Voltage- and time-dependent inhibitory effects on rat aortic and porcine coronary artery contraction induced by propafenone and quinidine.

1. Class I antiarrhythmic drugs (e.g. Na+ channel blockers) such as propafenone and quinidine also inhibit voltage-gated Ca2+ and K+ channels. In the present paper the voltage- and time-dependent inhibitory effects of propafenone and quinidine were studied on depolarization-induced vascular contractions and 45Ca2+ uptake in isolated endothelium denuded rat aorta and pig left descending coronary artery. 2. Quinidine and propafenone (10(-7) M -5 x 10(-5) M) produced a concentration-dependent relaxation of the contractions induced by 80 mM KCl. Propafenone was significantly more potent (P < 0.05) than quinidine in both rat aorta and pig coronary arteries but both drugs more potent (P < 0.05) in relaxing rat aorta than pig coronary arteries. In rat aortic rings, the relaxant effects of propafenone were unaffected by pretreatment with the Na+ channel blocker, tetrodotoxin. 3. The degree of inhibition produced after prolonged exposure (40 min) to propafenone and quinidine differed as the time of depolarization with 80 mM KCl was increased. Quinidine (3 x 10(-6) M, 10(-5) M and 3 x 10(-5) M) not only produced an inhibition at the very early stage of contraction, but also a time-dependent inhibition was observed. In contrast, propafenone (10(-6) M, 3 x 10(-6) M and 10(-5) M) produced a more marked concentration-dependent early block but only a mild time-dependent inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vasodilation produced by forskolin compared with that produced by adenosine in rabbit coronary artery

We compared the relaxing action of forskolin with that of adenosine in rabbit coronary artery and analyzed the pharmacological properties of this effect of forskolin. In preparations of this artery precontracted by 10-16 mM KCl, the addition of forskolin (10(-9)-10(-5) M) and adenosine (10(-9)-10(-4) M) to the organ bath produced dose-dependent relaxations. The mean EC50 for the relaxing action of forskolin was 4.5 X 10(-8) M and that of adenosine was 3.6 X 10(-7) M, forskolin being 10 times more potent than adenosine. Relaxation produced by forskolin was not affected by treatment with propranolol (10(-5) M), atropine (10(-6) M), or 8-phenyltheophylline (10(-6) M), but was competitively inhibited by ouabain (10(-6) M). The relaxation produced by adenosine was inhibited by 8-phenyltheophylline (10(-6) M) competitively and by ouabain (10(-6) M) noncompetitively. In preparations precontracted by a higher concentration of KCl, 40 mM, forskolin produced full relaxation with a shift of the dose-response curve to the right; adenosine did not produce full relaxation. Both forskolin and adenosine attenuated the maximum contraction produced by Ca2+. These findings indicate that the vasorelaxing effect of forskolin was not due to activation of beta-adrenoceptors, muscarinic receptors, or adenosine receptors, whereas that of adenosine was due to activation of adenosine receptors. Increased availability of intracellular Ca2+ competitively inhibited the relaxation induced by forskolin and noncompetitively inhibited the relaxation induced by adenosine. Both forskolin and adenosine noncompetitively inhibited contraction induced by Ca2+.     

Nitrate tolerance induced by nicorandil or nitroglycerin is associated with minimal loss of nicorandil vasodilator activity.

In the current study, the vasodilator and tolerance-inducing actions of a recently developed organic nitrate vasodilator, nicorandil, were compared to nitroglycerin (NTG) in an isolated coronary artery preparation. The order of potency for relaxing U46619-constricted bovine-isolated coronary artery rings was NTG greater than isosorbide dinitrate (ISDN) greater than nicorandil. NTG was approximately 250-fold more potent than nicorandil (mean EC50 values for relaxation; 0.044 and 11.2 microM, respectively; n = 6-8). Coronary artery rings preexposed for 60 min to NTG (30 microM) were subsequently markedly less responsive to the relaxant effects of NTG (7.5-fold increase in mean EC50 value, 68.4% decrease in Emax; p less than 0.001) and ISDN (14.1-fold increase in mean EC50 value; p less than 0.001), although only marginally less responsive to nicorandil (1.75-fold increase in mean EC50 value; p less than 0.05). Thus, the coronary artery relaxant actions of nicorandil were significantly less affected by NTG-induced tolerance than were the relaxant actions of the related organic nitrate compounds, NTG and ISDN. To compare the tolerance-inducing actions of NTG and nicorandil, the relaxant actions of a series of nitric oxide (NO)-containing vasodilators were determined in control coronary artery rings and in rings preexposed for 60 min to either 30 microM NTG or 5,000 microM nicorandil. Quantitatively, similar changes in coronary artery ring responsiveness were produced by tolerance induced by NTG and nicorandil; marked attenuation of responsiveness to NTG and to the nonnitrate compound 3-morpholinosydnonimine (SIN-1), but only marginal attenuation of responsiveness to nicorandil and NO.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of vasorelaxants in human basilar arteries and umbilical arteries

The vasodilatation produced by adenosine, sodium nitrite, and papaverine was compared on isolated human basilar arteries and umbilical arteries precontracted with 30 mM KCl or submaximal concentrations of serotonin and prostaglandin F2 alpha. The basilar artery was far more sensitive to the vasodilators than the umbilical vessel. Papaverine, for instance, was 133 times more effective (EC50 = 3.9 x 10(-6) M) in basilar arteries precontracted with KCl than it was in the umbilical artery (EC50 = 5.2 x 10(-4) M) and all dilators inhibited by at least 95% basilar arteries precontracted with serotonin or prostaglandin F2 alpha. In contrast, umbilical arteries precontracted with KCl or prostaglandin F2 alpha failed to relax significantly to adenosine and sodium nitrite at concentrations that exceeded 10(-3) M, and contractions elicited by serotonin were inhibited by less than 40%. The results support the concept that physiological mechanisms responsible for dilation in most vessels are deficient in the umbilical artery and that the deficiency may be related to the role this atypical vessel plays in closure of the extracorporeal circulation at birth.     

Characteristics of the vasorelaxing action of (3E)-4-ethyl-2-hydroximino-5-nitro-3-hexamide FK409, a new vasodilator isolated from microbial sources, in isolated rabbit arteries.

We examined the vasoinhibitory effect of (3E)-4-ethyl-2-hydroximino-5-nitro-3-hexamide FK409, a new vasodilator, on contractile responses in isolated rabbit arteries. FK409 (10(-8)-10(-5) M) inhibited contractile responses to norepinephrine (NE), histamine (His), and 5-hydroxytryptamine (5-HT) in rabbit aorta. The pattern of inhibition by FK409 was not competitive. The inhibitory effect of FK409 on the 5-HT response was much greater than that of nitroglycerin (NG). A high concentration of FK409 (10(-5) M) was necessary to inhibit the response to KCl (10-70 mM). The effect of combined treatment with FK409 (10(-5) M) and a subthreshold concentration of nifedipine (10(-9) M) on the KCl response was much greater than a single treatment with either agent. In addition, 3 x 10(-6) M D600, but not FK409 (10(-6) or 10(-5) M), inhibited the increase in the rate of 45Ca influx stimulated by a 40-mM KCl substituted solution. In a Ca2(+)-free medium containing EGTA and nifedipine, FK409 (10(-9)-10(-5) M) inhibited phasic responses to NE, His, and 5-HT, and subsequent sustained responses owing to addition of Ca2+. The response to caffeine in rabbit iliac arteries incubated in Ca2(+)-free medium was also inhibited by FK409 (10(-6) and 10(-5) M). In rabbit aorta precontracted with NE (10(-5) M) and partially inhibited by prior exposure to NG (10(-5) M), the relaxing effect of FK409 was slightly attenuated. Pretreatment of tissues with FK409 (10(-6) M) inhibited the relaxing action of NG much more than prior NG inhibited the relaxing action of FK409. Methylene blue (10(-5) M), but not hemoglobin (10(-6) M), inhibited the relaxing action of FK409, whereas M&B 22,948 (3 x 10(-4) M) potentiated it. FK409 caused a relaxation of precontracted aorta without endothelium that was inhibited by methylene blue. In rabbit aorta precontracted with NE, FK409 (10(-6) M) increased cyclic GMP but not cyclic AMP content. FK409 (10(-5) M) had no effect on the NE-mediated increase in tissue inositol monophosphate (IP). These results suggest that FK409 inhibits the responses attributed to both intracellular Ca2+ release and Ca2+ influx through receptor-operated channels. The inhibitory effect of FK409 on both the KCl contractile response and KCl-stimulated 45Ca influx appears to be different from that of nifedipine or D600. Furthermore, the inhibitory action of FK409 may be partially mediated by cyclic GMP.     

Effect of nitroglycerine in popliteal preparations from patients with peripheral occlusive arteriopathy precontracted with KCl or 5-hydroxytryptamine

1. At the present time, there are no studies in isolated arteries from patients suffering from peripheral occlusive arteriopathy (POA). In the present study, we attempt to characterize the effect of nitroglycerine (GTN) in isolated popliteal preparations obtained after leg amputation in 60-90-year-old men and women suffering from POA. 2. After surgical operation, arterial samples were stored in a refrigerator at 4 degrees C and, after 12-36 h, they were cut into rings and mounted in organ baths containing Krebs'-Henseleit solution at 37 degrees C and gassed constantly with 95% CO2 and 5% O2. Because noradrenaline elicited very poor contractile responses in these preparations, in the present study we evaluated the concentration-dependent contractions induced by KCl (15-90 mmol/L) and 5-hydroxytryptamine (5-HT; 10-7 to 10-4 mol/L) in arteriopathic popliteal rings and, when the corresponding maximum contractile effect had been obtained, we also evaluated the concentration-dependent relaxing effect produced by GTN (10-10 to 10-5 mol/L) in all precontracted preparations. As a reference, similar experiments were performed in popliteal preparations obtained following surgery on non-arteriopathic vascular tissue where it was necessary to resect a certain percentage of healthy vessel. 3. The responses to KCl and 5-HT were greater in healthy vessel than in arteriopathic rings. The relaxing effect of GTN was greater in preparations precontracted with 5-HT than in those preparations precontracted with KCl. In addition, preparations precontracted with KCl relaxed even less when they were obtained from patients with POA. 4. The present data indicate that GTN is a vasodilator with little effect on depolarized arteries. The results also indicate that the effect of this drug is even less in depolarized arteries from patients with POA.     

Endothelium-independent relaxation of rabbit coronary artery by 17 beta-oestradiol in vitro.

1. We assessed the relaxant effect of 17 beta-oestradiol (10(-7), 10(-6) and 10(-5) M) on rabbit isolated coronary arteries precontracted with prostaglandin F2 alpha (3 x 10(-6) M), high extracellular potassium (30 mM) and Bay K 8644 (10(-6) M) plus high extracellular potassium (15 mM) by measuring isometric tension. 17 beta-Oestradiol (10(-6) and 10(-5) M) induced significant relaxation in coronary arteries from male and female rabbits. No differences were seen between arteries with or without endothelium. There were also no differences between coronary arteries isolated from male and female rabbits. 2. Inhibitors of endothelium-derived relaxing factor and vasodilator prostanoids, namely, reduced haemoglobin, N omega-nitro-L-arginine methyl ester and indomethacin, did not affect the relaxation induced by 17 beta-oestradiol in endothelium-intact coronary arteries. 3. Methylene blue, an inhibitor of guanylate cyclase, did not affect the coronary artery relaxation induced by 17 beta-oestradiol. 4. The calcium concentration-dependent contraction curve in potassium-depolarization medium was shifted to the right by 17 beta-oestradiol (10(-6) and 10(-5) M) in the rabbit coronary artery and rat aorta. The -log EC50s of calcium in control and after incubation with 17 beta-oestradiol (10(-6) and 10(-5) M) were 3.7 +/- 0.09, 3.1 +/- 0.10 and 2.8 +/- 0.08 respectively in rabbit coronary arteries and 3.8 +/- 0.11, 3.3 +/- 0.14 and 2.9 +/- 0.15 in rat aorta. 5. The results indicate that 17 beta-oestradiol induces rabbit coronary artery relaxation by an endothelium-independent mechanism in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)     

5-Hydroxytryptamine does not release endothelium-derived relaxing factor in rat isolated coronary arteries

Removal of the endothelium from isolated rat proximal and distal coronary artery segments shifted the 5-HT concentration-response curve to the left without affecting, the maximal contractile response. 5-HT had no relaxing effect in 10(-5) M prostaglandin F2 alpha-precontracted vessels with an intact endothelium in the presence of 10(-5) M ketanserin. The spontaneous myogenic tone increased in both proximal and distal coronary artery segments after the endothelium had been removed. Indomethacin (10(-5) M) reduced the response of the proximal coronary artery segments to 5-HT by 35% but indomethacin had no effect on the 5-HT concentration-response curve of the distal coronary artery segments. Indomethacin relaxed precontracted (40 mM potassium) proximal coronary artery segments independently of the presence of the endothelium, suggesting a non-specific relaxing effect of indomethacin in these arteries. It is concluded that rat coronary artery endothelium is unresponsive to 5-HT because it lacks 5-HT1 receptors. The increased 5-HT sensitivity and spontaneous myogenic tone of endothelium-denuded rat coronary arteries is probably due to the elimination of the relaxing stimulus mediated by spontaneously released endothelium-derived relaxing factor.     

Regional heterogeneity in the contractile and potentiating effects of neuropeptide Y in rat isolated coronary arteries: modulatory action of the endothelium.

1. Neuropeptide Y (NPY) induced a concentration-dependent contraction of isolated rings of proximal epicardial (PC) and distal intramural (DC) coronary arteries of the rat, with an EC50 of ca. 1 x 10(-7) M. The NPY-induced contraction at 3 x 10(-7) M was significantly smaller in PC than DC arteries: 34% vs. 55% of the 125 mM K(+)-induced response, respectively. 2. NPY (2 x 10(-8) M) increased the sensitivity to noradrenaline (NA) and 5-hydroxytryptamine (5-HT) more in PC (4.2 and 2.8 fold, respectively) than in DC arteries (2.2 and 1.4 fold, respectively). The maximal contractile response to NA and 5-HT was increased more in DC (43% and 29%, respectively) than in PC arteries (20% and 12%, respectively). 3. Removal of the endothelium increased the sensitivity and maximal response to NPY as well as the spontaneous myogenic tone in PC but not in DC arteries. NPY had no relaxing effect on PC and DC arteries submaximally contracted with 10(-6) M prostaglandin F2 alpha, suggesting that spontaneous rather than stimulated release of endothelium-derived relaxing factor (EDRF) depresses the contractile action of NPY in PC arteries. 4. The results indicate a heterogeneity in the contractile and potentiating action of NPY in rat coronary arteries depending on size or location in the coronary circulation.     

Vasodilatory effect in rat aorta of eriodictyol obtained from Satureja obovata

The vasodilator effect of eriodictyol (5,7,3',4'-tetrahydroxyflavanone), isolated previously from the medicinal plant Satureja obovata Lag., was studied in rat thoracic aorta rings. Eriodictyol relaxed in a concentration-dependent manner the noradrenaline (10(-6) M) and KCl (80 mM) induced contractions. The relaxant effect was more potent in noradrenaline precontracted preparations (IC50 = 6.11 +/- 0.2 x 10(-5) M) than in those precontracted with KCl (IC50 = 2.96 +/- 0.1 x 10(-4) M). Eriodictyol produced weakly concentration-dependent inhibition of the phasic component induced by KCl and noradrenaline while the inhibition of the tonic phase of these contractions was more pronounced. These effects were endothelium independent. In addition, eriodictyol (10(-5) and 5 x 10(-5) M) inhibited CaCl2 cumulative concentration response curves. Eriodictyol weakly inhibited the release of calcium from the sarcoplasmic reticulum and its contribution to the relaxant effect seems to be slight. We have also observed the relaxant effect of eriodictyol on phorbol-12-myristate-13-acetate (PMA) (10(-7) M) induced contractions both in normal calcium (IC50 = 4.69 +/- 0.3 x 10(-5) M) and calcium-free medium (IC50 = 3.74 +/- 0.4 x 10(-5) M). Finally we studied the effects on protein kinase C (PKC) activity. This flavonoid did not show any activity. These results suggest that the vasodilator effect of eriodictyol in rat thoracic aorta could be partially related to the inhibition of calcium influx or other enzymatic protein subsequent to activation of PKC related to the activation of contractile proteins like myosin light chain kinase (MLCK).     

In vitro vascular effects of cicletanine in pregnancy-induced hypertension.

1. The vascular effects of cicletanine have been studied in vitro on ring preparations of inferior epigastric arteries from normotensive human females and human females with pregnancy-induced hypertension (preeclampsia). 2. Cicletanine (10(-7)-10(-3) M) elicited concentration-dependent relaxation of vessels precontracted with 10(-7) M noradrenaline (NA) or 60 mM K+ but was more potent in the former. Relaxation was significantly greater in rings from preeclamptic patients and was uninfluenced by endothelium removal. 3. The intracellular Ca-dependent contractile responses to 10(-5) M NA in Ca-free medium as well as the subsequent extracellular Ca-dependent contractions (on restoration of external Ca) were significantly attenuated dose-dependently by cicletanine (10(-5) M, 3 x 10(-4) M) in arterial rings from both normotensive and preeclamptic patients. Cicletanine also relaxed rings precontracted by 25 mM K+ but was ineffective against 80 mM K(+)-induced contractions. 4. The inhibition of intracellular Ca-dependent contractions was significantly greater in rings from preeclamptic than from normotensive patients whereas extracellular Ca-dependent contractions were comparably inhibited in both groups. Nifedipine, on the other hand, had little effect on the intracellular Ca-dependent contractions but significantly depressed extracellular Ca-dependent contractions. 5. Cicletanine-induced relaxation was uninfluenced by pretreatment with propranolol, ouabain, tetraethylammonium, procaine, indomethacin, cimetidine or tetrodotoxin but was antagonized by glibenclamide. 6. The results show that cicletanine inhibits contractile responses of human isolated inferior epigastric arteries by a mechanism unrelated to endothelial factors but associated with inhibition of calcium metabolism. An action of cicletanine on glibenclamide-sensitive K+ channels is also suggested. Cicletanine-induced inhibition was significantly greater in arteries from preclamptic patients.     

Two possible mechanisms underlying nitrate tolerance in monkey coronary arteries

1. Previous studies using isolated arteries have demonstrated cross-tolerance between nitric oxide (NO) donors such as nitroglycerin (NTG) and sodium nitroprusside (SNP). However, it remains unclear whether the vasorelaxing effect of atrial natriuretic peptide (ANP), an activator of particulate guanylate cyclase, is affected by treatment with NO donors. To investigate the cross-tolerance and interactions between NTG and ANP in coronary vasorelaxant responses, we used two models of monkey coronary arterial strips (Macaca fuscata). 2. In one model, which was induced by a 1 h treatment with 4.4 x 10(-4) mol/L NTG followed by washout of the agent for 1 h, the vasorelaxing effects of subsequent NTG were markedly attenuated, whereas those of ANP and NO were not affected. These findings suggest that the development of NTG tolerance is associated with a biotransformation process from NTG to NO. In the other model, which did not include washout after exposure to 3 x 10(-6) mol/L NTG, the vasorelaxant responses to 10(-8) mol/L ANP (31.1+/-5.4 vs 5.1+/-2.1%, respectively; P < 0.001), 10(-6) mol/L NO (61.5+/-2.4 vs 29.5+/-8.5%, respectively; P < 0.001) and 10(-8) mol/L SNP (49.4+/-6.4 vs 8.0+/-2.0%, respectively; P < 0.001) were significantly attenuated. The concentration- response curve for 8-bromo-cGMP (8-Br-cGMP) was shifted to the right, whereas responses to papaverine and forskolin were unchanged. These findings suggest that an intracellular process that occurs after the synthesis of cGMP is responsible for this interaction. 3. As a mechanism of NTG tolerance, two possible processes may be impaired: (i) biotransformation from NTG to NO; and (ii) an intracellular process that occurs after the synthesis of cGMP.     

Comparative cardiovascular actions of clentiazem, diltiazem, verapamil, nifedipine, and nimodipine in isolated rabbit tissues.

Clentiazem is an 8-chloro-substituted derivative of diltiazem. We compared the relative potency of clentiazem with that of diltiazem, verapamil, nifedipine, and nimodipine in isolated rabbit right atria and vascular smooth muscle removed from various arterial beds. In experiments with isolated right atria, calcium channel blockers were added cumulatively to study relative cardiodepressive potencies (as compared with vascular effects) with the following results: verapamil greater than or equal to diltiazem greater than clentiazem greater than or equal to nimodipine much greater than nifedipine. The aorta, pulmonary, renal, mesenteric, coronary, and basilar arteries were removed, cut helically in strips, and mounted in isolated tissue baths to measure isometric force. Vessels were contracted with either 40 mM KCl (opening voltage-operated calcium channels) or 1 x 10(-5) M norepinephrine (NE, opening receptor-operated calcium channels). Cumulative dose-response curves were generated for relaxation with each calcium channel blocker. All compounds were more potent at relaxing potassium-induced contractions than NE-induced contractions. In vessels precontracted with KCl, neither diltiazem, verapamil, or nifedipine showed selectivity for basilar artery as compared with the mesenteric artery. Both clentiazem and nimodipine were selective (6- and 30-fold, respectively) for basilar artery in blocking potassium-induced contractions. When NE was used to contract the arteries, clentiazem (12-fold), diltiazem (8-fold), verapamil (8-fold), and nifedipine (153-fold) were all more potent in relaxing the contraction in basilar artery than in mesenteric artery. Nimodipine failed to demonstrate selectivity for basilar artery as compared with mesenteric artery contracted with NE.(ABSTRACT TRUNCATED AT 250 WORDS)     

Different influence of endothelium in the mechanical responses of human and cat isolated cerebral arteries to several agents.

The present work was undertaken to elucidate the role of the vascular endothelium in the changes of isometric tension elicited by different compounds in isolated cylinders of human and cat cerebral arteries and cat pulmonary arteries. Endothelium removal by rubbing significantly reduced the relaxing response to acetylcholine (ACh) of isolated segments of all the arteries. The same treatment did not modify the contraction elicited by 5-hydroxytryptamine (5-HT) in the human and cat cerebral segments but increased the contractile effect of the amine in cat pulmonary arteries. The mechanical responses to vasopressin, ATP and adenosine in isolated segments of cat cerebral arteries were unaffected after removing the endothelial layer. L-Arginine, but not D-arginine (10(-5) M), enhanced significantly the relaxation induced by increasing doses of ACh in unrubbed cat cerebral arteries whereas it did not modify the response to ACh in rubbed ones. However, L-arginine had no effect on the dose-response curve to 5-HT in both kinds of preparation and did not change the tone in precontracted unrubbed cat cerebral segments. These results suggest that the endothelium of the cerebrovascular bed plays a minor role in regulating the mechanical response induced by several vasoactive agents, although it retains its ability to produce an endothelium-derived relaxing factor.     

Mechanisms of relaxing response induced by rat/mouse hemokinin-1 in porcine coronary arteries: roles of potassium ion and nitric oxide

Rat and mouse hemokinin-1(r/m hemokinin-1) is a recently described member of the tachykinin family whose cardiovascular functions are not fully understood. In this study, we investigated the mechanisms of the relaxing response induced by r/m hemokinin-1 in isolated porcine coronary arteries by using a specific antagonist of tachykinin NK(1) receptor (SR140333), a nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (L-NNA), and 1H-[1,2,4] Oxadiazolo [4,3-a] quinoxalin-1-one (ODQ), a blocker of cGMP production. r/m Hemokinin-1 (10(-12)-10(-6 )M) evoked a marked endothelium-dependent vasodilatation (E(max)=121.12+/-10.6% and 91.79+/-2.39% in 10(-6) M PGF(2)alpha and 30 mM KCl precontracted arterial rings, respectively) of coronary arteries mediated by activation of endothelial tachykinin NK(1) receptors. Two components contributed to this r/m hemokinin-1-elicited vasodilatation, the first of which was endothelium-derived hyperpolarizing factor (EDHF), which played a major role. This EDHF was identified as a potassium current through certain kinds of potassium channels on the endothelial cell membrane of porcine coronary arteries. Specific antagonists of Ca(2+)-activated K(+) channels (dequalinium and clotrimazole) did not have an inhibitory effect on the r/m hemokinin-1-induced vasodilatation, whereas they did on the substance P-induced vasodilatation. When potassium ion efflux was impaired by a high K(+) concentration (30 mM) or removal of K(+) from the surroundings, NO synthesis was triggered by r/m hemokinin-1 to produce an equivalent EDHF (K(+))-independent vasorelaxation as a compensatory mechanism.     

Vasorelaxant effects of eugenol on rat thoracic aorta

Eugenol is a natural pungent substance and the main component of clove oil, with vasorelaxant action. To elucidate some of the possible mechanisms involved in this action isometric tension was measured in aortic rings from male Wistar rats precontracted with phenylephrine (PHE, 10(-7) M) or KCl (75 mM). Responses to increasing concentrations of eugenol (10(-6)-10(-2) M) were obtained in the presence and absence of endothelium. In the presence of eugenol, dose-response curves to PHE (10(-9) to 10(-4) M) and KCl (5-125 mM) were displaced downwards. Concentration-dependent relaxation was observed in rings precontracted with PHE (10(-7) M) and KCl (75 mM). The tension increment produced by increasing external calcium concentration (0.25-3 mM) was also reduced by eugenol (300 microM) treatment. The inhibitory effects of eugenol (300 microM) were compared to those induced by nifedipine (0.01 microM), a selective Ca(2+) channel blocker, producing similar relaxant effects. Two other protocols were performed. After precontraction with PHE (10(-7) M), increasing concentrations of eugenol (10(-6)-10(-2) M) were used before and after N(w)-nitro-L-arginine (L-NAME, 10(-4) M) and methylene blue (10(-5) M) treatment. Eugenol-induced relaxation was reduced by endothelial damage (rubbing), L-NAME and methylene blue treatments. Results suggested that eugenol produces smooth muscle relaxation resulting from the blockade of both voltage-sensitive and receptor-operated channels that are modulated by endothelial-generated nitric oxide.     

Effects of the new potassium channel opener JTV-506 on coronary vessels in vitro and in vivo

The vascular effects of JTV-506 ((-)-(3S,4R)-2.2-bis(methoxymethyl)- 4-[(1,6-dihydro-l-methyl-6-oxo-3-pyridazinyl)amino]-3-hydroxychroman+ ++-6- carbonitrile hemihydrate, CAS 170148-29-5), a new potassium channel opener, was evaluated in isolated coronary arteries and anesthetized dogs. JTV-506 (1 nmol/l-3 mumol/l) produced a concentration-dependent relaxation in porcine isolated epicardial large coronary arteries precontracted with KCl (30 mmol/l), phenylephrine (3 mumol/l), histamine (3 mumol/l), serotonin (5-HT; 300 nmol/l), prostaglandin F2 alpha (PGF2 alpha; 10 mumol/l), U-46619 (100 nmol/l), endothelin-1 (ET-1; 30 nmol/l) and Bay K-8644 (100 nmol/l). JTV-506 was 2.5-8.5 and 13.3-81.5 times more potent than levcromakalim (CAS 94535-50-9) and nicorandil (CAS 65141-46-0), respectively, but was less potent than nifedipine (CAS 21829-25-4). JTV-506 and levcromakalim produced almost a complete relaxation in arteries precontracted with various kinds of vasoconstrictor, except for KCl. In contrast, nifedipine produced about 80-90% relaxation in arteries, precontracted with PGF2 alpha, U-46619 and ET-1. Thus, this potassium channel opener can be characterized as an agonist-nonselective vasorelaxant. The relaxing effects of JTV-506 and levcromakalim on coronary arteries precontracted with 30 mmol/l KCl was competitively antagonized by 3 mumol/l glibenclamide, an ATP-sensitive potassium (KATP) channel blocker. In canine isolated epicardial large coronary arteries, 10 mumol/l JTV-506, 10 mumol/l levcromakalim, 100 mumol/l nicorandil and 0.1 mumol/l nifedipine eliminated 10 mmol/l 3,4-diaminopyridine-induced rhythmic contractions. In anesthetized dogs, when administered directly into the coronary artery, JTV-506 induced dose-dependent increases in coronary arterial diameter and coronary blood flow. These results suggest that JTV-506 elicits coronary vasorelaxation through activation of the KATP channel. It is expected that JTV-506 might be useful in the treatment of coronary vasospasm in angina pectoris.     

Comparison of development of tolerance between nicorandil and nitroglycerin in anesthetized, open-chest dogs

Development of tolerance to nicorandil (NCR), N-(2-hydroxyethyl) nicotinamide nitrate (ester), was compared with that to nitroglycerin (NTG) in dogs. An intra-coronary arterial (i.a.) injection of NCR (30 micrograms) or NTG (3 micrograms) produced coronary vasodilation. Development of tolerance (including cross tolerance) was determined by examining whether the coronary vasodilating effect of i.a. injection of these drugs was attenuated by a 2 hr-infusion of NCR or NTG. The effect of i.a. injection of NCR was not affected by either NCR infusion (10 micrograms/kg/min, i.v.) or NTG infusion (1 or 3 micrograms/kg/min, i.v.). The effect of i.a. injection of NTG, however, was attenuated by the NTG infusion, while it was not affected by the NCR infusion. Additionally, the coronary vasodilating effect of NCR infusion (30 micrograms/kg/min, i.v.) was not attenuated by NTG infusion (3 micrograms/kg/min, i.v.). These results suggest that NCR does not produce tolerance, whereas NTG does, and that there is no cross-tolerance between NCR and NTG in terms of the coronary vasodilating effect.     

The enhanced endothelin-1-induced contraction in cultured coronary arteries from mature female pigs is not antagonized by 17beta-estradiol

We have previously reported that organ cultured coronary arteries from market-age pigs (6-9 months of age) exhibit an enhanced contraction to the atherosclerotic-associated peptide, endothelin-1 (ET-1). The objective of this study was to investigate the interaction of 17beta-estradiol with ET-1 in organ cultured coronary arteries from older female pigs (3-4 years old). A cumulative concentration-response relationship (1 x 10(-9) M to 3 x 10(-7) M) was generated to ET-1, and the isometric tension measured in fresh and organ cultured (4 days at 37 degrees C) arterial rings that were each pre-incubated for 50 min in different concentrations (1 x 10(-9) M to 1 x 10(-5) M) of 17beta-estradiol. Compared to freshly used arteries, culturing induced a 2-fold increase in tension development to ET-1 (3 x 10(-7) M). Although 17beta-estradiol previously relaxed pre-constricted (with a 60 mM KCl solution) arteries, it did not affect the constrictive response to ET-1. Also, using an ET-1 ELISA we found that 17beta-estradiol did not effect ET-1 production in intact arteries. Our results indicate that 17beta-estradiol does not attenuate the production and constrictive properties of ET-1 in coronary arteries demonstrating a dedifferentiated cell phenotype.     

Prevention and reversal of tolerance to nitroglycerine with N-acetylcysteine

A recent study demonstrated that the sulfhydryl donor N-acetylcysteine (NAC) potentiated hemodynamic responsiveness to nitroglycerine (NTG) in patients with ischaemic heart disease. The interaction between NTG and NAC in rings of bovine coronary artery was examined. Vasodilator responses to NTG were determined after elevation of tone with the thromboxane mimetic U46619 [(15S)-hydroxy-11 alpha, 9 alpha-(epoxymethano) prosta-5Z, 13E-dienoic acid]. NAC (1 microM-3 mM) induced no changes in tone of the preparation, but 10 microM NAC significantly potentiated responses to NTG (EC50 reduced from 0.69 +/- 0.19 microM to 0.22 +/- 0.06 microM; p less than 0.01). Increasing degrees of tolerance to NTG were produced at pH 7.4 by preincubating coronary rings with NTG in concentrations of 4.4 and 44 microM, and 0.22 mM. With 0.22 mM NTG, EC50 for subsequently administered NTG was increased to 11.0 +/- 1.8 microM (p less than 0.001 vs. control vessels). The degree of tolerance produced with this concentration of NTG was markedly attenuated by simultaneous (EC50 = 0.50 +/- 0.30 microM; p less than 0.001 vs. tolerant vessels) or subsequent (EC50 = 1.17 +/- 0.59 microM, p less than 0.001 vs. control vessels) incubation with 10 microM NAC. These data confirm that responses to NTG are modulated by sulfhydryl (or specifically cysteine) availability and suggest that in vitro tolerance to NTG is related to sulfhydryl (or cysteine) depletion. It is therefore possible that in vivo potentiation of NTG responses by NAC will be of clinical benefit in preventing or reversing loss of hemodynamic responsiveness to NTG.