Isolated Rabbit Aorta Research Articles

Localization of a constitutively active, phagocyte-like NADPH oxidase in rabbit aortic adventitia: enhancement by angiotensin II.

Superoxide anion (O2-) plays a key role in the endogenous suppression of endothelium-derived nitric oxide (NO) bioactivity and has been implicated in the development of hypertension. In previous studies, we found that O2- is produced predominantly in the adventitia of isolated rabbit aorta and acts as a barrier to NO. In the present studies, we characterize the enzyme responsible for O2- production in the adventitia and show that this enzyme is a constitutively active NADPH oxidase with similar composition as the phagocyte NADPH oxidase. Constitutive O2--generating activity was localized to aortic adventitial fibroblasts and was enhanced by the potent vasoconstrictor angiotensin II. Immunohistochemistry of aortic sections demonstrated the presence of p22(phox), gp91(phox), p47(phox), and p67(phox) localized exclusively in rabbit aortic adventitia, coincident with the site of staining for O2- production. Furthermore, immunodepletion of p67(phox) from adventitial fibroblast particulates resulted in the loss of NADPH oxidase activity, which could be restored by the addition of recombinant p67(phox). Further study into the regulation of this adventitial source of O2- is important in elucidating the mechanisms regulating the bioactivity of NO and may contribute to our understanding of the pathogenesis of hypertension.

Pharmacologic profiles of KRH-594, a novel nonpeptide angiotensin II-receptor antagonist.

We studied pharmacologic profiles of KRH-594, dipotassium (Z)-2-[[5-ethyl-3-[2'-(1H-tetrazol-5-yl)biphenyl-4-yl]methyl-1,3,4 -thiadiazolin-2-ylidene]aminocarbonyl]-1-cyclopentenecarb oxylate, a novel angiotensin II (AII)-receptor antagonist. KRH-594 potently displaced specific binding of [125I]-AII from AT1 receptor with a Ki of 0.39 nM in rat liver membranes, but not from AT2 receptor in bovine cerebellar membranes (Ki > 10 microM). KRH-594 exhibited no affinity for 21 other receptors and two enzymes [50% inhibitory concentration (IC50) > 10 microM], demonstrating its high specificity toward AT1 receptors. In isolated rabbit aorta, KRH-594 caused nonparallel shifts to the right of the dose-response curve to AII and decreased the maximal response with a pK(B) of 10.4. We evaluated the in vivo efficacy and the duration of action in freely moving rats under nonfasting conditions. In normotensive rats, orally administered KRH-594 inhibited AII-induced pressor responses with a 50% inhibitory dose (ID50) of 0.39 mg/kg. In spontaneously hypertensive rats (SHRs), both KRH-594 (1 mg/kg p.o.) and losartan (10 mg/kg p.o.) exerted similar blood pressure-reducing effects, and their effects were still significant at 24 h after drug administration. We concluded that KRH-594 is a specific and efficacious AT1 antagonist that may find its use in the treatment of human hypertension.

Pharmacological profile of YM358, a novel nonpeptide angiotensin AT1 receptor antagonist

The pharmacological profile of YM358, 2,7-diethyl-5-[[2-(1H-tetrazol-5-yl)biphenyl-4-yl]methyl]-5H-pyrazolo-[1,5- b][1,2,4]triazole potassium salt monohydrate, a novel non-peptide angiotensin AT 1 receptor antagonist, was studied in vitro and in vivo. YM358 competed with [ 125 I ][Sar 1, Ile 8]angiotensin II for angiotensin AT 1 receptors in rat liver membranes. YM358 displayed competitive kinetics and the p K i value was calculated as 8.79. In contrast, YM358 had little effect on the binding of [ 125 I ][Sar 1, Ile 8]angiotensin II to the angiotensin AT 2 receptor in bovine cerebellum. In isolated rabbit aorta, YM358 produced a parallel rightward shift in the concentration–response curve for angiotensin II with a pA 2 value of 8.82. YM358 had no effect on the contraction induced by KCl, norepinephrine, serotonin, histamine, prostaglandin F 2 α or endothelin-1 even at 10 −5 M. On the basis of p K i values in the binding assay and pA 2 values in the isolated tissues, YM358 was approximately 3–10 times more potent than losartan in antagonizing angiotensin AT 1 receptors. In pitched rats, intravenous administration of YM358 inhibited an increase in mean blood pressure induced by intravenous infusion of angiotensin II in a dose-dependent manner. In conscious normotensive rats, YM358 at 3–30 mg/kg p.o. inhibited the angiotensin II-induced pressor response in a dose-dependent manner. YM358 at 30 mg/kg caused maximum and complete inhibition 30 min after dosing, and inhibition lasted more than 24 h. These results demonstrate that YM358 is a potent, AT 1-selective and competitive nonpeptide angiotensin receptor antagonist. Moreover, YM358 is both orally active and long-lasting. This pharmacological profile suggests that YM358 would be suitable for the treatment of cardiovascular disorders such as hypertension and chronic heart failure.

Effects of SC-52458, an Angiotensin AT 1 Receptor Antagonist, in the Dog

We have previously reported on the basic pharmacologic properties of SC-52458 (5-[(3,5-dibutyl-1H-1,2,4-triazol-1-yl)methyl]-2-[2-(1H-tetrazol-5-ylphenyl)]pyridine), a novel angiotensin (AII) receptor antagonist that binds potently to AT 1 receptors in rat adrenal cortex and blocks AII-mediated contraction in isolated rabbit aorta. In the present study, the ability of SC-52458 to block AII pressor responses in conscious dogs was measured. In addition, we determined whether SC-52458 lowered mean arterial pressure in dogs with 2 kidney/1 clip renal hypertension when given daily for 4 days. In conscious, normotensive dogs, SC-52458 at 30 mg/kg orally, blocked the pressor response to AII (50 ng/kg, intravenously) with maximal inhibition (91%) observed 2 h after dosing. Plasma concentrations of SC-52458 measured by HPLC also were highest at the 2-h time point. After 24 h, the AII pressor response remained inhibited (by 35%) and SC-52458 was still measurable in plasma from treated dogs. In dogs made hypertensive by constriction of the left renal artery, SC-52458 lowered mean arterial pressure compared to vehicle treatment although heart rate was not different in the two groups. The maximal blood pressure lowering achieved with SC-52458 was similar to the maximal effect observed with the angiotensin converting enzyme inhibitor lisinopril. We conclude that SC-52458 blocks AII mediated pressor responses in normotensive, conscious dogs and SC-52458 is an efficacious antihypertensive agent in dogs with 2 kidney/1 clip renal hypertension.

Impairment of endothelium-dependent relaxation of rabbit aortas by cigarette smoke extract—role of free radicals and attenuation by captopril

The aim of this study was to examine the effects of the water soluble component of cigarette smoke extract (CSE) on endothelium-dependent relaxation (EDR) of isolated rabbit aortas. The incubation with CSE was found to inhibit EDR in a dose-dependent manner. Co-incubation of the aortic strips with superoxide dismutase (SOD), N-acetylcysteine, glutathione or dimetyl sulfoxide (DMSO), free radical scavengers, attenuated the CSE-induced inhibition of the arterial relaxation. Co-incubation of the strips with captopril (3 mM), an angiotensin converting enzyme inhibitor, also attenuated CSE-induced impairment of vasorelaxation. In parallel experiments using cultured human endothelial cells, CSE suppressed endothelial release of NOx, stable metabolites of nitric oxide (NO). SOD, DMSO and captopril attenuated the suppression of NO production by CSE in association with reduction of free radicals, superoxide anions and hydroxyl radicals, in CSE solution. Neither lactate dehydrogenase release from the cultured endothelial cells nor cell death estimated by trypan blue exclusion test was found after the incubation of the cultured endothelial cells with CSE. The results indicate that free radicals in CSE induce the impairment of EDR, which may be partly due to suppression of NO production and is not due to non-specific cytotoxicity by CSE. Captopril attenuates CSE-induced endothelial dysfunction partly through scavenging free radicals.

The role of endothelium in the calcium-induced reduction of the contractile response of the rabbit aorta

1. 1. Increase of Ca 2+ concentration in the bath solution diminishes the contractile response of isolated rabbit aorta rings to α 1-adrenoceptor agonists and KCI. 2. 2. In intact preparations the contractions of methoxamine and phenylephrine were maximal when a 0.3- to 0.6-mM Ca 2+ bath solution was used, and those of KCI were maximal with a 2.5-mM Ca 2+ concentration. 3. 3. The contractions of methoxamine and phenylephrine also were decreased by increasing the Ca 2+ concentration above 1.25 mM in disrupted endothelium preparations and in those incubated in indomethacin (10 −5 M), N ω-nitro-L-arginine methyl ester (10 −4 M), or methylene blue (10 −6 M). 4. 4. High organ bath Ca 2+ concentrations also caused a decrease in KCI contractions using endotheliumdenuded and the treated preparations, the responses being similar with 1.25 mM and 2.5-mM Ca 2+ in the methylene blue-treated preparations, whereas they were greater with 1.25 mM Ca 2+ in the others.

Pharmacological Profiles of a Novel Non-peptide Angiotensin II Type I Receptor Antagonist HR720 In Vitro and In Vivo

The pharmacological properties of 2-butyl-4-(methylthio)-1-[[2’-[[[(propylamino)carbonyl] amino]sulfonyl](1,1′-biphenyl)-4-yl]methyl]-1H-imidazole-5-carboxylate (HR720), a novel non-peptide angiotensin (Ang)II type I (AT1) receptor antagonist, were characterized in both in vitro and in vivo systems. In vitro autoradiography using 125I-[Sar1,Ile8]Ang II as a ligand revealed that HR720 competitively inhibited the specific binding of the ligand to the adrenal cortex. The IC50 value for the adrenal cortex was 1.5 × 10-8 M, and the IC50 for medulla was 1.4 × 10-6 M. Similar results were obtained in the adrenal cortex with CV-11974, a known potent AT1-receptor antagonist. Since AT1 receptors are known to predominate in the adrenal cortex and AT2-receptors in the adrenal medulla, it is considered that HR720 is highly selective for AT1 receptors. HR720 inhibited the Ang II-induced contraction of isolated rabbit aortic strips and human gastroepiploic arteries in a noncompetitive manner, pD’2=9.40 and 9.62 for rabbit aorta and human artery, respectively. With CV-11974, pD’2 values of 9.84 in isolated rabbit aorta and 10.00 in human artery were obtained. HR720 did not affect the norepinephrine-, serotonin- or KCl-induced contraction even at a concentration of 1 × 10-5 M. In anesthetized hamsters, HR720 induced a dose-dependent inhibition of the pressure response to Ang II. The potency of HR720 to antagonize the Ang II-induced pressure response was similar to that of CV-11974. These results demonstrate that HR720 is a potent and selective AT1-receptor antagonist.

Novel types of receptor antagonists from the medicinal plant Garcinia mangostana

A crude methanolic extract of the fruit hull of Garcinia mangostana L. inhibited the contraction of the isolated rabbit aorta induced by histamine and serotonin. The extract has been fractionated by silica gel chromatography, monitoring the pharmacological activity to give active compounds. On the basis of physicochemical data, the active substances were identified as alpha-mangostin and gamma-mangostin. To define the pharmacological properties of alpha-mangostin, the effect of alpha-mangostin on both histamine H1 and H2 receptors were examined by monitoring the mechanical responses of smooth muscles and measuring the radioligand binding to cultured vascular smooth muscle cells. The results suggest that alpha-mangostin acts as a selective and competitive histamine H1 receptor antagonist. The pharmacological actions of gamma-mangostin on 5-HT receptors were also investigated by using contractile response of vascular smooth muscle, platelet aggregation and radioligand binding studies. The results provide the evidence that gamma-mangostin is a selective and competitive 5-HT2A receptor antagonist. It is of great interest that the structures of alpha-mangostin and gamma-mangostin free from nitrogen atom are not resemble to the common structures of histamine and serotonin receptor antagonists. alpha-Mangostin and gamma-mangostin may become novel types of lead compounds for histamine and serotonin receptor antagonists.

Mechanism of hydroxyl radical-induced modulation of vascular tone

We investigated the effects of hydroxyl radicals ( OH) generated by a OH-generating system (dihydroxyfumarate [DHF], adenosine diphosphate [ADP], and FeCl 3) on isolated rabbit aorta suspended in Krebs–Ringer solution. The OH-generating system produced a concentration-dependent generation of OH. OH relaxed rabbit aorta and norepinephrine (NE)-precontracted aorta in a concentration-dependent manner. Mannitol completely prevented this relaxation. Relaxation was completely absent in preparations denuded of endothelium. The relaxant effect was reduced by 62% by an inhibitor of nitric oxide synthesis ( N G-monomethyl- l-arginine), by 58% by an inhibitor of guanylate cyclase (methylene blue), by 48% by an inhibitor of cyclooxygenase (indomethacin), and by 83% by an adenosine triphosphate (ATP)-sensitive K + channel blocker (glyburide). The inhibition of OH-induced relaxation by a combination of indomethacin, methylene blue, and glyburide was not greater than by each of the individual agents. These results indicate that OH produces a relaxation of the aorta that is completely endothelium-dependent and is partly mediated by an endothelium-derived relaxing factor (nitric oxide), vasodilatory arachidonic acid metabolites, and an ATP-sensitive K + channel. Copyright © 1996 Elsevier Science Inc.

Recombinant human erythropoietin enhances vasoconstrictor tone via endothelin-1 and constrictor prostanoids

Hypertension is the main side effect developing in patients suffering from renal anemia who are treated with recombinant human erythropoietin (rHuEPO). We investigated the effect of rHuEPO on the vascular tone of isolated rabbit aorta and carotid artery under isometric conditions. The production of prostacyclin and the vasoconstrictor prostanoids PGF2 alpha and TXB2 was investigated in arterial rings incubated with rHuEPO. Endothelial cells from human umbilical veins (HUVECs) were isolated and cultured in flasks (37 degrees C, 5% CO2). After incubation with rHuEPO, the formations of prostacyclin (as its stable metabolite 6-keto-PGF1 alpha), PGF2 alpha, PGE2, thromboxane (TX) B2 and of ET-1 were measured by radioimmunoassays. rHuEPO had no direct vasoconstrictor effect, but it enhanced noradrenalin-induced contractions. This effect was more prominent in rings with intact endothelium than in rings from which the endothelium had been mechanically removed, indicating that endothelial vasoactive factors might be involved. Relaxations to acetylcholine (ACh, 1 microM) were unaltered in the presence or absence of rHuEPO, suggesting that the endothelial NO-cGMP pathway was not impaired by rHuEPO. Incubation with rHuEPO (20 to 200 U/ml) increased the release of the vasoconstrictor mediators ET-1, PGF2 alpha and TXB2, and decreased prostacyclin formation in isolated rabbit arterial rings and in HUVECs, respectively. The cyclooxygenase inhibitor indomethacin abolished the rHuEPO-induced increase in vasoconstrictor prostanoid production. ET-1 formation by HUVECs was also increased by rHuEPO in a dose-dependent manner (maximal effect +90% by rHuEPO 200 U/ml, P < 0.05). Indomethacin and the selective ETA receptor antagonist BQ123 each partly inhibited the enhancement of vascular responsiveness to noradrenalin induced by rHuEPO in rabbit carotid artery, but simultaneous administration of rHuEPO with both antagonists completely abolished the force increment. In conclusion, these studies show that a dose-dependent shift in the balance of constrictor and relaxing prostanoids as well as an increased synthesis of ET-1 induced by rHuEPO lead to the enhanced vascular responsiveness to noradrenalin in isolated rabbit arteries. The increased vascular responsiveness to noradrenalin, which is in line with clinical observations, may contribute to the hypertensive side effect associated with rHuEPO therapy in patients with chronic renal failure.

Influence of sarmesin on the cardiac and vascular actions of angiotensin II

The angiotensin II (ANG II) receptor blocker properties of sarmesin and its influence on the homotropic cooperativity of ANG II receptors were studied in two experimental models: isolated rabbit aorta and isolated rabbit atria. The results show that: (i) sarmesin is a specific competitive antagonist of ANG II receptors, with high affinity (pA2=8.93 in the isolated aorta and 8.66 in the isolated atria); and (ii) the slope of the concentration-response curves to ANG II and the Hill coefficient increased in the presence of sarmesin, the latter suggesting an enhancement of the positive homotropic cooperativity of ANG II receptors. These results may be explained overall by the reciprocal negative modulation of receptor affinity between sarmesin and ANG II, due, possibly, to opposite effects on the binding of G-proteins to ANG II receptors.

Endothelium-independent contractile and relaxant responses to histamine in the rabbit aorta and common carotid, mesenteric, renal, and femoral arteries

1. 1. The role of the vascular endothelium in the relaxant and contractile responses to histamine of the isolated rabbit aorta; common carotid, mesenteric, renal, and femoral arteries; as well as receptor types mediating these responses were analyzed. 2. 2. Histamine (10 −8 to 10 −4mol/l) contracted resting rings and caused a further concentration. dependent contraction of rings of the arteries precontracted by phenylephrine. 3. 3. Pyrilamine abolished the contractile response to histamine in resting rings of the arteries, whereas it reversed that response into a concentration-dependent relaxant response in precontracted rings of the arteries. The relaxant effect of histamine was abolished by metiamide, but it was not affected by sotalol and atropine. Moreover, in control experiments, the phenylephrine-induced contractions and acetylcho-line-induced relaxations were not changed by pyrilamine and metiamide, respectively. 4. 4. Endothelial removal did not influence the contractile and relaxant responses of the arteries to histamine. 5. 5. These findings indicate that, in the isolated rabbit aorta and common carotid, mesenteric, renal, and femoral arteries, the contractile effect of histamine resulting from the activation of H 1 receptors overcomes its relaxant effect resulting from the activation of H 2 receptors. The effects of histamine are neither mediated nor modulated by the endothelial cells.

Influence of hypercholesterolaemia on the reactivity of isolated rabbit arteries to 15-lipoxygenase metabolites of arachidonic acid: comparison with platelet-derived agents and vasodilators

The lipoxygenase product 15-hydroxyeicosatetraenoic acid (15-HETE) was shown to be the most important eicosanoid formed in the atherosclerotic rabbit aorta. The aim of the present study was to compare the effects of 15-HETE and its hydroperoxy precursor 15-HpETE with those of other vasoconstrictor and vasodilator agents in arteries from rabbits fed either a control or a cholesterol-rich diet for 16 and 30 weeks. 5-Hydroxytryptamine (5-HT) aggregated platelets and thrombin caused contractions of isolated rabbit aortas. The contractile responses elicited by platelets from control animals were similar to those evoked by platelets from atherosclerotic rabbits. After 16 weeks of hypercholesterolemia, the contractile responses were either augmented (5-HT), unchanged (platelets) or reduced (thrombin). After 30 weeks of hypercholesterolemia, the responses to all contractile agents used had decreased. In both aortas and pulmonary arteries the endothelium-dependent relaxations to the calcium ionophore, A23167, and to acetylcholine were progressively lost and the endothelium-independent relaxations to nitroglycerin were reduced by the progressing hypercholesterolemia. The 15-lipoxygenase metabolites contracted the isolated thoracic aorta and pulmonary artery from control rabbits and to a lesser extent those of the cholesterol-fed rabbits. After raising the tone in these vessels with prostaglandin F 2α (PGF 2α) or noradrenaline, 15-HpETE induced relaxations which were not significantly influenced by the development of fatty streaks. Our data illustrate that the contractions of the blood vessel wall to 15-HETE, like those to other vasoconstrictors, are markedly reduced by developing atherosclerosis. In contrast, the relaxations to 15-HpETE in the rabbit arteries remain unaltered after 16 or 30 weeks of hypercholesterolemia. This is unlike the reactions to other vasodilators, which are markedly reduced.

Oxidative stress increases glyceraldehyde-3-phosphate dehydrogenase mRNA levels in isolated rabbit aorta.

We have recently shown that inhibition of endogenous Cu,Zn superoxide dismutase (SOD) by diethyldithiocarbamate (DDC) increased superoxide anion levels in isolated rabbit aortic rings, describing a useful experimental model to examine the effects of oxidative stress on the vessel wall. The present study examined the effects of oxidative stress on the steady-state mRNA levels of glyceraldehyde-3-phosphate dehydrogenase (GAPDH; EC 1.2.1.12). Aortic rings were incubated in physiological salt solution at 37 degrees C for up to 6 h. DDC (2 mM) decreased total SOD activity to < 5% of control levels and increased superoxide anion level ninefold. Steady-state mRNA levels of GAPDH were increased under comparable conditions. Although decreased biological activity of endothelium-derived nitric oxide was indicated by lower basal guanosine 3',5'-cyclic monophosphate levels in aortic rings treated with DDC compared with those in control rings (1.2 +/- 0.1 vs. 1.9 +/- 0.3 fmol/microgram protein, P < 0.05), neither endothelium denudation nor NG-nitro-L-arginine methyl ester had any effects on the steady-state mRNA levels of GAPDH. The cell.permeable iron chelator 1,10-phenanthroline completely prevented the increases in GAPDH mRNA levels induced by DDC. These results suggest that oxidative stress resulting from inhibition of endogenous Cu,Zn SOD causes induction of GAPDH gene expression and that the hydroxyl radical, produced through the iron-catalyzed Haber-Weiss reaction, is the intracellular reactive oxygen species responsible for the DDC-stimulated increase in GAPDH mRNA.

Vascular Smooth Muscle Relaxation by α-Adrenoceptor Blocking Action of Dobutamine in Isolated Rabbit Aorta

We investigated the mechanism of vascular relaxation produced by dobutamine, a positive inotropic agent with beta 1-adrenergic action. Dobutamine concentration-dependently (10 nM-10 microM) relaxed ring segments of rabbit aorta partially precontracted with 1 microM phenylephrine (PE) but did not relax those precontracted with 40 mM K+ or 5 microM prostaglandin F2 alpha (PGF2 alpha). The relaxation was not completely inhibited by pretreatment with 10 microM propranolol. Dobutamine did not significantly increase tissue cyclic AMP levels concomitantly with relaxation as does isoproterenol (ISO) in rabbit aorta. Dobutamine produced a parallel rightward shift in concentration-response curves to PE. The Schild plot analysis resulted in a linear regression of a slope of 1.077 +/- 0.077, which was not significantly different from unity. The pA2 value of dobutamine as compared with PE in rabbit aorta was 6.81 +/- 0.03. Dobutamine causes arterial dilatation mediated not only through a beta-adrenergic action but also through an alpha-adrenergic blocking action in rabbit aorta.

Pharmacology of XR510, a Potent Orally Active Nonpeptide Angiotensin II AT1 Receptor Antagonist with High Affinity for the AT2 Receptor Subtype

The angiotensin II (Ang II) type 1 receptor (AT1) mediates all known physiological effects of ANG II, whereas functions of the type 2 (AT2) receptor are not clear. Should undesirable AT2 effects be identified, it may be advantageous to combine antagonism of AT1 and AT2 receptors. XR510 was shown to inhibit the specific binding of [125I]Sar1,Ile8-Ang II for AT1 and AT2 subtype binding sites in rat adrenal membranes with IC50 of 0.26 and 0.28 nM, respectively, and in human tissues with subnanomolar binding affinity. In isolated rabbit aorta, XR510 exerted insurmountable Ang II antagonism with a Kb value of 4 nM. In conscious renal hypertensive rats, XR510 decreased blood pressure (BP) with intravenous (i.v.) and oral (p.o.) ED30 of 0.08 and 0.27 mg/kg, respectively. In spontaneously hypertensive rats (SHR), repeated daily oral dosing of XR510, losartan, and enalapril at 30 mg/kg/day decreased BP similarly. In conscious furosemide-treated dogs, XR510, given either intravenously or orally, decreased BP. These results suggest that XR510 is an orally active and selective Ang II receptor antagonist with equal binding affinities for AT1 and AT2 receptor binding sites.

Pharmacology of a potent long-acting imidazole-5-acrylic acid angiotensin AT 1 receptor antagonist

The angiotensin II antagonistic activity of SB 203220, [ E-α-[[2-butyl-1-(4-carboxyl-1-naphthalenyl)methyl]-1 H-imidazol-5-yl]-methylene]-2-thiophene-propanic acid], was examined in several in vitro and in vivo assays. SB 203220 displaced [ 125I]angiotensin II binding from a variety of tissues including the cloned human AT 1 receptor (IC 505–15 nM). SB 203220 (10 μM) did not interact with AT 2, endothelin (ET A and ET B) or calcitonin gene-related peptide receptors. [ 3H]SB 203220 bound with high affinity to the AT 1 receptor ( K d = 4.9 nM), but dissociated from the receptor at a much slower rate when compared to [ 3H]SK & F 108566. SB 203220 antagonized intracellular Ca 2+ mobilization induced by angiotensin II in rat vascular smooth muscle cells and exhibited a selective and partially insurmountable antagonism of angiotensin II-induced contraction in isolated rabbit aorta. In the aorta, SB 203220 produced a concentration-dependent parallel shift in the concentration-response curve to angiotensin II [EC 30 = 5.94 ± 1.6 10 −11 M] and depressed the maximal contractile response to angiotensin II by approximately 35%. The antagonistic effect of SB 203220 in rabbit aorta was slowly reversible compared to SK & F 108566. SB 203220 displayed no agonist activity and had no effect on the contractile responses to KCl, endothelin-1 or norepinephrine. In rats, SB 203220 at 10 mg/kg i.v. inhibited angiotensin II-induced aldosterone release. Intraduodenal or oral administration of SB 203220 (1–10 mg/kg) to conscious rats and dogs inhibited the pressor responses to exogenous angiotensin II. SB 203220 (3–10 mg/kg) also produced a long-lasting (≥ 12 h) antihypertensive response in renin-dependent hypertensive rats and dogs. These data demonstrate that SB 203220 is a potent and highly selective AT 1 receptor antagonist having good oral activity and duration of action in hypertensive models.

Mediation of H2O2-induced vascular relaxation by endothelium-derived relaxing factor.

We investigated the effects of H2O2 generated by glucose (G) and glucose glucose oxidase (GO) on the isolated rabbit aorta suspended in Krebs-Ringer solution. H2O2 produced contraction in small concentration and relaxation followed by contraction in large concentration. Contraction produced by large concentration was smaller than that produced by small concentration of H2O2. Relaxation was prevented by deendothelialization or NG-monomethyl-L-arginine, an inhibitor of nitric oxide synthesis. These results suggest that H2O2 in large concentrations produces relaxation followed by contraction, and that the relaxation is endothelium-dependent and is mediated by nitric oxide, an endothelium-derived relaxing factor.

Contribution of activation of K + channels to glyceryl trinitrate-induced relaxation of rabbit aorta

1. 1. Possible contribution of K + channel opening to the relaxation by glyceryl trinitrate (GTN) was examined using isolated rabbit aorta. 2. 2. While glibenclamide and apamin failed to affect relaxation by GTN, both charybdotoxin (ChTx) and iberiotoxin (IbTx) effectively attenuated GTN-induced relaxation. 3. 3. The increase in cGMP produced by GTN was not attenuated by ChTx and IbTx. 4. 4. The inhibitory effect of ChTx on GTN-induced relaxation was not reduced in the presence of zaprinast, indicating that cGMP but not GMP was responsible for activation of the K + channel. 5. 5. Okadaic acid, a selective inhibitor of protein phosphatase 2A, had no effect on the relaxation by GTN. These results indicate that, though small in degree, activation of a ChTx-sensitive K + channel (large conductance Ca 2+-activated K + channel) is involved in the GTN-induced relaxation in rabbit aorta.

Release of 3H-5-hydroxytryptamine from isolated rabbit aorta.

The main aim of the present investigation was to study systematically the passive and stimulation-evoked release of 3H-5-hydroxytryptamine (3H-5-HT) from rabbit isolated aorta. This was accomplished by preloading rings of aorta with 3H-5-HT (10(-6)M) and then monitoring by fractional collection the basal 3H-outflow and stimulation-evoked 3H-overflow. The basal 3H-outflow from aorta preloaded with 10(-6)M of either 3H-5-HT or (-)-3H-noradrenaline (3H-NA) leveled off about 100 min. after the onset of wash-out and remained almost constant thereafter (100-240 min.). The basal 3H-outflow from tissue preloaded with 3H-5-HT was almost 3-fold higher (70-240 min.) than that seen after preloading with 3H-NA. Cocaine (3 x 10(-5)M) did not alter the basal 3H-outflow (15-240 min.) from tissue preloaded with 3H-5-HT, while pargyline (5 x 10(-4)M) decreased it by about 66% (100-240 min.). Electrical-field stimulation (S1-S7, 200 mA, 600 pulses, 0.5 msec., 3 Hz) were applied to the tissue. The initial stimulation-evoked 3H-overflow from aorta preloaded with 3H-5-HT was higher than the subsequent ones (S1-S7; 100, 35, 35, 35, 35, 37, and 40%). Similar results to these were obtained with tissues preloaded with 3H-NA. The stimulation (S1-S7; 200 mA). 600 pulses, 0.5 msec, 3 Hz)-evoked 3H-overflow increased in an apparent linear manner with the amount of current used (50-200 mA).(ABSTRACT TRUNCATED AT 250 WORDS)