Furthermore, most of the pharmacological actions of these salts are

Transcription

1 PHARMACOLOGICAL ACTIONS OF QUATERNARY AM- MONIUM SALTS. By J. RAVENT6S (Fellow of the Junta para Ampliacion de Estudios, Madrid). From the Department of Pharmacology, University of Edinburgh. (Received for publication 3rd March 1937.) THE curariform action of the quaternary ammonium salts was recognised by Crum Brown and Fraser in the remarkable researches which they carried out in Ever since that date these salts have been a favourite subject for pharmacological investigations, and consequently the literature is very extensive. It has, however, been summarised by Trendelenburg [1923], Kulz [1923], Gasser [1930], and Ing [1936]. Furthermore, most of the pharmacological actions of these salts are well established, and hence references will be only given regarding points of special importance. The author has studied the actions on a variety of isolated muscles of four series of compounds, namely (i) the series Me4.N, Me3EtN, etc., which is termed the Me3.N.R series; (ii) the series Me4.N, Et4.N, etc.; (iii) Me4N, Me3EtN, Me2Et2N, etc.; (iv) Me3.N.Phen, Me3N.CH2Phen, Me3N-CH2-CH2-Phen. METHODS. The following isolated muscles were used: (a) Frog's auricle, (b) frog's rectus abdominis, (c) rat's gut, (d) dorsal muscle of leech. These preparations were used immersed in Ringer's fluid, Tyrode's fluid, or Locke's fluid modified for the leech, and their isotonic movements were recorded in the usual manner. (a) The following technique was employed in the case of the frog's auricle. Rana esc. Hung. were used, and a strip of auricle free from sinus was isolated and driven at about 12 contractions per minute by break induction shocks. The Ringer's fluid had the percentage composition: NaCl 0 65; KCi 002; CaCl2 0'018; Na2HPO (ph 7.5). The movements were recorded by a light lever on smoked paper. This preparation was found to be very suitable for comparative quantitative estimations of the action of drugs which produced depression of the mechanical response. (b) Frog's rectus abdominis, (c) rat's gut, and (d) dorsal muscle of leech. Standard techniques [Gaddum, 1936] were employed with these

2 362 Ravento's isolated tissues, which were suspended respectively in Ringer's fluid, Tyrode's fluid, and Locke's fluid modified for the leech. The isotonic response of the rectus abdominis and leech and the rise of tonus of the gut were the responses measured. As regards the drugs used, some were bought (Messrs British Drug Houses) and others were kindly presented by Dr H. R. Ing of University College, London, and by Dr H. King of the National Institute, Hampstead. Dr W. 0. Kermack of the Royal College of Physicians Laboratory, Edinburgh, and Drs A. Jacob and J. Madinaveitia of the Department of Medical Chemistry, University of Edinburgh, kindly prepared certain of the compounds. Action8 of the Serie8 Me3.N.R. Kulz [1923] measured quantitatively the threshold molar concentrations of the series Me3.N.R which produced a curariform effect on the isolated sciatic-gastrocnemius preparation of the frog (Rana temporaria). Table I., column 1, shows his results; the figures show the equiactive concentrations expressed as molar/ Ing and Wright [1931] made similar experiments. The author has calculated from their tables the ratios of equiactive concentrations, which are shown in Table I., column 2, and the order shown for the relative activity of the different members of the series is in accordance with the conclusions in the original papers [1931, 1934]. The only difference worth noting between the results of Kulz and those of Ing and Wright is that the latter found Me3N.Et to have a weaker action than Me3.N.Pr, whereas Kulz found the reverse. Kulz [1923] also measured the relative activity of the members of this series in depressing the frog's heart. Table I., column 3, shows the concentrations he found to be equiactive as expressed as molar/ The author repeated these experiments on the frog's auricle. A very wide individual variation in response was observed. Measurements of the concentrations needed to produce the 50 per cent. depression of the response showed that in the case of Me4N this value varied in different auricles from 10-3 to 10-5 molar. The activity of each member was therefore compared with that of Me4N in each auricle used. Table I., column 4, shows the ratio of equiactive concentrations thus determined (Me4N = 100). These ratios agree with those obtained by Kulz except that I found Me4N to be much weaker relative to the remainder of the series than did Kulz. I found that Me3N.Hex showed a peculiar action in that it produced a 40 per cent. depression of activity in concentrations equal to those of Me4N, but that increasing the concentration of the Me3N.Hex did not produce a depression of more than 40 per cent.

3 Pharmacological Actions of Quaternary Ammonium Salts 363 TABLE I.-RATIOS OF EQUIACTIVE MOLAR CONCENTRATIONS OF MEMBERS OF THE SERIES Me3N.R. Curariform action in frog's Depressant action nerve-muscle on frog's cardiac Contracture. preparation. tissue. (1) (2) (3) (4) (5) (6) (7) Author. Kfilz, Ing and Kuilz, Auhr Author. Frog 9 right, Ato. rectus Author Leh Rat's gut. abdo- Leh minis. Me3N.Me. 1* Et *0 300* Pr Bu. 1' Am Hex (100) Hep No action No action Oct ,, 5000 No action 80 Dodec Cet No action No action No action No action The higher members of the series (i.e. Me3N.Hep and upwards) produced no depression of the auricle, and the compounds Me3N.Hex to Me3N.Cet inclusive antagonised the inhibitory action produced by the lower members of the series. This effect has already been reported [Raventos, 1936]. The action of the members of this series in causing a rise of tonus of the isolated rat's gut was found to be essentially similar to their action on the frog's auricle (Table I., column 5). In this case Me3OctN produced a feeble contraction at a concentration of about 10-4 molar. A similar action was produced by Me4N at molar. This result shows a ratio of activity of 50 to 1, but whereas in the case of Me4N increase of concentration caused increase of the response, in the case of the Me3OctN increase in the concentration abolished the feeble response. No contracture was observed with Me3CetN in any concentration. In the case of Me4N a diphasic response of the gut was frequently observed (fig. 2), namely, an initial fall of tonus followed by a rise. Low concentrations sometimes produced the fall of tonus without the rise. This diphasic effect produced by a pure chemical substance is of interest because similar effects produced by serum and tissue extracts have been ascribed to the action of multiple active principles [Clark and Gross, 1923]. The action of the members of the series on the frog's rectus abdominis

4 364 Ravento's (Table I., column 6) showed a smaller variation in activity than on the heart and the gut. Me3HexN and Me3HepN were found to produce a feeble contracture, whilst Me3HexN to Me3CetN antagonised the action of the lower members of the series. These results agree approximately with the previous observations of Kulz [1923], who found that when 10-4 molar concentrations were used, Me3HepN produced a greater contracture than Me3EtN, but less than Me3PrN. He found that Me3OctN produced no contracture and acted as a powerful antagonist to the lower members of the series. Ing and Wright [1934] measured the isometric response of the rectus abdominis and found the following increase of tension with 1 mille molar solutions:- Me4N-Me3AmN Me3HepN Me30ctN * 25 g. 5 g. ig. The effects produced by members of this series on the leech muscle (Table I., column 7) are completely different from their effects on the MeN4N afier, Me, N Me 3 OctfN C0tN *E LC3._ C.) L. Plf Cd.L- CTI 1:105 i: Z,5 x105 1; I0S C.) C11) I) F:1Zcso Io` a e o t f5oxg et 1: ad5 nl5 th 1-cinofleNadM3NOCt on the frog's rectus abdornin's and on the leech. FIG.~~~ ~~

5 Pharmacological Actions of Quaternary Ammionium Salts 365 other muscles since the higher members of the series produce a contracture in the same manner as do the lower members. Fnirthermlore, there is a relatively small diftlerence in the activities of the different meml)ers of the series. Fio-,s. 1 amdl 2' illustrate the (lifferences in the responses of the various typles of muscle investigated. The responses shown inl Table I. will be seen to fall into three groups; (a) Curariforin action and contracture of leech muscle. (b) (ontractuire of rat's gut and inhibition of frog's heart. (c) The response of the rectus ab(lom-imis, AN-hich wxas in some Me, N a&tcr- MC4N Mc03OcLN Mc 5Oc~N 01 0) Ln He) Ln C) 1: iq 1: j05 CY,- FIG. 2. :10K3 I:1O0 1:10>X1 Actions of Me4N and Me3NOct on the frog's auricle and rat's gut. respects intermediate, but showed a greater resemblance to the second class of response. The contrast between the responses of the different tissues to the different members of this homologous series are summnarised in Table II. The quantitative aspects of the antagonism of the higher members of the Me3NR series to the lower members of the series will be discussed in a later paper, but it may be mentioned that the drugs showed an increasing potency of antagonistic action in the series: Me3HexN < Me3HepN < AMe3OctN < Me3CetN.

6 366 Ravent6s TABLE II. Curariform Cnrcueo action. Contracture Cotracte of Contracture rectus abdominis. gut muscle and leech muscle. depression of heart. Ratio of activities Me4N/Me.N.Am Activity of Me3N.Oct S i m i I a r a n d N o contracture, F e e b 1 e contracture, as compared with nearly equal. but strong anta- no inhibition, b u t Me4N gonism of Me4N. strong antagonism of Me4N. This order of activity was found both with the frog's heart and with the rat's gut. The effect of increasing the length of the side chain R in the Me3NR series is therefore at first to increase the action produced on the frog's heart and on the rat's gut. At a length of chain between C5 and C7 this action disappears and an antagonistic action appears, and the latter effect is increased when the length of the chain is increased further. Actions of the Series Me4N to But4N. Ing and Wright [1934] found that the relative intensity of the curariform action of this series was as follows: Me4N=ButN > Pr4N > Et4N. Jordan [1877], Jacobj and Hagenberg [1902], and Marshall [1914] found that Et4N did not inhibit the frog's heart, did not produce contracture of frog's muscle, but antagonised the action of Me4N in the latter preparation. Kulz [1923] found that Et4N antagonised the action of the Me3BuN in the frog's heart, and that it did not produce contracture of the rectus abdominis. Burn and Dale [ ] found that Et4N did not stimulate the isolated cat's gut, and Schuller [1920] found that Et4N produced an atropine-like action on the frog's rectum. Hunt and Renshaw [1925] found that Et4N paralysed the frog's vagus but did not antagonise acetylcholine in the frog's heart or in the intact cat (blood pressure). Their failure to observe an antagonism to acetylcholine may have been due to the dose of the latter being large. The results obtained by the author are summarised in Table III. It will be seen that all four compounds produce a curariform action. As regards the other tissues Et4N produces a contracture in the leech as well as Me4N, but in no other case do the three compounds Et4N, Pr4N, and But4N produce an action similar to that of Me4N.

7 Pharmacological Actions of Quaternary Ammonium Salts 367 TABLE III.-RATIOS OF MOLAR CONCENTRATIONS OF SERIES Me4N-But4N PRODUCING EQUAL EFFECTS IN VARIOUS PREPARATIONS. Me4N. Et4N. Pr4N. But4N. Curariform action [Ing and Wright, 1934] Action on leech [Author]- Contracture nil nil Antagonism of Me4N... nil nil nil Action onfrog'8 rectub abdomini8 [Author]- Contracture nil nil nil Antagonism of Me4N... nil + + Action on rat's gut [Author]- Contraction nil nil nil Antagonism of Me4N Action on frog'8 auricle [Author]- Inhibition nil nil nil Antagonism of Me4N On the other hand, Et4N, Pr4N, and But4N have no antagonistic action in the leech, but all three antagonise Me4N in all the other tissues with the single exception that the Et4N does not show this action in the case of the rectus abdominis. These results agree with those obtained by previous workers except that I was unable to confirm Marshall's conclusion that Et4N antagonises Me4N in the frog's rectus abdominis. Actions of the Series Me4N, Me3EtN, Me2Et2N, etc. Boehm [1910] and Marshall [1914] studied the curariform action of the compounds intermediate between Me4N and Et4N. Their results are shown in Table IV. The author measured the action of these drugs on the leech. The results were unexpected since Me2Et2N had no action and the activity ran in the series: Me4N > Me3EtN > Me2Et2N < MeEt3N < Et4N. This series is quite different from that of the curariform activity which runs: Me4N > Me3EtN > Me2Et2N > MeEt3N=Et4N. The author obtained the same results as Marshall as regards the power of these drugs to produce contracture of the rectus abdominis, but whereas Marshall found that Me2Et2N to Et4N antagonised Me4N, the author could not demonstrate this effect. The relative activities of this series in the frog's heart were similar

8 368 Raventos TABLE IV.-RATIOS OF EQUIACTIVE MOLAR SOLUTIONS OF SERIES Me4N, Me3EtN, Me2Et2N, ETC. Me4N. Me3EtN. Me2Et2N. MeEt3N. Et4N. Curariform action- Boehm [1910] molar Marshall [1914] molar Action on leech [Author]- Contracture nil Antagonism of Me4N. - nil nil nil nil Action on rectus abdominis [Author]- Contracture... 1, nil nil nil Antagonism of Me4N. - - nil nil nil Action on frog's auricle [Author]- Inhibition nil nil nil Antagonism of Me4N. - nil +. + Action on rat's intestine [Author] Contracture nil Antagonism of Me4N. + to those on the rectus abdominis, except that Me2Et2N and Et4N produced a feeble antagonism of Me4N. In the case of the rat's intestine the compounds all produced contracture except Et4N. The results with this series were more complex than the others studied, and indeed the ratios obtained with the rat's gut were more like the curariform ratios than were the ratios obtained with the leech. The piperidine compounds, Me2 piperidino and Et2 piperidino, both produced no Me4N-like action on the heart, but antagonised the action of acetylcholine. The introduction of the piperidine ring therefore appeared to have an effect similar to that of two ethyl groups. Phenyl Compounds. The series Me3NPhen, Me3N.CH2Phen, Me3N.CH2CH2Phen was studied and the results are shown in Table V. Ing and Wright [1931] found that the curariform action of Me3N.Phen was about equal to that of Me4N. The author found that the activity of Me3N.CH.Phen on the frog's auricle was about 50 times that of Me4N. In the rat's gut the relative activities of this series in producing contracture was found to be as follows: Me3N.Phen and Me3N.CH2CH2Phen were about equal to Me4N, and Me3N.CH2Phen was about 10 times as powerful as Me4N. The fact that Me3N.CH2CH2Phen has an action similar to Me4N

9 Pharmacological Actions of Quaternary Ammonium Salts 369 TABLE V.-EQUIACTIVE MOLAR CONCENTRATIONS OF PHENYL COMPOUNDS (CONCENTRATIONS OF Me4N PRODUCING EFFECT=1). Curariform action [Ing and 1 Wright, 1931] Inhibition of frog'8 auricle 0-02 [Author] Me3N.Phen. Me3N.CHPhen. Me3N.CH2CH2Phen. Contraction of rat'8 gut [Author] is of interest, because the side chain in this case contains 8 carbon atoms, and the compound Me3N.Oct which contains this number of carbon atoms antagonises Me4N. This proves that the change over from one form of action to another is not dependent solely on the number of carbon atoms in the side chain R. The result of the introduction of a phenyl group resembles more the addition of 3 carbon atoms in a straight side chain, than the addition of 6 carbon atoms. Action of Quaternary Ammonium Salts on Mammalian Blood-pressure. Marshall [1914] found in cats that the series Me4N to Et4N produced a fall of blood-pressure. The relative activity of the different members of the series was as follows: Me4N was slightly more powerful than Me3EtN, whilst Me2Et2N was much weaker. Both Me2Et2N and MeEt3N had a very feeble action; Et4N had a very slight action in very large doses, and such doses reversed the action of Me4N and caused it to produce a rise of blood-pressure. Burn and Dale [1915] differentiated between two actions of these compounds on the blood-pressure of mammals. Firstly, a muscarinelike vaso-depressor action, and secondly, a nicotine-like vaso-pressor action, which was produced in atropinised animals and was due to the stimulation of the sympathetic ganglia. They found that Me4N produced both effects and that Et4N produced neither, but antagonised the nicotine-like action of the Me4N. Kulz [1923] found that Me4N constricted frogs vessels, and that this action was antagonised by Me3OctN. Reid Hunt [1926] found that the activity of the Me3N.R series in depressing the cat's blood-pressure was as follows:- Me3But.N > Me3N.CH2Phen > Me3N.Phen > Me4N. The ratio of the activities as estimated by dosage was roughly 4: 2: 1-5: 1. Et4N produced no action. VOL. XXVI., NO

10 370 Ravento's These compounds raised the blood-pressure of atropinised cats presumably by a stimulant action on the sympathetic ganglia. As regards this latter action Me4N was about 10 times as active as Me3But.N, whilst Et4N was inactive. Hunt and Renshaw [1925] found that Et4N had no atropine action on the cat. The evidence available suggests that the muscarine-like action of the quaternary ammonium salts in the mammal resembles their action on the frog's heart and rat's gut. The nicotine-like action is quite different from the muscarine-like action and shows the ratio of activity: Me4N > Me3ButN > Et4N. This is unlike any series found by the author for the activity of these compounds. Actions of Quaternary Ammonium Salts on Denervated Mammalian Muscle. Dale and Gasser [1926] found that contracture of the denervated cat's muscle was produced by Me4N, nitrosocholine (synthetic muscarin), acetylcholine, potassium salts, and nicotine. This effect wai not produced by true muscarine or by Et4N. Gasser and Dale [1926 b] showed that the action of acetylcholine on denervated mammalian muscle was not antagonised by atropine. DIsCUSSION. The manner in which Me4N and acetylcholine are antagonised by quaternary ammonium compounds will be discussed in a later paper, in which further evidence will be presented. The chief point shown by the experiments described in this paper is that the isolated tissues can be divided into three groups as regards their response to the quaternary ammonium salts. Group A. Paralysis of motor-nerve endings and contracture of leech muscle. Group B. Contracture of frog's muscle. Group C. Contracture of gut, inhibition of frog's heart. The distinction between these groups is shown most clearly by their responses to the Me3N.R series, which are represented in fig. 3. Group A shows comparatively small variations in the relative activity of the whole series. Group C shows that the activity is increased 100-fold as the length of the chain R in Me3N.R is increased from 2 to 5 carbon atoms, and that a further increase in length reverses the action so that no positive effect is produced and the action of the lower members of the series is antagonised. Group B is intermediate between A and C.

11 Pharmacological Actions of Quaternary Ammonium Salts 371 The series Me4N, Et4N, Pr4N, and But4N show a similar contrast (Table III.). All these compounds produce a curariform action, whilst only in the leech does Et4N produce an effect qualitatively similar to Me4N. On both the frog's heart and the rat's gut Et4N, Pr4N, and But4N all produce well-marked antagonism of Me4N, whereas only the last two compounds produce this action on the rectus abdominis, and no antagonism is produced in the leech. The results obtained with the series Me4N, Me3EtN, Me2Et2N, etc., which are shown in Table IV., are more difficult to explain. The results obtained by the author are very complex, since they show that every tissue is different as regards the relative activities of the series of compounds tested. It is therefore only worth while considering the most suggestive point. The classification into which the responses to the Me3N.R series falls is surprising, in that the three forms of contracture (leech, rectus abdomini, and gut) fall into three separate classes, since the so-called curariform action shows most resemblance to the contracture of the leech and the inhibition of cardiac muscle shows a general resemblance to the response of the gut. The two last effects have, however, an important characteristic, namely, that they both are of the parasympathetic type. It would appear, therefore, that parasympathomimetic actions differ as regards their response to quaternary ammonium salts from the other effects described above. The results show clearly that the visible character of the effect recorded is of no service as a basis of classification. For example, the various contractures produced have a certain resemblance as is shown in figs. 1 and 2, but the analysis demonstrates that the drug action in these cases differs very widely. The receptors in the frog's heart and the rat's gut are the most specific since small changes in constitution can produce a great augmentation of action, whilst further changes may abolish the action and cause the drug to act as an antagonist. The results obtained with the Me3N.R series on the gut and on the heart (fig. 3, curve C) can be interpreted on the hypothesis that the group Me3N unites with certain receptors in the tissue, and that the ease with which they are fixed increases as the length of the chain R in the Me3NR series increases from C2H5 upwards. The additional hypothesis must be made that a positive effect (contraction of gut or inhibition of heart) is only produced when the length of the chain R does not exceed a certain figure (6 carbon atoms in the heart and 8 carbon atoms in the gut). When this length is exceeded the compounds occupy the receptors, but do not produce an action, and hence antagonise the action of the lower members of the series. It is necessary to assume that in these tissues the Et3N group can also occupy the receptors, but that compounds containing this group

12 372 Ravento's cannot produce a positive action. Kulz [1923] showed that the series Et3N.R had an atropine-like action, the intensity of which increased as R increased. N/4000 Et.N.Me produced an antagonism similar to that produced by N/60,000 Et3OctN. The same assumption must -2 A -o- -I~~~~~~~~~~~~~~- C I FI1G. 3.-Relative actions of Me3N.R series on various preparations. Abscissa: Number of carbon atoms in R. Ordinate: Intensity of action expressed as ratio 100/x: (x =equiactive molar concentrations expressed as per cent. of concentration of MeN). Curve A. Action on motor-nerve endings and on leech. Curve B. Action on rectus abdominis of frog. Curve C. Action on frog's heart and rat's gut. be made in the case of Pr4N and BUt4N, namely, that they can occupy receptors but cannot produce a positive action. The response of the leech may next be considered, because as a general rule the responses of the rectus abdominis are intermediate in character between the parasympathomimetic responses and the responses of the leech. The responses of the leech to the Me.N.R series (Table I.) show that any drug containing the Me3N group produces contracture and that the intensity of action is not greatly influenced by the length of the chain R. The only exception to this general rule is that Me.CetN is inactive; moreover, this compound does not antagonise the lower compounds. Since Et4N produces an action qualitatively similar to

13 Pharmacological Actions of Quaternary Ammonium Salts 373 Me4N it would appear that the receptors of the leech can combine with either the Me3N or the Et3N group. The fact that Me3Et2N, Pr4N, and But4N are all inactive both as regards positive action and antagonism show that the leech receptors are selective in certain respects, and is of particular interest because these are the most striking differences observed between the response of the leech and the curariform effect. Tables I., III., and IV. show that none of the drugs investigated produce an antagonistic effect on the leech. Antagonism to Me4N can, however, occur in the leech, since I found that curarine antagonised the contracture produced by Me4N and by Et4N. In general, however, it would appear that antagonism is a much less common effect with the leech than with the frog's heart or rat's gut. SUMMARY. 1. The author measured the actions of various quaternary ammonium salts on a series of isolated tissues, namely: leech muscle, frog's rectus abdominis, frog's auricle, and rat's gut. 2. The following series of compounds were tested: (1) Me3N.Me, Me3N.Et... Me3N.Oct, Me3N.Cet. (2) Me4N to Bu4N. (3) Me4N, Me3.EtN, Me2.Et2N, Me.Et3N, Et4N. (4) Me3N.Phen, Me3N.Benz, Me3N(CH2)2Phen. 3. The tissues studied fell into three groups as regards their responses to the Me3N.R series: Group A. Leech muscle, which resembled in its response the motor-nerve endings. Group B. The rectus abdominis. Group C. The frog's auricle and the rat's gut. In Group A all the compounds produced actions similar in kind but of varying intensity. In Group C a reversal of action occurred. An increase of R in the Me3N.R series caused increased activity up to Me3N.Am, but further increase abolished the action, and the higher compounds then inhibited the action of the lower members of the series. The responses of the rectus abdominis were intermediate in character between Groups A and C, but more nearly resembled those of Group C. 4. The responses to the series Me4N, Et4N, Br4N, and But4N showed a similar grouping, but the similarity between the leech response and the curariform action was less complete. 5. The series Me4N, Me3EtN, Me2Et2N, etc., showed more complex responses.

14 374 Pharmacological Actions of Quaternary Ammonium Salts 6. The compound Me3N.CHPhen resembled in its actions Me3BuN and not Me3OctN. Hence the effect of introducing a phenyl group resembled that of the addition of 3 carbon atoms in a straight chain rather than that of the addition of 6 carbon atoms. It is with pleasure that I put in record my indebtedness to Prof. A. J. Clark for his kind hospitality, and for his constant help and advice. The expenses of this research were in part defrayed by the Moray Fund, for which help I express my thanks. REFERENCES. BoEHM, R. (1910). Arch. exp. Path. Pharmak. 63, 176. BURN, J. H., and DALE, H. H. (1914). J. Pharmacol. Baltimore, 6, 417. CLARK, A. J., and GROSS, L. (1923). Arch. Inter. Pharmacodyn. 28, 243. CRUM BROWN, A., and FRASER, T. R. (1869). Proc. Roy. Soc. Edin. 6, 147. DALE, H. H., and GASSER, H. S. (1926 a). J. Pharmacol. Baltimore, 28, 287. DALE, H. H., and GASSER, H. S. (1926 b). Ibid. 29, 53. GADDUM, J. H. (1936). Gefa88erweiternde Stoffe der Gewebe. Thieme, Leipzig. GADDUM, J. H. (1937). J. Physiol. 89, 7 P. GASSER, H. S. (1930). Physiol. Rev. 10, 35. HUNT, R. (1926). J. Pharmacol. Baltimore, 28, 367. HUNT, R., and RENSHAW, R. R. (1925). Ibid. 25, 315. ING, H. R. (1936). Physiol. Rev. 16, 527. ING, H. R., and WRIGHT, W. M. (1931). Proc. Roy. Soc. London, B, 103, 337. ING, H. R., and WRIGHT, W. M. (1934). Ibid., B, 114, 48. JACoBJ, C., and HAGENBERG, J. (1902). Arch. exp. Path. Pharmak. 48, 48. JORDAN, S. N. (1877). Ibid. 8, 15. KJLZ, F. (1923). Ibid. 98, 339. MARSHALL, C. R. (1914). Trans. Roy. Soc. Edin. 50, 379, 481. RAVENTOS, J. (1936). J. Physiol. 88, 5 P. SCHtLLER, J. (1920). Arch. exp. Path. Pharmak. 90, 196. TRENDELENBURG, P. (1923). Heffter's Handb. exper. Pharmakol. 1, 564. Springer, Berlin.