Differences of Antagonism for a Selective α_<1D>-Adrenoceptor Antagonist BMY 7378 in the Rabbit Thoracic Aorta and Iliac Artery

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Based on the affinity of α_<1D>-adrenoceptor subtype for a selective antagonist BMY 7378, we studied its functional role in rabbit thoracic aorta and iliac artery, and evaluated the subtypes of the α_1-adrenoceptors that are activated by phenylephrine (a full agonist) and tizanidine (a partial agonist). In thoracic aorta, the concentration-response curves of phenylephrine and tizanidine were antagonized by BMY 7378 with low potency (pA_2 values 6.68±0.06 and 6.67±0.06, slopes of Schild plot 1.06±O.04 and 1.01±0.04, respectively). On the other hand, in iliac artery concentration-response curves for phenylephrine were potently antagonized by a low concentration of BMY 7378, and the slope (0.75±0.02) of the Schild plot was significantly different from unity. In iliac artery, a concentration-response curve of tizanidine was antagonized by BMY 7378 with low potency (pA_2 value 6.64±0.08, slope of Schild plot 1.01±0.05). These results suggest that an α_<1D>-adrenoceptor subtype contributes to α_1-adrenoceptor mediating muscle contraction in iliac artery, but not in thoracic aorta of rabbit, and that it is activated by a full agonist phenylephrine but not by a partial agonist tizanidine.
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