Hyoscyamine

HYOSCYAMINE, Hyoscyaminum, an alkaloid found along with others in plants of the family Solanaceae (Atropa, Hyoscyamus, Datura, etc.). The natural hyoscyamine obtained from plants is the levorotatory form; dextrorotatory hyoscyamine has been obtained artificially. l-Hyoscyaminum is similar in action to atropine and, by selectively acting on the endings of parasympathetic nerves, paralyzes them. d-Hyoscyaminum, on the contrary, appears to be of little activity in relation to the endings of the vagus nerve and other parasympathetic nerves (25-100 times weaker than the levorotatory form). At present, atropine is considered a racemic form consisting of equal parts of levorotatory and dextrorotatory hyoscyamine. The action of atropine on parasympathetic endings must be attributed almost entirely to the pharmacologically highly active l-hyoscyamine; since the latter is included in atropine in equal parts with the less active dextrorotatory form, it is understandable that atropine possesses half the potency of l-hyoscyamine. In relation to the central nervous system of warm-blooded animals, there is, according to Cushny, no such difference between levorotatory and dextrorotatory hyoscyamines, and therefore atropine exerts an action on the centers equal to that of the hyoscyamines. However, regarding the excitatory action on the spinal cord of a frog, d-Hyoscyaminum stands in first place, and l-Hyoscyaminum is many times inferior to it. Atropine, which in this case owes its action to the dextrorotatory ingredient, also stands here in strength between the two hyoscyamines. In terms of the strength of action on these elements, levorotatory hyoscyamine is almost twice as potent as atropine. In most of the plants mentioned above, both atropine and hyoscyamine are present, the latter in the predominant amount; however, during extraction performed without special precautions, levorotatory hyoscyamine partly converts into dextrorotatory, and a racemic form is formed—atropine. Due to the difficulty of obtaining hyoscyamine, pure preparations of it are not available for sale. Some clinicians attribute to hyoscyamine, unlike atropine, a depressing effect on the central nervous system; one must think that these observations relate to impure preparations containing an admixture of scopolamine. Hyoscyamine (l-Hyoscyaminum sulfuricum) is used in the form of eye drops (0.1-0.5%). Some believe that its mydriatic action occurs faster, but also passes sooner compared to atropine; however, it is unlikely that this applies to pure preparations of hyoscyamine. Hyoscyamine is detected in forensic cases in the same way as atropine (see).