Effects of reduced calcium ion concentration and of diltiazem on vasoconstrictor responses to noradrenaline and sympathetic nerve stimulation in rat isolated tail artery

Lysophosphatidylcholine potentiates vascular contractile responses by enhancing vasoconstrictor-induced increase in cytosolic free Ca2+ in rat aorta

We investigated the effects of palmitoyl-L-alpha-lysophosphatidylcholine on the contractile responses of the endothelium-denuded rat aorta to high K+, noradrenaline, UK14,304 (5-bromo-6-[2-imidazolin-2-ylamino]-quinoxaline) (a selective alpha2 adrenoceptor agonist) and phorbol 12-myristate 13-acetate (PMA). Lysophosphatidylcholine at concentrations from 10(-6) M to 10(-4) M did not contract aortic strips. However, lysophosphatidylcholine strongly potentiated the UK14,304-induced contraction. High K+ - and PMA-induced contractions were also potentiated. In contrast, the noradrenaline-induced contraction was only slightly potentiated by 10(-5) M lysophosphatidylcholine. In fura PE-3-loaded aortic strips, lysophosphatidylcholine (10(-5) M) markedly augmented the increase in both cytosolic free Ca2+ ([Ca2+]i) and contractile tension induced by UK14,304, high K+ and PMA. Nicardipine (10(-7) M) and 10(-6) M Ro-31-8220 (¿1-[3-(amidinothio)propyl-1H-indoyl-3-yl]-3-(1-methyl-1H-++ +indoyl-3-yl)-maleimide-methane sulfate) strongly inhibited the increase in [Ca2+]i and contractile tension induced by UK14,304 and in the presence of these inhibitors, the enhancing effects of lysophosphatidylcholine were attenuated. However, the enhancing effect on high K+ -induced contraction was not affected by Ro-31-8220. These results suggest that lysophosphatidylcholine may cause an augmentation of the increase in [Ca2+]i induced by UK14,304 which response is depend on protein kinase C activation and in this way potentiate contractile responses in the rat aorta. Protein kinase C independent mechanisms may also be involved in the enhancing effect of lysophosphatidylcholine on smooth muscle contraction.

Effect of alpha2-adrenoceptor stimulation on isolated canine Purkinje fiber contraction

We have recently identified the presence of postjunctional alpha2-adrenoceptors in canine Purkinje fibers. In this study, we examined the effects of alpha2-adrenoceptor stimulation on the contraction strength of isolated Purkinje fibers. Exposure to the alpha2-adrenoceptor specific agonist and antagonist, UK 14,304 (5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) and yohimbine (17-hydroxyyohimban-16-carboxylic acid methyl ester hydrochloride) alone at 0.1 microM respectively, did not produce any significant effect on Purkinje contraction strength. Purkinje contraction strength was augmented by isoproterenol (0.1 microM), forskolin (0.1 microM), or 8-bromo-adenosine cyclic 2',3'-monophosphate (8-bromo-cAMP, 10 microM). UK 14,304 significantly reversed the effects of isoproterenol and forskolin but not those of 8-bromo-cAMP on Purkinje contraction strength. After incubation with pertussis toxin, the positive inotropic effect of forskolin on Purkinje contraction strength remained intact, but the forskolin effect could no longer be reversed by UK 14,304. These results suggest that the postjunctional alpha2-adrenoceptors in canine Purkinje fibers are coupled to a pertussis toxin-sensitive G protein, probably Gi. Stimulation of the alpha2-adrenoceptor antagonizes the effect of beta-adrenoceptor stimulation on Purkinje contraction strength in an accentuated antagonism manner.

Alpha-adrenoceptors and vascular regulation: molecular, pharmacologic and clinical correlates

This manuscript is intended to provide a comprehensive review of the alpha-adrenoceptors (ARs) and their role in vascular regulation. The historical development of the concept of receptors and the division of the alpha-ARs into alpha 1 and alpha 2 subtypes is traced. Emphasis will be placed on current understanding of the specific contribution of discrete alpha 1- and alpha 2-AR subtypes in the regulation of the vasculature, selective agonists and antagonists for these receptors, the second messengers utilized by these receptors, the myoplasmic calcium pathways activated to initiate smooth muscle contraction, as well as the clinical uses of agonists and antagonists that work at these receptors. New information is presented that deals with the molecular aspects of ligand interactions with specific subdomains of these receptors, as well as mRNA distribution and the regulation of alpha 1- and alpha 2-AR gene transcription and translation.

Contribution of alpha-adrenoceptors to depolarization and contraction evoked by continuous asynchronous sympathetic nerve activity in rat tail artery

1. The effects of continuous but asynchronous nerve activity induced by ciguatoxin (CTX-1) on the membrane potential and contraction of smooth muscle cells have been investigated in rat proximal tail arteries isolated in vitro. These effects have been compared with those produced by the continuous application of phenylephrine (PE). 2. CTX-1 (0.4 nM) and PE (10 microM) produced a maintained depolarization of the arterial smooth muscle that was almost completely blocked by alpha-adrenoceptor blockade. In both cases, the depolarization was more sensitive to the selective alpha-adrenoceptor antagonist, idazoxan (0.1 microM), than to the selective alpha 1-adrenoceptor antagonist, prazosin (0.01 microM). 3. In contrast, the maintained contraction of the tail artery induced by CTX-1 (0.2 nM) and PE (2 and 10 microM) was more sensitive to prazosin (0.01) microM, than to idazoxan (0.01 microM). In combination, these antagonists almost completely inhibited contraction to both agents. 4. Application of the calcium channel antagonist, nifedipine (1 microM), had no effect on the depolarization induced by either CTX-1 or PE but maximally reduced the force of the maintained contraction to both agents by about 50%. 5. We conclude that the constriction of the tail artery induced by CTX-1, which mimics the natural discharge of postganglionic perivascular axons, is due almost entirely to alpha-adrenoceptor activation. The results indicate that neuronally released noradrenaline activates more than one alpha-adrenoceptor subtype. The depolarization is dependent primarily on alpha 2-adrenoceptor activation whereas the contraction is dependent primarily on alpha 1-adrenoceptor activation. The links between alpha-adrenoceptor activation and the voltage-dependent and voltage-independent mechanisms that deliver Ca2+ to the contractile apparatus appear to be complex.

Modification of adrenergic reactivity in rat tail artery by dietary lipids and calcium channel antagonists

1. The antiatherosclerotic activity of dihydropyridines (DHP), potent calcium antagonists, was studied with respect to prevention of hypercontractility of perfused rat tail arteries. 2. Used for 1 month, atherogenic diet increased pressor responses to norepinephrine (NE) in Ca(2+)-free physiological salt solution (PSS), and PSS containing Ca(2+). 3. When nifedipine (NIF) or nitrendipine (NIT) was administered simultaneously with an atherogenic diet, the contractile activity of NE in Ca(2+)-free PSS was attenuated. Moreover, vasoconstrictor responses to NE in PSS containing Ca2+ were inhibited after 1-month treatment with NIT and nimodipine (NIM). 4. NIF, NIT and NIM prevented atherosclerosis-induced vascular hyperreactivity to alpha-adrenoceptor agonists in rat tail artery.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

An amplifying effect of exogenous and neurally stored 5-hydroxytryptamine on the neurogenic contraction in rat tail artery

1. The interactions between sympathetic neuroeffector transmission and 5-hydroxytryptamine (5-HT) were investigated in segments of rat isolated tail artery. 2. Contractile responses to field stimulation of the artery segments were abolished by tetrodotoxin (3 x 10(-7) M). A subthreshold concentration of acutely applied exogenous 5-HT (10(-8) M) markedly enhanced the contractions induced by sympathetic nerve stimulation, through an action on postjunctional 5-HT2-receptors. 3. The amplifying effect of 5-HT involved an enhanced influx of extracellular calcium into the smooth muscle cells. In contrast, the neurogenic contractions in vessels not exposed to 5-HT were not dependent on extracellular calcium. 4. The adrenergic component of the amplified response involved postjunctional alpha 1- but not alpha 2- adrenoceptor activation. 5. Exposure of the vessels to 5-HT (5 x 10(-7) M) for 30 min resulted in uptake of the amine into the perivascular sympathetic nerves, as demonstrated by immunohistochemistry. After chemical sympathectomy with 6-hydroxydopamine in vitro or in vivo, or surgical sympathectomy, there was little or no uptake. 6. Exposure to 5-HT followed by repeated washing resulted in an enhancement of the neurogenic contraction, which was still fully tetrodotoxin-sensitive. The enhanced response was blocked by ketanserin (10(-8) M) and prevented by the presence of the 5-HT uptake blocker, paroxetine (3 x 10(-8) M), during the period of exposure to 5-HT.

Characterization of a neurogenic and a direct smooth muscle component in the contractile response to electrical field stimulation in rat tail artery

1. The extent to which neuronal transmitter release contributes to the contractions induced by transmural nerve stimulation of the rat tail artery at various stimulus intensities was characterized. 2. Using tetrodotoxin, which blocks conduction of the action potential along the nerves, and omega-conotoxin GVIA, a blocker of transmitter release from the nerve terminals, as well as chemical and surgical denervations of the perivascular sympathetic nerves, a neurogenic and a direct smooth muscle component could be clearly separated. 3. The neurogenic component was fast in onset, rise and decline (after the end of stimulus), and showed a voltage dependency only at lower stimulus intensities. The non-neurogenic component was slower in onset, rise and decline, and showed a strict voltage dependency throughout the whole stimulus range. This implies that the non-neurogenic component becomes increasingly prominent at high, non-physiological voltages. Mechanisms underlying the declining neurogenic contractile response at the stronger stimulus intensities are discussed. 4. We found no evidence supporting the existence of a possible tetrodotoxin- or omega-conotoxin GVIA-resistant contractile component originating from the perivascular nerves (sympathetic or non-sympathetic). Thus, in order to get a purely neurogenic response stimulus intensities should be minimized to give a contraction that is fully sensitive to these two agents. 5. Transmitter release from the perivascular sympathetic nerves was fully responsible for the purely neurogenic contractions. Activation of postjunctional alpha 1-adrenergic receptors was mainly involved, with a substantial contribution from alpha 2-receptors, and a minor contribution from neuropeptide Y receptors. There was no evidence for a contractile component linked to activation of so-called gamma-adrenergic receptors. 6. Beta-adrenergic receptors, serotonergic, cholinergic, prostanoic or purinergic mechanisms do not appear to contribute to the neurogenic (or the non-neurogenic) response. The neurogenic contraction does not utilize potential-sensitive calcium channels.

Vasoconstrictor responses of rat tail artery to sympathetic nerve stimulation contain a component due to activation of postjunctional beta- or alpha 2-adrenoceptors

The role of alpha 1-, alpha 2- and beta-adrenoceptors in vasoconstrictor responses to sympathetic nerve stimulation was investigated in perfused proximal segments of rat tail artery by using selective blocking drugs. Prazosin (1 nM) markedly reduced the responses but idazoxan (100 nM) did not, and propranolol (1 microM) significantly enhanced them, indicating that the vasoconstriction was due to activation of alpha 1-adrenoceptors and that it was partly counteracted by a vasodilator component due to activation of beta-adrenoceptors. In the presence of propranolol, idazoxan or reduction of the concentration of Ca2+ in the perfusing solution from 2.5 to 0.63 mM significantly reduced responses to sympathetic nerve stimulation, indicating that a component of the vasoconstrictor response was due to activation of alpha 2-adrenoceptors. Forskolin, which increases cyclic AMP levels independently of beta-adrenoceptors, reduced responses to sympathetic nerve stimulation to a greater extent in the presence than in the absence of propranolol and this effect was additive with that of prazosin but not idazoxan. It is concluded that activation of beta-adrenoceptors inhibits the component of responses to sympathetic nerve stimulation due to activation of alpha 2-adrenoceptors because of an inhibitory effect of cyclic AMP on Ca2+ channels linked to alpha 2-adrenoceptors.

Alpha 2-adrenoceptor agonists enhance vasoconstrictor responses to alpha 1-adrenoceptor agonists in the rat tail artery by increasing the influx of Ca2+

1. The alpha 2-adrenoceptor agonists TL99 (2-(N N-dimethyl)amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene) and UK14304 (5-bromo-6-[2-imidazoline-2-yl-aminol]-quinoxaline), in concentrations that are less than 1% of those producing vasoconstriction, enhance vasoconstrictor responses to noradrenaline and phenylephrine in isolated perfused preparations of the rat tail artery. 2. The enhancing effect was abolished when Ca2+ was absent and by the calcium channel blocking drug diltiazem. 3. alpha 2-Adrenoceptor agonists had no effect on the component of the responses to noradrenaline and phenylephrine that is attributable to mobilization of intracellular Ca2+, but enhanced the component attributable to influx of extracellular Ca2+. 4. These results suggest that the enhancing effect of alpha 2-adrenoceptor agonists on responses of the rat tail artery to alpha 1-adrenoceptor agonists involves an increase in Ca2+-influx into smooth muscle cells through Ca2+ channels that are opened when alpha 2-adrenoceptors are activated.

Amplification by serotonin of responses to other vasoconstrictor agents in the rat tail artery

1. A low concentration of serotonin (3 nmol/L), which did not exert a direct vasoconstrictor action, amplified the responses to certain other vasoconstrictor agents (alpha 1-adrenoceptor agonists, KCl, ATP and vasopressin) in isolated perfused segments of the rat tail artery. 2. Low concentrations of serotonin (0.3 and 1 nmol/L) amplified vasoconstrictor responses to sympathetic nerve stimulation, but higher concentrations of serotonin (10 and 30 nmol/L) produced vasoconstriction and reduced responses to sympathetic nerve stimulation. 3. The calcium channel blocking drug diltiazem (1 and 10 mumol/L) produced concentration-dependent reductions of vasoconstrictor responses to phenylephrine. The amplifying effect of serotonin on responses to phenylephrine was attenuated by 1 mumol/L and abolished by 10 mumol/L diltiazem, and was also abolished in a Ca2+-free medium. 4. Ketanserin (10 nmol/L) antagonized the vasoconstrictor action of serotonin and, to a lesser extent, the vasoconstrictor actions of phenylephrine and noradrenaline. It abolished the amplifying effect of a low concentration of serotonin on responses to noradrenaline and phenylephrine. 5. The amplification of vasoconstrictor response in the rat tail artery by serotonin appears to be due to activation of receptors of the 5-HT2 subtype which are coupled to an increase in Ca2+ influx into the vascular smooth muscle cells.

Alpha 2-adrenoceptor agonists enhance responses to certain other vasoconstrictor agonists in the rat tail artery

1. The effects of the alpha2-adrenoceptor agonists clonidine, rilmenidine, TL99 and UK14304 on the vasoconstrictor response to sympathetic nerve stimulation and on the concentration-response curves to noradrenaline and phenylephrine were compared in two isolated, perfused vascular tissues: the rat tail artery (which has both postjunctional alpha 1- and alpha 2-adrenoceptors), and the rabbit ear artery (in which only alpha 1-adrenoceptors are present postjunctionally). 2. In the rabbit ear artery, the first observable effect of alpha 2-adrenoceptor agonists was inhibition of vasoconstrictor responses to sympathetic nerve stimulation. This occurred with concentrations of the alpha 2-adrenoceptor agonists which were far below those producing vasoconstriction. Responses to noradrenaline were not affected. 3. In contrast, in the rat isolated perfused tail artery, alpha 2-adrenoceptor agonists, in concentrations that produced no other observable effects, enhanced the vasoconstrictor responses to sympathetic nerve stimulation and to noradrenaline. Much higher concentrations of alpha 2-adrenoceptor agonists produced vasoconstriction in most preparations and only then reduced the response to sympathetic nerve stimulation. The enhancing effect of alpha 2-adrenoceptor agonists was blocked by idazoxan, but not by prazosin. 4. Vasoconstrictor responses in the rat tail artery to the relatively selective alpha 1-adrenoceptor agonist phenylephrine were enhanced by alpha 2-adrenoceptor agonists. The enhancement of the response to phenylephrine was greater than that to the mixed alpha 1- and alpha 2-adrenoceptor agonist noradrenaline. 5. Vasoconstrictor responses in the rat tail artery to vasopressin, ATP and KCl, like those to alpha 1-adrenoceptor agonists, were enhanced by alpha 2-adrenoceptor agonists.2+owever, vasoconstrictor responses to

Inhibition of noradrenaline release by omega-conotoxin GVIA in the rat tail artery

1. The perivascular nerves of isolated tail arteries from Wistar rats were stimulated with field pulses (1 Hz, 2 pulses, every 2 min). omega-Conotoxin 10 nmol l-1 depressed neurogenically mediated contractions, but did not influence the contractions to noradrenaline 0.1-0.3 mumol l-1. 2. The inhibitory effect of omega-conotoxin was concentration-dependent (IC50 = 3.8 nmol l-1). It did not reach a steady-state during 30 min incubation and could not be reversed upon subsequent washout for another 60 min. 3. A gradual increase in the Ca2+ concentration of the medium from 1.25 mmol l-1 to 10 mmol l-1 enhanced vasoconstriction and attenuated the action of omega-conotoxin 10 nmol l-1. When a low stimulation intensity (120 mA) was used at high external Ca2+ (10 mmol l-1), similar contractile responses were obtained as under normal conditions (200 mA current, 2.5 mmol l-1 Ca2+). However, the inverse relationship between the effect of the toxin and external Ca2+ remained unchanged. 4. The time-course and degree of the inhibition by omega-conotoxin 3 nmol l-1 was identical in tail arteries of spontaneously hypertensive rats (SHR) and their normotensive controls (WKY). 5. When tail arteries of Wistar rats were preincubated with [3H]-noradrenaline, field stimulation (0.4 Hz, 24 pulses, every 16 min) evoked tritium overflow and vasoconstriction. omega-Conotoxin 30 nmol l-1 inhibited both responses to a similar extent. 6. Our results suggest that omega-conotoxin selectively blocks Ca2+ channels in the terminals of perivascular nerves and thereby reduces the release, but not the contractile effect of the sympathetic transmitter.

Effect of levonorgestrel and ethinyloestradiol on vasoconstriction in rat isolated vasculature

Female Sprague-Dawley rats were injected s.c. with 0.2 micrograms day-1 ethinyloestradiol (EE2) or 2.0 micrograms day-1 levonorgestrel (NG), procedures previously shown to increase systolic blood pressure. Increases in perfusion pressure to clonidine, phenylephrine, noradrenaline (NA) and angiotensin II (AII) were observed in rat isolated tail arteries, and to NA in the isolated mesenteric vasculature from steroid and vehicle-treated rats. NG treatment for four weeks produced increases in sensitivity to phenylephrine and NA in rat tail arteries; at 6 weeks the increases in sensitivity had largely disappeared but the maximum responses to clonidine and phenylephrine were increased. No change in sensitivity to AII was observed with NG. In contrast, EE2 treatment for six weeks produced increases in sensitivity to AII, and a decrease in sensitivity and maximum response to clonidine but not to phenylephrine or NA, in tail arteries. Responses to NA in the mesenteric vasculature were increased after 6 weeks NG treatment but unaffected after 12 weeks EE2 treatment. It is concluded that NG treatment stimulates alpha-adrenoceptor number, affinity or receptor-linked Ca2+ events which may contribute to its previously demonstrated hypertensive effect. The increased responsiveness to AII but not the decrease in alpha 2-adrenoceptor responsiveness may be associated with the chronic hypertension induced by EE2.

The alpha 2-adrenoceptor agonists clonidine, TL99 and DPI enhance vasoconstrictor responses to sympathetic nerve stimulation and noradrenaline in the rat tail artery preparation

1. Clonidine (1-30 nmol/l) enhanced the vasoconstrictor responses to sympathetic nerve stimulation in isolated perfused artery preparations from the rat tail. Higher concentrations of clonidine produced vasoconstriction in most artery preparations and reduced the response to sympathetic nerve stimulation. 2. The enhancing effect of clonidine was not affected by prazosin but was blocked by idazoxan, indicating that it was due to activation of alpha 2-adrenoceptors. 3. The alpha 2-adrenoceptor agonists DPI and TL99, like clonidine, enhanced vasoconstrictor responses to sympathetic nerve stimulation. 4. Vasoconstrictor responses to noradrenaline were enhanced by clonidine and TL99. 5. The findings suggest that activation of postjunctional alpha 2-adrenoceptors results in facilitation of the contractile response elicited by activation of alpha 1-adrenoceptors.

Effect of acidosis on alpha 1- and alpha 2-adrenoceptor-mediated vasoconstrictor responses in isolated arteries

We have investigated the effect of reducing the pH (from 7.5 to 7.0 by addition of HCl) on vasoconstrictor responses to noradrenaline in cat middle cerebral artery (in which responses are mediated almost entirely by alpha 2-adrenoceptors) and in rabbit pulmonary artery (in which responses are mediated by alpha 1-adrenoceptors). In the cerebral artery, a reduction in pH caused a pronounced inhibition of the responses to noradrenaline, and the antagonistic effect of idazoxan (100 nM) was increased 10-fold. In contrast, in the pulmonary artery, a reduction in pH had no effect on the responses to noradrenaline and the antagonistic effect of prazosin (100 nM) was not altered. We conclude that acidosis selectively reduces the vasoconstriction mediated by alpha 2-adrenoceptors in vitro.

Cocaine-calcium channel antagonist interactions

Diltiazem, a benzothiazepine calcium channel antagonist, was given to six healthy men as a single 60 mg oral dose 120 min before IV injection of cocaine (0.2 mg/kg) in a double-blind, placebo-controlled, two-session study. Diltiazem alone produced no significant effects. Cocaine increased blood pressure, heart rate, pupil size and subjective "high" ratings, and decreased skin temperature. Diltiazem pretreatment diminished the cocaine effect on skin temperature, but did not otherwise alter the response to cocaine. Calcium channel antagonists diminish the effects of cocaine in vitro and in animals. Dosage considerations may be critical because of the differential sensitivity of various tissues to calcium channel antagonists.

Effects of blockade of alpha 1- and alpha 2-adrenoceptors on vasoconstrictor responses to single and twin pulse stimulation in rat tail artery

Perfused/superfused proximal segments of Sprague-Dawley rat tail artery were stimulated at supramaximal voltage with two 1 ms square wave pulses. The pulse interval was either 10 or 20 s. With either interval the response to each pulse was similar, amounting to about 20 mm Hg increase in perfusion pressure. Prazosin (0.1 nM) approximately halved the response to both pulses whereas idazoxan (30 nM) was without effect. With an interval of 10 s, cocaine (4 microM) greatly increased the response to the first but not to the second pulse; in the presence of cocaine, prazosin (1 nM) again reduced both responses whereas idazoxan (30 nM) reduced the response to the first pulse but increased the response to the second. With an interval of 20 s, cocaine increased the responses to both pulses to a similar degree; in the presence of cocaine, idazoxan reduced the responses to both pulses. The results suggest that in rat tail artery, inhibition of the neuronal uptake process is required if noradrenaline released after stimulation with a single pulse is to reach smooth muscle alpha 2-adrenoceptors, and if feedback inhibition is to persist for more than 10 s.

Preferential antagonism by diltiazem of alpha 2-adrenoceptor mediated vasoconstrictor responses in perfused tail arteries of spontaneous hypertensive rats

Vasoconstrictor responses mediated by the alpha 2-adrenoceptor agonist TL99, were particularly sensitive to blockade by the calcium antagonist drug diltiazem in isolated perfused tail arteries of spontaneously hypertensive rats (SHR). In contrast, the vasoconstrictor responses induced by the alpha 1-adrenoceptor agonist methoxamine were significantly more resistant to antagonism by diltiazem. At higher concentrations (greater than 300 nmol/l) diltiazem became an effective antagonist of all alpha-adrenoceptor mediated responses. In normotensive Wistar Kyoto (WKY) or Sprague-Dawley (SD) rats diltiazem was significantly less potent against vasoconstrictor responses to TL99 than in SHR. The blockade of alpha 1-adrenoceptor mediated vasoconstriction by diltiazem was not significantly different when normotensive rats and SHR were compared. The vasoconstrictor responses evoked by 5HT in the perfused tail arteries were particularly resistant to blockade by diltiazem in SHR arteries. The responses to endogenously released noradrenaline, evoked by electrical field stimulation, were significantly antagonised by diltiazem (30 nmol/1-3 mumol/l) in SHR-tail arteries, while they were not modified in WKY-tail arteries. At the concentrations of diltiazem which blocked end organ responses to field stimulation, there was no modification of total tritium overflow from SHR-tail arteries after labelling the tissue with 3H-noradrenaline, indicating that diltiazem does not inhibit transmitter release at these concentrations. The tail artery preparation of SHR contains a population of postsynaptic alpha 2-adrenoceptors which mediate contraction in this blood vessel and the calcium entry blocker diltiazem is a potent antagonist of vasoconstrictor responses mediated by vascular alpha 2-adrenoceptors in hypertensive rats. These findings may be relevant to the antihypertensive action of diltiazem.