Abstract
The effects of 8 imidazoline derivatives on α-adrenoceptor-mediated contraction or relaxation of various kinds of smooth muscle preparations were investigated. In rabbit aortic strips, K-6341, K-6343 and K-4011 produced α-agonistic action which was antagonized by prazosin. In the study of α-antagonistic action of imidazoline derivatives, i.e. antagonism against norepinephrine- or phenylephyrine-induced contraction of the rabbit aortic strip, the order of potency series, prazosin > phentolamine > yohimbine > K-4011, K-3827, K-4300 > tolazoline was found. Reserpine pretreatment did not affect the results. In the study of α-antagonistic effect on the relaxing response to norepinephrine of methacholine-contracted rat ileum, the potency series was revealed as follows : prazosin > phentolamine > yohimbine > K-4011, K-3827, K-4300, tolazoline. A similar experiment was performed on histamine-contracted guinea-pig taenia coli, and the results were nearly identical to the case of the rat ileum, except that K-6341 and K-6343 also exhibited α-antagonistic action which was not detected in the rat ileum. This suggests that α2-receptor may exist in the postsynaptic α-adrenoceptor in the guinea-pig taenia coli. The results of the present study indicate that the potency of imidazoline derivatives to inhibit norepinephrineinduced contraction of some smooth muscle is almost identical to that which inhibited norepinephrine-induced relaxation of other smooth muscles. Furthermore, K-4300 was found to possess anti-acetylcholine and anti-histamine actions in the guinea-pig taenia coli and anti-serotonin action in the rat fundus strips.
