Pharmacological studies of imidazoline derivatives. III. Actions on cardiovascular system and acute toxicity

The effects of 8 imidazoline derivatives on the cardiovascular system and their acute toxicity were investigated. In anesthetized rats, K-6341 and K-6343 produced relatively long-lasting elevation of blood pressure. This hypertensive response was prevented by prazosin. In contrast, K-4011, K-3827 and K-4300 exerted long-lasting hypotensive actions. Norepinephrine-induced increase in blood pressure was competitively antagonized by these three derivatives. Therefore, the hypertensive and hypotensive actions of imidazoline derivatives are conceivably exerted by activating and blocking alpha 1-adrenoceptors. In the isolated guinea-pig atrial preparations, K-4011, K-3827, K-6341 and K-6343 produced positive inotropic responses which were independent of beta-adrenoceptor activation. The positive inotropic effect of K-6341, which was the most potent, was specifically attenuated or abolished by diltiazem, suggesting that a change in Ca2+ movement across the cell membrane is contributing to K-6341-induced increase in atrial contractions. The main symptoms of mice manifested when injected with imidazoline derivatives were a decrease in spontaneous movement, exophthalmos, an increase in palpebral opening, paralysis of four limbs and respiratory depression. Acute LD50 values of imidazoline derivatives in mice (i.v., i.p.) were between these of tolazoline and naphazoline.