Coronary Tone Research Articles

Endothelium-mediated relaxation of porcine collateral-dependent arterioles is improved by exercise training.

Endothelium-dependent modulation of coronary tone is impaired in the collateral-dependent coronary microcirculation. We used a porcine model of chronic coronary occlusion and collateral development to evaluate the hypothesis that exercise training enhances endothelium-mediated relaxation and increases endothelial nitric oxide synthase (ecNOS) mRNA levels of collateral-dependent microvasculature. Adult female miniature swine were subjected to chronic, progressive ameroid occlusion of the proximal left circumflex coronary artery (LCx); after 2 months, animals were randomly exposed to 16-week exercise-training (EX group; treadmill running) or sedentary (SED group; cage confinement) protocols. After completion of EX or SED programs, coronary arterioles ( approximately 100 microm in diameter) were isolated from collateral-dependent LCx (distal to occlusion) and nonoccluded left anterior descending coronary artery (LAD) regions of each heart. Arterioles were studied by in vitro videomicroscopy or frozen for ecNOS mRNA analysis (RT-PCR techniques). Relaxation to the endothelium-dependent vasodilator bradykinin was decreased (P<0.05) in arterioles isolated from collateral-dependent LCx versus nonoccluded LAD regions of SED animals. Bradykinin-mediated relaxation, however, was not different in LCx versus LAD arterioles isolated from EX animals. Nitroprusside-induced relaxation was unaffected by either chronic occlusion or exercise. Importantly, ecNOS mRNA expression was significantly decreased in arterioles isolated from LCx versus LAD regions of SED animals. After training, ecNOS mRNA expression was not different between LAD and LCx arterioles. These data indicate that exercise training enhances bradykinin-mediated relaxation of collateral-dependent LCx arterioles isolated after chronic coronary occlusion, most likely because of effects on ecNOS mRNA expression and increased production of NO.

Role of Endothelin-1 in the Active Constriction of Human Atherosclerotic Coronary Arteries

Atherosclerotic coronary arteries are prone to constriction but the underlying causes are incompletely understood. We tested the hypothesis that endothelin-1 (ET-1), a potent vasoconstrictor, contributes to the heightened tone of atherosclerotic human coronary arteries. In 8 patients with coronary artery disease (CAD) and 8 patients with angiographically smooth coronary arteries (normal), we infused BQ-123, an antagonist of the ET(A) receptor, into a major coronary artery (infused artery) at 40 nmol/min for 60 minutes. The infused artery in the CAD patients contained a >50% stenosis. Using quantitative angiography, we compared the dilation of the infused artery with another, noninfused coronary artery. To estimate the magnitude of the contribution of ET-1 to coronary tone, we compared the dilation to BQ-123 with that elicited by intracoronary nitroglycerin (200 microgram). BQ-123 induced significant dilation in the normal arteries (7.3% at 60 minutes, P<0.001 versus noninfused arteries) and a greater dilation in the CAD arteries (16.3% at 60 minutes, P<0.001 versus infused normal arteries). The dilation at stenoses was particularly pronounced (21.6% at 60 minutes, P<0.001 versus infused CAD arteries). Compared with the dilation from nitroglycerin, ET-1 contributed to 39% of the coronary tone in normal arteries, 74% of tone in CAD arteries, and 106% of tone at stenoses (P<0.01). ET-1 accounts for nearly all the resting tone in atherosclerotic coronary arteries, especially at stenoses. Inhibitors of ET-1, by relieving constriction, may significantly lessen the hemodynamic significance of coronary stenoses and thereby reduce myocardial ischemia.

Regulation of the coronary vasomotor tone: What we know and where we need to go

The control of coronary blood flow has been studied for decades, but despite our extensive efforts, the critical regulators of flow are largely unknown. One purpose of this review is to summarize some recent concepts about the control of coronary flow and also point out areas where additional knowledge must be acquired. A second purpose of this review is to highlight the need for additional noninvasive measurements of flow that undoubtedly will require further evolution of contemporary technologies, and also application of specific methods toward noninvasive measurements of coronary blood flow. Only after the development of such measurements will the scientific community begin to understand the intricacies of the regulation of coronary flow in human beings. (J Nucl Cardiol 2001;8:599-605.)

Role of endogenous endothelin on coronary flow in health and disease.

The role of the endothelium in the control of coronary flow has been demonstrated. Results of recent studies, both on animals and on humans, suggest that endogenous endothelin also plays an important role in basal coronary tone. Disease processes such as ischaemia-reperfusion injury, congestive heart failure, hypertension and atherosclerosis may be contributed to by an imbalance in, or excess of, release of endothelin. With the discovery of newer endothelin antagonists and endothelin converting enzyme inhibitors, especially with fewer hepatic side effects, there is the potential for much future research into novel therapeutic management of these common cardiovascular disorders.

Paraoxonase gene polymorphisms and coronary reactivity in young healthy men.

This study examined the relationships between paraoxonase genotypes, coronary artery reactivity, and indices of low-density lipoprotein oxidation in healthy men. Impairment in coronary flow reserve, as assessed by positron emission tomography, is associated with lipoprotein oxidation, which is affected by high-density lipoprotein bound enzyme, paraoxonase. Paraoxonase has two common polymorphisms (M/L55 and R/Q192) that change the activity of the enzyme. Forty-nine healthy men (mean age 35 +/- 4 years) were divided by paraoxonase genotype into low (Q192/Q192, or M55/M55, M55/L55) and high-active (R192/Q192, R192/R192, or L55/L55) groups and related to the myocardial blood flow, to the susceptibility of low-density lipoprotein to oxidation, and the autoantibody titer against oxidized low-density lipoprotein. The blood flow was measured by positron emission tomography at rest and during adenosine infusion. The low-active Q192/Q192 genotype was associated with higher resting blood flow corrected for rate-pressure product compared to the high-active R192/R192 and R192/Q192 genotypes (P=0.011). The blood flow stimulated by adenosine was not significantly different in the low- and high-active genotype groups. Paraoxonase genotypes had no effect on low-density lipoprotein susceptibility to oxidation or autoantibody formation against oxidized low-density lipoprotein. Genotypes of paraoxonase may not clearly contribute to the early changes in coronary reactivity. Coronary vasomotor tone at rest appears to be modulated by paraoxonase R/Q192 polymorphism through mechanism(s) unrelated to low-density lipoprotein oxidation.

Value and limitations of ambulatory ECG monitoring for assessment of myocardial ischemia.

Ambulatory ECG monitoring (AEM) is the only available method to assess the presence and severity of myocardial ischemia during daily life. Several investigators have shown that the recording systems currently used can detect ischemic changes with similar accuracy as treadmill exercise testing. Ischemic changes on AEM are, however, present in only 40%-60% of patients with coronary artery disease (CAD) and positive exercise tests. For this reason, and because of the high day-to-day variability in daily ischemic changes, AEM cannot be used as a screening tool for detecting CAD or for evaluating severity of ischemia in individual patients. In patients with proven CAD, ischemic changes on AEM are associated with an adverse outcome in patients with stable and unstable ischemic syndromes, and in postmyocardial infarction patients. Suppression of daily ischemia seems to be associated with improved outcome. The mechanism of daily ischemia is not identical to exercise-induced ischemia. In addition to increased demand, which is a major contributor to AEM detected-ischemia, increased coronary tone also seems to play a major role. AEM has been shown to be a useful and reliable tool to assess the efficacy of various antiischemic drugs.

Pulmonary adrenomedullin counteracts deterioration of coronary flow and myocardial performance evoked by pulmonary endothelins in experimental acute respiratory distress syndrome.

We recently showed that pulmonary endothelins may affect coronary circulation under various experimental and clinical conditions. Here, we investigated the effect of pulmonary mediators on coronary tone in experimental acute respiratory distress syndrome. We focused particularly on pulmonary endothelin-1, a major vasoconstrictor in acute respiratory distress syndrome, and on adrenomedullin, a potent vasodilator that is up-regulated by inflammatory stimuli. Controlled experiment that used isolated organs. Experimental laboratory. Wistar rats. The saline effluent from an isolated lung was used to serially perfuse the coronary vessels of an isolated heart. We compared serial perfusion after 2-hr pretreatment of lungs with vehicle or endotoxin (50 microg/mL), and we used the following drugs to elucidate the coronary response observed: the endothelin type A receptor antagonist BQ-123 (2 microM), the endothelin type B antagonist A-192621 (500 nM), the endothelin-converting enzyme inhibitor phosphoramidon (50 microM), the calcitonin gene-related peptide type-1 receptor antagonist hCGRP(8-37) (2 microM), and the adrenomedullin receptor antagonist hAM(22-52) (200 nM) (n = 6 each). In controls, serial perfusion decreased coronary flow to 87 +/- 3% of baseline (p < .05). BQ-123 and phosphoramidon prevented this effect, whereas blockade of endothelin type B and adrenomedullin-binding receptors had no effect. After endotoxin challenge, coronary flow significantly increased to 110 +/- 2%. This response was augmented by BQ-123 (124 +/- 2%) and phosphoramidon (123 +/- 3%); A-192621 had no effect. Application of hCGRP(8-37) and hAM(22-52) significantly decreased coronary flow to 81 +/- 3% and 88 +/- 2%, respectively. Flow decrease after blockade of both adrenomedullin-binding receptors (73 +/- 2%) significantly deteriorated peak left ventricular pressure, to 82 +/- 6% of baseline; rate of pressure increase, to 81 +/- 5%; and rate of pressure decline, to 77 +/- 6%. Endotoxin pretreatment elevated pulmonary venous big endothelin-1 (three-fold), endothelin-1 (two-fold), and adrenomedullin (five-fold). In experimental acute respiratory distress syndrome, pulmonary adrenomedullin--via calcitonin gene-related peptide type-1 receptor and adrenomedullin receptor--outweighs the coronary vasoconstrictor impact of pulmonary big endothelin-1 exerted via endothelin type A receptors after conversion to mature endothelin-1. The consequence is prevention of flow-related deterioration of myocardial performance.

Coronary vasoconstriction after coronary angioplasty is attenuated by endothelin a receptor antagonism

P studies have demonstrated distal coronary vasoconstriction immediately after percutaneous transluminal coronary angioplasty (PTCA). It was presumed that this coronary vasoconstriction is the result of intimal injury or endothelial dysfunction,1 abnormal autoregulatory tone,2–4 formation of vasoactive substances,5,6 impaired release of endothelialderived relaxing factor,7 and activation of adrenergic neural reflexes.8 Endothelin-1 (ET-1) levels have been found to be elevated after PTCA.9 ET-1 produces very potent and marked vasoconstriction in vitro as well as in vivo.10 ET-1 binds to at least 2 receptors: in blood vessels, endothelin A (ETA) receptors are present only on vascular smooth muscle cells and cause contraction, whereas endothelin B receptors are on endothelium, causing vasodilatation, and on vascular smooth muscle cells, causing contraction. In healthy men, the major receptor causing vasoconstriction, at least in the systemic circulation, is the ETA receptor.11–13 Endogenously produced ET-1 contributes to the maintenance of basal coronary artery tone in humans with and without coronary artery disease.14 The aim of this study was to test the hypothesis that ETA receptor stimulation contributes to the coronary artery vasoconstriction observed after PTCA. We studied 22 consecutive patients, mean age 58 6 7 years, scheduled to undergo elective PTCA for an isolated obstructive lesion and willing to participate. The study conformed to the principles outlined in the Declaration of Helsinki, and was approved by the hospital ethics committee. All patients gave informed consent. All patients fulfilled the following entry criteria: (1) history of chronic stable angina pectoris $3 months, and (2) angiographic appearance of lesions displaying an internal diameter reduction .50%. Exclusion criteria were unstable angina, a history of previous myocardial infarction, left main stem disease, angiographic evidence of coronary collaterals, total and subtotal occlusive lesions, lesions adjacent to a bifurcation, diabetes mellitus, depressed left ventricular function (,55%), valvular heart disease, systolic blood pressure ,100 mm Hg, or any concurrent illness. Patients were randomly allocated to receive an intracoronary infusion of BQ-123 (n 5 11) or normal saline 0.9% (n 5 11). The 2 groups had similar clinical characteristics (Table 1). All procedures were performed in patients in the fasting state, without the use of premedication. Antianginal medications were discontinued for $12 hours before the procedure. No other medication apart from heparin and BQ-123 was given until the end of the protocol. PTCA was performed using a standard technique.9 Heparin was administered as a bolus at the beginning (5,000 IU intra-arterially) and during the procedure to maintain an activated clotting time of .300 seconds. Two balloon inflations were performed for each patient with a 5-minute resting interval. If the clinical and angiographic result was satisfactory after the second balloon inflation, the patient was randomized to 1 of the 2 groups. Patients who developed dissections at the dilated site, needed stent implantation for any reason, or had an unsatisfactory angiographic result after the second balloon inflation were excluded from the study. Hemodynamic measurements (heart rate and blood pressure) and coronary arteriograms recorded by hand injection were recorded at baseline (before angioplasty initiation) and 5 and 25 minutes after the second balloon inflation, with the guidewire in place. The study was completed when the arteriogram was obtained 25 minutes after the second balloon inflation. Five minutes after the second balloon inflation, BQ-123 (Clinalfa, Switzerland) or saline was infused through the guiding catheter at a constant rate of 1 ml/min (300 nmol/min) for 20 minutes. This dose of BQ-123 (total dose of 6 mmol) has been shown not to From Onassis Cardiac Surgery Center, Athens, Greece; and Department of Medical Sciences, University of Edinburgh, Western General Hospital, Edinburgh, Scotland. Dr. Kyriakides address is: Onassis Cardiac Surgery Center, 356 Sygrou Avenue, Athens 17674, Greece. E-mail: zskyr@otenet.gr. Manuscript received September 7, 2000; revised manuscript received and accepted November 6, 2000. TABLE 1 Clinical and Angiographic Characteristics of the Patients Studied

Altered effects of potassium channel modulation in the coronary circulation in experimental hypercholesterolemia

Objective: To evaluate the role of potassium channels in the regulation of coronary hemodynamics in experimental hypercholesterolemia. Background: Potassium (K +) channels play an important role in coronary vasoregulation. It has previously been demonstrated that experimental hypercholesterolemia is associated with altered coronary vasomotion; however, the role of K + channels in modulating coronary blood flow in this pathophysiologic state has not been evaluated. Methods and results: Pinacidil (group 1, n=5) at 2 μg/kg per min, glibenclamide (group 2, n=5), or N-monomethyl- l-arginine (LNMMA) (group 3, n=4) at 50 μg/kg per min were infused into the left anterior descending artery of pigs prior to and following 10 weeks of 2% cholesterol diet. After 10 weeks of cholesterol feeding, intracoronary pinacidil resulted in a significant increase in coronary blood flow (CBF) and coronary artery diameter (CAD) compared to the normolipidemic state (111±10 versus 59±12%, and 6±1.1 versus 2.7±1.0%, respectively, P<0.05 for both comparisons), whereas intracoronary glibenclamide resulted in a significant decrease in CBF and CAD compared to the normolipidemic state (−17±5 versus 5±6%, and −0.8±1.4 versus 3.6±1.6%, respectively, P<0.05 for both comparisons). The effect of intracoronary LNMMA on CBF and CAD was significantly attenuated after 10 weeks of cholesterol feeding as compared to the normolipidemic state (−47±5.4 versus −0.8±6.8%, and −19.4±5.7 versus −2.3±3.3%, respectively, P<0.05 for both comparisons). Furthermore, pretreatment with intracoronary LNMMA did not alter the CBF response to pinacidil in normal pigs (group 4, n=4) (57.4±19 versus 59±12%, P=NS). Conclusions: The current study demonstrates an enhanced effect of coronary K + channel modulation and confirms the attenuated basal NO activity previously reported in experimental hypercholesterolemia. Acute withdrawal of basal NO activity alone, however, does not explain the enhanced effect of coronary K + channel modulation. These findings underscore the importance of the K + channel pathway in the regulation of coronary vasomotor tone in pathophysiologic states.

Role of K(ATP)(+) channels in regulation of systemic, pulmonary, and coronary vasomotor tone in exercising swine.

The role of ATP-sensitive K(+) (K(ATP)(+)) channels in vasomotor tone regulation during metabolic stimulation is incompletely understood. Consequently, we studied the contribution of K(ATP)(+) channels to vasomotor tone regulation in the systemic, pulmonary, and coronary vascular bed in nine treadmill-exercising swine. Exercise up to 85% of maximum heart rate increased body O(2) consumption fourfold, accommodated by a doubling of both cardiac output and body O(2) extraction. Mean aortic pressure was unchanged, implying that systemic vascular conductance (SVC) also doubled, whereas pulmonary artery pressure increased almost in parallel with cardiac output, so that pulmonary vascular conductance (PVC) increased only 25 +/- 9% (both P < 0.05). Myocardial O(2) consumption tripled during exercise, which was paralleled by an equivalent increase in O(2) supply so that coronary venous PO(2) was maintained. Selective K(ATP)(+) channel blockade with glibenclamide (3 mg/kg iv), decreased SVC by 29 +/- 4% at rest and by 10 +/- 2% at 5 km/h (both P < 0.05), whereas PVC was unchanged. Glibenclamide decreased coronary vascular conductance and hence myocardial O(2) delivery, necessitating an increase in O(2) extraction from 76 +/- 2% to 86 +/- 2% at rest and from 79 +/- 2% to 83 +/- 1% at 5 km/h. Consequently, coronary venous PO(2) decreased from 25 +/- 1 to 17 +/- 1 mmHg at rest and from 23 +/- 1 to 20 +/- 1 mmHg at 5 km/h (all values are P < 0.05). In conclusion, K(ATP)(+) channels dilate the systemic and coronary, but not the pulmonary, resistance vessels at rest and during exercise in swine. However, opening of K(ATP)(+) channels is not mandatory for the exercise-induced systemic and coronary vasodilation.

Cardiostimulant effects of urotensin-II in human heart in vitro.

The effects of the recently identified human peptide urotensin-II (hU-II) were investigated on human cardiac muscle contractility and coronary artery tone. In right atrial trabeculae from non-failing hearts, hU-II caused a concentration-dependent increase in contractile force (pEC(50)=9.5+/-0.1; E(max)=31.3+/-4.8% compared to 9.25 mM Ca(2+); n=9) with no change in contraction duration. In right ventricular trabeculae from explanted hearts, 20 nM hU-II caused a small increase in contractile force (7.8+/-1.4% compared to 9.25 mM Ca(2+); n=3/6 tissues from 2 out of 4 patients). The peptide caused arrhythmic contractions in 3/26 right atrial trabeculae from 3/9 patients in an experimental model of arrhythmia and therefore has less potential to cause arrhythmias than ET-1. hU-II (20 nM) increased tone (17.9% of the response to 90 mM KCI) in 7/7 tissues from 1 patient, with no response detected in 8/8 tissues from 2 patients. hU-II is a potent cardiac stimulant with low efficacy.

Vasoactive intestinal peptide: cardiovascular effects.

Vasoactive intestinal peptide (VIP) is present in the peripheral and the central nervous systems where it functions as a nonadrenergic, noncholinergic neurotransmitter or neuromodulator. Significant concentrations of VIP are present in the gastrointestinal tract, heart, lungs, thyroid, kidney, urinary bladder, genital organs and the brain. On a molar basis, VIP is 50-100 times more potent than acetylcholine as a vasodilator. VIP release in the body is stimulated by high frequency (10-20 Hz) nerve stimulation and by cholinergic agonists, serotonin, dopaminergic agonists, prostaglandins (PGE, PGD), and nerve growth factor. The VIP peptide combines with its receptor and dose-dependently activates adenylyl cyclase. The vasodilatory effect of VIP in different vascular tissues or species also may be due to increases in nitric oxide, cyclic GMP, and other signaling agents. In the heart, VIP immunoreactive nerve fibers are present not only in the epicardial coronary arteries and veins, but also the sinoatrial node, atrium, interatrial septum, atrioventricular node, intracardiac ganglia, and ventricles (right ventricle >> left ventricle). In the coronary arterial walls, VIP may contribute to the regulation of normal coronary vasomotor tone. In research animals and in humans, VIP, administered into the coronary artery or intravenously, increases the epicardial coronary artery cross-sectional area, decreases coronary vascular resistance, and significantly increases coronary artery blood flow. High frequency parasympathetic (vagal) nerve stimulation also releases endogenous VIP in the coronary vessels and heart and significantly increases coronary artery blood flow. In addition, the release of VIP in the heart is increased during coronary artery occlusion and during reperfusion where VIP may promote local blood flow and may have a free-radical scavenging effect. VIP also has a primary positive inotropic effect on cardiac muscle that is enhanced by its ability to facilitate ventricular-vascular coupling by reducing mean arterial pressure by 10-15%. In concentrations of 10(-8)-10(-5) mol, VIP augments developed isometric force and increases atrial and ventricular contractility. The presence of VIP-immunoreactive nerve fibers in and around the sinus and the atrioventricular nodes of mammals strongly suggests that this peptide can affect the heart rate. In this regard, endogenously released or exogenous VIP can significantly increase the heart rate and has a more potent effect on heart rate than does norepinephrine. The presence and significant cardiovascular effects of VIP in the heart suggests that this peptide is important in the regulation of coronary blood flow, cardiac contraction, and heart rate. Current investigations are defining the physiological role of VIP in the regulation of cardiovascular function.

Melatonin potentiates contractile responses to serotonin in isolated porcine coronary arteries.

The present study was designed to determine the effects of melatonin on coronary vasomotor tone. Porcine coronary arteries were suspended in organ chambers for isometric tension recording. Melatonin (10(-10)-10(-5) M) itself caused neither contraction nor relaxation of the tissues. Serotonin (10(-9)-10(-5) M) caused concentration-dependent contractions of coronary arteries, and in the presence of melatonin (10(-7) M) the maximal response to serotonin was increased in rings with but not without endothelium. In contrast, melatonin had no effect on contractions produced by the thromboxane A(2) analog U-46619 (10(-10)-10(-7) M). The melatonin-receptor antagonist S-20928 (10(-6) M) abolished the potentiating effect of melatonin on serotonin-induced contractions in endothelium-intact coronary arteries, as did treatment with 1H-[1, 2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10(-5) M), methylene blue (10(-5) M), or N(G)-nitro-L-arginine (3 x 10(-5) M). In tissues contracted with U-46619, serotonin caused endothelium-dependent relaxations that were inhibited by melatonin (10(-7) M). Melatonin also inhibited coronary artery relaxation induced by sodium nitroprusside (10(-9)-10(-5) M) but not by isoproterenol (10(-9)-10(-5) M). These results support the hypothesis that melatonin, by inhibiting the action of nitric oxide on coronary vascular smooth muscle, selectively potentiates the vasoconstrictor response to serotonin in coronary arteries with endothelium.

The physiological role of endogenous endothelin in the regulation of human coronary vasomotor tone

OBJECTIVESThe study was done to investigate the physiological role of endogenous endothelin-1 in the human coronary circulation by studying the effect of an intracoronary infusion of the specific endothelin receptor subtype A (ETA) receptor antagonist BQ123 on coronary vasomotor tone.BACKGROUNDEndothelin-1 contributes to the maintenance of peripheral vascular tone in humans. However, its physiological role in the human coronary vasculature is unknown.METHODSWe studied 12 patients (mean age 54.7 ± 2.5 years, 3 men) undergoing cardiac catheterization for investigation of atypical chest pain, with angiographically normal coronary arteries. Coronary artery cross-sectional area was measured with digital quantitative coronary angiography, and coronary blood flow was assessed with an intracoronary Doppler flow wire. Flow-mediated (adenosine, 18 μg) and agonist-mediated (substance P, 20 pmol/min for 2 min) endothelial responses were measured prior to study. BQ123 (40 nmol/min for 15 min and monitored for a further 15 min) was infused into the left coronary artery.RESULTSThe BQ123 caused significant dilation of the proximal (artery cross-sectional area: 8.08 ± 0.9 to 8.88 ± 0.9 mm2; p < 0.05), mid (5.32 ± 0.8 to 6.49 ± 0.8 mm2; p < 0.001) and distal study vessel (2.11 ± 0.2 to 2.50 ± 0.2 mm2; p < 0.05). There was an increase in coronary blood flow (26.8 ± 2.8 to 32.8 ± 3.4 ml/min; p < 0.001) but no change in systemic hemodynamics. Baseline flow- or substance P-induced epicardial vasodilation did not correlate with the degree of vasodilation induced by BQ123.CONCLUSIONSThese data uncover a role of endogenous endothelin-1 in the maintenance of basal vasomotor tone in patients with angiographically normal coronary arteries.

Endothelial and myogenic regulation of coronary artery tone in the mouse

Ventricular septal (150–200 μm) arteries were isolated from the hearts of six-week-old CD-1 mice and mounted on a pressure myograph. Equilibration of the vessels at 70 mm Hg for 60 min resulted in the development of spontaneous myogenic tone. Maximum tone observed in these vessels greatly exceeded that previously reported in septal arteries from rats. Inhibition of endothelin ET A and endothelin ET B receptors with bosentan (1 and 10 μM) reduced basal tone. Endothelin release required intact endothelial cells. The α 1-adrenceptor selective agonists phenylephrine and methoxamine did not cause change in coronary tone, while the α 2-adrenceptor selective agonists 6-allyl-2-amino5,6,7,8-tetrahydro-4 H-thiazolo-[4,5-d]azepin-dihydrochloride (BHT 920) and clonidine produced vasodilatation. Noradrenaline (1 nM–10 μM) induced a concentration-dependent vasodilatation, which was inhibited by concurrent treatment with yohimbine (10 μM) and propranolol (20 μM). Vasodilatation due to BHT 920 was abolished with vessel denudation, indicating the endothelial location of α 2-adrenoceptors. Acetylcholine (1 nM–10 μM) caused an endothelium dependent vasodilatation; inhibition of nitric oxide synthase with N ω-nitro- l-arginine methyl ester attenuated this response. The endothelium-dependent vasodilators bradykinin and substance P produced no vasomotor effect in mouse coronary arteries. Differences between human and murine responses may impact on the relevance of the mouse coronary artery for use as a potential model of human coronary vessel diseases.

BK(Ca) channels compensate for loss of NOS-dependent coronary artery relaxation in cardiomyopathy.

Previously, we showed that development of myocardial necrotic lesions is associated with impaired endothelium-dependent coronary artery relaxation in young cardiomyopathic hamsters. Since active necrosis declines with aging, this study was designed to determine whether coronary artery endothelium-dependent relaxation to ACh is restored and to identify the mechanisms mediating this effect. Intraluminal diameter was recorded in coronary arteries (150-250 micrometer) from control (C, 297 +/- 5 days old) and cardiomyopathic (M, 296 +/- 4 days old) hamsters. Relaxation to ACh (10(-9)-3 x 10(-5) M) was similar in vessels from C and M hamsters. However, mechanisms mediating relaxation to ACh were altered. Inhibition of nitric oxide synthase (NOS) activity with N-nitro-L-arginine (1 mM) had a greater inhibitory effect in vessels from C hamsters, indicating a reduction in NOS-dependent relaxation in vessels from M hamsters. Conversely, inhibition of large Ca(2+)-dependent K(+) (BK(Ca)) channels with charybdotoxin (CTX, 0.1 microM) had a greater inhibitory effect in vessels from M hamsters. In the presence of both N-nitro-L-arginine and CTX, relaxation to ACh was abolished in both groups. CTX (0.1 micrometer) produced a 50 +/- 4 and 30 +/- 3% contraction of vessels from M and C hamsters, respectively, indicating an enhanced role for BK(Ca) channels in regulation of coronary artery tone in M hamsters. Finally, vasodilatory cyclooxygenase products contributed to ACh-induced relaxation in vessels from M, but not C, hamsters. In conclusion, NOS-dependent relaxation of coronary small arteries is reduced in the late stage of cardiomyopathy. An increase in relaxation mediated by BK(Ca) channels and vasodilatory cyclooxygenase products compensates for this effect.

Prologue: new insights into the regulation of the coronary microcirculation.

The third Special Topic of papers on new regulatory mechanisms in the coronary microcirculation is focussed on the control of coronary vasomotor tone. Papers with both a basic scientific and clinical orientation are included, because in the aggregate, the studies show the importance of the coronary

Enhanced phospholipase C activity in the cultured skin fibroblast obtained from patients with coronary spastic angina: possible role for enhanced vasoconstrictor response

OBJECTIVESWe measured phospholipase C (PLC) activity in the cultured skin fibroblasts obtained from patients with and without coronary spasm and examined its correlation with coronary artery vasomotility.BACKGROUNDCoronary artery vasomotility is enhanced in coronary spastic angina (CSA), but no information is available for the intracellular signaling. In spontaneously hypertensive rats, PLC activity in the skin fibroblasts has been shown to be enhanced.METHODSSkin fibroblasts obtained from 24 patients with CSA—14 with organic coronary artery disease (CAD) and 12 control subjects—were cultured by the explant method. Activity of PLC was determined by incubating the membrane fraction with 3H-phosphatidyl inositol bisphosphate and by quantifying 3H-inositol trisphosphate. In patients with CSA and control subjects, the relations between PLC activity and coronary artery basal tone and constrictor response to intracoronary acetylcholine (ACh) were examined.RESULTSActivity of PLC (pmol/protein [mg] per min) was 1.74 ± 0.19 in patients with CSA; 0.90 ± 0.12 in patients with CAD; and 0.65 ± 0.07 in control subjects (p < 0.001, patients with CSA vs. patients with CAD and control subjects; p = NS, patients with CAD vs. control subjects). According to the Lineweaver-Burk plot, Michaelis constant (μmol/liter) of PLC was 28 ± 4 in patients with CSA; 49 ± 14 in patients with CAD; and 56 ± 10 in control subjects (p < 0.05, patients with CSA vs. control subjects), whereas the maximal velocity was not different between the three groups. There were significant positive correlations between PLC activity and both basal tone (p = 0.0108) and response to ACh (p = 0.0053). Western blot analysis using membrane fraction demonstrated that 89% of PLC isoenzymes detected was of the δ1 isoform.CONCLUSIONSBecause the PLC activity measured was genetically defined and was positively correlated with coronary artery vasomotility, enhanced PLC activity may be involved in the pathogenesis of coronary spasm.

Role of pertussis toxin-sensitive G protein in metabolic vasodilation of coronary microcirculation.

We have previously demonstrated that pertussis toxin (PTX)-sensitive G protein (G(PTX)) plays a major role in coronary microvascular vasomotion during hypoperfusion. We aimed to elucidate the role of G(PTX) during increasing metabolic demand. In 18 mongrel dogs, coronary arteriolar diameters were measured by fluorescence microangiography using a floating objective. Myocardial oxygen consumption (MVO(2)) was increased by rapid left atrial pacing. In six dogs, PTX (300 ng/ml) was superfused onto the heart surface for 2 h to locally block G(PTX). In eight dogs, the vehicle (Krebs solution) was superfused in the same way. Before and after each treatment, the diameters were measured during control (130 beats/min) and rapid pacing (260 beats/min) in each group. Metabolic stimulation before and after the vehicle treatment caused 8.6 +/- 1. 8 and 16.1 +/- 3.6% dilation of coronary arterioles <100 microm in diameter (57 +/- 8 microm at control, n = 10), respectively. PTX treatment clearly abolished the dilation of arterioles (12.8 +/- 2. 5% before and 0.9 +/- 1.6% after the treatment, P < 0.001 vs. vehicle; 66 +/- 8 microm at control, n = 11) in response to metabolic stimulation. The increases in MVO(2) and coronary flow velocity were comparable between the vehicle and PTX groups. In four dogs, 8-phenyltheophylline (10 microM, superfusion for 30 min) did not affect the metabolic dilation of arterioles (15.3 +/- 2.0% before and 16.4 +/- 3.8% after treatment; 84.3 +/- 11.0 microm at control, n = 8). Thus we conclude that G(PTX) plays a major role in regulating the coronary microvascular tone during active hyperemia, and adenosine does not contribute to metabolic vasodilation via G(PTX) activation.

High level of cholesterol increases coronary vasomotor tone during exercise.

Coronary vasomotor tone plays an important role in the regulation of myocardial perfusion and influences ischemic threshold significantly. Endothelial dysfunction occurs in the presence of coronary risk factors and is closely linked to the development of atherosclerosis affecting myocardial perfusion and decreasing ischemic threshold. To study the effect of hypercholesterolemia on coronary vasomotor tone in normal and stenotic coronary arteries at rest and during exercise. In total 48 patients were included in the present analysis. Patients were divided into two groups according to the actual levels of serum cholesterol: 18 patients had normal (mean 181 +/- 28 mg%; group 1) and 30 had elevated (mean 263 +/- 46 mg%; group 2) levels of serum cholesterol according to the 4S criteria with a cutoff level of 213 mg% (5.5 mmol/l). Coronary vasomotor tone at rest and during supine bicycle exercise was calculated by dividing mean aortic pressure by radius of coronary vessel obtained using biplanar quantitative coronary angiography. A normal as well as a stenotic vessel segment in each patient were studied. Normal vessel segments in patients with normal levels of cholesterol (group 1) exhibited no exercise-induced change in coronary vascular tone (+3%, NS), whereas a significant increase in tone (+24%, P < 0.01 versus rest) occurred in those with high levels of cholesterol (group 2). In contrast, stenotic segments in members of both groups exhibited an increase in vascular tone irrespective of the actual level of serum cholesterol. Hypercholesterolemia causes a pathologic increase in coronary vasomotor tone of angiographically normal vessel segments during exercise. A similar pathologic response occurs in stenotic arteries, but this is independent of the actual level of serum cholesterol. These findings suggest that hypercholesterolemia influences vasomotor tone of the nonstenosed coronary arteries in patients with coronary artery disease probably through the occurrence of endothelial dysfunction.