alpha(1)-Adrenoceptor subtypes mediating inotropic responses in rat heart

We studied the distribution of alpha(1)-adrenoceptor subtypes by radioligan d binding assays using I-125-labeled 2-beta(4-hydroxyphenyl)- ethylaminomet hyl)-tetralone (BE2254) and RNase protection assays, and determined the rol e of each subtype in mediating the inotropic response in rat heart. Chloret hyl-clonidine preincubation causes a similar to 72% decrease in the maximal binding capacity (B-max). On the other hand, protection from phenoxybenzam ine 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-aff inity binding sites for the alpha(1A)- and alpha(1D)-selective antagonists in rat ventricles, with the alpha(1B)-adrenoceptor estimated as 45%. Consis tent with the receptor-binding result, a similar distribution of mRNAs enco ding alpha(1A), alpha(1B), and alpha(1D) (22, 39, and 39%), based on RNase protection assays, was observed. In addition, we demonstrated that the nora drenaline response through alpha(1)-adrenoceptor was antagonisted by 5-meth yl-urapidil, RS-17053, BMY7378, and WB4101 in contraction functional experi ments. K-I values for the above compounds were defined for all three alpha( 1)-adrenoceptor subtypes expressed in the human embryonic kidney 293 cell s tably, and were further compared with the corresponding pA(2) values. Inter estingly, 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 exper iments. Because the potential of alpha(1D) measured to be 21% based on prot ection from phenoxybenzamine- caused inhibition by BMY7378, the combined po tential of alpha(1A) and alpha(1B) can be estimated as similar to 80%. Take n together, these results suggest that the three alpha(1)-adrenoceptor subt ypes coexist in rat heart, with alpha(1A) and alpha(1B) playing a more prom inent role in the positive inotropic response to noradrenaline.