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K A Edman

Publications and source records attributed to K A Edman.

At least 37 records · Page 2Linked to original sources
PubMed1989

Effects of amrinone on twitch, tetanus and shortening kinetics in mammalian skeletal muscle.

The contractile effects of amrinone were studied on twitch and tetanus responses of mouse lumbrical muscles. Amrinone (1.1 mM) increased the maximum force level during the tetanus, reduced the rate of rise of force during the onset of tetanus and prolonged the time from the last stimulus to half-relaxation. The rate of redevelopment of force after a release during the tetanus plateau was likewise reduced. Amrinone (greater than or equal to 0.5 mM) increased the time to peak twitch force and the time to half-relaxation during the twitch. The peak twitch force was not significantly changed. The force-velocity relation was markedly affected by amrinone: there was a decrease in maximum velocity of shortening, an increase in maximum isometric force and a decrease in curvature of the force-velocity relation. The results suggest that amrinone modulates the kinetic properties of the myosin cross-bridges by exerting a specific effect upon the myofilament system. The latter effects are mainly responsible for the contractile changes produced by amrinone in mouse skeletal muscle. Amrinone appears to have little effect on the metabolism of activator calcium in this particular preparation.

Amrinone↗
PubMed1989

Effects of fatigue and reduced intracellular pH on segment dynamics in 'isometric' relaxation of frog muscle fibres.

1. Longitudinal movements of marked segments of single fibres from the anterior tibialis muscle were recorded during tetanus and relaxation under isometric (fixed-end) conditions. 2. During relaxation, shortening and lengthening of different segments occurred simultaneously, starting at about the same time as the end of the linear fall of force (shoulder on the force record). 3. Variations in intracellular pH, measured with pH-sensitive microelectrodes, along the length of fibres were not statistically significant, and are unlikely to be responsible for the non-uniform behaviour of different segments. 4. As expected from earlier studies, both fatigue (produced by increasing tetanus duration or decreasing the time between tetani) and intracellular acidification (produced by raised extracellular CO2), reduced the tetanus force and prolonged the linear phase of force decline in relaxation. Each treatment delayed the start and markedly reduced the amount of segment movement in relaxation. 5. Fatigue and intracellular acidification have a smaller effect on force during stretching than on force produced under isometric conditions. This may contribute to making the segments behave in a more uniform way during relaxation under these conditions. 6. Changes in the Ca2+ uptake mechanisms are also discussed as possible causes for the changes in segment behaviour in relaxation.

Animals↗
PubMed1988

The consequences of fibre heterogeneity on the force-velocity relation of skeletal muscle.

The consequences of fibre heterogeneity on the collective force-velocity properties of bundles of parallel fibres were examined in a simulation model. The model was tested by comparing the actual force-velocity curve of a bundle of three fibres, each of which had been individually characterized, with the force-velocity curve predicted by the model for the bundle based on the individual fibre properties. The predicted and measured force-velocity curves were in excellent agreement. The curvature of the force-velocity relation for a muscle, as indicated by a/P0 in Hill's (1938) hyperbolic equation, increases with increasing heterogeneity in the maximum shortening velocities (Vmax(i] of the individual fibres in the muscle. In a muscle that is heterogeneous with respect to Vmax(i), the maximum shortening velocity determined by the slack test method (V0) can be expected to represent the fastest fibre(s) in the muscle. The maximum velocity of shortening (Vm), determined by extrapolation from a hyperbola that is fitted to force-velocity data at finite loads, is substantially lower than V0. The difference in estimates of V0 and Vm is a function of: (i) the degree of heterogeneity of the muscle with respect to Vmax(i) and the curvature of the force-velocity relationship of the individual fibres, and (ii) the force range used to establish the hyperbola from which Vm is derived. The ratio of Vm to V0 can be used as an index to estimate the degree of variability in the maximum velocity of shortening among individual fibres in a muscle.

Animals↗
PubMed1988

Maximum velocity of shortening related to myosin isoform composition in frog skeletal muscle fibres.

1. The velocity of unloaded shortening (V0), the myofibrillar ATPase activity and the immunoreactivity to two monoclonal antibodies (A1 and A2) that were raised against the myosin heavy chains were studied in single fibres of the anterior tibialis muscle of Rana temporaria. V0 was recorded for the fibre as a whole using the slack-test method. Myofibrillar ATPase activity was determined by means of a quantitative histochemical technique. 2. A highly significant, direct relationship was found to exist between V0 and the myofibrillar ATPase activity recorded in the same single fibres. Both V0 and the myofibrillar ATPase activity changed in proportion to the cross-sectional area of the fibres. 3. Muscle fibres that had first been characterized with respect to V0 and myofibrillar ATPase activity were exposed to monoclonal antibodies A1 and A2. Thin fibres, having relatively low V0 and low myofibrillar ATPase activity, reacted preferentially with A1. Thick fibres, on the other hand, exhibiting relatively high V0 and high myofibrillar ATPase activity, were preferentially stained by A2. A third category of fibres reacted with both A1 and A2. The results support the view that the variability in shortening velocity and myofibrillar ATPase activity that exists among twitch fibres in frog skeletal muscle is based on differences in myosin heavy-chain composition. 4. Attempts were made to elucidate further the previous observation (Edman, Reggiani & te Kronnie, 1985) that the velocity of unloaded shortening (V0) differs along the length of individual muscle fibres. To this end discrete segments (0.5-0.7 mm in length) of intact fibres were delineated by opaque markers of hair that were placed on the fibre surface. The change in length between two adjacent markers (one segment) was recorded photo-electrically while the fibre was released to shorten against a very small load between 2.2 and 2.0 micron sarcomere lengths. In the majority of fibres (eight out of eleven preparations), V0 and myofibrillar ATPase activity exhibited similar patterns of variation along the fibre. Pooled data from thirty-three segments of twelve fibres showed a positive correlation between V0 and myofibrillar ATPase activity (P less than 0.05). 5. The possibility was explored that the myosin isoform composition might vary along the length of an individual muscle fibre. For this purpose bundles of fibres were cross-sectioned at 0.5-1 mm intervals along their entire length and the reactivity to monoclonal antibody A2 was tested at each location.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Triphosphatases↗
PubMed1988

Double-hyperbolic force-velocity relation in frog muscle fibres.

1. The relationship between force and velocity of shortening was studied at 2.10 micron sarcomere length during fused tetani (1-3 degrees C) in single fibres isolated from the anterior tibialis muscle of Rana temporaria. The speed of shortening was recorded from the whole fibre and, in some experiments, simultaneously from a short (ca. 0.6 mm) segment, while the preparation was released to shorten isotonically at selected force levels ('load-clamp' recording). The segment was defined by opaque markers of hair that were placed on the fibre surface. The distance between the markers was recorded by means of a photo-electric detector system. 2. The force-velocity relation had two distinct regions, each one exhibiting an upwards concave shape, that were located within the ranges 0-78 and 78-100% of the measured isometric force (P0), respectively. The two portions of the force-velocity relation could be fitted well by hyperbolic functions or by single-exponential functions. The curvature was more pronounced in the high-force region than at low-intermediate loads. The transition between the two portions of the force-velocity relation (the 'break point' of the force-velocity curve) occurred at 78.4 +/- 0.4% of P0 (mean +/- S.E. of mean, n = 12) corresponding to 10.9 +/- 0.4% of maximum velocity of shortening (Vmax). The general shape of the force-velocity curve, and the appearance of a break point near 78% of P0, was the same when measurements were made from the whole fibre and from a short segment along the same fibre. 3. The 'negative' branch of the force-velocity relation was delineated for loads ranging from P0 to 1.6-1.8 P0 in five experiments. The negative branch formed a smooth continuation of the force-velocity relation recorded between 0.78 P0 and P0. The force-velocity relation was nearly flat between 0.90 P0 and 1.20 P0, the difference in speed of shortening or elongation being 1.8 +/- 0.3% (mean +/- S.E. of mean, n = 5) of Vmax over this range. 4. An increase in sarcomere length from 1.85 to 2.60 micron did not affect Vmax but caused a steady decrease in curvature of the force-velocity relation, both at low-intermediate loads and in the high-force range. Similar changes in shape of the force-velocity relation were produced by osmotic compression of the fibre in a Ringer solution made hypertonic by addition of 98 mM-sucrose.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Double-hyperbolic nature of the force-velocity relation in frog skeletal muscle.

Load-clamp recordings were performed on single fibres of the anterior tibialis muscle of Rana temporaria to further characterize the relationship between force and velocity of shortening. With the techniques used the speed of shortening could be recorded at different loads both from the fibre as a whole and from individual segments (ca. 0.5 mm in length) along the intact fibre. In accordance with previous observations the force-velocity relation was found to have a break point near 78% of the measured isometric force, P0. The results furthermore demonstrate that the force-velocity relation is composed of two hyperbolic functions, above and below the break point, respectively. The hyperbola in the high-force region has a more pronounced curvature than that observed at low and intermediate loads. An increase in sarcomere length from 2.10 to 2.60 microns makes both portions of the force-velocity relation less curved and reduces, to a great extent, the difference in curvature between the two hyperbolas. Similar effects are produced by raising the tonicity of the extracellular medium. Addition of caffeine (0.5 mM), or changing the temperature within the range 1-11 degrees C, does not affect the shape of any portion of the force-velocity relation. The fact that the above features of the force-velocity relation appear in very short segments as well as in the whole fibre suggests that the double-hyperbolic shape of the force-velocity curve does represent the contractile behaviour at sarcomere level. The system behaves as if a fraction of myosin cross-bridges were neutralized, or abolished (leading to a relative decrease in force), as the speed of filament sliding is reduced below approximately 10% of the maximum.

Animals↗
PubMed1987

The sarcomere length-tension relation determined in short segments of intact muscle fibres of the frog.

1. Single fibres isolated from the tibialis anterior muscle of Rana temporaria were stimulated to produce a 1 s fused tetanus, while a short (ca. 0.5 mm) segment of the fibre was held at constant length. The segments were defined by opaque markers of hair that were placed on the fibre surface. The distance between two adjacent markers (one segment) was monitored by means of a photo-electric recording system. The length of a given segment could be controlled to within 0.2% of the segment's length by adjusting the over-all length of the fibre by means of an electromagnetic puller and servo system. 2. Segments producing constant force (no 'tension creep') during length-clamp recording were studied at different striation spacings within the following ranges of sarcomere length: 2.20-3.70, 1.90-2.45 and 1.50-2.20 microns. The absence of tension creep suggested (Edman & Reggiani, 1984 a) that the sarcomere pattern remained stable within the length-clamped segment during contraction at different lengths. 3. The tetanic force of a given length-clamped segment was consistently found to increase, as the sarcomere length was reduced from 2.20 to 1.98-2.02 microns, the mean increase in force being 6.9 +/- 0.4% (S.E. of mean, thirty-two segments). By further decreasing the sarcomere length active force was reduced. 4. The increase in force-producing capability between 2.20 and 2.00 microns sarcomere length was further explored by recording the maximum rate of force redevelopment, dF/dtmax, after a quick release during the plateau of a fixed-end tetanus. dF/dtmax varied with sarcomere length between 2.20 and 2.00 microns in the same way as the isometric force of a short, length-clamped segment, increasing by ca. 10% over this range. This finding provides further support for the view that the fibre's capacity to produce force is not constant between 2.20 and 2.00 microns sarcomere length. 5. The descending limb of the length-tension relation extended between 2.00 and approximately 3.65 microns sarcomere length. Its middle, straight portion (between 2.30 and 3.30 microns sarcomere spacings) extrapolated to zero tension at 3.49 microns sarcomere length. The upper and lower portions of the descending limb were slightly curved (at sarcomere lengths less than 2.30 and greater than 3.30 microns, respectively) giving the descending limb a symmetrical sigmoid appearance.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1985

Effects of amrinone on the contractile behaviour of frog striated muscle fibres.

The effects of amrinone (0.005-2.1 mM) were studied on isolated single muscle fibres of the frog (0.6-4.3 degrees C). In the concentrations used amrinone potentiated the isometric twitch, increased the initial rate of force development and prolonged the time to peak twitch force and the time from peak force to half relaxation. There was a moderate but statistically significant (P less than 0.001) enhancement of the maximum tetanic force and a marked slowing of the relaxation phase of the tetanus. Amrinone depressed the velocity of unloaded shortening by 33 +/- 2% (mean +/- SE, n = 6) and reduced the curvature of the force-velocity relation leading to an increase of a/Po in Hill's (1938) hyperbolic equation. In accordance with the effects of amrinone on the force-velocity relation, there was a decrease in the rate of redevelopment of force after the fibre was released to zero tension during the tetanus plateau. The evidence suggests that the drug affects the contractile properties of frog muscle fibres by two different mechanisms: (I) amrinone affects the excitation-contraction coupling causing enhanced calcium release in response to membrane excitation and (2) amrinone exerts a direct effect on the kinetics of turnover of the myosin cross-bridges. The latter aspect of the drug effect was evaluated in terms of A.F. Huxley's (1957) cross-bridge model by means of a computer program.

Aminopyridines↗
PubMed1985

Differences in maximum velocity of shortening along single muscle fibres of the frog.

The velocity of 'unloaded' shortening (V0) and the force-velocity relation were studied during fused tetani (0.5-2.0 degrees C) in short successive segments along the entire length of single fibres isolated from the tibialis anterior muscle of Rana temporaria. The segments were defined by opaque markers of hair that were placed on the fibre surface, 0.5-0.8 mm apart, from one tendon insertion to the other. The change in distance between two adjacent markers (one segment) was monitored by means of a photoelectric recording system, while the fibre was released to shorten isotonically between 2.2 and 2.0 micron sarcomere lengths. The accuracy of the V0 measurement was better than 4% in all parts of the fibre. V0 varied along the length of the fibre, each fibre having a unique velocity pattern that remained constant throughout the experiment. The difference between the highest and lowest values of V0 within the fibre varied between 11 and 45% of the fibre mean in thirty-two preparations (mean difference 23 +/- 2%, S.E. of mean). An attempt was made to relate the V0 pattern to the fibre's orientation in the body in fourteen complete experiments. The highest values of V0 were obtained near the proximal end of the fibre, and there was a clear trend for V0 to assume lower values towards the distal end. The V0 pattern along the fibre did not correlate with the segments' capacities to produce force nor with the passive viscoelastic properties of the segments. Force-velocity data obtained from individual segments provided a good fit to Hill's (1938) hyperbolic equation at loads less than 80% of the measured tetanic force. The curvature of the force-velocity relation, defined by alpha/P0 in Hill's equation (P0 being the isometric force calculated from the hyperbolic function) varied between 0.09 and 0.46 in sixteen segments of six different fibres. V0 was inversely related to alpha/P0 according to the following regression: V0 = 3.21 - 3.22. (alpha/P0), correlation coefficient, 0.72; P less than 0.005. No clear correlation between V0 and alpha/P0 existed at the whole-fibre level. The results support the view that the kinetic properties of the myofilament system differ from one region to another along the length of a muscle fibre.

Animals↗
PubMed1984

Length-tension-velocity relationships studied in short consecutive segments of intact muscle fibres of the frog.

Length changes of consecutive, 0.5-0.8 mm long segments of frog single muscle fibres were studied by photoelectric recording of opaque markers placed on the fibre surface. There was a marked redistribution of segment length during an ordinary isometric contraction (fixed fibre ends) at both 2.15 and 2.6-2.8 microns sarcomere length. This length redistribution can explain the tension 'creep' that occurs during standard isometric contractions on the descending limb of the length-tension relation. Length clamp of individual segments eliminated tension creep completely. Active force of length-clamped segments was investigated within the range 2.20-3.65 microns sarcomere length. The descending limb of the length-tension relation (determined in segments where no tension creep occurred) was not strictly linear but had a slightly sigmoid shape. Active force was reduced to zero at a sarcomere length close to 3.65 microns. While isometric force varied only moderately between different segments, the velocity of unloaded shortening (V0) was found to vary greatly (by 22-50%) along the length of a fibre. V0 did not correlate with the passive resistance to a length change, the isometric force or the cross-sectional area of the individual segments. Local differences of the internal milieu and/or coexistence of myosins of different kinetic properties within a single fibre may account for the observed differences in V0.

Animals↗
PubMed1984

Stretch of contracting muscle fibres: evidence for regularly spaced active sites along the filaments and enhanced mechanical performance.

Single frog skeletal muscle fibres were stretched during fused tetanic contractions. The force increase during stretch exhibited a breakpoint at a mean critical length change of 16.6 nm per half sarcomere that was independent of stretch velocity and sarcomere length. The early decaying extra force after stretch (component 2) was removed by a small quick release, leaving a longer lasting component (component 3). The amplitude of release required increased with time up to the angle in the force record during stretch, was constant for the remainder of the stretch and decreased with time after the end of stretch; it was consistently less than the critical amplitude of stretch (above). Component 3 occurred at sarcomere lengths above 2.3 microns and was amplitude dependent. The final force after stretch was usually higher than the isometric force at the starting length of the stretch. Non-uniformity as a cause of this component was examined by (a) laser diffraction studies which showed sarcomere stretch at all locations and (b) 0.6-0.7 mm long segments along the entire fibre which all elongated during stretch. After stretch the sarcomeres and segments were significantly more stable than during control isometric tetani. Segments which were clamped by a servo system demonstrated component 3. Shortening during contraction followed by stretch back to the starting length led to nearly as much force enhancement as stretch alone, suggesting that component 3 is not due to a passive elastic element recruited during activation. An increase in temperature decreased components 1 (velocity dependent force during stretch) and 2 but increased component 3. The critical length features of component 2 suggest a cross-bridge mechanism. However, the sarcomere length dependence of all components differs from that of isometric force and from predictions based on filament overlap.

Animals↗
PubMed1984

Redistribution of sarcomere length during isometric contraction of frog muscle fibres and its relation to tension creep.

Changes in length of successive 0.5-0.8 mm segments along single muscle fibres of Rana temporaria were recorded during 3 s isometric (fixed fibre ends) tetani at 2.15 and 2.60 micron sarcomere length. The measurements were performed by means of a photo-electric detector system which recorded the distance between opaque markers (ca. 60 microns in width) that were attached to the upper surface of the fibre. The segment length change had an initial rapid phase (1) which coincided with the steep rise of force and a subsequent slow phase (2) which coincided with the upper, rounded portion of the force myogram and the 'plateau' of the tetanus. At 2.15 micron sarcomere length the majority of the central segments (comprising approximately 90% of the fibre) shortened to various degrees during phase 1. A considerable redistribution of length occurred during phase 2 in that some segments shortened at the expense of others which were forcibly stretched. The central region, taken as a whole, shortened by 0.1-0.5% during phase 2. The end segments were consistently found to elongate during phase 1. However, they were able to hold the tension, without further elongation, during phase 2. The pattern of length changes within the central region of the fibre observed at 2.15 micron sarcomere spacing remained largely the same after increasing the sarcomere length to 2.60 micron. However, in contrast to the situation at 2.15 micron sarcomere length there was an over-all (0.4-1.5%) elongation of the central region of the fibre during phase 2 at the great fibre length. This elongation of the central region was associated with marked shortening of the end segments. The sarcomere length of the end segments (s.1.e) was compared to that of the central region of the fibre (s.l.c) at various fibre rest lengths. There was no significant difference between s.l.e and s.l.c when the fibre was just taut, i.e. at approximately 2.1 micron sarcomere length. The following relationship between s.l.e and s.l.c was found to apply for values of s.l.c ranging between 2.2 and 2.7 micron: s.l.e = 0.636 s.l.c + 0.744 (correlation coefficient, 0.93). The possibility was explored that redistribution of sarcomere length along the fibre causes the slow climb of force ('tension creep') that occurs during a tetanus at great (greater than 2.2 micron) sarcomere lengths. Tension creep could be reproduced, after peak force had been attained, during an isometric tetanus by releasing the fibre to shorten within the range 2.6-2.3 micron sarcomere length.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1983

Diazepam, a highly effective twitch potentiator in isolated muscle fibres of the frog.

The contractile effects of diazepam (10-200 microM) were studied on twitch and tetanus responses of isolated fibres of the semitendinosus muscle of Rana temporaria (3.5-5.0 degrees C). Diazepam (100-200 microM) enhanced the twitch amplitude to 95-100% of maximum tetanic force and increased the rate of rise of force, the time to peak twitch force and the total duration of the relaxation phase. The maximum tetanic output was unaffected by diazepam but the drug increased the rate of rise of the tetanic force and delayed the onset of force decay after the last stimulus. However, the kinetics of relaxation was unaffected by the drug. Diazepam had no effect on either threshold, submaximum or maximum contracture responses to caffeine and to increased potassium concentration. Diazepam in concentrations producing full twitch potentiation caused only a moderate (ca 30%) increase in action potential duration. The results are in line with the idea that 1: diazepam enhances the twitch response by increasing the rate of release of activator calcium without affecting the rate of calcium resequestration and 2: diazepam acts by modulating a mechanism in the excitation-contraction coupling that responds specifically to membrane excitation.

Action Potentials↗
PubMed1982

Residual force enhancement after stretch of contracting frog single muscle fibers.

Single fibers from the tibialis anterior muscle of Rana temporaria at 0.8-3.8 degrees C were subjected to long tetani lasting up to 8 s. Stretch of the fiber early in the tetanus caused an enhancement of force above the isometric control level which decayed only slowly and stayed higher throughout the contraction. This residual enhancement was uninfluenced by velocity of stretch and occurred only on the descending limb of the length-tension curve. The absolute magnitude of the effect increased with sarcomere length to a maximum at approximately 2.9 micrometers and then declined. The phenomenon was further characterized by its dependence on the amplitude of stretch. The final force level reached after stretch was usually higher than the isometric force level corresponding to the starting length of the stretch. The possibility that the phenomenon was caused by nonuniformity of sarcomere length along the fiber was examined by (a) laser diffraction studies that showed sarcomere stretch at all locations and (b) studies of 9-10 segments of approximately 0.6-0.7 mm along the entire fiber, which all elongated during stretch. Length-clamped segments showed residual force enhancement after stretch when compared with the tetanus produced by the same segment held at the short length as well as at the long length. It is concluded that residual force enhancement after stretch is a property shown by all individual segments along the fiber.

Animals↗
PubMed1982

Shortening induced deactivation of skinned fibres of frog and mouse striated muscle.

The depressant effect of active shortening, previously established in intact muscle fibres, was studied during calcium induced contractures of chemically skinned fibres from the semitendinosus muscle of Rana temporaria and the psoas muscle of the mouse. The decrease in contractile activity was determined by comparing the rate of force redevelopment (at a given tension level) after a large (test) and a small (control) release step. Under standard experimental conditions (ionic strength: frog 135 mM, mouse 190 mM; Ca2+ 3.0 microM; Mg2+: frog 25 microM, mouse 100 microM; MgATP2-: frog 1.0 mM, mouse 2.0 mM) active shortening of 0.15 microns per sarcomere (in excess of control release) reduced the contractile activity by approximately 50% of the control in both frog and mouse muscle fibres. Full contractile activity was regained within less than 4 s during isometric activity after the shortening phase. The depressant effect of shortening was steadily reduced, to almost complete disappearance of the effect, by increasing the free calcium concentration within the range 1.5-12.0 microM. Similarly, an increase in ionic strength from 105 to 235 mM reduced the depressant effect by approximately 40%. In contrast, there was a progressive enhancement of the shortening effect as the magnesium ion concentration was increased from 25 to 590 microM. It is proposed that interaction between the myosin cross-bridges and the thin filament during sarcomere shortening leads to a decrease in troponin-calcium binding resulting in a temporary deactivation of the contractile system.

Adenosine Triphosphate↗
PubMed1982

Laser diffraction studies of sarcomere dynamics during 'isometric' relaxation in isolated muscle fibres of the frog.

1. A study has been made of changes in sarcomere length and tension which occur during relaxation from isometric (;fixed ends') tetani in isolated muscle fibres of the frog. Sarcomere lengths were calculated from measurements of the separation of the zero-to-first-order intensity maxima in diffraction patterns generated by illuminating small segments of fibre with a He-Ne laser. Diffraction spectra were recorded continuously on cine-film using the method of ;streak' photography.2. Many sarcomeres in a muscle fibre are found to undergo active shortening during relaxation, at the expense of others located elsewhere which become passively extended. The time of onset of changes in sarcomere length coincides with the well known ;shoulder' on the tension record, and their amplitude is maximal at the time when isometric force approaches zero. The original pattern of sarcomere lengths is only re-established after tension has disappeared.3. The variability in the pattern of sarcomere length changes during relaxation was studied in a sample of twenty fibres, by making ;streak' recordings from successive 1 mm segments along the entire length of each fibre. This survey showed that segments which elongate are located predominantly, though not exclusively, close to the fibre ends. The fractional length of the fibre which underwent shortening was found to vary considerably in different preparations. In many ;streak' recordings the first order line fragmented into several distinct intensity maxima.4. The effects of varying the initial fibre length on the time course of the tension decay and on the accompanying changes in sarcomere length were studied. The range of fibre lengths investigated corresponded with sarcomere spacings of 1.9-3.2 mum. The rate constant, R, for the exponential (later) component of the tension decay decreased with increasing fibre length and this was accompanied by a reduction in the mean amplitude of sarcomere shortening [Formula: see text] and lengthening [Formula: see text]. However, the time interval during which sarcomeres shortened or elongated increased, and so the mean velocities of sarcomere length changes ([Formula: see text] and [Formula: see text]) also displayed an inverse dependence upon fibre length.5. The effects of altering temperature (0-20 degrees C) on the time course of the changes in sarcomere length and tension were investigated. The time to onset of the length changes, t(1), the time to maximum amplitude, t(2), and the time to full recovery, t(3), all decreased exponentially with increasing temperature, but to differing extents. Temperature co-efficients for the velocity of sarcomere length changes during the interval t(2) - t(1) and for the fast tension decay were 2.47 ([Formula: see text]), 2.54 ([Formula: see text]) and 2.45 (R). The rate of the slow tension decay (before the tension ;shoulder') also increased with temperature, with a Q(10) of 2.43.6. The complex patterns of sarcomere length changes seen during relaxation appear to be due to variations in the duration of mechanical activity in different fibre segments. A model is presented which shows that the characteristic form of the tension decay can be accounted for by the progressive emergence of local inequalities in the relative strengths of adjacent sarcomeres, as relaxation proceeds.

Animals↗