Relationship of alpha receptor types to hypotension and renal vasodilation caused by alpha blockers in conscious dogs

Pharmacologic profile of urapidil

Urapidil, a phenylpiperazine-substituted derivative of uracil, is a selective alpha 1-adrenoceptor antagonist and therefore decreases blood pressure and peripheral resistance. Apart from its alpha 1-adrenoceptor-blocking potency, which is somewhat less selective than that of prazosin, urapidil appeared to display substantial central hypotensive activity. Initially, this central mechanism was assumed to be the same as that of clonidine, i.e., agonistic activity toward central alpha 2 adrenoceptors. This view, however, appeared to be incorrect for the following reasons: (1) In radioligand binding studies urapidil did not show any affinity for alpha 2 adrenoceptors; and (2) the central hypotensive activity of urapidil, injected into feline vertebral arteries remained unaffected by the alpha 2-adrenoceptor-blocking agents yohimbine and rauwolscine, which are known to suppress the central hypotensive effect of clonidine. The central hypotensive effect of urapidil so far remains unknown in detail, although a few (but not all) experimental arguments would plead for the 5-hydroxytryptamine (5-HT 1A)-agonistic potency of urapidil as an explanation of its central activity. The central hypotensive mechanism of urapidil will be discussed in more detail later in this issue. A presynaptic alpha 2-agonistic activity can only be demonstrated in certain models but not in the classic pithed rat preparation. It is unlikely to play a relevant role in urapidil's hypotensive activity, and also because of the poor affinity of the drug for alpha 2 adrenoceptors. Finally, a weak beta 1-adrenoceptor activity of urapidil can be demonstrated, but this mechanism is unlikely to contribute significantly to the drug's hypotensive activity.

A comparison of the effects of doxazosin and alfuzosin with those of urapidil on preganglionic sympathetic nerve activity in anaesthetised cats

Preganglionic sympathetic nerve activity, blood pressure, heart rate and femoral arterial conductance were recorded in anaesthetised, paralysed cats. Urapidil, doxazosin and alfuzosin were infused i.v. for 1 h into different animals at the rate of 2 mg kg-1 h-1. All three drugs caused a fall in thoracic preganglionic sympathetic nerve activity along with blood pressure. Although urapidil had a greater hypotensive action than doxazosin and alfuzosin its sympathoinhibitory action was delayed and weaker. Since all three drugs are alpha 1-adrenoceptor antagonists it appears that antagonism at this receptor type may cause central sympathoinhibition as well as a decrease in total peripheral resistance. However, the different effects of urapidil suggest that its action on central sympathetic tone and therefore its hypotensive action cannot be due entirely to its ability to block alpha 1-adrenoceptors.

Beta 1-adrenoceptor antagonism by urapidil prior to and after the alpha 2-antagonist rauwolscine in anesthetized dogs

The purpose of this investigation was to evaluate the in vivo beta-adrenoceptor antagonistic properties of urapidil, a new antihypertensive alpha 1-adrenoceptor blocking agent. In dogs anesthetized with pentobarbital, the cardioaccelerator nerve was isolated, decentralized and stimulated electrically to elicit adrenergically mediated increases in heart rate. Dobutamine was administered (3-30 micrograms/kg i.v.) to elicit increases in heart rate by the direct stimulation of beta-adrenoceptors. Urapidil 2 mg/kg i.v. had no significant effect on cardioaccelerator nerve-induced tachycardia, but 5 mg/kg i.v. decreased the response by 27%. Heart rate responses to dobutamine were suppressed slightly by the low dose of urapidil and to a greater degree by the high dose of urapidil, whereas the pressor response to dobutamine was markedly attenuated at both dose levels. Neurally or dobutamine-elicited tachycardia remaining after urapidil treatment was eliminated by propranolol (1 mg/kg i.v.). When the selective alpha 2-antagonist rauwolscine was administered (1000 micrograms/kg i.v.), tachycardic responses to nerve stimulation increased 49% above control, and urapidil (5 mg/kg) given subsequently, caused a 48% reduction in the response. These data indicate that urapidil has weak beta 1-blocking activity more clearly seen after blockade of alpha 2-adrenoceptors.

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.