Exhaustive swimming differentially inhibits P2X1 receptor- and α1-adrenoceptor-mediated vasoconstriction in isolated rat arteries

Aim:

To investigate the effects of exhaustive swimming exercise on P2X1 receptor- and α1-adrenoceptor-mediated vasoconstriction of different types of arteries in rats.

Methods:

Male Wistar rats were divided into 2 groups: the sedentary control group (SCG) and the exhaustive swimming exercise group (ESEG). The rats in the ESEG were subjected to a swim to exhaustion once a day for 2 weeks. Internal carotid, caudal, pulmonary, mesenteric arteries and aorta were dissected out. Isometric vasoconstrictive responses of the arteries to α,β-methylene ATP (α,β-MeATP) or noradrenaline (NA) were recorded using a polygraph.

Results:

The exhaustive swimming exercise did not produce significant change in the EC₅₀ values of α,β-MeATP or NA in vasoconstrictive response of most of the arteries studied. The exhaustive swimming exercise inhibited the vasoconstrictive responses to P2X1 receptor activation in the internal carotid artery, whereas it reduced the maximal vasoconstrictive responses to α1-adrenoceptor stimulation in the caudal, pulmonary, mesenteric arteries and aorta. The rank order of the reduction of the maximal vasoconstriction was as follows: mesenteric, pulmonary, caudal, aorta.

Conclusion:

Exhaustive swimming exercise differentially affects the P2X1 receptor- and α1-adrenoceptor-regulated vasoconstriction in internal carotid artery and peripheral arteries. The ability to preserve purinergic vasoconstriction in the peripheral arteries would be useful to help in maintenance of the basal vascular tone during exhaustive swimming exercise.     

The role of α1- and α2-adrenoceptors in the coronary vasoconstrictor responses to neuronally released and exogenous noradrenaline in the dog

Summary 1. Coronary vasoconstriction was examined in response to the neuronal release of noradrenaline produced by bilateral carotid occlusion and the infusion of tyramine (5 – 50 Erg/kg/min i. v.) in anaesthetized dogs which had been vagotomized and treated with the -adrenoceptor antagonist propranolol (1.0 mg/kg i. v.). These responses were compared to those produced by the infusion of noradrenaline (0.1 – 0.5 g/kg/min i. v.). 2. Similar increases in late diastolic coronary resistance were produced by bilateral carotid occlusion (0.70 ± 0.25 mmHg min/ml), and intravenous infusions of tyramine, 20 g/kg/min (0.70 ± 0.12 mm Hg min/ml) and noradrenaline, 0.5 gg/kg/min (0.59 ± 0.11 mm Hg min/ml). 3. Selective antagonism at ₁-adrenoceptors with prazosin (0.5 mg/kg i. v.) attenuated the coronary constrictor response to bilateral carotid occlusion (0.36 ± 0.09 mm Hg min/ml), tyramine (0.12 ± 0.06 mm Hg min/ml) and noradrenaline (0.18 ± 0.07 mm Hg min/ml). Antagonism at ₂-adrenoceptors with idazoxan (1 mg/kg i. v.) attenuated the coronary vasoconstriction produced by bilateral carotid occlusion (0.30 ± 0.06 mmHg min/ml), tyramine (0.17 ± 0.08 mmHg min/ml) and noradrenaline (0.12 ± 0.03 mm Hg min/ml). Combined antagonism at both ₁- and ₂-adrenoceptors with prazosin and idazoxan abolished the responses to bilateral carotid occlusion, tyramine and noradrenaline. 4. These results show that coronary vasoconstriction produced by either neuronally released or exogenous noradrenaline is mediated by both ₁ and ₁-adrenoceptors. It appears that in the coronary resistance vessels of the dog postjunctional ₁- and ₂-adrenoceptors are both innervated by sympathetic nerves.     

Effects of isoprenaline,adrenaline and selective α1- and α2- adrenoceptor stimulation on hypoxic pulmonary vasoconstriction in rat isolated perfused lungs

In the rat pulmonary vasculature perfused with blood in situ vasoconstriction induced by hypoxia was reversed by isoprenaline (doses > 1 ng) and adrenaline (doses > 30 ng) and exacerbated by phenylephrine but not UK 14304. Doses of adrenaline < 30 ng had no effect, except in the presence of propranolol (1 μM) or phentolamine (3 μM) when they caused vasoconstriction and vasodilation respectively, showing that, at dose levels < 30 ng, adrenaline's β- adrenoceptor vasodilator properties were balanced by its α- adrenoceptor vasoconstrictor properties. The pressor effects of adrenaline, in the presence of propranolol, were antagonised by prazosin (0.1 μM) but not by equi-molar concentrations of rauwolscine. These results suggest that the α- adrenoceptor agonist property of adrenaline is of benefit to its use as an inhaled bronchodilator because unopposed β- adrenoceptor stimulation can reverse hypoxic pulmonary vasoconstriction in poorly ventilated regions of the lung, promote further ventilation/ perfusion mismatching and lower PaO₂. They further suggest that adrenaline affects pulmonary vascular tone in the rat via α₁- adrenoceptors, stimulation of α₂- adrenoceptors having no effect.     

Effect of converting enzyme inhibition and angiotensin receptor blockade on the vasoconstriction mediated by α1 and α2 stimulation in pithed normotensive rats

Summary The effect of functional impairment of the renin-angiotensin system on the vasoconstriction mediated by postsynaptic ₁ and ₂-adrenoceptors in pithed normotensive rats was studied. Selective ₁-adrenoceptor stimulation was induced by intravenously administered cirazoline, whereas B-HT 920 was used as a selective agonists at ₂-adrenoceptors. The angiotensin converting enzyme was inhibited by intravenous treatment of the pithed rats with captopril, teprotide or enalapril. Blockade of angiotensin receptors was produced by intravenously applied [Sar¹ Ala⁸]angiotensin II (saralasin). Pretreatment with angiotensin converting enzyme inhibitors or with saralasin in doses which produced a maximal reduction in basal diastolic blood pressure, only slightly attenuated the hypertensive response to cirazoline. In contrast, these drugs provoked a most significant reduction of the ₂-adrenoceptor mediated vasoconstriction. Restoration of the basal diastolic blood pressure by intravenous infusion with angiotensin II or with vasopressin completely reversed the inhibitory effect of captopril on the vasopressor response to B-HT 920. One hour after bilateral nephrectomy, captopril still reduced the ₂-adrenoceptor mediated vasoconstriction. However, 18–24 h after bilateral nephrectomy, captopril had no additional inhibitory effect on the vasopressor response to selective ₂-adrenoceptor stimulation. It is concluded that in pithed normotensive rats the pressor response to ₂-adrenoceptor stimulation is significantly potentiated by endogenous angiotensin II, even at low circulating levels of the octapeptide. The modulatory action of angiotensin II on the -adrenoceptor mediated vasoconstriction probably represents an effect on the basal arteriolar muscular tone rather than a specific interaction.     

Actions of tizanidine on α1- and α2-adrenoceptors in the peripheral tissues

• 1.1. Tizanidine behaved as the partial agonist on the α₁-adrenoceptor in high doses (10⁻⁶−10⁻⁴ M) and as the α₂-adrenoceptor agonist in low doses (3 × 10⁻⁹−10⁻⁶ M).
• 2.2. Tizanidine is about one-third as potent as clonidine in α₂-agonistic effects.

     

Neural ATP release and its α2-adrenoceptor-mediated modulation in guinea-pig vas deferens

Summary Contractions, release of previously stored [³H]-noradrenaline (measured as overflow of total tritiated compounds) and release of ATP elicited by electrical field stimulation (210 pulses, 7 Hz) were studied in the superfused vas deferens of the guinea pig. Prazosin and suramin were used to suppress non-neural ATP release, and effects of bromoxidine and rauwolscine on the neural release thus isolated were examined.Electrical stimulation elicited reproducible contraction, tritium overflow and ATP overflow. Both prazosin (0.03–3 M) and suramin (30–300 M) reduced contractions as well as the evoked overflow of ATP. No visible contraction remained in 21 of 28 tissues exposed to prazosin 0.3 M combined with suramin 300 M. The evoked overflow of ATP under these conditions was about 17% of that observed in the absence of drugs. In the presence of prazosin 0.3 M and suramin 300 M, bromoxidine (0.01–1 M) decreased and rauwolscine (0.1–10 M) increased the evoked overflow of both tritium and ATP. Rauwolscine increased the evoked overflow of tritium to a significantly greater extent than the overflow of ATP.It is concluded that the overflow of ATP elicited by electrical (neural) stimulation in the presence of prazosin 0.3 M and suramin 300 M reflects purely neural release of ATP. This release of ATP, like the release of noradrenaline, is modulated through prejunctional ₂-adrenoceptors. The ₂-adrenoceptor modulation of the release of noradrenaline seems to be more marked than the modulation of the release of ATP. Correspondence to B. Driessen at the above address     

Effects of the irreversible α-adrenoceptor antagonists phenoxybenzamine and benextramine on the effectiveness of nifedipine in inhibiting α 1- and α 2-adrenoceptor mediated vasoconstriction in pithed rats

Summary In pithed normotensive rats, i.v. injection of the selective ₁-adrenoceptor agonist cirazolien produced vasoconstriction which was largely resistant to inhibition by nifedipine. On the other hand, the pressor effects of the selective ₁-adrenoceptor agonists St 587 and Sgd 101/75 were much more effectively blocked by nifedipine, although not as effectively as the pressor effects to the selective ₂-adrenoceptor agonist B-HT 920. The sensitivity to inhibition of vasoconstriction in pithed rats to the different agonists increased in the order cirazoline St 587 ₁-, but not to ₂-adrenoceptor activation was dose-dependently enhanced. The potency of nifedipine to inhibit ₁-vasoconstriction by cirazoline, St 587 and Sgd 101/75 was increased maximally to the level of efficacy at which nifedipine antagonized B-HT 920-induced vasoconstriction. The dose of phenoxybenzamine required to maximally increase the potency and efficacy of nifedipine to antagonize vasoconstriction of the ₁-adrenoceptor agonists was inversely related to the level of sensitivity to blockade by nifedipine of the vasoconstriction they produced. In contrast, pretreatment of rats with the irreversible antagonist, benextramine (10 mg/kg, i.v., –100 to –60 min) did not increase the potency or efficacy of nifedipine to antagonize vasoconstriction to cirazoline, St 587, Sgd 101/75 or B-HT 920, despite irreversible blockade of ₁- and ₂-adrenoceptors. These data suggest that phenoxybenzamine, but not benextramine, selectively inhibits the ₁-adrenoceptor mediated vasoconstrictor mechanism that is independent of influx of extracellular calcium. Moreover, the results show that the existence of receptor reserve or the number of ₁-adrenoceptors activated does not determine the relative contribution of calcium influx-independent mechanisms in ₁-adrenoceptor-mediated vasoconstriction.Preliminary data were communicated at the Joint Meeting of the French and German Pharmacological and Toxicological Societies, Freiburg i. Br., September 19–22, 1983 (Timmermans et al. 1983a) and at the Winter Meeting of the British Pharmacological Society, London, January 1984 (De Jonge et al. 1984)     

Regional differences of β1- and β2-adrenoceptor-mediated functions in feline heart

The effects of (-)-adrenaline and (-)-noradrenaline were studied on isolated preparations of kitten heart. To define the contribution of beta 1-adrenoceptors (beta 1AR) and beta 2-adrenoceptors (beta 2AR) we used as tools the highly beta 1AR-selective antagonist CGP 20,712 A and non-linear analysis of antagonism. The beta 2AR-mediated responses to the catecholamines, disclosed by CGP 20,712 A, were verified by blockade with the beta 2AR-selective ICI 118,551. The relative density and contribution of beta 1AR and beta 2AR to (-)-adrenaline- and (-)-noradrenaline-induced adenylyl cyclase stimulation was also estimated in right ventricular membranes. 1. In the sinoatrial pacemaker (-)-adrenaline caused positive chronotropic effects through both beta 1AR and beta 2AR while (-)-noradrenaline does so predominantly through beta 1AR. During beta 1AR blockade (-)-adrenaline did cause the same maximum effects through beta 2AR as (-)-noradrenaline did through beta 1AR. 2. In left atria (-)-adrenaline caused positive inotropic effects predominantly through beta 1AR. CGP 20,712 A also uncovered a beta 2AR component at high (-)-adrenaline concentrations comprising one third of the maximum beta 1AR-mediated response. 3. Receptor binding assays revealed that 80% of right ventricular beta AR were beta 1AR and 20% beta 2AR. Consistent with this finding, around 80% of the adenylyl cyclase stimulation by both (-)-noradrenaline and (-)-adrenaline was mediated through beta 1AR, around 20% through beta 2AR. The positive inotropic effects of (-)-noradrenaline appeared to be nearly exclusively mediated through beta 1AR in right ventricular papillary muscles. 4. The positive inotropic effects of (-)-adrenaline were quite variable with regard to beta 1AR and beta 2AR in right ventricular papillary muscles. Although beta 1AR-mediated effects are predominant in many muscles with only a small contribution of beta 2AR, in some muscles beta 2AR mediated around 50% of the maximum effect elicited through beta 1AR. In 3 out of 17 muscles beta 2AR mediated the same maximum effect of (-)-adrenaline as beta 1AR. 5. On occasion, we found marked beta AR heterogeneity amongst two muscles from the same right ventricle. One muscle only exhibited beta 1AR-mediated effects of (-)-adrenaline whereas in the other muscle maximal effects could be elicited through beta 2AR.(ABSTRACT TRUNCATED AT 400 WORDS)     

Tyrosine kinases participate in α_(1A)-adrenoceptor-mediated vasoconstriction in perfused rat hindlimb

目的：研究酪氨酸激酶是否参与α１Ａ肾上腺素受体引起血管平滑肌收缩的信号传导．方法：灌流大鼠后肢血管床标本，观察酪氨酸激酶抑制剂对去甲肾上腺素（ＮＥ）引起收缩反应的影响．结果：酪氨酸激酶抑制剂ｔｙｒｐｈｏｓｔｉｎ和ｇｅｎｉｓｔｅｉｎ均显著抑制ＮＥ引起的收缩反应，但对ＫＣｌ引起的收缩反应无影响；酪氨酸磷酸酶抑制剂Ｎａ３ＶＯ４显著加强ＮＥ引起的收缩反应；ｔｙｒｐｈｏｓｔｉｎ和ｇｅｎｉｓｔｅｉｎ对蛋白激酶Ｃ激动剂ｐｈｏｒｂｏｌ１２ｍｙｒｉｓｔａｔｅ１３ａｃｅｔａｔｅ引起的收缩反应均无影响，但均抑制Ｇ蛋白激动剂ＮａＦ引起的收缩反应．结论：Ｔｙｒｐｈｏｓｔｉｎ和ｇｅｎｉｓｔｅｉｎ敏感的酪氨酸激酶参与α１Ａ肾上腺素受体介导的大鼠后肢血管床收缩反应     

Presynaptic α2-autoreceptors in brain cortex: α2D in the rat and α2A in the rabbit

Summary Presynaptic ₂-autoreceptors in rat and rabbit brain cortex were compared by means of antagonists and agonists. Brain cortex slices were preincubated with [³H]-noradrenaline and then superfused and stimulated by 3 (rat) or 4 (rabbit) pulses at a frequency of 100 Hz.The ₂-adrenoceptor agonist bromoxidine (UK 14 304) reduced the electrically evoked overflow of tritium with EC₅₀ values of 4.5 nmol/l in the rat and 0.7 nmol/l in the rabbit. The antagonists phentolamine, 2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazole (BRL 44408), rauwolscine, 1,2-dimethyl-2,3,9,13b-tetrahydro-1H-dibenzo(c,f)imidazo(1,5-a)azepine (BRL 41992), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), 6-chloro-9-[(3-methyl-2-butenyl)oxy]-3-methyl-1H-2,3,4, 5-tetrahydro-3-benzazepine (SKF 104078), imiloxan, prazosin and corynanthine did not per se increase the evoked overflow of tritium but shifted the concentration-inhibition curve of bromoxidine to the right in a manner compatible with competitive antagonism. Up to 4 concentrations of each antagonist were used to determine its dissociation constant K_D. The K_D values correlated only weakly between the rat and the rabbit. Dissociation constants K_A of bromoxidine were calculated from equieffective concentrations in unpretreated brain slices and slices in which part of the ₂-adrenoceptors had been irreversibly blocked by phenoxybenzamine. The K_A value was 123 nmol/l in the rat and 7.2 nmol/l in the rabbit.The results confirm the species difference between rat and rabbit brain presynaptic ₂-autoreceptors. Comparison with data from the literature indicates that the rat brain autoreceptors can be equated with the _2D subtype as defined by radioligand binding, whereas the rabbit brain autoreceptors conform to the _2A subtype. For example, the antagonist affinities for the rat autoreceptors correlate with their binding affinities for the gene product of ₂-RG20, the putative rat _2D-adrenoceptor gene (r = 0.97; P<0.01), but not with their binding affinities for the gene product of ₂-C10, the putative human _2A-adrenoceptor gene. Conversely, the rabbit autoreceptors correlate with the ₂-C10 (r = 0.98; P<0.001) but not with the ₂-RG20 gene product. Since presynaptic ₂-autoreceptors are also _2D in rat submaxillary gland and perhaps vas deferens and _2A in rabbit pulmonary artery, the possibility arises that the majority of ₂-autoreceptors generally are _2D in the rat and _2A in the rabbit. Moreover, receptors of the _2A/D group generally may be the main mammalian ₂-autoreceptors.Correspondence to: N. Limberger at the above address     

α1A/B-Knockout mice explain the native α1D-adrenoceptor's role in vasoconstriction and show that its location is independent of the other α1-subtypes

Background and purpose:

Theoretically, three α₁-adrenoceptor subtypes can interact at the signalling level to alter vascular contraction or at the molecular level to alter each other''s cellular location. The α_1A/B-adrenoceptor knockout mouse (α_1A/B-KO) was used to study the isolated α_1D-adrenoceptor to consider these potential interactions in native tissue.

Experimental approach:

Pharmacological analysis of carotid and mesenteric arteries employed wire myography and fluorescent ligand binding (α₁-adrenoceptor ligand BODIPY FL-prazosin, QAPB).

Key results:

α_1A/B-KO carotid had clear α_1D-adrenoceptor-induced contractions. In WT carotid α_1D-adrenoceptor dominated but all three α₁-subtypes participated. α_1A/B-KO mesenteric had α_1D-adrenoceptor responses with high sensitivity and small maximum, explaining how α_1D-adrenoceptor could determine agonist sensitivity in WT. In both arteries α_1A/B-KO fluorescence levels were reduced but pharmacologically more consistent with ‘pure’α_1D-adrenoceptors. α_1D-Adrenoceptor binding in α_1A/B-KO was observed on the cell surface and intracellularly and was present in a high proportion of smooth-muscle cells in both strains, regardless of artery type.

Conclusions and implications:

‘Pure’α_1D-adrenoceptor pharmacology in α_1A/B-KO provides a quantitative standard. Functionally, the α_1D- and α_1A-adrenoceptors produce additive responses and do not significantly compensate for each other. α_1D-Adrenoceptor contributes to sensitivity even in resistance arteries. In α_1A/B-KO, the loss of α_1A- and α_1B-adrenoceptors is reflected by a general decrease in fluorescence, but similar binding distribution to WT indicates that the α_1D-adrenoceptor location in native smooth-muscle cells is not influenced by other α₁-adrenoceptors. Equivalent levels of receptors did not correspond to equivalent responses. In conclusion, α₁-subtypes do not interact but provide independent alternative signals for vascular regulation.     

Binding sites of α1- and α2-adrenoreceptors

Conclusions The models constructed for the binding sites of rat brain ₁-AR and ₂-AR satisfy all the steric requirements and energy characteristics of interaction with known ligands, cited in [5–7, 14]. The differences detected in the arrangement and orientation of the functional groups of the binding sites permit an explanation of a whole series of typical differences in the interaction of adrenoactive substances with both subtypes of-AR.Our analysis showed that the greatest contribution to the interaction with the receptor is made by ionic, donor-acceptor, and hydrophobic bonds. The role of van der Waals forces in the interactions examined is evidently extremely negligible. The most effective and specific preparations prove to be compounds that not only form the maximum number of donor-acceptor bonds with the receptor but also orient their own hydrophobic fragments in such a way that the ionic and donor-acceptor bonds formed between the molecule and the receptor are shielded from contact with the aqueous phase. The energy effects of hydrophobic interactions of this type may be rather substantial (3.9–3.12).The production of new synthetic preparations, for which the conditions of complementarity to the-AR will be most fully satisfied, can be carried out taking the requirements of structural correspondence of the topography of the binding sites into account.Translated from Khimiko-farmatsevticheskii Zhurnal, Vol. 18, No. 8, pp. 904–912, August, 1984.     

The effect of selective α1- and α2-adrenoceptor stimulation on intraocular pressure in the conscious rabbit

Summary The influence of topically applied selective ₁- and ₂-adrenoceptor agonists on intraocular pressure and the diameter of the pupil was investigated in conscious rabbits. Selective stimulation of the ₁-subtype of receptors induced an elevation in intraocular pressure, accompanied by mydriasis, whereas stimulation of the ₂-subtype caused a marked and dose-dependent ocular hypotensive response, which was blocked by the selective ₂-adrenoceptor antagonist yohimbine. ₂-Agonists induced neither macroscopic ocular side effects, nor an effect on the pupil size. Possibly, the subclass of ₂-adrenoceptor stimulating drugs represent a group of new antiglaucomatous agents.     

Involvement of adenylate cyclase and protein kinase C in the α2-adrenoceptor-mediated inhibition of noradrenaline and 5-hydroxytryptamine release in rat hypothalamic slices

Summary In superfused rat hypothalamic slices prelabelled with [³H]-noradrenaline, the ₂-adrenoceptor agonist UK 14304 inhibited in a concentration-dependent manner the electrically-evoked release of tritium. This inhibition was antagonized by the ₂-adrenoceptor blocking agent idazoxan, which by itself increased the electrically-evoked tritium overflow. Exposure to forskolin, an adenylate cyclase activator, increased the electrically-evoked release of [³H]-noradrenaline. In the presence of forskolin (1 mol/l), both the inhibitory effect of UK 14304 and the increasing effect of idazoxan on the electrically-evoked release of [³H]-noradrenaline were less pronounced than in the absence of the adenylate cyclase activator. Exposure to forskolin and to the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine shifted to the right the concentration-effect curve for UK 14304 in a similar manner as that observed in the presence of forskolin alone. Exposure to phorbol-12,13-dibutyrate (0.01–10 mol/l), a drug which activates protein kinase C, increased the electrically-evoked release of [³H]-noradrenaline. In the presence of phorbol-12,13-dibutyrate (0.1 and 1 mol/l), the concentration effect curve for UK 14304 on tritium overflow was significantly shifted to the right. The increasing effect of idazoxan on tritium overflow was significantly less pronounced in the presence of 1 mol/l phorbol-12,13-dibutyrate.In superfused rat hypothalamic slices prelabelled with [³H]-5-hydroxytryptamine, the ₂-adrenoceptor agonist UK 14304 significantly inhibited the electrically-evoked release of tritium. Exposure to forskolin increased in a concentration-dependent manner [³H]-5-hydroxytryptamine overflow, but did not modify the UK 14304-mediated inhibition. Exposure to 3-isobutyl-1-methylxanthine enhanced the electrically-evoked release of [³H]-5-hydroxytryptamine. In the presence of both forskolin (1 mol/l) and 3-isobutyl-l-methylxanthine (1 mmol/l), the concentration-response curve for UK 14304 was significantly shifted to the right. Exposure to phorbol-12,13-dibutyrate (0.01–10 mol/l) enhanced in a concentration-dependent manner the electrically-evoked overflow of [³H]-5-hydroxytryptamine. In the presence of phorbol-12,13-dibutyrate (0.1 and 1 mol/l), UK 14304 was significantly less potent to inhibit tritium release than in the absence of the protein kinase C activator.It is concluded that both cyclic AMP and phosphoinositide turnover are involved in the modulation of noradrenaline and 5-hydroxytryptamine release by presynaptic ₂-adrenoceptors in rat hypothalamic slices. However, these interactions do not represent definitive proof for a cause-effect relationship for the second messengers mediating the ₂-adrenoceptor induced inhibition of transmitter release either as autoreceptor or as heteroreceptor.Send offprint requests to S. Z. Langer at the above address     

Influence of calcium entry blockade on α 1 - and α 2-adrenoceptor mediated vasoconstriction in the forearm of hypertensive patients

Summary The influence of treatment with the calcium entry blockers PY 108-068 (PY) and PN 200-110 (PN) on ₁- and ₂-adrenoceptor mediated vasoconstriction has been investigated in the forearms of hypertensive patients. Changes in forearm vascular resistance (FVR) in response to the intra-arterial infusion of drugs were determined at the end of a placebo period and after 2–4 weeks of treatment with PY or PN. The drugs used were the selective agonists methoxamine (₁) and B-HT 933 (₂). During placebo, basal FVR was dose-dependently increased by methoxamine and B-HT 933. Basal blood pressure was lowered during PN but not during PY. Treatment with the calcium entry blockers did not influence the effect of methoxamine, but the vasoconstriction induced by B-HT933 was attenuated by both of the calcium entry blockers. These results confirm the findings in animal studies that calcium entry blockers preferentially inhibit the ₂-adrenoceptor mediated vasoconstriction induced by selective agonists.     

β-Adrenoceptors potentiate α1-adrenoceptor-mediated inotropic response in rat left atria

AIM: To investigate whether stimulation of β-adrenoceptor (AR) and its subtypes augment α1-AR-evoked positive inotropic response and inositol phosphate (InsP) accumulation in isolated rat left atria. METHODS: Inotropic response was determined by contractile function experiment in isolated electrically driven rat left atria. ^3H-InsP accumulations were measured by ^3H-inositol incorporation and column chromatography. RESULTS: (1) Stimula-tion of α1-AR by phenylephrine (PE) or norepinephrine (NE) in the presence of propranolol (Prop) evoked positive inotropic response and ^3H-InsP accumulations, while stimulation of β-AR by isoprenaline (ISO) or NE in the presence of phentolamine (Phen) only evoked positive inotropic response, but not ^3H-InsP accumulations. (2) Simultaneous stimulation of α1- and β-AR by NE or ISO plus PE significantly shifted the concentration-dependent inotropic response curves and ^3H-InsP accumulation curves to the left and upward compared with individual α1-AR stimulation by PE or NE in the presence of Prop. (3) In the presence of ICI118551 (selective β2-AR antagonist) or CGP12177 (selective β1-AR antagonist), stimulation of either β1- or β2-AR did not change α1-AR-evoked inotropic response and ^3H-InsP accumulations. CONCLUSION: Stimulation of β1-AR and β2-AR potentiates α1-AR-mediated positive inotropic response and InsP accumulation in isolated rat left atria.     

Yohimbine antagonises α1A- and α1D-adrenoceptor mediated components in addition to the α2A-adrenoceptor component to pressor responses in the pithed rat

We have recently shown that responses to pressor nerve stimulation in the pithed rat are mediated by α(1A)- and α(1D)-adrenoceptors, with no evidence for α(2)-adrenoceptor involvement, and that responses previously identified as α(2)-adrenoceptor mediated are actually α(1D)-adrenoceptor mediated. We have now re-examined the subtypes of α-adrenoceptor involved in pressor responses produced by exogenous agonists in the pithed rat preparation to confirm whether α(2)-adrenoceptors are involved in these responses. The α(2)-adrenoceptor and α(1D)-adrenoceptor antagonist yohimbine (1mg/kg) and the α(2A)-adrenoceptor antagonist methoxy-idazoxan (5 mg/kg) significantly shifted, but the α(1D)-adrenoceptor antagonist BMY 7378 (8-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspir o[4.5]decane-7,9-dione dihydrochloride) (1 mg/kg) did not affect, the pressor potency of the α(2)-adrenoceptor agonist xylazine. α(1)-adrenoceptor antagonists showed low potency against pressor responses to xylazine. The pressor potency of the α(1)-adrenoceptor agonist amidephrine was not affected by BMY 3778 (1 mg/kg) but significantly shifted by prazosin (0.01 mg/kg) and by yohimbine (1 mg/kg). In contrast, the pressor potency of phenylephrine was significantly shifted by both yohimbine and BMY 7378 (1 mg/kg), but to a greater extent by the α(1A)-adrenoceptor antagonist RS 100329 (5-Methyl-3-[3-[3-[4-[2-(2,2,2,trifluroethoxy) phenyl]-1-piperazinyl]propyl]-2,4-(1H,3H)-pyrimidinedione] hydrochloride) (0.1 mg/kg). In conclusion, we have identified and separated α(1A)-, α(1D)- and α(2A)-adrenoceptor antagonist actions of yohimbine against pressor responses. Pressor responses to exogenous agonists in the pithed rat involve both α(1A)- and α(1D)-adrenoceptors and in addition, α(2A)-adrenoceptors.     

Involvement of α1- and α2-adrenoceptors in the responses of proximal and distal segments of the canine saphenous vein to exogenous and endogenous noradrenaline

Summary In the present study the relationship between adrenergic nerve terminals and postjunctional -adrenoceptors mediating the responses to the endogenous transmitter was compared at proximal and distal levels of the canine saphenous vein.Concentration-response curves to noradrenaline and to tyramine as well as frequency-response curves to electrical stimulation were compared at both levels of the vessel, in the absence and presence of either prazosin (100nmol·l^–1) or yohimbine (100nmol·l^–1) The influence of inhibition of neuronal uptake by cocaine (12 µmol·l^–1) on the responses to noradrenaline in the presence of prazosin (56 nmol·l^–1) or yohimbine (20 nmol·l^–1) was compared at the proximal level. The results show that, at the proximal level, the maximal responses to electrical stimulation and tyramine reached 80.1±2.2 (n = 18) and 74.2±1.9 (n = 18)%, respectively, of the maximal responses to noradrenaline, and 70.3±0.8 (n = 15) and 53.1 ± 1.2 (n = 14) %, respectively, at the distal level. Furthermore, the proximal strips were more sensitive to electrical stimulation than the distal ones. Prazosin had a much greater inhibitory effect on the contractile responses to noradrenaline than on those to electrical stimulation, at both levels. At proximal level, the shifts (to the right) of the concentration (frequency)-response curves (at EC₅₀) amounted to 0.58±0.02 (n = 16) and 0.18±0.02 (n = 8) log units, respectively (P<0.05), but, at the distal level, to 1.12±0.03 (n = 16) and 0.28±0.08 (n = 8) log units, respectively (P< 0.05). At the proximal level, yohimbine antagonizes about equally the responses to noradrenaline and the responses to electrical stimulation. However, at the distal level, the shift of the concentration-response curve to noradrenaline was much larger than that of the frequency-response curve to electrical stimulation [1.12±0.07 and 0.80±0.10 (n = 6) log units at EC₅₀, respectively (P<0.05)]. The leftward shift of the concentration-response curve to noradrenaline caused by cocaine was more pronounced in the presence of prazosin than in the presence of yohimbine: 0.95±0.15 and 0.69±0.12 (n = 12) log units, respectively (P<0.05).We conclude that, in the canine saphenous vein: 1) noradrenaline released from the adrenergic nerve terminals by electrical stimulation and by tyramine preferentially activates ₂-adrenoceptors at both proximal and distal levels; 2) the effectiveness of ₂-adrenoceptor stimulation is greater at the proximal than at the distal level; 3) ₁-adrenoceptors at the distal level seem to be different from those at the proximal one. Send offprint requests to S. Guimarães at the above address     

β-Adrenoceptors potentiate α_1-adrenoceptor-mediated inotropic response in rat left atria

AIM: To investigate whether stimulation of β-adrenoceptor (AR) and its subtypes augment α1-AR-evoked positive inotropic response and inositol phosphate (InsP) accumulation in isolated rat left atria. METHODS: Inotropic response was determined by contractile function experiment in isolated electrically driven rat left atria. 3H-InsP accumulations were measured by 3H-inositol incorporation and column chromatography. RESULTS: (1)     

Investigation of α1-adrenoceptor subtypes mediating vasoconstriction in rabbit cutaneous resistance arteries

1. Cutaneous resistance arteries (c.r.a.) (internal diameter=240.94±5.42 μm, n=67/25 (number arteries/number animals)) from New Zealand white rabbits were mounted in wire myographs and a normalization procedure followed.
2. Cumulative concentration-response curves (CCRCs) were constructed for the α-adrenoceptor agonists noradrenaline (NA), (R)A61603 and phenylephrine (PE) in the presence of cocaine (3 μM), propranolol (1 μM) and corticosterone (10 μM). The effects of competitive α₁-adrenoceptor antagonists, prazosin, WB4101, 5-methyl-urapidil, HV723, BMY7378 and the irreversible α_1B selective compound chloroethylclonidine (CEC) were examined versus the potency and maximum response of the c.r.a.s to noradrenaline.
3. The high potency of A-61603 relative to PE has been shown to differentiate both functional and binding site α_1A- or α_1B-adrenoceptors from α_1D-adrenoceptors: A-61603 was 944 times more potent than phenylephrine (at EC₅₀) suggesting the presence of a functional α_1A or α_1B as opposed to an α_1D-subtype.
4. Exposure to chloroethylclonidine (CEC; 100 μM) decreased the maximum response to noradrenaline but did not significantly change noradrenaline sensitivity indicating that a substantial part of noradrenaline-induced vasoconstriction in rabbit cutaneous arteries is CEC-insensitive.
5. The potencies of prazosin (pA₂=9.14) and WB4101 (pA₂=9.30) indicate the involvement of prazosin-sensitive functional α₁-adrenoceptors. The slopes of corresponding Schild plots for prazosin and WB4101 did not include negative unity which implies the possible involvement of more than one functional α₁-adrenoceptor subtype in noradrenaline-induced vasoconstriction in rabbit cutaneous resistance arteries. In contrast to this, in the case of 5-methyl-urapidil and HV723, the Schild plot slope parameters were not significantly different from negative unity over the range of concentrations used; the low pA₂ value for 5-methylurapidil (7.27) suggests the non-involvement of an α_1A- or an α_1D-adrenoceptor; the low pA₂ value for HV723 (8.47) was similar to that against responses postulated as α_1L.
6. We conclude that rabbit cutaneous resistance arteries express a prazosin-sensitive functional α₁-adrenoceptor resembling the α_1B and another low affinity site for prazosin which on the basis of the functional antagonism produced by HV723 most closely resembles the α_1L-adrenoceptor; the low pA2 value for HV723 (8.47) is similar to that against responses postulated as α_1L.