Effects of some alpha-adrenoceptor agonists and antagonists on the guinea-pig ileum

The purpose of the present study was to further characterize the alpha-adrenoceptors located on parasympathetic fibres. Segments of guinea-pig ileum were stimulated by transmural electrical pulses, and the ensuing contractions, which are due to the release of acetylcholine from postganglionic parasympathetic fibres, were monitored. Clonidine and tramazoline, which are thought to act preferentially on presynaptic alpha-adrenoceptors, reduced the contractions, whereas phenylephrine and methoxamine, postsynaptic alpha-adrenoceptor agonists, were ineffective. Contractions induced by acetylcholine were not changed by clonidine but were abolished by atropine. Yohimbine, piperoxan, phentolamine and the thymoxamine reversed or prevented the inhibitory effect of clonidine. Prazosin and AR-C239 did not antagonize this effect. The inhibitory effect of tramazoline was antagonized by piperoxan but not AR-C239 or by prazosin. Naloxone did not alter the action of clonidine, and piperoxan did not change the inhibitory effect of morphine. In conclusion, these experiments suggest the presence on cholinergic postganglionic fibres of both opiate receptors and alpha-adrenoceptors.. The latter appear to resemble more closely alpha 2-adrenoceptors than alpha 1-adrenoceptors.