Determination of alpha 1-adrenoceptor subtype selectivity by [3H]-prazosin displacement studies in guinea-pig cerebral cortex and rat spleen membranes.
1. [3H]-prazosin homogeneously labels alpha 1-adrenoceptors in guinea-pig cerebral cortex and rat spleen membranes with dissociation constants of 1.28 and 1.49 x 10(-10) M respectively. 2. Phentolamine and WB 4101 displacement studies show that guinea-pig cerebral cortex contains 30% alpha 1A- and 70% alpha 1B-adrenoceptor subtypes, whereas rat spleen contains a virtually homogeneous alpha 1B-adrenoceptor subtype population. The alpha 1-adrenoceptor population of rat thoracic aorta is predominantly of the alpha 1A-adrenoceptor subtype, and in guinea-pig thoracic aorta it is mainly of the alpha 1B-adrenoceptor subtype. 3. Half of the compounds displacing [3H]-prazosin bound to guinea-pig cerebral cortex membranes display alpha 1A-adrenoceptor selectivity. Among these compounds, WB 4101 and methoxamine are most selective, displaying selectivity ratios of approximately 38 and approximately 26 respectively. 4. The affinity constants of the non-selective compounds for the alpha 1-adrenoceptor in guinea-pig cerebral cortex membranes correlate well with the affinity constants obtained for alpha 1B-adrenoceptors in rat spleen membranes. The affinities of selective compounds for the alpha 1B-adrenoceptor subtype in guinea-pig cerebral cortex correlate very well with their affinity for alpha 1B-adrenoceptor in the rat spleen homogenate. Both regression lines coincide with the line of identity. The affinity constants of selective compounds for the alpha 1A-adrenoceptors in guinea-pig cerebral cortex only apparently correlate with the affinity for either the alpha 1B-adrenoceptors in guinea-pig cerebral cortex or in the rat spleen. Regression analyses indicate a straight line relationship (r2>0.9) between pKEA and Pk1B but the regression lines deviate from the line of identity.
Department of Pharmacology, Faculty of Pharmacy, Utrecht University, The Netherlands.
D M
DM
Veenstra
K J
KJ
van Buuren
F P
FP
Nijkamp
D000316:Adrenergic alpha-Agonists / Q000378:metabolism; D000317:Adrenergic alpha-Antagonists / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D002540:Cerebral Cortex / Q000378:metabolism*; D004146:Dioxanes / Q000378:metabolism; D006168:Guinea Pigs; D008297:Male; D010646:Phentolamine / Q000378:metabolism; D011224:Prazosin / Q000378:metabolism*; D011869:Radioligand Assay; D051381:Rats; D017208:Rats, Wistar; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D013154:Spleen / Q000378:metabolism*
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D004146:Dioxanes; D011942:Receptors, Adrenergic, alpha; C010654:(2-(2',6'-dimethoxy)phenoxyethylamino)methylbenzo-1,4-dioxane; D011224:Prazosin; D010646:Phentolamine