Identification of α-1L Adrenoceptor in Rabbit Ear Artery

The α-1L adrenoceptor (AR) was identified in rabbit ear artery by both functional and ligand binding studies. In functional studies using arterial rings, the contractile response to NS-49 [(R)-(-)-3′-(2-amino-1-hydroxyethyl)-4′-fluorometh-anesulfonanilide hydrochloride] (α-1A and α-1L AR-selective agonist) was competitively antagonized with low affinities by prazosin, RS-17053 [N-[2-(2-cyclopropylmethoxyphenoxy) ethyl]-5-chloro-α,α-dimethyl-1H-indole-3-ethamine hydrochloride], and 5-methylurapidil but with high affinities by tamsulosin and KMD-3213 [(-)-1-(3-hydroxypropyl)-5-[(2R)-2-({2-[(2,2,2-trifluoroethoxy)phenoxy]ethyl}amino)propyl]-2,3-dihydro-1H-indole-7-carboxamide]. In contrast, the response to noradrenaline (nonselective α-1 AR agonist) was inhibited noncompetitively by these antagonists (except 5-methylurapidil) with Schild slopes different from unity. These results suggest that the response to NS-49 was mediated predominantly via α-1L ARs, whereas the response to noradrenaline was produced through two distinct α-1 AR subtypes (presumably α-1B and α-1L ARs). In binding studies with intact segments of rabbit ear artery, [3H]KMD-3213 bound with high affinity (pKD = 9.7) to α-1 ARs, which were subdivided by prazosin, RS-17053, and 5-methylurapidil into two subtypes (α-1A and α-1L ARs). In contrast, [3H]prazosin binding sites in ear artery segments (pKD = 9.8) were identified as α-1A and α-1B ARs. In conventional binding studies using isolated rabbit ear artery microsomal membranes, [3H]KMD-3213 binding sites were identified as α-1A ARs with high affinities for prazosin, RS-17053, and 5-methylurapidil. Our study indicates that an α-1L AR having a unique pharmacological profile coexists with α-1A and α-1B ARs in rabbit ear artery and can be identified either functionally or by binding studies using intact tissues but not microsomal membrane preparations.