Different subtypes of alpha 1A-adrenoceptor mediating contraction of rat epididymal vas deferens, rat hepatic portal vein and human prostate distinguished by the antagonist RS 17053.

ABSTRACT

1. The alpha 1-adrenoceptor subtype mediating contraction of the rat hepatic portal vein to phenylephrine was characterized by use of competitive antagonists previously shown to have selectivity between the expressed alpha 1-subtype clones. Prazosin competitively antagonized the phenylephrine contractions with a pA2 value of 9.2, as did WB 4101 (pA2 9.4), 5-methyl urapidil (pA2 8.6), indoramin (pA2 8.4) and BMY 7378 (pA2 6.5). 2. The pA2 values on the rat portal vein correlated highly with their previously published pA2 values for the alpha 1A-adrenoceptors mediating contraction of the rat epididymal vas deferens and human prostate and poorly with those for the alpha 1B- and alpha 1D-adrenoceptors mediating contraction of the rat spleen and aorta, respectively. The antagonist pA2 values on the rat portal vein correlated highly with their previously published pK1 values for the expressed alpha 1a-clone and poorly with those for the expressed alpha 1b- and alpha 1d-clones. Therefore the results show that contraction of the rat portal vein to phenylephrine is mediated by alpha 1A-adrenoceptors. 3. The novel alpha 1-adrenoceptor antagonist RS 17053 had a relatively high affinity for the alpha 1A-adrenoceptors mediating contraction of the rat epididymal vas deferens (pA2 9.5) compared with the alpha 1B-adrenoceptors in the rat spleen (pA2 7.2) or the alpha 1D-adrenoceptors in the rat aorta (pKB 7.1), in agreement with its selectivity for the expressed alpha 1a-clone. However, RS 17053 had over 100 fold lower affinity for the alpha 1A-adrenoceptors mediating contraction of the rat portal vein (pKB 7.1) and human prostate (pKB 7.1) compared with its affinity for the alpha 1A-adrenoceptors in the rat epididymal vas deferens or the expressed alpha 1a-clone. 4. The difference in affinity of RS 17053 between the rat epididymal vas deferens and rat portal vein cannot be explained by a species difference in the receptor. Therefore RS 17053 may distinguish between subtypes of the alpha 1A-adrenoceptor in the rat portal vein and human prostate compared with those in the rat epididymal vas deferens or the expressed alpha 1a-clone.