Active Plant Constituents

Their mode of Action

In order to understand the uses of medicinal plants and their mode of action, it is necessary to know something about their active constituents and the effectiveness of these.

The active principles present in a plant are very variable in amount; they may even be entirely absent if, for example, the plant has been grown under very unfavourable conditions or if the plant belonged to a race low in active constituents.

On the other hand, the amount of active principles at times may be higher than normal and under such circumstances the plant will have a stronger action.

For this reason the pharmacist has to standardise the majority of drugs that he uses in order to provide the physician and the patient with drugs of good average quality.

In the following paragraphs some of the important groups of active constituents are considered, together with their mode of action:

Among the mineral constituents of the body the salts of potassium and of calcium are specially important.

Potassium salts possess diuretic properties, while calcium salts contribute to bone structure, to the regulation of the nervous system and to the resistance of the patient to infection. The salts of potassium are found in abundance in almost all plants and they are generally present in soluble form.

Calcium salts are much less soluble and reach the body only in small amounts when tisanes are prepared and administered.

Silicic acid is also present in practically all plants and some contain large amounts (e.g. horsetail, knotgrass, lungwort). This acid acts as a strengthener of conjunctive tissues, especially of the lungs, thus providing some slight increase in resistance to pulmonary tuberculosis.

A balanced diet will provide sufficient intake of mineral salts. Those supplied by medicinal plants are associated with the other active principles and the medicinal action of such mineral constituents is of no great importance.

Organic acids (malic, citric, tartaric, oxalic, etc.) are also common constituents of plants; they accumulate, for example, in fruits. They act in certain cases as mild laxatives, especially tartaric acid and its salts.

Mucilage in plants has the property of swelling in water to produce plastic masses or viscous solutions; it is this property that produces their laxative effect: water is retained in the intestine, which prevents the contents from hardening and on the contrary acts as a lubricant; at the same time the content of the intestine increases in volume, increasing its pressure on the walls of the organ and favouring peristaltic movement.

In addition, the mucilage forms a protective coat on the mucosa so that irritants such as acids, salts, etc. are unable to come in contact with inflamed or diseased areas. For these reasons mucilaginous drugs are used as laxatives and for the protection of the inflamed mucosa of the digestive tract; sometimes these soothing properties are used for diarrhoeas, especially those caused by certain bacterial actions or by drastic irritants.

Mention must also be made of their use as hot compresses (poultices): a mucilage retains a large amount of water and hence maintains an elevated temperature, which penetrates the tissues progressively. The main mucilage-containing drugs are linseed, fenugreek, mallow leaves and flowers, Iceland moss and lime flowers. Mucilages are degraded to sugars by prolonged heat and so lose their activity.

Glycosides are substances that are decomposed into a non-sugar part and one or several sugars when hydrolysed by enzymes, by dilute acids or alkalis or by boiling. Their medicinal action is due to the non-sugar part of the molecules which are chemically very diverse. The sugar part of the molecule generally influences the solubility in water, and hence its absorption by the body. Many plant glycosides are of no therapeutic significance, but others, such as those present in foxglove, black hellebore or lily-of-the-valley are very cardio-active and at the same time increase diuresis; they are among the most active toxic substances found in plants. A special group comprises the anthraquinone glycosides found in buckthorn, alder buckthorn, monk's rhubarb as well as in Chinese rhubarb and senna; these are powerful laxatives.

Glycosides derived from salicylic acid comprise another group; they are found in some willows, in meadowsweet, violets, etc. Their action is febrifuge, anti-inflammatory, antiseptic and analgesic; they are used in the treatment of rheumatism.

For several years there has been an interest in a group of colourless or pale yellow glycosides, the flavonoids. They strengthen the blood capillaries and prevent the small cutaneous haemorrhages so frequent in the aged. Some of them relieve cramps of the smooth muscles while others improve circulation in the coronary arteries.

Saponins are also glycosides. Their outstanding physical character is that their aqueous solutions froth greatly; this is the reason for their use as detergents and it explains their name (Sapo, in Latin, means soap). Large doses in the blood stream are dangerous and may prove fatal by dissolving the red blood corpuscles (haemolysis). But since they are only feebly absorbed from the gastro-intestinal tract, their administration by mouth is generally without danger.

In the intestine they facilitate the resorption of certain substances (foods or medicines). They are mild laxatives, diuretics and ex;pectorants. Saponins are widespread in the plant kingdom, they are abundant in rupturewort, soapwort root, cowslip 'root, mullein and polygala. Like all other glycosides they are destroyed and lose their activity if their aqueous solutions are boiled.

As distinct from other saponins those of herb paris and corn cockle are well absorbed by the intestine and readily enter the blood stream, giving rise to haemolysis that may be fatal (especially the berries of herb paris).
When tisanes are prepared from drugs containing glycosides (including saponins), it is necessary to avoid prol ged boiling, since this will Place their efficacy.

Tannins have the property of precipitating proteins. For this reason they convert animal skins into leather. They are widespread through the plant kingdom, as for example in oak bark, walnut leaves, the willows, smartweed, roses, bilberry, lady's mantle, tormentilla and other Potentilla species, etc. In the free state and in large doses they irritate the mucosa; in small doses they precipitate small amounts of proteins in the cells of the mucosa which are thus rendered impermeable; other irritants are thus prevented from penetrating to the deeper layers of damaged mucosa, hence healing is aided.

This property also explains the use of tannins as antidiarroeals and in the treatment of certain burns. By an analogous process tannins prevent the development of bacteria since the proteins necessary for their nutrition are removed and also their own protein contents are precipitated. Tannins also contract the blood capillaries and so prevent certain haemorrhages. Tannins react with atmospheric oxygen and are converted to inactive substances; they are also destroyed by prolonged boiling in water.

Volatile Oils or Essences are among the most utilised products in popular medicine. They are very volatile, especially in steam. Their presence is the principal cause of characteristic plant odours. They are irregularly distributed throughout the plant kingdom: some families contain practically none, while others (Umbelliferae, Labiatae, Cornpositae, etc.) contain more or less significant amounts in many of their species. The volatile oil occurs most frequently in special glands, either within the tissues or on the epidermal surface.

Their medicinal activity is very variable. Some act on the central nervous system e.g. anise oil (carminative) or oil of wormwood (stimulant). Many increase the secretion of gastric juices (saliva, stomach and intestinal juices, bile) and hence increase appetite. They aid digestion and regularise intestinal action. When placed on the mucosa, on wounds or even on intact skin they can increase the flow of blood, especially of leucocytes (hyperaemia).

This property, associated with the bactericidal properties of certain oils, is the basis of their antiseptic action. Alcoholic solutions prepared from drugs containing volatile oils (tinctures) may be used as liniments for rheumatism; they act by increasing the flow of blood in the areas so treated.

Some volatile-oil-containing plants, e.g. juniper, lovage, stimulate secretion of urine; these are used to reduce accumulation of water in the body (dropsy).

Resins are secreted by special glands similar to those that produce volatile oils and frequently at the same time as these. They are not volatile; they are used as skin irritants.

Alkaloids are nitrogenous compounds that have a more or less marked action on the central nervous system and often, also, on the peripheral nervous system. Some alkaloids are among the most powerful poisons known. Only a few of the plants containing alkaloids are used in popular medicine, although henbane is used as a liniment to relieve pain. Belladonna and aconite are two well known plants containing alkaloids.

Bitter principles do not comprise one chemical group for their only common property is their bitter taste. But this property is of therapeutic significance; taken by mouth they increase the secretion of digestive juices and so increase the appetite of the patient. Among the bitter drugs are wormwood, gentian root, centaury, buckbean and Iceland moss.

Very many plants have a bitter taste and it must not be overlooked that some of these have other medicinal effects, they may be violent poisons as for example the tropical plant nux vomica which contains strychnine. But only those plants are used as bitters that, in suitable doses, are not toxic.

Some plants contain both bitter principles and volatile oils, e.g. wormwood or gentian. The two groups of constituents increase secretion of digestive juices and so increase appetite.

Antibiotics that are extracted from the so-called lower plants (of which the best known is penicillin) are of the utmost medicinal importance since they cure a number of infectious dis-eases. Although the majority of antibiotics are extracted from moulds some have also been found in certain 'higher' plants, e.g. burdock, which explains the anti-infective action attributed to these plants.

The study of medicinal plants for these properties is only just beginning, and at the present time only few can be named with antibiotic constituents. If the higher plants are not often used in the manufacture of antibiotics, it must be remembered that their growth is very slow in comparison with the moulds that yield penicillin.

The toxicity of certain medicinal plants has alreaay been mentioned. We shall return to this in every instance when it is necessary, as we consider the individual plants. This toxicity shows clearly that medicinal plants should be used only with due care. But on the other hand they must be used for those diseases that they can relieve.

On this topic it must be stressed that there is no difference in principle between the toxicity of medicinal plants and that of synthetic chemicals. Some plants are virtually non-toxic, while others become poisonous in higher than normal doses and one must always be aware of this.....

Because an active constituent is of natural origin, it does not mean that it is non-toxic; obviously both the active constituent and the plant itself may be poisonous.