Abstract

We studied the distribution of α₁-adrenoceptor subtypes by radioligand binding assays using ¹²⁵I-labeled 2-β(4-hydroxyphenyl)-ethylaminomethyl)-tetralone (BE2254) and RNase protection assays, and determined the role of each subtype in mediating the inotropic response in rat heart. Chlorethylclonidine preincubation causes a ∼72% decrease in the maximal binding capacity (B_max). On the other hand, protection from phenoxybenzamine alkylation by 5-methyl-urapidil or BMY7378 decreasedB_max by 59 and 70%. By competitive inhibition, we have identified 19 to 28% and 30% high-affinity binding sites for the α_1A- and α_1D-selective antagonists in rat ventricles, with the α_1B-adrenoceptor estimated as 45%. Consistent with the receptor-binding result, a similar distribution of mRNAs encoding α_1A, α_1B, and α_1D (22, 39, and 39%), based on RNase protection assays, was observed. In addition, we demonstrated that the noradrenaline response through α₁-adrenoceptor was antagonisted by 5-methyl-urapidil, RS-17053, BMY7378, and WB4101 in contraction functional experiments. K_I values for the above compounds were defined for all three α₁-adrenoceptor subtypes expressed in the human embryonic kidney 293 cell stably, and were further compared with the corresponding pA₂ values. Interestingly, the correlation was significantly higher for α_1A(r² = 0.73) and α_1B(r² = 0.66) than α_1D(r² = 0.35) in these experiments. Because the potential of α_1D measured to be 21% based on protection from phenoxybenzamine-caused inhibition by BMY7378, the combined potential of α_1A and α_1B can be estimated as ∼80%. Taken together, these results suggest that the three α₁-adrenoceptor subtypes coexist in rat heart, with α_1A and α_1B playing a more prominent role in the positive inotropic response to noradrenaline.