Alpha(1)-adrenoceptor subtypes mediating inotropic responses in rat heart

We studied the distribution of alpha(1)-adrenoceptor subtypes by radioligand binding assays using (125)I-labeled 2-beta(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 approximately 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 alpha(1A)- and alpha(1D)-selective antagonists in rat ventricles, with the alpha(1B)-adrenoceptor estimated as 45%. Consistent with the receptor-binding result, a similar distribution of mRNAs encoding alpha(1A), alpha(1B,) and alpha(1D) (22, 39, and 39%), based on RNase protection assays, was observed. In addition, we demonstrated that the noradrenaline response through alpha(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 alpha(1)-adrenoceptor subtypes expressed in the human embryonic kidney 293 cell stably, and were further compared with the corresponding pA(2) values. Interestingly, the correlation was significantly higher for alpha(1A) (r(2) = 0.73) and alpha(1B) (r(2) = 0.66) than alpha(1D) (r(2) = 0.35) in these experiments. Because the potential of alpha(1D) measured to be 21% based on protection from phenoxybenzamine-caused inhibition by BMY7378, the combined potential of alpha(1A) and alpha(1B) can be estimated as approximately 80%. Taken together, these results suggest that the three alpha(1)-adrenoceptor subtypes coexist in rat heart, with alpha(1A) and alpha(1B) playing a more prominent role in the positive inotropic response to noradrenaline.