AbstractChlordimeformN′‐(4‐choro‐O‐tolyl)‐N,N‐dimethyl‐formamidine; CDF), an acaracide‐insecticide, relaxed helically cut strips of rabbit thoracic aorta and deadventitiated aorta strips. Relaxations by CDF of aorta strips contracted by 5 × 10−7Mprostaglandin F2αwere not altered by the antimuscarinic agent atropine (10−5M), thebeta‐adrenergic antagonist propranolol (10−5M), the histamine H2antagonist cimetidine (2 × 10−4M), the dopamine antagonist haloperidol (10−5M) or the Na+‐K+‐ATPase inhibitor ouabain (5 × 10−6M). CDF further relaxed deadventitiated strips contracted by 10−6MNE or 40 mMpotassium and then partially relaxed by 10−5Mverapamil. CDF (10−5M) potentiated contractions caused by the addition of calcium to potassium depolarized strips in zero calcium media, but higher concentrations reduced calcium‐induced contractions. The highest CDF concentration (10−2M) not only prevented tension increase on calcium addition, but also progressively decreased tension to below baseline levels. The addition of 10−2MCDF to aorta strips not contracted by a vasoactive agent decreased resting tension. This relaxation was usually maximal at 10−3Mand less at 10−2M. Thus, CDF relaxes vascular smooth muscle, not by antagonism at the usual vascular relaxant
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