Ki values of OPC-28326 and yohimbine determined by ligand binding assay with [3H]RX821002 (n = 3) and by luciferase assay with epinephrine (n = 5, mean ± S.D.), using the CHO cells overexpressing rat α2D-adrenoceptors, α2B-adrenoceptors, or α2C-adrenoceptors