THE MECHANISM OF TONUS IN UNSTRIATED MUSCLE

Transcription

1 THE MECHANISM OF TONUS IN UNSTRIATED MUSCLE BY SUNIrA II,~)EPJIT SINGH AND INDERnT SINGH, F.A.Sc. (From the Physiological Laboratory, Medical College, Agra) Received March 14, 1950 UNSTRIATED muscle exhibits two kinds of tone (Singh and Singh, 1946; 1947; 1948 a, b; 1949 a, b; Singh, 1949); they have been termed as lactic and alactic tones respectively. There appear to be separate contractile mechanisms for these tones. In the present research, further evidence is presented about the existence of these two kinds of tonic contractions and their mechanism elucidated. Experimental methods were as described previously; stomach muscle from Rana tigrina and unstriated muscle from dog's stomach have been used. RESULTS Tonus in frog's stomach.--the cardiac and pyloric halves behave differently as regards their tonic properties. The muscle from the pyloric half, when put under tension of about g., can maintain this tension for hours. The tension is not due to visco-elastic properties, as when relaxed, the muscle can lengthen more without exciting any tension. The stomach is taken out immediately after the animal is killed with a blow on the head, so that the tonic contraction described above represents normal tone of the muscle. The above tonic contraction has got all the properties of alactic tone described previously. It is resistant to asphyxia or cyanide and adrenaline (1 in ) does not produce the slightest inhibition, though it may produce a contraction. It can be slightly diminished by glucose or alternating current. The cardiac portion, especially from near the cardio-cesophageal sphincter exhibits all the properties of lactic tone. It is sensitive to asphyxia or cyanide. It ~s very sensitive to adrenaline; 1 in l0 T may almost completely abolish it. Muscle from intermediate portions may exhibit intermediate properties. The tension due to tonic c6ntraction can be destroyed by forcible stretching without affecting the twitch contraction produced by alternating current (Fig. 1), showin~ that the two are due to separate contractile elements. The 351

2 352 Sunita Inderjit Singh and Inderjit Singh FIe. 1. Frog stomach muscle from pytoric region.--the muscle is being stimulated with alternating current 12 volts/ 10 sec. per minute. The muscle is put under tension. The tension is then destroyed by forcible stretching at X and IT. Note that the twitch contractions at* uneffected. properties of the tonic contraction of the pyloric end of the stomach theretore exactly resemble those of the asphyxial contraction, and these experiments therefore confirm the views mentioned previously regarding the nature of alactic tone. In dog's stomach there are similar difference between the properties of the muscles from cardiac and pyloric ends, but they are not so marked as in the frog's stomach. The variability of the tone of the stomach probably accounts for the variable action of sympathetic and vagus nerves. Sympathetic stimulation would result in contraction or there would be no response

3 The Mechanism ol c Tonus in Unstriated Muscle 353 if the muscle is in alaetic tone; it wou.d result in inhibition if in lactic tone. Similarly vagus stimulation would result in contraction of a relaxed muscle, and if already contracted it would relax the muscle, as lactic tone may be neutralised as result of stimulation. The achalasia of some sphincters in the body is probably due to change in the nature of the tone; a change of lactic to alactie tone would cause a failure of relaxation on the arrival of an inhibitory impulse. Vascular hypertension may also be due to change in the nature of tone of the blood vessels. The reaction to stretch.--distilled water causes an unloaded frog's muscle to elongate activity. It has been found that if the pyloric half of the stomach muscle is put under tension of about 30 to 40 g. and then immersed in distilled water, this elongation is prevented. The greater the tension, the greater its antagonistic effect on elongation; this is shown by using an isotonic lever. Thus the muscle may relax with small loads (5 g.) but not with larger loads (20 g.). The muscle may swell by more than 100p.o. and yet not elongate, showing that the swelling is not the cause of elongation. The pylorie and cardiac halves of the frog's stomach muscle differ remarkably in their reaction to stretch and distilled water. The pyloric portion does not relax at all when put under tension and then immersed in distilled water (Fig. 2), whilst the cardiac portion begins to relax immediately. The relaxation must be distinguished from the linear decrease of tension due to the viscoelastic properties of the muscle. The muscle from the cardior sphincter relaxes most. When reimmersed in saline or isotonic sodium chloride solution, the pyloric portion relaxes and the cardiac portion contracts, though after 24 hours immersion it may relax instead of contract. These reactions are produced by living as well as dead muscle. They may be produced after half to one hour's immersion in distilled water, a procedure which does not kill the muscle or they may be produced after 24 hours' immersion, a procedure which kills the muscle. These experiments with distilled water and the behaviour of dead muscle suggest that in unstriated muscle there are two kinds of proteins responsible for the two kinds of tone respectively. The dog's stomach muscle, whether from the cardiac or the pyloric portion contracts when put under tension and immersed in distilled water, so that it predominantly contains the protein responsible for alactic tone. The gut muscle behaves oppositely, so that it predominantly contains the second kind of protein. Various muscles probably contain various mixtures of these proteins and thus differ in behaviour.

4 354 Sunita-Inderjit Singh and Inderjit Singh Fio. 2. Frog stomach muscle from pyloric region.--the muscle is put under tension and then immersed in distilled water. Drum stopped at X for 24 hours. Isotonic NaC1 added at Y. Drum stopped at Z for an hour, and distilled water add at W. The muscle begins to contract again. Tension decreases due to its viscoelastic properties. The muscle elongates in isotonic potassium chloride and other solutions (Singh and Singh, 1949 c) and stretching antagonises this elongation. The behaviour of living muscle.--the decrease of tension produced in muscle immersed in distilled water by increasing the osmotic pressure can also be produced in living muscle immersed in saline. Thus if the muscle is put under tension in saline, then by doubling the osmo.tic pressure of the saline by adding sodium chloride produces similar reduction of tone, showing that the proteins of living muscle behave like those of the dead muscle immersed in distilled water. The muscle therefore contains a mechanism 9 whereby it is able to maintain tension without expenditure of energy. This

5 The Mechanism o/" Tonus in Unstriated Muscle 355 is also proved by the fact that the oxygen consumption of the muscle (Rao and Singh, 1940) and lactic acid production (Bharadwaj and Singh, 1950) diminishes if the muscle is stretched. The diminution of metabolism is due to the fact that the alactic mechanism takes over from the lactic one. Stretching also produces a contraction which requires energy so that it may stimulate the mechanism of lactic tone. Stretching, therefore, induces a change in the contractile protein of the muscle which increases the tendency to contraction and diminishes that to inhibition (Fig. 3). It is remarkable that asphyxia also produces similar eft~cts, inhibition being converted into contraction (Singh and Singh, 1949ar) g Length ram. Fzo. 3. Frog stomach muscle.--effect of initial length on contraction produced by alternating current (A.C.) and inhibition produced by adrenaline, 1 in I06 (A.D.). With small in initial length there is inhibition, and as the length is increased inhibition is converted into contraction. DISCUSSION Previously it has been described how unstfiated muscle reams to dimi~ nished oxygen supply. These experiments show that the muscle acts similarly if the demand is excessive. When increased demand is made on the

6 356 Sunita Inderjit Singh and Inderjit Singh muscle with normal oxygen supply or normal demand with diminished oxygen supply, it puts into action a contractile mechanism which does not require energy. The muscle thereby is enabled to maintain tension without extra expenditure of energy. As a matter of fact the energy expenditure is diminished, because the alactic mechanism takes over from the ]actic one. To understand the properties and reactions of unstriated muscle, the nature of tone has to be taken into account. SUMMARY 1. Frog's and dog's stomach show the predominance of alacfic tone in the pylofic regeion and lactic tone in the cardiac region. 2. The tone in the pyloric region has the properties of the asphyxial contraction. 3. The tension due to tonic contraction of the pyloric muscle can be destroyed without affecting the twitch contraction suggesting that these two are mediated by different contractile mechanisms. 4. Stretching of muscle antagonises elongation produced by distilled water and other substances. 5. The muscle from the pyloric and cardiac regions behave differently in distilled water. The former contracts and the latter relaxes. These reactions are produced both in living as well as dead muscles, so it is concluded that unstriated muscle contains two kinds of contractile proteins for lactic and alacfic tones respectively. 6. Stretching increases the tendency to contraction and decreases that to inhibition. Asphyxia also produces similar effects. 7. When increased demand is made on unstriated muscle with normal oxygen supply or normal demand with diminished oxygen supply, it puts into action a contractile mechanism which does not require energy. Bharadwaj, U. R., and. Singh, I. Rao, M. S., and Singb, I. Singh, I. and Singh, I. REFERENCES.,. Unpublished observations... jr. PhysioL, 1940, 98, Proc. Ind. Acad. Sci., 1949, 29, Curt. $ci., 1947, 16, Proc. Ind. Acad. Sci., 1948 a, 27, Curr. Sci., 1948 b, 17, Proc. Ind. Acad. Sci., 1949 a, 30, Ibid., 1949 b, 30, Ibid., 1949 c, 30, IBid., 1949 d, 30, 215. a8.50 Pmnted at The Banj[alere Press, Mysore Road, Bang~lo~ City, by G. Smnlvasa Rao, Supemntenden~, and Pabhshed by The Indhan Academy of'~o~lel. 8anna]ore