Experimental Physiology (1992), 77, 539-552 Printed in Great Britain

REVIEW ARTICLE

ENHANCEMENT OF MECHANICAL PERFORMANCE OF STRIATED MUSCLE BY STRETCH DURING CONTRACTION M. I. M. NOBLE Academic Unit of Cardiovascular Medicine, Charing Cross and Westminster Medical School, 17 Horseferry Road, London SW1P 2AR (MANUSCRIPT RECEIVED 30 SEPTEMBER 1991, ACCEPTED 6 JANUARY 1992)

CONTENTS

PAGE

Introduction The experimental data Force enhancement during and after stretch Component 1: velocity-dependent force enhancement during stretch Component 2: a force component recruited by a critical sarcomere extension (not increased by further extension), which decays with time Component 3: residual force enhancement after stretch of contracting muscle Structural explanations of force enhancement by active stretch: connectin and nebulin X-ray diffraction studies Energetic considerations: heat production and ATP consumption Simulation of force enhancement by active stretch by muscle models Conclusion References

539 540 540 541 542 544

546 546 547 547 550 550

INTRODUCTION

Most physiologists and medical students know that when a striated muscle is stimulated, if that muscle has been stretched at rest beyond the optimum length for overlap of actin and myosin filaments, there is a reduction in force production (Ramsey & Street, 1940; Edman, 1966; Gordon, Huxley & Julian, 1966). Fewer know that stretch of striated muscle whilst contracting beyond the optimum length for overlap of actin and myosin filaments increases force production which then remains at a higher level (Fenn, 1924; Abbot, Aubert & Hill, 1951; Hill & Howarth, 1959; Deleze, 1961; Edman, Elzinga & Noble, 1978). The reason for this discrepancy may lie in the fact that the former result is acquired in the comfortable knowledge that it is compatible with the standard 'accepted' theory of cross-bridge action whereas, as Deleze (1961), working at University College London, was uncomfortably aware, the latter fact is incompatible with that theory without extra ad hoc assumptions. The purpose of the present review is to remind readers of the facts.

20

EPH 77

540

M. I. M. NOBLE

A

Force

B

Jnln

2 3

Force sum

Length Fig. 1. Components of the response to stretch during active contraction represented diagrammatically. A, only components 1 and 2 are present on the plateau of the length-tension diagram. The sum of the forces (i.e. force sum) corresponds to the response found experimentally. B, all three components are present on the descending limb of the length-tension diagram at sarcomere lengths above 2 3 ,um. Force sum (1 + 2 + 3) corresponds to the response found experimentally, i.e. increasing force with decreasing overlap of thick and thin filaments. THE EXPERIMENTAL DATA

The possible importance of our own experiments (Edman et al. 1978) and those of Hill (1977) lies not so much in their originality, as in the fact that they were carried out in single fibres, allowing a more accurate delineation of the features of the phenomenon. Our study (1978) also established that the phenomenon was accompanied by an increase in the entire force-velocity curve, rather than just force - hence the term 'mechanical performance' in my title. Such an increase in the force-velocity curve denoted an underlying mechanism that is not just of a passive elastic nature. In that study we also showed that there were at least three components to the response to stretch of contracting striated muscle; these we subsequently designated components 1, 2 and 3 (Edman, Elzinga & Noble, 1979) and are illustrated in Fig. 1. As these results have stood the test of the intervening time, and have subsequently been repeatedly confirmed, it seems appropriate to reconsider the importance of the findings today. Force enhancement during and after stretch The application of a steady stretching ramp at constant velocity during the plateau of a tetanus causes a sharp initial increase in force (Fig. 1). After a certain amount of stretch there is a break in the force record so that the initial sharp rising phase is followed by a plateau or slow rise of tension. Following the end of stretch, force remains higher than in isometric contraction at the short (initial) or long (final) length; this force enhancement after stretch decays with time. The peak force achieved during stretch is dependent on the velocity of stretch whereas that after stretch is not. This identifies a separate component during stretch which is velocity dependent and which we have designated component 1 (Fig. 1). After stretch there is a different behaviour of the force enhancement depending on whether the overlap of filaments is optimal or not. The main difference is that component 2, present at optimal overlap, decays completely with time, whereas component 3 which is added to component 2 at lengths corresponding to reduced overlap, remains for as long as one can practically

observe it.

STRETCH OF MUSCLE DURING CONTRACTION

541

Tension relative to isometric (P/IP,) 10 12 14 1-6 18 20

0-1

0'2 > 0-2 a)

Fig. 2. The 'negative' portion of the force-velocity curve. Data points in the figure are taken partly from Edman et al. (1978) (-), partly from the study of Lombardi & Piazzesi (1990) (*) and partly from a recent presentation to the Physiological Society by Curtin & Edman (1991) (U).

Component 1. velocity-dependent force enhancement during stretch It is well known that there is a 'negative' part of the force-velocity curve (Katz, 1939; Aubert, 1956). However, it is not well appreciated that the force recorded during this part of the curve is supplemented by an extra force which is still present when stretching stops and which we have designated component 2. This problem is likely to account for the discrepancies between different authors. It seems likely to me that the form of the curve is as illustrated by the continuous line in Fig. 2; the isolated single fibre data points in that figure are taken partly from Edman et al. (1978) (0), partly from the study of Lombardi & Piazzesi (1990) (*) and partly from a recent presentation to the Physiological Society by Curtin & Edman (1991) (U). Lombardi & Piazzesi (1990) also studied a higher range of velocities of lengthening at which a transient extra force is produced in excess of that at the plateau (Fig. 1). In order to obtain clues as to the underlying mechanism of component 1, it would have been gratifying to have found experiments studying the effects of interventions upon this curve. Unfortunately, very little has been done along these lines. Although Mansson (1989) does not present the negative force-velocity curve, he clearly shows that hypertonicity causes an increase in force enhancement during stretch, with a greater drop in force immediately after the end of stretch, i.e. an increased component 1. Curtin & Edman (1991) showed that the force values on the negative force-velocity curve were markedly increased by fatigue; this was in contrast to the lower forces of the positive part of the curve. Fatigue and increased tonicity of the bathing fluid are associated with increased intracellular ionic strength, with fatigue also producing other effects such as intracellular acidosis which are inconvenient for the present discussion (see below). Nevertheless, it seems reasonable to me, within the present limitation of available data, to conclude that increased ionic strength causes the increase in component 1 of force enhancement during stretch and the changes in the force-velocity curve depicted in Fig. 3. A less clear-cut conclusion can be drawn from the data on the effect of intracellular acidosis caused by CO2 administration (Curtin, 1990). In this paper, whole muscle was used 20-2

542

M. I. M. NOBLE Tension relative to isometric

10)

15

(PIP,) 20

0-04

>

01i2 L

Fig. 3. The 'negative' portion of the force velocity curve (0) and the effect of fatigue (*). Data replotted from Curtin & Edman (1991).

rather than single fibres, making the published tracings less easy to interpret. There is no doubt that normalized force during stretch is enhanced by hypercapnia; however, I am not able to be sure that this is a component I increase rather than a component 2 increase. The intervention chosen is also less useful for my purpose since intracellular acidosis has effects on the contractile system over and above those of increased ionic strength alone. The impression given above is strengthened by the contrast to the effects of adding phosphate (Steinen, Versteeg, Papp & Elzinga, 1991). This intervention causes a decrease in force on the negative part of the force-velocity curve as well as the positive part. This is consistent with a decreased power generation by the myosin ATPase (Hibberd, Dantzig, Trentham & Goldberg, 1985), and indicates that the mechanism induced by hypertonicity and fatigue (above) is completely different.

Conpolnent 2: a fioce (component rec-iited hbY a critical sarcomnere extension (not inicreased bhi finrther extensioni), whliich (leca vs wtith tinme The characteristics of this component are illustrated in Fig. 1, and described in detail by Edman, Elzinga & Noble (1981). The extra force contributed by this component is not dependent on velocity of stretch, but only upon sarcomere length up to a critical extension. For this reason, force records show a typical break point at which the rapid rising phase is replaced by a plateau of force. The critical sarcomere extension required to achieve this break averages 16 6 nm which is independent of sarcomere length between 1 8 and 2 8 /im. The study of Lombardi & Piazzesi (1990) shows that when the velocity of lengthening is increased above 0-3 /im s 1 half-sarcomere 1, an extra peak of force occurs at progressively lower sarcomere extensions. This indicates that the apparent lack of effect of velocity on the critical extension required to reach the break point (Edman et al. 1981) may be due to the low range of velocities used in that study. In whole muscle and in single fibres to which a series compliance has been added, there is a 'give' in the sarcomere length record at an extension of about 12 nm (Flitney & Hirst, 1975, 1978h; Bottinelli, Eastwood & Flitney, 1989). This is not found in single fibres studied without series compliance (Edman et al. 1978; Lombardi & Piazzesi, 1990). It seems reasonable to suppose that this occurs at the critical break point whenever the sarcomere length cannot be constrained as rigidly as in the modern type of length- or segment-clamp

STRETCH OF MUSCLE DURING CONTRACTION

543

A

268

AL\

Sarcomere length

(jIm) 2 76

1

L

0 30 Force (N mm-2) 0

05s

AL

B

026 _

Force (N mm-2)

II

0 24 _

It

022

020 22

25 24 23 Sarcomere length (,um)

26

Fig. 4. A, a force recording of two tetani, one isometric at 2-76 /im sarcomere length and the other with stretch of sarcomeres from 2 68 to 2 76 ,im. At a critical amplitude of stretch, AL, there is a break in the force record followed by a slower rising phase. B, force after the end of stretch during tetani as a function of amplitude of stretch, superimposed upon the length-tension diagram (continuous line) established by length changes between tetani. At a critical amplitude of stretch, AL, there is a break in the relationship followed by a slower rising phase. Replotted from data of Edman et al. 1981.

single fibre experiments. The critical length for sarcomere 'give' is of the same order of magnitude as the critical extensions measured by other methods. The break point during stretch is also associated with an arrest of the anisotropic decrease in intensity of laser light transmitted through the tissue (Eastwood & Flitney, 1985, 1986). This indicates that there is a change in the internal ordering of the sarcomere structure from the isometric to the stretch condition. Its decay after stretch with a similar time course to the decay of force (see below) seems to indicate that this is important for component 2. Eastwood & Flitney very reasonably postulate that this phenomenon could be a manifestation of cross-bridge distortion. After the end of the stretch, there is a decaying force component. By using different amplitudes of stretch we were able to derive a force-extension curve for this component, which is similar in form to that just described during stretch (Fig. 4). The mean critical amplitude of stretch to reach the break point in this phenomenon was 11 5 nm; it was also independent of sarcomere length between 1 8 and 2 8 pm. This extra force after stretch disappeared when a small release was applied. We therefore devised a method of release to a load clamp with zero velocity of shortening and found that this could be applied to the whole time course of the tetanus. When this was done, we found that the critical amplitude of release increased with time up to the angle in the force record during stretch, and decreased with time after the end of stretch; it was consistently less than the critical amplitude of stretch required to reach the break point of force enhancement during

M5 M.M. M. NOBLE

544

stretch, but was also independent of sarcomere length. The force drop accompanying the critical release increased a little with sarcomere length to an optimum at 24-27,am, and decreased at longer lengths. Mansson (1989) studied instantaneous stiffness with and without stretch in control and hypertonic solution. During stretch, force falls in an almost identical manner with step amplitude of release (see his fig. 2). He interpreted this as a slight increase in instantaneous active stiffness caused by hypertonicity during stretch, but with similar total extension of the undamped fibre elasticity; this was in contrast to marked reduction in the extension of the undamped elasticity by hypertonicity in the absence of stretch. It is not straightforward to interpret these experiments (in which releases to zero force were imposed) in terms of critical release to the zero velocity load required to characterize component 2. I have not found it possible to interpret Mansson's records with respect to the effect of hypertonicity on the velocity-independent component 2 force. There is no isometric comparison myogram, but it appears that the force enhancement after stretch is increased in proportion to isometric force before stretch (see his fig. 1). However, I am not sure whether this is attributable to component 2 or 3 or both. Further studies of changes in ionic strength are required using the critical release method. Lombardi & Piazzesi (1990) also studied instantaneous stiffness, but only during stretch (component 1 also present) and without interventions. They found that stiffness was slightly greater during lengthening than during the plateau of the isometric tetanus. A remarkable experiment has been reported by Elzinga's group (Versteeg, Stienen, Papp & Elzinga, 1990; Stienen, Versteeg, Papp & Elzinga, 1992) who studied the influence of phosphate on the break point during stretch in skinned psoas and soleus fibres. The effect on reduction of force by phosphate was much greater in psoas fibres (a 42 % reduction in force) than in those of soleus. The critical sarcomere extension required to reach the break point increased from 15 nm (in control conditions for both muscles and in soleus fibres in the presence of phosphate) to 29 nm per half-sarcomere in psoas fibres in the presence of

phosphate.

Component 3: residualforce enhancement after stretch of contracting muscle This component is only seen on the descending limb of the length-tension curve. It is illustrated in Fig. 1 and described in detail by Edman, Elzinga & Noble (1982). The absolute magnitude of the effect increases with sarcomere length to a maximum at about

2-9,um and declines at greater lengths. The magnitude of the effect is (more or less linearly) dependent on amplitude of stretch. With an adequate amplitude of stretch, the final force level is greater than the isometric force level corresponding to the starting length of the stretch. Releasing a fibre to an isotonic load removes component 2; thus the force-velocity relationships obtained result from the action of component 3. It was found that velocity of shortening was increased after stretch (Cavagna & Citterio, 1974; Edman et al. 1978; Sugi & Tsuchiya, 1981) compared with tetani with no stretch, at loads except zero (i.e. there was no change in As one expects for an aspect of component 3, this effect is only seen on the descending limb of the length-tension curve (Edman et al. 1978). Much effort has gone into demonstrating that this effect is not due to non-uniformity of sarcomere length along the fibre length, even though that mechanism would not explain the enhancement of velocity of shortening. However, stretch of a clamped short segment of fibre produces the same result (Edman et al. 1982). Component 3 does not develop with time as nonuniformities develop; it can be revealed at full amplitude immediately after stretch by applying a small critical release to remove component 2. Isotonic lengthening under a load

V.ax).

STRETCH OF MUSCLE DURING CONTRACTION

545

A

280

Sarcomere

length

(um)

287

L

ft /Component 2 B

Component 3 0 25

Force

[Isometric__

(N mm 2) 5s S

S

Fig. 5. A, length change imposed on the fibre to demonstrate component 3. B, force tracings. After stretch, the imposition of a critical release causes a transient force drop followed by a maintained increase of force above the isometric value. These responses are compared with a force record of stretch without release, and an isometric tetanus obtained at the final length; the length records for these last two myograms are omitted. Redrawn from data of Edman et al. (1982).

greater than isometric tetanic force causes the fibre to shorten, i.e. against a load higher than isometric tetanic force (Sugi & Tsuchiya, 1981). The most elegant way of demonstrating component 3 and separating it from component 2 is by use of a critical release (see discussion of component 2 in previous section, and Fig. 5). An effect of an intervention on force always arouses interest in the question of concomitant changes in fibre stiffness, because the latter, according to cross-bridge theory, resides in the cross-bridges. According to this hypothesis, tension and stiffness should change proportionately. However, force enhancement by active stretch is accompanied by a decrease in muscle fibre stiffness (Sugi & Tsuchiya, 1981). Of great interest is the question of intrasarcomere structural change during active stretch. Hill's (1977) study showed that the stretch occurred within the I band, rather than the A band, indicating that indeed the overlap of thick and thin filaments decreases. This should be confirmed by quick-freeze or quick-fix experiments to measure thin filament lengths. Such experiments have now been performed (Susuki, Tsuchiya, Oshimi, Takei & Sugi, 1989; Brown & Hill, 1991; K. A. P. Edman, personal communication). When fibres are fixed and subjected to electron microscopy, disordering during the fixation process is likely to occur as ATP becomes depleted and rigor bonds form. Brown & Hill (1991) studied three fibres by this method and unfortunately used much higher velocities of stretch than those under consideration in this review. Not surprisingly, this leads to considerable longitudinal derangement of the filaments, with no overlap at all (and presumably no tension) in some half-sarcomeres. Stretch at lower velocities during activity appears to induce some disordering of the lattice in cross-section (Susuki et al. 1989). My interpretation of Edman's description of his quick-freeze experiments (using the same velocities of stretch as those of Edman et al. (1978)) is that the filament lengths do indeed remain constant, compatible with the decreasing overlap hypothesis which requires the assumption of poorly extensible actin filaments. Edman also describes greater staggering of the myofilaments with active stretch than is seen with isometric contraction alone. How this can produce a higher force is not clear, but Edman points out that the staggering of myofibrils he sees is

546

M. I. M. NOBLE

relevant because it implies strain of passive structures that could provide a basis for his concept of a parallel elastic element formed during activity (see below). STRUCTURAL EXPLANATIONS OF FORCE ENHANCEMENT BY ACTIVE STRETCH: CONNECTIN AND NEBULIN

An attractive idea that occupied our thinking during the original experiments was that there might be some kind of extra parallel elastic element brought into being by activation. Such an element, when stretched, would give an extra force proportional to the strain, and would add velocity at finite loads when subsequent shortening occurred. These are indeed the characteristics of component 3. Our original idea was that the extra parallel elastic element might be the sarcolemma, but this is disproved by the failure of swelling of the fibre with hypotonic solution to produce a disproportionate increase in component 3. Compatible with that result, however, would be an element stretching from the thick filament to the Z-disc, i.e. the C-filament consisting of connectin (also called titin) and nebulin, an elastic structure which stretches when the sarcomere is stretched. The connectin filament remains bound to the thick filaments in active fibres, but its elastic properties are unaltered by activity, calcium ions and cross-bridge activity (Horowits, Maruyama & Podolsky, 1989). This would appear to rule out the C-filament as the source of component 3. The failure of activation to affect C-filament elasticity is apparently in contradiction to the conclusions of Horowits, Kempner, Bisher & Podolsky (1986) who inactivated titin and nebulin by irradiation (and by freezing and thawing) in skinned fibres. However, this experiment actually depressed resting force, active force, and stretch-induced force disproportionately. This was thought to be caused by breakage of the C-filaments by the treatment. No evidence for an increase in C-filament stiffness by activation emerges from this or any other experiment of which I am aware. This explanation therefore seems to be an attractive possibility, but one for which such

evi'dence as there

is is

negative. X-RAY DIFFRACTION STUDIES

A motivation for the retention of the active parallel elastic element hypothesis for component 3 is that it allows a separate explanation from that for component 2. Component 2 requires an explanation at cross-bridge level because the critical distances involved are independent of sarcomere length, indicating a dependence on regularly spaced structures along the filaments. Since explanations in terms of cross-bridges are virtually impossible for component 3 (Deleze, 1961), a separate elastic element explanation is preferable and leaves one to tackle the understanding of component 2 on it own. The break point of component 2 was postulated to result from cross-bridge deformation by Flitney & Hirst (1975, 1978a, h). One type of measurement which is popularly interpreted in terms of cross-bridge mechanics is that obtained from the diffraction of Xrays passed through muscle. In particular, the equatorial reflections I1.0 and I1, have been interpreted in terms of cross-bridge attachment (Huxley, 1968; Haselgrove & Huxley, 1973). However, in studies of active stretch, components 2 and 3 are both present. The effect of stretch during activity and active-stretch-induced force enhancement on the I .( and I,l were studied by Tanaka, Hashizume & Sugi (1984). There was a small decrease in the value of I,l attributed to decreased regularity of the filament lattice. There was no change in the ratio of I,.I 1 indicating that stretch does not produce changes in myosin head orientation.

STRETCH OF MUSCLE DURING CONTRACTION

547

Matsubara & Yagi (1985) studied the intensity of the 14 3 nm meridian reflection caused by the myosin heads which protrude from the myosin filament at 14-3 nm intervals and are arranged in a helical manner along the filament. He observed a decrease in the intensity of this reflection during active stretch. Difficulties arise in the interpretation of this result because ordinary tetanic stimulation of the muscle caused an increase in 14 3 nm intensity in Matsubara & Yagi's records, and a decrease in that intensity in the records of Sugi, Tanaka, Wakabayashi, Kobayashi, Iwamoto, Hamanaka, Mitsui & Ameniya (1986). A decrease in intensity could therefore be interpreted as a change similar or opposite in sign to the change induced by activation. The paper of Sugi et al. (1986) also presents results of sinusoidal changes in length, which is a complicated intervention. Unlike the previous paper from this group (Tanaka et al. 1984, above), the length increase part of the sine wave causes decreases in I,l intensity sufficiently to affect the I,1 /I, ratio. As with Matsubara & Yagi (1985) the 14 3 nm meridian intensity decreased during the stretch phase. The interpretation of all these diffraction pattern changes in terms of myosin head behaviour in not straightforward, especially as a conformational change upon activation will cause a separation of electrostatic charge and subsequent movement of potassium ions; this constitutes a considerable mass movement

(Iwazumi, 1970).

ENERGETIC CONSIDERATIONS: HEAT PRODUCTION AND ATP CONSUMPTION

During stretch of contracting muscle the net energy output (heat plus work) is negative (Fenn, 1924; Abbott & Auber, 1951; Abbott et al. 1951; Hill & Howarth, 1959), leading to the postulate that stretch of actively contracting muscle causes energy to be absorbed into chemical resynthesis. However, Curtin & Davies (1972) showed that there was continued breakdown of high-energy phosphate compounds, although at a considerably reduced rate. More modern heat measurements indicate that components 1 and/or 2 are associated with increased heat output, but that component 3 is not, in that Curtin & Woledge (1979) failed to find residual enhancement of heat production in the presence of residual enhancement of force. The net energy release was not any different after stretch. They thought that performance of the experiment at different temperatures would differentiate between active and passive processes, but this experiment produced unexpected results. The higher temperature decreased component 2 and increased component 3 (Edman, Elzinga & Noble, 1984). The finding of a critical distance that characterizes component 2 and is independent of sarcomere length (see above) places this component rather unequivocally in the myosin cross-projections. However, although a rise in temperature would increase their metabolic rate (turnover rate), this results in a decrease of force! By contrast component 3, which is not associated with increased enzymatic activity or heat production (Curtin & Woledge, 1979), is increased by higher temperature! SIMULATION OF FORCE ENHANCEMENT BY ACTIVE STRETCH BY MUSCLE MODELS

It will be apparent from the foregoing that some difficulties arise in 'explaining' these

phenomena according to the cross-bridge model of physical attachment and detachment (Huxley 1957; Huxley & Simmons, 1971; Haselgrove & Huxley, 1973). One should perhaps not completely ignore more simple primitive models, such as a two-element model in which the force in one element rises with stretch to a maximal value at which it is held for the remainder of the stretch by a frictional resistance. This force would decay when stretching

548

M. 1. M. NOBLE

ceased. At longer lengths a third (parallel) element as postulated by Edman (Tsuchiya & Edman, 1990) could impose a length-dependent additional force (component 3). However, it is necessary for such a parallel elastic element to be formed during activation with unstretched properties determined by the sarcomere length at which activation occurs. This possibility was disproved by Edman et al. (1982) by activating at a longer length, and then releasing the fibre before allowing force to redevelop prior to stretch; component 3 was unaffected by this manoeuvre. Another point of interest in relation to simple element models is the elastic behaviour of resting muscle. This is similar, with a break point, but with lower force development than in active muscle and enhancement by hyperosmolarity (Flitney, 1975; Mansson, 1989). This would seem to indicate that, if the stretch response is dependent on cross-bridge function (not dependent on how the cross-bridge acts), the resting muscle response is due to low-level cross-bridge activity, equivalent to the 'short-range elastic component' of Hill (1968). The negative part of the force velocity curve (component 1) is certainly accounted for, both in the original exposition of the model, and in more recent analyses (Lombardi & Piazzessi, 1990). However, the effect of fatigue in increasing relative force at any given velocity of lengthening (Fig. 3) remains unexplained. There also remains the problem that during lengthening, cross-bridge cycling must increase (Lombardi & Piazzessi, 1990) but ATPase splitting decreases. This leads to the necessity to assume that during lengthening there is a fundamental change in myosin biochemistry, e.g. in the model of Lombardi & Piazzessi, by assuming that the cycling can occur in stretch without ATP splitting. The question of the break point in component 2 is an interesting test of the theory. The break during stretch is assumed to be the result of detachment of cross-bridges after a period when they are strained by the critical lengthening of about 10 14 nm (Lombardi & Piazzessi, 1990). However, this is not compatible with the compliance of the cross-bridges as assessed by instantaneous step changes in length (Lombardi & PiaLzzessi, 1990) from which the maximum possible strain is only about 3 nm at a corresponding force of up to double the isometric value. It then becomes necessary to assume an extra 10 nm of extension due to back rotation of the cross-bridge (Tsuchiya & Edman, 1990). Another major difficulty with providing an adequate cross-bridge model for component 2 is that the break point occurs at different sarcomere lengthenings in different situations, e.g. 16-6 nm in our experiments during stretch, 12 nm during stretch in Flitney's experiments, al continuously increasing and decreasing amount during and after stretch as measuled by the critical release method (Edman et al. 1981), and a varying amount during stretch depending on velocity of stretch in Lombardi's experiments. It is difficult to explain why this should vary so much, even in the same experiment, if it represents the distance the cross-bridge must be stretched to induce detachmeints. In the case of components 2 and 3, a flavour of the conceptual difficulty can be obtained by recalling a discussioll of the subject between K. A. P. Edman and H. F. Huxley in 1984 (discussion of Edman, Elzinga & Noble, 1984). Huxley proposed that there was a region of cross-bridge distortions where the detachment rate wa1s extrem-nely low and at the same time supposed that when the cross-bridges were forced even further (i.e. an even greater force was applied to them), they did begin to detalch. The ones that were in this intermediate range would detach extremely slowly and give rise to the long-lasting force (component 3). Then the extensive release necessary to release that force would probably correspond to the length over which that low detachment rate took place. There would be no reason why that should necessarily be the same distance as would be required to stretch the muscle in the

STRETCH OF MUSCLF DURING CONTRACTION

549

first place, before coming to the break in the tension curve. The events after the break point postulated to depend on moving the cross-bridges beyond their normal attachment point, with most of them continuously detaching. As the extent to which they were distorted increased, their detachment rate increased to the point where the detachment rate was very high, and at that point most of the bridges kept coming off, so that fixed the initial tension reached. There were always some bridges which would not have come off by then, and they then got carried into the region where they detached extremely slowly. Edman points out that there are a number of observations which are difficult to fit with the Huxley concept. For instance, residual force enhancement after stretch increases steadily with amplitude of stretch, and the stretch is much larger than the presumed working range of a cross-bridge. Also this component 3 increases with sarcomere length with a maximum at about 3 0 lim sarcomere length, i.e. the amount of extra force produced by stretch increases with decreasing area of overlap (discussion of Edman, Elzinga & Noble,

were

1984). Such an unsatisfactory outcome to the debate in 1984 may have resulted from the fact that the participants (including myself) ignored the work of Hatze. He produced a convincing simulation of the stretch responses (Hatze, 1981), using a mathematical model (Hatze, 1977). What physico-chemical phenomena might be embodied in such equations which are demonstrably close in effect to the muscle responses? Hatze (1981) states that the process assumed for the cross-bridges resembles the effect produced by a sudden displacement of a current-carrying coil in a magnetic field: the force resisting the displacement immediately rises, and energy stored in the electric circuit of the coil increases. He further assumes that movement of the cross-bridge in the direction of lengthening increases its electric charge, and that this charge increases non-linearly with stretching

velocity. These equations are similar to those that would arise from the assumption of lwazumi

(Iwazumi, 1970; Iwazumi & Noble, 1989) that the force exerted by the cross-bridge arises from a local electrostatic field acting on the tip of a dielectric thin filament. Hatze, however, still applies his equations to proposed attachments and detachments of a radically different nature to those envisioned by H. E. Huxley (discussion cited above). This does not seem to me strictly necessary. The Iwazumi model predicts that the electrostatic field strength of the cross-bridge will be increased by stretch through electrostatic induction in relation to velocity of lengthening (component 1). Staggering of myofilaments is predicted due to the fact that thin filament tips have to line up more and more with the myosin cross-projection as the field strength increases with stretch; this also places increasing strain on the Z-disc structure which is really more like a net than a disc. The distance at which a break point occurs (component 2) is not constant but is related to the longitudinal dimension of the enhanced field; thin filament tips line up to the (sarcomere) central side of the myosin projections during contraction and have to traverse right through the field to the distal side with stretch. Thus, the critical distance must vary with field dimension, and therefore with field strength and force, during and after stretch. This behaviour differs, in contrast, from the instantaneous force-length relationship occurring with a step change in length; this reflects the distances required for the thin filament tip to exit the field on the central side only, while at the same time the electrostatic induction effect is very much enhanced by the very high velocities of movement. If the field strength is weak, it may build up over a longer period of time during stretch so that the thin filament tips traverse to the next set of cross-projections, i.e. after an additional stretch of 14 3 nm per half-sarcomere (this remarkably predicts the result of

550

M. I. M. NOBLE

Versteeg et al. (1990), see above). Finally Iwazumi's longitudinal field synthesis provides a concave overall distribution of calcium ions along the thick filament (modified by an energy-saving vernier effect of the troponin distribution on the thin filament). This produces a higher calcium ion concentration at the tip of the thin filament to increase the activation of the myosin cross-projection at that location. Only this determines the force at increased sarcomere length (component 3) for any given starting activation length. CONCLUSION

The effects of stretching actively contracting muscle remain perplexing and unexplained. A full understanding of the underlying mechanism could well reveal basic evidence concerning the mechanism of cross-bridges force generation. The puzzlement of Deleze (1961) is reiterated with little modification by myself, after reviewing the literature of the intervening 30 years! The author is grateful for financial support from the Garfield Weston Trust. REFERENCES

ABBOT, B. C. & AUBERT, X. M. (1951). Changes of energy in a muscle during very slow stretches. Proceeding,s of the RoYal SocietY B 139, 104- 117. ABBOTT, B. C., AUBERT, X. M. & HILL, A. V. (1951). The absorption of work by a muscle stretched during a single twitch or a short tetanus. Proceedingps of the Royal SocietY B 139, 86-104. AUBERT, X. (1956). Le Coupl/age Energtique die la Contraction Musculaire. These d'Agregation de l'Enseignement Superieur. Editions Arsaia, Bruxelles. BOTTINELLI, R., EASTWOOD, J. C. & FLITNEY, F. W. (1989). Sarcomere give during stretch of frog single muscle fibres with added series compliance. Qluarterl/i Jou-rnal of Experimental Phy siology 74, 215-217. BROWN, L. & HILL, L. (1991). Some observations on variations in filament overlap in tetanized muscle and fibres stretched during a tetanus, detected in the electron microscope. Jolur-nbal of Muiscle Research anid Cellular Motility 12, 171-182. CAVAGNA, G. D. & CITTERIO, G. (I1974). Effect of stretching on the elastic characteristics and the contractile component of frog striated muscle. Jourtnal of Physiology 239, 1-14. CURTIN, N. A. (1990). Force during stretch and shortening of frog sartorius muscle: effects of intracellular acidification due to increased carbon dioxide. Jourtnal of Musc/e Research anid Cell/ula Motilit'y 11, 251-257. CURTIN, N. A. & DAVIES, R. E. (1972). Chemical and mechanical changes during stretching of activated frog skeletal muscle. Cold Sprinlg Hatrbor Symposia on Quantitative Biology 37, 619-626. CURTIN, N. A. & EDMAN, K. A. P. (1991). Force-velocity relation for shortening and lengthening of fatigued isolated muscle fibres from frog. Journal of Physiology, 438, 48 P. CURTIN, N. A. & WOLEDGE. R. C. (1979). Chemical change, production of tension and energy following stretch of active muscle of frog. Journial of PhYsiology 287, 539-550. DELEZE, J. B. (1961). The mechanical properties of the semitendinosus muscle at lengths greater than its length in the body. Journal of Physiology 158, 154-164. EASTWOOD, J. C. & FLITNEY, F. W. (1985). Laser spectroscopy studies of actin--myosin interaction in living frogs' muscles. Journal of Physiology 367, 76P. EASTWOOD, J. C. & FLITNEY, F. W. (1986). Laser spectroscopic studies of actin myosin interaction in isolated active frog's muscle: wavelength dependence of transparency changes during stretch. Jolurntial of PhYsiology 380, 28 P. EDMAN, K. A. P. (1966). The relation between length and active tension in isolated semitendinosus fibres. Jolunal of PhYsiology 183, 407 417. EDMAN, K. A. P., ELZINGA, G. & NOBLE, M. I. M. (1978). Enhancement of mechaLnical performance by stretch during tetanic contractions of vertebrate skeletal muscle fibres. Journtial o/ Phy,siology 281, 139-155.

STRETCH OF MUSCLE DURING CONTRACTION

551

EDMAN, K. A. P., ELZINGA, G. & NOBLE, M. I. M. (1979). The effect of stretch of contracting skeletal muscle fibres. In Cross-bridge Mechanism in Muscle Contraction, ed. SUGI, H. & POLLACK, G. H., pp. 297-308. University of Tokyo Press, Tokyo. EDMAN, K. A. P., ELZINGA, G. & NOBLE, M. I. M. (1981). Critical sarcomere extension required to recruit a decaying component of extra force during stretch in tetanic contractions of frog skeletal muscle fibres. Journal of General Physiology 78, 365-382. EDMAN, K. A. P., ELZINGA, G. & NOBLE, M. I. M. (1982). Residual force enhancement after stretch of contracting frog single muscle fibres. Journal of General Physiology 80, 769-784. EDMAN, K. A. P., ELZINGA, G. & NOBLE, M. I. M. (1984). Stretch of contracting muscle fibres: evidence for regularly spaced active sites along the filaments and enhanced mechanical performance. In Contractile Mechanisms in Muscle, ed. POLLACK, G. H. & SUGI, H., pp. 739-751. Plenum Press, New York. FENN, W. 0. (1924). The relationship between the work performed and the energy liberated in muscular contraction. Journal of Physiology 59, 373-395. FLITNEY, F. W. (1975). Light scattering associated with tension changes in the short-range elastic component of resting frog's muscle. Journal of Physiology 255, 1-14. FLITNEY, F. W. & HIRST, D. G. (1975). Tension responses and sarcomere movements during length changes applied to contracting frog's muscle. Journal of Physiology 251, 66-68 P. FLITNEY, F. W. & HIRST, D. G. (1978a). Filament sliding and energy absorbed by the cross-bridges in active muscle subjected to cyclical length changes. Journal of Physiology 276, 467-479. FLITNEY, F. W. & HIRST, D. G. (1978b). Cross-bridge detachment and sarcomere 'give' during stretch of active frog's muscle. Journal of Physiology 276, 449-465. GORDON, A. M., HUXLEY, A. F. & JULIAN, F. J. (1966). The variation in isometric tension with sarcomere length in vertebrate muscle fibres. Journal of Physiology 184, 170-192. HASELGROVE, J. C. & HUXLEY, H. E. (1973). X-ray evidence for radial cross-bridge movement and for the sliding filament model in actively contracting skeletal muscle. Journal of Molecular Biology 77, 549-568.

HATZE, H. (1977). A myocybernetic control model of skeletal muscle. Biological Cybernetics 25, 103-119.

HATZE, H. (1981). Analysis of stretch responses of a myocybernetic model muscle fibre. Biological Cybernetics 39, 165-170. HIBBERD, M. G., DANTZIG, J. A., TRENTHAM, D. R. & GOLDMAN, Y. E. (1985). Phosphate release and force generation in skeletal muscle fibers. Science 228, 1317-1319. HILL, A. V. & HOWARTH, J. V. (1959). The reversal of chemical reactions in contracting muscle during an applied stretch. Proceedings of the Royal Society B 151, 169-193. HILL, D. K. (1968). Tension due to interaction between the sliding filaments in resting striated muscle. The effect of stimulation. Journal of Physiology 199, 637-680. HILL, L. (1977). A-band length, striation spacing and tension change on stretch of active muscle. Journal of Physiology 266, 667-685. HOROWITS, R., KEMPNER, E. S., BISHER, M. E. & PODOLSKY, R. J. (1986). A physiological role for titin and nebulin in skeletal muscle. Nature 323, 160-164. HOROWITS, R., MARUYAMA, K. & PODOLSKY, R. J. (1989). Elastic behaviour of connectin filaments during thick filament movement in activated skeletal muscle. Journal of Cellular Biology 109, 2169-2176. HUXLEY, A. F. (1957). Muscle structure and theories of contraction. Progress in Biophysics and Biophysical Chemistry 7, 255-318. HUXLEY, A. F. & SIMMONS, R. M. (1971). Proposed mechanism of force generation in striated muscle. Nature 233, 533-538. HUXLEY, H. E. (1968). Structural difference between resting and rigor muscle: evidence from intensity changes in the low-angle X-ray diagram. Journal of Molecular Biology 37, 507-520. IWAZUMI, T. (1970). A new field theory of muscle contraction. Ph.D. Thesis, University of Pennsylvania: University Microfilms Inc., Ann Arbor, MI, USA. IWAZUMI, T. & NOBLE, M. I. M. (1989). An electrostatic mechanism of muscular contraction. International Journal of Cardiology 24, 267-275. KATZ, B. (1939). The relation between force and speed in muscular contraction. Journal ofPhysiology

96, 54-64. LOMBARDI, V. & PIAZZESI, G. (1990). The contractile response during steady lengthening of stimulated

frog muscle fibres. Journal of Physiology 431, 141-171.

552

M. I. M. NOBLE

MANSSON, A. (1989). Changes in force and stiffness during stretch of skeletal muscle fibers, effects of hypertonicity. Bioplhvsica/ Jolurn1al 56, 429-433. MATSUBARA, I. & YAGI, N. (1985). Movements of cross-bridges during and after slow length changes in active frog skeletal muscle. Jolurnlal of Ph_'sio1og.i 361. 151 163. RAMSEY, R. W. & STREET, S. F. (1940). The isometric length-tension diagram of isolated skeletal muscle fibres of the frog. Journ-lalof Cc/lltl/ar 'oComparatUve Phlsiologv 15, 11 34. STIENEN, G. J. M., VERSTFEG, P. G. A., PAPP, Z. & ELZINGA, G. (1992). Mechanical properties of skinned rabbit psoas and soleus muscle fibres during lengthening: effects of phosphate and Ca2Joiollal of Phyvsiology, 451, 503-523. SUGI, H., TANAKA, H., WAKABAYASHI, K., KOBAYASHI, T., IWAMOTO. H., HAMANAKA, T., MITSUI, T. & AMEMIYA, Y. (1986). Structural changes during contraction in vertebrate skeletal muscle as studied by time-resolved X-ray diffraction technique. Biomnedica et Biochimica Acta 45, SI 522. SUGI, H. & TSUCHIYA, T. (1981). Enhancement of mechanical performance in frog muscle fibres after quick incrcascs in load. Journ-tial of Phy_siology 319, 239-252. SUZUKI, S., TSUCHIYA, T., OSHIMA, Y., TAKEI, T. & SUGI, H. (1989). Electron microscopic studies on the stretch-induced disordering of the myofilament lattice in tetanized frog skeletal muscle fibres. Journial of Electron Microscopy 38, 60 63. TANAKA, H., HASHIZUMI, H. & SUGI, H. (1984). Factors affecting the equatorial X-ray diffraction pattern from contracting frog skeletal muscle. In Contractile MechanLisms in Muscle, ed. Pollack, G. H. & Sugi, H., pp. 193-202. Plenum Press, New York. TSUCHIYA, T. & EDMAN, K. A. P. (1990). Mechanism of force enhancement after stretch in intact single musclc fibres of the frog. Acta Phlvlsiologica Scandinavica 140, 23A. VERSTEEG. P. G. A., STIINEN, G. J. M., PAPP, Z. & ELZINGA, G. (1990). Influence on the forcc response during controlled lengthening of skinned psoas and soleus muscle fibres. In Muscle and Motility, ed. MARIYHEL, G. & CARRARO, U.. pp. 253 256. Interccpt, Andover, UK.