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 (Bmax). On the other hand, protection from phenoxybenzamine alkylation by 5-methyl-urapidil or BMY7378 decreasedBmax by 59 and 70%. By competitive inhibition, we have identified 19 to 28% and 30% high-affinity binding sites for the α1A- and α1D-selective antagonists in rat ventricles, with the α1B-adrenoceptor estimated as 45%. Consistent with the receptor-binding result, a similar distribution of mRNAs encoding α1A, α1B, and α1D (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. KI 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 α1A(r2 = 0.73) and α1B(r2 = 0.66) than α1D(r2 = 0.35) in these experiments. Because the potential of α1D measured to be 21% based on protection from phenoxybenzamine-caused inhibition by BMY7378, the combined potential of α1A and α1B can be estimated as ∼80%. Taken together, these results suggest that the three α1-adrenoceptor subtypes coexist in rat heart, with α1A and α1B playing a more prominent role in the positive inotropic response to noradrenaline.
Footnotes
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Send reprint requests to: Dr. Chide Han, Institute of Vascular Medicine, Third Hospital, Beijing Medical University, Beijing 100083 China. E-mail: hanqd{at}mail.bjmu.edu.cn
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↵1 This work was supported by grants from the National Science Foundation of China and by China Medical Board of New York Inc. Grant 93–591.
- Abbreviations:
- WB4101
- 2-(2,6-dimethoxphenoxyethyl)-aminomethyl-1,4 benzodioxane
- BE2254
- 2-β(4-hydroxyphenyl)-(ethylaminomethyl)-tetralone
- CEC
- chlorethylclonidine
- BMY7378
- 8-[2-[4[-(2-methoxyphenly)-1-piperazinyl]-8-azaspiro]4,5]decane-7,9-dione dihydrochloride
- 5-MU
- 5-methyl-urapidil
- POB
- phenoxybenzamine
- RBA
- radioligand binding assays
- RS-17053
- N-[2–2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-α,α-dimethyl-1H-indole-3-ethanamine hydrochloride
- HEK
- human embryonic kidney
- bp
- base pair
- Received May 21, 1999.
- Accepted August 4, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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