Principles of General Physiology

Mention may also be made of the substance extracted by rain from grass, which has been shown by Pickering (1911, see also Russell, 1912, p. 112) to be injurious to apple trees. They should not, in fact, be surrounded by growing grass, as is common in orchards. There are some of these substances which will receive mention in the present section for two reasons. The first is that, although the subject properly belongs to pharmacology, it is clear that the mode of action of the hormones of the previous section cannot be understood until we know more of the action of drugs on cells. The second is that certain alkaloids and ot&er active principles are of great value as means of investigation, owing to their action as excitants or paralysers of particular kinds of cells or nerve endings.

Their Mode of Action. — The preparation by Barger and Dale (1910) of a series of amines, which were found to possess the power which adrenaline has of stimulating sympathetic endings, but in different degrees, gave the opportunity of comparing this property with their chemical structure. Details of the latter will be found in Barger's monograph (1914). It was found that approximation in structure to that of adrenaline was associated with increased intensity of action, and more definite restriction to the sympathetic system. The optimum carbonskeleton for this purpose consists of a benzene ring with a side-chain of two carbon atoms, of which the terminal one is attached to an NH2 group. This is further intensified by the presence of two hydroxyls on the benzene ring in the 3 : 4 position relative to the side-chain. These substances are, therefore, catechol derivatives. Of these bases, those with a methyl-ammo group, including adrenaline, produce the inhibitory effects of the sympathetic, such as that on the intestine, more powerfully than the excitatory effects on the blood vessels, etc. The opposite is true of the primary amines of the same series. It is to be noted, however, that a catechol nucleus is not essential. Catechol itself has no action of the kind referred to ; while not only parahydroxyphenyl-ethylamine, but also iso-amylamine, are powerfully active.

As has often been pointed out, in comparing the activity of a series of related substances it must not be forgotten that, in altering the chemical composition, we alter in many ways the physical properties also. We have to reckon with changes in solubility, in ability to pass through the cell membrane, in approximation to the colloidal state, in surface tension, in rate of diffusion, and so on. Barger and Dale, in the paper mentioned, give a valuable discussion of the theoretic aspect of the question, from which I take the following considerations. They show that the ease of oxidation has nothing to do with activity. Since there is evidence that the excitatory and inhibitory effects can be varied independently, reason is given for the belief that the myo-neural junctions

concerned with inhibition are not identical, in their relations to chemical substances, with those concerned with excitatory effects. Ergotoxine, as we shall see presently, is of interest in this respect. It is difficult to reconcile the view that the sympathetic nerves produce their effect by the liberation of adrenaline at the myo-neural junction with the fact that a base, which only differs from adrenaline by the absence of the methylation of the amino group, is as active. Why should it set free an allied substance, but not a more active one? The difficulty is further increased by the fact that certain inhibitory effects, as on the non-pregnant uterus of the cat, are relatively more easily produced by adrenaline than by nerve stimulation, whereas some motor effects, such as pilo-motor action, are more easily produced by nerve stimulation than by adrenaline.

The fact that these bases have very definite relations to the cells .of a certain morphological system shows that there "must be something in these cells, or connected with them and them only, which has a strong affinity for these amines." But it is pointed out that this property is by no means necessarily the same as that which confers stimulant activity on the amines. As I pointed out in connection with catalytic action, the adsorption of a substance on a surface is independent of the chemical action it may exert on the material of the surface after adsorption. Barger and Dale further call attention to atropine and pilocarpine, whose localisation is practically identical, while their action is opposite. Thus also the peculiar distribution of the action of nicotine, or of the sympathomimetic amines, does not necessarily depend on the existence of specific chemical receptors in the cells peculiarly sensitive to them. It may be that in some cells the stimulant substance easily reaches the site of action. The authors further find "the theory of receptive side-chains very difficult to apply " to their results. If the relation is one of chemical union, it seems that the points of constitution common to all the active bases should give an indication of the nature of the chemical receptor in the cells which combines with them. But there is only one common complex, namely : —

and this "exists in innumerable bases with no sympathomimetic activity." That physical factors intervene is indicated by the fact that differences in the relative activity of pairs of substances appear in the course of an experiment, and occasionally in an individual cat as compared with other cats. On the purely chemical view, it would be necessary to assume that there is a different chemoreceptor for each amine, and that these may vary independently of each other ; a view very difficult to maintain, since " the number of possible sympathomimetic amines is indefinitely large." The conclusion • arrived at is that " the least unsatisfactory view seems to us to be that which regards the existence of stimulant activity as dependent on the possession of some chemical property, the distribution and, in the main, the intensity of activity as due to a physical property."

The remarks of Straub (1912, p. 4) are also of interest. "The theory of the selective distribution of active substances in the organism has, as a necessary foundation, a purely material taking up of them by the cell. What happens in the cell in presence of the substance, how it gets there, and why it is held fast is the next question. It is frequently answered (as by Ehrlich) by the statement that the substance, as a chemical individual, reacts with chemical constituents of the chosen cell, with satisfaction of affinities and formation of a chemical compound. I hold this explanation, in its general aspect, as too far-reaching and inappropriate, and, in its results, as unfruitful. There are an indefinite number of substances which have a constitution scarcely capable of reactions in the organism, such as nitrous oxide, carbon dioxide, potassium salts, and many of those substances called indifferent narcotics, on account of their passivity ; one cannot imagine with what cell molecules they are*to show chemical affinity, since this affinity of the cell molecules arises merely from ordinary organic chemistry. If one is to operate

with ' chemo-receptors ' for all of these numerous cases, one mystery is merely changed into another. The existence of chemo-receptors for poisons is not to be denied, but this is not a universal fact, and therefore no general theory can be based upon it." There are also some definite experimental facts which show that it does not apply to certain typical cases. Straub himself (1907) has shown that the action of muscarine on the heart of Aplysia, and Neukirch (1912) that the action of pilocarpine on the mammalian intestine, are absent after the drug has entered the cells, and are only manifest when it is in the act of either entering or leaving. Further, Neukirch (p. 166) shows that no perceptible diminution in the strength of dilute solutions of pilocarpine is produced by the lying of the intestine in them, a fact difficult to understand if it were taken up in chemical combination in the cells.

Additional interesting facts come out in the investigations of Straub (1910) on strophanthin, a glucoside of the digitalis group. When injected into the organism, its action is most marked on the ventricle of the heart, next on the other heart cavities, and finally on the blood vessels. The active doses are extraordinarily small. Straub had previously found that the action of alkaloids is reversible, in that they can be regained from the organs which had stored them up, by simple washing with water. But on testing the case of strophanthin, he found that it could not be regained by washing out. It seemed possible that it had entered into a definite chemical compound with the heart muscle, or some constituent thereof, although it is difficult to see what kind of a compound, undecomposed by boiling, such a chemically inert substance as a glucoside could form with cell materials. On this account Straub thinks it always necessary to test other possibilities before making the assumption of a chemical union. On proceeding to investigate the question further, the unexpected result was obtained that, contrary to the case of the alkaloids, the reason why no glucoside could be extracted from the cells was because there was none there. The cells had not taken it up at all. Thus, if 1 c.c. of a solution containing O'Ol mg. of the drug, which is sufficient to produce a powerful action without killing the ventricle, be perfused backwards and forwards, it is found, by applying it to a second heart, that it is quantitatively as powerful as at first. The method of measurement was carefully worked out, and will be found in the paper. It was also found that the intensity of the action was always proportional to the concentration of the solution, not to the total amount present.

It is obvious that some slight loss must take place in such a powerful action, and, by repeated perfusion of a solution through six hearts, it was found that each heart had used 0'002 mg. This amount could not possibly be detected, and hence the reason why it seemed impossible to wash any out, since there was none in sufficient quantity to be detected if it were washed out. It is to be remembered that this quantity taken up by the heart would have no perceptible action if applied to another heart. Straub is inclined to think that the results show that an adsorption at the cell membrane is responsible for the activity, since these glucosides are allied to the saponines, which have great power of lowering surface energy, as we have seen. It will be remembered that, in adsorption, the amount taken up is in proportion to the concentration, just as the action of strophanthin was found to be ; and that a certain minimum concentration would be necessary in order that the quantity necessary for action should be adsorbed.

On the view that the cell behaves as a " giant molecule " in the chemical sense, it might be held that one molecule of an active drug would be sufficient to enter into chemical reaction with a cell. It is possible to calculate from Clark's data (1912), together with some which he has kindly given me, what must be the molecular weight of the compound with which strophanthin combines. This glucoside is favourable for the purpose, since it acts directly on the muscle cell. Clark had a more powerful preparation than Straub's, and found that 0*00008 mg. was sufficient to act on 5 mg. of heart muscle (dry weight). The molecular weight of strophanthin is 922, so that a simple proportion gives us the result : —

whence we obtain 57,625,000 for the " molecular weight" of the muscle cell, truly a " giant " amongst molecules. The participation in a chemical reaction of such a molecule is difficult to realise, as also how a comparatively small molecule attached at one point should influence the whole of the giant. Naturally, the molecule with which combination takes place may be only one of those present in the cell, but then we have to give up the theory of giant molecules.

The only remaining point of general application to which I would call attention is the nature of " specificity " in relation to these and similar systems. In connection with enzymes, I have discussed the question at some length in an appendix to my monograph on " Enzyme Action," and shown that the preferential action of an enzyme on a particular substrate is, apparently, only a matter of rate, and if so, that the " lock and key " illustration is not quite appropriate. If a key does not fit it will never open a lock, however long a time be given for it to do so. In fact, it seems to me that such a point of view, met with also in the form of a " moulding to templates," is not applicable to reactions in the organism, if to any chemical reaction at all. The kinetic view of velocities of reaction is more in accordance with facts.

In a previous page (page 60), in speaking of " specific " adsorption, it was pointed out that, in this process, there are innumerable possibilities of interaction of the various forces acting at surface boundaries, without the necessity of calling in the provisions of chemical groups which are to be supposed capable of combination with groups on the substance adsorbed, and with these particular groups only. Attention may also be directed to the work of Barger and W. W. Starling (1915) on the adsorption of iodine by certain organic compounds. This shows itself to be closely related to the chemical composition of these substances and therefore to that of their surfaces. But this relationship does not result in chemical combination nor in abolition of the nature of the process as an adsorption. It would appear that those properties of the surface, such as electric charge and so on, which control the degree of adsorption, are dependent on the chemical nature of the surface. This dependence need cause us no surprise, since the physical properties of a substance, inclusive of surface energy, are so closely related to its chemical composition.

When we have to deal with colloidal solutions, the considerations brought forward by Wolfgang Ostwald (1912) are to the point. We saw (page 107) that, when two colloids have charges of opposite electrical sign, it is usual to find that they mutually precipitate one another. Also that a colloidal solution is precipitated by ions of the opposite electrical sign to themselves. Now, Michaelis and Davidsohn (1912) found that the precipitation phenomena in cases of some " precipitins " and " agglutinins " are nearly independent of the hydrogen ion concentration in the solution, and, therefore, of the electrical charge of the particles. The conclusion drawn is that there is a specific chemical affinity between the substances concerned. But, as Wolfgang Ostwald points out, all that the experiments show is that processes of purely electrical neutralisation are insufficient for the purpose. But it is not to be supposed that electrical charges are the only properties of surfaces that concern colloidal systems. We have only to remember the " coagulation " by neutral salts, by non-electrolytes, by the results of " mechanical " adsorption on surfaces, and so on. Moreover, even when electrical charges are present, the precipitation is not always determined by them. For example, tannin precipitates gelatine better in the presence of acid ; gold sols are not necessarily precipitated when deprived of charge. The chief variable, surface tension, is altered, not only by electrical charge, but also by chemical composition, temperature, degree of dispersion, degree of solvation, and so on. The amount of electrolyte required to precipitate sulphur sols, as mentioned above (page 94), depends on the size of the particles.

It is not permissible, in fact, to refer all hitherto unexplained phenomena to chemical relations, as is customary. Substances are frequently assumed to be chemically different because they have some different physical property, although no chemical difference can be detected. Tannin, for example, has a higher optical activity in a higher dispersed condition than in a coarsely dispersed one, and has, therefore, been stated to be a mixture of "chemically different tannins," although no chemical difference has been shown to exist. Wolfgang Ostwald justly insists that the "chemical" conception is here a purely negative one, and that to be satisfied with the fact that the chemical composition of such substances as "immune bodies" is so complex that one may quietly ascribe all their properties to it, is the antithesis of a view leading to progress.

The growth of excised tissues in plasma is of interest in this connection. It was thought at one time that such tissues would only grow in plasma of the same individual, an extreme case of specific relationship. But Walton finds (1914) that this is not the determining factor ; any plasma, of the same or another individual, may contain certain substances which inhibit the growth of tissue, together with others which favour growth. The former are destroyed by freezing the plasma for one to three days, the latter by a longer period of freezing, six to eight days. Further, D. and J. G. Thomson (1914) have found that tissue from certain human tumours can be cultivated successfully in the blood-plasma of the fowl, to which has been added extract of embryo chick. Champy et Coca (1914) find the plasma of the cat is toxic for the tissues of the pigeon, while rat tissue grows excellently in tortoise plasma. The toxicity, in fact, is merely accidental. Reference may also be made to the work of Margaret R. Lewis (1915, p. 155) who obtained excellent growth in Locke's Ringer solution.

We may now proceed to refer, briefly, to certain examples of drugs which have a physiological interest. It is remarkable how great a variety of these active substances are formed by plants. It seems evident that they must be more or less accidental products of chemical change. A very small number would suffice for protection of .the plant from being consumed by animals for food. Similar conclusions may be drawn from the occurrence of adrenaline and a substance related to digitalin in the " parotoid " glands of a tropical toad, described by Abel (1911). It is impossible to see what use to a toad a rise of blood pressure in the animal which attacks it could be.

Acetyl-choline. — Dale (1914) finds that this substance produces vascular dilatation in extraordinarily small doses, much smaller than those of adrenaline required to raise the blood pressure. The perfused heart of the frog also shows a distinct inhibition with a dilution of one in a hundred millions. It excites especially the nerve endings of the cranial and sacral autonomic systems, causing stimulation of the vagus, secretion of saliva, contraction of the oesophagus, stomach, and intestine, and of the bladder. The effects, although so powerful, last but a short time. The ester is probably hydrolysed into its relatively inert components. It has scarcely any action on the plain muscle known to be innervated by the sympathetic system. Since it produces vascular dilatation, the non-sympathetic origin of vaso-dilators in general seems to be indicated.

Strychnine. — It is unnecessary here to say more about this alkaloid than to call attention to its peculiar physiological property of converting inhibition into excitation in the phenomena of reciprocal innervation, a property which has already been described (page 427). Nicotine. — It was shown by Langley and Dickinson (1889) that nicotine has the property, in moderate doses, of paralysing the nerve cells of sympathetic ganglia, without affecting the peripheral endings of the fibres. The effect appears to be exerted on the synapse, and is not confined to sympathetic ganglia. It serves, therefore, as a valuable means of discovering whether there is any cell station for given fibres in any situation to which the drug can be applied. Langley has made considerable use of it for this purpose. The synapses of different nerves require different doses, and also the sensibility to it of different species of animal is not identical.

Preceding the paralysis, there is a stage of stimulation, so that, amongst other phenomena, a large rise of arterial pressure results from intravenous injection of nicotine. Atropine and Pilocarpine. — The interest of these two alkaloids, the first in paralysing secretory nerves, the second in stimulating them, has been referred to above (page .344). Another useful property of atropine is to paralyse the vagus endings in the heart, and also that of paralysing the mechanism of accommodation in the eye, and causing dilatation of the pupil. Certain peculiarities in the mode of action of pilocarpine have been mentioned above (page 143).

lli/oscine. — This alkaloid, allied to atropine, is of interest in connection with the different activities of the natural alkaloids and their optical isomers. The latter, although not inactive, are less so than the former. Now hyoscine contains two optically active groups, both of which are physiologically active, so that four different isomers exist, each with a different degree of activity. Cushny finds that by assigning to each of the four constituents a definite value, that of each of the four hyoscines can be deduced. The values differ less than those of the two adrenalines. The question has some theoretical interest in connection with the doctrine of fitting to templates or " lock and key." If this were the correct point of view, it would be expected that only one of the isomers would be active, whereas they only differ in degree.

Veratrine. — The peculiar effect of this alkaloid in producing tonic contraction of skeletal muscle has been mentioned above (page 417). Lamm (1911) has described some interesting experiments which throw light on its mode of action. The conclusions which he draws are as follows. When a muscle is immersed in a solution of a salt of veratrine, it takes up small amounts of the poison, probably as free alkaloid, since the effect is more powerful in alkaline solution. A solution can be exhausted by means of a series of muscles. The drug has no effect until the muscle is stimulated independently. If the toxic action is small, the veratrine " tetanus " does not come on until the initial twitch is over. There is no reason to suppose that the kind of fibrillation taking place is due to stimulation of any different kind of substance (sarcoplasm) than that responsible for the twitch. It appears to be due to some kind of reaction between the poison and some product of metabolism of the active muscle. Thus a solution which has served for action on one muscle is found to be increased in activity. If a muscle is disintegrated in a veratrine solution, it is found that apparently more alkaloid can be extracted from the mass than was originally present in the solution. It is suggested that the exciting action depends on an increase in permeability of the cell membrane, since calcium salts markedly increase the amount necessary for an effect.

Ergotoxine. — Dale (1906) and Barger and Dale (1907) have obtained from ergot a preparation which has interesting properties. It paralyses those sympathetic endings which have an excitatory function, leaving intact those with an inhibitory function. Both kinds of fibres in the cranial and sacral autonomic nerves are left untouched. Thus the vaso-constrictors of the sympathetic are paralysed, but the inhibitory effect of the splanchnics on the small intestine is not affected. Adrenaline, after ergotoxine, causes a fall of blood pressure, but it is not quite certain whether this is actually due to a stimulation of sympathetic vaso-dilators, or to constrictor endings whose function has been reversed by the ergotoxine.

Cushny 's book (1910, 2), will be found to contain any further information required by the reader regarding the action of drugs. Toxins are the poisonous substances produced by micro-organisms. They may pass out into the culture fluid or be retained in the bodies of the microbes, only to be obtained from these by disintegration. Their chemical nature is unknown, since they can only be obtained in such small quantities. They are the substances responsible for the numerous diseases produced by the agency of micro-organisms. Pasteur showed, for example, that a culture of the bacterium of chicken cholera produced the symptoms of the disease, even after filtering off the organisms themselves. For further details, the reader is referred to the book by Burnet

Along with certain other substances, all of protein nature, so far as reliable evidence goes, toxins have the property of producing, when injected into the living organism, certain substances which have the power of neutralising their action. These are known as antitoxins, or, in general, as antibodies, and those substances capable of exciting their production are called their antigens. Statements have been made that substances other than proteins may act as antigens ; we have seen (page 3 16) that there is no satisfactory evidence that enzymes can act thus, although some lipoids and glucosides have been said to do so ; the evidence, however, that the preparations were free from protein is not sufficiently clear.

The mechanism by which "specific" antitoxins are produced is not yet understood. Very complex colloidal reactions are certainly involved. Two cases may be mentioned. The blood of the crayfish injected into a mouse makes it immune against scorpion toxin ; but the crayfish itself is more sensitive to the toxin than the mouse is, and its blood does not protect another crayfish. Rabbits can be immunised against tetanus by inoculation with tetanus bacilli in repeated small doses. Yet their serum has no effect in neutralising the toxin in vitro. •

The reader is recommended to consult the papers by Gengou (1908) for various facts to be taken into consideration. The "side-chain" theory of Ehrlich has now ceased to be of much help, otherwise than in the invention of new names, valuable as it has been in the past. It was found by Portier and Richet (1902) that a toxin could be prepared from the tentacles of the sea anemone which caused intense vascular congestion in the viscera of dogs, leading to death after some hours. They found also that, if a dose insufficient to cause death was given, so that the animal recovered, a second dose of only one-twentieth the amount of the first, if given subsequently, caused extremely severe symptoms, vomiting, diarrhoea, paralysis, and so on.

This effect was obtained only if a certain minimum number of days (eight to twelve) had elapsed between the first and second injections. It will be remembered that, in the usual process of immunisation to a toxin, injections are given at intervals of about four days, without the appearance of severe symptoms, so that the manifestation of this supersensitive state, called by Richet, " anaphylaxis" is prevented by the development of the ordinary state of immunity. We shall see presently that the two processes are intimately connected.

A remarkable fact is that non-toxic substances, such as egg-white, are also able to produce severe symptoms of anaphylaxis. But a necessary condition seems to be that of the colloidal state ; a similar condition applies to the property of acting as antigens, in general. What evidence have we as to the nature of this interesting phenomenon ? The monograph by Richet (1912) gives a good general account of the subject, but the most important experimental work is that of Dale (1912). The value of these investigations consists in the fact that they were made chiefly on an isolated organ, the virgin uterus of the guinea-pig, so that the conditions could be accurately controlled ; the results are of great importance, not only in the interpretation of anaphylaxis, but in the theory of immunity in general.