Influence of structural changes in the imidazolidine ring of clonidine on hypotensive activity

2-(2,6-Dichlorophenylimino)piperimidine with HNO3 (St-404), 2-(2,6-dichlorophenyl)-5,6-dihydroimidazo[2,1-b]thiazole fumarate (compound 44-549) and 1,2,3,5-tetrahydroimidazo[2,1-b]quinazoline with HCl (TIQ) were studied with respect to their effects on blood pressure and heart rate in the anaesthetized, normotensive rat following intravenous administration and in the chloralose-anaesthetized cat by means of infusions via the left vertebral artery. St-404 and compound 44-549 possess central hypotensive and bradycardic activities and display modes of action similar to that of clonidine. Central alpha-adrenergic receptors are presumably involved. In the anaesthetized, normotensive rat St-404 is about 3,400 times less active than clonidine, whereas compound 44-549 is 5 times more effective in lowering arterial pressure. The hypotensive effect of TIQ is brought about by a mechanism of action which is different from that of clonidine. The alpha-sympatholytic properties of TIQ suggested previously by others are not confirmed by the experiments presented in this paper.