Evidence for a functional alpha 1A- (alpha 1C-) adrenoceptor mediating contraction of the rat epididymal vas deferens and an alpha 1B-adrenoceptor mediating contraction of the rat spleen

Abstract

1. The alpha 1-adrenoceptor subtype mediating contraction of the rat epididymal vas deferens and rat spleen has been investigated by use of alpha 1-adrenoceptor antagonists that have shown selectivity between the different cloned receptor subtypes. 2. In the rat epididymal vas deferens the potency of noradrenaline and phenylephrine was increased in the presence of neuronal and extra-neuronal uptake blockers, cocaine and beta-oestradiol, but these did not alter that of methoxamine. The order of potency of the agonists in the presence or absence of uptake blockade was noradrenaline > phenylephrine > methoxamine. In the rat spleen the potency of these agonists was not altered in the presence of cocaine and beta-oestradiol, and their order of potency was the same as in the vas deferens. 3. The non subtype selective alpha 1-adrenoceptor antagonist prazosin (up to 1 x 10(-7) M) was found to antagonize contractions to noradrenaline in the vas deferens competitively (pA2 9.2), but only in a non competitive manner in the spleen. Contractions to phenylephrine in the spleen however were competitively antagonized by prazosin (up to 1 x 10(-7) M) with a pA2 of 9.2. This suggests that there is an alpha 1- and a non alpha 1-adrenoceptor response to noradrenaline in the rat spleen. 4. Pretreatment with chlorethylclonidine (10(-4) M for 30 min) did not alter the noradrenaline contractions in the vas deferens, but contractions to noradrenaline and phenylephrine in the spleen were shifted 30 and 300 fold to the right of the control curve, respectively. This suggests that only the contractions in the spleen were mediated by alpha 1B-adrenoceptors. 5. The noradrenaline contractions in the vas deferens were competitively antagonized by WB 4101 (pA29.6), 5-methyl-urapidil (pA2 8.7), phentolamine (pA2 8.3), benoxathian (pA2 9.4), spiperone (pA2 7.5),indoramin (pA2 8.4) and BMY 7378 (pA2 6.7), consistent with the affinities of these antagonists in binding studies on tissue alpha 1A-adrenoceptors. These values correlated best with their published affinities on the expressed alpha 1c-adrenoceptor clone and poorly with those at either the expressed alpha lb- or alpha 1d adrenoceptor clones. Therefore the classical alpha 1A-adrenoceptor appears to be the same as the expressed alpha lc-adrenoceptor clone.6. The phenylephrine contractions in the spleen were competitively antagonized by WB 4101 (pA2 8.1),5-methyl-urapidil (pA2 7.1), phentolamine (pA2 7.3), benoxathian (pA2 7.4), spiperone (pA2 7.9),indoramin (pA2 7.5) and BMY 7378 (pA2 7.4), consistent with the affinities of these antagonists in binding studies on tissue alB-adrenoceptors. The pA2 values correlated best with the published affinities of these compounds on the expressed alb-adrenoceptor clone and poorly with those at either the expressed alpha ld- or alpha lc-adrenoceptor clones. Therefore the alpha lB-adrenoceptor appears to be the same as the expressed alpha lb-adrenoceptor clone.7. The results provide pharmacological evidence that the alpha1-adrenoceptor mediating noradrenaline contractions in the epididymal portion of the rat vas deferens is the alpha 1A-(alpha lC) subtype and that contractions to phenylephrine in the rat spleen are mediated by the alpha 1B-subtype.