SELECTIVITIES OF SOME AGONISTS ACTING AT α1‐ AND α2‐ADRENORECEPTORS IN THE RAT VAS DEFERENS

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Abstract
1. In this paper, estimates of the selectivities of a series of twelve sympathomimetic agents acting at postjunctional .alpha.1- and prejunctional .alpha.2-adrenoreceptors were investigated, using epididymal and prostatic segments of the rat vas deferens. 2. The relative order of potency for the twelve agonists at prejunctional .alpha.2-adrenoceptors mediating inhibition of field-stimulation-induced contractions in the prostatic segment of the vas deferens was: clonidine > (-)-adrenaline > xylazine .gtoreq. (-)-noradrenaline > (+)-adrenaline > dopamine .gtoreq. phenylephrine .gtoreq. metaraminol .gtoreq. (+)-noradrenaline > (-)-isoprenaline > methoxamine > (+)-isoprenaline. 3. The relative order of potency for the agonists at postjunctional .alpha.1-adrenoceptors mediating contraction of smooth muscle in epididymal segments of the vas deferens was: (-)-adrenaline .gtoreq. (-)-noradrenalien > phenylephrine > clonidine .gtoreq. (+)-adrenaline .gtoreq. methoxamine .gtoreq. (+)-noradrenaline .gtoreq. metaraminol .gtoreq. dopamine .gtoreq. (-)-isoprenaline .gtoreq. xylazine: (+)-isoprenaline was inactive. 4. (+)-Noradrenaline, the stereoisomers of adrenaline and isoprenaline, dopamine, clonidine, xylazine and metaraminol displayed .alpha.2-selectivity whereas phenylephrine and methoxamine displayed .alpha.1-adrenoreceptor selectivity. (-)-Noradrenaline possessed a similar potency at both .alpha.1- and .alpha.2-adrenoreceptors thus making it non-selective by the criteria used in this study. 5. Estimates of the potencies of optical isomers of catecholamines and of dopamine on epididymal and prostatic segments of the vas deferens indicated the steric orientation of the .beta.-hydroxyl substituent is equally important at both subtypes in this tissue.

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