Vol. 291, Issue 2, 829-836, November 1999

₁-Adrenoceptor Subtypes Mediating Inotropic Responses in Rat Heart¹

You-Yi Zhang Kai-Ming Xu² and Chide Han

Institute of Vascular Medicine, Third Hospital, Beijing Medical University, Beijing, China

We studied the distribution of ₁-adrenoceptor subtypes by radioligand binding assays using ¹²⁵I-labeled 2-(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 decreased B_max by 59 and 70%. By competitive inhibition, we have identified 19 to 28% and 30% high-affinity binding sites for the _1A- and _1D-selective antagonists in rat ventricles, with the _1B-adrenoceptor estimated as 45%. Consistent with the receptor-binding result, a similar distribution of mRNAs encoding _1A, _1B, and _1D (22, 39, and 39%), based on RNase protection assays, was observed. In addition, we demonstrated that the noradrenaline response through ₁-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 ₁-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 _1A (r² = 0.73) and _1B (r² = 0.66) than _1D (r² = 0.35) in these experiments. Because the potential of _1D measured to be 21% based on protection from phenoxybenzamine-caused inhibition by BMY7378, the combined potential of _1A and _1B can be estimated as ~80%. Taken together, these results suggest that the three ₁-adrenoceptor subtypes coexist in rat heart, with _1A and _1B playing a more prominent role in the positive inotropic response to noradrenaline.