Sympathomimetics impart their effects via .alpha. and .beta. receptors. Of the sympathomimetics, there are natural and synthetic substances which are predominantly effective either via .beta. or via .alpha. receptors.
The activity of sympathomimetics which act predominantly via .alpha. receptors consists, for example, in the contraction of the smooth muscle of the blood vessels and uterus, the sphincters in the gastrointestinal tract and the M. dilatator Pupillae (mydriasis). They also have a relaxant effect on the longitudinal muscle of the gastrointestinal tract and cause glycogenolysis in the liver.
The .alpha.-mimetics are known to be useful systemically for producing hypotensive regulation of the circulation, and locally for reducing inflammation of the mucous membranes in the nose, for stopping diffuse bleeding, as a mydriatic and as an additive to local anaesthetics.
It is known that in some species of laboratory animals, intravenous administration of clonidine results in a brief (limited to a few hours) but substantial increase in the production of growth hormones and that intramuscular administration in monkeys leads to an increase in appetite and a consequent increase in food intake and a brief weight gain. The food intake in rats has also been shown to increase after intracerebral administration of this drug.