Stereoselective Binding of Niguldipine Enantiomers to Alpha 1A-adrenoceptors Labeled with [3H]5-methyl-urapidil

[3H]5-Methyl-urapidil, a potent antihypertensive derivative of urapidil, binds to alpha 1A-adrenoceptors in rat brain cortex membranes with a dissociation constant (KD) of 0.89 nM and a Bmax of 116 fmol/mg protein. The ligand does not bind to purified liver cell membranes (alpha 1B-adrenoceptors). [3H]5-Methyl-urapidil also labels 5-HT1A receptors in brain membranes (KD: 0.84 nM and Bmax: 235 fmol/mg protein). (+/-)-Niguldipine, a novel 1,4-dihydropyridine with Ca2+-antagonistic as well as alpha 1A-adrenoceptor blocking properties, is a competitive inhibitor of [3H]5-methyl-urapidil binding to alpha 1A-adrenoceptors. In contrast to those for prazosin, the Ki values for niguldipine were highly dependent on the membrane protein concentration, indicating partitioning of niguldipine into hydrophobic compartments unavailable for alpha-adrenoceptor interaction. The extrapolated, 'true' Ki values were as follows: (+/-)-niguldipine: 0.298 nM, (-)-niguldipine: 3.12 nM, (+)-niguldipine: 0.145 nM.