Assessment of alpha-adrenergic receptor subtypes in isolated rat aortic segments

Elevated Muscle Sympathetic Nerve Activity Contributes to Central Artery Stiffness in Young and Middle-Age/Older Adults

Muscle sympathetic nerve activity (MSNA) influences the mechanical properties (ie, vascular smooth muscle tone and stiffness) of peripheral arteries, but it remains controversial whether MSNA contributes to stiffness of central arteries, such as the aorta and carotids. We examined whether elevated MSNA (age-related) would be independently associated with greater stiffness of central (carotid-femoral pulse wave velocity [PWV]) and peripheral (carotid-brachial PWV) arteries, in addition to lower carotid compliance coefficient, in healthy men and women (n=88, age: 19-73 years, 52% men). We also examined whether acute elevations in MSNA without increases in mean arterial pressure using graded levels of lower body negative pressure would augment central and peripheral artery stiffness in young (n=15, 60% men) and middle-age/older (MA/O, n=14, 43% men) adults. Resting MSNA burst frequency (bursts·min^-1) was significantly correlated with carotid-femoral PWV ( R=0.44, P<0.001), carotid-brachial PWV ( R=0.32, P=0.004), and carotid compliance coefficient ( R=0.28, P=0.01) after controlling for sex, mean arterial pressure, heart rate, and waist-to-hip ratio (central obesity), but these correlations were abolished after further controlling for age (all P>0.05). In young and MA/O adults, MSNA was elevated during lower body negative pressure ( P<0.001) and produced significant increases in carotid-femoral PWV (young: Δ+1.3±0.3 versus MA/O: Δ+1.0±0.3 m·s^-1, P=0.53) and carotid-brachial PWV (young: Δ+0.7±0.3 versus MA/O: Δ+0.7±0.5 m·s^-1, P=0.92), whereas carotid compliance coefficient during lower body negative pressure was significantly reduced in young but not MA/O (young: Δ-0.04±0.01 versus MA/O: Δ0.001±0.008 mm²·mm Hg^-1, P<0.01). Collectively, these data demonstrate the influence of MSNA on central artery stiffness and its potential contribution to age-related increases in stiffness of both peripheral and central arteries.

The effect of chlorpyrifos on isolated thoracic aorta in rats

This study investigated the effect of chlorpyrifos on thoracic aorta and on the level of NO in plasma and aorta. The effect of chlorpyrifos on thoracic aorta in organ bath was determined in 10 rats. Another 45 rats were assigned to 3 groups with 15 rats each: control group 1 received distilled water, control group 2 was given corn oil, and the last group was given 13.5 mg/kg chlorpyrifos dissolved in corn oil every other day for 8 weeks orally. Chlorpyrifos (10⁻¹⁰ M–10⁻⁵ M) showed no effect on isolated thoracic aorta. Plasma AChE activity was decreased, while LDH, ALT, GGT, and AST activities were increased in chlorpyrifos group compared to control groups. Plasma NO level was increased in chlorpyrifos group compared to control groups. iNOS expression was present in all groups in the cytoplasm of the endothelia and in the smooth muscle cells of aorta. According to semiquantitative histomorphological analysis, iNOS immunopositive reactions were seen in the decreasing order in chlorpyrifos, control 2, and control 1 groups. eNOS immunopositive reactions were observed in the endothelial cell cytoplasm, rarely in the subintimal layer, and the smooth muscle cells of aorta. There were no differences among the groups in terms of eNOS immunostaining. In conclusion, chlorpyrifos induced NO production in aorta following an increase in NOS expression.

Contractions induced by phenylephrine and noradrenaline are differently affected by endothelium-dependent relaxation in rat aorta

In rings of rat aorta precontracted with phenylephrine (10 μm) or noradrenaline (10 μm), addition of carbachol (10 μm) produced an endothelium-dependent relaxation. However, regardless of the concentration of agonist tested, both the intensity and duration of the relaxation were significantly less when noradrenaline, rather than phenylephrine, was used as the precontracting agent. The different responses observed do not appear to be related to destruction of endothelium-derived relaxing factor by autoxidation of noradrenaline since neither EDTA (30 μm) nor superoxide dismutase (30 units mL−1) improved the relaxation to carbachol. In addition, in endothelium-free rings, the noradrenaline (1 μm)-induced contraction was less sensitive than the phenylephrine (1 μm)-induced contraction to sodium nitroprusside (0·1 μm) or to 8-Br-cGMP (300 μm). With phenylephrine-, but not noradrenaline-, induced contraction, the relaxation triggered by carbachol was significantly reduced by pretreatment of the aortic rings with chloroethylclonidine (50 μm), which inactivates a subpopulation of α1-adrenoceptors. Thus, the results confirm that both alkylation sensitive and resistant α1-adrenoceptors exist in rat aorta and indicate that EDRF may discriminate between these two α1-adrenoceptor subtypes which are differently affected by phenylephrine and noradrenaline.

New insights in the contribution of voltage-gated Na(v) channels to rat aorta contraction

Background

Despite increasing evidence for the presence of voltage-gated Na⁺ channels (Na_v) isoforms and measurements of Na_v channel currents with the patch-clamp technique in arterial myocytes, no information is available to date as to whether or not Na_v channels play a functional role in arteries. The aim of the present work was to look for a physiological role of Na_v channels in the control of rat aortic contraction.

Methodology/Principal Findings

Na_v channels were detected in the aortic media by Western blot analysis and double immunofluorescence labeling for Na_v channels and smooth muscle α-actin using specific antibodies. In parallel, using real time RT-PCR, we identified three Na_v transcripts: Na_v1.2, Na_v1.3, and Na_v1.5. Only the Na_v1.2 isoform was found in the intact media and in freshly isolated myocytes excluding contamination by other cell types. Using the specific Na_v channel agonist veratridine and antagonist tetrodotoxin (TTX), we unmasked a contribution of these channels in the response to the depolarizing agent KCl on rat aortic isometric tension recorded from endothelium-denuded aortic rings. Experimental conditions excluded a contribution of Na_v channels from the perivascular sympathetic nerve terminals. Addition of low concentrations of KCl (2–10 mM), which induced moderate membrane depolarization (e.g., from −55.9±1.4 mV to −45.9±1.2 mV at 10 mmol/L as measured with microelectrodes), triggered a contraction potentiated by veratridine (100 µM) and blocked by TTX (1 µM). KB-R7943, an inhibitor of the reverse mode of the Na⁺/Ca²⁺ exchanger, mimicked the effect of TTX and had no additive effect in presence of TTX.

Conclusions/Significance

These results define a new role for Na_v channels in arterial physiology, and suggest that the TTX-sensitive Na_v1.2 isoform, together with the Na⁺/Ca²⁺ exchanger, contributes to the contractile response of aortic myocytes at physiological range of membrane depolarization.

Acute dilation to alpha(2)-adrenoceptor antagonists uncovers dual constriction and dilation mediated by arterial alpha(2)-adrenoceptors

BACKGROUND AND PURPOSE

In mouse tail arteries, selective alpha(2)-adrenoceptor antagonism with rauwolscine caused powerful dilation during constriction to the alpha(1)-adrenoceptor agonist phenylephrine. This study therefore assessed phenylephrine's selectivity at vascular alpha-adrenoceptors and the mechanism(s) underlying dilation to rauwolscine.

EXPERIMENTAL APPROACH

Mouse isolated tail arteries were assessed using a pressure myograph.

KEY RESULTS

The alpha(2)-adrenoceptor agonist UK14,304 caused low-maximum constriction that was inhibited by rauwolscine (3 x 10(-8) M) but not by the selective alpha(1)-adrenoceptor antagonist prazosin (10(-7) M). Concentration-effect curves to phenylephrine, cirazoline or noradrenaline were unaffected by rauwolscine but were inhibited by prazosin, which was more effective at high compared with low levels of constriction. In the presence of prazosin, rauwolscine inhibited the curves and was more effective at low compared with high levels of constriction. Although rauwolscine alone did not affect concentration-effect curves to phenylephrine, noradrenaline or cirazoline, it caused marked transient dilation when administered during constriction to these agonists. Dilation was mimicked by another alpha(2)-adrenoceptor antagonist (RX821002, 3 x 10(-8) M), was dependent on agonist selectivity, and did not occur during adrenoceptor-independent constriction (U46619). During constriction to UK14,304 plus U46619, rauwolscine or rapid removal of UK14,304 caused transient dilation that virtually abolished the combined constriction. Endothelial denudation reduced these dilator responses.

CONCLUSIONS AND IMPLICATIONS

Inhibition of alpha(2)-adrenoceptors caused transient dilation that was substantially greater than the contribution of alpha(2)-adrenoceptors to the constriction. This reflects a slowly reversing alpha(2)-adrenoceptor-mediated endothelium-dependent dilation and provides a rapid, sensitive test of alpha(2)-adrenoceptor activity. This approach also clearly emphasizes the poor selectivity of phenylephrine at vascular alpha-adrenoceptors.

alpha2-adrenoceptor agonists as nasal decongestants

Nasal congestion, one of the major disease features of rhinitis, is induced by the filling of venous sinusoids causing mucosal engorgement with resultant obstruction of nasal airflow. The only available drugs that directly target the underlying vascular features driving nasal obstruction are the sympathomimetic alpha-adrenoceptor agonists due to their vasoconstrictor action. However, standard decongestants are nonselective alpha-adrenoceptor agonists, which have the potential for side-effects liabilities such as hypertension, stroke, insomnia and nervousness. In the present study, the effects of nonsubtype selective alpha(2)-adrenoceptor agonists BHT-920 and PGE-6201204 were evaluated in several isolated nasal mucosa contractile bioassays including dog, pig and monkey, and in a real-time tissue contractility assay using isolated pig nasal explants for BHT-920. The decongestant activity of PGE-6201204 was evaluated in vivo in a cat model of experimental congestion. Our results showed that alpha(2)-adrenoceptor agonists (1) contract nasal mucosa of different species, (2) exert a preferential vasoconstrictor effect on the capacitance vessels (veins and sinusoids), and (3) elicit decongestion. In conclusion, a selective alpha(2)-adrenoceptor agonist causing constriction preferentially in the large venous sinusoids and veins of nasal mucosa and producing nasal decongestion is expected to show efficacy in the treatment of nasal congestion without the characteristic arterio-constrictor action of the standard nonselective sympathomimetic decongestants.

Pharmacological characterization of alpha 2-adrenoceptor-mediated responses in pig nasal mucosa

1. Pig nasal mucosal strips were incubated with alpha-adrenoceptor antagonists followed by alpha2-adrenoceptor agonist concentration-response curves. 2. Contractions elicited by the alpha2-adrenoceptor agonists BHT-920 (pD2 = 6.16 +/- 0.07), UK 14,304 (pD2 = 6.89 +/- 0.13) and PGE-6201204 (pD2 = 7.12 +/- 0.21) were blocked by the alpha2-adrenoceptor antagonist yohimbine (0.1 microm). In contrast, the alpha1-adrenoceptor antagonist prazosin (0.03 microm) had no effect on the BHT-920-, UK 14,304- and PGE-6201204-induced contractions, but blocked the contractile response to the alpha(1)-adrenoceptor agonist phenylephrine (pD2 = 5.38 +/- 0.04) and the mixed alpha1- and alpha2-adrenoceptor agonist oxymetazoline (pD(2) = 6.30 +/- 0.22). 3. The alpha2-adrenoceptor antagonist yohimbine (0.01-0.1 microm, pA2 = 8.04), alpha2B/C-adrenoceptor antagonist ARC 239 (10 microm, pK(b) = 6.33 +/- 0.21), alpha2A/C-adrenoceptor antagonist WB 4101 (0.3 microm, pK(b) = 8.01 +/- 0.24), alpha2A-adrenoceptor antagonists BRL44408 (0.1 microm, pK(b) = 6.82 +/- 0.34) and RX 821002 (0.1 microm, pKb = 8.31 +/- 0.35), alpha2C-adrenoceptor antagonists spiroxatrine (1 microm, pKb = 7.32 +/- 0.32), rauwolscine (0.1 microm, pKb = 8.16 +/- 0.14) and HV 723 (0.3 microm, pKb = 7.68 +/- 0.14) inhibited BHT-920-induced contractions in pig nasal mucosa. 4. The present antagonist potencies showed correlations with binding affinity estimates (pKi) obtained for these antagonists at the human recombinant alpha2A- and alpha2C-adrenoceptors (r = 0.78 and 0.83, respectively) and with binding affinity estimates (pKd) obtained in pig native alpha2A- and alpha2C-monoreceptor assays (r = 0.85 and 0.78, respectively). No correlation was observed for the alpha2B-subtype. 5. In conclusion, contractile responses to phenylephrine, BHT-920, UK 14,304, PGE-6201204 and oxymetazoline indicate that alpha1- and alpha2-adrenoceptors are present and mediate vasoconstriction in pig nasal mucosa. Furthermore, correlation analysis comparing antagonist potency in pig nasal mucosa with affinities for human recombinant alpha2-adrenoceptors and native pig alpha2-adrenoceptors suggest that alpha2A- and alpha2C-adrenoceptor subtypes constrict pig nasal mucosa vasculature.

Influence of several methodological procedures utilized to obtain in vitro vascular preparations on endothelial activity

Several maneuvers usually employed to set up isolated vascular preparations could effect the endothelium-dependent relaxation (EDR). The effects of five such maneuvers were studied in rings of rat aorta: 1) Type of anesthesia, 2) Cold storage of the vessels, 3) Length of the stabilization period, 4) Repeated contractions during stabilization, and 5) Performance of washouts during stabilization. Repeated contractions with norepinephrine (NE) 0.1 microM after stabilization altered neither the contraction nor the EDR induced by acetylcholine (Ach) 1 microM. Pentobarbital anesthesia and cold storage of the preparations for 24 h significantly decreased the EDR without effecting the contractile response of the rings. The absence of washouts during stabilization increased the contractions to either NE 0.1 microM or KCl 80 mM by nearly 50%. This increase was prevented by endothelial disruption or, in the presence of intact endothelium, by repeated washouts or by incubation with Bosentan 22 microM. It is concluded that 1) Anesthesia of the animals and cold storage of the preparations can alter the EDR even in the absence of contractile changes in the smooth muscle, and 2) Accumulation of endothelin during the incubation period, even if not producing changes in the resting tension, can substantially alter the subsequent response to vasoactive interventions.

Agmatine, an endogenous ligand at imidazoline binding sites, does not antagonize the clonidine-mediated blood pressure reaction

Since agmatine has been identified as a clonidine displacing substance (CDS), the aim of this study was to investigate whether agmatine can mimic CDS-induced cardiovascular reactions in organ bath experiments, pithed spontaneously hypertensive rats (SHR) and anaesthetized SHR. Intravenously-administered agmatine significantly reduced the blood pressure and heart rate of anaesthetized SHR at doses higher than 1 and 3 mg kg(-1), respectively. These effects are probably mediated via central mechanisms, since there was an approximate 8 fold rightward shift of the dose-response curve in the pithed SHR (indicating a weakened cardiovascular effect). Moreover, in organ bath experiments, agmatine failed to alter the contractility of intact or endothelium-denuded aortal rings. When agmatine was administered i.c.v. to anaesthetized SHR, blood pressure was increased without any alteration of heart rate, whereas blood pressure was unchanged and heart rate was increased after injection into the 4th brain ventricle. This suggests that haemodynamic reaction patterns after central application are related to distinct influences on central cardiovascular mechanisms. Agmatine reduces noradrenaline release in pithed SHR while alpha(2)-adrenoceptors are irreversibly blocked with phenoxybenzamine, but not while I(1)-binding sites are selectively blocked with AGN192403. This suggests that agmatine may modulate noradrenaline release in the same way that clonidine does, i.e. via imidazoline binding sites; this involves a reduction in sympathetic tone which in turn reduces blood pressure and heart rate. Finally, CDS-like cardiovascular activity appears not to be due to agmatine, since (i) blood pressure in anaesthetized SHR is decreased by agmatine and clonidine, and (ii) agmatine did not antagonize the blood pressure reaction to clonidine in pithed or anaesthetized SHR.

Postjunctional alpha(2C)-adrenoceptor contractility in human saphenous vein

The postjunctional alpha(2)-adrenoceptor-mediated contractility was characterized in human saphenous vein derived from coronary artery bypass graft surgery. Human saphenous vein contracted to alpha(2)-adrenoceptor selective agonists BHT-920 (5,6,7,8-Tetrahydro-6-(2-propenyl)-4H-thiazolo[4,5-d]azepin-2-amine dihydrochloride; pD(2)=6.7+/-0.1) and UK 14,304 (5-Bromo-6-(2-imidazolin-2-ylamino)quinoxaline; pD(2)=7.2+/-0.1). BHT-920-induced contractions were inhibited by the alpha(2)-adrenoceptor antagonist yohimbine (17-Hydroxy-yohimban-16-carboxylic acid methyl ester hydrochloride; pA(2)=8.7+/-0.5), but not by the alpha(1)-adrenoceptor antagonist prazosin (1-[4-Amino-6,7-dimethoxy-2-quinazolinyl]-4-[2-furanylcarbonyl]-piperazine hydrochloride; 300 nM). In contrast, prazosin (pK(b)=7.9+/-0.2) potently antagonized contractions elicited by the alpha(1)-adrenoceptor agonist phenylephrine ((R)-3-Hydroxy-alpha-[(methylamino)methyl] benzenemethanol hydrochloride; pD(2)=4.9+/-0.1), indicating that both alpha(2)- and alpha(1)-adrenoceptor evoke human saphenous vein contractions. Functional antagonist activity estimates (pA(2) or pK(b)) obtained for the alpha-adrenoceptor antagonists ARC 239 (2-[2-(4-(2-Methoxyphenyl)piperazin-1-yl)ethyl]-4,4-dimethyl-1,3-(2H,4H)-isoquinolindione dihydrochloride), WB 4101 (2-(2,6-Dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride) and HV 723 (alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy) ethyl)amino)propyl)benzeneacetonitrile) against BHT-920-induced human saphenous vein contractions were 7.0+/-0.6, 8.3+/-0.6 and 7.7+/-0.3, respectively. The alpha(2)-adrenoceptor subtype affinities (pK(i)) obtained in recombinant human alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptor competition binding assays were 8.6, 8.3 and 8.6 for yohimbine; 6.3, 8.4 and 7.0 for ARC 239; 8.4, 7.5 and 8.4 for WB 4101 and 7.5, 7.4 and 7.9 for HV 723, respectively. Taken together, the binding and functional antagonist activity estimates obtained in these investigations indicate that alpha(2C)-adrenoceptor is the predominant postjunctional alpha(2)-adrenoceptor subtype in human saphenous vein.

Alpha-adrenoceptor agonists produce Ca2+ oscillations in isolated rat aorta: role of protein kinase C

We investigated the relationship between tension development and the cytosolic free Ca2+ level ([Ca2+]i) in responses to norepinephrine (NE) and selective alpha2-adrenoceptor agonist, UK14,304 of the endothelium-denuded rat aorta loaded with fura PE-3. NE (3 x 10(-8) M) evoked a rapid increase in [Ca2+]i followed by slight decreasing to a steady state level and produced a contraction. After the NE-induced increase in [Ca2+]i had reached a maximum, the [Ca2+]i showed persistent oscillations. The Ca2+ oscillations were superimposed on the sustained increase in [Ca2+]i. UK14,304 (3 x 10(-6) M) also evoked an increase in [Ca2+]i and produced a contraction. However, the UK14,304-induced effect on [Ca2+]i was characterized by pronounced oscillations, and the amplitude of the sustained increase in [Ca2+]i was less than that seen with NE. Protein kinase C inhibitor, Ro31-8220 (3 x 10(-6) M) and verapamil (10(-5) M) abolished both NE and UK14,304-evoked Ca2+ oscillations. UK14,304-induced contractions were also strongly inhibited by Ro31-8220 and verapamil. However, NE induced contractions were partly inhibited by these inhibitors. The sustained increases in [Ca2+]i evoked NE and UK14,304 were not significantly inhibited by Ro31-8220 and verapamil. These results suggest that NE and UK14,304 produce Ca2+ oscillations during sustained contractions in rat aorta. The alpha2 adrenoceptor agonist, UK14,304-induced sustained contraction and Ca2+ oscillations may be due to PKC activation and opening of voltage-dependent L type Ca2+ channels.

Pharmacological, electrophysiological and toxicity studies of Limacia scanden Lour. (Menispermaceae)

Pharmacological studies showed that Limacia scanden Lour. extracts have sympathomimetic activities similar to noradrenaline (NA). A crude extract of Limacia scanden injected intravenously as a single bolus induced a dose-dependent increase in arterial blood pressure in anaesthetized rats and cats. Pretreatment with a non-specific alpha blocker phentolamine (10(-5) M) blocked this effect, whereas the beta blocker propanolol (10(-5) M) did not. The extract also reduced intestinal motility and this response could be blocked by pretreatment with phentolamine (10(-5) M) and specific alpha1-blocker, prazosin (10(-5) M). In superfused rabbit aorta preparations, it induced an increase in contractions. This effect was blocked by pretreatment with prazosin (10(-5) M), whereas the alpha2-blocker yohimbine (10(-5) M) had only a slight effect. The effects of NA on superfused aorta strip contraction were similar to extract. Toxic symptoms were manifested in less than 5 min when the mice were given 465 mg/kg of extract intraperitoneally. Physiological and behavioural changes observed in dying mice implicated serious malfunctioning of the autonomic nervous system and motor activity. Electrophysiological studies on the tonically autoactive neuron (TAN) of the snail Achantina fulica Férussac revealed that crude extract of Limacia scanden induced excitatory responses which were similar to those of serotonin (5-HT) stimulation. Studies with different ionic compositions of the bathing saline revealed that this excitatory effect of Limacia scanden could be attributed either to release of endogenous serotonin or inhibition of 5-HT reuptake in the CNS. This observation could tentatively be used to provide the framework towards elucidating the mechanism and rationale for the use of this plant in traditional medicine in the treatment of depression and affective disorders.

Quantitative changes in pharmacodynamic parameters of noradrenaline in different rat aorta preparations: influence of endogenous EDRF

1. The aim of the present study was to assess the role of endothelial cells in the modulation of vasocontractile responses to noradrenaline in rat isolated aorta when cut as standard helical strips or as ring segments. 2. Noradrenaline-potency in helical strip preparations evaluated as -logEC50 was greater than that obtained in endothelium-intact ring preparations (9.45 +/- 0.28 versus 8.69 +/- 0.09, respectively) (P < 0.05). The maximum contractile response of helical strips was significantly higher than the response of ring preparations (P < 0.05). 3. Subsequent experiments were performed on helical strips and ring preparations where the endothelium was removed by rubbing the luminal surface of the aorta with filter paper. Removal of the endothelium potentiated the noradrenaline-induced contraction in ring preparations, but not in the helical strips. 4. The nitric oxide synthase inhibitors L-NAME (3 x 10(-5)-3 x 10(-4) M) or L-NNA (1 x 10(4)-3 x 10(-4) M) which were added to the tissue bath potentiated the noradrenaline-induced contraction in the endothelium-intact ring preparations, although only L-NNA induced a statistically significant potentiation. Both L-NAME and L-NNA had no effect on the noradrenaline-contraction induced in rings without endothelium, or in helical strips with or without endothelium. 5. Vascular acetylcholine-induced relaxation is dependent on endothelium derived relaxing factor (nitric oxide). Acetylcholine (10(-9)-10(-6) M) induced a concentration-dependent relaxation in noradrenaline preconstricted intact rings. The relaxant response was strongly reduced by L-NAME (3 x 10(-5)-1 x 10(-4) M). The relaxant response to acetylcholine was very weak in ring and helical strip preparations without endothelium, but also, surprisingly, in unrubbed standard helical strips. 6. The present results suggest that the endothelium of standard helical strip preparations may be greatly damaged, a view confirmed by morphological studies. The structural and functional damage of the endothelium induced very important changes in pharmacodynamic parameters such as in the potency and the maximal responses of vascular preparations to noradrenaline. Therefore, caution must be observed when the potency and intrinsic activity of agonists evaluated on different preparations are compared, even if these come from the same vascular segment.

Increased vascular responsiveness to alpha 2-adrenergic stimulation during NOS inhibition-induced hypertension

Increased vascular resistance during systemic nitric oxide synthase (NOS) inhibition is dependent on adrenergic vasoconstriction. This study tested the hypothesis that increased vascular sensitivity to adrenergic agonists contributes to this vasoconstriction. Superior mesenteric arteries and thoracic aortae from male Sprague-Dawley rats drinking water containing N omega-nitro-L-arginine (L-NNA; 14 days, 60 mg.kg-1.day-1) and control rats were-cut into helical strips, and endothelium was removed for contractile experiments. L-NNA arteries were more sensitive to UK-14304 (alpha 2-adrenergic agonist) and norepinephrine (NE), whereas responses to phenylephrine (PE) were not different concentration causing 50% maximal response (EC50), L-NNA vs. control: UK-14304, 0.071 vs. 0.71 mumol/l; NE, 1.15 vs. 9.95 nmol/l]. Yohimbine, an alpha 2-selective antagonist, caused a concentration-dependent inhibition of contraction to NE only in L-NNA arteries (EC50 = 6.3 vs. 1.6 nmol/l at 1 nmol/l yohimbine), whereas prazosin shifted NE curves similarly in arteries from both groups. Yohimbine (10 nmol/l) inhibited contractions to UK-14304 (EC50 = 59 mumol/l vs. 17 mumol/l) but not contractions to PE, whereas prazosin inhibited both. These data indicate that L-NNA-induced hypertension leads to increased sensitivity of prazosin-sensitive alpha 2-adrenoceptors, an upregulation that could cause the increased vasoconstrictor response to NE in this model of hypertension.

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.

Effects of hydralazine on contractile responses to alpha 1 and alpha 2-adrenoceptor agonists in isolated rubbed rat aorta

1. Effects of hydralazine on contractile responses to noradrenaline (an alpha 1- and alpha 2-adrenoceptor agonist) to phenylephrine and methoxamine (both selective alpha 1-adrenoceptor agonists) and to clonidine and BHT-920 (both relatively selective alpha 2-adrenoceptor agonists) were examined in isolated rat aorta deprived of endothelium. Hydralazine (1 mM) produced a rightward shift with depression of the maximal tension of the concentration-response curves for all the agonists tested. The effects on curves for clonidine and BHT-920 (partial agonists) were greater than on curves for noradrenaline, phenylephrine and methoxamine (full agonists). 2. The inhibitory effect of prazosin (pA2, about 10) was much greater than that of yohimbine (pA2, about 7) for all the agonists. 3. In tissues pretreated with phenoxybenzamine, hydralazine (1 mM) inhibited the residual response to all the agonists. The inhibitory effect on residual response to full agonists was similar to that observed on response to partial agonists in tissues not treated with phenoxybenzamine. 4. The relationship between maximal response and percentage receptor occupancy was nonlinear for full agonists, but near-linear for partial agonists. 5. These results indicate that the responses induced by noradrenaline, phenylephrine, methoxamine, clonidine and BHT-920 in the rat aorta are due to the activation of alpha 1-adrenoceptors and confirm the vasorelaxant action of hydralazine. 6. These results also suggest that the differential effects of hydralazine on the responses to alpha-adrenoceptor agonists may be due to differences in the amount of receptor reserve available available in this blood vessel for full agonists (noradrenaline, phenylephrine or methoxamine) and partial agonists (clonidine or BHT-920).

Pharmacological evidence for interactions between 5-HT1A receptor agonists and subtypes of alpha 1-adrenoceptors on rabbit aorta

This study was designed to determine if alpha 1-adrenoceptors are involved in the vascular responses to 5-HT1A receptor agonists. Buspirone (3.1 x 10(-7)-3.1 x 10(-5) M) and 8-hydroxy-2(di-N-propylamino)tetralin (8-OH-DPAT; 3.1 x 10(-6)-10(-4) M) elicited contractions of rabbit aorta rings which were blocked by prazosin (10(-9)-5.6 x 10(-9) M), but which were unaffected by reserpine pretreatment (1 mg/kg i.p.). 5-Methylurapidil (10(-7) and 10(-6) M) blocked contractions elicited by 8-OH-DPAT and by buspirone, whereas chloroethylchonidine (10(-5) and 10(-4) M) inhibited only the effect of buspirone. In addition, these 5-HT1A receptor agonists relaxed arteries precontracted with alpha-adrenoceptor agonists in a similar range of concentrations in which they elicited contraction. Moreover, 8-OH-DPAT and buspirone protected the alpha-adrenoceptors from the irreversible blockade provoked by phenoxybenzamine (10(-7) M), as judged by the norepinephrine contraction and stimulated phosphatidylinositol labeling. According to these results the contractile and relaxant effects elicited by 5-HT1A receptor agonists are a consequence of a direct interaction with alpha 1-adrenoceptors. The contraction elicited by 8-OH-DPAT may be mediated by alpha 1A-adrenoceptors, whereas both alpha 1A- and alpha 1B-adrenoceptors may mediate the effect of buspirone in rabbit aorta.

α1-adrenoceptor blocking properties of spiroxatrine in rat aorta

This preliminary study has analyzed the potential ability of the 5-HT1A ligand spiroxatrine to interact with vascular alpha 1-adrenoceptors. Norepinephrine and the selective alpha 1-adrenoceptor agonist, methoxamine, elicited concentration-dependent contractions of rat aortic rings. In contrast, (+/-)-spiroxatrine (from 10(-8) to 3.1X10(-7) M) was devoid of any effect on vascular tone per se, but shifted the concentration-response curves of norepinephrine and methoxamine to the right in a concentration-dependent manner with pA2 values of 8.48 +/- 0.22 and 8.93 +/- 0.33, respectively. Endothelium removal did not significantly affect the above pA2 values of (+/-)-spiroxatrine. These data, taken in concert, support the contention that (+/-)-spiroxatrine displays alpha 1-adrenoceptor blocking properties in rat aortic rings.

Characterization of histamine-induced contraction in rat isolated aorta

High concentrations of histamine (greater than 10 microM) contract rat aortic rings and the effect is greatly enhanced when the endothelium is removed. The present study was aimed at characterizing the histamine-induced contractions of de-endothelialized rat aortic rings. These contractions were poorly inhibited by the histamine H1-receptor antagonist, mepyramine (1 and 10 microM) and insensitive to the histamine H2-receptor antagonist, cimetidine (10 microM), and to the cyclooxygenase inhibitor, indomethacin (5 microM). In contrast, the alpha-adrenoceptor antagonists, prasozin and pentholamine, antagonized these contractions in a concentration-dependent manner (respective apparent pKB values 9.7 and 7.9) and nifedipine (3 microM) reduced them by about 75%. Pretreatment of de-endothelialized rings with 8-bromo-cyclic GMP and of intact rings with methylene blue resulted in respective inhibition and enhancement of histamine-induced contractions, quite similarly to the effects in the presence and in the absence of endothelium, respectively. Histamine elicited endothelium-dependent relaxation of aortic rings precontracted by prostaglandin F2 alpha. This relaxation was abolished in the presence of mepyramine (1 microM). However, mepyramine failed to mimic the enhancing effect of endothelium removal on histamine-induced contractions of resting aortic rings. It is concluded that, in rat aorta, (1) contractions induced by high concentrations of histamine (greater than 10 microM) are probably mediated by alpha 1-adrenoceptors; and (2) spontaneous, but not histamine-stimulated, release of endothelium-derived relaxing factor is mainly involved in the modulation of histamine-induced contractions.

Effects of aging on postsynaptic alpha 1-adrenoceptor mechanisms in rat aorta

1. Effects of aging on alpha 1-adrenoceptor and S2-serotonin receptor mechanisms in rat aorta were studied. 2. In rat aorta, the potency (pD2 value) of norepinephrine or phenylephrine increased with age from 3 to 10 weeks, but decreased thereafter with age from 10 to 80 weeks. The affinity (pKA value) of norepinephrine or phenylephrine and of prazosin (pA2 value) did not alter with aging. 3. In rat vas deferens, the efficacy of norepinephrine and the maximum binding sites of [3H]prazosin increased with age from 3 to 18 weeks, but decreased thereafter with age from 18 to 60 weeks. The affinity (pKA value) of norepinephrine and the dissociation constant (KD value) of prazosin did not alter with aging. 4. In rat aorta, the potency (pD2 value) and affinity (pKA value) of serotonin, and affinity (pA2 value) of ketanserin did not alter with aging. 5. There is no significant difference between slopes of regression lines between a cytosolic free Ca2+ level [( Ca2+]i) and tension in the presence of phenylephrine in aorta strips from 10- and 60-week-old rats. 6. These results suggest that changes in alpha 1-adrenoceptor mechanisms with aging are due to changes in receptor density or receptor reserve, but not to changes in affinity of drugs to alpha 1-adrenoceptor or sensitivity of contractile system to Ca2+ mediated through alpha 1-adrenoceptor, and that S2-serotonin receptor mechanisms in rat aorta do not alter with aging.

Effects of pregnancy and endothelial cell removal on alpha-adrenoceptor-mediated responses of rat thoracic aortae

1. The possibility was examined that changes in sensitivity of the aorta to alpha-adrenoceptor agonists during pregnancy in the rat are due to changes in smooth muscle receptor-mediated responses and/or endothelial cell-mediated responses. 2. Maximum constrictor responses to phenylephrine (PE) of both endothelium-intact and denuded thoracic aortic rings were greater when tissues were obtained from 20-day pregnant rats compared with non-pregnant ones. For endothelium-denuded thoracic aortic rings, the pA2 value for phentolamine as an antagonist of PE was not significantly different for rings from 20-day pregnant rats compared with rings from non-pregnant rats. 3. Prazosin (1-50 nM) markedly depressed maximum contractile responses to PE of endothelium-intact (but not denuded) thoracic aortic rings from both non-pregnant and 20-day pregnant rats. However, this marked depression of maximum responses by prazosin did not occur in the additional presence of yohimbine (10 nM), and yohimbine alone (10-500 mM) did not affect maximum responses to PE. There was no significant difference between the pA2 value for prazosin against PE-induced contractions of endothelium-denuded aortic rings obtained from either non-pregnant or 20-day pregnant rats (in the presence of yohimbine, 10 nM). 4. These results provide no evidence for a change in aortic smooth muscle alpha-adrenoceptor affinity during pregnancy, although maximum responsiveness of aortic smooth muscle to PE is increased on day 20 of pregnancy (c.f. non-pregnant controls).

Hypotension induced by growth-hormone-releasing peptide is mediated by mast cell serotonin release in the rat

Growth-hormone-releasing peptide (GH-RP-6) is a synthetic hexapeptide that selectively releases growth hormone (GH) when administered to a number of animals species. In the rat, maximal GH release occurs after intravenous administration of 100 micrograms/kg GH-RP-6. Intravenous administration of 5 mg/kg GH-RP-6 produced 100% lethality within 2-5 min of drug administration. Further investigative studies demonstrated that the lethal effect of GH-RP-6 was preceded by an initial hypertensive episode, followed by a rapid, profound hypotension and bradycardia. The rise and fall in blood pressure also were observed in pithed rats treated with GH-RP-6, suggesting that the central nervous system was not responsible for the changes in blood pressure. However, the GH-RP-6-induced bradycardia was not observed in pithed rats, indicating the fall in heart rate was mediated through a central reflex mechanism. No direct effects of GH-RP-6 were seen in the isolated rat aorta or canine saphenous vein. Pretreatment of conscious rats with naloxone (10 mg/kg, iv), an opiate receptor antagonist, did not prevent the hypertensive response to GH-RP-6, but the hypotension and lethality were attenuated. Pretreatment with cyproheptadine (2.5 mg/kg, iv), a dual serotonin/histamine antagonist, or ketanserin (3 mg/kg, iv), a selective serotonin antagonist, prevented the GH-RP-6-induced hypotension and lethality. Cyproheptadine unmasked a 40 mm Hg rise in mean arterial pressure which persisted for over 10 min. In addition, degranulation of mast cells with compound 48/80 inhibited the toxicity of GH-RP-6, suggesting that mast cell degranulation and the subsequent release of autocoids is responsible for the cardiovascular effects of GH-RP-6. In vitro, GH-RP-6 (10(-5) - 10(-3) M) produced a concentration-related release of histamine from rat peritoneal mast cells. However, the histamine release by GH-RP-6 (10(-4) M) was not inhibited by naloxone (10(-4) M) in isolated mast cells, suggesting either that peritoneal mast cells are not responsible or that the mast cell degranulation in vitro is not opiate mediated. In conclusion, it appears that GH-RP-6 degranulates mast cells releasing serotonin, which produces hypotension, bradycardia, and death. This degranulation of mast cells is apparently inhibited by naloxone in vivo, suggesting that opiate receptors are involved in the hypotension and lethality associated with the administration of GH-RP-6.

Effects of endothelial cell removal on alpha-adrenoceptor-mediated responses of aortae of pregnant rats

1. The possibility that changes in sensitivity of the aorta occurring during pregnancy in the rat are due to changes in uptake mechanisms, alpha- and/or beta-adrenoceptor-mediated responses was investigated. 2. Thoracic aortic rings from 20 day pregnant rats showed increased sensitivity to the constrictor effects of phenylephrine, with increased maximum responses when compared with those from non-pregnant animals. Removal of endothelium caused leftward shifts of the log concentration-response curves with further increases in maxima to the same extent in rings of both non-pregnant and 20 day pregnant rats. 3. Propranolol, beta-oestradiol, and nisoxetine did not significantly alter the CR-curves to phenylephrine of endothelium-intact or denuded thoracic aortic rings obtained from either non-pregnant or 20 day pregnant rats. 4. BHT-920 failed to cause marked constriction of endothelium-intact rings. After removal of endothelium, significant constrictor responses to BHT-920 occurred which were of similar magnitude for rings from both non-pregnant and 20 day pregnant rats. 5. Relaxant responses to BHT-920 of endothelium-intact rings preconstricted with phenylephrine were not significantly different between those from non-pregnant and 20 day pregnant rats. Removal of endothelium resulted in rightward shifts of the curves together with decreased maximum responses. 6. These results support earlier suggestions that the endothelium plays a role in controlling vascular reactivity to phenylephrine. The increased maximum response of thoracic aortic rings to phenylephrine seen during pregnancy does not appear to be due to marked changes in amine uptake mechanisms or beta- or alpha 2-adrenoceptor-mediated responses.

Alpha-1 and alpha-2 adrenoceptor agonists induce vasoconstriction of the normotensive rat caudal artery in vitro by stimulation of a heterogeneous population of alpha-1 adrenoceptors

Although alpha-2 adrenoceptor agonists rapidly induce arterial vasoconstriction in vivo, such responses have proven difficult to obtain in vitro. We have investigated the vasoconstrictor effects of various alpha-1 and alpha-2 adrenoceptor agonists in the perfused superfused caudal artery of the normotensive rat. Intrinsic activities were; methoxamine; 1, phenylephrine; 0.94, noradrenaline; 0.93, guanfacine; 0.88, clonidine; 0.47, UK 14,304 [5-bromo-6-(2-imidazoline-2-ylamino)-quinoxaline tartrate]: 0.10, azepexole; 0. Antagonism by the selective alpha-1 agent, prazosin of the vasoconstrictor responses provoked by methoxamine, guanfacine or clonidine, showed a high affinity with--log KB values in the range of 8.5 to 9.4. There were no significant differences between the KB values obtained with the three agonists. Antagonism by the selective alpha-2 antagonist, yohimbine showed a low affinity with KB values between 6.7 to 7.6 for the three agonists. The calcium entry blocker, nicardipine, antagonized responses to clonidine at nanomolar concentrations and those to phenylephrine at micromolar concentrations. We conclude that vasoconstrictor responses in this isolated tail artery preparation are primarily mediated via an alpha adrenoceptor which can be classified, on the basis of the results with specific antagonists, as being of the alpha-1 type. The results obtained with nicardipine suggest that the population of alpha adrenoceptors is not, however, homogeneous.

Enhancement of contractile responses to partial alpha-adrenoceptor agonists during warming in rat aorta

We examined the effects of warming on the contractile responses to full and partial alpha-adrenoceptor agonists in rat aorta. The contractions elicited by norepinephrine and methoxamine were not affected during warming (40 degrees C, 42 degrees C), whereas those induced by clonidine and St 587 were significantly enhanced. KCl-induced contractions of rat aorta were not affected by warming. The dissociation constants of clonidine and St 587 at 40 degrees C were not different from those at 37 degrees C. At 40 degrees C, the receptor occupancy-contractile response curve of clonidine was a hyperbolic curve similar to that of methoxamine at 37 degrees C, although at 37 degrees C the curve was almost linear. The responses of St 587 at both 37 degrees C and 40 degrees C were related inversely hyperbolic to the receptor occupancy, but the receptor occupancy-contractile response curve was shifted to the left and upward during warming. Clonidine and St 587 elicited equal responses at lower fractional occupancies at 40 degrees C than at 37 degrees C. The relative efficacies of clonidine and St 587 to methoxamine were significantly augmented during warming. It is suggested that the contractile responses to partial alpha-adrenoceptor agonists in rat aorta are enhanced during warming, and that this effect is related to the intrinsic efficacy of the agonists rather than to any function of their relative selectivity for alpha 1- or alpha 2-adrenoceptors. Such enhancement is due to augmentation of the efficacy rather than to augmentation of the affinity of the agonists.

Characterization of post-junctional alpha-adrenoceptors in the rat isolated perfused femoral artery

A comparison was made of contractile responses to alpha-adrenoceptor agonists in the rat aorta and in the rat isolated perfused femoral artery. Dose-response curves were constructed to noradrenaline (alpha 1/alpha 2), methoxamine (alpha 1-selective) and B-HT 920 (alpha 2-selective). Methoxamine behaved as a full agonist in both tissues as compared with noradrenaline, while B-HT 920 was only a partial agonist in the aorta and produced small responses in the femoral artery preparation which were not dose-dependent. pA2 or -log KB values were calculated for prazosin and idazoxan against noradrenaline and methoxamine. Similar -log KB values for prazosin against both agonists were obtained in both tissues, while idazoxan was approximately ten times more potent in the femoral artery preparation than in the aorta. These results suggest that the aorta contains a single population of alpha 1-adrenoceptors, while the perfused femoral artery preparation contains predominantly alpha 1-adrenoceptors but also a small population of alpha 2-adrenoceptors.

Interactions of MEN 935 (adimolol), a long acting beta- and alpha-adrenolytic antihypertensive agent, with postsynaptic alpha-adrenoceptors in different isolated blood vessels--influence of angiotensin II

MEN 935 [1-(3-[3-(1-naphthoxy)-2-hydroxypropyl) amino)-3,3-dimethylpropyl)-2-benzimidazolinone-hydrochloride monohydrate, adimolol] is a long acting antihypertensive agent with beta- and alpha-adrenolytic properties. Preliminary experiments in pithed rats had led to the suggestion that the alpha-adrenolytic activity was of the alpha 2-subtype. The alpha-adrenolytic properties of MEN 935 were now tested in isolated vascular preparations of rat aorta, rabbit vena ischiadica and rabbit vena cava inferior against the selective alpha 1-adrenergic agonist phenylephrine (PE) and the selective alpha 2-adrenergic agonist B-HT 920 [2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo-(4,5-d)azepine]. The experiments were performed in absence and in presence of 5 X 10(-9) mol/l angiotensin II (A II). MEN 935 antagonized contractions to phenylephrine as well as those to B-HT 920 in each vessel. A twofold shift to the right of the concentration-response curves to both agonists was obtained with concentrations between 1.9 X 10(-8) and 1.4 X 10(-5) mol/l, depending on the vessel under investigation. A II modulated the adrenolytic properties of MEN 935 in each vessel. However, irrespective of the presence or absence of A II, no pharmacologically relevant difference between antagonism against PE or B-HT 920 could be seen. In isolated vessels, MEN 935 exerts a nonselective alpha-adrenergic antagonism. In receptor binding studies in rat cerebellar cortex, MEN 935 showed a Ki of 5.2 X 10(-7) mol/l at alpha 1-adrenoceptors and a Ki of 1.3 X 10(-5) mol/l at alpha 2-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for and against heterogeneity of alpha 1-adrenoceptors

Recent experimental evidence has suggested that the alpha 1 adrenoceptor may need to be further subdivided. It can no longer be stated categorically that alpha 1-adrenoceptors are present only at postjunctional sites, in view of several reports of alpha 1-mediated modulation of adrenergic and cholinergic neurotransmission. Furthermore, comparison of the pharmacologic characteristics of the alpha 1-adrenoceptor in different species and/or tissues can show clear differences in sensitivity to selective agonists and antagonists, and differences in the degree of dependence on extracellular calcium. However, in other cases, alpha 1-adrenoceptors at diverse sites have been found to have identical characteristics. Furthermore, the subcategories identified by the various selective agents do not fall into the same discrete groups, in contrast to division of alpha-adrenoceptors into alpha 1 and alpha 2-adrenoceptors. Therefore, at this time it seems premature to subdivide the alpha 1-adrenoceptor further.

Increased Ca2+ sensitivity of alpha 1-adrenoceptor-stimulated contraction in SHR caudal artery

Potassium and alpha-receptor-stimulated contractile responses of caudal artery rings of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were measured under conditions in which norepinephrine (NE) uptake and K+-induced NE release by nerve terminals were eliminated. The maximum isometric tension developed by SHR arterial rings was significantly more compared with WKY arterial rings when arteries were stimulated with NE but not when stimulated with K+. The Ca2+ sensitivity of NE-stimulated arterial rings was about twofold higher compared with WKY arterial rings. However, the Ca2+ sensitivity of K+-depolarized arterial rings was comparable between WKY and SHR. This increase in Ca2+ sensitivity was specifically due to changes in the alpha 1-receptor-mediated mechanisms in SHR. The 50% effective concentration (EC50) values for both NE and alpha 1-specific agonist, methoxamine hydrochloride, were comparable between WKY and SHR, suggesting that alpha 1-receptor sensitivity is not altered in SHR. The relative contributions of postsynaptic alpha 1- and alpha 2-receptors in caudal artery contractions as calculated from the experiments with alpha 1- and alpha 2-receptor agonist and antagonists were 80 and 20% in WKY and 95 and 5% in SHR, respectively. Nifedipine inhibition of caudal artery contractions was significantly greater (P less than 0.05) in SHR when stimulated with NE but not when stimulated with potassium. Our results indicate that the mechanisms involved in K+ depolarization-dependent contractions are not altered in SHR. However, the mechanisms involved in the coupling of alpha 1-adrenergic receptor and smooth muscle contractions may be altered in SHR caudal artery rings.

Enhancement of central and peripheral alpha 1-adrenoceptor sensitivity and reduction of alpha 2-adrenoceptor sensitivity following chronic imipramine treatment in rats

After chronic imipramine treatment (20 mg/kg i.p., once daily for 14 days) the dose-response curve of the isolated rat anococcygeal muscle to phenylephrine shifted to the left, and furthermore, the -log KA value (affinity) for phenylephrine was significantly increased without affecting the affinity for guanfacine. On the other hand, such treatment caused a shift to the right of the dose-response curve to guanfacine on aortic strips and the affinity of the alpha-adrenoceptor for guanfacine was lowered without any accompanying changes in the affinity value for phenylephrine. However the relative efficacies of phenylephrine or guanfacine were not influenced by imipramine in either preparation. The ability of phenylephrine to displace [3H]prazosin from its specific binding sites was significantly enhanced after chronic imipramine treatment. These results may indicate that following chronic imipramine treatment the alpha 1-adrenoceptors of both central and peripheral tissues responded with supersensitivity to an alpha 1-preferential agonist, and the alpha 2-adrenoceptors with reduced sensitivity to an alpha 2-preferential agonist.

Effect of removing the endothelial cells on the reactivity of rat aortic segments to different alpha-adrenoceptor agonists

The influence of removing the endothelial cells on the alpha-receptor-mediated contractile response in segments of rat aorta was investigated using agonists with a range of affinity for alpha 1- and alpha 2-receptors. The preferential alpha 1-agonists were methoxamine, cirazoline, ST 587, and Sgd 101/75 and the preferential alpha 2-agonists were B-HT 920, clonidine, and guanfacine. When the endothelium was intact, the intrinsic activity (compared to noradrenaline) varied widely (0.0-0.7) for both groups of agonists. After removal of the endothelium the intrinsic activity was increased in each case to that of noradrenaline, or close to it. Furthermore, an increase in potency was obtained for each agonist, although to different degrees. No correlation, however, was found between the selectivity of the agonists and the degree of enhancement caused by the removal of the endothelium, in terms of either the intrinsic activity or the potency. Moreover, the use of the selective alpha 2-receptor antagonist rauwolscine on intact tissues did not mimic the effect of removing the endothelium. Therefore, the alpha-receptors of the endothelium could not be classified as either of the alpha 1- or alpha 2-subtype.

The contractions induced in rat and guinea-pig aortic strips by the alpha 2-adrenoceptor selective agonists B-HT 920 and UK 14,304 are mediated by alpha 1-adrenoceptors

The alpha-adrenergic action of the selective alpha 2-adrenoceptor agonists B-HT 920 and UK 14,304 was investigated on helically cut preparations of rat and guinea-pig isolated aorta. The alpha 1-adrenoceptor selective stimulant (-)-phenylephrine was included for comparison. All agonists induced concentration-dependent contractions in both preparations. Calcium entry blockade by D 600 almost abolished the contractions evoked by B-HT 920 and UK 14,304 in rat aorta while those evoked in guinea-pig aorta were less strongly affected. Contractions elicited by (-)-phenylephrine were moderately impaired by D 600 in rat aorta whereas there was only a limited effect in guinea-pig aorta. Analysis of the prazosin and yohimbine antagonism of B-HT 920- and UK 14,304-evoked contractions showed the involvement of alpha 1-like adrenoceptors in rat and guinea-pig aorta, prazosin being approximately 1000 times more potent that yohimbine. The results show that B-HT 920 and UK 14,304 contract rat and guinea-pig aorta via alpha 1-like adrenoceptors which are not identical. It is submitted that rat and guinea-pig alpha 1-adrenoceptors activate different contractile processes.