Isolated Rabbit Thoracic Aorta Research Articles

Vasodilatory Effect of Guanxinning Tablet on Rabbit Thoracic Aorta is Modulated by Both Endothelium-Dependent and -Independent Mechanism.

Vasodilatory therapy plays an important role in the treatment of cardiovascular diseases, especially hypertension and coronary heart disease. Previous research found that Guanxinning tablet (GXNT), a traditional Chinese compound preparation composed of Salvia miltiorrhiza (Danshen) and Ligusticum chuanxiong (Chuanxiong), increase blood flow in the arteries, but whether vasodilation plays a role in this effect remains unclear. Here, we found that GXNT significantly alleviated the vasoconstriction of isolated rabbit thoracic aorta induced by phenylephrine (PE), norepinephrine (NE), and KCl in a dose-dependent manner with or without endothelial cells (ECs). Changes in calcium ion levels in vascular smooth muscle cells (VSMCs) showed that both intracellular calcium release and extracellular calcium influx through receptor-dependent calcium channel (ROC) declined with GXNT treatment. Experiments to examine potassium channels suggested that endothelium-denuded vessels were also regulated by calcium-activated potassium channels (Kca) and ATP-related potassium channels (KATP) but not voltage-gated potassium channels (kv) and inward rectifying potassium channels (KIR). For endothelium-intact vessels, the nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) contents in vascular tissue obviously increased after GXNT treatment, and pretreatment with the NO synthase inhibitor Nw-nitro-L-arginine methyl ester (L-NAME) or guanylyl cyclase inhibitor methylthionine chloride (MB) significantly inhibited vasodilation. An assessment of NO-related pathway protein expression revealed that GXNT enhanced the expression of phosphorylated endothelial NO synthase (eNOS) in a dose-dependent manner but had no effect on total eNOS, p-Akt, Akt, or PI3K levels in human umbilical vein ECs (HUVECs). In addition to PI3K/AKT signaling, Ca2+/calmodulin (CaM)-Ca2+/CaM-dependent protein kinase II (CaMKII) signaling is a major signal transduction pathway involved in eNOS activation in ECs. Further results showed that free calcium ion levels were decreased in HUVECs with GXNT treatment, accompanied by an increase in p-CaMKII expression, implying an increase in the Ca2+/CaM-Ca2+/CaMKII cascade. Taken together, these findings suggest that the GXNT may have exerted their vasodilative effect by activating the endothelial CaMKII/eNOS signaling pathway in endothelium-intact rings and calcium-related ion channels in endothelium-denuded vessels.

Royal jelly causes hypotension and vasodilation induced by increasing nitric oxide production.

Among royal jelly’s (RJ) various biological activities, its possible antihypertension and vasorelaxation effects deserve particular attention, but the underlying mechanisms of action remain unclear. Therefore, this study used the spontaneously hypertensive rats (SHR) hypertension model and the isolated rabbit thoracic aorta rings model to explore the mechanisms underlying the hypotension and vasorelaxation effects of RJ. Rats were divided into the following groups (n = 6): WKY‐control group, SHR‐control group, and SHR‐RJ group. SHR‐RJ group was received 1 g/kg of RJ via oral administration daily for 4 weeks. Systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), and nitric oxide (NO) level were detected. In addition, the mechanism of vasodilation of RJ was investigated using an isolated rabbit aortic ring technique. RJ significantly reduced SBP and DBP as well as increased NO levels of SHR in vivo. RJ caused vasorelaxation of the isolated aorta rings, and this effect was inhibited by atropine (M3 receptor blocker), L‐NAME (nitric oxide synthase inhibitor), methylene blue (guanylate cyclase inhibitor), and indomethacin (cyclooxygenase inhibitor). Moreover, RJ could markedly suppress the NE‐induced intracellular Ca2+ releases and high K+‐induced extracellular Ca2+ influx in denuded aortic rings. In addition, RJ can also increase cGMP levels and the production of NO in isolated aortic rings. The present study showed that RJ has antihypertensive effects and was associated with increased NO production. In addition, RJ contains muscarinic receptor agonist, possibly an acetylcholine‐like substance, and induces vasodilation through NO/cGMP pathway and calcium channels.

Pharmacological Characterization of BR-A-657, a Highly Potent Nonpeptide Angiotensin II Receptor Antagonist

The pharmacological profile of BR-A-657, 2-n-butyl-5-dimethylamino-thiocarbonyl-methyl-6-methyl-3-{[2-(1H-tetrazole-5-yl)biphenyl-4-yl]methyl}-pyrimidin-4(3H)-one, a new nonpeptide AT1-selective angiotensin receptor antagonist, has been investigated in a variety of in vitro and in vivo experimental models. In the present study, BR-A-657 displaced [(125)I][Sar(1)-Ile(8)]angiotensin II (Ang II) from its specific binding sites to AT1 subtype receptors in membrane fractions of HEK-293 cells with an IC50 of 0.16 nM. In a functional assay using isolated rabbit thoracic aorta, BR-A-657 inhibited the contractile response to Ang II (pD'2: 9.15) with a significant reduction in the maximum. In conscious rats, BR-A-657 (0.01, 0.1, 1 mg/kg; intravenously (i.v.)) dose-dependently antagonized Ang II-induced pressor responses. In addition, BR-A-657 dose-dependently decreased mean arterial pressure in furosemide-treated rats and renal hypertensive rats. Moreover, BR-A-657 given orally at 1 and 3 mg/kg reduced blood pressure in conscious renal hypertensive rats. Taken together, these findings indicate that BR-A-657 is a potent and specific antagonist of Ang II at the AT1 receptor subtype, and reveal the molecular basis responsible for the marked lowering of blood pressure in conscious rats.

Inhibitory Effect of Fentanyl on Phenylephrine-Induced Contraction on Rabbit Aorta

This in vitro study was designed to assess the effects of fentanyl on isolated rabbit thoracic aorta rings contracted with phenylephrine. Methods included contraction of aorta rings with phenylephrine (10–5 M) and recording the changes after increasing concentrations of fentanyl (10–9 M – 10–5 M). Similar experiments were done after incubation with Nω- nitro-L-arginine methyl ester (10–4 M), indomethacin (10-5 M), naloxone (10–5 M), ouabain (10–5 M), TEA (10–4 M) and glibenclamide (10–5 M). It was revealed that, fentanyl causes relaxation in rabbit aorta rings precontracted with phenylephrine. Removal of endothelium significantly reduces the relaxant response to fentanyl. Nitric oxide synthase inhibitor L-NAME, K+ channel blocker glibenclamide and Na+/K+ ATPase inhibitor ouabain inhibits the relaxant effect of fentanyl in endothelium intact aorta rings. These results suggest that fentanyl causes dose dependent vasodilatation in the rabbit aorta via activation of KATP channels and Na+-K+ -ATPase, and nitric oxide released from endothelium.

Protective Effect of Propofol on Endothelial Damage Induced by Reactive Oxygen Species in Rabbit

Background: Reactive oxygen species (ROS) induce lipid peroxidation and tissue damage in the isolated rabbit thoracic aorta. The aim of this study was to explore the influence of the propofol and midazolam against ROS in the isolated rabbit thoracic aortic endothelium. Methods: Eighteen white male rabbits (weighing 2.0-2.5 kg) were used. The thoracic aorta was dissected free and cut into rings (3-4 mm) and then suspended in a organ bath filled with 10 ml Krebs solution bubbled with 5% 95% at 37. Aortic rings were then equilibrated for 90 min, and a resting tension of 1.5 g was applied. The Krebs solution was changed every 15 min. Isometric tension was recorded with transducer coupled to a data acqusition system (Biopac Inc. USA) on a PC. After precontraction with norepinephrine (NE, M), changes in tension were measured following the cumulative administration of acetylcholine (ACh , and M) and nitroglycerin (NTG, M). Data are expressed as percentage of the M NTG-induced relaxation (ACh/NTG). The ACh/NTG, before and after electrolysis were defined as the control and the experimental groups. The aortic rings were pretreated with propofol (, , and M, n= 8, 10, 15, 13), midazolam ( M, n = 7), catalase (1,000 U/ml, n = 12), mannitol ( M, n = 5) or not pretreated group (Free, n = 6). After 30 minutes, the aortic rings were exposed to ROS generated by electrolysis (DC 9 V, 20 mA, aortic rings 1 cm away from electrode) in Krebs solution for 2 minutes, which was then changed for physiologic buffered salt solution. The aortic rings were precontracted with NE and vasorelaxation was induced with ACh and NTG at the above mentioned concentrations. Results: Propofol produced vasorelaxation of NE-precontracted thoracic aorta in a dose-dependent fashion in all groups of propofol (, , and M) even after ROS attack (P ＜ 0.05 vs control value). Catalase produced vasorelaxation after ROS attack (P ＜ 0.05 vs control value). On the other hand, ACh-induced significant endothelium-dependent vasorelaxation were not observed in the midazolam or mannitol pretreated group or the non-pretreated group (P ＜ 0.05 vs control group). Conclusions: These findings suggest that propofol and catalase preserve ACh induced endothelium-dependent vasorelaxation and that propofol has a concentration dependent ROS scavenging effect like catalase.

Sympatholytic properties of several AT1-receptor antagonists in the isolated rabbit thoracic aorta.

To evaluate the facilitating effect of angiotensin II on sympathetic neurotransmission to quantitatively compare the sympatho-inhibitory potencies of the selective AT1 -receptor antagonists losartan, irbesartan and telmisartan in the isolated rabbit thoracic aorta. To investigate the influence of pharmacological compounds on pre-junctional sympathetic transmission, the quantification of sympathetic transmitter release is the most straightforward approach. To investigate the sympatholytic properties of AT1 -blockers, we studied their effects on the enhancement by angiotensin II of electrical field stimulation (EFS)-evoked (2 Hz) sympathetic transmission in a modified spillover model. Angiotensin II (0.01 nmol/l-0.1 micromol/l) caused a concentration-dependent enhancement of EFS-evoked noradrenaline release (control versus concentrations 0.1 nmol/l-0.1 micromol/l, P<0.05). The maximal augmentation, by almost 100%, was observed at a concentration of 1 nmol/l (FR2/FR1, 2.03 +/- 0.11 versus control, 0.99 +/- 0.03). Higher concentrations (up to 0.1 micromol/l) produced less than maximal facilitation. The AT1 -receptor antagonists losartan (0.1 nmol/l-0.1 micromol/l), telmisartan (0.01-10 nmol/l) and irbesartan (0.1 nmol/l-0.1 micromol/l) concentration dependently attenuated the angiotensin II-mediated (1 nmol/l) enhancement of EFS-evoked sympathetic outflow. The concentrations that reduced the enhancement by 50% (IC50 values, expressed as -log mol/l +/- SEM) were 9.05 +/- 0.16 losartan, 10.28 +/- 0.20 telmisartan and 9.20 +/- 0.23 irbesartan. Accordingly, the order of potency with respect to sympatho-inhibition proved telmisartan> irbesartan = losartan (where > signifies P<0.05). The facilitating effect of angiotensin II on the sequelae of neuronal stimulation appears to be mediated by pre-synaptically located AT1 -receptors. Facilitation can be concentration dependently attenuated by AT1 -blockade. The order of potency with respect to sympatho-inhibition is telmisartan irbesartan = losartan. These differences may be explained by differences in affinity for the pre-synaptic AT1 -receptor.

In vitro effects of dantrolene sodium and verapamil on noradrenaline-induced contractions of rabbit aorta and their interactions.

The effects of dantrolene sodium (0.1 to 10 microM) and verapamil (0.01 to 1 microM) administered alone or together (1 microM verapamil, 0.1 to 10 microM dantrolene sodium) were investigated in isolated rabbit thoracic aorta precontracted with 0.1 microM noradrenaline (NA). Verapamil plus dantrolene sodium produced a dose-dependent inhibition of aortic strips contractions evoked by NA, and all concentrations of dantrolene sodium significantly decreased the inhibitory effect of 1 microM verapamil (p < 0.001, ANOVA). In conclusion, dantrolene sodium and verapamil inhibited 0.1 microM noradrenaline-evoked aorta contractions, and all doses of dantrolene sodium decreased the inhibitory effect of 1 microM verapamil in a dose-dependent manner.

Characterisation of a modified approach to the study of sympathetic neurotransmission and its presynaptic modulation in the isolated rabbit thoracic aorta

Introduction: The quantification of [ 3H]noradrenaline spillover from electrically stimulated, sympathetic nerves is a widely used method to study presynaptic effects of hormones, transmitters and drugs. Although a straightforward approach, the execution of the experiments is not trivial. This holds true mainly for a reliable control of the experimental conditions, a major pitfall of the commonly used superfusion setup, and problems concerning the sampling of the tritium containing probes. Methods: These difficulties prompted us to develop a variant of this method, which is based on a classical organ bath setup. Rabbit thoracic aortic rings were incubated with [ 3H]-labeled noradrenaline. Instead of being constantly washed away by superfusion, the [ 3H]noradrenaline is allowed to accumulate in the medium. Results: Electrical field stimulation (EFS) (2 Hz, 3 ms, 150 mA) caused a significant increase of [ 3H]noradrenaline outflow by approximately a factor 4.2 ( P<.05). The fractional release of noradrenaline during consecutive periods of stimulation remained unaltered (FR 2/FR 1 0.99±0.03). The EFS-evoked release could be nearly abolished by the selective sodium channel blocker tetrodotoxin (1 μM) (FR 2/FR 1 0.06±0.03, P<.05). The N-type calcium antagonist ω-conotoxin GVIA (0.3 μM) abolished the stimulation-evoked outflow (FR 2/FR 1 0.01±0.06, P<.05), whereas the antisympathotonic agent guanethidine (10 μM) attenuated the EFS-evoked noradrenaline outflow by approximately a factor 2 (FR 2/FR 1 0.46±0.07, P<.05). Angiotensin II (0.1 and 1 nM) enhanced the EFS-evoked [ 3H]noradrenaline outflow by nearly a factor 1.5 and 2, respectively (FR 2/FR 1 of 1.43±0.11 (0.1 nM) and 2.03±0.11 (1 nM); n=6–8, P<.05). All agents failed to influence basal outflow. Discussion: Our modified experimental approach appears to be suitable to study presynaptic influences on sympathetic transmission in the rabbit thoracic aorta. In addition to optimal control of the experimental conditions, the method offers the advantage of a safe and reliable sampling.

Effects of lidocaine on rabbit isolated thoracic aorta

Lidocaine has been demonstrated to modify both contraction and relaxation of the vascular smooth muscle. Although lidocaine has been shown to inhibit endothelium-independent relaxations, the effects of lidocaine on arterial relaxation induced by peroxynitrite, a reaction product of superoxide and nitric oxide, have not been studied. The current study was designed to evaluate the effects of lidocaine on endothelium-dependent and -independent relaxations in isolated rabbit thoracic aorta. Rings of the rabbit thoracic aorta with or without endothelium were mounted for isometric force recording. Concentration–response curves to calcium ionophore A23187 ( 10−9to 3 × 10−6m), acetylcholine ( 10−9to 10−3m), sodium nitroprusside (SNP, 10−9to 10−3m), and peroxynitrite ( 10−9to 10−3m) were obtained in a cumulative manner. Lidocaine ( 10−6to 10−4m) was applied 15 min before addition of phenylephrine. Under resting force, lidocaine produced contractions at high concentrations ( 10−5to 10−2m) in endothelium-intact and -denuded arteries but removal of the endothelium did not significantly affect contractile activity. In phenylephrine-precontracted arteries, lidocaine caused concentration-dependent relaxations in both endothelium-intact and -denuded arteries. Inhibition of nitric oxide synthase or removal of endothelium did not affect the relaxations to lidocaine. Lidocaine suppressed the endothelium-independent relaxations of peroxynitrite, also poly (ADP-ribose) synthetase (PARS) enzyme activator, and SNP at high concentrations. Concentration-dependent vascular relaxations to A23187 and acetylcholine were significantly inhibited by lidocaine. These results suggest that lidocaine can depress vascular relaxations by a complex mechanism including inhibition of PARS enzyme activity.

Human urotensin II mediates vasoconstriction via an increase in inositol phosphates

The cyclic peptide urotensin II has recently been cloned from human and reported to potently constrict primate blood vessels. To elucidate the cellular signalling mechanisms of this peptide, we investigated a possible relationship of vasomotor effects of human urotensin II and phosphoinositide turnover in isolated rabbit thoracic aorta. Human urotensin II produced a slowly developing increase in isometric contractile force ( pEC 50=9.0) that was endothelium-independent. The contractile effect of urotensin II was significantly inhibited by the phospholipase C inhibitor, 2-nitro-4-carboxyphenyl- N, N,-diphenylcarbamate (NCDC), but not by the cyclooxygenase inhibitor, indomethacin. In slices of rabbit thoracic aorta, human urotensin II increased phosphoinositide hydrolysis, and this effect was also inhibited by NCDC. The potency of urotensin II ( pEC 50=8.6) was similar to that found in the contractile studies. Thus, vasoconstrictor effects of human urotensin II appear to be mediated by a phospholipase C-dependent increase in inositol phosphates, suggesting that the peptide acts via a G q protein-coupled receptor.

Pharmacological Analysis of Noncholinergic Action of 2-sec-Butylphenyl N-Methylcarbamate Insecticide on the Isolated Rabbit Aorta

We investigated noncholinergic actions of 2-sec-butylphenyl N-methylcarbamate (BPMC) on the isolated rabbit thoracic aorta to help determine the mechanisms responsible for its unique toxicological properties, which are characterized by cardiovascular collapse and low lethality compared to its anticholinesterase (anti-ChE) activity. BPMC inhibited K+-induced contraction more effectively than it did norepinephrine (NE)-induced contraction. The inhibitory effect on K+-induced contraction was not altered by changing the external K+ concentration, but it was decreased by adding an L-type Ca2+ channel agonist, BAY K 8644. Simultaneous measurement of tension and cytosolic Ca2+ levels elevated by K+ stimulation revealed that BPMC decreased the Ca2+ levels prior to and parallel to the tension. The magnitude of the inhibitory effect on Ca2+ levels was increased by treating BPMC before Ca2+ application in the depolarized preparation. However, BPMC did not inhibit caffeine- or NE-induced transient contraction in Ca2+-free medium. On the other hand, BPMC produced tonic contraction in the resting aorta. The contraction to BPMC did not develop after removing the adventitia. The contraction was inhibited by phentolamine or guanethidine but not by atropine or tetrodotoxin. These results suggest that BPMC inhibits depolarization- or agonist-induced contraction by inhibiting Ca2+ entry through L-type Ca2+ channels, whereas it produces vascular contraction in the resting state by releasing NE from sympathetic nerve terminals. These apparently opposing mechanisms may contribute to the unique noncholinergic toxicological properties of BPMC.

Angiotensin II Receptor Assays

AbstractThis unit describes the use of an isolated rabbit thoracic aorta preparation for determining the nature, potency, and selectivity of antagonists to the agonism of angiotensin II at AT1 receptors by measuring their effects on the tension of the tissue.

Pharmacological properties of α-mangostin, a novel histamine H 1 receptor antagonist

In the isolated rabbit thoracic aorta and guinea-pig trachea, α-mangostin inhibited histamine-induced contractions in a concentration-dependent manner in the presence or absence of cimetidine, a histamine H 2 receptor antagonist. But KCl-, phenylephrine- or carbachol-induced contractions were not affected by α-mangostin. The concentration-contractile response curve for histamine was shifted to the right in a parallel manner by α-mangostin. In the presence of chlorpheniramine, a histamine H 1 receptor antagonist, α-mangostin did not affect the relaxation of the rabbit aorta induced by histamine. In the guinea-pig trachea, α-mangostin had no effect on the relaxation induced by dimaprit, a histamine H 2 receptor agonist. α-Mangostin caused a concentration-dependent inhibition of the binding of [ 3H]mepyramine, a specific histamine H 1 receptor antagonist to rat aortic smooth muscle cells. Kinetic analysis of [ 3H]mepyramine binding indicated the competitive inhibition by α-mangostin. These results suggest that α-mangostin is a novel competitive histamine H 1 receptor antagonist in smooth muscle cells.

Endothelium-dependent hyperpolarization. Beyond nitric oxide and cyclic GMP.

Blood flow and blood pressure are determined by an integration of reflex, humoral, and local vascular control mechanisms. Knowledge of these mechanisms has mushroomed over the past 15 years, particularly in the area of local endothelium-dependent vasomotor control. This has stemmed from the pioneering report in 1980 by Robert Furchgott1 demonstrating that the endothelium releases a vasodilator substance in response to acetylcholine. This concept has been expanded with knowledge that the endothelium releases a variety of relaxing and contracting factors that regulate the underlying smooth muscle.2 The most widely known endothelium-derived relaxing factor, nitric oxide, is released from endothelial cells in response to shear stress or stimulation of different receptors for a variety of neurohumoral mediators on the endothelial cell surface.3 The increase in endothelial cell calcium initiated by these stimuli increases the activity of a constitutively expressed enzyme, nitric oxide synthase, which converts l-arginine to nitric oxide and citrulline.4 5 6 7 Nitric oxide thus formed diffuses to and inhibits contraction of the underlying vascular smooth muscle.3 The physiological significance of the production of endothelial nitric oxide is suggested by the vasoconstriction observed in most vascular beds8 9 10 11 12 13 and the increase in systemic arterial blood pressure,14 15 16 which occurs on infusion of inhibitors of nitric oxide synthase. This observation further implies that under normal conditions, endothelial cells are locally liberating nitric oxide which effectively inhibits vasoconstriction arising by other mechanisms. Thus, normal vascular homeostasis depends in the periphery on a balance between neurally and humorally mediated vasoconstriction in skeletal muscle, mesentery, and the kidney, and local endothelium-dependent vasodilatation. In the truly vital areas of the heart, brain, and genitalia, vasodilator neural mechanisms reinforce the vasodilator influence of the endothelium.17 All is not known regarding the …

Muscarinic receptor characterization and evaluation of a series of novel muscarinic ligands in the isolated rabbit thoracic aorta

Muscarinic receptor characterization and evaluation of a series of novel muscarinic ligands in the isolated rabbit thoracic aorta

P-227 - The effects of U-41123F (adinazolam), a novel anti-panic disorder drug, and its main metabolite, U-42352 in the isolated rabbit thoracic aorta.

P-227 - The effects of U-41123F (adinazolam), a novel anti-panic disorder drug, and its main metabolite, U-42352 in the isolated rabbit thoracic aorta.

Kinetics of relaxation responses to vasorelaxants in intact and adventitia- and endothelium-denuded isolated rabbit thoracic aorta

1. 1. Although the relaxation responses to 5′- N-ethylcarboxamidoadenosine and sodium nitroprusside were stable, the relaxation response to isoproterenol partially desensitized as was observed by the partial regaining of tissue tension. 2. 2. The apparent rate constant for relaxation by all three vasorelaxants was decreased in the presence of the adventitia and endothelium. 3. 3. The magnitude of the relaxation responses for all three vasorelaxants and the apparent rate constant for isoproterenol desensitization were similar in intact and denuded tissues.

Distribution and metabolism of angiotensin I and II in the blood vessel wall.

The demonstration of all components of the renin-angiotensin system in vascular tissue has raised questions as to the precise location of the local angiotensin II generation within the vascular wall. We investigated the metabolism of angiotensin I to angiotensin II in the vascular wall in the isolated rabbit thoracic aorta. Angiotensin I (3 x 10(-9) M) applied into the aortic lumen was partially converted to angiotensin II (14% after 60 minutes), but most of the luminal angiotensin I was degraded to peptide fragments or diffused as intact angiotensin I, peptide fragments, or both, into the vessel wall. Incubation studies with [3H]angiotensin I revealed that angiotensin I or angiotensin I fragments mainly diffused into the medial layer of the aorta and to a lesser degree into the adventitia and the endothelium. After removal of the endothelium, angiotensin II generation could no longer be detected. Addition of the angiotensin converting enzyme inhibitor ramiprilat (10(-7) M) to the incubation medium led to a complete blockade of angiotensin II generation by endothelial angiotensin converting enzyme. Our results underline the importance of the endothelium for conversion of angiotensin I to angiotensin II and provide evidence that conversion of angiotensin I to angiotensin II is predominantly achieved by endothelial cells. They also support the concept of an endocrine versus autocrine/paracrine renin-angiotensin system where the endothelium of the vasculature is the critical target site for angiotensin II production by both systems and, thus, the most important site for the actions of angiotensin converting enzyme inhibitors.

Responses of isolated rabbit thoracic aortae to endothelin-1 and several vasoactive substances a new blood perfusion circuit under the controlled conditions of resistance and flow

Analysis of the reactivity of isolated rabbit thoracic aorta to isoproterenol and norepinephrine injected into a blood perfusion circuit intraarterially (i.a.) under controlled conditions of resistance and flow using conscious support rabbits revealed that with pressure loads of 10–20 mmHg for a vessel-holding chamber and 60 mmHg for a Starling pneumatic resistance set, a suitable vasodilation or vasoconstriction can be obtained. This leads us to assume that the system could be used to assess the direct influence of vasoactive substances on vasomotor tone and, furthermore, the support rabbit could be used for systemic hemodynamic variables and behavior observation. The i.a. injection of increasing doses of endothelin (ET) (0.01–0.3 μg) caused a weak but long-lasting and dose-dependent vasoconstriction of the thoracic aorta; its effect was less potent than that of angiotensin II or norepinephrine when their peak responses were compared. The duration of vasoconstriction caused by 0.3 μg i.a. of ET was ∼ 30 min and much longer than that of angiotensin II and norepinephrine. Histamine, serotonin, and prostaglandin E 2 produced noticeable dose-dependent vasoconstriction. When ET (0.1–3 μg) was injected i.v. to conscious rabbits, the systemic blood pressure, which first dropped and then rose, was accompanied by significant changes in the heart rate in a reciprocal way. The observed rise in the blood pressure with the accompanying decrease in the heart rate lasted for at least 30 min following 1 μg i.v. of ET. The maximal tolerable dose of ET in conscious rabbits was about 1 μg i.v. In summary, it may be concluded that ET is a long-acting vasoconstrictor and might be more capable of producing an effect downstream of its release site; however, it is very toxic in the rabbit.

Pharmacological Characterization of Contractile Responses Induced by α1-Agonists, Norepinephrine and Clonidine, by Selective Antagonists of Their Subtypes in Rabbit Thoracic Aorta

In the rabbit isolated thoracic aorta, WB 4101 and 5-methylurapidil dose-dependently shifted the concentration-response curves for norepinephrine to the right. Schild plots showed that the inhibition of responses for WB 4101 and 5-methylurapidil was biphasic, implying that norepinephrine acted through two receptor populations. Clonidine produced a concentration-dependent contraction in the isolated rabbit thoracic aorta. WB 4101 and 5-methylurapidil antagonized the contractions for clonidine, and the Schild plot to both antagonists against clonidine yielded a monophasic slope. Schild plots of the results obtained from the inhibition by WB 4101 and 5-methylurapidil for norepinephrine in strips pretreated with chloroethylclonidine yielded a straight line with a slope of unity. Specific binding of [3H]prazosin in the aortic membrane preparations was saturable. The Hill coefficient obtained from the inhibition curves for clonidine was significantly different from unity. Clonidine interacted with two binding sites labelled by [3H]prazosin, but the low affinity site was completely eliminated by pretreatment with 10 μM chloroethylclonidine. These results suggest that the subtype activated by norepinephrine is different from that activated by clonidine, and that norepinephrine-induced contraction through both α1A- and α1B-subtypes and clonidine through only the α1A-subtype in the rabbit thoracic aorta.