A new method for the visualization of subepicardial coronary arteries in small isolated mammalian hearts

A model is described which permits direct visualization of large coronary arteries in a supported modified perfused heart preparation, using a perfluorochemical (FC-43) as perfusate. Filling of a large coronary artery with Patent Blue Dye is recorded by gated photography (color arteriography). The technique is applicable to the study of reactivity (spasm) of coronary arteries in hearts of small and large animals (rats, rabbits, dogs). The technique has the following advantages: preservation of vascular endothelium, adequate oxygenation, avoidance of major surgical intervention to implant sensors for the detection of changes in coronary diameter, quantitative evaluation of time dependent changes in geometry of large coronary arteries and simultaneous measurement of large coronary vessel and total coronary vascular resistance.     

Coronary artery surgery.

Coronary artery disease has been described as the largest public health problem in Western society. In spite of the many advances in recent years in its medical management, many patients remain disabled even after optimal medical therapy. The aortocoronary bypass operation, introduced in the mid-1960s, has been shown to have consistent subjective and objective effects on the course of the disease in a large proportion of patients. The procedure consists of inserting a portion of the saphenous vein into both the aorta and a coronary artery to bypass the obstruction. It is usual now to bypass obstructions in several coronary arteries at the same operation if necessary. The prognosis for patients with ischemic heart disease with medical management depends on the extent of the disease. Patients with obstruction of only one coronary artery have a prognosis very little different from normal. On the other hand, obstruction of several arteries is consistently associated with a mortality approaching or exceeding 10% per year.     

Role of ET-1 in the regulation of coronary circulation

Given that circulating ET levels in heart failure, in particular, may reach physiological threshold for coronary constrictor responses, the primary objective of the present review is to consider coronary vessels as an important target for circulating and locally produced endothelin(s). In healthy vessels, ET-1 causes biphasic coronary responses characterized by a transient dilation of large and small arteries followed by a sustained constriction. ETB receptors are pivotal in the early dilation of resistance vessels, whereas dilation of conductance vessels may be a secondary phenomenon triggered by flow increases. Exogenous ET-1 causes coronary constriction almost exclusively through ETA receptor activation. Human and canine large epicardial coronary vessels display significant baseline ET-1 dependent tone in vitro and in vivo, an ETA-dependent process. In contrast, ETB receptors located on smooth muscle cells are apparently less important for producing constrictor responses. NO production may serve as an important counter-regulatory mechanism to limit ET-dependent effects on coronary vessels. Conversely, in a dysfunctional endothelium, the loss of NO may augment ET-1 production and activity. By lifting the ET-dependent burden from coronary vessels, ET receptor blockade should help to ensure a closer match between cardiac metabolic demand and coronary perfusion.     

Diameter asymmetry of porcine coronary arterial trees: structural and functional implications

The coronary vasculature is characterized by highly asymmetric diameters at bifurcations, which may be an important determinant of flow distribution. To facilitate accurate reconstruction of the coronary network for hemodynamic analysis, we introduce a statistical data set of the diameter asymmetry at bifurcations based on morphometric data of the porcine coronary arterial and venous trees. The bifurcation asymmetry data were represented by the diameter ratio of the daughters relative to mother vessel and by an area expansion ratio (AER) at each bifurcation. A novel asymmetry ratio matrix was introduced to describe the diameter asymmetry of daughters to mother vessels. The relations between AER and flow velocity, and asymmetry ratio matrix and flow distribution, were considered. The results indicate that the ratio of large daughter to mother vessel has a minimum value at order 5 (mean diameter of approximately 70 microm), whereas the ratio of small daughter to mother vessel decreases monotonically with increase in order number. The AER was found to be fairly uniform for larger vessels and to increase from order 5 toward the capillaries. At order 5, we observe a transition in asymmetric bifurcation pattern that may mark a hemodynamic transition from transmural to perfusion subnetworks. The functional implications of these structural transitions are considered.     

Intermediate coronary artery stenosis: evidence-based decisions in interventions to avoid the oculostenotic reflex

The prevalence of intermediate coronary artery stenosis (defined as a diameter stenosis of 40% to 70%) is quite large in patients undergoing PTCA. The coronary angiogram is considered the 'gold standard' for the definition of coronary anatomy, in spite of various limitations associated with its use. In recent years, sensor tipped guidewire based methods of physiologic assessment of stenosis severity, like myocardial fractional flow reserve, and poststenotic coronary flow reserve had established their role in the decision making in catheterization laboratory. The decision making should combine morphologic and physiologic assessment as better evidence based approach in guiding therapy to avoid the 'oculostenotic reflex'.     

Influence of coronary artery diameter on eNOS protein content

The purpose of this study was to test the hypothesis that the content of endothelial nitric oxide synthase (eNOS) protein (eNOS protein/g total artery protein) increases with decreasing artery diameter in the coronary arterial tree. Content of eNOS protein was determined in porcine coronary arteries with immunoblot analysis. Arteries were isolated in six size categories from each heart: large arteries [301- to 2,500-microm internal diameter (ID)], small arteries (201- to 300-microm ID), resistance arteries (151- to 200-microm ID), large arterioles (101- to 150-microm ID), intermediate arterioles (51- to 100-microm ID), and small arterioles(<50-microm ID). To obtain sufficient protein for analysis from small- and intermediate-sized arterioles, five to seven arterioles 1-2 mm in length were pooled into one sample for each animal. Results establish that the number of smooth muscle cells per endothelial cell decreases from a number of 10 to 15 in large coronary arteries to 1 in the smallest arterioles. Immunohistochemistry revealed that eNOS is located only in endothelial cells in all sizes of coronary artery and in coronary capillaries. Contrary to our hypothesis, eNOS protein content did not increase with decreasing size of coronary artery. Indeed, the smallest coronary arterioles had less eNOS protein per gram of total protein than the large coronary arteries. These results indicate that eNOS protein content is greater in the endothelial cells of conduit arteries, resistance arteries, and large arterioles than in small coronary arterioles.     

Action of agonists and antagonists on adrenergic receptors in isolated porcine coronary arteries

The adrenergic receptors of porcine coronary arteries were investigated in helically cut strips of small (less than or equal to 0.5 mm outer-diameter (od), medium (0.8-1.2 mm od), large (1.5-2.5 mm od), and very large (greater than 4 mm od) coronary arteries. Both the beta1 agonist dobutamine and the beta2 agonist terbutaline relaxed coronary arteries partially contracted by 25 mM of KCl. Dobutamine contracted small coronary arteries at 10(-5) M concentrations, then relaxed them at 10(-4) M. The beta1-adrenoceptor antagonist metoprolol contracted coronary arteries relaxed by either dobutamine or terbutaline, but the beta2 antagonist H35/25 did so only in high and probably nonselective concentrations. Alpha1-adrenoreceptor stimulating concentrations of phenylephrine did not contract any of the arteries. Metoprolol and high concentrations of H35/25 further contracted large coronary arteries partially contracted by 25 mM potassium. These contractions were blocked by verapamil and papaverine but not by atropine, phentolamine, yohimbine, mepyramine, or methysergide. This seems to indicate that beta-adrenergic receptors in porcine coronary arteries are beta1-receptors, or closely resemble beta1-receptors. They differ from many other beta1-receptors, however, in that they are stimulated by terbutaline. Alpha1 adrenoreceptors seem not to be present in these porcine coronary arteries to a significant extent. Metoprolol and high concentrations of H35/25 have a direct contractile effect in large porcine coronary artery that is not mediated by alpha-adrenergic, muscarinic, histaminergic, or serotonergic receptors but requires verapamil-sensitive calcium.     

Adrenergic control of coronary arteries

This review of adrenergic control of coronary arteries is based on studies conducted on isolated vessels. Both alpha- and beta-adrenergic receptors are present in the coronary vasculature but with different distributions. The large coronary arteries have a larger percentage of alpha receptors, which mediate contraction, whereas the small coronary arteries are equipped almost exclusively with beta receptors, which mediate relaxation. The beta receptors are of the subclass beta 1. Both alpha 1 and alpha 2 receptors are present in the large coronaries. The alpha 2 receptors are prejunctional and inhibit neuronal release of norepinephrine (NE), and are also postjunctional and mediate contraction of the vascular smooth muscle. Evidence is reviewed suggesting that coronary alpha and beta receptors may be different conformational states of the same parent macromolecule in which the alpha state predominates at lower temperatures. Contrary to these results we have observed that whereas the response of the large coronary artery to NE at 39 C is predominantly constriction, it is predominantly dilation at 29 C. The characteristic of the adrenergic receptor is clearly temperature dependent.     

Role of the vagus in control of the major conduit coronary artery in the dog.

Using El Badawi and Schenk's modification of Karnovski's method for the demonstration of acetylcholinesterase, the authors found cholinergic fibres both in the perivascular connective tissue and directly in the wall (in the adventitia) of the major coronary arteries; the fibres were distributed regularly around the circumference of the arteries. In the case of the smaller intramyocardial arteries, the cholinergic fibres were concentrated at two poles of the blood vessel; none were present in the wall of the veins. The shape and topography of the coronary cholinergic arterial plexus resemble the shape and topography of the coronary sympathetic adrenergic system. In apparent contradiction of this finding, stimulation of the cervical vagus did not affect the diameter of the large coronary arteries. Since acetylcholine (6--10 micrograms/kg i.v.) produced a mean 7.4% increase in the diameter of the ramus interventricularis ventralis, we concluded that there are no postgangliar cholinergic fibres of vasomotor significance for the large coronary arteries in the cervical vagus. The specific acetylcholinesterase activity found in the wall of these vessels belongs either to cholinergic terminals whose ganglion cells are not located in the vagal ganglion, or to cholinergic axones terminating outside the wall of the large coronary arteries.     

Bifurcation asymmetry of the porcine coronary vasculature and its implications on coronary flow heterogeneity

The branching pattern of the coronary arteries and veins is asymmetric, i.e., many small vessels branch off of a large trunk such that the two daughter vessels at a bifurcation are of unequal diameters and lengths. One important implication of the geometric vascular asymmetry is the dispersion of blood flow at a bifurcation, which leads to large spatial heterogeneity of myocardial blood flow. To document the asymmetric branching pattern of the coronary vessels, we computed an asymmetry ratio for the diameters and lengths of all vessels, defined as the ratio of the daughter diameters and lengths, respectively. Previous data from silicone elastomer cast of the entire coronary vasculature including arteries, arterioles, venules, and veins were analyzed. Data on smaller vessels were obtained from histological specimens by optical sectioning, whereas data on larger vessels were obtained from vascular casts. Asymmetry ratios for vascular areas, volumes, resistances, and flows of the various daughter vessels were computed from the asymmetry ratios of diameters and lengths for every order of mother vessel. The results show that the largest orders of arterial and venous vessels are most asymmetric and the degree of asymmetry decreases toward the smaller vessels. Furthermore, the diameter asymmetry at a bifurcation is significantly larger for the coronary veins (1.7-6.8 for sinus veins) than the corresponding arteries (1.5-5.8 for left anterior descending coronary artery) for orders 2-10, respectively. The reported diameter asymmetry at a bifurcation leads to significant heterogeneity of blood flow at a bifurcation. Hence, the present data quantify the dispersion of blood flow at a bifurcation and are essential for understanding flow heterogeneity in the coronary circulation.     

Endogenous prostaglandin release contributes directly to coronary artery tone.

An in vitro coronary artery preparation of beef heart was found to synthesize and release continuously large amounts of a prostaglandin of the E type. Inhibition of prostaglandin synthesis with aspirin, indomethacin, or eicosa-5,8,11,14-tetraynoic acid induced a sustained contraction of the coronary artery, and pretreatment with indomethacin diminished markedly the output of prostaglandin into the bathing medium. It appears that prostaglandin E1, generated from within the vessel wall itself, may act as an intrinsic regulator of coronary artery tone in the beef heart, and that blockade of this function leads to vasospasm.     

Animal pharmacology of nitroglycerin

Nitroglycerin, first introduced over a hundred years ago, is now finding wider clinical applications. To a large degree, the renewed interest in the clinical pharmacologic usage of nitroglycerin is due to the availability of new formulations and drug delivery systems. The current review focuses on the physiological and pharmacological actions of nitroglycerin in mammals. Routes of nitroglycerin metabolism, biochemistry and absorption are discussed. The phenomenon of nitroglycerin tolerance is illustrated and related to specific quantitative alterations occurring at the cellular level. The cardiovascular effects of nitroglycerin are discussed in terms of its effects on coronary flow, the myocardium itself, and on the peripheral vasculature. The early speculation of Murrell (3) that nitroglycerin “would probably prove of service in the treatment of angina pectoris…and other vascular disorders…” has now been realized.     

Hypercholesterolemia inhibits L-type calcium current in coronary macro-, not microcirculation.

Hypercholesterolemia (HC) is a mary risk factor for the development of coronary heart disease. Coronary ion regulation, especially calcium, is thought to be important in coronary heart disease development; however, the influence of high dietary fat and cholesterol on coronary arterial smooth muscle (CASM) ion channels is unknown. The purpose of this study was to determine the effect of diet-induced HC on CASM voltage-gated calcium current (I(Ca)). Male miniature swine were fed a high-fat, high-cholesterol diet (40% kcal fat, 2% wt cholesterol) for 20-24 wk, resulting in elevated serum total and low-density lipoprotein cholesterol. Histochemistry indicated early atherosclerosis in large coronary arteries. CASM were isolated from the right coronary artery (>1.0 mm ID), small arteries ( approximately 200 microm), and large arterioles ( approximately 100 microm). I(Ca) was determined by whole cell voltage clamp. L-type I(Ca) was reduced approximately 30% by HC compared with controls in the right coronary artery (-5.29 +/- 0.42 vs. -7.59 +/- 0.41 pA/pF) but not the microcirculation (small artery, -8.39 +/- 0.80 vs. -10.13 +/- 0.60; arterioles, -10.78 +/- 0.93 vs. -11.31 +/- 0.95 pA/pF). Voltage-dependent activation was unaffected by HC in both the macro- and microcirculation. L-type voltage-gated calcium channel (Ca(v)1.2) mRNA and membrane protein levels were unaffected by HC. Inhibition of I(Ca) by HC was reversed in vitro by the cholesterol scavenger methyl-beta-cyclodextrin and mimicked in control CASM by incubation with the cholesterol donor cholesterol:methyl-beta-cyclodextrin. These data indicate that CASM L-type I(Ca) is decreased in large coronary arteries in early stages of atherosclerosis, whereas I(Ca) in the microcirculation is unaffected. The inhibition of calcium channel activity in CASM of large coronary arteries is likely due to increases in membrane free cholesterol.     

A novel technique for estimating the compact myocardium in fishes reveals surprising results for an athletic air-breathing fish, the Pacific tarpon

A method for quickly assessing the relative proportion of compact myocardium in the ventricle of teleosts is introduced and used in juvenile Pacific tarpon Megalops cyprinoides , a member of the only air-breathing elopomorph teleost genus. The proportion of compact myocardium increased with body mass, reaching up to 60% of the ventricular mass. The finding for tarpon was a surprising discovery since recent literature has suggested that air breathing evolved primarily as means of supplying oxygen to the fish heart during activity. The present data, which represent the first quantitative assessment of the compact myocardium for any air-breathing fish, suggest that myocardial oxygen supply in the tarpon is supplemented by the coronary circulation associated with compact myocardium during exercise, while air breathing is important during aquatic hypoxia. Compact myocardium was also measured as a point of reference in an extant representative from a more ancient fish lineage than the elopomorphs, the water-breathing spiny dogfish Squalus acanthias and found to be only 9% of ventricular mass. In conclusion, the presence of a coronary circulation in extant elasmobranchs may mean that the coronary circulation evolved well before air breathing in fishes and, for tarpon at least, the coronary oxygen supply to the ventricular myocardium has not necessarily been superseded by air breathing.     

Echocardiographic assessment and percutaneous closure of multiple atrial septal defects

Percutaneous coronary intervention can be associated with distal embolization of thrombotic material causing myocardial necrosis and infarction. We discuss the role of intravascular imaging to guide the use of a distal protection device by describing the outcome of a young woman presenting with non-ST elevation myocardial infarction. Coronary angiography demonstrated an isolated minor stenosis in the proximal left anterior descending coronary artery with slight haziness beyond the lesion. Intravascular ultrasound confirmed an extensive thrombus overlying a bulky atherosclerotic plaque. A distal filter wire was therefore successfully used to reduce the risk of distal embolization. The use of intravascular ultrasound in patients presenting with acute coronary syndrome may reveal large thrombi that are difficult to image using conventional angiographic techniques. Intravascular ultrasound can therefore be used as a tool to select lesions requiring distal protection.     

Debate: at what level of coronary heart disease risk should a statin be prescribed?

3-Hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) are effective treatments for the primary and secondary prevention of coronary heart disease, but an outstanding issue is determining who should have such treatment. The benefit from treatment with statins appears to be proportional to the underlying risk of coronary heart disease and independent of the factors increasing risk. Most benefit will therefore be achieved by treating people at increased risk of coronary heart disease. Statins reduce coronary morbidity even when the risk of coronary heart disease is relatively low (6% over 10 years), but reduction in all-cause mortality, the true measure of safety has been shown only when the risk of a major coronary heart disease event is 15% over 10 years or greater. At this level of risk patients appear willing to take treatment to gain the benefit expected from statin treatment, and the cost effectiveness of statin treatment is within the range accepted for other treatments. The major impediments to the systematic introduction of statin treatment at this level of risk are the very high overall cost and the large workload in countries like Britain, where the population risk of coronary heart disease is high. For this reason, recent British guidelines correctly advise statin treatment for secondary prevention and primary prevention when the 10 year coronary heart disease risk is 30% or greater as the first priority, moving to a lower coronary heart disease threshold for primary prevention only when resources permit.     

Effects of the collateral circulation of the heart

Collateral circulation minimizes the myocardial injury which results from narrowing of a coronary artery. A large collateral circulation has disadvantages, however. It may divert so much of the limited blood flow through the adjacent nonarteriosclerotic coronary artery that the blood supply of the normal muscle supplied by that artery may be inadequate during heavy exercise. In the presence of a large collateral circulation, both the normal and ischemic regions of the heart may be extremely vulnerable to small arteriosclerotic changes narrowing the patent artery near the aorta. The effective increase in flow which results from arteriolar vasodilatation produced by drugs may be much greater in the presence of a small collateral circulation than a large one.     

Training induces nonuniform increases in eNOS content along the coronary arterial tree.

Exercise training produces enhanced nitric oxide (NO)-dependent, endothelium-mediated vasodilator responses of porcine coronary arterioles but not conduit coronary arteries. The purpose of this study was to test the hypothesis that exercise training increases the amount of endothelial NO synthase (eNOS) in the coronary arterial microcirculation but not in the conduit coronary arteries. Miniature swine were either exercise trained or remained sedentary for 16--20 wk. Exercise-trained pigs exhibited increased skeletal muscle oxidative capacity, exercise tolerance, and heart weight-to-body weight ratios. Content of eNOS protein was determined with immunoblot analysis in conduit coronary arteries (2- to 3-mm ID), small arteries (301- to 1,000-microm ID), resistance arteries (151- to 300-microm ID), and three sizes of coronary arterioles [large (101- to 150-microm ID), intermediate (51- to 100-microm ID), and small (<50-microm ID)]. Immunoblots revealed increased eNOS protein in some sizes of coronary arteries and arterioles but not in others. Content of eNOS was increased by 60--80% in small and large arterioles, resistance arteries, and small arteries; was increased by 10--20% in intermediate-sized arterioles; and was not changed or decreased in conduit arteries. Immunohistochemistry revealed that eNOS was located in the endothelial cells in all sizes of coronary artery. We conclude that exercise training increases eNOS protein expression in a nonuniform manner throughout the coronary arterial tree. Regional differences in shear stress and intraluminal pressures during exercise training bouts may be responsible for the distribution of increased eNOS protein content in the coronary arterial tree.     

Velocity and wall shear stress patterns in the human right coronary artery.

Blood flow dynamics in the human right coronary artery have not been adequately quantified despite the clinical significance of coronary atherosclerosis. In this study, a technique was developed to construct a rigid flow model from a cast of a human right coronary artery. A laser photochromic method was used to characterize the velocity and wall shear stress patterns. The flow conditions include steady flow at Reynolds numbers of 500 and 1000 as well as unsteady flow with Womersley parameter and peak Reynolds number of 1.82 and 750, respectively. Characterization of the three-dimensional geometry of the artery revealed that the largest spatial variation in curvature occurred within the almost branch-free proximal region, with the greatest curvature existing along the acute margin of the heart. In the proximal segment, high shear stresses were observed on the outer wall and lower, but not negative, stresses along the inner wall. Low shear stress on the inner wall may be related to the preferential localization of atherosclerosis in the proximal segment of the right coronary artery. However, it is possible that the large difference between the outer and inner wall shear stresses may also be involved.     

Na+−Ca2+ exchange and Ca2+ channel characteristics in bovine aorta and coronary artery smooth muscle sarcolemmal membranes

Tension generation and Ca²⁺ flux in smooth muscle varies depending upon the diameter of a vessel and its location. The purpose of the present investigation was to determine if the biochemical characteristics of the Na⁺–Ca²⁺ exchanger and the Ca²⁺ channel differ in sarcolemmal membrane preparations isolated from a large conduit vessel (thoracic aorta) or from large and small coronary arteries. We also investigated the possibility of differences between sarcolemmal membranes isolated from coronary arteries dissected from the right and left ventricles. The purification of the sarcolemmal membranes was of a similar magnitude amongst the different groups. Contamination of the sarcolemmal membranes with other membranous organelles was negligible and similar amongst the groups. The K_m and V_max of Na⁺-dependent Ca²⁺ uptake in sarcolemmal vesicles was similar amongst the groups. Calcium channel characteristics were examined by measuring [³H]PN200-110 binding to sarcolemmal vesicles. The right coronary artery membranes from both large and small caliber vessels exhibited a higher K_d and the small right coronary artery sarcolemmal preparation had a lower maximal binding density for [³H] PN200-110. The results suggest that the right coronary artery, and in particular the small diameter right coronary artery, possesses altered Ca²⁺ channel characteristics in isolated sarcolemmal membranes.