Central and peripheral cardiovascular actions of urapidil in normotensive and Goldblatt hypertensive animals

Systemic and regional hemodynamic effects of alpha-adrenoceptor blockade in chronic left ventricular dysfunction in the conscious dog

In seven dogs with long-standing left ventricular dysfunction induced 16 weeks earlier by repetitive transmyocardial direct current (DC) shock, the acute hemodynamic effect of the alpha 1-adrenoceptor antagonist urapidil was studied. Left ventricular end-diastolic pressure (LVEDP) was significantly increased from preshock levels at the time of study and cardiac output was reduced. Plasma norepinephrine was significantly increased from control levels and was not altered by urapidil infusion. The mean arterial pressure fell in response to alpha 1-blockade from 111 to 85 mm Hg, the LVEDP fell from 16 to 9 mm Hg, and cardiac output increased from 2.90 to 3.70 L/min (all p less than 0.01). Regional blood flows measured by microsphere injection revealed an increase in blood flow to skeletal muscle, which had not been significantly decreased by the left ventricular dysfunction in this model, and further decreases in splanchnic flow, which was already depressed compared with that in normal dogs. Therefore acute alpha-adrenoceptor blockade improves central hemodynamics in experimental heart failure but does not normalize the resting blood flow maldistribution in this model.

Effect of urapidil on the performance of ischemic myocardium in anesthetized dogs

Urapidil (URA) is used to treat acute hypertension in patients with coronary artery disease, but the effect of URA on the performance of ischemic myocardium has not yet been investigated. The present study was intended to assess the function of ischemic myocardium following URA administration. In eight anesthetized (piritramide) open-chest dogs systolic contraction (dL) and end-diastolic length (edL) of myocardium supplied by the left descending (LAD) and circumflex (LCA) coronary arteries were measured by sonomicrometry simultaneously with aortic pressure (AoP), left ventricular end-diastolic pressure (LVedP), heart rate (HR), stroke volume (SV), and LAD-flow (QLAD). QLAD was reduced by LAD stenosis to about 50% of control, decreasing dLLAD by 55%. Concomitantly, edLLAD increased by about 9% and LVedP by 22%, whereas AoP decreased by 5%. Then, URA was given i.v. (0.25 + 0.25 + 0.50 + 1.0 mg/kg) in 15-min intervals. Following URA, the performance of the non-ischemic area was not systematically affected, but dLLAD increased by about 50%. This could neither be related to the significant reduction in afterload (AoP: -8%), nor to an increase in preload (LVedP and edLLAD did not change significantly), nor to an improved oxygen supply via the LAD (QLAD even decreased), although an increased collateral flow the LCA could not be excluded. The increase in systolic shortening correlated very closely to a decrease in heart rate (r = -0.92). It is concluded that the improved function of ischemic myocardium following urapidil resulted from a reduced oxygen demand in consequence to the decrease in heart rate.

New alpha 1-adrenergic receptor antagonists for the treatment of hypertension: role of vascular alpha receptors in the control of peripheral resistance

The pharmacology, clinical efficacy and safety of new alpha-adrenergic receptor antagonists for the treatment of hypertension was reviewed (Table XIV). Although all these agents block alpha 1 receptors, some of them have additional effects on histamine, serotonin, dopamine, and alpha 2 receptors. These other actions account for the differences in the side effect profiles observed, i.e., increased incidence of central nervous system side effects found with indoramin, ketanserin, and urapidil, as well as for some additional beneficial effects of ketanserin (i.e., antiplatelet aggregation activity). The magnitude of BP reduction observed with antagonists of alpha 1-adrenergic receptors is modest. In most studies, the degree of BP reduction is comparable to that of prazosin, but less than that achieved with thiazide diuretics, beta-receptor antagonists, or methyldopa. Studies on the comparative efficacy and safety of new alpha 1 antagonists with converting enzyme inhibitors or calcium-channel blockers are not available. In general, alpha 1 antagonists produce greater reductions in standing than in supine BP, an effect due to the venodilatory action of these drugs. New alpha 1 antagonists appear to have equal efficacy in black and white hypertensive individuals. Their comparative efficacy and safety in young vs elderly hypertensive individuals requires further investigation. No information about the possible development of tolerance during treatment with new alpha 1 blockers was encountered. The effects of alpha 1 antagonists on HR are variable and depend on how long after the oral dose the measurements were obtained. In most studies, no significant HR changes are noticed for readings obtained 24 hours post dose; whereas tachycardia has been observed at the time of peak hypotension. Since alpha 1 antagonist-induced tachycardia is most likely of reflex nature, i.e., mediated to an increase in sympathetic activity, the increased HR may be associated with increases in myocardial contractility and in myocardial oxygen consumption. Consequently, a 24-hour HR monitoring during treatment with alpha 1 antagonists should be required for evaluation of new agents. The hemodynamic, humoral, and hormonal effects of the newer alpha 1-receptor antagonists are comparable to those of prazosin. The most consistent finding is a reduction in total peripheral resistance associated with either no change or with only small increases in cardiac index. These agents have been shown either not to change or to increase renal blood flow.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of urapidil, clonidine, and prazosin on sympathetic tone in conscious rats

To test the hypothesis that the hypotensive action of urapidil is in part related to a direct action on the brain, the central (intracerebroventricular) and peripheral (intravenous) effects of urapidil were studied and compared with those obtained with clonidine and prazosin. All studies were conducted in conscious, chronically instrumented stroke-prone spontaneously hypertensive rats (SHRSP). Efferent sympathetic nervous system activity was estimated by means of a bipolar electrode implanted on the splanchnic nerve. Only clonidine, administered intracerebroventricularly and intravenously, decreased sympathetic nerve activity. Urapidil and prazosin either did not affect sympathetic nerve activity after central administration or increased it after peripheral administration at low and high doses, respectively. Centrally administered urapidil and prazosin lowered blood pressure but also blocked the response to intravenously administered phenylephrine; this result suggests a peripheral effect. Centrally administered urapidil decreased heart rate. Urapidil given either intracerebroventricularly or into the cisterna magna had no influence on baroreceptor responses. Intravenous infusions of urapidil and prazosin in sufficient doses to lower blood pressure in spontaneously hypertensive rats by 50 mm Hg completely blocked the actions of phenylephrine. These data suggest that in conscious SHRSP urapidil lowers blood pressure through peripheral blockade of alpha 1-adrenergic receptors rather than by means of central sympathetic suppression. In this regard urapidil resembles prazosin rather than clonidine; however, the effect of urapidil on heart rate is consistent with a central mode of action.

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.

Deviating central hypotensive activity of urapidil in the cat

Urapidil, a novel antihypertensive drug, as well as possessing a peripheral alpha 1-adrenoceptor antagonistic effect, also has a significant degree of central hypotensive activity. When injected into the left vertebral artery of chloralose-anaesthetized cats the dose-dependent hypotensive effect of the drug was much stronger than after its systemic administration. Prior treatment, also via the vertebral artery, with various receptor antagonists (yohimbine, prazosin, diphenhydramine, metiamide, sulpiride, pirenzepine, R 56413, naloxone) did not antagonize the central hypotensive effect of urapidil administered subsequently. Accordingly, the central hypotensive action of urapidil is not mediated by central receptors of the following types: alpha 2- or alpha 1-adrenoceptors; H1- and H2-histamine; dopaminergic; muscarinic; 5-hydroxytryptamine; opiate. As such the mechanism of the central hypotensive action remains unexplained. It obviously deviates from that of classical centrally acting antihypertensive drugs like clonidine, guanfacine and alpha-methyldopa, which are agonists of central alpha 2-adrenoceptors.