Isometric contraction, isotonic contraction and passive contraction in smooth muscles.
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Comparison of the efficiency of rat papillary muscles during afterloaded isotonic contractions and contractions with sinusoidal length changes.
The results of previous studies suggest that the maximum mechanical efficiency of rat papillary muscles is lower during a contraction protocol involving sinusoidal length changes than during one involving afterloaded isotonic contractions. The aim of this study was to compare directly the efficiency of isolated rat papillary muscle preparations in isotonic and sinusoidal contraction protocols. Experiments were performed in vitro (27 degrees C) using left ventricular papillary muscles from adult rats. Each preparation performed three contraction protocols: (i) low-frequency afterloaded isotonic contractions (10 twitches at 0.2 Hz), (ii) sinusoidal length change contractions with phasic stimulation (40 twitches at 2 Hz) and (iii) high-frequency afterloaded isotonic contractions (40 twitches at 2 Hz). The first two protocols resembled those used in previous studies and the third combined the characteristics of the first two. The parameters for each protocol were adjusted to those that gave maximum efficiency. For the afterloaded isotonic protocols, the afterload was set to 0.3 of the maximum developed force. The sinusoidal length change protocol incorporated a cycle amplitude of +/-5% resting length and a stimulus phase of -10 degrees. Measurements of force output, muscle length change and muscle temperature change were used to calculate the work and heat produced during and after each protocol. Net mechanical efficiency was defined as the proportion of the energy (enthalpy) liberated by the muscle that appeared as work. The efficiency in the low-frequency, isotonic contraction protocol was 21.1+/-1.4% (mean +/- s.e.m., N=6) and that in the sinusoidal protocol was 13.2+/-0.7%, consistent with previous results. This difference was not due to the higher frequency or greater number of twitches because efficiency in the high-frequency, isotonic protocol was 21.5+/-1.0%. Although these results apparently confirm that efficiency is protocol-dependent, additional experiments designed to measure work output unambiguously indicated that the method used to calculate work output in isotonic contractions overestimated actual work output. When net work output, which excludes work done by parallel elastic elements, rather than total work output was used to determine efficiency in afterloaded isotonic contractions, efficiency was similar to that for sinusoidal contractions. The maximum net mechanical efficiency of rat papillary muscles performing afterloaded isotonic or sinusoidal length change contractions was between 10 and 15%.
Animals↗
Fatigue of mouse diaphragm muscle in isometric and isotonic contractions.
Fatiguabilities of mouse diaphragm muscle in vitro in isometric and isotonic contractions were compared in this study. Isolated mouse diaphragm muscle was stimulated repetitively to induce fatigue during both isometric and isotonic contractions. The supramaximal electrical stimulation used was a train of 100-Hz, 0.5-ms pulses delivered to the muscle every 2 s for 0.5 s. The percentage decrease in isometric tension from beginning to end of the fatiguing process was used as the index of fatigue. The experiments were carried out at different PO2 levels in both normal and zero-glucose Ringer solutions. It was found that fatigue developed more rapidly in isotonic contractions than in isometric ones. Also, the extracellular glucose level demonstrated little effect on the muscle's short-term fatiguability, whereas reductions in the extracellular PO2 exerted a profound effect, especially in the case of isotonic fatigue.
Aerobiosis↗
The effects of isotonic contractions on the rate of fatigue development and the resting membrane potential in the sartorius muscle of the frog, Rana pipiens.
The goal of this study was to characterize how isotonic contractions affect the rate of fatigue development. Muscle bundles dissected from frog sartorius muscles were stimulated with 100-ms long train of pulses (0.5 ms, 6 V, 140 Hz). To measure the effect of the isotonic contractions, isometric tetanus were elicited at regular time intervals during the stimulation to fatigue. In general, isotonic contractions caused a faster decrease in tetanic force than isometric contractions. The difference in tetanic force between an isotonic and isometric fatigue increased gradually over a 20-min period to 7.9 and 13.5% at 0.04 and 0.1 trains/s (TPS), respectively. At 0.2, 0.5, and 1.0 TPS, the decrease in tetanic force was also faster during an isotonic fatigue, which resulted in an initial difference in tetanic force between the two types of fatigue. The difference did not exceed 18.5% and did not persist throughout the stimulation period; i.e., the difference disappeared before the end of the fatigue stimulation. The half-relaxation time was prolonged during fatigue development, and the prolongation was greater during an isotonic fatigue, except at 0.04 TPS. The increases in the half-relaxation time at 0.2, 0.5, and 1.0 TPS were followed by a decrease, and the decreases were especially pronounced during an isotonic fatigue at 0.5 and 1.0 TPS. The results showed for the first time that isotonic contractions cause a faster rate of fatigue development in frog sartorius muscles, and this effect depends on the frequency of stimulation.
Animals↗
Muscle fiber conduction velocity at different states of isotonic contraction.
Conduction velocity (CV), relative twitch force (RTF) and contraction time (CT) of single muscle fibers (SF) and small muscle fiber bundles (FB) were measured at different states of isotonic contraction with double impulse stimuli at varying interstimulus intervals (ISI) from 0 to 1000 ms in the biceps brachii muscle in vivo. During an isotonic contraction, muscle fibers conducted the action potential on average 0.18 m/s faster along the muscle fiber membrane than did relaxed muscle fibers. This difference was statistically significant (P < 0.001). There was a significant positive correlation between the degree of isotonic contraction and fiber bundle conduction velocity (FBCV) with a first peak at the maximum RTF at an ISI of 9 ms and a second peak at the maximum CT at an ISI of 100 ms.
Adult↗
Isotonic contraction as a result of cooperation of sarcomeres--a model and simulation outcome.
The molecular level of the functional structure of the contractile apparatus of cross-striated muscle has been mapped out almost minutely. Most authors accept the basic principles of the theory of sliding filaments and the theory of operation of molecular generators of force which, of course, are progressively updated by integrating new knowledge. The idea of the model delineated below does not contradict these theories, for it refers to another level of the system's hierarchy. The definition of the system, hereafter referred to Ideal Sarcomere (IS), takes into account the fact that, during isotonic contraction, a large number of not wholly independently working sarcomeres and molecular generators of force is active in a synergistic way. The shortening velocity of isotonically contracting IS is determined by the relation between quantities conveying different tasks of active generators of force and the influence of the system parameters. Although IS is derived from simple axiomatic predicates, it has properties which were not premediated in defining the system and which, in spite of this, correspond to some properties of the biological original. The equations of the system allow us to calculate the shortening velocity of 'isotonic contraction' and other variables and parameters and show, inter alia, an alternative way to derive and interpret the relations stated in Hill's force-velocity equation. The simulation results indicate that the macroscopic manifestations of isotonic contraction may be also contingent on the properties of the cooperating system of the multitude of sarcomeres, which also constitutes one part of the functional structure of muscle.
Animals↗
Structural changes in smooth muscle cells during isotonic contraction.
Smooth muscle cells of the guinea-pig taenia coli were studied in light and electron microscopy, in condition of mild stretch or of isotonic contraction. During contraction the cells increase in transverse sectional area and their packing density passes from 94,000-mm-2 to 18,000-mm-2. The percentage increase in transverse sectional area of the taenia is approximately the same as the percentage decrease in length. Measurements of cell transverse sectional area suggest that the individual cells shorten and fatten more than the taenia as a whole. Whereas stretched muscle cells run parallel to each other and show a fairly smooth surface, isotonically contracted cells are twisted and entwine around each other. Their surfaces are covered with myriad processes and folds. Longitudinal, transverse or oblique stripes are seen in light microscopy in the contracted muscle cells and it is suggested that they are related to the characteristics of the cell surface. In electron microscopy a complex pattern of interdigitating finger-like and laminar processes is observed. Caveolae are mainly found on the evaginated parts of the cell surface, dense patches are mainly (but not always) found on the invaginated parts. Desmosome-like attachments between contracted cells are frequent. The collagen fibrils run approximately parallel to the stretched muscle cells; on the other hand, they run obliquely and transversely around the isotonically contracted cells.
Animals↗
[Isotonic contraction of rabbit superior rectus muscle].
It is well known that the mammalian extraocular muscle has slow muscle fiber morphologically. But the contraction speed of slow muscle fiber has not been mentioned in previous reports. We studied the isotonic contraction of slow muscle fiber and compared it with fast muscle fiber. Each fiber bundle was isolated from the rabbit superior rectus muscle. Both light microscopic findings and the reaction to isotonic contraction in Ca2+ free Ringer's solution could identify each of the two muscle fiber types. Contraction speed depended on the frequency of stimuli (40-200 Hz) in both slow and fast muscle fiber. Maximum velocities of slow and fast fiber bundles were 4.3 +/- 1.53 mm/sec and 26.9 +/- 3.48mm/sec at 200Hz, respectively. It was suggested that the relaxation of slow muscle fiber did not interfere with the contraction of the fast muscle fiber. The relation between contraction velocity and afterload showed an approximately right angle hyperbolic curve.
Animals↗
Energy production in cardiac isotonic contractions.
The energy output of rabbit papillary muscle is examined and it is shown that there is more energy liberated in an afterloaded isotonic contraction than in an "equivalent" isometric contraction. This statement holds true regardless of whether equivalence is based on the proposition that tension or the time integral of tension is the best index of muscle energy expenditure. Besides the external work performed there is additional heat production in isotonic contractions and this heat increases as the afterload is decreased. The additional heat is more evident when tension rather than the time integral of tension is made the determinant of energy expenditure. It is shown in single contractions that the rate of isotonic heat production, regardless of afterload size, never exceeds the heat rate recorded in an isometric contraction at the same initial length. Experiments reveal no simple linear correlation between isotonic energy output and contractile element work. Problems associated with the compartmentalization of the energy output of a contraction are discussed.
Analysis of Variance↗
[Isotonic contraction and contracture of the isolated right rat ventricle. Effect of La3+, prenylamine, ATP, Mg2+ (author's transl)].
1. The isolated right rat ventricle was immersed in Tyrode solution (25) ml) and stimulated electrically at a frequency of 60/min. Changes in the amplitude of isotonic contractions and contractures were assessed. 2. Isotonic contractions were reduced (after Prenylamine, ATP, Mg), or suppressed (by LaCl3 = 10 mM). Contracture was caused by Prenylamine (4 mg/25 ml Tyrode solution or more). 3. Addition of 100 mM KCl (NaCl reduced equimolarily to 37 mM did not influence contracture in the presence of the investigated substances with the exception of MgCl2 = 10 to 15mM, where only the rate of development of potassium contracture was reduced. 4. After previous immersion of the tissue in the presence of LaCl3 - 10 mM and Prenylamine 1-4 mg/25 ml Tyrode solution the contracture developed after removal of NaCl (substitution by sucrose 270 mM), which, however, declined after addition of NaCl only when previously treated with Prenylamine and not when treated with La. 5. Addition of 100 mM KCl (naCl reduced to 37 mM) after previous immersion of the tissue in the presence of the investigated substances caused contracture the rate of rise of which was smaller after La than in controls: after the remaining substances it did not differ. 6. Reduction of the contracture after reduction of KCl from 100 to 5.6 mM developed only after previous immersion of the tissue in the presence of ATP and Mg and not after previous immersion in the presence of Prenylamine and La. 7. The results are compared with biochemical findings. They suggest a shift of contractile Ca in the heart.
Adenosine Triphosphate↗
Time-resolved X-ray diffraction studies of myosin head movements in live frog sartorius muscle during isometric and isotonic contractions.
Using the facilities at the Daresbury Synchrotron Radiation Source, meridional diffraction patterns of muscles at ca 8 degrees C were recorded with a time resolution of 2 or 4 ms. In isometric contractions tetanic peak tension (P0) is reached in ca 400 ms. Under such conditions, following stimulation from rest, the timing of changes in the major reflections (the 38.2 nm troponin reflection, and the 21.5 and 14.34/14.58 nm myosin reflections) can be explained in terms of four types of time courses: K1, K2, K3 and K4. The onset of K1 occurs immediately after stimulation, but that of K2, K3 and K4 is delayed by a latent period of ca 16 ms. Relative to the end of their own latent periods the half-times for K1, K2, K3 and K4 are 14-16, 16, 32 and 52 ms, respectively. In half-times, K1, K2, K3 lead tension rise by 52, 36 and 20 ms, respectively. K4 parallels the time course of tension rise. From an analysis of the data we conclude that K1 reflects thin filament activation which involves the troponin system; K2 arises from an order-disorder transition during which the register between the filaments is lost; K3 is due to the formation of an acto-myosin complex which (at P0) causes 70% or more of the heads to diffract with actin-based periodicities; and K4 is caused by a change in the axial orientation of the myosin heads (relative to thin filament axis) which is estimated to be from 65-70 degrees at rest to ca 90 degrees at P0. Isotonic contraction experiments showed that during shortening under a load of ca 0.27 P0, at least 85% of the heads (relative to those forming an acto-myosin complex at P0) diffract with actin-based periodicities, whilst their axial orientation does not change from that at rest. During shortening under a negligible load, at most 5-10% of the heads (relative to those forming an acto-myosin complex at P0) diffract with actin-based periodicities, and their axial orientation also remains the same as that at rest. This suggests that in isometric contractions the change in axial orientation is not the cause of active tension production, but rather the result of it. Analysis of the data reveals that independent of load, the extent of asynchronous axial motions executed by most of the cycling heads is no more than 0.5-0.65 nm greater than at rest.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
Ca2+-dependent facilitated shortening in isotonic contraction of trachealis muscle.
We compared isotonic shortening with isometric force generation as a function of external Ca2+ in 166 tracheal smooth muscle (TSM) strips from 27 mongrel dogs in vitro. Concentration-response curves were generated with muscarinic stimulation (acetylcholine, ACh), alpha-adrenergic receptor activation (norepinephrine after beta-adrenoceptor blockade, NE), serotonin (5-HT), and KCl-substituted Krebs-Henseleit solution. The concentrations of 5-HT causing half-maximal shortening (ECS50, 1.54 +/- 0.14 X 10(-7) M) and half-maximal active isometric tension (ECT50, 1.72 +/- 0.30 X 10(-7) M) were similar (P = NS). Likewise, ECS50 (21.9 +/- 0.7 mM) and ECT50, (22.0 +/- 0.9 mM) were similar for KCl. In contrast, facilitated isotonic shortening (i.e., greater isotonic shortening for comparable degrees of force generation) was elicited with ACh and NE for all levels of force generation between 15 and 85% of maximum and for all concentrations of ACh from 3 X 10(-8) to 3 X 10(-5) M (P less than 0.05 for all points). Facilitated isotonic shortening also was elicited for all concentrations of NE from 10(-8) to 10(-6) M (P less than 0.05 for all points). Removal of Ca2+ from the perfusate substantially reduced the potency of ACh (P less than 0.001) and abolished differences between ECS50 (2.23 +/- 0.28 X 10(-5) M) and ECT50 (2.50 +/- 0.46 X 10(-5) M, P = NS). We demonstrate that for comparable degrees of force generation, muscarinic and alpha-adrenergic receptor activation cause greater isotonic shortening than KCl or 5-HT and that this facilitated shortening is associated with the concentration of external Ca2+.
Acetylcholine↗
Indices of myocardial oxygen consumption for isotonic contraction.
To investigate the model-independent mechanical determinants of energy expenditure, a respirometer was constructed to study isolated feline papillary muscles. Mechanical parameters recorded were the distance of shortening (deltaL), peak velocity of shortening (Vp), mean velocity of shortening (V), tension-time index (TTI), afterload (P), and the integral of the contraction portion of the phase plane trajectory of velocity and length (integral of VdL). Oxygen consumption (Vo2) during 15-min isotonic contraction periods was monitored with a polarographic electrode. Vp, V, and delta L were inversely related to Vo2 in a curvilinear manner. P and TTI were directly related to Vo2 in a curvilinear fashion. Integral of VdL was inversely related to Vo2 in a linear manner. In several experiments the contractile state of the muscles was augmented by addition of norepinephrine (7 X 10(-8) M). The relationship between integral of VdL and Vo2 was shifted above and parallel to that for the control muscles. These experiments indicate that the index integral of VdL is linearly related to the oxygen consumed in isotonic contraction of isolated mammalian ventricular myocardium at a given level of contractile state.
Animals↗
Force-velocity shifts with repetitive isometric and isotonic contractions of canine gastrocnemius in situ.
The force-velocity (F-V) relationships of canine gastrocnemius-plantaris muscles at optimal muscle length in situ were studied before and after 10 min of repetitive isometric or isotonic tetanic contractions induced by electrical stimulation of the sciatic nerve (200-ms trains, 50 impulses/s, 1 contraction/s). F-V relationships and maximal velocity of shortening (Vmax) were determined by curve fitting with the Hill equation. Mean Vmax before fatigue was 3.8 +/- 0.2 (SE) average fiber lengths/s; mean maximal isometric tension (Po) was 508 +/- 15 g/g. With a significant decrease of force development during isometric contractions (-27 +/- 4%, P < 0.01, n = 5), Vmax was unchanged. However, with repetitive isotonic contractions at a low load (P/Po = 0.25, n = 5), a significant decrease in Vmax was observed (-21 +/- 2%, P < 0.01), whereas Po was unchanged. Isotonic contractions at an intermediate load (P/Po = 0.5, n = 4) resulted in significant decreases in both Vmax (-26 +/- 6%, P < 0.05) and Po (-12 +/- 2%, P < 0.01). These results show that repeated contractions of canine skeletal muscle produce specific changes in the F-V relationship that are dependent on the type of contractions being performed and indicate that decreases in other contractile properties, such as velocity development and shortening, can occur independently of changes in isometric tension.
Animals↗
[The inotropic and bathmotropic effects of beta stimulation: a study comparing dobutamine and dopamine on the guinea-pig papillary muscle in isotonic contraction].
The study was aimed at comparing the effects of dobutamine (dob) and dopamine (dop) on isotonic contraction and rhythmicity of isolated guinea-pig papillary muscles (in oxygenated Tyrode at 37 degrees C), by taking into account: 1) the rate of stimulation (50% above the diastolic threshold) at 5 fixed periods: (RR: 1600, 1200, 1000, 800 and 400 ms); 2) 7 log concentrations (logC) of the index amine (from 10(-9) to 10(-3) M). To this end, a dose-relation protocol which explored the effects of all 5 RR and 7 logC was designed and 15 adult female Guinea-pigs (250 to 350 g) were randomized to either the dob (n = 8) or the dop (n = 7) arm. This enabled a total of 525 sets of data to be analyzed: in 38 sets (7.2%) premature contractions (CP) were coded. CP were sustained (freq: > 3) in 25 of these latter 38 sets (4.8%). Compared to the basal state, the amplitude (AMP%) and the log of percent amplitude (logAMP%) and time to peak (TP%) changes of the isotonic (Gould transducer) twitch were calculated along with the log of this latter variable (logTP%). AMP%, log AMP%, TP% and logTP% were linearly correlated with logC at all RR. In the range 1600-400 RR, for both amines, significant linear correlations (magnitude of 0.15 > r < magnitude of 0.70, 0.001 > p < 0.022) were seen for plots of AMP%, logAMP% and TP%:steeper correlations were observed for dop. This was confirmed in multivariate analysis (BMDP-9R) whereby AMP%, logAMP%, TP%, logTP%, CP, and freqCP were dependent variables and coded variables were included to either define the type of treatment (dop versus dob) or logC. In these analyses, logC (t > 11) and dop (t > magnitude of 3) might be used to explain (0.28 > r2 < 0.42, 0.00001 > p < 0.0025) AMP% and logAMP%, meaning that a different inotropic (isotonic) efficacy exists between these 2 amines, at all logC. On the other hand, when CP and freqCP were coded, explanatory variables were AMP% and logAMP% (4.86 > t < 6.95, 0.06 > r2 < 0.09, p < 0.00001), but not the variable used to code the type of treatment (dob versus dop).(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
Catchlike-inducing train activation of human muscle during isotonic contractions: burst modulation.
Stimulation trains that exploit the catchlike property [catchlike-inducing trains (CITs)] produce greater forces and rates of rise of force than do constant-frequency trains (CFTs) during isometric contractions and isovelocity movements. This study examined the effect of CITs during isotonic contractions in healthy subjects. Knee extension was electrically elicited against a load of 10% of maximum voluntary isometric contraction. The stimulation intensity was set to produce 20% of maximum voluntary isometric contraction. The muscle was tested before and after fatigue with a 6-pulse CFT and 6-pulse CITs that contained an initial doublet, triplet, or quadruplet. For prefatigue responses, the greatest isotonic performance was produced by CITs with initial doublets. When the muscles were fatigued, triplet CITs were best. CITs produce greater excursion, work, peak power, and average power than do CFTs, because CITs produced more rapid rates of rise of force. Faster rates of rise of force enabled the preload on the muscle to be exceeded earlier during the stimulation train.
Adult↗
Changes of tracheal smooth muscle stiffness during an isotonic contraction.
Stiffness of the series elastic component (SEC) of canine tracheal smooth muscle in isotonic contraction and relaxation was measured by applying small force perturbations to the muscle and measuring the resulting length perturbations. The quick, elastic length transient was taken as the change in length of the SEC (delta L). The force perturbation was a train of 10-Hz rectangular force waves varying from 0 to 10% maximum isometric tension (Po) in magnitude (delta P = 10% Po). Stiffness of the SEC was estimated by the ratio delta P/delta L. The change in SEC stiffness with respect to the change in muscle length was further studied by obtaining the stress-strain curves of the SEC at different muscle lengths using the load-clamping method. The clamps were applied at a fixed time (10 s after stimulation). Length of the muscle 10 s after contraction was controlled by the magnitude of the isotonic afterload. It was found that the apparent SEC stiffness increased as muscle length decreased. This stiffness increase is not likely due to an increase in the number of attached cross bridges, but it is probably due to the gradual diminution of the SEC length itself during muscle shortening.
Animals↗
Force-velocity relationship of human elbow flexors in voluntary isotonic contraction under heavy loads.
The force-velocity relationships of the human elbow flexors as one muscle in maximal voluntary contraction were determined using isotonic lever systems for seven men (19-38 yrs). The external loads used for the contraction weighed 0%, 20%, 40% 60%, 80% and 90% of maximum voluntary isometric force (Fo). The force and velocity were determined at an elbow angle of 90 degrees. The effect of the inertia of the forearm on the force was corrected using the angular acceleration of the forearm during an elbow flexion. The effect of muscular fatigue on the velocity during an elbow flexion was minimized by adjusting the initial elbow angle at the onset of the elbow flexion to the size of the load and reducing the time duration of the elbow flexion with a heavy load. Hill equation fitted fairly well to the force-velocity data observed in the experiments up to 90% Fo. The maximum isometric force predicted using Hill equation was larger by 6% (mean) than that observed in the experiments.
Adult↗