OPC-28326, a Selective Femoral Vasodilator, Is an α_2C-Adrenoceptor-Selective Antagonist

Abstract

OPC-28326 has been reported to selectively increase femoral blood flow in open-chest dogs and autoperfused canine femoral artery preparations. Preliminary data indicated that OPC-28326 has a high affinity at the α₂-adrenoceptor. In the present study, we tested OPC-28326 in isoflurane anesthetized rats at a dose of 3 mg/kg of body weight, given intraduodenally. OPC-28326 significantly increased femoral blood flow, by 44.7 ± 13.8%, 45 min after drug administration, whereas carotid blood flow increased by only 3.6 ± 5.5% (n = 6). Chinese hamster ovary cell lines overexpressing rat α_2D-, α_2B-, or α_2C-adrenoceptor were established. These cells also coexpress luciferase, driven by cAMP elevation. In radioligand binding assays using cell membrane preparations, OPC-28326 dose dependently competed with [³H]RX821002 binding, with calculatedK_i values of 3840 ± 887, 633 ± 46, and 13.7 ± 1.9 nM on α_2D-, α_2B-, and α_2C-adrenoceptor, respectively. A similar affinity and rank order of potency were also found for OPC-28326 on the α₂-subtypes using epinephrine as agonist in luciferase assays. No agonistic effect of OPC-28326 was detected on any of the α₂-adrenoceptors. Finally, in situ hybridization performed on skeletal muscle tissue sections collected from rat hind limb (musculus gastrocnemius) demonstrated a high level expression of α_2C in the vascular tissues. Thus, the abundance of α_2C in the skeletal muscle may account for the selective effect of OPC-28326 in increasing femoral blood flow.