Evidence that the population of postjunctional-adrenoceptors mediating contraction of smooth muscle in the rabbit isolated ear vein is predominantly α2

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Abstract
1 Noradrenaline (NA), phenylephrine and UK-14304 elicited concentration-dependent contractions of the rabbit isolated ear vein of similar maximal magnitude. The rank order of potency, UK-14304 > noradrenaline > phenylephrine, is consistent with that of an effect mediated through an α2-subtype. 2 The potent and highly selective α1-adrenoceptor antagonists prazosin and YM-12617, at concentrations as high as 1 μm, produced less than a 4 fold rightward displacement of the NA concentration-response curve. 3 The selective α2-adrenoceptor antagonists rauwolscine, Wy-26703 and CH-38083 antagonized responses to noradrenaline in a competitive manner. For all three antagonists, the pA2 values were consistent with an effect at α2-adrenoceptors. However, 0.1 μm YM-12617 increased the potency of rauwolscine 2 fold indicating the presence of a small population of postjunctional α1-adrenoceptors. 4 The relative antagonist potency of the yohimbine diastereoisomers rauwolscine and corynanthine against noradrenaline (rauwolscine 30 fold > corynanthine) is also consistent with an effect at α2-adrenoceptors. 5 Contractions elicited by noradrenaline in the rabbit isolated ear vein appear to be mediated predominantly by postjunctional α2-adrenoceptors.

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