Quantitative structure-activity relationships of imidazolidine derivatives related to clonidine at peripheral...

The effects of clonidine, oxymetazoline and thirteen substituted phenyliminoimidazolidine analogues of clonidine were studied and compared on postjunctional alpha-adrenoceptors in guinea-pig and rabbit aorta and on prejunctional alpha-adrenoceptors in guinea-pig atria. In the aorta all compounds were partial agonists at postjunctional alpha-adrenoceptors. Correlation of agonist potency with various combinations of their physicochemical parameters showed that, with the present series of compounds, physicochemical parameters alone determined 92% of the variance in the data with potency being primarily and highly correlated with pKa. Similarly in atria, all compounds, with the possible exceptions of St 95 and St 1943, appeared to be partial agonists at prejunctional alpha-adrenoceptors; transmitter noradrenaline release could either be inhibited (field stimulation at 1 Hz for 5 s) or enhanced (5 Hz for 30 s). The results indicate that 2,6-substitution in the phenyl ring appears to be a specific non-physicochemical activity-enhancing factor at prejunctional but not postjunctional alpha-adrenoceptors. A comparison with data in the literature giving the acute hypotensive potencies of the compounds suggests that central alpha-adrenoceptors may more closely resemble atrial prejunctional alpha-adrenoceptors than aortic postjunctional alpha-adrenoceptors. Quantitative correlations between centrally-mediated hypotensive and peripheral vasoconstrictor effects of clonidine-like imidazolidines do not provide a sufficiently sensitive method of distinguishing between alpha-adrenoceptor subtypes.