Isolated Rabbit Aorta Research Articles

The optical isomers of the 1,4-dihydropyridine BAY K 8644 show opposite effects on Ca channels

The optical isomers of the 1,4-dihydropyridine BAY K 8644 were studied in isolated rabbit aorta and heart preparations. The (−)-enantiomer has the known vasoconstricting and positive inotropic properties of the Ca agonistic compound. In contrast, its antipode shows at about 10–50 times higher concentrations the vasodilating and negative inotropic effects of Ca antagonistic drugs. It is concluded that neither simple chemical nor physical actions can be responsible for the opposite effects of Ca antagonistic and Ca agonistic dihydropyridines.

Differences in norepinephrine activation and diltiazem inhibition of calcium channels in isolated rabbit aorta and mesenteric resistance vessels.

The mechanisms of norepinephrine stimulation of calcium ion entry in isolated rabbit aorta and mesenteric resistance vessels were studied through measurements of effects on calcium-45 influx, tension, and membrane potential. The resistance vessels were considerably less sensitive to norepinephrine than the aorta. The aorta exhibited complex dose-response curves for norepinephrine-stimulated calcium influx and contraction, whereas these were simple in the arterioles. Both vessels were depolarized with increasing concentrations of potassium. Norepinephrine did not depolarize the aorta, whereas it did depolarize the mesenteric resistance vessels. This result supports the contention that norepinephrine opens receptor-operated channels to induce calcium entry in the aorta, while it may activate potential sensitive calcium channels in the mesenteric resistance vessels. However, the maximum depolarization with norepinephrine (10(-4) M) in the arterioles was completely blocked by 10(-5) M diltiazem, whereas that induced by 80 mM potassium was unaltered by the diltiazem. Furthermore, 10(-4) M norepinephrine was able to stimulate virtually the same contraction and calcium influx in 80 mM potassium-depolarized arterioles as in normal polarized tissues. These results are consistent with norepinephrine opening of receptor-operated channels to allow calcium entry in the rabbit mesenteric resistance vessels. That the behavior of norepinephrine-activated channels in the aorta is more complex than in the arterioles is further illustrated by a dramatically decreasing sensitivity of norepinephrine-stimulated calcium influx to diltiazem with increasing norepinephrine in the aorta but not in the arterioles.(ABSTRACT TRUNCATED AT 250 WORDS)

Possible Mechanisms of Inhibitory Action of Sodium Nitroprusside on Histamine-Induced Contractile Responses in Isolated Rabbit Aorta Compared with Basilar Artery and Taenia Coli

Possible mechanisms of the inhibitory actions of sodium nitroprusside (NaNP) on histamine-induced contractile responses of isolated rabbit aorta, basilar artery and taenia coli were studied. NaNP (3×10−5 M) reduced maximum contractile response to histamine in the aorta, whereas a concentration (10−4 M) of NaNP slightly shifted the concentration-response curve for histamine towards its higher concentrations, but did not influence the maximum response in the basilar artery and taenia coli. The reduction in the maximum response of the aorta by NaNP was reversed by removal of the endothelium, or treatment with indomethacm (5 × 10−6 M) or quinacrine (5×10−6 M), but not by nordihydroguaiaretic acid (NDGA) (5×10−5 M). Prostaglandin (PG) E1 PGE2 and PGI2 produced a relaxation in histamine-contracted basilar artery and taenia coli, whereas PGE1 and PGE2 produced a weak relaxation in histamine-contracted aorta. PGD2 relaxed taenia coli contracted by histamine, but had no influences in aorta and basilar artery. From these results, it is concluded that the inhibitory action of NaNP on histamine-induced contractile response in isolated rabbit aorta may be mediated at least partly by endothelium-derived arachidonic acid metabolite(s) via the cyclooxygenase pathway which must not involve PGI2. Furthermore, the difference in the inhibitory actions of NaNP between the tissues used in this study must be attributed to the absence of such mechanisms both in the basilar artery and taenia coli.

Studies on YM-12617: a selective and potent antagonist of postsynaptic alpha 1-adrenoceptors.

YM-12617, 5-[2-[[2-(2-ethoxyphenoxy)ethyl]-amino]propyl]-2 -methoxybenzenesulfonamide HCl is a structurally new type of extremely potent alpha 1-adrenoceptor antagonist. Its alpha-adrenoceptor blocking properties have been compared with those of prazosin, phentolamine and yohimbine using both pharmacological and 3H-ligand binding techniques in vitro and in vivo. In the isolated rabbit aorta, a tissue known to contain mainly alpha 1-adrenoceptors at postjunctional sites, YM-12617 competitively antagonized noradrenaline-induced contraction with a pA2 value of 10.11. Although YM-12617 was also a competitive antagonist toward clonidine at prejunctional alpha 2-adrenoceptors in the isolated rat vas deferens, its affinity for these receptors (pA2 = 6.41) was 5,000 times lower than that displayed for the postjunctional alpha 1-adrenoceptors in the isolated rabbit aorta. YM-12617 displaced both 3H-WB 4101 and 3H-clonidine binding to rat brain membranes; however, the affinity of YM-12617 for alpha 1-adrenoceptors (pKi = 9.64) was 3800 times higher than that for alpha 2-adrenoceptors (pKi = 6.06). Based on pA2 values obtained in the isolated tissues and pKi values in the binding assays, YM-12617 was 2-18, 36-117 and 1,740-5,750 times more potent than prazosin, phentolamine and yohimbine in antagonizing alpha 1-adrenoceptors, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of bitis nasicornis venom on isolated rabbit aorta: By A.K. TILMISANY, O.H. OSMAN and A. ABDEL AZIZ. Department of Pharmacology, College of Medicine, King Abdulaziz University, Jeddah, P.O. Box 9029, Saudi Arabia

Effect of bitis nasicornis venom on isolated rabbit aorta: By A.K. TILMISANY, O.H. OSMAN and A. ABDEL AZIZ. Department of Pharmacology, College of Medicine, King Abdulaziz University, Jeddah, P.O. Box 9029, Saudi Arabia

Pharmacological action of 5'-methylthioadenosine on isolated rabbit aorta strips.

The effects of 5'-methylthioadenosine (MTA) and adenosine analogues on isolated rabbit thoracic aorta strips were studied in vitro. High concentrations (500-1,000 microM) of adenosine analogues produced dose-dependent relaxation in isolated rabbit thoracic aorta strips. The relative potencies of relaxant effect were MTA greater than N6-phenylisopropyladenosine greater than 2-chloroadenosine on a molar basis. MTA (50-1,000 microM) suppressed the contraction induced by norepinephrine in isolated rabbit thoracic aorta strips in a concentration-dependent manner. Nucleoside uptake inhibitor dipyridamole did not impair the MTA actions. Pretreatment of the aorta strips with theophylline, an adenosine receptor antagonist, blocked the actions of MTA. MTA showed a relaxant effect in KCl-contracted aorta suggesting that MTA did not affect the metabolism or reuptake of norepinephrine. The present experiments suggest that MTA has a pharmacological action on the arterial smooth muscle cells mediated through adenosine receptors.

P4H0210) - Effect of Ca-blockers on 86Rb-efflux and 45Ca-uptake in isolated rabbit aorta durine high K stimulation

P4H0210) - Effect of Ca-blockers on 86Rb-efflux and 45Ca-uptake in isolated rabbit aorta durine high K stimulation

Studies on the mechanism of action of the vasoconstrictive dihydropyridine, CGP 28392

The effects of the vasoconstrictive dihydropyridine, CGP 29392, upon tension development and 45Ca 2+ uptake by vascular smooth muscle were studied using isolated rabbit aorta. CGP 28392 did not elicit contractile responses when administered alone but lowered the threshold for the contractile response to elevated extracellular K +. CGP 28392 also increased the magnitude of the contractions elicited by submaximal concentrations of K +. This effect was competitively antagonized by PY 108-068, a Ca antagonist of the dihydropyridine class. Similarly, the relaxing effects of PY 108-068 upon KCl-constricted rabbit aorta were competitively antagonized by CGP 28392. The calcium content of unstimulated tissues was only minimally increased by CGP 28392 but the 45Ca 2+ uptake stimulated by depolarizing levels of K + was markedly augmented by this agent. Further analysis of the data indicated that CGP 28392 did not alter the relationship between tension development and 45Ca 2+ uptake. In contrast, CGP 28392 was much less effective in augmenting the contractile response and 45Ca 2+ uptake elicited by noradrenaline. These results thus support the hypothesis that CGP 28392 predominantly facilitates Ca 2+ entry through potential-operated Ca 2+ channels.

A comparative study on possible calcium antagonistic properties of indapamide and other drugs potentially interfering with calcium transport in isolated vascular smooth muscle

In the present study the effects of indapamide (IN) were compared with those of chlorthalidone (C) and of drugs potentially interfering with calcium transport, e.g. verapamil (V), papaverine (P), phentolamine (PH) and diazoxide (D) using isolated rabbit aorta in order to detect calcium antagonistic properties. IN and C failed to show any relaxation effect towards either noradrenaline (NA)- or high K +-precontracted aorta strips, whereas V and P reduced the K + contraction dose dependently and PH, V, P and D induced a significant relaxation of the NA contraction. Preincubation with IN or C did not affect the dose-response curve of NA-induced contractions. PH antagonized the NA contraction strongly whereas V and P shifted the dose-response curve to the right only at the highest concentration. In a Ca 2+-depleted and high K +-depolarized aorta preincubation with V was able to antagonize Ca 2+-induced contraction effectively in a dose-dependent fashion whereas P only showed partial inhibition at the highest concentration. The effects of the drugs on responses to NA in a Ca 2+-free medium were also investigated. This type of contraction is likely to be due to mobilization of internally located Ca 2+. The results demonstrate that PH was able to counteract this type of contraction effectively whereas only a high concentration of V, P and D was effective in reducing the Na contraction in Ca 2+-free medium. Both IN and C failed to affect this type of contraction. In conclusion, the present results indicate that neither IN nor C possess calcium entry and or intracellular calcium antagonistic properties under different conditions as measured in a conduit vessel of rabbits. Moreover, the results point to an additional site of calcium antagonistic action for V, P and D, especially at higher concentrations.

Purification, sequencing and synthesis of natriuretic and vasoactive rat atrial peptide.

Mammalian atria contain potent natriuretic and diuretic substances which exist in high- and low-molecular-weight forms and which appear to be associated with atrium-specific granules. The natriuretic effect of atrial extract is largely accountable for by its renal haemodynamic effects; atrial extracts also antagonize hormone- and non-hormone-induced contraction of the isolated rabbit aorta and isolated rat kidney vasculature. We have completely purified a low-molecular-weight natriuretic and vasoactive substance from rat atria and characterized it as a 24-amino acid peptide. Synthetic peptide, produced by solid-phase synthesis, mimics biological effects of crude atrial extract and purified peptide; its activity is enhanced by slow oxidation, suggesting a disulphide (Cys 4-Cys 20) configuration for the native peptide. If secreted into blood, this atrial natriuretic peptide (' auriculin B') could be a novel peptide hormone of considerable importance to renal and cardiovascular homeostasis.

Action of oxmetidine on the rabbit aorta: Comparison with some calcium entry blockers and nitroglycerin

The novel histamine H2 receptor antagonist, oxmetidine, was tested on isolated rabbit aorta for its activity on calcium ion transport and/or utilization suggested from previous experiments. Its effect was compared with that of some "classical" calcium entry blockers (verapamil, D-600 and nifedipine) and of nitroglycerin which acts predominantly on the intracellular calcium. The effect of all these compounds was evaluated against the contraction elicited by KCl, angiotensin and noradrenaline acting by different spasmogenic mechanisms. Oxmetidine was found to antagonize to approximately the same extent the effect of the three stimulatory substances: however concentrations required for this effect were from 10 to 100 times as high as those required to inhibit histamine H2 receptors. Nitroglycerin behaved quite similarly but was approximately 10.000 times more potent than oxmetidine. Verapamil and D-600 (which were virtually identical) were always more potent (about 10 times) than oxmetidine but less efficacious on the angiotensin-induced contraction. Finally nifedipine was the most active compound on the KCl induced contraction but was almost ineffective against angiotensin. It is concluded that oxmetidine, though with a still unclear mechanism, possesses a weak but definite inhibitory effect on the availability of the intracellular calcium resembling the effect of nitroglycerin. Whether or not an action on the influx of calcium is also present cannot be surely established as long as calcium fluxes are not measured. Differences in the behaviour of the "classical" calcium entry blockers suggest that each of them possesses, together with the main well recognized effect, additional effects responsible for apparently surprising results obtained against different spasmogenic compounds.

Angiotensin Tachyphylaxis in the Isolated Rabbit Aorta

The effect of modifications of the N-terminal end of the angiotensin II (AII) molecule on its ability to induce tachyphylaxis in the isolated rabbit aorta was studied by analyzing the effects of several analogs of AII. No tachyphylaxis to AII was observed, but (1-sarcosine)-AII, (1-guanido-acetic)-AII and (1-glycine)-AII induced marked tachyphylaxis. (1-Betaine)-AII, (2-lysine)-AII, (2-ornithine)-AII and (1-sarcosine,2-lysine)-AII were unable to induce tachyphylaxis in the rabbit aorta. A positively charged N terminus and the guanidino group of the Arg2 side chain appear to be needed for the manifestation of the tachyphylactic property, while the Asp1 side chain is responsible for the absence of tachyphylaxis to AII. It is proposed that the analogs that are able to induce tachyphylaxis, after binding to the receptor to produce the agonistic response, induce a slow conversion of the hormone-receptor complex into a tachyphylactic state, and that this conversion is promoted by the interaction of the N terminus and the guanidino group of the analog with their complementary sites.

Characterization of the Thrombin-Induced Contraction of Vascular Smooth Muscle

Purified human alpha-thrombin induced a sustained contraction of isolated rabbit aorta and dog coronary arteries. These vascular tissues also exhibited a refractoriness towards a second thrombin exposure. The extent of tachyphylaxis exhibited by the aorta correlated with the initial concentration of thrombin and the length of time the tissue was exposed to thrombin. The thrombin-induced contraction in the aorta was not blocked by phospholipase or cyclooxygenase inhibitors, but it was inhibited in the presence of hirudin, heparin, nitroglycerin, and nitroprusside. Nitroglycerin, nitroprusside, and hirudin also inhibited the contraction in the dog coronary artery. Ca++ channel blockers did not inhibit the thrombin-induced contraction in the coronary artery, although a small inhibition was observed in Ca++-free media. In both tissues, equivalent contractile responses were obtained using equimolar quantities of beta-, tetranitromethane-, and alpha-thrombin, even though the latter's coagulant activity was 30-40 times that of the modified thrombins. However, if the catalytic activity of thrombin was inhibited by modification with Tos-Lys-CH2Cl, hirudin, or heparin/antithrombin III, the vasoconstrictor activity was also lost. These studies suggest that alterations of the fibrinogen-binding site do not affect the contractile activity of thrombin. The contraction may be the result of a proteolytic interaction of the active site of the enzyme with vascular smooth muscle.

Activated complement and anaphylatoxins increase the in vitro production of prostacyclin by rabbit aorta endothelium.

The effect of activated human serum complement and highly purified anaphylatoxins on the production of prostacyclin (PGI2) by endothelium of the isolated rabbit aorta was investigated. The results indicated that complement activation with endotoxin (LPS) or cobra venom factor (CVF) led to the generation of principles that stimulated PGI2 formation. A similar effect was seen with tryptic cleavage products of complement factors C5 and C3, suggesting the possible involvement of anaphylatoxins. Indeed, on molar base purified porcine C5a and C5a des Arg were at least 1000 times more potent than other vasoactive inflammatory mediators, as stimulators of vascular PGI2 release. Therefore we suggest that complement-mediated stimulation of vascular PGI2 production contributes to the decreased peripheral vascular resistance during endotoxic shock in rabbits. We further propose that C5a-dependent stimulation of PGI2 formation may dilatate resistance vessels, thereby increasing local blood flow. Together with the vascular permeability effects of C5a, this may provide a local regulatory mechanism for histamine- and bradykinin-independent oedema formation during inflammatory reactions.

Effects of vanadate on isolated vascular tissue: biochemical and functional investigations.

Vanadate is a potent inhibitor of Na+,K+-ATPase derived from bovine aorta. The Ca2+, Mg2+-ATPase of the same preparation was inhibited at 10 times higher concentrations. Compared with [3H]ouabain, 48V bound quickly to bovine aortic microsomes. Equilibrium binding experiments revealed one high-affinity, low-capacity and one low-affinity binding site for 48V, whereas [3H]ouabain possessed only one binding site of high affinity. A high NADH-vanadate reductase activity was measured in the same preparation, suggesting that, in this tissue, vanadate may be converted to vanadyl, a form to which the Na+,K+-ATPase is relatively insensitive. An increase in the contractile force of isolated rabbit aorta was measured with the following potency: phenylephrine greater than ouabain greater than vanadate. The order in intrinsic activity was as follows: phenylephrine congruent to ouabain greater than vanadate. The action of vanadate was rapid in onset and stable over several hours, while that of ouabain was slow and transitory. Vanadate increased tension in isolated rabbit veins to an extent similar to phenylephrine, but at concentrations two orders of magnitude higher. Vanadate action decreased with decreasing (Ca2+)0, but remained constant at a constant ratio of (Ca2+)0/(Na+)2(0). Vanadate-induced increases in tension were decreased by verapamil by about 43% and persisted in a solution in which Na+ was replaced by Li+. Vanadate increased electrically stimulated contractions. It is concluded that most of the effect of vanadate is due to an increase in calcium influx into the smooth muscle cell and that the effect of vanadate on Na+,Ca2+ exchange is of minor importance.

Ca-antagonistic substance from soft coral of the genus Sarcophyton.

A 14-membered ring diterpenoid named cembrane, possessing Ca-antagonistic action on the isolated rabbit aorta, has been isolated from a soft coral of the genus Sarcophyton, and its structure established by X-ray and spectroscopic data.

Effects of an extract of Ginkgo biloba and diverse substances on the phasic and tonic components of the contraction of an isolated rabbit aorta

1. 1. The effects of phentolamine, propranolol, D 600, theophylline, papaverine and an extract of Ginkgo biloba were studied with respect to the two phases of the contractile response induced by norepinephrine in an isolated rabbit aorta. 2. 2. Phentolamine (3 × 10 −6 M) inhibits the rapid phase of the contraction of the rabbit aorta brought on by norepinephrine (NE) 10 −5 M more strongly than the tonic phase. Propranolol (10 −6 M) potentiates this rapid phase. 3. 3. D 600 inhibits the slow phase with an EC 50 = to 3.8 × 10 −8 M. 4. 4. Papaverine and theophylline increase the relaxation that follows the rapid phase of contraction. The slow phase is inhibited only by papaverine. 5. 5. The extract of Ginkgo biloba (Gb) at a concentration of 3 mg/ml has the same type of effect as papaverine 3 × 10 −5 M.

Modification by cadmium ions of 45calcium uptake by isolated rabbit aortae.

Treatment with 0.5 mM Cd++ suppressed the K+ (30 mM)-stimulated uptake and influx of 45Ca and abolished the K+-induced contraction in helical strips of rabbit aortae. The rapid and slow efflux of 45Ca previously accumulated was not appreciably affected by Cd++. It is concluded that interference with the transmembrane influx of Ca++ is a major mechanism of Cd++ action.

Generation of a Vasoactive Substance in Human Plasma during Coagulation

A stimulant of vascular smooth muscle contraction was generated in fresh, citrated human plasma during activation of the clotting system. Plasma, exposed briefly to thromboplastin and Ca<sup>++</sup>, induced a contraction of isolated rabbit aorta and dog coronary arteries that was slow in development and persisted after washout. The contractile activity was not blocked by phenoxybenzamine, atropine, or angiotensin inhibitor, but was blocked when heparin or hirudin was incubated with the plasma. The contractile stimulant produced in the plasma was short-lived (<3 min) and paralleled the appearance of thrombin in plasma. Purified human α-thrombin also induced a sustained contraction in these blood vessels that was not inhibited by phenoxybenzamine, atropine, or angiotensin inhibitor, but was blocked by hirudin. Partial relaxation of the thrombin-treated blood vessel was achieved by the addition of heparin. These results suggest that this vasoactive component of thromboplastin-activated human plasma is α-thrombin. Because of its potent and persistent effects, thrombin-induced vasospasm may be an important mechanism in the etiology of ischemic heart disease.

α1-Adrenoceptors induce Ca influx and intracellular Ca release in isolated rabbit aorta

α1-Adrenoceptors induce Ca influx and intracellular Ca release in isolated rabbit aorta