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Drew G M. 
“Effects of a-Adrenoceptor Agonists and Antagonists on Pre- and Postsynaptically Located a-Adrenoceptors”. 
Eur.J.Pharmacol.. 1976;36(2):313-20.
The effects of a-adrenoceptor agonists and antagonists on preand postsynaptically located a-adrenoceptors in the pithed rat, were investigated. Rats (200-400 g) were anesthetized, pithed, and given atropine sulphate and tubocurarine EICl (Wellcome) and the vague nerves cut. B.P. and heart rate were monitored. The preganglionic nerves to the heart were continuously stimulated. The doses of agonists which reduced the stimulated heart beat by 50 beats/min -(presynaptic effect) and increased diastolic B.P. by 50 mm Hg postsynaptic effect) were determined. The presynaptic effects of antagonists were evaluated against clonidine-induced bradycardia stimulated hearts. The effects of antagonists at postsynaptic adrenoceptors were determined against clonidine-induced vasopressor response in rats without cardiac nerve stimulation. Agonists tested were salts of LSD (Sandoz), BAY-1470 (Bayer), clonidine (Catapress; Boehr.Ingelheim), oxymetazoline (MerckDarmstadt), naphazoline (Ciba-Geigy), methoxamine and phenylephrine (Koch-Light). Antagonists were phentolamine (Ciba-Geigy), yohimbine (Sigma), piperoxan, tolazoline (Ciba-Geigy), chlorpromazine (May+Baker), thymoxamine (Opilan; Warner), phenoxybenzamine (SK+F), bulbocapnine (K+K), metoclopramide (Primperan; Berck), cyproheptadine (Merck-Sharpe+Dohme), and metiamide. Clonidine was equipotent in reducing heart rate and elevating B.P. but LSD and BAY-1470 were more effective in reducing heart rate. Oxymetazoline, naphazoline and methoxamiae produced pressor responses at doses lower than those which reduced heart rate. Phentolamine was the most potent antagonist at presynaptic a-adrenoceptors. Piperoxan, ychimbine and tolazoline were 3 to 7 x less potent. Chlorpromazine and phenoxybenzamine were only weakly active and thymoxamine was 1000 x less potent than phentolamine. Other drugs had little or no effect. Phentolamine was the most potent antagonist of clonidine vasopressor response. Piperoxan, yohimbine and tolazoline were 3-7 times less active and thymoxamine was 10 times less potent. Conclusion Pre- and postsynaptic a-adrenoceptors may be of different types.
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