This section is from the book "Materia Medica And Therapeutics - Vegetable Kingdom", by Charles D. F. Phillips. Also available from Amazon: Materia Medica And Therapeutics: Vegetable Kingdom.
Active Ingredients. - There is still a considerable degree of uncertainty as to the part taken by the individual constituents of rhubarb in its action on the body. The first ingredient that requires notice is Chrysophanic acid, C14H10O4, now known to be the same substance which, under various impure forms, has received the names of Rhabarber-bitter, Rhabarberine, Rheumnine, Rhabarbergelb, Rhabarbergelbsaure, Rheine, Rhaponticine, Lapathine, and Rumicine (Peckolt). Chrysophanic acid, as such, does not perhaps exist to any large extent in rhubarb, but is formed by the splitting up of Chrysophane (to be presently described). It crystallizes out of alcohol in small orange-yellow prismatic and lustrous needles of a somewhat bitter taste. It is friable by heat, slightly soluble in water, freely soluble in ether and hot alcohol. From its solution in benzol, on slow evaporation, it is deposited in rather paler yellow crystals which are oblique rhombic tables. Concentrated sulphuric acid dissolves it red; water added to this solution separates the acid in yellow masses unchanged. Aqueous solutions of fixed alkalies and ammonia change it to a beautiful red; the ammonia solution, being evaporated, leaves un-changed chrysophanic acid, but the potash solution, if evaporated, leaves red chrysophanate of potash. (Chrysophanic acid is chiefly obtained from a vegetable powder, called Goa or Poh di Bahia, which is the product of some unknown Brazilian plant. The powder contains from 70% to 80% of acid.)
The other vegetable acid present in rhubarb is a peculiar tannic acid, C,6H16O14, a yellowish brown powder which attracts water very readily; is easily soluble in water and spirit, but not in ether. The brown colored aqueous solution has an acid reaction, precipitates perchloride of iron blackish green, reduces gold and silver salts, precipitates lime and albumen, but does not give a precipitate in solution of tartar emetic.
The neutral body Chrysophane, C16H18O8, is a glucoside, which, when boiled with dilute sulphuric or hydrochloric acid, splits up into sugar and Chrysophanic acid. Chrysophane is obtained in microscopic orange-yellow prisms; it is present in the proportion of about 9 or 10 grains to the pound of rhubarb-root, is highly soluble in water, insoluble in ether.
Concentrated sulphuric acid gives it a brownish color; from water it subsides in green flocky masses.
Emodine, C40H30O13, is an extra product of the (De La Rue) process for the extraction of chrysophanic acid; it forms deep orange-red crystals varying in size up to oblique rhombic prisms of two inches in length; these do not melt and decompose below a temperature of 482° F. Alkaline watery solutions turn it red; ammonia gives a violet-red.
Phaoretine, C16H16O7, is an extra product of the process for extracting the rhubarb tannin; it is a dull brown shining mass which pulverizes to yellow brown; is tasteless, melts on heating, and evolves yellow fumes that smell like rhubarb. It is insoluble in water, ether, and chloroform, very soluble in spirit, also in warm acetic acid.
Aporetine and Erythroretine are resinoid bodies which are possibly mere mixtures of other rhubarb constituents; at any rate, nothing accurate is known about them.
Physiological Action. - Rhubarb furnishes a striking instance of the widely, though not universally operative law, that changes in dosage not only alter the degree in which a medicament will act upon the body, but, when carried beyond a certain point, altogether change the mode of action. Taken in small doses, such as 4 to 8 grains of the powder, not only does it exert no purgative action, but, as will be presently shown, it is efficient in checking chronic diarrhoea; it exercises in addition a remarkable tonic influence upon primary digestion, increasing appetite, and enabling the food to be disposed of without discomfort in cases where previously there had always been sluggish digestion and flatulence. On the contrary, if taken in large doses, such as a scruple to a drachm, rhubarb induces none of these tonic effects upon the stomach, and acts as a direct laxative. The operation commences in from 4 to 8 hours after the medicine has been taken. According to the magnitude of the dose, and conditionally upon its having been repeated or otherwise, there may be more or fewer evacuations; but on the whole the character of the stools is fairly constant. They are of loose, but not of watery consistence, and usually of a markedly yellow-brown color. When the number of evacuations has been considerable, a brief period of constipation usually follows.
When these actions of rhubarb are further inquired into, it is found impossible as yet to decide as to the intimate working of the drug. Is the purgative action produced simply by a direct stimulation of the muscular fibres of the stomach and intestines, pushing their contents more steadily and rapidly onward? Most observers are inclined to adopt this idea; and if it were correct, the same loose character of the stools might be accounted for without the supposition of any unusual outpour of secretion, by the theory that the faeces simply passed onward too rapidly to allow of the normal mucous secretions being reabsorbed, a process which certainly contributes to the dryness of normal, and still more of constipated, dejections. And it is remarkable that, even in the largest doses, rhubarb does not appear, like jalap, scammony, and other resinous purgatives, to cause inflammation of the mucous membrane; at least such a result has never to my knowledge been recorded. The usual results of an unnecessarily large purgative dose are limited to the production of more or less violent colicky pains, such as do not attend the milder laxative operation.
On the other hand, it has been asserted by several writers that rhubarb specially affects the mucous secretions of the duodenum as well as its peristaltic action, and also that it increases the flow of bile. The former assertion must be incapable of proof, and the latter very probably rested partly on the absurd and effete doctrine of signatures, and partly on the fact that the stools contain a large quantity of yellow coloring matter. As to this coloring matter, however, it is in all likelihood only the same which makes its appearance in the urine, the sweat, and the milk of persons who have taken rhubarb.