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

Abstract

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. The relative order of potency for the twelve agonists at prejunctional alpha 2-adrenoreceptors mediating inhibition of field-stimulation-induced contractions in the prostatic segment of the vas deferens was: clonidine greater than (-)-adrenaline greater than xylazine greater than or equal to (-)-noradrenaline greater than (+)-adrenaline greater than dopamine greater than or equal to phenylephrine greater than or equal to metaraminol greater than or equal to (+)-noradrenaline greater than (-)-isoprenaline greater than methoxamine greater than (+)-isoprenaline. The relative order of potency for the agonists at postjunctional alpha 1-adrenoreceptors mediating contraction of smooth muscle in epididymal segments of the vas deferens was: (-)-adrenaline greater than or equal to (-)-noradrenaline greater than phenylephrine greater than clonidine greater than or equal to (+)-adrenaline greater than or equal to methoxamine greater than or equal to (+)-noradrenaline greater than or equal to metaraminol greater than or equal to dopamine greater than or equal to (-)-isoprenaline greater than or equal to xylazine; (+)-isoprenaline was inactive. (+)-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.(ABSTRACT TRUNCATED AT 250 WORDS)