Pharmacological studies of imidazoline derivatives. I. A comparison of the effect on pre- and postsynaptic alpha-adrenoceptors in the rat vas deferens.

The effects of 8 imidazoline derivatives on pre- and postsynaptic alpha-adrenoceptors and [3H]norepinephrine release were investigated in the rat vas deferens and were compared with those of reference drugs, yohimbine, phentolamine, tolazoline, and prazosin. K-6341 and K-6343 activated postsynaptic alpha 1-adrenoceptor and were antagonized by prazosin. The order of potency series as alpha 1-antagonist, prazosin greater than phentolamine greater than yohimbine greater than K-4011 greater than K-3827 greater than K-4300, tolazoline was found in antagonism against phenylephrine. Many derivatives exhibited an antagonistic action against clonidine, and the potency series as alpha 2-antagonist was revealed as follows: phentolamine greater than yohimbine greater than K-3827 greater than K-4011 greater than K-6341 greater than K-6343 greater than K-4300, tolazoline greater than prazosin greater than K-4299. The antagonistic effect was more selective on presynaptic (alpha 2) than postsynaptic (alpha 1) adrenoceptor and the rank order of selectivity (KB-post/KB-pre) was K-3827 greater than yohimbine greater than K-4011 greater than tolazoline, K-4300 greater than phentolamine greater than prazosin. Imidazoline derivative increased [3H]norepinephrine release, the actions of K-3827, K-6341 and K-4300 being particularly potent. From these results, the structure-activity relationship was summarized. First, when 2',3'-methylenedioxyphenyl group is connected with the imidazoline ring via an amino or methylene residue, these compounds become alpha 1-agonist. Secondly, when the methylenedioxyphenyl group and the imidazoline ring are connected via an amino residue, these compounds show a weak agonistic and potent antagonistic characteristics at the presynaptic alpha 2-adrenoceptor. Thirdly, an addition of a methylenedioxy radical to the phenyl ring increased the affinity of the compounds to presynaptic alpha 2-receptor.