Subclassification of alpha 1-adrenoceptor recognition sites by urapidil derivatives and other selective antagonists.
1. The affinities of urapidil derivatives and other antagonists for alpha 1-adrenoceptors labelled by [3H]-prazosin were determined on membranes of six different rat tissues. 2. Urapidil and its 5-acetyl-, 5-formyl- and 5-methyl-derivative displaced [3H]-prazosin from alpha 1-adrenoceptor binding sites in a concentration-dependent manner which varied with tissue. IC50 values were lower in vas deferens, hippocampus and cerebral cortex than in heart, liver and spleen. For 5-methyl-urapidil, binding to two distinct sites could be demonstrated with mean K1 values of about 0.6 and 45 nM. Saturation binding studies with [3H]-prazosin in the presence of 5-methyl-urapidil indicated a competitive type of interaction between 5-methyl-urapidil and [3H]-prazosin. 3. The proportion of [3H]-prazosin binding sites with high affinity for 5-methyl-urapidil was 58% in vas deferens, 69% in hippocampus, 41% in cerebral cortex and 23% in myocardium. In liver and spleen virtually no high affinity sites were found. These values were in good agreement with the percentages of binding sites with high affinities for WB-4101 and phentolamine, indicating that all these antagonists bind to the same subtype of alpha 1-recognition sites, whereas other alpha-antagonists like BE 2254, yohimbine and unlabelled prazosin did not discriminate between two binding sites. 4. Preincubating membranes of the cerebral cortex with chloroethylclonidine preferentially inactivated [3H]-prazosin binding sites with low affinity for 5-methyl-urapidil. 5. The antagonist potencies of 5-methyl-urapidil and WB-4101 against alpha 1- adrenoceptor-mediated contractile responses were higher in vas deferens than in myocardium. The alpha 1-mediated effects in vas deferens but not in the heart were highly susceptible to nitrendipine. 6. Using 5-methyl-urapidil, the existence of two distinct alpha 1-adrenoceptor recognition sites could be demonstrated which correspond to the proposed alpha 1A- and alpha 1B-subtypes. Since 5-methyl-urapidil is one of the ligands with most selectivity between these subtypes in binding studies it may serve as a valuable tool for such investigations.
Hanft
G
G
Department of Pharmacology, University of Essen, Federal Republic of Germany.
Gross
G
G
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000818:Animals; D000959:Antihypertensive Agents / Q000494:pharmacology*; D004146:Dioxanes / Q000494:pharmacology; D006321:Heart / Q000187:drug effects; D066298:In Vitro Techniques; D008297:Male; D009206:Myocardium / Q000378:metabolism; D010627:Phenethylamines / Q000494:pharmacology; D010646:Phentolamine / Q000494:pharmacology; D010879:Piperazines / Q000494:pharmacology*; D011224:Prazosin / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000378:metabolism*; D044024:Tetralones; D015016:Yohimbine / Q000494:pharmacology
D000317:Adrenergic alpha-Antagonists; D000959:Antihypertensive Agents; D004146:Dioxanes; D010627:Phenethylamines; D010879:Piperazines; D011942:Receptors, Adrenergic, alpha; D044024:Tetralones; C057446:5-methylurapidil; D015016:Yohimbine; C010652:BE 2254; C015568:urapidil; C010654:(2-(2',6'-dimethoxy)phenoxyethylamino)methylbenzo-1,4-dioxane; D011224:Prazosin; D010646:Phentolamine