literature with reference to the effect on the stomach of the

Transcription

1 REVERSAL OF THE GASTRIC VAGUS. BY G. L. BROWN AND R. C. GARRY. (From the Department of Physiology, University of Leeds.) GARRY [1930 a, b] and Shafer, Underwood and Gaynor [1930] found that amytal (iso-amyl ethyl barbituric acid) had the peculiar property of removing the inhibitory effect of the vagus on the heart. The effect was demonstrated in animals anaesthetized with amytal and in decapitate preparations which had received subaneesthetic doses of the drug by intraperitoneal injection or by intravenous infusion. It appeared to us that considerable interest would attach to the investigation of the action of the drug on the response of the stomach to vagus stimulation. There exists a considerable body of very contradictory literature with reference to the effect on the stomach of the vagus. This has recently been reviewed by McSwiney [1931]. In general it appears that both excitation and inhibition may result from vagus stimulation, a factor in determining the direction of the response being the state of activity of the stomach at the time of stimulation. The active contracted stomach is stated to respond by relaxation, and the relaxed organ by contraction [Carlson, Boyd and Pearcy, 1922; McCrea, McSwiney and Stopford, 1925; McCrea and McSwiney, 1926]. A number of observers have found that the frequency and strength of the stimulation may play a part in determining the effect. Thus, Veach [1925] is of the opinion that the effect of the vagus on the lower cesophagus and stomach is of the nature of Wedensky inhibition. Veach, Schwartz and Weinstein [1930] have suggested the term "inhibition by fatigue" as being more applicable to the effects of the vagus on the stomach. McSwiney and Wadge [1928] have brought forward evidence to disprove the existence of Wedensky effects. We hoped by the use of a drug such as amytal, which appeared to have a specific action on the vagal inhibitory mechanism, to elucidate some of the problems presented by these conflicting views.

2 214 G. L. BROWN AND R. C. GARRY. METHODS. Cats were employed throughout the investigation, since they afforded a preparation which is reliable and which gives consistent results. In many experiments the spinal cat was used: it was prepared under ether by transection of the cord at the atlanto-axial joint and destruction of the brain with sinus forceps. Artificial ventilation on oxygen was given B A Fig. 1. 1cm. Diagram of vulcanite fluid electrode. by a Starling pump; the ventilation was kept as low as possible compatible with adequate arterialization of the blood, and a sufficient dead space was provided. After preparation of the animal, the pylorus was ligated through a small abdominal incision, the abdomen was closed, and the stomach was washed out through a tube passed through a cervical cesophagotomy. In those experiments in which it was wished to secure activity of the

3 REVERSAL OF THE GASTRIC VAGUS. stomach, 60 c.c. of meat extract in warm water were administered immediately after washing out. A water float manometer connected to the stomach tube was used for recording the gastric movements, the whole system being filled with warm 0 9 p.c. saline. The peripheral end of the cut vagus nerve was stimulated both in the neck and below the heart in the thorax, and, in certain experiments, the thoracic sympathetic chain was also stimulated. To expose the infracardiac vagus, the ribs were exposed through a longitudinal incision through the skin and latissimus dorsi, an intercostal space was opened and the ribs held apart., One of the larger branches of the vagus was identified on the cesophagus, tied and dissected downwards for some 2 cm. The cesophageal vagus in the cat consists usually of a main right and left branch with one or more finer twigs lying posteriorly. When necessary, the sympathetic chain was dissected out through the same incision. All nerve stimulation was carried out by means of fluid electrodes filled with the warmed defibrinated blood of the animal under experiment. The electrodes were constructed of vulcanite, electrical contact being made by silver plugs (Fig. 1). The nerve is drawn through on a fine ligature and the lower hole filled with vaseline or a mixture of hard paraffin and vaseline. The lower chamber is then filled with blood by means of the side tube A, which is then plugged. The upper chamber is filled through B, and the thread holding the nerve is secured by the plug. After application to the nerve, the electrodes are allowed to rest in the thorax, and the thoracic wall is closed. In these conditions the nerve retains its excitability practically unaltered for as long as 6 hours, spread of current is effectively eliminated, and the nerves may be stimulated without any interference with the animal under experiment. Stimuli of frequencies between 2 and 60 break shocks per sec. were applied by means of a calibrated induction coil and the contact breaker described by Brown and Lees [1931]. RESULTS. 215 In all experiments in which amytal was administered to the spinal animal, the normal reaction to vagus stimulation was determined prior to the administration of the drug. We consider that the discrepancies in the literature and the rarity with which animals not under the influence of anaesthetics have been employed for the investigation of the effect of vagus stimulation on the stomach justify a separate consideration of the results obtained.

4 216 G. L. BROWN AND R. C. GARRY. Normal response to the vagus. The account given below applies to the effects of stimulating both the cervical and the infracardiac vagus unless statement is made to the contrary. In general, the results may be divided into three classes. (a) When the stomach shows definite tone and activity. As a criterion of the tonus of the stomach we have used the ability of the organ to maintain a head of pressure in the manometer [Mc Cre a, Mc Swiney and Stopford, 1925]. When the stomach is atonic, the introduction of as much as 200 c.c. of fluid does not cause the entogastric pressure to rise above about 5 cm. When, on the contrary, the gastric tonus is high, the stomach will maintain a pressure of as much as 15 cm. with the introduction of only 60 c.c. of fluid. The term activity we have used in reference to the presence of rhythmic movements. Adequate rhythmic movements are usually, but not always, associated with good tone. When the stomach shows good tone and activity, the response to vagal stimulation is predominantly one of inhibition (Fig. 2 A). Frequently the inhibitory effect is preceded by a sharp contraction of short duration. In some instances, with prolonged stimulation, the inhibition tends to escape and the record regains its previous level. The inhibition is sometimes followed by some augmentation of rhythmic movements and a rise of the record above its previous base line. In experiments of this class, inhibition results from all strengths and frequencies of stimulation, a frequency as low as 2 per sec. causing a profound and lasting inhibition. (b) When the stomach is definitely atonic. The response to vagus stimulation is predominantly one of contraction and augmentation or initiation of rhythmic movements. The contraction is frequently preceded by a relaxation of very short duration. (c) When the condition of the stomach is indefinite, motor responses are common at the outset of the experiment. This response frequently becomes more and more of the nature of an inhibition as the stimulation is repeated and the tone and activity rise. In these experiments, variations in the response due to alterations in frequency, duration and strength of the stimulating current are most commonly observed. In the majority of our experiments, low-frequency stimulation favoured inhibition and high-frequency favoured contraction. In one experiment alone, results which might agree with the "Wedensky" hypothesis of Veach [1925] were observed. The effects of duration of stimulation are of interest. A short duration stimulus of high frequency produced

5 REVERSAL OF THE GASTRIC VAGUS. 217 inhibition as a post-stimulation phenomenon. Prolongation of such a stimulus carried the response through the inhibition to a marked contraction. It is, indeed, frequently possible to obtain any response desired by the adjustment of the duration and type of stimulation. This is rendered especially easy when the final response, be it contraction or relaxation, is preceded by an initial response of short duration of the opposite sign. For instance, relaxation is, as previously pointed out, Fig. 2. Spinal cat. Stimulation of left vagus in the neck: A. Before amytal, 50 per sec., coil 0. B. Immediately -after administration of 40 mg. of amytal per kg., 50 per sec., coil 0. C. Three hours after amytal, 50 per sec., coil 0. frequently preceded by a sharp contraction; suitable stimulation at a sufficiently low frequency will then produce a series of such preliminary contractions, giving the appearance of a true motor response. Similarly, as shown above, a short stimulation may evoke only the short relaxation which frequently precedes an otherwise pure motor reaction. In general, no significant differences were observed between the effect of stimulating the vagus in the neck and below the heart, and, in fact, the records obtained from each site of stimulation are frequently superimposable. In one experiment alone, neck stimulation produced contraction and thoracic relaxation.

6 218 G. L. BROWN AND R. C. GARRY. To summarize, then, it appears that the predominant factor determining the direction of the reaction of the stomach to vagus stimulation is the condition of activity and tonus of the organ. Where this is indeterminate, the response may be controlled by the frequency and duration of the stimulating current. In many experiments, low frequency stimulation is more favourable to relaxation than high frequency. It is of interest to note in this connection that Dickinson and McSwiney [1932] find that in the isolated innervated preparation of the tortoise stomach, low-frequency stimulation is always more effective in the production of an inhibitory response to vagus stimulation. Effects of amytal. (1) In the spinal animal. (a) Vagus response in the neck. The initial experiments were performed in order to find whether amytal had any effect on the motor response of the stomach to vagus stimulation. Accordingly, spinal cats were prepared, and the administration of meat extract after ligation of the pylorus was omitted. In these circumstances, the usual response to vagus stimulation is one of contraction. The administration of amytal did not interfere in any way with the augmentor effect of vagus stimulation. Preparations were then made for the study of the inhibitory vagus effect. A constant inhibitory response to vagus stimulation was first established. Amytal was then either injected intravenously in small volume or given in dilute solution by slow intravenous infusion. The immediate effect of the drug is to cause a considerable fall of bloodpressure, which later recovers, and frequently a rise in tone of the stomach. This rise of stomach tone is most frequently seen after small doses. Doses of an anaesthetic value may cause some depression of gastric tone and activity. After administration of the drug, vagus stimulation of all strengths and frequencies causes contraction of the stomach (Fig. 2), the previous inhibition being completely reversed. This contraction may be preceded by a small preliminary inhibition. Fig. 2 also shows the partial disappearance of the action of the drug after the passage of 2 hours. It is interesting to note that the reappearance of the gastric inhibitory effect is synchronous with the reappearance of the cardiac inhibition. It must be pointed out, however, that the effects of the drug on the cardiac vagus and on the gastric vagus do not necessarily run parallel, since reversal of the gastric vagus may be produced by doses of the

7 REVERSAL OF THE GASTRIC VAGUS. 219 drug insufficient to abolish the inhibitory effect of the vagus on the heart. (b) Vagus response in the thorax. It appeared possible that the reversal of the gastric vagus effect might be connected in some way with the abolition of the inhibition of the heart. We accordingly stimulated Fig. 3. Spinal cat. Stimulation of right infracardiac vagus: A. Before amytal, 60 per sec., coil 10. B. After 30 mg. of amytal per kg., 60 per sec., coil 10. the vagus below the heart before and after the administration of amytal. Fig. 3 shows the effect on the vagus response of such an injection of amytal: there is, as in the neck, complete reversal of the inhibitory effect. The reversal of the infracardiac vagus is, however, not always as clear cut as the reversal from stimulation of the cervical vagus. Preliminary inhibitions are more prominent and persist after large doses of the drug.

8 220 G. L. BROWN AND R. C. GARRY. (2) In the animal ancesthetized with amytal. Stimulation of the cervical vagus in the animal anwisthetized with amytal caused in all cases contraction of the stomach. No inhibitory effects were observed. When the vagus is stimulated below the heart, a very interesting result is observed (Fig. 4). In the majority of experiments, the first stimulation in the thorax is definitely inhibitory. On repeating the stimulation when the stomach record has reached its previous level, the response is one of diminished relaxation or even Fig. 4. Cat anassthetized with amytal. Consecutive stimulations of left infracardiac vagus, 10 per sec., coil 10; showing development of reversal. contraction. With succeeding stimulations, the inhibitory effect becomes successively smaller until pure contraction alone is the final result of stimulation. Once the motor effect has been elicited, it persists throughout succeeding stimulations, if they are given in close sequence. If, however, no stimulation be applied for a period of at least 20 min., the first stimulation after this interval causes inhibition, contraction again occurring on repeating the stimulation. Amytal and the sympathetic. The action of amytal appears to be restricted to the inhibitory vagal mechanism of the stomach. In animals anaesthetized with the drug and in spinal animals which have received amytal, stimulation of the thoracic

9 REVERSAL OF THE GASTRIC VAGUS. 221 sympathetic chain causes inhibition with high frequency stimuli, while the vagus both in the neck and in the thorax causes contraction (Fig. 5). It must, however, be pointed out that amytal, like all anesthetics, may, in large doses, produce sufficient depression of the stomach to reverse Fig. 5. Spinal cat after administration of 62 mg. of amytal per kg.: A. Stimulation of left cervical vagus, 60 per sec., coil 7. B. Stimulation of right thoracic sympathetic chain, 60 per sec., coil 10. Previous to administration of the drug, both nerves produced relaxation. the action of the sympathetic. If amytal in dilute solution be allowed to infuse slowly into a vein, the complete sequence of the reversal may be observed. At the beginning of the experiment, stimulation of both nerves causes relaxation of the stomach. As the concentration of amytal rises, the response to vagal stimulation becomes contraction, the sympathetic remaining inhibitory. Further increase of the drug concentration causes reversal of the sympathetic.

10 222 G. L. BROWN AND R. C. GARRY. DISCUSSION. The response of the stomach of the spinal animal to vagus stimulation has been investigated with a view to the establishment of the normal reaction. It appears clear that, as far as our present knowledge goes, the main factor determining the direction of the response of the stomach to vagus stimulation is the condition of activity of the peripheral mechanism. The frequency of stimuli applied to the nerve may play a part in determining the response, but only when the peripheral conditions are, as it were, in the balance. In these circumstances, frequencies between 2 and 20 per sec. are most effective in causing inhibition. We have found no evidence in favour of the existence of We densk y effects in our preparations. We wish to draw attention to the very significant differences which exist between the response of the stomach to vagus stimulation and the effects of sympathetic stimulation. As Brown, McSwiney and Wadge [1930] have shown, the response of the stomach of the spinal animal to sympathetic stimulation bears little relation to the activity of the peripheral mechanism, the main factor being the frequency of stimuli incident upon the nerve. This difference is very well shown in the behaviour of the isolated innervated stomach preparation. Mc SWiney and Robson [1931 a, b] and Brown and McSwiney [1932] were able to obtain inhibition and contraction of the stomach strips at will by altering the frequency of stimuli applied to the peri-arterial (sympathetic) nerves. In the case of the vagal innervated strip, contraction was readily obtained, but inhibition occurred only after the addition of atropine to the preparation [McSwiney and Robson, 1929]. Further very striking differences between vagus and sympathetic effects are observed in the form of the reactions in the intact animal. The inhibitions and contractions evoked by sympathetic stimulation are almost invariably simple in form. Those produced by vagus stimulation are almost as frequently complicated by preliminary reactions opposite in sign to the final response. A striking feature of these precursory reactions is their phasic character, the total duration being seldom more than 30 sec. These preliminary effects add considerably to the difficulty of interpretation of records, since stimuli of short duration may be sufficient to elicit only the preliminary effect. It is clear, then, that the vagus and the sympathetic effects on the stomach present many gross differences. In general it appears that the vagus system is very much more sensitive to external influences than

11 REVERSAL OF THE GASTRIC VAGUS. 223 the sympathetic, and that the vagal inhibitory mechanism is largely disabled by the environmental change incident in the preparation of the isolated innervated strip. It is tempting to attribute these differences to the presence of the peripheral neurone in the vagus system. The effects of amytal on the response of the stomach to vagus stimulation present several points of interest. The gastric peripheral mechanism is peculiarly susceptible to depressant drugs, and persistent motor effects are not uncommon in ansesthetized animals. The action of amytal, however, appears to be much more specific than due simply to general depression of gastric function. In spinal animals which have received small doses of the drug, there may be an actual increase in tonus and activity, and, moreover, in animals anaesthetized with the drug, the initial reaction to vagus stimulation in the thorax may be inhibitory. It has been suggested that the inhibitory effects on the stomach of stimulation of the cervical vagus might be associated with the effect of the vagus on the heart, either mechanically on account of the fall of general blood-pressure, or by transmission by the blood stream of specific substances liberated in the heart [Brinkmann and Van der Velde, 1925]. The normal response of the stomach is apparently identical whether the vagus be stimulated in the cervical region or below the heart. There exist, however, certain differences when the effects of amytal are recorded. It appears that it is more difficult to reverse the vagus with amytal when stimulation is carried out below the heart. This phenomenon, however, does certainly not suggest that the cardiac effects do in any way contribute directly to the gastric inhibition. The limitation of the action of amytal to the vagal system is of interest. Brown and McSwiney [1932] have recently shown that the inhibitory effects on the stomach of sympathetic stimulation may be reversed by the administration of luminal, and that the reversal is confined largelv to the sympathetic system, since when the action of the sympathetic is purely motor, inhibition can still be obtained by vagus stimulation. In the case of amytal, the reverse holds good. When the vagus is causing contraction only, sympathetic stimulation still produces relaxation. It must be pointed out, however, that, with both drugs, pushing of the dose too far can produce abolition of the inhibitory effects of both the vagus and sympathetic. The present work shows that there exist very marked differences both in the response of the stomach to vagus and sympathetic stimulation and in the relative susceptibility of the two systems to certain anaesthetics. Kiss [1931] has recently suggested that the vagus conveys

12 224 G. L. BROWN AND R. C. GARRY. to the stomach and intestines fibres which are entirely sympathetic in origin, and that the actions of the so-called parasympathetic system are merely a "negative phase" of true sympathetic activity. While we are not in a position to criticize Kiss's morphological findings as to the central origin of the anatomical vagus, this work shows that the peripheral effects, at least, of the two systems are very different and suggest a distinct mechanism for their actions. The same criticism is applicable to the work of Kure [1930, 1931], in that he suggests that the motor effects of splanchnic stimulation are due to admixed fibres of parasympathetic origin. Brown and McSwiney [1932] have suggested that the motor and inhibitor responses of the stomach to the sympathetic and their reversal by luminal might be explained on the assumption of the liberation peripherally of a specific substance, the direction of response depending on the rate of liberation. We have not sufficient evidence at our disposal to be able satisfactorily to explain the vagus phenomena on the same basis. It is, however, significant that we have found the optimum frequency of stimuli to produce inhibition by the vagus to be between 2 and 20 per sec., in view of the findings of McSwiney and Robson [1929] that maximum summation of stimuli takes place in the vagal peripheral neurone when the shocks applied to the nerve fall at an interval of 01 sec. This suggests that inhibition in the intact animal is associated with the maximum peripheral effect. SUMMARY. The normal reactions of the stomach of the spinal cat to stimulation of the cervical and infracardiac vagus are described. Stimulation may result in either contraction or relaxation. The predominant factor determining the direction of response is the condition of activity of the peripheral neuro-muscular mechanism. When this is indeterminate, the direction of response may be controlled by alteration in the frequency and duration of the stimulation; stimuli falling between 2 and 20 per sec. are most effective in producing relaxation. No significant differences were observed between the effects of vagus stimulation in the neck and below the heart. Administration of amytal (iso-amyl ethyl barbituric acid) in subanaesthetic doses reverses the inhibitory action of the vagus, leaving unaffected the response to sympathetic stimulation. The differences

13 REVERSAL OF THE GASTRIC VAGUS. 225 between the effects on the stomach of vagus and of sympathetic stimulation are discussed. We wish to express our thanks to Prof. B. A. McSwiney for his advice and criticism throughout this investigation. The expenses of this investigation were defrayed in part by a grant made to Prof. B. A. McSw,iney by the Government Grants Committee of the Royal Society. REFERENCES. Brinkman, R. and Van der Velde, J. (1925). Pfluegers Arch. 209, 383. Brown, G. L. and Lees, D. S. (1931). J. Phy8iol. 72, 17 P. Brown, G. L. and McSwiney, B. A. (1932). Ibid. 74, 179. Brown, G. L., McSwiney, B. A. and Wadge, W. J. (1930). Ibid. 70, 253. Carlson, A. J., Boyd, T. E. and Pearcey, J. F. (1922). Amer. J. Physiol. 51, 14. Dickinson, S. and McSwiney, B. A. (1932). Personal communication. Garry, R. C. (1930 a). J. Physiol. 69, 12 P. Garry, R. C. (1930 b). J. Pharm. Exp. Ther. 39, 129. Kiss, F. (1931). Arch. Anat. Hist. Embryol. 13, 165. Kure, K., Saegusa, G., Kawaguchi, K. and Shiraishi, K. (1930). Quart. J. Exp. Physiol. 20, 51. Kur6, K., Ichiko, K. and Ishikawa, K. (1931). Ibid. 21, 1, 103, 119. McCrea, E. D. and McSwiney, B. A. (1926). J. Physiol. 61, 28. McCrea, E. D., McSwiney, B. A. and Stopford, J. S. B. (1925). Quart. J. Exp. Physiol. 15, 201. McSwiney, B. A. (1931). Physiol. Rev. 11, 478. McSwiney, B. A. and Robson, J. M. (1929). J. Physiol. 68, 124. McSwiney, B. A. and Robson, J. M. (1931 a). Ibid. 71, 194. McSwiney, B. A. and Robson, J. M. (1931 b). Ibid. 73, 141. McSwiney, B. A. and Wadge, W. J. (1928). Ibid. 65, 350. Shafer, G. D., Underwood, F. J. and Gaynor, E. P. (1930). Amer. J. Physiol. 91,461. Veach, H. 0. (1925). Ibid. 71, 229. Veach, H. O., Schwartz, L. L. and Weinstein, M. (1930). Ibid. 92, 453. PH. LXXV. 15