Isolated Rabbit Aorta Research Articles

Rp-8-Br-guanosine-3′,5′-cyclic monophosphorothioate inhibits relaxation elicited by nitroglycerin in rabbit aorta

To ascertain whether the activation of cyclic GMP-dependent protein kinase is involved in the relaxant effects of nitroglycerin, the effects of Rp-8-Br-guanosine-3′,5′-cyclic monophosphorothioate (Rp-8-Br-cGMPS), an inhibitor of activation of G-kinase by cyclic GMP, were studied. In the isolated rabbit aorta contracted by phenylephrine, Rp-8-Br-cGMPS (30 μM) competitively inhibited the relaxation elicited by 8-Br-cGMP, but not that elicited by 8-Br-cyclic AMP, indicating that Rp-8-Br-cGMPS is a specific inhibitor of activation of cyclic GMP-dependent protein kinase by cyclic GMP. The relaxation elicited by nitroglycerin was inhibited by Rp-8-Br-cGMPS.

Studies on zwitter-ionization of drugs. III. Synthesis and pharmacological activities of N-alkylcarboxylic acid derivatives of 1,2,3,4,10,14b-Hexahydrodibenzo[c,f]-pyrazino[1,2-a]azepine and 2,3,4,9-tetrahydro-1H-dibenzo[3,4: 6,7]cyclohepta[1,2-c]pyridine

The N-alkylcarboxylic acids of 1,2,3,4,10,14b-hexahydrodibenzo[c,f]pyrazino[1,2-alpha]az epi ne (6a) and 2,3,4,9-tetrahydro-1H-dibenzo[3,4: 6,7]cyclohepta[1,2- c]pyridine (6b) were synthesized and examined for pharmacological activities in vitro: an inhibitory effect on the monoamine [noradrenaline (NA) and 5-hydroxytryptamine (5-HT)] uptake into the rat crude synaptosome, an inhibitory effect on the 5-HT- and NA-induced contraction in the isolated rabbit aorta and on the histamine- and acetylcholine-induced contraction in the isolated guinea-pig ileum, and binding affinity for alpha 2-adrenoceptor and D2-receptor. The in vitro tests indicated that zwitter-ionization was capable of maintaining antihistaminic activity while greatly reducing other pharmacological activities such as effects on central nervous system. 3-[2,3,4,9-Tetrahydro-1H-dibenzo[3,4: 6,7]cyclohepta[1,2- c]pyridin-2-yl]propionic acid (6b-2), selected as a candidate antiallergic agent having equally potent activities in rats and guinea-pigs, exhibited strong inhibitory effects on 48 h homologous passive cutaneous anaphylaxis (PCA) in rats (ED50 = 0.012 mg/kg, p.o.) and on histamine-induced bronchoconstriction in anesthetized guinea-pigs (ED50 = 0.0088 mg/kg, p.o.).

In vitro cardiac pharmacology of the new histamine H2-receptor agonist amthamine: Comparisons with histamine and dimaprit

The cardiac activity of the novel histamine H2-receptor agonist amthamine was investigated in a variety of isolated heart preparations from guinea pigs and humans and in the isolated rabbit aorta. Amthamine caused an increase in the sinus rate of spontaneously beating guinea-pig atria (pD2 = 6.72) and in the contractility of the electrically driven guinea-pig papillary muscle (pD2 = 6.17) and of the human atrium (pD2 = 5.38). In all these systems, amthamine behaved as a full agonist with a potency comparable to or slightly higher than that of histamine and 10 times higher than that of dimaprit. The positive effects of amthamine were competitively antagonized by ranitidine which had pA2 values (6.46 and 6.25 in the guinea-pig atria and papillary muscle, respectively) comparable with those calculated against histamine and dimaprit. In the isolated rabbit aorta amthamine was devoid of H1-mediated activities up to 3 x 10(-4) M. These results indicate that amthamine is a potent and selective histamine H2-receptor agonist which can be considered a valuable tool for investigating H2-receptor mediated effects in cardiac tissues.

Effect of TMB-8 on α-adrenoceptor agonist and KCl induced-contractions in isolated rabbit aorta

1. 1. TMB-8 (10 −6 M–10 −4 M) depresses the contractile effect of the selective α 1-adrenoceptor agonists methoxamine and phenylephrine in the isolated rabbit aorta. 2. 2. TMB-8 also depresses contractions evoked by 80 mM KCl in this tissue when used at similar concentrations. 3. 3. The calcium antagonist nifedipine potentiates the inhibitory effect of TMB-8 on the α 1-contractions. 4. 4. In preparations mounted in Ca-free solution containing 0.5 mM EGTA, 10 −4 M TMB-8 markedly depressed the contractions caused by both α 1-adrenoceptor agonists. 5. 5. The Ca 2+ agonist BAY K 8644 (10 −6 M) partially prevented the inhibitory effect of TMB-8 on 80 mM KCl contractions.

Kinetic characterization of adenosine A 2 receptor-mediated relaxation in isolated rabbit aorta

Previous studies in our laboratory (Wiener et al., 1991, Soc. Neurosci. Abstr. 17, 989) have addressed aspects of the functional antagonism between the responses mediated by activated adenosine A 2 receptors and α 1-adrenoceptors in adventitia- and endothelium-denuded rabbit thoracic aortic rings by steady-state protocols which ignore the time course of response generation. In the present communication we describe aspects of the time-dependent kinetics of relaxation responses to adenosine A 2 receptor agonists in tissues pre-contracted with the α 1-adrenoceptor agonist phenylephrine. The results were analyzed by application of the model originally developed by Keitz et al. (1990, J. Pharmacol. Exp. Ther. 255, 650) to describe the relaxation response, to a β-adrenoceptor agonist, as a first-order exponential decrease in tissue tension over time to estimate the apparent rate constant for relaxation (k rel) and the magnitude of relaxation at equilibrium. The magnitude of the relaxation responses to adenosine, N 6-cyclohexyladenosine, N 6-methyladenosine, 5′-N-ethylacarboxamidoadenosine, and R(−)-N 6-(2-phenyl-isopropyl) adenosine were agonist concentration-dependent and saturable, as were the apparent rate constants for relaxation. In addition, the magnitude of the apparent rate constants for relaxation and the relaxation responses were inversely proportional to the fractional occupancy of the α 1-adrenoceptor. The hypothesis put forth by Keitz et al. that the maximal value of the apparent rate constant for relaxation may serve as the kinetic definition of agonist efficacy was also tested and found to be invalid for the adenosine A 2 receptor. We propose that this pair of activated receptors and tissue preparation is a good model to study quantitative aspects of functional antagonism by kinetic paradigms.

Extraneuronal uptake and O-methylation of 3H-adrenaline in the rabbit aorta.

The influence of uptake2 inhibitors on the O-methylation and accumulation of 3H-adrenaline by the isolated rabbit aorta was studied. Strips were incubated with 0.05 mumol/l 3H-(-)-adrenaline during 15 min. Monoamine oxidase and uptake1 were inhibited and the 3H-adrenaline present in the tissue was measured as well as the metabolites found in the tissue and in the incubation fluid. In another series of experiments, monoamine oxidase, uptake1 and catechol-O-methyl transferase (COMT) were inhibited, and tritium accumulation was measured in the tissue. When COMT was inhibited, inhibitors of uptake2 produced a maximal reduction of 3H-adrenaline accumulation that did not exceed 50%. When COMT was intact, inhibitors of uptake2 diminished total 3H-removal and, more markedly, O-methylation and concomitantly increased the tissue content of 3H-adrenaline. Mineralocorticoids (corticosterone and deoxycorticosterone acetate) inhibited 3H-adrenaline uptake (when COMT was inhibited) and 3H-metanephrine formation (when COMT was functional) as effectively as did sexual steroids (17-beta-oestradiol, progesterone and testosterone); hydrocortisone (hemisuccinate or phosphate) had no effect (for concentrations up to 120 mumol/l). At the end of the incubation some strips were washed out with amine-free solution. Compartmental analysis of the efflux showed that the amine had distributed into three extraneuronal compartments (compartment I, II and III, with half times of 0.4, 4 and 15 min, respectively). Corticosterone (120 mumol/l) decreased the amount of 3H-adrenaline in compartment III and simultaneously increased the amount of the amine in compartment I (extracellular space).(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of zooxanthellatoxins, novel vasoconstrictive substances from the zooxanthella Symbiodinium sp

New polyhydroxypolyenes with potent vasoconstrictive activity, zooxanthellatoxin-A and -B, were isolated from a symbiotic marine alga Symbiodinium sp. These compounds caused sustained contractions of isolated rabbit aorta at concentrations above 7 × 10 −7 M; this effect was abolished in Ca 2+-free solution or in the presence of verapamil. Both compounds were relatively large molecules (mol. wt about 2900), containing a large number of oxygen atoms and olefinic carbons, thus differing from two other vasoconstrictive marine toxins, palytoxin and maitotoxin, in containing more olefins than palytoxin, and fewer ethereal rings than maitotoxin.

Evidence that [Sar 1]angiotensin II behaves differently from angiotensin II at angiotensin AT 1 receptors in rabbit aorta

Three peptide analogues, [Sar 1]angiotensin II, angiotensin II and [Asn 1,Val 5]angiotensin II, that act at angiotensin AT 1 receptors were compared in an isolated rabbit aorta assay. Significant differences have been found among them in agonist profiles and agonist-antagonist interactions with losartan, a nonpeptide antagonist selective for AT 1 receptors. Most significantly, underestimation of the antagonist potency for losartan with a flat Schild plot was obtained with [Sar 1]angiotensin II. These findings were confirmed in further examinations with representative peptide antagonists including [Sar 1]angiotensin II. These failure of PD123177, a nonpeptide antagonist selective for AT 2 binding sites, to induce any significant difference in the complex antagonism of [Sar 1,Phe(Br 5) 8]angiotensin II to angiotensin II appeared to rule out significant involvement of AT 2 binding sites in the differences observed among the agonists, as well as in the complex antagonism. On the basis of the present findings it is speculated that either a saturable agonist removal process or heterogeneous sub-populations of AT 1 receptors may be involved.

Effect of ofloxacin on adrenergic contractile mechanisms in isolated rabbit aorta

Effect of ofloxacin on adrenergic contractile mechanisms in isolated rabbit aorta

O-299 - inhibitors of sarcoplasmic reticulum (SR) Ca2+ pump ATPase inhibit the nitroglycerin (NG)-induced relaxation in the isolated rabbit aorta.

O-299 - inhibitors of sarcoplasmic reticulum (SR) Ca2+ pump ATPase inhibit the nitroglycerin (NG)-induced relaxation in the isolated rabbit aorta.

Pharmacological characterization of abbott-81282, a novel, non-peptide angiotensin-II antagonist selective for type-1 receptors

Abbott-81282 (A-81282) has been identified among a series of related compounds as being a highly potent and selective antagonist of angiotensin receptors. At AT 1 receptors of the rabbit aorta, A-81282 exhibited a pA 2 of 9.64 (±0.33) vs. angiotensin-II, and demonstrated characteristics consistent with competitive antagonism of this receptor. These results were supported in radioligand binding assaya in which A-81282 inhibited the binding of [ 125I]-Sar 1-lle 8-Angiotensin-II to rat liver membranes with a pK I of 8.505 (±0.102). Selectivity of this agent for AT 1 receptors was validated by its lack of activity at other receptor sites, such as α 1 receptors of isolated rabbit aorta. Moreover, A-81282 lacked affinity for AT 2 receptors of bovine cerebellar membranes or for α or β adrenergic receptor sites in radioligand binding assays. A-81282 lowered blood pressure significantly in vivo in renal artery-ligated rats at doses of 1 mg/kg i.v. or 5 mg/kg p.o. The compound was slowly and moderately absorbed from the duodenum of anesthetized rats and demonstrated low first-pass metabolism in the rat liver. Because of its selectivity and potency for antagonizing AT 1 receptors, and its activity in lowering blood pressure in experimental animals, A-81282 has the potential to be a useful antihypertensive agent in man.

Effects of lysolipids and oxidatively modified low density lipoprotein on endothelium-dependent relaxation of rabbit aorta.

Exposure of isolated arteries to oxidatively modified low density lipoprotein (LDL) has been reported to suppress endothelium-dependent relaxation (EDR). To determine whether lipid degradation products in oxidized LDL contribute to impaired relaxation, we have tested the responsiveness of isolated rabbit aortas to endothelium-dependent relaxants (acetylcholine, ATP, and calcium ionophore A23187) and nitroglycerin before and after 2-hour incubations with selected lipids and LDL preparations. Concentrations (10 microM) of lecithin, phosphatidylserine, lysophosphatidylserine, sphingomyelin, phosphatidic acid, palmitate, arachidonate, and auto-oxidized arachidonate had no effect on EDR. Concentrations (10 microM) of lysolecithin, lyso-platelet activating factor, and sphingosine significantly suppressed endothelium-dependent relaxation. Native LDL (100 micrograms/ml incubation buffer) containing only small amounts of lysophosphatidylcholine exerted no effect on EDR. In contrast, LDL preparations oxidatively modified by exposure to cultured endothelial cells or copper inhibited EDR. When modified LDL was depleted of its lysolecithin by treatment with a selective phospholipase B (lysolecithinase), the inhibitory effects were attenuated. In contrast, native LDL accumulating lysolecithin under the influence of a phospholipase A2 (lecithinase) exerted inhibitory effects mimicking those of oxidized LDL. Lipids and lipoproteins had no effect on the responsiveness to nitroglycerin, an endothelium-independent vasodilator. We conclude that lysolecithin in oxidatively modified LDL contributes importantly to its vasomotor effects.

Role of glycolytic and oxidative metabolism in endothelium-dependent relaxation in isolated rabbit aorta

Role of glycolytic and oxidative metabolism in endothelium-dependent relaxation in isolated rabbit aorta

Kinetics of relaxation responses to vasorelaxants in isolated rabbit aorta

Transient responses of isolated tissues to drugs are best studied by application of non-steady-statc protocols in which the data collected are analyzed using kinetic models. The time dependence of the relaxation response of the adventitia- and endothelium-denuded rabbit aorta to four vasorelaxants (nitroglycerin, sodium nitroprusside, 5'-N-ethylca;boxamidoadenosine and isoprotcrcnol) was analyzed by an exploratory kinetic model. A rapid relaxation ( t 1 2 = 1–3 min ) was elicited by all vasorelaxants. An apparent dcscnsitization or fade of the relaxation response to nitroglycerin or isoprotcrcnol was visualized as the partial regaining of tissue tone ( t 1 2 = 2–3 min ). The relaxation responses to sodium nitroprusside or 5'-N-ethyIcarboxami-doadcnosine were stable for at least 60 min and did not exhibit an apparent regaining of tension. Tissues rendered desensitized by cither isoprotcrcnol or nitroglycerin responded fully to sodium nitroprusside or 5'-N-cthylcarboxamidoadenosinc. The rate constant for relaxation was vasorelaxant concentration-dependent and saturable for all vasorelaxants. For isoproterenol, nitroglycerin, and 5'-N-ethylcarboxamidoadenosine the rate constant for relaxation was inversely proportional to the contractile stimulus, as was the magnitude of relaxation for all vasorelaxants. Although the magnitude and rate constant of the fade was not concentration-dependent for isoproterenol, it was inversely proportional to the nitroglycerin concentration. The rate constant of the fade was proportional to the contractile stimulus for isoproterenol and nitroglycerin, and the magnitude of the fade was proportional to the contractile stimulus for nitroglycerin. We propose that kinetic studies of responses in isolated vasculature supersede studies performed under steady-state conditions, for they extend our knowledge of the manner by which the steady-state is achieved and allow for a quantitative analysis of the time-dependent changes which should assist in elucidating the biochemical basis of the observed physiological response.

Mechanism of muscular relaxation induced by ATP and adenosine on isolated rabbit aorta

Mechanism of muscular relaxation induced by ATP and adenosine on isolated rabbit aorta

Pharmacological characterization of ATP receptors mediating vasodilation on isolated rabbit aorta

1. 1. The effects of adenosine 5′-triphosphate (ATP), 2-methyl-thio-ATP and adenosine on rabbit aorta were examined in isolated preparations precontracted by noradrenaline, both in vessels where the endothelium was present and mechanically removed. 2. 2. In the presence of endothelium, ATP (30 μM-3 mM) induced a relaxation that was reduced by removal of the endothelium. 3. 3. The maximum endothelium-dependent relaxation of ATP was twice the maximum endothelial activity of the potent agonist at P 2y-purinoceptors, 2-methyl-thio-ATP. 4. 4. Adenosine which acts on P 1 purinoceptors, induced a relaxant effect at 1 mM concentration, both in the vessels with and without endothelium. 5. 5. It is concluded that relaxation by ATP is induced both via the endothelial P 2y-purinoceptor and via a “nucleotide” receptor that is located on endothelium and on smooth muscle of rabbit aorta.

Acetylcholine-induced relaxation of the rabbit aorta is preload-independent but markedly dependent upon the degree of excitatory agonist-induced tone.

1. The aim of the present study was to investigate the relationship between endothelium-dependent relaxation evoked by acetylcholine, tissue preload and the degree of noradrenaline-induced tone in the isolated rabbit aorta. 2. In the aorta preload-response curves were bell-shaped with increasing preload augmenting responses to noradrenaline up to a certain point (optimal value) and then declining. Removal of the endothelium significantly increased responses to low concentrations of noradrenaline (less than 0.1 microM) but did not significantly affect the maximum response of the aorta to this amine or the preload-response curves generated at several concentrations of noradrenaline. 3. The optimal preload value was around 10 g for the aorta and changes in preload did not influence the sensitivity of the tissue to noradrenaline as assessed by pEC50 values to this agonist. 4. Acetylcholine (0.01-10 microM) evoked endothelium-dependent relaxations which in absolute terms increased as the tissue preload was increased. This relationship was much less evident when acetylcholine responses were measured in terms of the percentage inhibition of the respective noradrenaline contraction, when little or no change in the acetylcholine responses was noted. 5. When acetylcholine relaxations were expressed in terms of a percentage of the maximum noradrenaline-induced response evoked at each preload setting, the results confirmed that preload changes had little or no influence upon acetylcholine responses. 6. In contrast, tissue sensitivity to, and the extent of acetylcholine-induced relaxation, were markedly affected by the level of excitatory agonist-induced tone. As noradrenaline-induced tone increased, maximum responses and pIC50 values to acetylcholine were reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of oxodipine on isolated rabbit aorta and mesenteric resistance vessels

The inhibitory effects of the dihydropyridine Ca 2+ antagonist, oxodipine, on contractions and 45Ca 2+ influx stimulated by noradrenaline (NA) and high K + in rabbit aorta were compared to the same parameters measured in mesentric resistance arteries. In aortic rings oxodipine, 10 −11−10 −6 M, inhibited in a concentration-dependent manner the contractions induced by high K + (IC 50 = 9.0±4.0×10 −10M) or by Ca 2+ in high K + solution (IC 50 = 6.2±2.4×10 −9M), while responses to NA were only slightly affected (IC 50>10 −6 M). In mesenteric resistance vessels oxodipine inhibited the contractions induced by high K + and NA but was more effective against NA- than high K +-induced contractions (IC 50 = 5.2±3.1×10 −10 and 1.2±1.8×10 −8 M respectively). The concentration-inhibition curves for high K +-induced contraction and 45Ca 2+ influx in aorta were almost superimposable (I 50 = 2.2±2.0×10 −9 M), whereas NA-induced contractions were inhibited less than 45Ca 2+ influx (I 50 = 8.2±2.6×10 −8 M), In mesentric resistance vessels the curves for contraction and 45Ca 2+ influx stimulated by high K + and NA were also superimposable, but 45Ca 2+ influx stimulated by NA was more sensitive to oxodipine than that stimulated by high K + (I 50 = 3.9±2.0×10 −10 and 2.2±1.2×10 −8 M, respectively). It is concluded that the effects of oxodipine can be attributed to its ability to inhibit Ca 2+ entry through both potential- and receptor-operated pathways. The higher sensitivity of NA-induced contraction and 45Ca 2+ influx in mesenteric resistance vessels to oxodipine as compared to aorta may be related not only to the fact that the contraction induced by NA in resistance vessels is nearly completely dependent on extracellular Ca 2+ but also to the high sensitivity of their NA-activated Ca 2+ entry.

Inhibition of nitric oxide synthase specifically enhances adrenergic vasoconstriction in rabbits.

1. The effect of inhibition of nitric oxide biosynthesis using N-nitro-L-arginine (NOLA) was examined in conscious rabbits and rabbit isolated aortae. 2. In autonomically blocked conscious rabbits intravenous infusion of NOLA (15 mg/kg) significantly increased arterial pressure and hindlimb vascular resistance but did not affect heart rate. Depressor and hindlimb vasodilator responses to acetylcholine (3-12 micrograms/kg per min) were significantly attenuated in the presence of NOLA. In contrast, NOLA significantly enhanced responses to intravenous infusion of glyceryl trinitrate (10-40 micrograms/kg per min) in vivo. 3. Infusion of noradrenaline (1-4 micrograms/kg per min) or the release of neuronal noradrenaline in response to the infusion of tyramine (80-320 micrograms/kg per min) increased arterial pressure and hindlimb vascular resistance in autonomically blocked conscious rabbits. After the administration of NOLA, the vasoconstrictor responses to both noradrenaline and tyramine were significantly enhanced. 4. In isolated rabbit aortae, NOLA (10 mumol/L) significantly impaired relaxant responses to acetylcholine but did not affect responses to glyceryl trinitrate. NOLA enhanced contractile responses to the adrenoceptor agonists noradrenaline and phenylephrine but did not affect the contractile responses to the thromboxane-mimetic U46619. 5. These data indicate that in autonomically blocked conscious rabbits, NOLA causes systemic vasoconstriction, impairs dilator responses to acetylcholine and enhances dilator responses to glyceryl trinitrate. In addition, NOLA enhances constrictor responses to both exogenous and neuronally-released noradrenaline. These results suggest that nitric oxide is important in the regulation of normal vascular tone and in the modulation of vascular responses to vasodilator and vasoconstrictor agents.

Vasodilating properties of KRN2391: structural basis of a new pyridine-type potassium channel opener with a nitrate moiety

The vasodilating mechanism of a new compound, cyanoimino-3-pyridylmethylaminoethyl nitrate methanesulfonate (KRN2391), a derivative of nicorandil, was examined in the isolated rabbit aorta. To elucidate the structure activity relationship, a comparison was made with the two denitrated derivatives: cyanoimino-3-pyridylmethylaminoethyl acetate methanesulfonate (Ki4032) and cyanoimino-3-pyridylmethylaminoethyl alcohol (Ki3315). In preparations precontracted with phenylephrine (10(-7) mol/l), KRN2391, Ki4032 and Ki3315 caused concentration-dependent relaxation. pD2 values (-log [EC50]) were 6.74 +/- 0.03, 5.67 +/- 0.05 and 3.63 +/- 0.03, respectively. Both methylene blue and glibenclamide produced a shift to the right of the concentration-response curves for KRN2391. The shift by glibenclamide became greater in the presence of methylene blue. An elevation of the cGMP content was not detected until the concentration of KRN2391 was increased to a level enough to produce a full relaxation (3 x 10(-6) mol/l). In contrast, in the case of Ki3315 a parallel shift to the right of the concentration-response curve was observed after glibenclamide (10(-5) mol/l). Methylene blue (10(-5) mol/l) had no effect on the concentration-response curve, and there was no increase in cyclic GMP (cGMP) with 10(-3) mol/l of the compound. The concentration-response curve of Ki4032 was also attenuated by glibenclamide. Though this compound lacks the nitrate moiety, it (10(-4) mol/l) showed a slight tendency to increase the cGMP content, and methylene blue slightly but significantly modified the concentration-response curve of this compound. However, the co-administration of glibenclamide and methylene blue resulted in no further modification of the concentration-relaxation curve.(ABSTRACT TRUNCATED AT 250 WORDS)