We examined the dependence of force on CSA in our skinned fibres to see whether force per CSA is greater for small than large fibres, as has been reported for skinned fibres from humans Gilliver et al. The slopes were significantly different for the two muscles. Again the slopes were different and the residual plot Fig. Dependence of force and power on size of skinned fibres.
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D Residual scatter about the lines in C versus fibre volume. The comparison between intact and skinned force and power data is shown in Fig. The skinned fibres and intact fibre bundles are very different in size, so the values are plotted on a log—log scale to ensure that the individual points are visible. In each figure, the blue line was fitted through the results for skinned fibres from both ED-V and PL muscles and can be compared with the red line for the intact fibre bundles from ED-V and PL.
In both graphs, the two lines are almost continuous with each other. The constants of proportionality for the skinned and for intact fibres given in the figure legend are not significantly different. Dependence of force and power on size of skinned fibres and intact fibre bundles. A Isometric force as a function of cross-sectional area CSA of skinned fibres blue and intact fibre bundles red.
The data are plotted on log scales. B Power versus volume. Colours and symbols as in A. The isometric force and maximum power are not significantly different between skinned and intact fibre preparations. However, the force at which maximum power is produced is significantly greater in intact fibre bundles than in skinned fibres. Also the force intercept of the power versus force curve is significantly greater for intact fibre bundles than for the skinned fibres.
The main aim of this study was to compare the mechanical performance of intact fibres and skinned fibres under as similar conditions as possible. Maximum power output and isometric force were chosen because they are aspects of mechanical performance particularly relevant to muscle function in vivo and whole animal performance. Thus, the results can be taken as mainly representative of the population of fast-twitch fibres.
We found that maximum isometric force produced by intact and skinned fibres is the same within the limits of experimental errors. Here, we report one such other method: the slope of the force versus CSA relationship Fig.
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This method gives a value for the group of force and CSA values that is different from the mean of the ratios for the individual preparations. However, it is worth noting that the accuracy of such comparisons of power between intact and skinned fibres is limited by a number of factors. First, the methods used to estimate the amount of contractile material is different for intact and skinned preparations.
For intact fibre bundles, volumes were obtained from the masses of the blotted muscle and the density of muscle Mendez and Keys, Whereas, for skinned fibre segments the volumes were obtained from measurements under the microscope of width, depth and length. Second, each of these methods is an imperfect estimate of the amount of myofibrillar material.
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The volume of an intact fibre bundle includes extracellular space. Also damaged and non-excitable muscle fibres in the bundle add volume but do not produce power. Fibres swell during skinning and so have a volume greater after skinning than before. The amount of swelling is highly variable Godt and Maughan, A better comparison between intact and skinned preparations might be achieved based on the mass of the contractile proteins. A possible method is to measure dry mass of the fibres or fibre segment after fixation in alcohol and washout of salts and use this as an estimate of mass of contractile proteins.
We have used this method successfully on large single skinned fibres from dogfish West et al. However, the CSA of rabbit fibres is much smaller and we only used a short segment of fibre. Another possible method is SDS-PAGE with stain-free technology, which may provide accurate quantification of fibre protein content and thus the size of skinned fibre segments.
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For the purpose of comparing maximum power output of skinned and intact fibres, we have also used power expressed in relative units, which is less subject to the problem of measuring the amount of contracting myofibrillar material. The relevant point here is that relative power does not require measurement of fibre CSA or bundle volume, but instead uses maximum isometric force as measure of the amount of contracting myofibrillar material per CSA.
CV for skinned fibres was 0. Other comparisons of intact and skinned fibre performance can be made on the basis of literature values. This agreement contrasts with results obtained at lower temperatures, where skinned fibres maintain their performance well enough that reliable measurements of maximum power can be made. Comparisons of published data show that intact fibre preparations consistently produce more power than skinned fibres from the same species.
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Similar large differences are seen also in mammalian muscle at low temperature. This prompts the question of how the powers compare at even higher temperatures. In contrast, skinned fibre performance declines rapidly with repeated activation, which prevents reliable measurements of maximum power Bottinelli et al. We speculate that the factor is lost, or the mechanism is inactivated in skinned fibres, thus compromising their ability to produce force and power at these higher temperatures. However, it is important to recognise that the in vivo behaviour of muscle cannot be duplicated in skinned fibre experiments, at least with current methods.
In particular, it is not possible to rapidly turn off contraction of skinned fibres. Intact fibres have a definite advantage in this context because at the end of electrical stimulation action potentials cease and contraction relaxes, as in vivo. Consequently, the normal cycles of length change with intermittent contraction that occur during most locomotion can be duplicated with intact, but not with skinned fibres.
Eighteen rabbits were used 9 male and 9 female. Their mass ranged from 0. Successful experiments were done on bundles of intact fibre from 11 rabbits. Skinned fibres were prepared from 16 rabbits.
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The ED-V and PL muscles were usually dissected from both legs and then a part of each muscle was prepared for experiments. Fibre bundles [average length A fragment of tendon at each end of the fibre bundle was held in an aluminium foil clip. Supra-maximal stimulus strength V , 1. L o is the fibre length at which maximum isometric active force is produced.
Passive force i. At the end of the experiment, the fibre bundle was fixed in alcohol while pinned at L o so that fibre length could be measured. Tendons were removed and the preparation was then blotted and weighed. Preparation volume was calculated from the blotted mass assuming a density of 1. BDM is used to prevent contraction which can occur during skinning. We found part way through this set of experiments that such contractions do not occur in the absence of BDM under the conditions we used and therefore we discontinued using BDM.
Exposure to BDM did not affect mechanical properties: maximum values of isometric force, power and shortening velocity were not significantly different between the groups 96 fibres with BDM, 45 fibres no BDM. Constituents of the solution for preparing skinned fibres. Individual fibres were transferred to solution A, containing 0. The A apparatus was positioned on the x , y moving stage of an inverted microscope. L o , the standard length, is the fibre segment length at which SL is 2. Fibre depth and width were measured using the B camera software. The fibre depth was visualised by using a prism built in the bath of the A apparatus.
Each value of depth and width was recorded as the mean of three separate measurements along the fibre length. CSA was calculated from the fibre width and depth assuming an elliptical cross-section.
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Skinned fibre CSA was correlated with the mass of the rabbits. We did not make a correction to the CSA for swelling that may have occurred as a result of skinning for the following reasons.
The variability is likely to be dependent on factors such as sarcomere length Matsubara and Elliott, and solution ionic strength West et al. Constituents of pre-activating, activating and relaxing solutions for skinned fibre experiments. The shortening velocity during the force-clamp was measured and used to calculate power output.
To accumulate a data series with a range of shortening velocities, the T-jump activation was repeated on each fibre between 5 and 8 times, each time with force clamped to a different level. Some of the intact fibre bundles and some skinned fibres produced low isometric forces, probably because of damage during dissection and mounting. Out of 26 intact fibre bundles, 10 were rejected and from skinned fibres, 2 were rejected.
In skinned fibre experiments the isometric force in successive activations decreased at an average rate of 2. Force values in mN were divided by the corresponding isometric force F IM. For examining the dependence of isometric force on CSA and of power on volume, we used the Model 2 regression method York et al. Note that is not the same as the isometric force Edman et al. Woledge who died 13 March This Issue. Download PDF. NOTE: We only request your email address so that the person you are recommending the page to knows that you wanted them to see it, and that it is not junk mail.
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