Autoradiographic study of the alpha-adrenoceptors of rat aorta and tail artery

Effects of Platelet Activating Factor on Contractions and Ca Influx Induced by Noradrenaline and Potassium in Rat Rubbed and Intact Aorta. Comparison with Its Hypotensive Effect in Anaesthetized Normotensive Rats

In order to clarify the mechanism of hypotensive activity of platelet activating factor (PAF), the effects of this drug on blood pressure in anaesthetized normotensive rats, on KCl- and noradrenaline-induced 45Ca uptake and contractile responses in rat aorta rings with and without endothelium were studied. PAF (3 μg kg−1, i.v.) showed long-lasting hypotensive effects in anaesthetized normotensive rats accompanied by a significant increase in heart rate. PAF (0·1–10 μm) did not relax the contractions induced by noradrenaline (10 μm) or K+ (60 Mm) in rubbed or intact rat aorta. PAF did not affect the basal uptake of 45Ca2+ nor that induced by the two vasoconstrictor agents. In experiments in a calcium free medium, PAF (10 μm) had no effect on the noradrenaline- (10 μm) induced contractions. These results suggest that the hypotensive activity of PAF in normotensive anaesthetized rats is not due to a direct effect on rubbed and intact rat aorta rings (acting within the cell or blocking Ca2+ influx through l-type transmembrane calcium channels).

Lack of involvement of pertussis toxin-sensitive G-proteins in norepinephrine-induced vasoconstriction of rat aorta smooth muscle

Several studies have shown that stimulation of pertussis toxin (PTX)-sensitive G-proteins amplified alpha-adrenoceptor (alpha-AR) agonist-induced vasoconstriction in small muscular and resistance arteries. The aim of this study was to assess the potential involvement of PTX-sensitive G-proteins in norepinephrine (NE)-induced constriction in a large diameter artery, the rat aorta. PTX (1 microg/mL, 2 hr; 3 microg/mL, 4 hr) did not modify concentration-response curves to NE in endothelium-denuded aortic rings. However, several lines of evidence suggested that aortic smooth muscle cells (SMC) had a PTX-sensitive G-protein pathway. [alpha-(32)P]ADP-ribosylation of G(i/o)-proteins by PTX (3 microg/mL, 4 hr) was demonstrated in situ in the intact aorta without endothelium. alpha(i/o) subunits were identified in vitro by both immunoblotting and ADP-ribosylation experiments in rat aorta SMC membranes. The measurement of G(i/o)-specific GTPase activity evidenced an effective coupling between NE receptors and G(i/o)-proteins, as NE induced an increase in basal G(i/o)-specific GTPase activity (20.7 +/- 2.8 vs 7.2 +/- 2.2 pmol P(i)/mg protein at 5 min; P < 0.05 vs basal). Co-immunoprecipitation revealed the in vitro coupling between alpha(1D)-ARs and G(i)-protein in rat aorta SMC membranes. In conclusion, we identified a PTX-sensitive G(i/o)-protein pathway in rat endothelium-denuded aorta. We showed an effective coupling between NE receptors and G(i)-proteins via alpha(1D)-ARs. Since PTX has no effect on NE-induced vasoconstriction, the PTX-sensitive G(i)-protein pathway does not play a predominant role in NE-induced responses in rat aorta SMC in contrast to small diameter muscular and resistance arteries.

Acute vasorelaxant effects of metformin and attenuation by stimulation of sympathetic agonist release

We recently discovered 1) that intravenous injection of the antidiabetic drug metformin in the rat rapidly reduces arterial pressure elevations maintained by the alpha-adrenoceptor agonist phenylephrine (PE) and 2) that direct administration of metformin to isolated rat tail arterial tissue rings rapidly relaxes PE-induced contractions. To further characterize this potential direct vasodilator action, we examined effects of metformin on contractions induced not only by PE but also by norepinephrine (NE) and by nonadrenergic agonists (5-hydroxytryptamine, 5HT; arginine vasopressin, AVP). Also, because the rat tail artery contains abundant adrenergic nerve endings we conducted these tests not only in arterial rings with nerve endings intact but in rings in which they had been removed by pretreatment with 6-hydroxydopamine. In intact rings, metformin at levels from approximately 0.2 to 20 mmol/L rapidly relaxed half-maximal contractions induced by PE and NE similarly and to a markedly greater degree than contractions induced by 5-HT (p<0.05). Metformin did not relax AVP-induced contractions. In addition, removal of adrenergic nerve endings facilitated metformin's relaxant effects (p<0.05). Thus, the acute vasodilator action of metformin appears 1) to be selectively more powerful on arterial smooth muscle contractions induced adrenergically versus nonadrenergically and 2) to be buffered by a possible metformin-induced release of endogenous NE from adrenergic nerve endings. Such results were not seen during relaxation produced by either the calcium channel inhibitor nifedipine or the nitrovasodilator nitroprusside suggesting that metformin's effects are mediated by other mechanisms.

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.

Disparate effects of antidiabetic drugs on arterial contraction

Type II diabetic patients and others with insulin resistance are at risk for development of hypertension characterized by elevated peripheral vascular resistance and loss of insulin's normal vasodilating activity. Oral antidiabetic drugs have recently been recognized to have disparate effects on arterial pressure in such patients and in related rodent models. Sulfonylureas (e.g., glyburide), which stimulate insulin secretion, have been reported either to increase or not to affect arterial pressure, whereas nonsulfonylurea agents with insulin-sensitizing properties, the biguanide metformin and various thiazolidinediones (eg, pioglitazone), have been reported to decrease arterial pressure in humans and rodents. To help elucidate these disparate effects, we investigated these agents for direct actions on arterial vascular contractility and its sensitivity to insulin. Preincubation of intact rat tail arterial tissue rings for 2 hours with known therapeutically effective antidiabetic concentrations of metformin and pioglitazone significantly attenuated the force of contractions produced by either potassium (membrane depolarization) or norepinephrine ([NE] adrenergic receptor activation). Glyburide did not influence these contractions. Preincubation with metformin also induced an attenuating (vasodilating-like) action of insulin on arterial tissue rings contracted by potassium. Conversely, glyburide induced an accentuating action of insulin on potassium-mediated contractions. These results are consistent with measures of vascular function obtained in the past after oral administration of the drugs, which suggested but did not prove that they may exert direct effects on arterial vascular contractility. Thus, metformin and thiazolidinediones may decrease arterial pressure partly by direct vasorelaxant mechanisms, with metformin having an additional effect of inducing vasorelaxation by insulin. In contrast, sulfonylureas may directly induce a paradoxical vasoconstrictor response to insulin.

Intracellular Ca2+ and contractile responses to alpha 1-adrenoceptor subtype activation in rat aortic vascular smooth muscle

To simultaneously and rapidly measure intracellular Ca2+ concentration ([Ca2+]i) and contraction in vascular smooth muscle, the Ca2+ fluorophore, fura-2/acetoxymethyl ester, was incorporated into an intact sample of rat aorta. Noradrenaline produced a biphasic [Ca2+]i response (phase-1 and phase-2) which was different to the monophasic contractile response. Phase-1 of the [Ca2+]i response was a large, fast, transient increase which usually clearly preceded contraction. Phase-2 of the [Ca2+]i response was slower, peaked between 20-40 s after addition of noradrenaline, and often subsequently declined whilst contraction continued to increase. Contraction followed phase-2 of the [Ca2+]i response to noradrenaline more closely than phase-1. WB 4101 (alpha 1A-adrenoceptor antagonist) produced a major reduction in phase-1 of the [Ca2+]i response to noradrenaline, a lesser reduction of phase-2 of the [Ca2+]i response to noradrenaline and least reduction of contraction. Chlorethylclonidine (alpha 1B-adrenoceptor antagonist) reduced phase-1 and phase-2 of the [Ca2+]i response and contraction to noradrenaline to a similar degree. We conclude that noradrenaline produces a biphasic [Ca2+]i increase and that neither alpha 1-adrenoceptor subtype is specifically linked to phase-1 or phase-2 of the [Ca2+]i response to noradrenaline in the rat aorta. However, selective alpha 1B-adrenoceptor activation shows a higher force/[Ca2+]i relationship in comparison to alpha 1A-adrenoceptor activation.

Postjunctional alpha 2-adrenoceptors in blood vessels: effect of age

To investigate the impact of age on function of vascular postjunctional alpha 2-adrenoceptors in tail arteries of F-344 rats aged 6 and 20 months, vasoconstrictor responses of isolated ring segments were studied. In both 6 and 20 months old animals, norepinephrine concentration response curves were significantly shifted to the right in the presence of the alpha 2-adrenoceptor antagonists idazoxan (300 nM) or rauwolscine (50 nM). Furthermore, in both age groups alpha 2-adrenoceptor agonists UK14304 (300 nM) or BHT920 (25 nM) produced significant enhancements in the vasoconstrictor response to the selective alpha 1-adrenoceptor agonist, methoxamine. Concentration response curves for norepinephrine and methoxamine were not different with age. These data verify the existence of postjunctional alpha 2-adrenoceptors and demonstrate that the function of these receptors does not change with age. This supports the proposition that overall vascular adrenergic function in blood vessels is maintained with advancing age.

Frequency- and train length-dependent variation in the roles of postjunctional alpha 1- and alpha 2-adrenoceptors for the field stimulation-induced neurogenic contraction of rat tail artery

The present paper examines the roles of postjunctional alpha 1- and alpha 2-adrenoceptors for the noradrenaline (NA)-induced neurogenic contractile response to field stimulation mainly with 1-100 pulses at 2 or 20 Hz, in the tail artery of adult normotensive rats. Pharmacological tools were employed to isolate and characterize the alpha 1- and alpha 2-adrenoceptor-mediated components of this response. The degree to which the drugs influenced NA release or reuptake was assessed by their effects on the electrochemically determined, stimulation-induced rise in the NA concentration at the innervated outer surface of the media. This response was unaffected by alpha,beta-methylene ATP (10 microM) or suramin (500 microM), added to desensitize or block P2-purinoceptors, respectively prazosin (0.1 microM) or SK&F 104078 (6-chloro-9-[(3-methyl-2-butenyl)oxyl]-3-methyl- 1H-2,3,4,5-tetrohydro-3-benzazepine, 0.1 microM), used to block postjunctional alpha 1- and alpha 2-adrenoceptors respectively, nifedipine (10 microM), blocker of Ca2+ influx through L-type channels, and ryanodine (10 microM), which blocks mobilization of Ca2+ from intracellular stores; it was moderately enhanced by yohimbine (0.1 microM), blocker of pre- and postjunctional alpha 2-adrenoceptors, and strongly enhanced by cocaine (3 microM) or desipramine (1 microM), blockers of NA reuptake. Judging from their inhibitory effects on the contractile responses to the alpha 1- and alpha 2-adrenoceptor agonists, phenylephrine and xylazine, prazosin (0.1 microM) and SK&F 104078 (0.1 microM) could be used to selectively block alpha 1- and alpha 2-adrenoceptors respectively, while yohimbine (0.1 microM) was less selective, strongly depressing alpha 2- and slightly depressing alpha 1-adrenoceptor-mediated responses. The alpha 1-adrenoceptor-mediated component of the contractile response to short trains at 20 Hz was fast in onset, brief in duration and abolished by ryanodine; that mediated by alpha 2-adrenoceptors was more delayed, prolonged and insensitive to ryanodine. Both components were dose-dependently depressed by nifedipine (0.1-10 microM). The small contractile responses to single pulses, or up to 50 pulses at 2 Hz, or short train (< 4 pulses) at 20 Hz, were more markedly depressed by 0.1 microM yohimbine or SK&F 104078 than by 0.1 microM prazosin and, hence, mediated mainly by alpha 2-adrenoceptors. The reverse was true of the much larger response to longer trains at 20 Hz, which thus probably was mediated mainly by alpha 1-adrenoceptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Presence of beta- but not alpha-adrenoceptors in the rat jugular vein: autoradiographical evidence

Previous functional studies with the rat jugular vein have shown alpha-adrenergically mediated contraction was minimal to nonexistent, yet this tissue relaxed in response to norepinephrine via beta 1 and to isoproterenol via beta 2-receptor activation. Unlike the rat jugular vein, the rat aorta markedly contracted to norepinephrine and relaxed to beta-adrenoceptor stimulation. This study qualitatively examined the alpha- and beta-receptor distributions in the rat jugular vein, as compared to the rat aorta, with autoradiographical methods. Cross-sections of blood vessels were labelled with the alpha-receptor ligand, [125I]HEAT ([beta-(4-hydroxy-3-iodophenyl)-ethyl-aminomethyl]-tetralone) and the beta-receptor ligand, [125I]ICYP (iodo-cyanopindolol), and subsequently were opposed to emulsion coated coverslips. Specific binding of both alpha- and beta-receptors was demonstrated in the rat aorta. While specific beta-receptor binding was demonstrated in the rat jugular vein, this tissue showed no detectable alpha-receptors. These autoradiographic studies indicate that the lack of alpha-adrenergically mediated contraction in the rat jugular vein is likely due to a virtual absence of alpha-receptor binding sites, rather than poor functional alpha-receptor coupling.

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.

Influence of the vascular endothelium on agonist-induced contractions and relaxations in rat aorta

The influence of the vascular endothelium on agonist-induced contractions and relaxations has been measured using intact segments of rat aorta. Contiguous rubbed segments were used as controls. Angiotensin II, histamine, noradrenaline, U46619 and UK14304 contracted both rubbed and intact tissues. The threshold spasmogenic concentrations of these agonists were lower in rubbed tissues than in intact preparations. The sensitivity and responsiveness of tissues to angiotensin II, histamine, noradrenaline and UK14304 were greater in rubbed than in intact tissues. Acetylcholine and histamine relaxed the established spasms of intact tissues but not those of rubbed preparations, These relaxant effects of acetylcholine were abolished by pre-incubation with haemoglobin. In the presence of prazosin, noradrenaline or UK14304 relaxed established contractions in intact tissues. These effects were antagonized by idazoxan or by pre-incubation with haemoglobin. In intact preparations, idazoxan had no effect on the spasmogenic sensitivity and responsiveness to UK14304. Pre-incubation with haemoglobin augmented the spasmogenic actions of noradrenaline, U46619 or UK14304 in intact tissues, but had no effect on these responses in rubbed preparations. Tissue concentrations of cyclic GMP were greater in intact than in rubbed tissues. A concentration of acetylcholine (10 microM) evoking just maximal mechanical inhibition produced a significant increase in cyclic GMP concentration in intact preparations. However, no detectable changes in cyclic GMP concentration were produced by UK14304 (10 microM) or by acetylcholine (30 nM), concentrations which were equi-effective in inhibiting mechanical activity. In the presence of threshold spasmogenic concentrations of noradrenaline, the contractile effects of angiotensin II were augmented and became comparable to those observed in rubbed preparations. In the presence of greater concentrations of noradrenaline, angiotensin II always produced an additional contraction. It is concluded that the presence of the vascular endothelium limits the spasmogenic action of a variety of agonists. Although spasmogens like noradrenaline and UK14304 can stimulate the release of endothelium-derived relaxing factor (EDRF) via alpha 2-adrenoceptors, the inhibitory effects of EDRF largely result from the spontaneous release of this substance.

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.

Autoradiographic localization of alpha-adrenoceptors, muscarinic acetylcholine receptors and opiate receptors in the heart

Using in vitro autoradiography the distribution of [3H]rauwolscine, [3H]prazosin and [3H]quinuclidinyl benzilate binding sites has been demonstrated in cardiac tissue taken from the cat and rat. A similar distribution of both alpha 1- and alpha 2-adrenoceptor sites was seen but the distribution of muscarinic acetylcholine sites was markedly different. alpha-Adrenoceptors were present predominantly in ventricular muscle whereas muscarinic acetylcholine receptors exhibited a greater density in atrial tissue compared to ventricular muscle. Opiate receptors were absent from cardiac tissue.