Length dependence of changes in sarcoplasmic calcium concentration and myofibrillar calcium sensitivity in striated muscle fibres

Frequency- and length-dependent tension development in rat heart muscles exposed to isoflurane and halothane

Using 22 isolated rat ventricular muscle preparations, we investigated whether or not the increase in preload and/or contraction frequency may counteract the negative inotropy of both isoflurane (2.0%) and halothane (1.0%). Increases in preload from 94% of Lmax (the length where muscles produce the maximal tension) to Lmax did not alter significantly the percent decrements in tension development caused by either isoflurane or halothane. The increases in contraction frequency from 0.1 to 0.6 Hz augmented the depressant effect of isoflurane significantly ( P < 0.001), while the depressant effect of halothane was not altered at these contraction frequencies. Small but significant counteraction occurred in the depressant effects of halothane at 0.8 and 1.6 Hz ( P = 0.002). These changes in intracellular mechanism(s), resulted from the increase in contraction frequency, interacted with the two anesthetics on tension development, while these may not be the case for the increase in preload.

Quantitative model for Schädler's isometric oscillations in insect flight and cardiac muscle

Schädler and colleagues (1969, 1971) and Steiger (1977a) have found that tetanized insect fibrillar and cardiac muscles exhibit damped isometric oscillations in tension following a quick stretch. This behaviour cannot be explained by the conventional sliding filament model at full activation, or by including stretch activation in the obvious way. However, it is predicted by a sliding filament model which allows these muscles to be further activated by an increase in thin-filament tension even at high calcium levels (above 10(-5) M), providing the strength gamma of strain-activation coupling exceeds a critical value. Calculations from a comprehensive model of the actin-myosin contraction cycle suggest that this can be achieved if the phosphate release and head rotation steps are both regulated by calcium and thin-filament tension. The model also predicts a delayed tension rise following a quick release for subcritical values of gamma. Current knowledge of sarcomere structure and regulation of contractility in striated muscle indicates that this strain-activation mechanism alone cannot account for all stretch-activation phenomena, although many can be predicted if the regulatory filament is allowed to carry passive tension.

Cellular mechanisms of fatigue in skeletal muscle

Prolonged activation of skeletal muscle leads to a decline of force production known as fatigue. In this review we outline the ionic and metabolic changes that occur in muscle during prolonged activity and focus on how these changes might lead to reduced force. We discuss two distinct types of fatigue: fatigue due to continuous high-frequency stimulation and fatigue due to repeated tetanic stimulation. The causes of force decline are considered under three categories: 1) reduced Ca2+ release from the sarcoplasmic reticulum, 2) reduced myofibrillar Ca2+ sensitivity, and 3) reduced maximum Ca(2+)-activated tension. Reduced Ca2+ release can be due to impaired action potential propagation in the T tubules, and this is a principal cause of the tension decline with continuous tetanic stimulation. Another type of failing Ca2+ release, which is homogeneous across the fibers, is prominent with repeated tetanic stimulation; the underlying mechanisms of this reduction are not fully understood, although several possibilities emerge. Changes in intracellular metabolites, particularly increased concentration of Pi and reduced pH, lead to reduced Ca2+ sensitivity and reduced maximum tension, which make an important contribution to the force decline, especially with repeated tetanic stimulation.

Differential effects of cardiac hypertrophy and failure on right versus left ventricular calcium activation

We studied calcium responsiveness of skinned muscle preparations from the right and left ventricles of rats with cardiac hypertrophy and cardiac hypertrophy plus failure. To test the hypothesis that differences in contractile function are due to changes in myofilament calcium responsiveness, we compared preparations from spontaneously hypertensive rats with cardiac failure, spontaneously hypertensive rats without cardiac failure, and age-matched normotensive Wistar-Kyoto control rats 18-24 months of age. Rats with failure had pleural/pericardial effusions, left atrial thrombi, and right and left ventricular hypertrophy. Muscles were skinned by saponin (250 micrograms/ml) and activated with a series of calcium buffers. Data were plotted as pCa (-log[Ca2+]) versus isometric force and then fit to a modified Hill equation. Values for 50% maximal activation (calcium sensitivity), maximal calcium-activated force, and the slope of the calcium-force relation were compared. Our data indicate that with the development of hypertrophy, calcium sensitivity of left ventricular muscles remains unaffected, but maximal calcium-activated force is increased. In contrast, maximal calcium-activated force declines toward control levels with the development of left ventricular failure, despite the continued presence of significant hypertrophy. In the normotensive rats, the left ventricle is more sensitive to calcium than the right ventricle (pCa50 = 6.0 +/- 0.05 versus 5.7 +/- 0.09; p less than 0.05); however, both the calcium sensitivity and maximal calcium-activated force of the right ventricle increase with the development of compensatory hypertrophy secondary to left ventricular failure. These changes that occur in rats with cardiac hypertrophy and failure may represent important physiological adaptive mechanisms.

Length-dependence of isometric twitch tension potentiation and myosin phosphorylation in mouse skeletal muscle

The effect of changes in muscle length on post-tetanic isometric twitch tension potentiation and myosin P-light chain phosphorylation was studied at 23 degrees C in the mouse extensor digitorum longus muscle. The length-tension relationship was determined for the same muscles after a 30 min period of quiescence and between 30 s and 3 min after a 1.5 s tetanus at L0. Isometric twitch tension is increased at all muscle lengths after the tetanus; however, the fractional increase in twitch tension rises from 0.2 at L0 to a maximum of 0.3 at 1.2 L0. The fractional increase in twitch tension measured at any fixed muscle length is constant between 30 s and 3 min post-tetanus. P-light chain phosphorylation remains constant between 30 s and 3 min post-tetanus followed by a slow decline to basal values. Under fixed length conditions, there is linear relationship between the relative magnitude of the twitch tension and the extent of P-light chain phosphorylation. Net myosin phosphorylation measured after a 1.5 s tetanus at 1.23 L0 is 35% less than that obtained under the same conditions at L0. Thus, contraction-induced phosphorylation of P-light chain decreases with increased muscle length and post-tetanic potentiation at a constant level of P-light chain phosphorylation increases with increasing muscle length. These observations may be consistent with alterations in the sarcoplasmic Ca2+ ion transient as the muscle is lengthened.

Effects of muscle length and load on intracellular Ca2+ in tracheal smooth muscle

Canine tracheal smooth muscle strips were loaded with the bioluminescent Ca2+ indicator aequorin, mounted in a tissue bath, and attached to an electromagnetic lever to investigate the effect of changes in muscle length and load on cytosolic free Ca2+. The intracellular Ca2+ concentration ([Ca2+]i) was inversely correlated with muscle length and active force developed during isometric contractions elicited by electrical field stimulation. In addition, quick release to either constant length or constant load at any time point during an active contraction resulted in an increase in [Ca2+]i. These observations are consistent with the possibility that the binding of Ca2+ to contractile or regulatory proteins is decreased when muscle length or active force development is decreased, resulting in the release of bound Ca2+ into the cytoplasm. However, the possibility that changes in muscle length or load affect other mechanisms that regulate [Ca2+]i cannot be excluded.

Dissociation of force from myofibrillar MgATPase and stiffness at short sarcomere lengths in rat and toad skeletal muscle

1. Single fast-twitch fibres from the extensor digitorum longus muscle of the rat, Rattus norvegicus, and single twitch fibres from the iliofibularis muscle of the cane toad, Bufo marinus, were mechanically skinned and then used to measure maximally Ca2+-activated [( Ca2+] greater than 0.03 mmol l-1) isometric force production, myofibrillar MgATPase activity and fibre stiffness at different sarcomere lengths. MgATP hydrolysis was linked by an enzyme cascade to the oxidation of NADH (nicotinamide adenine dinucleotide, reduced form) and was monitored by a microfluorimetric system. Fibre stiffness was measured from the amplitude of force oscillations generated by small sinusoidal length changes. 2. At sarcomere lengths which were optimal for isometric force production (around 2.7 microns for rat and 2.2 microns for toad fibres) the myofibrillar MgATPase activity (mean +/- S.E.M.) at 21-22 degrees C was found to be 3.80 +/- 0.53 molecules MgATP hydrolysed s-1 per myosin head for eight rat fibres and 6.35 +/- 0.77 s-1 per myosin head for four toad fibres. 3. At sarcomere lengths shorter than 2.7 microns in rat fibres and 2.2 microns in toad fibres, MgATPase and stiffness remained elevated and close to their respective values at 2.7 microns in rat fibres and 2.2 microns in toad fibres even when the isometric force decreased to near zero levels. 4. The dissociation at short sarcomere lengths of myofibrillar MgATPase activity and fibre stiffness from isometric force suggests that the cross-bridge cycle is not greatly affected by double actin filament overlap with the myosin filaments at short sarcomere lengths. Moreover, the results suggest that cross-bridges can be formed by myosin with actin filaments projecting from the nearest Z-line and from the Z-line in the other half of the sarcomere. 5. These results help to reconcile energetic and mechanical data obtained by others at short sarcomere lengths and can be explained within the framework of the sliding filament theory.

Contractile activation and the effects of 2,3-butanedione monoxime (BDM) in skinned cardiac preparations from normal and dystrophic mice (129/ReJ)

Small cardiac myofibrillar preparations were obtained from the right ventricle of normal (129/ReJ) and dystrophic (129/ReJ dy/dy) mice and were chemically skinned in a relaxing solution by exposure to Triton X-100 (3% v/v). (2) The isometric force produced in these skinned cardiac preparations at different sarcomere lengths was measured in solutions of different [Ca2+] and ionic strength. The effect of the negative inotropic drug 2,3-butanedione monoxime (BDM), which is known to act at the myofibrillar level was also investigated. (3) The murine cardiac preparation from normal animals was found to develop 50% maximal force at a pCa (= -log10[Ca2+]) of 5.59 +/- 0.08 and 5.94 +/- 0.03 (mean +/- SD) under physiological (ionic equivalents concentration, I = 154 mM; pH 7.10; [Mg2+] 1 mM) and low ionic strength (I = 94 mM; pH 7.10; [Mg2+] 1 mM) conditions respectively. The isometric force curves were significantly shallower at low ionic strength (Hill coefficient, 1.8 +/- 0.1) than at physiological ionic strength (Hill coefficient, 2.6 +/- 0.3) and the sarcomere length effect on the force-pCa relation was markedly reduced at lower ionic strength. (4) Increasing BDM concentrations in solutions up to 100 mM reduced the maximum Ca2+-activated force of cardiac preparations from normal mice to less than 6% of the control values in a dose dependent fashion. BDM also rendered the cardiac preparations less sensitive to Ca2+ by a factor of up to 1.5 in a process which showed saturation at BDM concentrations higher than 15 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Length and myofilament spacing-dependent changes in calcium sensitivity of skeletal fibres: effects of pH and ionic strength

The calcium sensitivity of force was measured in glycerinated rabbit psoas fibres at sarcomere lengths (SL) from 2.3 to 3.4 micron. Increased SL caused calcium sensitivity to increase and the slope of force-calcium relations to decrease. We have hypothesized that length-dependent changes in myofilament lattice spacing and the presence of fixed charge on the myofilaments are important in determining calcium sensitivity. Lattice spacing changes were monitored by measuring fibre diameter (D). D was decreased by increasing SL, decreasing bathing solution pH and by osmotic compression with 3% PVP. 3% PVP caused D to decrease by about 15% at all SLs and pH values tested. Force-calcium relations were measured at different SLs and pH values, with and without 3% PVP in the bathing solutions. At all pH values D at SL 2.3 micron with 3% PVP was comparable to the value at 3.4 micron, without PVP. At pH 7.5 and 7.0 calcium sensitivity was about the same at both SL, although the slope of the force-calcium relation was less at longer SL. The similarity of the calcium sensitivity at the same D, but much different SL, indicates that lattice spacing is important in determining calcium sensitivity, while SL and the degree of myofilament overlap are important in determining the slope of force-calcium relations. In order to test for the role of myofilament charge in determining calcium sensitivity, pH and ionic strength were varied. Decreasing pH caused decreased maximum force and calcium sensitivity. In addition, the influence of SL on calcium sensitivity decreased as pH was lowered, with minimal SL dependence at pH 5.5; even though lattice spacing still decreased with increasing SL. When D was decreased with PVP, calcium sensitivity increased at all SLs in pH 7.5 and 7.0 while the same lattice spacing changes at pH 6.0 and 5.5 resulted in greatly reduced shifts in calcium sensitivity. These results indicate that the effect of lattice spacing on calcium sensitivity depends on myofilament charge. At pH 6.0, even though osmotic compression of the lattice has no effect, increasing SL causes about half the shift in calcium sensitivity seen at pH 7.0. Lowering ionic strength from 200 to 110 mM caused an increase in both the magnitude and length dependence of calcium sensitivity at pH 7.0, while at pH 5.5 both decreased.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of troponin C on the cooperativity in Ca2+ activation of cardiac muscle

This study describes the effects of exchanging native cardiac troponin C (CTnC) from the right ventricular muscle of Syrian hamster for purified skeletal (S) TnC from fast twitch muscles in triggering cardiac contraction. Ca2+ sensitivity of the myocardium became decreased with STnC to 62% of the original value with CTnC. Furthermore, the slope of the pCa-force curve of cardiac muscle was found to be increased with STnC. The results show that the TnC moiety, as part of the switching mechanism during activation, also regulates thin-filament cooperativity in muscle. Modifications in both the Ca2+ sensitivity and cooperativity are associated with alterations in the primary structure of TnC.

Mechanics and energetics of overstretch: the relationship of altered left ventricular volume to the Frank-Starling mechanism and phosphorylation potential

Isovolumic perfused rat hearts containing an intraventricular balloon were used to assess the effects of incremental balloon volumes on developed pressure, oxygen consumption, coronary flow, phosphorylation potential obtained by P-31 nuclear magnetic resonance, wall thickness obtained by two-dimensional echocardiography, and diastolic wall stress. Three phases in developed pressure were noted: (1) volumes from 0 to 150 microliter resulted in a continuous increase in developed pressure; (2) with volumes from 150 to 250 microliter, developed pressure remained constant whereas developed (systolic) and diastolic wall stress rose sharply; and (3) with volumes from 250 to 400 microliter, developed pressure fell whereas developed (systolic) and diastolic wall stress continued to rise. The ln [(PCr)/(Pi)] was in synchrony with oxygen consumption at 0 and 50 microliter balloon volumes, and then diverged at volumes greater than 100 microliter. Oxygen consumption increased from 0 to 50 microliter, was constant from 50 to 250 microliter balloon volume, and then declined. The ln [(PCr)/(Pi)] fell precipitously at balloon volumes greater than 100 microliter, most likely limited by oxygen consumption. Coronary flow did not change significantly until 250 microliter or more of water was added to the balloon, and then it started to decline. Volumes greater than 100 microliter result in overstretch of myofibers, as observed by the precipitous decline in ln [(PCr)/(Pi)], and the steep increase in diastolic wall stress. With excessive volume loading, the drop in phosphorylation potential, ln [(PCr)/(Pi)], appears to contribute to the decrease in developed pressure.

Hysteresis and the length dependence of calcium sensitivity in chemically skinned rat cardiac muscle

1. The relationship between pCa (-log10[Ca2+]) and steady-state isometric tension has been investigated in saponin- or Triton-treated (chemically 'skinned') cardiac muscle of rat. 2. Hysteresis exists in the relationship such that the muscle is less sensitive to Ca2+ during increasing activation (as [Ca2+] is stepped upward) than during reducing activation (as [Ca2+] is stepped downward). 3. The extent of the hysteresis is insensitive to interventions that increase overall calcium sensitivity by chemical means, such as caffeine, carnosine or increased pH. 4. The extent of the hysteresis is sensitive to sarcomere length. The phenomenon is virtually absent above sarcomere lengths of about 2.2-2.3 microns but becomes progressively greater at shorter sarcomere lengths. 5. The effect of sarcomere length on calcium sensitivity is restricted to the upward-going (increasing activation) part of the pCa-tension loop below 2.2 microns. The downward-going (decreasing activation) part of the hysteretic relationship is virtually unaffected by sarcomere length up to 2.2 microns. 6. Significant alterations in sarcomere length do not occur during tension development in the experiments described here: the phenomenon is not attributable to experimental artifacts of this kind. 7. Hysteresis develops sufficiently rapidly to be consistent with a physiological relevance during the normal heart beat. 8. The effects of sarcomere length show that the phenomenon is not due to force per se since, for example, greater peak force produces less hysteresis as sarcomere length is increased towards 2.2 microns. 9. Tonicity increase (by high-molecular-weight dextran), which shrinks the myofilament lattice, increases calcium sensitivity but reduces the effect of sarcomere length on calcium sensitivity. 10. The results suggest that lattice shrinkage is the mechanism which accounts for hysteresis in, and the sarcomere length dependence of, calcium sensitivity in cardiac muscle.

Effect of sarcomere length and filament lattice spacing on force development in skinned cardiac and skeletal muscle preparations from the rabbit

Skinned cardiac and skeletal muscle freeze-dried preparations were activated in solutions strongly buffered for Ca2+. The response of single skeletal muscle fibres or thin strips of papillary muscle was investigated in relation to changes in Ca content of the perfusate. Sarcomere length was set and controlled during the experiments. The relation between the negative logarithm of the Ca concentration, the pCa, and the normalized developed force proved to be sigmoidal. The exact position of these curves proved to be dependent upon both sarcomere length and the distance between the filaments. The latter was shown by means of osmotic compression of the fibres using dextran. As a consequence of these observations, it was concluded that the length-tension relation is dependent upon the actual Ca concentration. The results are discussed in terms of cross-bridge interaction.

Caffeine suppresses length dependency of Ca2+ sensitivity of skinned striated muscle

Freeze-dried skinned cardiac and skeletal muscle preparations of the rabbit were immersed in Ca2+-containing solutions with different concentrations of caffeine. The relation between the negative logarithm of the Ca2+ concentration (pCa) and normalized developed force was studied. The exact position of these Ca2+-sensitivity functions proved to be dependent on both the sarcomere length (monitored by means of laser diffraction) and caffeine concentration. High concentrations of caffeine induce a reversible fall in tension, particularly at low binding site saturation (low pCa) and long sarcomere lengths. At a concentration of 10 mM caffeine, the sarcomere length dependency of the Ca2+-sensitivity curves is markedly reduced for the rising part of the curve. Only the depressive effect of caffeine at high pCa remains. A possible mechanism of caffeine action is discussed.

Molecular basis for the influence of muscle length on myocardial performance

According to Starling's law of the heart, the force of contraction during the ejection of blood is a function of the end-diastolic volume. To seek the molecular explanation of this effect, a study was made of the effects of length on Ca2+ sensitivity during tension development by isolated demembranated cardiac muscle in which the cardiac form of troponin C was substituted with skeletal troponin C. The results of troponin C exchange were compared at sarcomere lengths of 1.9 and 2.4 micrometers. Enhancement of the myocardial performance at the stretched length was greatly suppressed with the skeletal troponin C compared with the cardiac troponin C. Thus the troponin C subunit of the troponin complex that regulates the activation of actin filaments has intrinsic molecular properties that influence the length-induced autoregulation of myocardial performance and may be a basis for Starling's law of the heart.

Spontaneous oscillatory contraction of sarcomeres in skeletal myofibrils

We found that the lengths of all sarcomeres spontaneously oscillated in an isolated skeletal myofibril, when both ends were fixed, submillimolar to millimolar concentrations of ATP, ADP and inorganic phosphate (Pi) were present, and Ca2+ was removed. Narrowing and widening of an H-zone and an I-band were observed corresponding to the shortening and lengthening of a sarcomere, suggesting that thick and thin filaments slide past each other. The oscillation of each sarcomere was asymmetrical, consisting of a rapid lengthening phase and a slow shortening phase. The period of oscillation was about 3s; the peak-to-peak amplitude of oscillation reached as much as 30% of the average sarcomere length. The propagation of the sarcomere oscillation along the long axis of the myofibril was observed occasionally in single myofibrils and frequently in bundles of myofibrils. The 'state'-diagram showing the concentration range of ADP and Pi in which contraction, oscillation or relaxation of myofibrils occurs in the presence of ATP and the absence of Ca2+ suggested that the oscillation is a third state of skeletal muscle located in between the contracting and relaxing states.

Skinned fibers of human atrium and ventricle: myosin isoenzymes and contractility

Different myosin isoenzymes of pig and human atrium and ventricle and rat ventricle were characterized by two approaches: pyrophosphate polyacrylamide gel electrophoresis (PP-PAGE) and analysis of the myosin P light chains by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). We further investigated the relation between atrial and ventricular myosin isoenzymes of human, pig, and rat, and the maximum (unloaded) shortening velocity (Vmax) and the Ca2+ sensitivity of chemically skinned fibers of the same species. The myosin isoenzymes of both human and pig atrium comigrated in the PP-PAGE with rat V2 isomyosin, whereas the ventricle of human and pig comigrated with rat V3. In both human and pig ventricle, a myosin P light chain polymorphism exists (two phosphorylatable P light chains with the same molecular weight but different isoelectric points). In contrast, we found no P light chain polymorphism in the atrium of human and pig and in the ventricle of rat (one phosphorylatable P light chain only). A correlation exists between Vmax, Ca2+ sensitivity, and atrium- and ventricle-specific myosin isoenzymes of human and pig. Vmax was determined by the slack-test method. Plots of delta l versus delta t of atrial and ventricular skinned fibers were well fitted by a single straight line up to delta l = 15% and delta l = 13%, respectively. Vmax of skinned ventricular fibers was lower than Vmax of skinned atrial fibers in both human and pig. Ca2+ sensitivity of skinned fibers of ventricle, however, was higher than Ca2+ sensitivity of atrial skinned fibers in both human and pig.

Chemomechanics of altered perfusion pressure in rat hearts

In an apex-ejecting isolated perfused working rat heart, as well as isovolumic preparations of rat hearts, perfusion pressure was studied independent of afterload. A decrease in perfusion pressure caused an immediate decrease in developed pressure (10s). There was a significant increase in free Pi and the phosphorylation potential after 20-30 min of perfusion at a reduced coronary flow induced by a reduction in perfusion pressure. Developed pressure decreased prior to the phosphorylation potential and inorganic phosphate; however, the phosphorylation set a limit to maximum work performance. At a perfusion pressure of 140 cm H2O and an afterload of 140 cm H2O, work imposed on the heart was maximum; there was no further increase in work.

Starling's law of the heart is explained by an intimate interaction of muscle length and myofilament calcium activation

The results of several different types of investigations over the last decade clearly indicate that muscle length modulates the extent of myofilament calcium ion (Ca2+) activation. Similarly, the fiber length during a contraction, which is determined in part by the load encountered during shortening, also determines the extent of myofilament Ca2+ activation. Thus, "contractile" or "inotropic" state as it refers to the extent of myofilament activation can, in theory, no longer be considered independent of the muscle length, as was formerly thought to be the case. Accordingly, terms such as preload, afterload and myocardial contractile state as they pertain to cardiac muscle properties lose part of their significance in light of current knowledge.

Evidence for a force-dependent component of calcium binding to cardiac troponin C

The duration of activation in cardiac muscle is a function of the load. On the basis of studies of Ca2+ transients in muscles subjected to quick release, it has been suggested that force or shortening-mediated changes in Ca2+-troponin C affinity may provide a mechanism for a contraction-activation feedback. This study was designed to test the hypothesis that the formation of force-generating complexes between actin and myosin enhances the affinity of cardiac troponin C for Ca2+. This was done by first establishing the normal relationship between Ca2+ binding and force development in chemically skinned bovine ventricular muscle bundles and then comparing the Ca2+-saturation curves obtained with relaxed and contracting muscle bundles. A double isotope technique was used to measure Ca2+ binding during ATP-induced force generation and during relaxation maintained by the phosphate analogue vanadate. The results showed that the generation of force was associated with an enhanced binding of Ca2+ to the Ca2+-specific regulatory site of cardiac troponin C. These data provide direct evidence that feedback between force and activation in the heart may be mediated by the Ca2+-regulatory site of troponin C.

Extra calcium on shortening in barnacle muscle. Is the decrease in calcium binding related to decreased cross-bridge attachment, force, or length?

Barnacle single muscle fibers were microinjected with the calcium-specific photoprotein aequorin. We have previously shown (Ridgway, E. B., and A. M. Gordon, 1984, Journal of General Physiology, 83:75-104) that when barnacle fibers are stimulated under voltage clamp and length control and allowed to shorten during the declining phase of the calcium transient, extra myoplasmic calcium is observed. The time course of the extra calcium for shortening steps at different times during the calcium transient is intermediate between those of free calcium and muscle force. Furthermore, the amplitude increases with an increased stimulus, calcium transient, and force. Therefore, the extra calcium probably comes from the activating sites on the myofilaments, possibly as a result of changes in calcium binding by the activating sites. The change in calcium binding may be due, in turn, to the change in muscle length and/or muscle force and/or cross-bridge attachment per se. In the present article, we show that the amount of the extra calcium depends on the initial muscle length, declining at shorter lengths. This suggests length-dependent calcium binding. The relation between initial length and extra calcium, however, parallels that between initial length and peak active force. The ratio of extra calcium to active force is therefore virtually independent of initial length. These data do not distinguish between a direct effect of length on calcium binding and an indirect effect owing to changes in cross-bridge attachment and force through some geometrical factor. The amount of extra calcium increases with the size of the shortening step, tending toward saturation for steps of greater than or equal to 10%. This experiment suggests that calcium binding depends on muscle force or cross-bridge attachment, not just length (if at all). There is much less extra calcium seen with shortening steps at high force when the high force results from stretch of the active muscle than when it results from increased stimulation of muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of stretch on sarcoplasmic free calcium of frog skeletal muscle at rest

The concentration of free calcium in the sarcoplasm of a resting frog skeletal muscle is increased by stretch. The magnitude of the rise in free calcium increased with the degree of stretch and the ambient temperature and it was enhanced by caffeine. This phenomenon might play a role in the twitch potentiation and enhanced metabolic rate evoked by stretch.

The influence of free calcium on the maximum speed of shortening in skinned frog muscle fibres

The influence of [Ca2+] on the maximum velocity of shortening (Vmax) was examined in mechanically skinned Rana pipiens and Rana temporaria fibres using improved force clamps and the slack test techniques. All measurements were made at 7.5 degrees C. At low relative loads (P/P0 less than 0.1), maximally activated R. pipiens fibres shortened more rapidly than did submaximally activated fibres. At higher relative loads, however, little difference in the speed of shortening was observed. Vmax (determined by the slack test) of R. pipiens fibres increased as the level of activation increased. Over sarcomere lengths 1.8-2.1 microns it was 2.28 muscle lengths/s (m.l./s) (S.E. of mean +/- 0.25, n = 5) at 20-35% activation, 2.89 m.l./s (+/- 0.22, n = 7) at 40-60% activation, and 4.18 m.l./s (+/- 0.25, n = 6) at 100% activation. At longer sarcomere lengths (2.2-2.6 microns), higher Vmax values were observed at all levels of activation, but the influence of Ca2+ on Vmax persisted. Vmax was 3.54 m.l./s (+/- 0.41, n = 4) at 20-30% activation and 5.15 m.l./s (+/- 0.22, n = 5) at 100% activation. In R. temporaria fibres, Vmax (determined by force clamps over sarcomere lengths 1.8-2.1 micron) also increased as the level of activation increased, from 3.47 m.l./s (+/- 0.06, n = 6) at 13-29% activation to 5.62 m.l./s (+/- 0.17, n = 6) at 100% activation. Vmax was also determined (using the slack test) in mechanically and chemically skinned rabbit soleus fibres. Vmax at 15 degrees C (1.05 m.l./s, +/- 0.11, n = 5) at full activation decreased by more than 3-fold as the level of activation was reduced to 10%. We conclude that the level of activation influences the Vmax of skinned skeletal muscle fibres. This has now been demonstrated in three different preparations and by a variety of techniques. This effect is most pronounced at low relative loads, and might not be observed if there are experimental limitations which prevent making velocity measurements at low relative loads.

Comparison between the sarcomere length-force relations of intact and skinned trabeculae from rat right ventricle. Influence of calcium concentrations on these relations

To investigate the extent to which the properties of the cardiac myofibrils contribute to the length-force relation of cardiac muscle, we determined the sarcomere length-force relations for rat ventricular trabeculae both before and after the muscles were skinned with the detergent Triton X-100. Sarcomere length was measured continuously by laser diffraction. In the unskinned trabeculae stimulated at 0.2 Hz, the relation between active force and sarcomere length at an extracellular calcium concentration of 1.5 mM was curved away from the sarcomere length axis, with zero force at sarcomere length of 1.5-1.6 micron. At 0.3 mM calcium, the sarcomere length-force relation was curved toward the sarcomere length axis. Chemical skinning of the muscle with 1% Triton X-100 in a "relaxing solution" caused an increase in intensity and decrease in dispersion of the first order diffraction beam, indicating an increased uniformity of sarcomere length in the relaxed muscle. During calcium-regulated contractures in the skinned muscles, the central sarcomeres shortened by up to 20%. As the calcium concentration was increased over the range 1-50 microM, the relation between steady calcium-regulated force and sarcomere length shifted to higher force values and changed in shape in a manner similar to that observed for changes in extracellular calcium concentration before skinning. The sarcomere length-force relations for the intact muscles at an extracellular calcium concentration of 1.5 mM were similar to the curves at calcium concentration of 8.9 microM in the skinned preparations, whereas the curves at an extracellular calcium concentration of 0.3 mM in intact muscles fell between the relations at calcium concentrations of 2.7 and 4.3 microM in the skinned preparations. A factor contributing to the shape of the curves in the skinned muscle at submaximal calcium concentrations was that the calcium sensitivity of force production increased with increasing sarcomere length. The calcium concentration required for 50% activation decreased from 7.71 +/- 0.52 microM to 3.77 +/- 0.33 microM for an increase of sarcomere length from 1.75 to 2.15 micron. The slope of the force-calcium concentration relation increased from 2.82 to 4.54 with sarcomere length between 1.75 and 2.15 micron. This change in calcium sensitivity was seen over the entire range of sarcomere lengths corresponding to the ascending limb of the cardiac length-force relation. It is concluded that the properties of the cardiac contractile machinery (including the length-dependence of calcium sensitivity) can account for much of the shape of the ascending limb in intact cardiac muscle.

Ca2+ sensitivities and transient tension responses to step-length stretches in feline mechanically-stripped single-fibre jaw-muscle preparations

At a muscle length, Lo (just taut), isometric tension at constant levels of various Ca2+ activations and transient tension responses to rapid length stretches (less than 1 per cent of Lo within 2 ms) at maximal Ca2+ activation level were measured in temporal, masseter and digastric (anterior belly) muscles (2-3 mm long and 24-48 micron in diameter). Steady isometric tension increased in a sigmoid fashion with increasing Ca2+ concentration from about pCa 7.28 to 4.49 in temporalis, from about pCa 6.18 to 4.40 in masseter and from about pCa 5.82 to 4.40 in digastric. The maximum tension was 75.5 +/- 10.2 g/mm2 in temporalis, 44.7 +/- 14.1 g/mm2 in masseter, and 46.1 +/- 20.1 g/mm2 in digastric. In the resting state, the sarcomere length at Lo was 2.34 +/- 0.06 micron in temporalis, 2.20 +/- 0.08 micron in masseter, and 2.20 +/- 0.00 micron in digastric. When the sarcomere length was stretched from 2.20 to 2.34 micron (the sarcomere length of temporalis at Lo) in the masseter and digastric, the Ca2+ sensitivity increased without significant change of the maximum tension in either muscle. The transient tension responses in all three muscles showed two distinct phases; an immediate tension increase coincident with the length stretch followed by an exponential tension decrease. The mean value of the time constant in the second phase was 58.5 +/- 19.7 ms in temporalis, 58.5 +/- 12.6 ms in masseter, and 362.6 +/- 16.8 ms in digastric. Thus temporalis showed a higher Ca2+ sensitivity at Lo and a greater maximum tension-producing capability than the other muscles and the cross-bridge turnover rate appears to be slower in digastric than in the others.