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.
Zhang
Y Y
YY
Institute of Vascular Medicine, Third Hospital, Beijing Medical University, Beijing, China.
Xu
K M
KM
Han
C
C
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D001667:Binding, Competitive; D019976:Cloning, Organism; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004622:Embryo, Mammalian / Q000502:physiology; D006352:Heart Ventricles / Q000187:drug effects*; D006801:Humans; D066298:In Vitro Techniques; D007668:Kidney / Q000502:physiology*; D009119:Muscle Contraction / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology; D010627:Phenethylamines / Q000494:pharmacology*; D011485:Protein Binding; D011869:Radioligand Assay; D051381:Rats; D018340:Receptors, Adrenergic, alpha-1 / Q000145:classification* / Q000502:physiology; D012260:Ribonucleases / Q000378:metabolism; D044024:Tetralones
D000316:Adrenergic alpha-Agonists; D010627:Phenethylamines; D018340:Receptors, Adrenergic, alpha-1; D044024:Tetralones; C010652:BE 2254; D012260:Ribonucleases; D009638:Norepinephrine