PROGRESS IN BIOPH Y SICS - Molecular and Cellular Biophysics

M 11S(11«1«1 8'I' IL I> ( 'I'1>1( l«l A N » ' I ' l l I«'(> IL I I«1 i (> V (,'(>N 'I' IL«L O'I' i» N

V. 1. 6. General discnssion of ectlychanf/es

The discussion in the last few sections has led to postulating three steps
in the activation process,apart,from membrane changes and the
inward spread along th e 7~ meinbranes. T h ese steps, which agree
r ather closely wit h t h ose postulated by SANnow ( 1947) in or der t o
account for the early tension changes, are:

(1) Some changes which allows the r( i actioii A X P --> A t- X + PO4
to takeplace (reaction 3,p. 288).

(2) The reaction AX I ' ~ A + X + PO4 itself ; t h i s r e a c t io » i s
accompanied b y t he l i b e r a t io n o f " ac t i v a t i on. h eat " a n d b y t h e
lengthening whose early stages show up as the latency relaxation.

(3) The formation of the links between actin and inyosin by which
tension or shortening is generated. The "alpha process" would corresponcl
probably only to the earlier part of this step: as more links are formed,
an increase of pressure would be progressively loss able to cause the
r elative motion b etween the tw o s ets of f i l aments on w h ich i t w a s
assumed. (p. 302) that the effect depends. The increase of torsional
rigidity would presumably also correspond to this step.

S tep (1} must p r o ceed wit h a t i m e c o n stant o f o n e o r t w o m i l l i -
seconds in frog muscle at O'C, since thc rate of step (2), Ls I»(lic(Lto<t by
heat production, reaches its maximum in a time of this order (Hn,i„
19494, 1963a).

The time constant of step (2) might bo sot equal to that of t)io early
fall in the heat rate (after shortening heat has been deducted); from
A. V. HDL,'s work (1949O) this appears:to be roughly 25 insec. This
may well be an overestimate since the activation heat may contain
an element corresponding to stop (8).

According to th e hypothesis developed on pp. 281 — 288, the rate
constant for stop (8) is (f - ~-r/). T)Iis v;L<i(:s;Lcc(>rding to thc relntiv(<
positions of th e r eactive sites; a » a v erage VL(iio for f/ was t alo;» oii

p. 296 IL>I 0'7 soc" , ILIid (J - ~ - f/)/f/ was g i v ol i l «llo v«LI<io«J'88, s(> tll«Ll

(f + (/) would b e 8 6 0 sec ' , c o r r esponding t o a t i m e c o n st<L»t of

28 iiisoc.
Tl>OH(> l,iiii(> (;(>i>sl„»ilH I » > ; < l(«l»I (") I»i(l (8) Iir(' ; i l » ><il, i Ig>)it 1 (> IL(;(:(>ii»l

f or tho rise oi' tuisio»a) iigidity (ass»»iod to correspoiul to stop 8)
bohlg (',»lrliiloto :Ll I l l»>«I, l()0 »is(;(,:LI't( i' l li(; if,ii»I<I<is (;LII ti»1(;s 1(.f'(ii
to frog musclesat O'C).

V. 2. Decreased exfractabilLty foproteins

It has bee» known for a loiig tii»e that the fibrillar l» otei»s of muscle are
much moro easily got i»to solutio» fru»i fresh musclo th(LIL from muscle
in fatigue or rigor (Deuticke-Kamp e6'ect). At t h e same time a new
component appears in the extracts (contractine, DvzmssoN, 1950).

804

O Tii E I L P H E N O M E N A I N MU ROI « E

BANGA and SzENT-GYQRGYI (1943} showed. that the decrease of extrac-
tability is largely explained by the union of actin and myosin, supposed
t,o be separate in the resting 6bre, to form actomyosin, but the observa-
tions of H A s sznEAcH (1S68) suggest that other factors may also be
involved. He found that a solution of ionic strength 0 0, containing
pyrophosphate, will extract the inyosin from minced muscle, leaving
6laments presumably of actin but removing the 8 lines. If the residues
are broken up with a blender i» the same solution, the actin is dissolved
as well (H Asszr,EAcH and >SGHNzmzz, 1951 ); similarly, this solution
dissolves both the actin and. the myosin from 6brils prepared from
fresh muscle (PEEEY, 1965). On the other hand, fibrils prepared from
inuscle in rigor mortis lose only their myosin on extraction with this
solution (HAsszLEAcH, 1S63), the actin filaments and the 8 lines being
retained.; fibrils from glycerol-extracted muscle appear to behave in
lho same way (HANsoN and H. E. HUxr.zY, 1953). Thus, both the 8
) ines and. the actin filaments are protected. by rigor from being dissolved,
while the actin 61aments, but not the 8 lines, are preserved in muscles
wliich has been minced but not broken up into fibrils. Clearly some
change has taken place in the material of the 8 line on rigor. Possibly
tl»s isenough to explain also the increased resistance to extraction of
the actin filaments, or an alteration may have taken. place in them
too, which is an interesting possibility in connection with an earlier
suggestion that the actin 61aments may undergo a lengthening when the
inuscle is activated (p. 301).

The existence of a change in the physical properties of the 8 l i ne
material further suggests that changes in t hi s structure may con-
ceivably be involved insome of the mechanical accompaniments of
activation (e.g. the increase in torsional rigidity). On the other hand,
i t also suggests that rigor mortis involves definite activation of t he
contractile substance (since the function of the 8 line appears to be to
tra»smit activation) a»d not merely a union between actin and myosin
rosiilting directly f ro m t h o r e duced AT P c o ncentration. S u g gestions
tliat, activation occurs in other kinds of rigor have recently been put
l '(»'w>Lr(l on ot her g r o u n d s ( SANI>ow IL»d > )GFINRYEIL (1965) for i o d o -
;L(;(;t;Llo rigor, a»d .I1AILNILN, Difzv aiul T i f @zt,l>Ar,i. (1966) for dinitro-
1>lie<i ol cont ractures).

V. 3. OpILcal cILan//esA(doringa fwL/ch

V, 3. 1. ScatterLnf/of light

The early decrease in the amount of light di6racted by the striations,
discovered. by D. K. HD.i. (194S), was mentioned on p. 300. This early
change issoon masked by a much larger OKect, which appears to be a
decrease in the amount of light scattered by the muscle, and which

805

follows roughly the tiinc course of tiie tensioii (H<'IiAEincR and Copi<ERT,
1937; D. K . H l ' r L , 1040, 1053). T h i s scattering occurs almost entirely
in the direction at right angles to the long axis of the fibres, and must
therefore be due t o l o n g itudinal elements of t h e m u scle structure.
These are presumably (a) the outlines of the fibres themselves, and (6)
t he threads of sarcoplasm that lie between the groups of fibrils. I f w e
set aside the possibility that the change depends on an alteration in
the shape of thefibrcs,then the decrease is>scattering is probably due
to a decrease in the difFerence of refractive index between sarcoplas»i
and fibrils. I n i s olated fibres under the interference microscope, it is
quite clear that the sarcoplasm has a higher refractive index than any
part of th e fi br ils (A . I<. HiTxLzv and. NrEDERGERKE, 1054 ), so that a
decrease i n t h e r e f r a c tive i n d ex d i f fe rence c ould i n d i c a te e i t h er a
transfer of dissolved substances from sarcoplasm to fibrils, or of water
in the opposite direction; the second seems a more likely possibility.
There is no basis at present for estimating tlie magnitude of the shift.

V; 8, 2. Decreasefobirefringence

It is well established that the strength of the birefringence of striated
muscle falls during an. isometric tw i t cli ( voN MU RAI.'r, i<832; i) u z i .I.>t
and COTTRELL, 1087 ), the amount of t h e fall being a maxi niu m ( a bout
30 per cent) if. the muscle is near its natural length. This effect niight
also be accounted for by the shift of water from fibrils to sarcoplasi»
which was suggested in the last section as an explanatio» of the decrease
in scattering. T his could act in two ways: fi r st, by concentrating the
material which lies between the filaments in the fibrils, so reducing the
refractive index difFerence which causes the form component of t he
birefringence, and second, by reducing any contribution, analogous to
form birefringence, which may be made directly by the refractive index
< lifference between the threads of sarcoplasm and the fibrils. I f t h e
»Iovement ofwater.went so farthat thc refractive i»<lcx <liffcrciice w:Is
actually reversed in either of t h ese cases, then thc for»i b i rcl'ringo>ice
would increase again (since its strength is roughly proportional to the

sill>It>'< of tlio r<',I'!'>I« tive i»<1<!x <lifl'<!rci>« !); l l>i>> is<t 1><>s>il><l< ! <!Nl>hw»ii I<»>
I<>I' l l><! <1<>l>l>l<>-I>>I»>1><!<1 <'.III'v<t< I I» tl w<'!I'<! <>I>tel>l><!<I I>v v< >N M I > li s I 'I'.

V . I . 'I'<AI»s<s(<»isi<»i

V • 4 . 1 . Zl IJsOl'Ill <!'I'<<>l i<'s

The absolute values for the tetanus tension pei unit cross-sectional
area that are to bc f ound in t h e l i t erature for vertebrate muscle vary
over a wide range. I<"rog muscle at O'C appears to give from 1 5 to

2kg/cm' (H I L L, 10 88; H A z D U, 1 051); a t r o o m t e m p erature, HA y DU

finds 2 5 kg/cms, while RAMszv and STREET (1040) obtained 3 5 kg/cms
8n6













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