Blood platelet adrenoceptor: aggregatory and antiaggregatory activity of imidazoline drugs.

The effects of seven commonly used imidazoline drugs on human blood platelet aggregation and their inhibitory activity against the adrenaline-induced process were studied. The alpha-adrenoceptor reacting drugs, prazosin and yohimbine, were used as the standards. Low agonistic properties were found in the case of antazoline, clonidine and tetryzoline, whereas those three drugs along with naphazoline, phentolamine, tolazoline and xylometazoline exhibited marked antagonistic activity against the adrenaline-induced aggregation. The results are discussed from the point of view of the existing classification of the alpha-receptors. The conclusion is that the human blood platelet alpha-adrenoceptors, although resembling the properties of the alpha 2-subtype, form either a separate subclass of the receptors or a heterogeneous population of the alpha 2-subtype and an adrenergic receptor different from both alpha 1- and alpha 2-subclasses.