Hidenobu MASHIMA Department of Physiology,School of ...

Jap.J.Physiol.,27,321-335,1977

TETANIC CONTRACTION AND TENSION-LENGTH

RELATION OF FROG VENTRICULAR MUSCLE

Hidenobu MASHIMA

Department of Physiology,School of Medicine,Juntendo University,Hongo,Bunkyo-ku,Tokyo,113 Japan

Abstract Complete tetanic contraction was generated in frog ventri-

cular muscle by repetitive electrical stimulation.The maximum stimulus

was a transverse alternating current at 10Hz and 17-20 V/cm in peak to

peak voltage in an external solution containing 9 mM Ca2+.The maxi-

mum isometric tension thus obtained was twice or more than that of

the twitch tension at 20•Ž.The tetanic tension and its rate of rise

declined with decreasing external Ca2+concentration and less than half

of the maximum tension was generated at 1.8mM Ca2+.Various tetanic

tensions less than the maximum were obtained in the partially depolarized

muscle in excess K+solution by reducing the stimulus intensity.Adren-

aline(5•~10-6g/ml)potentiated the submaximal tetanic tension as

well as the twitch tension,although no effect was observed for the maxi-

mum tetanic tension.The tension-length relation for the tetanic con-

traction of ventricular muscle was similar to that of the skeletal muscle,

but the tension fell almost linearly at shorter lengths than 0.9Lm,where

Lm is the optimum length at which the maximum tension,Fm,was

generated.Fm was 4.6g/mm2,while the sarcomere length at Lm was

2.0-2.2ƒÊm.

Previous studies on the mechanical properties of the cardiac muscle had thedifficulty of the inability to tetanize the muscle.Some extrapolations or approxi-mations have always been made which allow measurements to be taken at differenttimes in a twitch because the active state of the muscle varies during the twitch.The first attempt to tetanize the cardiac muscle was made by HENDERSON et al.

(1971)in rat papillary muscles.They applied a train of electrical pulses at 25-50Hz in the presence of 0.6-7.5mm of external calcium and obtained a low tetanicforce which preceded by the initial twitch and partial relaxation.FORMAN et al.

(1972)effected repetitive electrical pulses in cat papillary muscles and found thatsmooth tetani could be obtained with repetitive electrical stimulation in the

Received for publication March 11,1977

真島英信

321

322 H.MASHIMA

presence of both 10mM caffeine and 10mM calcium.Attempts to tetanize catpapillary muscles were unsuccessful in the presence of either caffeine or increasedconcentrations of calcium alone,although either of these conditions alone workedfor rat muscles.On the other hand,GOTO et al.(1967)observed in bullfrogcardiac muscles that a marked sustained contraction was generated by massivestimulation of an alternating current at 20Hz.In a previous report(MASHIMA,1976),the author successfully obtained high tetanic force in frog ventricularmuscle by applying an alternating current at 10Hz in the presence of 9mM calciumwithout adding caffeine.

The present study is aimed at establishing the optimum conditions for electri-cal stimulation to obtain maximum tetanic force in frog cardiac muscle and todetermine the effects of length and adrenaline on the tension-length relation fortetanic contraction.

METHODS

Experiments were performed on a small strip prepared from the ventricle of

the frog(Rana nigromaculata),about 10mm in length and less than 1mm in

diameter.First,a circular preparation was dissected from near the coronary

sulcus,and opened by cutting an appropriate portion of the ring.Both ends of

the preparation were ligated with thin threads.The experimental set-up was

almost similar to that described in a previous report(MASHIMA and KUSHIMA,

1971).The muscle preparation was mounted horizontally in a polystyrol bath

(3•~7•~1.5cm)containing 10ml Ringer's solution.A pair of platinum foil elec-

trodes(7•~1.5cm)were placed on opposite walls of the bath in parallel with the

muscle.Thus,the whole length of the muscle was stimulated simultaneously by

a transverse electric field between these massive electrodes.Square pulses(SP)

or alternating current(AC)at various frequencies were applied with a high-

current stimulator(about 30 Watts).The temperature of the Ringer's solution

(110mM NaCl,12mM NaHCO3,2mM KCl,1.8mM CaCl2,pH 7.2)was main-

tained at 20•Ž with a thermoelectric heat exchanger throughout the experiments,

and was constantly bubbled with 5% CO2 and 95% O2 gas mixture.One end of

the muscle was penetrated near the ligation by a stainless steel needle attached to

the tip of the isometric lever by which the muscle tension was conveyed to the

anodal pin of an RCA 5734 tube,and the ligature of the muscle was tied tightly

to the needle under a binocular microscope.The other end of the muscle was

also penetrated by a stainless steel needle connected to the isotonic lever.In the

present study the isotonic lever was fixed by a stopper in order to measure iso-

metric tension.The muscle length was varied with an accuracy of 0.1mm by

moving the isometric lever which was mounted on a sliding vernier scale.The

tension of the muscle was displayed on an ink-writing rectigraph(Nihon Kohden

RJG-3022).

TETANUS OF VENTRICULAR MUSCLE 323

The sarcomere spacing was measured by means of routine electronmicros-

copy,fixing the preparation with osmium solution at the optimum length,Lm,

at which the muscle developed maximum tension,Fm.In order to measure the

cross section of the Dreparation,the muscle was frozen with dichlorodifluoro-

methane spray at Lm, cut into 16ƒÊm slices with a cryostat in a cold chamber

(-20•Ž).Each slice was dried on a slide glass at room temperature and the

diameter of the preparation was measured under a microscope.The value ƒÎab/4

was taken as the cross section,where a is the largest diameter and b is the smallest

one.Usually,the cross section was measured at the thinest portion of the prep-

aratlon.

RESULTS

1. Tetanic contraction generated by repetitive electrical stimulation

When the muscle was stimulated by repetitive pulses or an alternating current

Fig.1 Fffect of frequency on the tetanic.tension of frog ventricle. Series A,repetitive

pulses 5msec,6V/cm;Al:single pulse,A2:1Hz,A3:5Hz,A4:10Hz,A5:20Hz,A6:40Hz,A7:100Hz,A8:AC 10Hz,20V/cm.Series B,AC 20v/cm;B1:1Hz,B2:2Hz,B3:5Hz,B4:10Hz,B5:20Hz,B6:40Hz,B7:80Hz.

324 H.MASHIMA

at more than 5 Hz in Ringer's solution, tetanic contraction was observed.Thistetanic tension increased with increasing external Ca2+ concentration up to 9mm

(see Fig.2,series A).Therefore,the experiments in this section were performedin the solution containing 9mm Ca2+.As seen in Fig.1,series A,completetetanus far larger than the twitch response was obtained at more than 5Hz.Atless than 5Hz,the summation of twitches was incomplete,and at more than40 Hz.the tetanic tension declined considerably and summation became incom-

plete.The optimum conditions for the repetitive pulses stimulation was 10Hzfrequency,6V/cm field intensity and 5-20 msec pulse duration. Under theseconditions,the tension curve attained maximum within 2sec.

An alternating current(AC)was more effective for obtaining high tetanictension,as shown in Fig.1,seriesB.At less than 3Hz,the summation was in-

complete,and at more than 20Hz,the tension was declined.Therefore,theoptimum frequency was around 10 Hz, similar to that for the repetitive pulses.The threshold intensity at 10Hz was about 1.5V/cm in peak to peak voltage.

Fig.2.Effects of external Ca2+ concentration and stimulus intensity on the tetanic tensionof frog ventricle.Series A,Ca2+ concentration;Al:1.8mM,A2:3.6mM,A3.7.2mM,A4:9mM,A5:12.6mM,A6:16.2mM,stimulus intensity:17V/cm.Series B,stimulusintensity;B1:1V/cm,B2:1.5V/cm,B3:2V/cm,B4:7V/cm,B5:14V/cm,B6:20V/cm,B7:30V/cm.


For the suDrathreshold intensity,the cardiac muscle responded in an almost all-

or-none manner as seen in Fig.2,series B.At less than 2V/cm,the summationwas not comDlete.but at more than 2v/cm,a smooth plateau tension was ob-

tained.The tetanic tension increased only slightly with increasing voltage up to17V/cm,but declined considerably at more than 20V/cm.Then,the optimumconditions for AC stimulation was 10Hz frequency,17-20V/cm intensity and asmooth plateau tension was obtained in 2sec.As shown in Fig.1,A8,the maxi-

mum tension generated by the maximum AC stimulation was much higher(7-8%)

than that by repetitive pulses.Moreover,AC had the advantage of quickerrecovery.It was possible to obtain identical tetanic tension curves repeatedly,

provided the interval between stimuli was 2-3min.The tetanic tension thusobtained was extremely constant for several hours,although the twitch tensiondeclined rather quickly with time.

2.•@ The effect of external Ca2+concentration upon tetanic tension

Tetanic tension and its rate of rise increased with an increase in external Ca2+

concentration up to 9mM,but only a slight increase was observed at more than

9mM as shown in Fig.2,seriesA.The tension curve at physiological Ca2+

concentration(Fig.2,Al)rises so slowly that it does not attain the plateaulevel within 4sec.The relative tension is plotted against the Ca2+ concentrationinFi9.3.The tension obtained in Ringer's solution containing 1.8mM Ca2+

immediately after dissection was usually more than 50% of the maximum tensionobtained in 9mM Ca2+solution,but it was less than 40% when the Ca2+con-

Fig.3.•@ Relation between external Ca2+concentration and tetanic tension of frog ventricle.

Two cases are shown. Arrows indicate the order of measurements.

326 H.MASHIMA

centration was reduced again to 1.8mm.Probably some inotropic substances,

such as noradrenaline from nerve terminals,are involved in the contraction of

fresh preparation and they are dissipated after several tetanic stimuli.Actually,

5•~10-6g/ml adrenaline added into the 1.8mm Ca2 solution slightly augmented

the tension.Therefore,it is likely that the curve for the early measurements in

Fig.3 includes some potentiating effects,but the curve for the late measurements

represents the pure effect of Ca2+concentration upon tetanic tension.This

later curve is quite similar to that described by LOTTGAU and NIEDERGERKE(1958)

in the twitch tension of frog cardiac muscle,except that their tension was saturated

at 4mM Ca2+in 100% Na+solution.It is notable that the maximum tension in

9mM Ca2+was not potentiated by adrenaline

From the above results,AC of 20V/cm at 10Hz in the solution containing

9mM Ca2+was adopted as the maximum stimulus in the present study.

3. Graded tetanic contractions in the depolarized cardiac muscle

When the external K+concentration was raised to more than 8mM,varioustetanic tensions less than the maximum tension were obtained by reducing thestimulus intensity as shown in Fig.4.In skeletal muscles,similar graded tetanushas already been shown by MASHIMA and TSUCHIYA(1968)by AC stimulation which

produced graded depolarization on the partially depolarized nonconductive musclemembrane.The tensions thus obtained were plotted against stimulus intensityin Fig.5.When the external K+ concentration was 8mM,the tension generatedbv the maximum stimulus decreased to 80% of the maximum tension and de-

creased further with decreasing stimulus intensity.In 12mm K+,the tension

A

D

B

E

C

F

Fig.4. Effect of stimulus intensity on the tetanic tension of depolarized frog ventricle.

K+concentration:8mm,Ca2+concentration:9mm,AC stimulation;A:20V/cm,B:15V/cm,C:10NT/cm,D:5V/cm,E:3V/cm,F:2NT/cm.


generated by the maximum stimulus decreased further to 65% of the maximum

tension.

The effect of adrenaline upon tetanic tension of such a partially depolarized

muscle was also examined.When 5•~10-6g/ml adrenaline was added to the ex-

ternal solution,the graded tension generated by weak stimulus was markedly

augmented after 5min as seen in Fig.5.The rate of augmentation was greater

Fig.5. Relation between stimulus intensity and tetanic tension of depolarized frog ventri-

cle.Maximum AC stimulation,Ca2+concentration:9mM,K+concentration;upper

curves:8mm,lower curves:12mm,open circle:without adrenaline,filled circle:5•~

10-6g/ml adrenaline was added.The maximum tension in 2mm K+was taken as 1.0.

at weaker stimulus intensities,although the absolute value of tension decreasedwith decreasing intensity.It is likely that the partial depolarization producedby weak stimulus can be increased by adrenaline,which is known to have a specificincreasing effect on the plateau phase of cardiac action potential in K+rich solu-tion(CARMELIET and VEREECKE,1969).Certainly,the twitch tension was markedlyaugmented by adrenaline as much as to 75% of the maximum tension(Fig.9),but the maximum tension was never augmented by adrenaline,although the rateof rise of tension was accelerated.

4. Tension-length relation for tetanic contractionIsometric tetanic tension generated by maximum stimuli were measured at

various muscle lengths.One of the results is shown in Fig.6.As the musclewas stretched,the resting tension markedly increased while the developed tension

328 H.MASHIMA

reached the maximum and then decreased.The length at which the maximumtension,Fm,was obtained was defined as the optimum length,Lm.It is known thatresting tension of the cardiac muscle is higher than that of the skeletal muscle,

probably because of a large amount of collagen between the fibers.Eventually,it was as large as the developed tension already at 110%Lm.However,the restingtension of overstretched muscle gradually declined as indicated by arrows in Fig.6.

Fig.6. Effects of length on the resting and developed tensions of frog ventricle.RT:

resting tension;1:extension curve of the resting muscle from 9mm to 12mm,2:from

9mm to 13mm,3:from 9mm to 14mm,DT:developed tension.

When the muscle was stretched from 9mm to 12mm,the resting tension im-mediately after the stretch was 7.5g,but gradually declined to 5.3g(curve 1).After the tetanic contraction for the measurement of developed tension,the restingtension still continued to decline.Therefore,when the muscle was released to theinitial length of 9mm and stretched again to 13mm,the extension curve changedto curve 2.Repeating the same procedure,curve 3 was obtained when the musclewas stretched from 9mm to 14mm.Apparently,the resting tension at a certainstretched length declined when previously overstretched.As a result,the restingtension at 90%Lm could be rendered sufficiently small compared with the devel-oped tension by previous stretching to 110-120%Lm.

The developed tension by maximum stimulus was measured at various musclelengths.After the contraction the muscle length was always returned to theinitial length(usually 90%Lm)at which the resting tension was less than 0.3g

(7% Fm),throughout the interval of stimuli.The developed tension-length rela-tion obtained in 11 muscles are summarized in Fig.7.After the experiments,


Fig.7. Developed tension-length curve for the tetanic contraction of frog ventricle.W:

twitch tension,broken line:tension-length curve for the skeletal muscle,bottom scale:

estimated sarcomere length.

some preparations were fixed at Lm and examined with an electron microscope in

order to measure the sarcomere spacing.Usually,the sarcomere length was 2.0-

2.2ƒÊm at Lm.In the other preparations the diameters at Lm were measured di-

rectly,or after cutting the frozen preparation into serial sections under a micro-

scope.As a result,the average value of Fm was 4.6g/mm2.

The tension-length diagram for the tetanic contraction of cardiac muscleshown in Fig.7 is almost similar to that of the skeletal muscle(broken line inFig.7)described by GORDON et al.(1966).However,in the region shorter than90%Lm,the tension decreased almost linearly.And it was difficult to stretch thecardiac muscle beyond 140%Lm because of the danger of rupturing the prepara-tion.Curve W in Fig.7 shows the tension-length relation for the twitch responseof the same preparations.This curve is essentially the same as the curves re-

ported by WALKER(1960)in frog cardiac muscle,WILLIAMS et al.(1965)in frogatrial muscle and SPIRO and SONNENBLICK(1965)in mammalian papillary muscle.

5. Effects of external Ca2+concentration and adrenaline on the tension-lengthrelation for tetanic contraction

The tension-length diagrams of the cardiac muscle in the solution containing9mM and 1.8mM Ca2+are shown in Fig.8,left.The relationship of relativetension to length are also plotted in Fig.8,right.Apparently,both curvesdecline at shorter lengths,but the degree of decline is greater in low Ca2+con-centrations.The reason is not clear,probably the overpassing position ofmyofilaments at shorter sarcomere length does not favor the rate of usage of freeCa2+ions at the active sites of thin filaments.In fact,the rate of rise as well asthe amplitude of tension in low Ca2+concentrations is so slow that the tension

330 H.MASHIMA

Fig.8. Effect of external Ca2+concentration on the developed tension-length relation for

the tetanic contraction of frog ventricle.Maximum AC stimulation,Ca2+concentra-

tion;upper curve:9mm,lower curve:1.8mm,left figure:absolute tension,right figure:

relative tension.The maximum tension of each curve was taken as 1.0.

Fig.9. Effect of adrenaline on the tension-length curve for the tetanic contraction of frog

ventricle.Ca2+concentration:9mM,W:twitch tension,A:twitch tension potentiated

by 5•~10-6g/ml adrenaline,T:tetanic tension generated by the maximum AC stimula-

tion.Tetanic tension was not potentiated by adrenaline.


curve hardly attains the plateau level in 4 sec(see Fig.2,Al).

The tension-length curve of the cardiac muscle in the solution containing

9mM Ca2+was not altered by adding 5•~10-6g/ml adrenaline to the external solu-

tion.Adrenaline does not affect the tetanic tension generated by the maximum

stimulus at any lengths as shown in Fig.9.On the other hand,the twitch tension

was markedly potentiated by adrenaline.Twitch tetanus ratio was about 2 in the

fresh preparation.However,as the twitch declined with time,the ratio soon

increased to more than 5.Adrenaline potentiated such a declined twitch several

times,but the maximum twitch tension under the effect of adrenaline did not

exceed 75%Fm(Fig.9).

DISCUSSION

Frog ventricular muscles could be tetanized by repetitive electrical stimulationat 10Hz in Ringer's solution and maximum tension was obtained in the presenceof 9mM of external Ca2+ concentration.It is assumed that a sufficiently rapidtrain of electrical stimuli induces a series of depolarizations,either conductive ornonconductive,at a rate which approximates to the electrical refractory period.These depolarizations trigger the supply of activator Ca2+to the myofilaments,which is derived both from internal stores and from the extracellular fluid.Thesustained tension implies that the net rate of Ca2+entry into the myofilament spaceis equal to its net rate of removal.The lower plateau tension at higher frequenciessuggests that internally sequestered Ca2+is not immediately available for sub-sequent release without an adequate interval between stimuli,or that the grade ofdepolarization will decrease when the subsequent stimulus falls within the re-fractory period.Therefore,the maximum tension could be obtained only around10Hz in frog ventricular muscle.The maximum tetanic tension thus obtainedwas twice as,or more,great as the twitch tension.In mammalian papillarymuscles,the maximum tetanic force was only about 5% greater than the twitchforce,which was potentiated by caffeine and increased calcium(FORMAN et al.,1972),and it was even smaller than the twitch force in the case without caffeine

(HENDERSON et al.,1971).The reason for this difference based on the species isnot clear,probably the electrical refractory period is relatively long in mammaliancardiac muscle.

According to MASHIMA and WASHIO(1968),the transmembrane potential offrog skeletal muscle showed the steady depolarization,which disappeared inNat+-free solution,during repetitive pulses or AC stimulation,and the absoluterefractory period was apparently shortened by hyperpolarizing half-cycle ofAC.Corresponding with this steady depolarization,MASHIMA and TSUCHIYA

(1968)obtained maximum tetanic contraction by applying AC at 200-500Hz.In the cardiac muscle of monkey,ANTONI et al.(1970)observed the similar steadydepolarization forming a constant plateau by strong AC stimulation at 50Hz.

332 H.MASHIMA

In the present study,AC at 10Hz was more effective than 50Hz for obtaining themaximum mechanical response of cardiac muscle.This difference in the optimumfrequency between skeletal and cardiac muscles could be explained by the dif-ference in the relative refractory period.In cardiac muscles the successive stimulishould be applied with interval of 100-200msec(or frequency of 5-10Hz)be-cause the refractory period is as long as 200msec,although with interval of 2-5msec(or frequency of 200-500Hz)in skeletal muscles because the refractory

period is about 5 msec.Several workers have shown that tensions greater thanthe twitch are obtained when the plateau potential of cardiac muscle is prolongedby a long pulse(TRAUTWEIN and DUDEL,1954;MORAD and TRAUTWEIN,1968;ANTONI et al.,1969;HENDERSON et al.,1971;GOTO et al.,1974).So,it is con-sidered that the cardiac muscle can be tetanized by repetitive electrical stimulationwhich results in the prolonged depolarization,provided with appropriate fre-

quency and sufficient intensity.Since the study of LUTTGAU and NIEDERGERKE(1958),it has been well known

that the twitch tension of cardiac muscle increases as the external Ca2+increases.The present study showed a similar S-shaped relationship between tension andCa2+concentration also for the tetanic contraction.CARMELIET and VEREECKE

(1969)found in calf and cow cardiac muscles that the amplitude of the plateaupotential increased by 17mV for a tenfold change in extracellular Ca2+,probablybecause of an increased Ca2+inward current during the plateau depolarization.MASCHER(1970)reported in cat and dog papillary muscles that the amplitude aswell as the rate of rise of slow regenerative membrane potential depended stronglyupon the external Ca2+concentration.HORACKOVA and VASSORT(1976)alsoobserved that the quantity of Cat+ entry upon depolarization was linearly relatedto peak twitch tension.These results indicate that increased external Ca2+causes increased tension through the potentiation of Cat+entry during depolariza-tion.However,the relative tension at shorter lengths was much more depressedin the low Cat+solution,as shown in Fig.8,right.Therefore,the effect of in-creased external Cat+upon tension must be modified not only with the amount ofdepolarization but also with the length of muscle.

On the other hand,adrenaline seems to affect tension through an increase ofCa2+inward current,as pointed out by CARMELIET and VEREECKE(1969).Theyobserved that adrenaline specifically increased the amplitude and duration of the

plateau phase of the cardiac action potential even in the depolarized and non-conductive preparations by an increased external potassium.Actually,adrenaline

potentiated the twitch tension or the submaximal tetanic tension of depolarizedmuscle generated by weak stimuli,but did not potentiate maximum tetanic tension.KAVALER and MORAD(1966)found that adrenaline augmented contractile tensionbut depressed the development of potassium contracture in frog ventricle.Inthe present study,the maximum tension decreased as the external K+increased asshown in Fig.5.Adrenaline clearly augmented the submaximal tension but the


absolute tension decreased to 65% of the maximum tension in the presence of

12mM K+.Another intracellular biochemical effect of adrenaline on cyclic AMP

(BOWMAN and NOTT,1969)was not assayed here.But it would be difficult to

imagine that the presence of excess K+depresses the intracellular inotropic ac-

tion,since the intracellular K+concentration is normally high.At least for the

effect of adrenaline on the tetanic tension generated by electrical stimulation,the

site of action must be mainly on the membrane.

The resting tension curve of cardiac muscle is significantly higher than that

of skeletal muscle particularly in the range beyond Lm.The relatively high

resting tension in cardiac muscle may be due to a large amount of collagen be-

tween the fibers(SPIRO and SONNENBLICK,1965).In the present results,how-

ever,the resting tension was greatly reduced when the muscle was once over-

stretched to 110-120% Lm,conforming to the observation of WALKER(1960).

Perhaps some of the nonmuscular tissues between fibers are plastic and do not

recover their original length after such nonphysiological extension.The poten-

tiating effect of prior stretching on the twitch tension described by WALKER(1960)

was not obvious for the tetanic tension generated by maximum stimulus.

The tension-length curve for the tetanus of ventricular muscle resembles the

curve for the twitch and also the curves that have been found for atrial muscle

(WILLIAMS et al.,1965),skeletal muscle(RAMSEY and STREET,1940;GORDON et al.,

1966)and smooth muscle(CsAPO,1955;MASHIMA and YOSHIDA,1965).SPIRO

and SONNENBLICK(1965)showed the constancy of A band width and the linearly

changing I band width relative to sarcomere length in cat papillary muscle and

estimated length of Lm was 2.2ƒÊm,although the I band width was often not

linearly related to over-all muscle length when extended to 120% Lm.POLLACK

and HUNTSMAN(1974)examined the sarcomere length in living rat papillary

muscle and reported that the sarcomere lengths corresponding to Lm averaged

2.30ƒÊm,a value higher than that previously reported for fixed tissues.However,

it may be reasonable to suppose that at lengths shorter than Lm in the fixed prep-

aration,the average sarcomere length could be estimated as shown in the bottom

scale of Fig.7.Comparing the developed tension-length curve for cardiac

muscle in Fig.7 with that of skeletal muscle described by GORDON et al.(1966),

the tension in cardiac muscle slightly declines at 90% Lm and falls almost linearly

at lengths shorter than 90% Lm which is about 2ƒÊm in sarcomere length,and a

shoulder is not prominent in the ascending limb of the curve,although the de-

scending limbs of both cardiac and skeletal muscles are almost the same.And

the maximum tension of frog cardiac muscle was 4.6g/mm2,about one-fourth of

that of the skeletal muscle in the same species(1.8kg/cm2).The reasons for the

smaller maximum tension and the smaller tension in the ascending limb are not

clear;probably the tissues between muscle fibers or relatively large amount of

nonfibrillar structures within the fiber would work as a disadvantage for the

cross-bridge movement in cardiac muscle,especially at lengths shorter than Lm.

334 H.MASHIMA

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Hidenobu MASHIMA Department of Physiology,School of ...

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