Afterloaded Isotonic Contractions Research Articles

Interventricular comparison of the energetics of contraction of trabeculae carneae isolated from the rat heart

We compare the energetics of right ventricular and left ventricular trabeculae carneae isolated from rat hearts. Using our work-loop calorimeter, we subjected trabeculae to stress-length work (W), designed to mimic the pressure-volume work of the heart. Simultaneous measurement of heat production (Q) allowed calculation of the accompanying change of enthalpy (H = W + Q). From the mechanical measurements (i.e. stress and change of length), we calculated work, shortening velocity and power. In combination with heat measurements, we calculated activation heat (Q(A)), crossbridge heat (Q(xb)) and two measures of cardiac efficiency: 'mechanical efficiency' ((mech) = W/H) and 'crossbridge efficiency' ((xb) = W/(H - Q(A))). With respect to their left ventricular counterparts, right venticular trabeculae have higher peak shortening velocity, and higher peak mechanical efficiency, but with no difference of stress development, twitch duration, work performance, shortening power or crossbridge efficiency. That is, the 35% greater maximum mechanical efficiency of right venticular than left ventricular trabeculae (13.6 vs. 10.2%) is offset by the greater metabolic cost of activation (Q(A)) in the latter. When corrected for this difference, crossbridge efficiency does not differ between the ventricles.

Optimal shortening velocities for in situ power production of rat soleus and plantaris muscles.

Force-velocity (FV) relationships have been used previously to calculate maximal power production and to identify an optimal velocity of shortening (V(opt)-fv) to produce such power in skeletal muscle. The cyclical nature of muscle position during locomotion for muscles such as the soleus and plantaris is such that either constant force or velocity is rarely attained. In the present study, the work loop technique, a technique developed to measure maximal attainable power output from muscles undergoing cyclic length changes, was undertaken to determine whether simulating in vivo function alters the power-velocity relationship of the soleus and plantaris and, in particular, the velocity of shortening that produces maximal power (V(opt)-wl). FV relationships were determined for both soleus (n = 4) and plantaris (n = 4) muscles in situ from adult female Sprague-Dawley rats by measuring shortening velocities during afterloaded isotonic contractions. The velocity that produced maximal power using FV relationships, V(opt)-fv, was 54.6 +/- 0.7 mm/s for the plantaris vs. 20.2 +/- 1.2 mm/s for the soleus. Then, the work loop technique was employed to measure net power from these same muscles at multiple cycling frequencies (1.5 to 4.0 Hz for the soleus; 4.0 to 8.0 Hz for the plantaris). Multiple power-velocity curves were generated (one at each cycle frequency) by varying the strain (1-8 mm). Thus, at each cycle frequency, V(opt)-wl could be identified. For both the plantaris and soleus, V(opt)-wl at each cycle frequency was not different from their respective V(opt)-fv value. Thus both fast and slow skeletal muscles have inherent optimal shortening velocities, identifiable with FV relationships, that dictate their respective maximal attainable mechanical power production using the work loop technique.

Preload release increases blood flow and decreases fatigue during repetitive isotonic muscle contractions

The effects of preload on blood flow (Q), O2 uptake (VO2), and fatigue were investigated in the canine gastrocnemius-plantaris muscle in situ. Repetitive (1 contraction/s, 200 ms duration) afterloaded (0.25-0.3 maximal active isometric tension) isotonic tetanic contractions were performed in high-preload (HP; 69 g/g, n = 5), low-preload (LP; 35 g/g, n = 6), and preload-release (PR; 0 g/g, n = 5) experiments. Maximal Q values (1.0, 1.6, and 2.1 ml.min-1.g-1, P < 0.05 for all comparisons) and Q2 delivery (8, 13, and 17 mumol.min-1.g-1, P < 0.05 for all comparisons) increased significantly with decreasing preload. The maximal VO2 of HP was 7.2 mumol.min-1.g-1, which is significantly lower than both LP (10.5 mumol.min-1.g-1, P < 0.05) and PR values (11.4 mumol.min-1.g-1, P < 0.05); these differences were sustained through 20 min of contractions. Fatigue, measured as a loss of power production, was 63, 37, and 23% at 20 min of contractions in HP, LP, and PR, respectively, indicating significantly less fatigue with decreasing preload (P < 0.05 for all comparisons). These data demonstrate that the preload, present as the level of passive tension maintained between contractions, can influence Q, VO2, and fatigue during repetitive isotonic tetanic contractions of muscle in situ by a mechanically determined metabolic modulation of dynamic muscle performance.

A simple Hill element-nonlinear spring model of muscle contraction biomechanics

The purpose of this study was to develop a model to predict the mechanical response of muscles during isometric tetanic, afterloaded isotonic and isovelocity shortening contractions. Two versions of the model were developed. Both incorporated a contractile element that obeyed a Hill force-velocity relationship and a series elastic element. In a quadratic spring version, the series elastic element force was represented as proportional to the square of the stretch; in a cubic spring version, it was represented as proportional to the cube of the stretch. Both versions provided closed-form equations for response predictions that involved four independent parameters. Once the four parameters were chosen, each of these responses could be predicted. Model validity was established by comparing predicted and observed responses in slow and fast hindlimb muscles of rodents. Significant model-predicted responses seldom differed by more than 15% from experimental values. The model can provide insights into how changes in individual properties affect the overall mechanical behavior of muscles in a variety of circumstances and reduce the need for collection of experimental data.

Contractile properties of the shortening rat diaphragm in vitro.

Diaphragmatic fatigue has been defined in terms of the failure of the muscle to continue to generate a given level of tension. Appropriate shortening of the diaphragm is, however, just as important for adequate ventilation. In this study we have examined in vitro the contractile properties of the rat diaphragm under afterloaded isotonic conditions and the effect of fatigue on the ability of the diaphragm to shorten. Shortening of the muscle strips was found to depend on size of afterload, frequency of stimulation, duration of stimulation, and initial length of the muscle. The afterloaded isotonic length-tension relationship coincided with the relationship between length and active isometric tension only for relatively small afterloads. Fatigue of the muscle strips, induced by isometric or afterloaded isotonic contractions, was associated with a decline in the extent of shortening as well as a decrease in active isometric tension. Ability to shorten and ability to develop isometric tension did not decrease to the same extent under all conditions. We conclude that active shortening, as well as active isometric tension, is decreased by muscular fatigue and that changes in these properties can be different depending on experimental conditions. The results suggest that the definition of diaphragmatic fatigue should be expanded to include the ability of the muscle to shorten by an appropriate amount. The results also suggest that measurement of isometric performance may not provide a complete estimate of the overall performance of the fatigued diaphragm.

Acute Energetic Effects of Daunomycin on Rabbit Heart Muscle

The acute effects of daunomycin on the mechanical and energetic outputs of rabbit papillary muscles have been examined at 27 degrees C using a myothermic technique. Contrary to expectations, daunomycin, in concentrations ranging from 10 to 160 micrograms/ml, acted as a positive inotropic agent in terms of peak stress development and work output. The rate of stress development was, however, slightly depressed, and the major mechanical effect of the drug was to prolong the relaxation phase so that there were large drug-dependent increments in the stress-time integral. Daunomycin at a concentration of 80 micrograms/ml produced a 22% increment in peak stress development and a 74% increment in stress-time integral. The linear relationship between total (active + passive) stress and heat production was altered such that there was a 73% increment in the activation heat component (intercept) and a 37% increase in the energy cost per unit stress development (slope). In afterloaded isotonic contractions, daunomycin increased the mean work output (W; averaged over all load levels) by 49%, but there was an even greater increment in the associated energy expenditure, ET, which rose by 66%. Consequently, the overall mechanical efficiency (W/ET X 100%) fell slightly. It is concluded that, in the rabbit, daunomycin in the acute situation increases total calcium delivery to the myofilaments and decreases the "apparent" transduction efficiency. These acute effects are the opposite of those reported in papillary muscles taken from rabbits in cardiac failure induced by chronic daunomycin administration. The short-term effects of daunomycin in the rabbit differ substantially from those seen in the rat and guinea pig.

Force-velocity constants in smooth muscle: afterloaded isotonic and quick-release methods.

In using pharmacologic stimuli, force-velocity (FV) curves are usually obtained by the method of quick release (QR) and redevelopment of shortening at peak tetanic tension; the advantage of the method being that the active state is at maximum. However, the QR may itself reduce the intensity of the active state and result in reduced values of FV constants. We tested this by delineating FV curves in canine tracheal smooth muscle using both conventional afterloaded isotonic contractions (ALI), and redevelopment of shortening after QR methods. For both these studies a supramaximal tetanizing electrical stimulus was used. The analysis of 11 experiments revealed that the latter method resulted in statistically significant reductions of all FV constants except for Po (maximum isometric tetanic tension). The means and standard errors for the sets of constants for the ALI and QR, respectively, are as follows: Vmax (maximum velocity of shortening) = 0.275 lo (optimal muscle length)/s +/- 0.024 (SE), and 0.216 lo/s + 0.023; a (hyperbolic constant with units of force) = 294 g/cm2 +/- 35 and 236 g/cm2 +/- 32; b (hyperbolic constant with units of velocity) = 0.059 lo +/- 0.004 and 0.039 lo/s +/- 0.005; a/Po = 0.214 +/- 0.028 and 0.182 +/- 0.026; and Po = 1.362 kg/cm2 +/- 0.106 and 1.294 kg/cm2 +/- 0.097. These data clearly show that the quick-release method for measuring force-velocity relationships in canine smooth muscle results in significant underestimates of muscle shortening properties.

What kinds of signals are perceived by vascular smooth muscles, including physical factors?

Contraction kinetics of smooth-muscle preparations (rat portal vein, rat tracheal muscle) were studied by means of force-velocity relations obtained from after-loaded isotonic contractions or from the analysis of the isometric tension increase. Furthermore, the time constant of the tension recovery following an inhibiting length vibration reflected the contraction kinetics as well. Signals that induce changes in the calcium concentration or the area of overlap between actin and myosin cause changes mainly in the number of acting cross-bridges. Decrease in temperature, pH reduction, and a long-term stimulation induce slower contraction kinetics probably because of a lesser myosin light-chain kinase activity. The results obtained give further evidence for different mechanisms controlling extent and rate of smooth-muscle contraction.

The force-velocity relation and stepwise shortening in cardiac muscle.

A series of experiments was carried out to determine the effects of load variation on the character of stepwise shortening. We imposed afterloaded isotonic contractions on rat ventricular trabeculae, and measured the effect of load on pause duration, i.e., on the duration of the periods during which there was no sarcomere shortening. Sarcomere lengths were measured by optical diffraction. Increases of load brought about increases of pause duration; the relation was linear. The relation did not appear to depend on time during contraction, but did depend on sarcomere length: for a given load, pauses were longer at shorter sarcomere lengths. In a supplementary protocol in which we measured the dynamics of the central segment of the muscle during muscle isometric contraction, we found that the velocity of sarcomere shortening during the shortening step was approximately independent of load. These results provide a framework for interpretation of muscle force-velocity relations: diminished velocity at high load may be the result of increased pause durations.

Effect of damaged ends in papillary muscle preparations.

The mechanical characteristics of the central segment of isolated cat papillary muscle were determined with recently developed equipment. Two small sharpened stainless steel pins, inserted transversely through the muscle, were used to mark the ends of a segment not damaged by attachments. Installation of the pins did not affect the performance of the muscle. The distance between the pins was measured and controlled to produce isometric and afterloaded isotonic contractions of the segment of the muscle between the pins. Data from such contractions were compared with traditional whole muscle measurements made on the same preparation. The isometric length-tension curve of the central segment was significantly higher than that of the whole muscle, and there was no plateau of developed force at long lengths in five of six muscles studied. In the resting state, the segment was more compliant than the whole muscle for physiologic lengths and much stiffer for longer lengths. Segment velocity and shortening were significantly higher than whole muscle velocity and shortening at comparable loads.

Auxotonic contractions in cardiac muscle segments.

The dynamics of segment shortening have been measured in the central regions of isolated papillary muscles during muscle isometric and after-loaded isotonic contractions. Segment lengths are inferred from muscle cross-sectional area using an assumption that the segments remain isovolumic. Area is assessed with a magnetic induction technique. Infused microspheres have been used as visual markers to corroborate the segment length measurement. The results confirm the existence of major segmental shortening during muscle isometric conditions. However, the time course of shortening is not the same as that of force development. Rather, the segments remain shortened until after force has fallen significantly from its peak value. This behavior appears in the force-segment length plane as counterclockwise loops. The relationship of peak force to segment length has been determined and found to depend on the mechanical conditions under which the muscle is equilibrated. These results demonstrate the utility of the new technique and indicate central segment behavior that is substantially different from that observed for the whole muscle.

Altered performance of rat cardiac muscle follows changes in mechanical stress during relaxation.

We compared two sets of loading conditions for their effect on the mechanical performance of isolated rat trabecular muscle: (1) loading which approximated that of the intact ventricle; (2) nonphysiological afterloaded isotonic contractions. Improved performance, manifested by increased shortening and rate of shortening at constant load, was seen when lengthening occurred at light loads, as in the intact heart. In contrast, lengthening occurring at the same load against with shortening had taken place was followed by diminished performance. Viscous elements in series with the muscle could not account for this phenomenon. The results suggest the presence of an autoregulatory response of myocardium to mechanical stress during relaxation.

Oxygen consumption and lactate production of the rat portal vein in relation to its contractile activity.

Energy turnover in the isolated rat portal vein was investigated by measurement of oxygen consumption (JO2) and lactate production (JLA) under simultaneous recording of mechanical activity. In spontaneous activity under aerobic conditions and at optimal muscle length JO2 and JLA were 0.55 and 0.62 micromol/min X g, respectively, corresponding to an ATP-production of 4.3 micromol/min X G. When muscle length was changed, an approximately linear relation was found between energy turnover and mean isometric tension. The tension-indpendent part of ATP-production was 3.0 micromol/min X g. In Ca2+-free solution the metabolic rate was 20% lower still. JO2 was nearly equal in isometric contractions and in afterloaded isotonic contractions from the same initial muscle length. During a maximal tonic contracture in 5+-depolarized portal vein JO2 increased to about twice that in spontaneous activity. Changes in contracture force by variations in muscle length or in [Ca2+]0 were associated with identical linear relations between JO2 and active tension. This relation was less steep than the corresponding relation for spontaneous activity. The anaerobic lactate production of the portal vein was 2.7 times theaerobic leve. The accelerated glycolysis did not compensate for eliminated oxidative metabolism. Under substrate-free aerobic conditions no lactate was produced by the muscle and compared to the control situation JO2 declined more than could be accounted for by reduced mechanical activity alone. The metabolic turnover rate in relation to isometric tension is high in the rat portal vein compared to that of tonic vascular smooth muscle from larger vessels. This correlates with differences in dyanmic mechanical properties. At comaparable tension levels in the portal vein, the rate of cross-bridge turnover may be higher in spontaneous phasic activity than in sustained contracture.

Force-velocity relations of the portal vein of hyperthyroid rats.

The effect of thyroxine on the elementary process of contraction of vascular smooth muscle was tested on 38 hyperthyroid and in 38 control rats. Hyperthyroidism was induced by i.p. injections of triiodthyronine or 1-htyroxine for 2 weeks. By means of afterloaded isotonic contractions of the tetanized portal vein force velocity relations were calculated. There was a slight increase in both the extrapolated velocity of shortening at zero load [from 0.93 +/- 0.04 ML/s (control rats) to 1.03 +/- 0.04 ML/s (hyperthyroid rats; P less than 0.05)], and the peak force generation (from 14.8 +/- 0.4 mN to 16.1 +/- 0.4 mN; P less than 0.005). The maximum of mechanical power development at a distinct extent of afterload was augmented from 1.35 +/- 0.06 muW/ML to 1.68 +/- 0.06 muW/ML (P less than 0.005). The influence of thyroxine on the elementary process of contraction in vascular smooth muscle is discussed in connection with the much stronger effect of hyperthyroidism on cardiac muscle contraction.

Force velocity relations in vascular smooth muscle: the influence of pH, pCa, and noradrenaline.

The kinetics of vascular smooth musclw activity was studied by means of afterloaded isotonic contractions of the tetanized rat portal vein at varied pH (8.0-5.9), pCa (3.4-2.1), and during noradrenaline incubation (0.4 mug/ml). Under control conditions (pH 7.3, pCa 2.6) the following parameters of the force velocity relation were calculated: a of Hill's equation (relating to the isometric peak tension) = 0.36; b (relating to the actual muscle length) = 0.19 ML/s; VM Trelating to the actual muscle length) = 0.56 ML/s. Within the range of pCa between 2.0 and 3.2 the amount of force generation (= delta P) depended on the extracellular calcium level whereas the extrapolated velocity of shortening of the unloaded preparation (= VM) did not. Also pH changes between 8.0 and 6.8 as well as noradrenaline incubation at a pH of 5.9 affected delta P quite considerably, but VM only scarcely. At a pH of 6.3, however, VM was distinctly diminished, and a reduced calcium sensitivity of the ATPase was inferred from the shift of ED50 of extracellular calcium from 0.66 mM Ca at a pH of 7.3 to 1.56 mM Ca at a pH of 6.3 (P less than 0.0005). It is concluded from these results that the experimental conditions-pCa between 2.0 and 3.2, pH between 8.0 and 6.8, and noradrenaline added at a pH of 5.9-obviously change the intracellular calcium concentration which influences the number of activated interaction sites rather than the velocity of crossbridge movement.

The force-velocity relation in phasic contractions of venous smooth muscle.

The force-velocity relation of the rat portal vein has been studied during regular spontaneous contractions induced by elevated levels of [K-+]O AND [Ca-2+]o. No satisfactory description of the force-velocity relation was obtained by measuring shortening velocity in afterloaded isotonic contractions. Therefore the method of isotonic quick release was used, as this permits mechanical studies at specified instants of the contraction-relaxation cycle. The temporal development of the force-velocity relation in the phasic response was investigated. During a time interval in the rising phase of the contraction, at about the time for maximal dP/dt, the force-velicity curves were practically identical, suggesting a plateau in the intensity of active state. At later stages of the twitch the curves were progressively displaced towards the origin, the intercepts on both the force and the velocity axis becoming smaller. At the time of the isometric peak the maximal shortening velocity had declined relatively more than the isometric force. This is presumably caused by inhomogeneous activation of the muscle at the beginning of relaxation, The maximal force-velocity relation in the rising phase of the contraction can be described by Hill's (1938) equation with the following parameters (at 37 degrees tc): a/Po = 0.73 plus or minus 0.04, b = 0.54 plus or minus 0.04 lengths/s, Vmax = 0.74 plus or minus 0.02 lengths/s (n = 7, mean plus or minus S.E,). The force-velocity relation of the portal vein in comparison with other kinds of muscle is discussed.

Force velocity relationships in vascular smooth muscle. The influence of temperature.

Force velocity relationships of isolated vascular smooth muscle preparations were examined in the tetanized rat portal--anterior mesenteric vein by means of afterloaded isotonic contractions. Lowering of the temperature from 37 degrees C to 25 degrees C caused an average decrease of the follwoing parameters: maximal velocity of shortening at zero load from 0.51 to 0.28 muscle length/sec; maximal rate of tension increase (dT/dt) from 847 to 362 dyn/sec; mechanical tension developed at the maximum of dT/dt from 43 to 34% of the peak force generation; constant b of Hill's equation from 0.18 to 0.09 muscle length/sec. The latency was prolonged from 0.33 to 0.56 sec, and the time-to-maximum of dT/dt from 0.9 to 1.6 sec. Between 27 degrees C and 37 degrees C the following Q10 values were calculated: for the maximal velocity of shortening at zero load 1.56; for the maximal rate of tension increase 1.88; for the latency 1.63. No distinct influence of temperature could be observed on the extent of isotonic shortening at zero load (3.69 muscle length at 37 degrees C), on the extent of peak force generation (1107 dyn at 37 degrees C), and on constant a of Hill's equation (0.35 at 37 degrees C). It is concluded that parameters of contraction velocity in vascular smooth muscle depend on temperature, whereas the extent of contraction is independent of temperature. These findings are discussed in connection with the theory of the sliding-filament mechanism.

Effect of ouabain on the energy output of rabbit cardiac muscle.

The effect of ouabain, 0.3 µg/ml, on the energy output of rabbit papillary muscles has been examined by a myothermic technique. The experiments were conducted at two temperature ranges, 19.2° to 22.8°C and 29.0° to 32.2°C, and both isometric and afterloaded isotonic contractions were studied. Temperature differences alone caused pronounced physiological changes, the higher temperature being associated with lower tension-independent heat and markedly higher active efficiency, external work /(external work + active heat production). The heat versus tension curve was rectilinear at higher temperatures but showed upward curvature at lower temperatures. At 19.2° to 22.8°C, ouabain increased the tension-independent heat by 23%, maximum tension development by 23%, and mean work output by 39%. Ouabain did not significantly alter the slope of the heat versus tension curve and increased mean efficiency only slightly. At 29° to 32.2°C, ouabain did not cause any significant change in the slope of the heat versus tension curve or in mean muscle efficiency. Ouabain produced significant increases in maximum tension development, mean work output, and the tension-independent heat. The effects of ouabain at the higher temperature were examined at two different calcium levels, 2.5 and 1.25 mM. In the isometric studies the effects of ouabain were independent of the calcium level, and the calcium level itself had no significant effect on the heat-tension relationship. In the isotonic studies, ouabain increased work output but more so at the 2.5 mM calcium level. Ouabain did not affect mechanical efficiency at either calcium level but muscle efficiency was higher at the 2.5 mM calcium level. It is concluded that any effects of the cardiac glycosides on energy expenditure are consequences of their inotropic actions and do not represent changes in the energy cost of contraction.

Force: velocity properties of mouse muscles.

THE force : velocity properties of extensor digitorum longus (EDL) and soleus (SOL) muscles of the mouse have been determined at birth and four weeks after birth. The methods and recording equipment used in this work have been described in detail elsewhere1. Contractions of muscles in four-week-old animals were recorded in vivo; the anaesthetic was 100 mg pentobarbital sodium/kg body-weight injected intraperitoneally followed by about 40 per cent of the initial dose every hour. Neonatal muscles were examined in vitro; in these conditions the average decline in the maximum isometric tetanic tension (P0) in the course of a series of measurements lasting several hours was only 7 per cent for EDL and 4 per cent for SOL. The speed of shortening was determined from distance : time curves of after-loaded isotonic tetanic contractions. These contractions were recorded with light levers during repetitive indirect stimulation at the optimal frequency (fo). The equivalent masses of the levers were 22 mg and 114 mg for muscles from neonatal and four-week-old animals respectively. All measurements were made with the muscles at 35° C to 35.5° C. Curves describing the relationship between the speed of shortening and the load were fitted to the data for each muscle by the method of least squares using Hill's equation2 ((P + a)V = b(P0 − P), where P0 = maximum isometric tetanic tension, P = load, V = speed of shortening, and a and b are constants). The anatomical arrangement of tendon and muscle fibres in mouse EDL and SOL is the same as that described for rat muscles1.