Molecular and Cellular Aspects of Muscle Function. Proceedings of the 28th International Congress of Physiological Sciences Budapest 1980, (Including the Proceedings of the Satellite Symposium on Membrane Control of Skeletal Muscle Function)

414 Pages·1981·22.907 MB·English

ADVANCES IN PHYSIOLOGICAL SCIENCES
Proceedings of the 28th International Congress of Physiological Sciences
Budapest 1980
Volumes
1 - Regulatory Functions of the CNS. Principles of Motion and Organization
2 - Regulatory Functions of the CNS. Subsystems
3 - Physiology of Non-excitable Cells
4 - Physiology of Excitable Membranes
5 - Molecular and Cellular Aspects of Muscle Function
6 - Genetics, Structure and Function of Blood Cells
7 - Cardiovascular Physiology. Microcirculation and Capillary Exchange
8 - Cardiovascular Physiology. Heart, Peripheral Circulation and Methodology
9 - Cardiovascular Physiology. Neural Control Mechanisms
10 - Respiration
11 - Kidney and Body Fluids
12 - Nutrition, Digestion, Metabolism
13 - Endocrinology, Neuroendocrine logy, Neuropeptides - I
14 - Endocrinology, Neuroendocrinology, Neuropeptides - II
15 - Reproduction and Development
16 - Sensory Functions
17 - Brain and Behaviour
18 - Environmental Physiology
19 - Gravitational Physiology
20 - Advances in Animal and Comparative Physiology
21 - History of Physiology
Satellite symposia of the 28th International Congress of Physiological Sciences
22 - Neurotransmitters in Invertebrates
23 - Neurobiology of Invertebrates
24 - Mechanism of Muscle Adaptation to Functional Requirements
25 - Oxygen Transport to Tissue
26 - Homeostasis in Injury and Shock
27 - Factors Influencing Adrenergic Mechanisms in the Heart
28 - Saliva and Salivation
29 - Gastrointestinal Defence Mechanisms
30 - Neural Communications and Control
31 - Sensory Physiology of Aquatic Lower Vertebrates
32 - Contributions to Thermal Physiology
33 - Recent Advances of Avian Endocrinology
34 - Mathematical and Computational Methods in Physiology
35 - Hormones, Lipoproteins and Atherosclerosis
36 - Cellular Analogues of Conditioning and Neural Plasticity
(Each volume is available separately.)
ADVANCES IN
PHYSIOLOGICAL SCIENCES
Proceedings of the 28th International Congress of Physiological Sciences
Budapest 1980
(including the proceedings of the satellite symposium on Membrane Control
of Skeletal Muscle Function)
Volume 5
Molecular and Cellular Aspects
of Muscle Function
Editors
E. Varga
A. Köver
T. Kovâcs
L. Kovâcs
Debrecen, Hungary
PERGAMON PRESS AKADEMIAI KIADO
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Copyright © Akadémiai Kiado, Budapest 1981
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British Library Cataloguing in Publication Data
International Congress of Physiological Sciences.
(28th : 1980 : Budapest)
Advances in physiological sciences.
Vol. 5 : Molecular and cellular aspects of muscle function.
1. Physiology - Congresses
I. Title II. Varga, E.
591.1 QP1 80-42101
Pergamon Press ISBN 0 08 026407 7 (Series)
ISBN 0 08 026817 X (Volume)
Akadémiai Kiado ISBN 963 05 2691 3 (Series)
ISBN 963 05 2731 6 (Volume)
In order to make this volume available as economically and as
rapidly as possible the authors' typescripts have been reproduced in
their original forms. This method unfortunately has its typographi-
cal limitations but it is hoped that they in no way distract the reader.
Printed in Hungary
FOREWORD
This volume is one of the series published by Akadémiai Kiado, the
Publishing House of the Hungarian Academy of Sciences in coédition with
Pergamon Press, containing the proceedings of the symposia of the 28th
International Congress of Physiology held in Budapest between 13 and 19
July, 1980. In view of the diversity of the material and the "taxonomic"
difficulties encountered whenever an attempt is made to put the various
subdisciplines and major themes of modern physiology into the semblance of
some systematic order, the organizers of the Congress had to settle for 14
sections and for 127 symposia, with a considerable number of free communi-
cations presented either orally or as posters.
The Congress could boast of an unusually bright galaxy of top names
among the invited lecturers and participants and, naturally, the ideal would
have been to include all the invited lectures and symposia papers into the vol-
umes. We are most grateful for all the material received and truly regret that
a fraction of the manuscripts were not submitted in time. We were forced
to set rigid deadlines, and top priority was given to speedy publication even
at the price of sacrifices and compromises. It will be for the readers to judge
whether or not such an editorial policy is justifiable, for we strongly believe
that the value of congress proceedings declines proportionally with the gap
between the time of the meeting and the date of publication. For the same
reason, instead of giving exact transcriptions of the discussions, we had to
rely on the introductions of the Symposia Chairmen who knew the material
beforehand and on their concluding remarks summing up the highlights of
the discussions.
Evidently, such publications cannot and should not be compared with
papers that have gone through the ordinary scrupulous editorial process of
the international periodicals with their strict reviewing policy and high
rejection rates or suggestions for major changes. However, it may be refresh-
ing to read these more spontaneous presentations written without having to
watch the "shibboleths" of the scientific establishment.
September 1, 1980 J. Szentâgothai
President of the
Hungarian Academy of Sciences
ν
PREFACE
Though the programme of the International Congress of Physiological
Sciences was available to members during the congress, it contained, as a
matter of course, a rather heavily summarized set of information.
There is, as we can feel, a general demand to have the papers of the
invited lecturers published, and in order to satisfy this, we included the
lectures by the invited lecturers at the Muscle Section and at the 5 congress
symposia as well as the concluding remarks by the chairmen at each session.
This volume also contains the papers delivered at the satellite sym-
posium on 'Membrane Control of Skeletal Muscle Function' held in Debre-
cen before the ICPS.
The editors would like to express their sincere gratitude to the authors
for having contributed their precious articles to this publication.
The Editors
xiii
Adv. Physiol. Sei. Vol. 5. Molecular and Cellular Aspects of Muscle Function
E. Varga, A. Kovér, T. Kovécs, L. Kovàcs (eds)
THE MECHANICAL PROPERTIES OF
CROSS-BRIDGES AND THEIR RELATION TO
MUSCLE CONTRACTION
A. F. Huxley
Department of Physiology, University College London, Gower Street, London WC1E 6BT., England
It is widely (though not universally) supposed that force or shortening
in striated muscle is generated by cross-bridges acting more or less
independently of one another within each overlap zone. Evidence for this
proposition has been recently reviewed (A.F.Huxley, I98O) and will only be
summarised here. The principal points, established on isolated twitch
muscle fibres from the frog at sarcomere lengths from 2.0 μπι upwards
(Gordon, Huxley & Julian, 1966), are (i) that isometric tension is
directly proportional to the number of cross-bridges overlapped by thin
filament, and (ii) that the speed of unloaded shortening is independent
of overlap. The first of these propositions has been contested recently
by ter Keurs, Iwazumi & Pollack (1978), the point at issue being whether
it is appropriate to take the tension reached in the initial rapid tension
rise or the substantially higher tension reached after the "creep" phase
which occurs in isometric tetani at sarcomere lengths greater than that at
which all cross-bridges are overlapped. Gordon et al (1966) took the
former of these values, and disregarded the creep, attributing it to
progressive development of non-uniformity of overlap in different half-
sarcomeres. This interpretation was first proposed by A.V.Hill (1953)
who pointed out that the negative slope of this part of the ^Length-tension
relation implied an instability since any region with, less than average
sarcomere length would be capable of producing more force and would
therefore shorten, increasing its ability to generate tension still
further, while stretching other parts of the fibre. Th'is process certainly
occurs in more than one form: (a) end regions of the fibre, where- the
sarcomeres are commonly shorter than in the middle of the fibre, may
shorten greatly (Huxley & Peachey, 196l), and (b) regions of extreme,
irregular shortening may develop slowly in other parts of the fibre
(Huxley & Peachey, 19^1; Julian, Sollins & Moss, 1978). Reasons for
believing that this instability is the origin of the creep are (a) that
Gordon et al (1966) found that the creep is reduced if, instead of holding
the tendon stationary, the length of a selected region of the fibre is held
constant by a servo system, and (b) that there is no creep in a tetanus
performed at a sarcomere length of about 2.1 μιη (frog muscle) where the
length-tension relation is flat (Ramsey & Street, 19^0; Gordon et al.1966).
ter Keurs et al. (1978) have recorded sarcomere length during tetani with
creep by scanning a laser beam along the fibre and recording the deflection
and width of the first-order diffracted beam; they find an increasing
scatter of sarcomere length but not enough to account for the increase
tension on the basis of proportionality to overlap. In the opinion of the
1
lecturer, however, this does not exclude non-uniformity of overlap as the
cause of the creep because there are many forms of non-uniformity which
would not show up as change of position or width of the diffracted beams,
e.g. (a) shortening in a region obscured by tendon at the fibre end,
(b) regions of gross irregularity smaller than the beam diameter, (c)
random variation of lengths of sarcomeres (this would introduce back-
ground scatter without broadening the diffracted beams), (d) displacement
of thick filaments relative to the mid-point between successive Ζ lines.
Clearly, however, the matter needs investigation by electron microscopy
after rapid fixation (work on these lines is in progress by Brown & Hill
(1978)) and by measurement of the tension rise produced by a given rate
of stretch.
The evidence that each cross-bridge acts cyclically within a single
contraction, undergoing cycles of attachment, exertion of force, and
detachment, has also been reviewed by Huxley (I98O). This proposition
will be assumed in this lecture to be true.
Mechanical properties of cross-bridges
Estimates of the mechanical properties of the cross-bridges can be
obtained by applying a sudden change of either load or length during the
plateau of an isometric tetanus. The earliest experiments with adequate
time resolution to show the complex phenomena which occur were those of
Podolsky (i960) and Civan & Podolsky (1966), in which shortening against
a preset load was suddenly permitted and the length changes were recorded.
We have repeated this type of experiment and obtained generally similar
results but we have concentrated on the converse type of experiment in
which a sudden length change is imposed and the time course of tension
is recorded. Typical records are shown in Fig.l. The later stages are
attributable to detachment and re-attachment of cross-bridges; I shall
not say more about them in this lecture but will concentrate on the
initial tension change (phase l) and the early tension recovery (phase 2).
Phase 1 consists of a change of recorded tension occurring more or
less simultaneously with the applied length change, and therefore
resembles the response of a pure compliance. Detailed analysis of the
records shows in fact (Ford et al. 1977, 1980) that the time course of
tension change during a shortening step is accounted for with great
precision (discrepancies of a very few microseconds for a given tension
level) on the assumption of an ideal compliance which obeys Hooke's law,
when allowance is made for (a) "early tension recovery" beginning before
the step is complete, (b) frequency response of the force transducer,
(c) effect of inertia of the fibre, and (d) effect of viscosity and
inertia of the surrounding solution. (First-order effects of tendon
compliance were eliminated in these experiments by the use of the
"spot-follower" to signal the length of a selected region of the fibre).
The value of the compliance is such that instantaneous shortening of a
half-sarcomere by k nm would bring tension from its isometric level to
zero. In quick stretches, however, there are small but systematic
discrepancies which suggest that in this range the compliance element
does not behave linearly but becomes stiffer the greater the stretch
(Ford et al. 198Ο).
2
0-2 ms
Fig.l. Records of tension change in response to sudden
shortening by 6 nm per half-sarcomere during an otherwise isometric
contraction. Upper panel: recorded at slow speed on an ultraviolet
recorder. Phase 1 (initial drop of tension nearly to zero) is too
rapid to be recorded. Phase 2 (early tension recovery) brings the
tension back to about 80$ of the isometric value. Phase 3 is the
slight drop of tension lasting approx. 7 ms. Phase *f is the final
asymptotic approach to a tension close to the original value, and
is nearly complete at the end of the record. Lower panel (a different
contraction in the same experiment): lower trace, tension, and upper
trace, change of length between two markers attached to the fibre.
Fast cathode-ray tube record to show phases 1 and 2. Isolated fibre
from m. tibialis anterior of frog; + 2.5 °C.
The next question is, where in the fibre structure does this compliance
reside - how much is in the cross-bridges, the thick filaments, the thin
filaments, or the Ζ line? In our first note on these transients (Huxley
& Simmons, 1970) we suggested that it was in the filaments, the tension
recovery which follows being attributable to action of the cross-bridges.
We soon showed, however (Huxley & Simmons, 1971a), that when the overlap
was reduced to about J>0& by working at an increased sarcomere length, the
amplitude of the tension change, for a given length change, was reduced
in the same proportion. This is what is to be expected if all the
compliance resided in the cross-bridges while the filaments and Ζ line
were perfectly rigid, and we concluded that this is the case. However,
stiffness would fall with increase of initial sarcomere length even if
the cross-bridges were rigid (for a rapid step) and the compliance
resided in the filaments, because the thick and thin filaments would be
locked together in parallel wherever they overlap and as sarcomere length
is increased the amount of overlap decreases so that progressively more
of^ the lengths of the filaments are in series giving increased length
change for a given tension change, i.e. reduced stiffness.
3
Theoretical curves of stiffness against overlap for different assumptions
about relative stiffness of cross-bridges and filaments fall fairly close
to one another and may intersect at intermediate degrees of stretch, so
that it is possible to estimate cross-bridge compliance from measured
fibre compliance at different sarcomere lengths only if (a) measurements
are made at three or more sarcomere lengths and (b) high precision is
achieved. The latter is difficult because (a) inertial effects are
relatively greater the longer the sarcomere length and (b) at long
sarcomere length, allowance has to be made for the passive stiffness of
the fibre. Appropriate records were taken during the experiments
described by Ford et al. (1977)» and have now been analysed (Ford et al.
19δθ) by comparing the recorded tension with the output of an analogue
circuit consisting of a ladder network in which inertia was represented
by inductances, viscosities by resistors, and compliances by capacitors
shunted by resistors to mimic the "early tension recovery" or the stress
relaxation in the passive stiffness of the fibre. An attempt to analyse
the responses to quick stretch in this way led to a conclusion (mentioned
informally at scientific meetings but not published) that cross-bridge
compliance accounted for only about 50$ of the total compliance measured
in fibres at a sarcomere length where there is complete overlap, the
remainder being due to filament compliance. However, as already indicated,
the fit was not satisfactory, the discrepancies suggesting that the
compliance did not obey Hooke's law. The responses to quick shortening,
however, were very well matched on the assumption that filaments and
Z-line were almost completely rigid and that the properties of individual
cross-bridges are identical at all sarcomere lengths (Ford et al. I98O).
There must of course be some compliance in the filament structure, but we
estimate that the cross-bridge compliance is at least 80$ and probably 90$
of the value calculated on the assumption of complete rigidity in the
filaments.
The smallness of the effect of thin-filament compliance has been
confirmed in recent experiments (unpublished) in collaboration with
Dr.V.Lombardi and Dr.L.D.Peachey. The tension change was recorded in
releases of the same magnitude made during tetani at initial sarcomere
lengths of 2.2 and 2.0 urn. At both of these lengths, all the myosin
heads are overlapped by thin filament, and to a first approximation :he
only difference is that at 2.2 μπι each half-sarcomere contains an
additional 0.1 μπι length of thin filament. The peak tension change was
I.5 - 2$ smaller at the longer sarcomere length, and this is about the
value to be expected if filament compliance (equal for thick and thin
filaments) accounts for 10$ of total observed compliance.
We may conclude that the cross-bridges (in frog twitch fibres at 0 °C)
contain a compliance which obeys Hooke's law in releases, is almost
completely undamped, and has a value such that instantaneous sliding
motion of thick relative to thin filaments by about 3-6 urn is sufficient
to bring tension from its isometric value to zero.
A further conclusion is that stiffness is a measure of the number of
attached cross-bridges per overlap zone. It must be emphasised, however,
that the relevant stiffness for this purpose is the instantaneous
stiffness of the fibre itself, and a "stiffness" measured as peak tension
change divided by length change per half-sarcomere may be a serious under-
estimate of the true value because of (a) tendon compliance, and (b)
"early tension recovery" beginning before the length change is complete;
4
further, the degree o'f this under-estimation will vary according to the
conditions (e.g. during steady shortening the fibre compliance is higher
(Julian & Sollins, 1975) so tendon compliance is relatively less important
but the early tension recovery is more rapid so the degree of truncation
of the tension response is greater).
Two types of explanation have been given for phase 2 of the transient,
the early tension recovery. Podolsky & Nolan (1973) have suggested that it
is due to attachment, with force generation, of cross-bridges which were
free during the isometric state and which are brought within range of a
thin-filament site by the sliding which occurs during the imposed shorten-
ing. My colleagues and I, on the contrary, have supposed (Huxley & Simmons
1971b) that this phase is due to changes occurring in cross-bridges that
are already attached. The latter idea is supported by the observations
that (a) the stiffness.is not increased at the end of this phase (Ford,
Huxley & Simmons, 197**) and (b) that stiffness is reduced during steady
shortening (Julian & Sollins, 1975)* whereas Podolsky's theory requires
the stiffness to be increased in both of these situations.
Another observation which suggests that the number of attached cross-
bridges is smaller during steady shortening than during the isometric state
is the "pull-out" (a period of extremely rapid elongation) which occurs
when a force equal to the isometric tension is suddenly re-applied during
steady shortening under a light load (Huxley, 1971i fig.6). This phenomenon
is presumably analogous to the "give" which occurs when a load approximate-
ly double the isometric value is applied during an isometric contraction
(Katz, 1939); both phenomena would be easily explained if a cross-bridge
detaches whenever the force on it exceeds a certain value, and the number
of attached cross-bridges is reduced during steady shortening. The aspect
of this phenomenon which is not so simple to explain is the fact that this
rapid pull-out does not continue indefinitely if the load is maintained but
slows down and stops; a possible explanation is suggested in a later para-
graph. . ,
A
-12 -9 -6 -3 • 3 nm
Step amplitude per half-sarcomere
Fig.2. Variation with step amplitude of two features of the
tension transient. T^, extreme tension reached, and tension
reached at end of early recovery phase, expressed as fractions
of the isometric tension Τ . Open symbols: step complete in
0.2 ms (as in Fig.l). Filled symbols: step complete in
1.0 ms; Τ is unchanged. Temperature +1 °C.
5