt II). The evaluation is based on the experimentally determined force-velocity relation as well as on the characteristic of the series-elastic stiffness. The changes in the isometric mechanogram of the hypertrophied heart muscle observed in the experiment can be mathematically predicted solely by means of the alterations in the force-velocity relationships induced by the hypertrophy, whereby the characteristic of the series-elastic stiffness is considered to be unchanged.

Cardiomegaly↗