Coronary Rings Research Articles

Functional coronary microvascular injury evident as increased permeability due to brief ischemia and reperfusion.

Although morphological studies suggest that coronary vascular injury is a result of prolonged ischemia and subsequent reperfusion, whether functional coronary microvascular injury develops during brief in vivo ischemia is unclear. In other organs, permeability is a sensitive indicator of functional vascular injury. Therefore, a new double-indicator method of assessing vascular protein permeability, a method that is both sensitive and specific for vascular injury, was used to investigate the effects of ischemia of graded duration followed by reperfusion on coronary microvascular function. To help confirm functional coronary vascular injury, endothelium-dependent vasodilation of isolated coronary vascular rings also was examined. Microvascular permeability was quantitatively assessed as a protein leak index by measuring the rate of extravascular accumulation of radiolabeled protein (indium 113m transferrin) normalized for vascular surface area (technetium 99m erythrocytes). Anesthetized dogs underwent 0 (control), 15, 30, or 60 minutes of left anterior descending coronary artery occlusion followed by 60 minutes of reperfusion. Even 15 minutes of ischemia increased the protein leak index by 50% (3.16 +/- 0.30 ischemic vs. 2.09 +/- 0.11 control). Longer periods of ischemia increased the protein leak index in proportion to the duration of ischemia. The protein leak index increased threefold (6.51 +/- 0.60) after 60 minutes of ischemia. At each duration of ischemia, there was significant regional variation in the protein leak index that correlated with the severity of ischemic blood flow to that region measured with microspheres. Endothelial injury also was evident after 15 and 30 minutes of ischemia as impaired vasodilation of isolated coronary rings in response to the endothelium-dependent vasodilators acetylcholine and the calcium ionophore A23187. Electron microscopy and in vitro direct immunofluorescence revealed evidence of vascular injury after 60 minutes but not after 15 minutes of ischemia. We conclude that even brief ischemia and reperfusion cause functional coronary vascular injury evident as increased microvascular permeability and impaired endothelium-dependent vasodilation and that regional differences in the degree of microvascular injury correlate with differences in the severity of ischemia.

Vascular reactivity of the coronary artery in steelhead trout ( Oncorhynchus mykiss

1. The responses of vascular rings from the coronary artery in steelhead trout, Oncorhynchus mykiss, were measured using isometric force transducers. 2. Coronary rings contracted with acetylcholine. 3. Nitroglycerine and sodium nitroprusside relaxed coronary rings. 4. Adenosine, ADP and ATP caused contractions. 5. A comparison is made with the reactivity of the coronary artery in rainbow trout.

Coronary reperfusion in dogs inhibits endothelium-dependent relaxation: Role of superoxide radicals

Previous studies indicate that release of superoxide radicals during coronary reperfusion following occlusion may relate to the loss of endothelium-dependent coronary arterial relaxation. We examined coronary arterial ring relaxation in dogs subjected to temporary circumflex (Cx) coronary artery occlusion and treated with saline or the superoxide radical scavenger superoxide dismutase (SOD). In dogs treated with saline, Cx coronary ring relaxation in response to leukotriene D 4 (LTD 4) and acetylcholine (ACh) was attenuated ( p < 0.01), but coronary relaxation in response to nitroglycerin was preserved, suggesting loss of endothelium-dependent relaxation following coronary reperfusion. In contrast, Cx coronary relaxation in response to LTD 4 and ACh was preserved in the SOD-treated dogs ( p < 0.01 compared to saline-treated dogs). To further examine the role of superoxide radicals in the loss of endothelium-dependent relaxation, normal nonischemic canine coronary artery and rat aortic rings were exposed to a superoxide radical generating system of xanthine and xanthine oxidase in vitro. Xanthine plus xanthine oxidase treatment caused a significant ( p < 0.01) decrease in the relaxant effects of ACh. Pretreatment of rat aortic rings with SOD protected against the loss of ACh-induced relaxation. These observations suggest that release of superoxide radicals during reperfusion is the basis of loss of endothelium-dependent coronary arterial relaxation. Treatment with superoxide radical scavengers prior to coronary reperfusion protects against this loss.

Coronary endothelial dysfunction from ischemia and reperfusion: Effect of reactive oxygen metabolite scavengers

Using anesthetized mongrel dogs exposed to 60 min of ligation of the left anterior descending coronary artery followed by 60 min of reperfusion, we examined the effect of superoxide dismutase (SOD) and dimethylthiouirea (DMTU) on evidence of endothelial injury in coronary rings studied in vitro. In 13 dogs treated with saline rings from the normal left circumflex coronary artery (LCF) relaxed by 98 ± 4% when exposed to 10 −5 M acetylcholine whereas rings from the left anterior descending coronary artery (LAD) relaxed by 79 ± 7% ( p<0.05). In the same rings maximum relaxation with the ionophore A23187 was 107 ± 5% versus 87 ± 8% ( p<0.05) for the LCF and the LAD, respectively. Comparisons of concentration-response curves through a range of doses of both acetylcholine and A23187 revealed significant differences for both vasodilators between the LCF and the LAD ( p<0.01 for each). Nine dogs were treated with bovine SOD infused in the left5 atrium the last 20 min of ligation and throughout reperfusion (140 units/kg/min) and six other dogs were treated with DMTU 500 mg/kg i.v. given the last 30 min of the ligation period. Neither SOD nor DMTU prevented endothelial injury in the LAD. Despite pretreatment with these agents, there were significant reductions in maximum relaxation and in total concentration-response curves in the LAD as compared with the results in rings from the LCF with both acetylcholine and A23187. There were normal responses to nitroprusside in both the LCF and LAD in all three experimental groups. Therefore, endothelial injury occurs with exposure of coronary arteries to ischemia and reperfusion but neither SOD, a scavenger of superoxide anion, nor DMTU, a scavenger of hydrogen peroxide and hydroxyl anion, attenuated or prevented this abnormality.

Attenuated coronary relaxation after reperfusion: effects of superoxide dismutase and TxA2 inhibitor U 63557A

Previous studies demonstrate endothelium-dependent leukotriene D4 (LTD4)-induced relaxation of canine coronary arterial rings in vitro. We now show that coronary occlusion followed by reperfusion attenuates (P less than 0.01) the relaxation of canine coronary artery rings in response to LTD4 as well as acetylcholine (ACh), suggesting loss of endothelium-dependent coronary reactivity (n = 6 dogs). Since superoxide anions have been shown to cause breakdown of endothelium-derived relaxing factor (EDRF), we wondered whether treatment of dogs with superoxide anion scavenger superoxide dismutase (SOD) would modulate the effects of LTD4 and ACh on reperfused coronary artery rings. Indeed, treatment of dogs (n = 5) with SOD before coronary reperfusion resulted in preservation of LTD4- and ACh-induced relaxation of coronary rings. Treatment of another five dogs with selective thromboxane-synthetase blocker U 63557A before coronary reperfusion also resulted in preservation of coronary ring relaxation in response to LTD4 and ACh. To determine the mechanism of U 63557A-induced preservation of coronary reactivity, canine neutrophil superoxide anion generation in the presence of U 63557A was measured. Although U 63557A had no effect on superoxide anion generation in neutrophils alone, it markedly (P less than 0.02) inhibited superoxide anion generation in neutrophils in the presence of platelets, most likely via shunting of accumulated cyclic endoperoxide in platelets toward formation of prostacyclin, which inhibits neutrophil superoxide anion production. Thus SOD and U 63557A protect against loss of endothelium-mediated vascular relaxation by LTD4 and ACh after coronary occlusion and reperfusion.

Comparison of responses to acetylcholine and serotonin on isolated canine and human coronary arteries

Canine and human coronary arteries were studied in organ baths to compare the responses to acetylcholine and serotonin in the two species. The human coronary rings were isolated from seven patients without cardiac disease (mean age 15 years, range 7-20). In one set of experiments canine and human preparations were incubated with phentolamine, propranolol and ketanserin (all at 1 mumol.litre-1 concentration) and precontracted with prostaglandin F2 alpha (PGF2 alpha 1-2 mumol.litre-1). Acetylcholine (0.1-10 mumol.litre-1) and serotonin (0.1-100 mumol.litre-1) relaxed canine preparations dose dependently, the maximum responses (expressed as % of depression of PGF2 alpha response) being 84 (SEM 6)% (n = 9) and 51(5)% (n = 6) respectively. In the same experimental conditions, acetylcholine and serotonin failed to relax the human coronary rings (n = 11) while substance P and bradykinin induced relaxations of 72(4)% (n = 11) and 66(7)% (n = 11) of PGF2 alpha response respectively. In another set of experiments, dose-contraction curves were constructed for acetylcholine or serotonin (in presence of phentolamine and propranolol). On human rings with endothelium, methylene blue (10 mumol.litre-1), a non-specific inhibitor of endothelium derived relaxing factor (EDRF), potentiated these dose-contraction curves: markedly for serotonin, the EC50 decreasing from 1.2(0.2) to 0.22(0.08) mumol.litre-1 (n = 11, p less than 0.01) with a significant increase in the maximal response); and slightly for acetylcholine, EC50 decreasing from 0.84(0.11) to 0.40(0.13) mumol.litre-1 (n = 10, p less than 0.05) without significant change in the maximal response.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin-1 from Pulmonary Artery and Micro vessels Acts on Vascular and Airway Smooth Muscle

The conditioned medium (CM) from microvascular and macrovascular endothelial cell (EC) cultures was tested for constrictor activity. Sheep coronary artery rings, under 1 g tension, and sheep tracheal smooth muscle strips, under 2 g tension, were hung in organ baths containing Krebs-Henseleit solution (39 degrees C) and were equilibrated with 95% O2 and 5% CO2. Isometric force was measured in response to 80 mM KCl and constrictor responses to 100% CM were expressed as a percentage of maximum KCl response. Serum-free CM from confluent microvascular endothelial cells caused a sustained, slow-onset contraction of the coronary rings similar to that obtained with CM from macrovascular ECs. Indomethacin (5 microM) added to either the microvascular or macrovascular CM enhanced the constrictor responses by 1.6- and 1.8-fold, respectively, and the constrictor effects of both media were reduced by the calcium antagonist gallopamil (10 nM). CM from macrovascular EC caused a sustained contraction of tracheal strips similar to microvascular CM. In both cases, constriction was preceded by a brief relaxation as noted in vascular smooth muscle. Unconditioned medium had no constrictor activity on either vascular or airway smooth muscle. Microvascular-derived constrictor activity was heat stable. There was a slight loss of activity in the heat-treated CM from the macrovascular cells compared with control CM. Constrictor effects on tracheal smooth muscle were partially reversed by 1 microM gallopamil.

Persistence of the Response to SIN1 on Isolated Coronary Arteries Rendered Tolerant to Nitroglycerin In Vitro or In Vivo

Experiments were performed on isolated canine and human coronary arteries to provide more insight into the mechanisms responsible for the vascular tolerance to nitroglycerin that is induced under in vitro or in vivo conditions. In vitro tolerance was produced after an incubation of coronary ring segments with nitroglycerin (10 microM for 30 min at physiological pH). After elevation of tone with KCl (15 mM), dose-response curves were constructed for nitroglycerin or SIN1 (3-morpholino-syndnonimin) on control and tolerant rings. On canine tolerant rings the dose-response curve for nitroglycerin-induced relaxations was significantly (p less than 0.001) shifted to the right, and 50% of the maximal relaxation (ED50) increased from 55 +/- 9 nM to 1.2 +/- 0.2 microM. Pretreatment of tolerant rings with N-acetylcysteine (NAC, 10 microM 10 min before KCl-induced contraction) partially restored the responsiveness to nitroglycerin, with ED50 reducing to 0.56 +/- 0.03 microM (p less than 0.02). On the other hand, the dose-response curves to SIN1 were not significantly altered. Similar results were obtained on human preparations. On isolated canine coronary rings rendered tolerant in vivo by subcutaneous injections of 15 mg/kg nitroglycerin (two times daily for 4 consecutive days), ED50 for nitroglycerin was 0.67 +/- 0.08 microM (p less than 0.001 versus control rings), and NAC again partially restored the responsiveness to nitroglycerin. As for the in vitro tolerance, the relaxations to SIN1 were not significantly altered on these canine rings rendered nitrate tolerant in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y Is a Vasoconstrictor of Human Coronary Arteries

Neuropeptide Y (NPY) is a 36-amino-acid polypeptide which coexists with catecholamines in many adrenergic and noradrenergic neurons. It has been demonstrated to exert pressor effects in the perfused guinea pig heart and to constrict large cerebral and coronary blood vessels in animal studies. To determine if NPY might be a human coronary vasoconstrictor, the authors studied its effect on postmortem human coronary arteries. Proximal epicardial coronary rings were studied in a superfusion apparatus in Krebs-Ringer bicarbonate buffer (37 degrees C, pH 7.4) presaturated with 95% O2-5% CO2. Concentration-response curves were obtained using NPY in 0.1% bovine serum albumin in buffer and the responses were compared to those obtained in the presence of alpha 1, beta, and cyclooxygenase antagonists. A dose-related constrictor effect was obtained with NPY, which was significantly more potent than noradrenaline, constriction often being seen at 10(-12) M concentration. A vasorelaxant effect was seen in nonatherosclerotic vessels at higher concentrations. The vasoconstriction produced by noradrenaline was potentiated by subthreshold concentrations of NPY. The vasoconstrictor effect of NPY was not inhibited by prazosin (10(-6) M), and the vasodilatory effect was not inhibited by propranolol (10(-5) M). Indomethacin (3 X 10(-6) M) did not alter either vasoconstriction or vasorelaxation. The authors conclude that NPY is a potent constrictor of the human coronary artery at concentrations that may be achievable in vivo; it may thus be a contributor to sympathetic enhancement of coronary artery tone.

Beta-adrenoceptors on endothelial cells do not influence release of relaxing factor in dog coronary arteries.

1. The aim of this study was to determine if stimulation of a beta-adrenoceptor on endothelial cells could release an endothelium-derived relaxing factor (EDRF) similar to that released by acetylcholine. 2. In dog coronary rings preconstricted with PGF2 alpha or serotonin, removal of the endothelium did not alter the relaxant responses to isoprenaline. However, in rings preconstricted with the thromboxane-mimetic U46619, removal of the endothelium enhanced the response to isoprenaline. 3. The vasorelaxant responses to isoprenaline in endothelium-denuded rings were inhibited in a concentration-dependent manner by the specific beta 1-adrenoceptor antagonist CGP-20712A (3-100 nmol/l), but were not altered by the beta 2-adrenoceptor antagonist ICI-118551 (30 nmol/l). 4. The vasorelaxant responses to isoprenaline in the presence of CGP-20712A (30 nmol/l) or ICI-118551 (30 nmol/l) were not impaired by removal of the endothelium. 5. Endothelium-dependent vasorelaxant responses to acetylcholine and bradykinin were not altered by isoprenaline (0.1 mumol/l) in the presence of CGP-20712A (30 nmol/l). 6. Therefore, the beta-adrenoceptors on dog coronary artery smooth muscle which produce relaxation are predominantly of the beta 1-subtype. Stimulation of any beta 2-adrenoceptors on the endothelium in dog coronary artery does not release EDRF, and does not modulate endothelium-dependent vasodilatation induced by other agents.

Is there an abluminal release of endothelium-derived relaxing factor (EDRF)?

In superfusion bioassay systems of rabbit thoracic aorta and femoral artery ring segments as well as canine femoral artery and coronary ring segments the release of endothelium-derived relaxing factor (EDRF) was stimulated by acetylcholine in donor segments with intact endothelium. The transition of EDRF through the vessel wall ("abluminal release") was not revealed in both bioassay systems. Externally applied acetylcholine did not reach the luminal surface of rabbit thoracic aorta but induced EDRF release in canine femoral artery segments.

Reperfusion after acute coronary occlusion in dogs impairs endothelium-dependent relaxation to acetylcholine and augments contractile reactivity in vitro.

Endothelial injury may contribute to the augmented coronary vascular tone seen in myocardial ischemia by impairing endothelial production or release of vasodilators. In vitro reactivity of arterial rings was studied after 60 min of coronary occlusion and 60 min of reperfusion in anesthetized dogs. Ischemia without reperfusion blunted contractile reactivity to potassium chloride (KCl), whereas ischemia plus reperfusion augmented contractile responses to both KCl and ergonovine. The response to acetylcholine, an endothelium-dependent vasodilator, was abolished in reperfused arteries, whereas the response to nitroprusside, an endothelium-independent vasodilator, was intact. Verapamil pretreatment restored KCl contractile responses to normal in reperfused coronary rings and partially restored endothelium-dependent relaxation. Electron microscopy revealed a nondenuding epicardial coronary endothelial injury in reperfused arteries. These data support the hypothesis that reperfusion of ischemic myocardium augments reactivity to vasoconstrictor agents by causing endothelial cell damage, excessive calcium influx, and loss of modulating vasodilator function.

Adenine nucleotides and 5-hydroxytryptamine released by aggregating platelets inhibit adrenergic neurotransmission in canine coronary artery.

The purpose of this study was to determine the effect of vasoactive substances released by aggregating platelets on adrenergic neurotransmission in canine coronary arteries. Isometric tension was recorded in isolated ring segments of coronary artery denuded of endothelium and the release of [3H]norepinephrine was measured from strips of coronary artery preincubated with the radiolabeled transmitter. Transmural electrical field stimulation and exogenously added norepinephrine caused beta adrenergic relaxations of coronary rings contracted by prostaglandin F2 alpha. In coronary rings further contracted by the addition of aggregating platelets in numbers less than that present in blood, the response to electrical stimulation was inhibited and the sensitivity to norepinephrine reduced. Micromolar concentrations of adenosine diphosphate, adenosine triphosphate, and 5-hydroxytryptamine were released by platelets under these experimental conditions. The reduced response to electrical stimulation was in part due to inhibition of the stimulated release of [3H]-norepinephrine. The combination of the serotonergic antagonist, methiothepin, and the purinergic antagonist, theophylline, attenuated the inhibition of the responses of coronary rings; either antagonist alone failed to do so, but did significantly block the reductions caused by 5-hydroxytryptamine and adenosine diphosphate, respectively. In addition, only the combination of the two antagonists significantly attenuated the inhibition of norepinephrine release caused by platelets. These data suggest that both adenine nucleotides and 5-hydroxytryptamine are important mediators of the prejunctional and postjunctional inhibition of coronary beta adrenergic neurotransmission caused by platelets.

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.

Effects of a novel calcium channel agonist dihydropyridine analogue, Bay k 8644, on pig coronary artery: biphasic mechanical response and paradoxical potentiation of contraction by diltiazem and nimodipine.

Bay k 8644 is a structural analogue of the 1,4-dihydropyridines whose pharmacological actions on heart and vascular smooth muscle are opposite from those of nifedipine and other similar calcium antagonists. We have examined the action of Bay k 8644 ("calcium channel agonist") on isolated porcine coronary artery rings. The interactions between Bay k 8644 and the vasodilators isosorbide dinitrate (ISDN), diltiazem, and nimodipine were quantitated. Bay k 8644 produced a biphasic, dose-dependent mechanical response, with contraction occurring over the concentration range of 1-350 nM (ED50 = 11.4 nM) and relaxation observed at concentrations greater than 350 nM (IC50 = 5.7 microM). ISDN, diltiazem, and nimodipine relaxed, in a dose-dependent manner, maximal Bay k 8644-induced contractions. When the coronary rings were pretreated for 25-90 min with 80% inhibitory concentrations of these vasodilators, there was little or no effect by ISDN on Bay k 8644-induced contractions; however, there was a surprising potentiation by diltiazem and by nimodipine. Pretreatment of coronary rings with higher concentrations of ISDN or diltiazem caused an inhibition of Bay k 8644-induced contraction, while pretreatment with higher concentrations of nimodipine caused further potentiation of contraction elicited by Bay k 8644. Bay k 8644 increased the tension developed in response to high potassium (potential-operated channel activation) or histamine (receptor-operated channel activation). To account for the biphasic response to Bay k 8644 (dose-dependent contraction and relaxation), and the unexpected potentiation of Bay k 8644-induced contraction by nimodipine and by diltiazem, a molecular model is proposed for vascular smooth muscle in which Bay k 8644 functions as a partial calcium channel agonist at two functionally distinct 1,4-dihydropyridine "receptor sites."

Role of atherosclerosis in spontaneous periodic contractions of isolated postmortem human coronary arteries.

The role of atherosclerosis in coronary artery spasm was studied by morphometrical examinations of isolated postmortem human coronary arteries that were used for the observation of spontaneous periodic contractions. Isometric tension of coronary ring segments from seventy-six cadavers was recorded in a bath containing physiological saline solution. According to the degree of atherosclerosis, the segments were classified into three groups: minimum intimal lesion (Group I), moderate intimal lesion (Group II) and severe atherosclerotic lesion (Group III). Results were analyzed by a chi-square test. The incidence of the spontaneous periodic contractions was 45% in the proximal portion. Of the segments in which the contractions occurred, 83% were in Group II. There were significant differences in the incidence between Group I and II (33% vs 80%; p less than 0.02) and Group II and III (80% vs 25%; p less than 0.001), respectively. According to the characteristics of the atherosclerosis, there were significant differences in incidence of the contractions between circumferential thickening and eccentric thickening in Group II (63% vs 28%; p less than 0.001). In conclusion, the spontaneous periodic contractions occur most frequently in the segments in which (1) the degree of atherosclerosis is moderate rather than minimum or severe and (2) the characteristics of atherosclerosis is circumferential thickening instead of eccentric thickening.

Effects of diltiazem and nitroglycerin on prostaglandin F2 alpha-induced periodic contractions of isolated human coronary arteries.

This study characterizes the inhibiting effect of diltiazem and nitroglycerin on periodic contractions of isolated human coronary arteries. Isometric force of coronary ring segments from sixty-nine cadavers was recorded in a muscle bath. To quantify the experimental results, we used 3 X 10(-6) M prostaglandin F2 alpha to induce the periodic contractions of a certain force. When diltiazem was added during the periodic contractions, the amplitude of oscillations gradually decreased until eventually oscillations ceased completely. The process prior to the cessation of the oscillations was characterized mainly by the inhibition of the contraction phase. The inhibition rate at the time of the complete cessation of oscillations was 49.3 +/- 6.3% at 5 X 10(-7) M. The time required for complete disappearance of oscillations was dependent on the diltiazem concentration. When nitroglycerin was added during periodic contractions, the oscillations did not disappear. Compared to the contraction phase, the relaxation phase was appreciably inhibited. With only 10(-7) M diltiazem, the rate of inhibition of the contraction phase was 22.0 +/- 7.7%, whereas the preliminary treatment with 5 X 10(-8) M nitroglycerin led to a complete cessation of the oscillations, and suppression of the level of the contractions to a significantly greater extent, viz. 58.7 +/- 5.8% (p less than 0.001). Therefore, it is considered more effective in the treatment and prevention of coronary spasm to use diltiazem and nitroglycerin simultaneously rather than individually.

Quantitative pharmacologic responses of normal and atherosclerotic isolated human epicardial coronary arteries.

We studied quantitative aspects of coronary artery contraction in isolated epicardial coronary ring segments from 49 human hearts. The order of maximal tension developed by drugs in normal calcium (ionized calcium, 1.26 mM) solution was U-44069 (a prostaglandin endoperoxide analog) greater than histamine greater than carbachol greater than serotonin greater than phenylephrine greater than ergonovine. In Ca2+-free solution these same drugs mediated a lesser degree of contraction, which demonstrates that the human coronary artery uses both "intracellular" and "extracellular" calcium in hormone receptor-activated contraction. U-44069, histamine, carbachol, and phenylephrine produced calcium-free/normal calcium maximal responses of 62.9% 48.7%, 39.8%, and 37.2%, respectively. Morphologic characteristics of the atherosclerotic plaques within the vessel lumen and the degree of myocardial dysfunction did not qualitatively alter these contractile responses. However, severely atherosclerotic coronary segments were supersensitive to histamine, but not to carbachol or calcium. In conclusion, the human epicardial coronary artery is a highly reactive vessel that uses at least two calcium pools to couple contraction. Receptor-coupled agonists differ in their abilities to mediate contraction and in the degree to which each calcium pool is used, and the presence of atherosclerosis potentiates the contractile response to histamine.

Responses of dog large coronary arteries to constrictor and dilator substances: Implications for the cause and treatment of variant angina pectoris

Canine circumflex coronary artery ring segments were mounted in organ baths and contracted by ergonovine (ergometrine), serotonin, phenylephrine, norepinephrine (in the presence of propranolol), potassium (K +) and a thromboxane A 2 mimetic, U46619. Ergometrine was classified as a serotonin agonist since concentration-response curves were competitively inhibited by methysergide but not by alpha-adrenoceptor antagonists. Arteries precontracted to 80% of maximum were relaxed by verapamil, nifedipine and glyceryl trinitrate. However, nifedipine was 100-fold more potent against K + than either serotonin- or phenylephrine-contracted vessels. Verapamil showed some selectivity for K + and serotonin contractions compared with phenylephrine and was almost ineffective against U46619. Glyceryl trinitrate was without selectivity. Electrical field stimulation (2 and 20 Hz, 10-second) caused a weak contraction or relaxation or biphasic response in the isolated coronary ring. Contraction was amplified by propranolol (up to 1 μM) or metoprolol (50 ng/ml) and inhibited by alpha-adrenoceptor antagonists. In a blood-perfused left anterior descending coronary artery preparation, external diameter was measured by sonomicrometry under conditions of controlled flow and resistance. Intraarterial serotonin, U46619 and ergometrine infusions decreased the diameter by up to 18% without causing spasm (0 lumen diameter). Lowering the perfusion pressure from 90 to 60 mm Hg increased the decrease in diameter during serotonin infusions. Verapamil (50 to 150 ng/ml) and nifedipine (15 to 45 ng/ml) inhibited, to a similar degree, the decrease in coronary artery diameter in response to serotonin infusions. However, verapamil was more effective than nifedipine in depressing the sinoatrial node, abolishing reflex tachycardia and lengthening the atrioventricular conduction time. These studies suggest that verapamil and nifedipine are only selective antagonists of constrictor agents in the dog coronary artery, that both low perfusion pressure and beta-adrenoceptor blockade may amplify constrictor responses, and that nifedipine has fewer cardiotoxic effects than verapamil.

Specific binding of [ 3H]nitrendipine to membranes from coronary arteries and heart in relation to pharmacological effects. Paradoxical stimulation by diltiazem

High affinity binding sites for the calcium channel inhibitor [ 3H]nitrendipine have been identified in microsomes from pig coronary arteries (K D=1.6 nM; B max=35 fmol/mg) and in purified sarcolemma from dog heart (K D=0.11 nM; B max=230 fmol/mg). [ 3H]nitrendipine binding to coronary artery microsomes was completely inhibited by nifedipine, partially by verapamil and D600 and, surprisingly, was stimulated by d-cis-diltiazem but not by 1-cis-diltiazem, a less active isomer. Half-maximal relaxation of KCl-depolarized coronary rings occurred in a slow process at 1 nM nitrendipine or 100 nM d-cis-diltiazem. In dog trabecular strips, nitrendipine caused a negative inotropic response (ED 50=1μM). These results suggest that there may be multiple binding sites for different “subclasses” of calcium channel inhibitors, and that drug binding sites may be different molecular entities from the putative calcium channels.