Effects of cadralazine on contractions induced by norepinephrine, serotonin, angiotensin II and K+ in rabbit aortic and renal arterial strips

Antagonistic effects of the new antihypertensive agent cadralazine (ethyl(+/-)-6-[ethyl(2-hydroxypropyl)-amino]-3-pyridazinecarbazate ) and its metabolite ISF-2405 [+/-)-6-[ethyl(2-hydroxypropyl)amino]-3-hydrazinopyridazine) on norepinephrine (NE), 5-hydroxytryptamine (serotonin), angiotensin II (angio II) and KCl induced contractions of rabbit abdominal aortic and renal arterial strips were compared with those of hydralazine. Substantially, cadralazine does not exert any effect on cumulative dose-response curves of these agonists in both vessel preparations even with the highest concentration of 10(-4) mol/l. ISF-2405 and hydralazine at concentrations of 10(-5) and 10(-6) mol/l showed non-competitive antagonism, depending not only on the dose but also on the length of the pretreatment time, on NE-induced contractions of abdominal aorta and renal artery. The two drugs attained maximal pD2 values with 60 min pretreatment without showing significant difference between the two vessel preparations, suggesting that the inhibitory effect of these drugs does not show vascular bed-related difference against NE-induced contractions. 60 min pretreatment with 10(-6) and 10(-5) mol/l of ISF-2405 and hydralazine also manifested non-competitive antagonism on contractile responses to serotonin, angio II, and K+ for both compounds. The degree of antagonistic effects of ISF-2405 and hydralazine on these agonists is similar, the order being angio II greater than serotonin greater than NE greater than K+. These results suggest that ISF-2405 and hydralazine exert direct vasodilating effects by the same mode of action at a site other than receptors against Ca2+ mobilization.