Influence of urapidil on alpha- and beta-adrenoreceptors in pithed rats.

The interaction of urapidil with pre- and postsynaptic alpha-adrenoreceptors and with postsynaptic beta-adrenoreceptors was studied in pithed normotensive rats and compared to the effects of clonidine, prazosin, and atenolol. I.v. injection of urapidil did not substantially change blood pressure, while clonidine raised blood pressure. Urapidil dose-dependently antagonized the pressor effects of the alpha 1-agonist L-phenylephrine (pDR2 6.8) and of the alpha 2-agonist azepexole (pDR2 5.2). Compared to urapidil, prazosin was a more potent antagonist of phenylephrine at postsynaptic vascular alpha 1-adrenoreceptors (pDR2 8.7) and of azepexole at alpha 2-adrenoreceptors (pDR2 5.6). Urapidil inhibited the tachycardia produced by discontinuous or continuous electrical stimulation of the thoracic spinal outflows (ID50 4.8 and 27.2 mumol/kg, respectively). In contrast to the inhibitory action of clonidine (ID50 0.039 and 0.023 mumol/kg, respectively), the inhibition by urapidil was not reversed by the selective alpha 2-antagonist rauwolscine (10 mumol/kg). Prazosin did not change stimulation-evoked tachycardia but atenolol caused pronounced inhibition (ID50 0.158 mumol/kg, discontinuous stimulation). Urapidil dose-dependently antagonized the tachycardic effect of isoprenaline at beta 1-adrenoreceptors (pDR2 5.1) but also exhibited intrinsic activity by increasing basal heart rate (maximum effect of urapidil was 30% of that of isoprenaline). Urapidil did not change the vasodilatory beta 2-adrenoreceptor-mediated effect of isoprenaline. The results suggest that urapidil is an antagonist at postsynaptic vascular alpha 1- and alpha 2-adrenoreceptors, with a greater potency against alpha 1-adrenoreceptors. An agonistic interaction of urapidil with presynaptic alpha 2-adrenoreceptors could not be demonstrated in pithed rats. Instead, the inhibition by urapidil of stimulation-evoked tachycardia could be accounted for by its beta 1-adrenoreceptor antagonistic effect.