Antagonism of alpha 1-adrenoceptor-mediated vascular contraction by urapidil in isolated arterial strips of spontaneously hypertensive rats.

Antagonism of alpha 1-adrenoceptor-mediated vascular contraction by urapidil was examined in helical strips of femoral and mesenteric arteries isolated from 13-week-old Aoki spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto (WKY) rats, since this agent has antihypertensive effect through antagonizing peripheral alpha-adrenoceptors. Schild plot analyses clearly demonstrated the existence of only alpha 1-adrenoceptors in these arteries from both strains. Therefore, it is possible to demonstrate alpha 1-adrenoceptor blocking effects of nonselective alpha-adrenoceptor antagonists as well as selective alpha 1-adrenoceptor antagonists. Urapidil antagonized the alpha 1-adrenoceptor-mediated vascular contraction in a competitive fashion. The pA2 value for urapidil against alpha 1-adrenoceptors was not significantly different between SHR and WKY rats. The addition of 10(-5) M norepinephrine (NE) produced a sustained contraction in a SHR femoral artery, whereas in a WKY rat femoral artery this agonist produced a transient contraction followed by a sustained relaxation. Urapidil elicited a dose-dependent relaxation with a IC50 value of 6.50 in the SHR femoral artery precontracted with NE. In the presence of 3 x 10(-7) M timolol, a beta-adrenoceptor antagonist, femoral arteries from both strains exhibited similar magnitude of contraction in response to the stimulation with 10(-5) M NE. Under these conditions, urapidil elicited a similar extent of relaxation between SHR and WKY rats. On the other hand, the addition of 10(-5) M NE produced a sustained contraction in mesenteric arteries from both SHR and WKY rats. The contraction expressed as a ratio to the maximum developed by KCl depolarization was significantly greater in SHR than in WKY rats. In these arteries, the relaxing effect of urapidil was more evident in SHR than in WKY rats. Contractile responses to NE and relaxing effects of urapidil were not affected by timolol. These results suggest that urapidil effectively antagonized enhanced alpha 1-adrenoceptor responses seen in SHR arteries.