Introductory RemarksAuthor(s): A. F. HuxleySource: Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 160, No.981, A Discussion on the Physical and Chemical Basis of Muscular Contraction (Oct. 27, 1964),pp. 434-437Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/75234 .

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A. F. Huxley (Discussion Meeting) A. F. Huxley (Discussion Meeting)

P. C. CALDWELL Calcium and the contraction of Maia muscle fibres 512

Discussion. A. V. Hill 516

DOROTHY M. NEEDHAM, F.R.S. Proteins of the contractile mechanism of maim- AND CATHERINE F. SHOENBERG malian smooth muscle and their possible

location in the cell 517 Discussion. Jean Hanson and J. Lowy,

Annemarie Weber 523

J. LowY, B. M. MILLMAN Structure and function in smooth tonic muscles AND JEAN HANSON of lamellibranch molluscs 525

J. C. REEGoG Tropomyosin-paramyosin system and 'pro- longed contraction' in a molluscan smooth muscle 536

Introductory remarks

BY A. F. HUXLEY, F.R.S.

Department of Physiology, University College London

When Hugh Huxley and I, as organizers of this meeting, came to choose a title for it, we found it difficult to avoid repeating word for word the title of the Discussion Meeting (Hill I949) that was arranged by the Royal Society in July 1949 under the leadership of A. V. Hill. In the end, we altered the wording a little, but the

scope of the present meeting is very much the same as that of its predecessor. The continuity of work in this field is also emphasized by several of the names of people taking part. A. V. Hill is again present, and his son D. K. Hill will be reading a paper. H. H. Weber, who followed A. V. Hill at the 1949 meeting, has unfortunately been prevented from coming, but he is represented by his daughter Annemarie Weber and his pupil Hasselbach. Dorothy Needham read a paper in 1949, and will be doing so again at this meeting. Two others who we might have hoped would take part again were Astbury, who died two and a half years ago, and Kenneth Bailey, who died earlier this year.

The main problems discussed at the 1949 meeting were: first, the contractile proteins and their arrangement in the fibril; second, the identity of the primary energy-yielding reaction; and third, the nature of the link from excitation in the membrane to activation of the contractile material. It so happens that major advances in each of these three topics have been made in the last few months and we are fortunate to have as speakers several who have taken leading parts in these advances. The meeting has been confined to these topics because of limitation of time; for example, important fields such as membrane phenomena, and the

enzymology of the contractile proteins, have been completely omitted. As regards the contractile proteins, it seems that all the speakers at the 1949

meeting assumed that muscle contained continuous filaments which shortened by folding or coiling. There was no suggestion of a sliding-filament idea, or even of the idea, widely current in the nineteenth century, that the A bands contained

P. C. CALDWELL Calcium and the contraction of Maia muscle fibres 512

Discussion. A. V. Hill 516

DOROTHY M. NEEDHAM, F.R.S. Proteins of the contractile mechanism of maim- AND CATHERINE F. SHOENBERG malian smooth muscle and their possible

location in the cell 517 Discussion. Jean Hanson and J. Lowy,

Annemarie Weber 523

J. LowY, B. M. MILLMAN Structure and function in smooth tonic muscles AND JEAN HANSON of lamellibranch molluscs 525

J. C. REEGoG Tropomyosin-paramyosin system and 'pro- longed contraction' in a molluscan smooth muscle 536

Introductory remarks

BY A. F. HUXLEY, F.R.S.

Department of Physiology, University College London

When Hugh Huxley and I, as organizers of this meeting, came to choose a title for it, we found it difficult to avoid repeating word for word the title of the Discussion Meeting (Hill I949) that was arranged by the Royal Society in July 1949 under the leadership of A. V. Hill. In the end, we altered the wording a little, but the

scope of the present meeting is very much the same as that of its predecessor. The continuity of work in this field is also emphasized by several of the names of people taking part. A. V. Hill is again present, and his son D. K. Hill will be reading a paper. H. H. Weber, who followed A. V. Hill at the 1949 meeting, has unfortunately been prevented from coming, but he is represented by his daughter Annemarie Weber and his pupil Hasselbach. Dorothy Needham read a paper in 1949, and will be doing so again at this meeting. Two others who we might have hoped would take part again were Astbury, who died two and a half years ago, and Kenneth Bailey, who died earlier this year.

The main problems discussed at the 1949 meeting were: first, the contractile proteins and their arrangement in the fibril; second, the identity of the primary energy-yielding reaction; and third, the nature of the link from excitation in the membrane to activation of the contractile material. It so happens that major advances in each of these three topics have been made in the last few months and we are fortunate to have as speakers several who have taken leading parts in these advances. The meeting has been confined to these topics because of limitation of time; for example, important fields such as membrane phenomena, and the

enzymology of the contractile proteins, have been completely omitted. As regards the contractile proteins, it seems that all the speakers at the 1949

meeting assumed that muscle contained continuous filaments which shortened by folding or coiling. There was no suggestion of a sliding-filament idea, or even of the idea, widely current in the nineteenth century, that the A bands contained

434 434

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Introduction

rods of fixed length. Indeed, the early electron micrographs of fragmented muscle are quoted as showing 'that the finest resolvable myofibrils run straight on through all the bands and still remain taut after contraction'. The arrangement of myosin and actin molecules in the fibril was actively discussed, but without much positive result. In contrast with the unexpected progress that has been made in this

respect, two speakers suggested that it was at least worth trying to obtain X-ray diffraction pictures from a muscle undergoing a twitch, but I do not believe that this experiment has yet been attempted.

As regards the identification of the immediate energy-giving reaction, A. V. Hill in 1949 challenged biochemists to detect the splitting of A TP in a twitch which was

postulated. Dorothy Needham suggested a spectrophotometric method for

attempting this, while Kenneth Bailey stated that he had attempted the experi- ment which R. E. Davies has now successfully carried out, namely, poisoning a muscle so that resynthesis of ATP would be completely prevented. Bailey's attempt was unsuccessful because the poisons which achieved this result on muscle mince did not penetrate the intact muscle fibre.

As regards activation of the contractile material, the position in 1949 was that it was clearly recognized that there was a problem, in that A. V. Hill had shown that the whole cross-section of each fibre became active much quicker than could be accounted for by inward diffusion of some activating agent released at the surface by the action potential. No contributions towards the solution of this

problem were made at that meeting, although there were already strong indications that the final activation of actomyosin might be due to calcium ions. Heilbrunn

(1943) had proposed this before the war, and had shown (Heilbrunn & Wiercinski

1947) that solutions of Ca salts would cause contraction when introduced inside a fibre by microinjection, and Kenneth Bailey (1942), on the basis of measurements of the effects of various ions on myosin ATPase, had proposed that stimulation of the contractile material 'is connected with the availability of activating Ca ions to the myosin adenosinetriphosphatase fibrillar surface'. 'It is legitimate to

assume', he wrote, 'that the living cell in the resting state can provide a mechanism for the separation of enzyme and activator, until they are brought together as a result of excitation.'

Many of the advances that have been made since 1949 have been due to the renewed interest in structure that has developed in many fields of biology. This has been largely due to the electron microscope, though Zernike's phase-contrast microscope, and the interference microscopes that followed it, also played a substantial part. Very little attention was paid by physiologists and biochemists to the microscopical structure of muscle during the 'twenties and 'thirties, but three people in particular come to mind as having kept the torch burning and

anticipated some of the important results of the post-war years. Two of them, Fritz Buchthal and G. M. Frank, are at this meeting, the other, H. H. Weber, as I have already mentioned, was prevented from coming. Weber (I935; Noll & Weber

1935) showed that the birefringence of muscle is due to the 'myosin' that it con-

tains, and concluded that therefore the myosin was located in the A bands. Buchthal has consistently pursued the optical and microscopical study of isolated

28-2

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A. F. Huxley (Discussion Meeting)

living muscle fibres. And G. M. Frank in 1927 demonstrated the constancy of A band width in frog muscle fixed during tetanic contractions at various lengths.

The only experimental result I would like to present now is a re-determination of the isometric length-tension diagram of isolated fibres from the semitendinosus muscle of the frog that has been made by A. M. Gordon, F. J. Julian and myself. The chief respects in which our method differed from that used by Ramsey & Street (1940) in their well-known experiments are: (1) that we measured the striation spacing directly by photographing the fibre with a high-power microscope; and (2) that we used a device which eliminates the complications due to the fact that fibres do not stretch uniformly when they are passively extended, but come to have a much smaller striation spacing near their ends than over the main part

1.0-

1 08-

0'

0.2- 0 J6-

/

J1 2 3 4 sarcomere length (/~m)

FIGURE 1. Tension developed, during isometric tetani at various sarcomere lengths, by a uniform region in the middle of an isolated fibre from frog striated muscle. Composite diagram showing results of separate series of experiments covering different parts of the range.

of their length (Huxley & Peachey I96I). This device (Gordon, Huxley & Julian

I963) uses a photoelectric servo to measure continuously the separation between two pieces of gold leaf stuck to the fibre, followed by an electromechanical servo

pulling on one tendon of the fibre which keeps this separation constant. The markers are stuck to the fibre at the ends of a part of its length within which the striation spacing has been found by microscopic observation to be practically uniform; for example, the two markers might be one-quarter and three-quarters of the length of the fibre from one of its ends.

The results are summarized in figure 1. The relationship is seen to consist of

segments which are practically straight, separated by angles which are as nearly sharp as can be expected in view of the residual non-uniformities in striation

spacing and other errors. The positions of the angles may be interpreted in terms of the values for the dimensions of various constituents of the fibril which will be

presented in the contributions by H. E. Huxley and Sally Page. On this basis we

may take the length of the thick filaments as 1*60 um, length of thin filament

(including Z lines) as 2-05 Mm, length of region without bridges at centre of thick filaments as 0.15 um, and thickness of Z line as 0-05 /m. The diagram shows:

(i) that tension falls to zero at a sarcomere length close to 3-65 [m where overlap

436

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between thick and thin filaments is expected to cease; (ii) that a plateau of tension exists below 2-20 /m which is the sarcomere length (2-05 + 0415 [m) at which each thin filament just overlaps all the bridges at its own end of the thick filament; (iii) the plateau ends at about 2.0 Km, which may be related either to the length (2*05 [im) at which thin filaments meet at the middle of the sarcomere, or to the length (1-90 /m = 2-05 - 0-15 m) where each thin filament begins to overlap the bridges at the far end of the thick filament, or both; and (iv) the decline of tension with shortening becomes much steeper at a sarcomere length close to 1.65 jum, the length at which the ends of thick filaments come into contact with the Z lines. The speed of shortening under a light load also begins to drop markedly when the sarcomere length reaches about 1.65 Mm. We may conclude that, at striation spacings above 2-0 Mm, the isometric tension is proportional to the number of bridges overlapped by each thin filament, and that the drop of tension at lengths below 2-0 /um is largely due to the structural events which cause the appearance of 'contraction bands' at the position of the M- and Z-lines.

REFERENCES (A. F. Huxley) Bailey, K. I942 Biochem. J. 36, 121. Frank, G. I927 Pfiig. Arch. ges. Physiol. 218, 37. Gordon, A. M., Huxley, A. F. & Julian, F. J. i963 J. Physiol. 167, 42P. Heilbrunn, L. V. I943 Outline of general physiology, 2nd ed., ch. 37. Philadelphia: Saunders. Heilbrunn, L. V. & Wiercinski, F. J. I947 J. Cell. Comp. Physiol. 29, 15. Hill, A. V. I949 Proc. Roy. Soc. B, 137, 40. Huxley, A. F. & Peachey, L. D. I96I J. Physiol. 156, 150. Noll, D. & Weber, H. H. I935 Pfiig. Arch. ges. Physiol. 235, 234. Ramsey, R. W. & Street, S. F. 1940 J. Cell. Comp. Physiol. 15, 11. Weber, H. H. 1935 Pfiig. Arch. ges. Physiol. 235, 205.

The contractile proteins of skeletal muscle

BY A. J. ROWE

Medical Research Council Laboratory of Molecular Biology, Cambridge

[Plates 74 to 77]

The study of the molecular size and shape of the fibrous proteins isolated from skeletal muscle has an interest in its own right: but in addition it has a particular importance in that these proteins are believed to provide the ultimate molecular 'building blocks' for the contractile system of living muscle. Myosin, actin and tropomyosin ('water-soluble tropomyosin', or 'tropomyosin B') together comprise almost all the fraction of soluble, fibrous proteins isolated from muscle. Whilst it

between thick and thin filaments is expected to cease; (ii) that a plateau of tension exists below 2-20 /m which is the sarcomere length (2-05 + 0415 [m) at which each thin filament just overlaps all the bridges at its own end of the thick filament; (iii) the plateau ends at about 2.0 Km, which may be related either to the length (2*05 [im) at which thin filaments meet at the middle of the sarcomere, or to the length (1-90 /m = 2-05 - 0-15 m) where each thin filament begins to overlap the bridges at the far end of the thick filament, or both; and (iv) the decline of tension with shortening becomes much steeper at a sarcomere length close to 1.65 jum, the length at which the ends of thick filaments come into contact with the Z lines. The speed of shortening under a light load also begins to drop markedly when the sarcomere length reaches about 1.65 Mm. We may conclude that, at striation spacings above 2-0 Mm, the isometric tension is proportional to the number of bridges overlapped by each thin filament, and that the drop of tension at lengths below 2-0 /um is largely due to the structural events which cause the appearance of 'contraction bands' at the position of the M- and Z-lines.

REFERENCES (A. F. Huxley) Bailey, K. I942 Biochem. J. 36, 121. Frank, G. I927 Pfiig. Arch. ges. Physiol. 218, 37. Gordon, A. M., Huxley, A. F. & Julian, F. J. i963 J. Physiol. 167, 42P. Heilbrunn, L. V. I943 Outline of general physiology, 2nd ed., ch. 37. Philadelphia: Saunders. Heilbrunn, L. V. & Wiercinski, F. J. I947 J. Cell. Comp. Physiol. 29, 15. Hill, A. V. I949 Proc. Roy. Soc. B, 137, 40. Huxley, A. F. & Peachey, L. D. I96I J. Physiol. 156, 150. Noll, D. & Weber, H. H. I935 Pfiig. Arch. ges. Physiol. 235, 234. Ramsey, R. W. & Street, S. F. 1940 J. Cell. Comp. Physiol. 15, 11. Weber, H. H. 1935 Pfiig. Arch. ges. Physiol. 235, 205.

The contractile proteins of skeletal muscle

BY A. J. ROWE

Medical Research Council Laboratory of Molecular Biology, Cambridge

[Plates 74 to 77]

The study of the molecular size and shape of the fibrous proteins isolated from skeletal muscle has an interest in its own right: but in addition it has a particular importance in that these proteins are believed to provide the ultimate molecular 'building blocks' for the contractile system of living muscle. Myosin, actin and tropomyosin ('water-soluble tropomyosin', or 'tropomyosin B') together comprise almost all the fraction of soluble, fibrous proteins isolated from muscle. Whilst it

Introduction Introduction 437 437

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