Functional characteristics of the coronary microcirculation

Coronary venous therapy to improve microvascular dysfunction

The coronary circulation is a complex system in which vascular resistances are determined by an interplay of forces in at least three compartments: the epicardial, the microvascular, and the venous district. Cardiologists, and particularly interventional cardiologists, normallly place the focus of their attention on diseases of the epicardial coronary circulation as possible causes of coronary syndromes and neglect the importance of the other two compartments of coronary circulation. The study of the coronary microcirculation, an increasingly recognized source of ischemia, has long been disregarded, but is witnessing a revival since the (re-)introduction of diagnostic tools in the better equipped catheterization laboratories. Unfortunately, to date our understanding of coronary microvascular disease remains incomplete and the numerous proposed classifications fail to reflect its complexity. Further, no specific therapy for these disorders is available. The coronary venous circulation is an even more neglected third vascular district. Its role in regulating coronary resistances is almost completely unexplored, but inital evidence suggests that the modulation of venous pressure might help improve coronary perfusion. Coronary sinus interventions are a group of invasive techniques (both surgical and catheter-based) that are designed to treat ischaemic heart disease by increasing coronary venous pressure and therefore redistributing coronary blood flow towards the endocardium. In this review paper, we revise the role of these interventions with particular focus on acute and chronic coronary microvascular disease.

Shaker-related voltage-gated K+ channel expression and vasomotor function in human coronary resistance arteries

OBJECTIVES

K_V channels are important regulators of vascular tone, but the identity of specific K_V channels involved and their regulation in disease remain less well understood. We determined the expression of K_V 1 channel subunits and their role in cAMP-mediated dilation in coronary resistance arteries from subjects with and without CAD.

METHODS

HCAs from patients with and without CAD were assessed for mRNA and protein expression of K_V 1 channel subunits with molecular techniques and for vasodilator response with isolated arterial myography.

RESULTS

Assays of mRNA transcripts, membrane protein expression, and vascular cell-specific localization revealed abundant expression of K_V 1.5 in vascular smooth muscle cells of non-CAD HCAs. Isoproterenol and forskolin, two distinct cAMP-mediated vasodilators, induced potent dilation of non-CAD arterioles, which was inhibited by both the general K_V blocker 4-AP and the selective K_V 1.5 blocker DPO-1. The cAMP-mediated dilation was reduced in CAD and was accompanied by a loss of or reduced contribution of 4-AP-sensitive K_V channels.

CONCLUSIONS

K_V 1.5, as a major 4-AP-sensitive K_V 1 channel expressed in coronary VSMCs, mediates cAMP-mediated dilation in non-CAD arterioles. The cAMP-mediated dilation is reduced in CAD coronary arterioles, which is associated with impaired 4-AP-sensitive K_V channel function.

Three-dimensional microvasculature in rat and human hearts using a non-injection Ca2+-ATPase method on thick and ultra-thick sections

Currently there are no methods available for staining rat and human myocardial microvasculature on thick sections that would allow for specific staining and differentiation of arterioles, venules, and capillaries. A non-injection technique is described that allows for labeling of the microvascular bed (MVB) in formalin-fixed pieces of the myocardium from humans and the white rat Rattus norvegicus, as well as human full-mount pericardium. Vessel staining is based on the activity of phosphatases (ATPases) and the precipitation of the released phosphate with calcium ions at high pH (pH 10.5-11.5). The resulting precipitate subsequently is converted to black or brown lead sulfide. The specificity of this reaction to vessels of the MVB allows arterioles, venules, capillaries, and pre- and postcapillaries to be clearly visualized in thick (60-100 µm) and ultra-thick (300-500 µm) sections against an unstained background of muscle and connective tissue. In addition, smooth muscle cells of arterioles are also stained allowing for differentiation between arteriolar and venular beds. These observations have not been reported in rat or human myocardium using other methods. This procedure should benefit studies of coronary microcirculation in experimental and pathological conditions, as well as in pharmacological investigations.

Oxygen Sensing, Cardiac Ischemia, HIF-1α and Some Emerging Concepts

Oxygen plays a critical role in the perpetuation and propagation of almost all forms of life. The primary site of cellular oxygen consumption is the mitochondrial electron transport chain and in addition, oxygen is also used as a substrate for various enzymes involved in cellular homeostasis. Although our knowledge of the biochemistry and physiology of oxygen transport is century old, recent development of sophisticated tools of biophysical chemistry revealed that tissue oxygenation and oxygen sensing is a highly evolved process, especially in mammals. Perturbation of normal oxygen supply is associated with diseases like tumorigenesis, myocardial infarction and stroke. Available information suggests that when tissue oxygen supply is limited, mitochondria emanate signals involving reactive oxygen species generation which in turn stabilizes oxygen sensing transcription factor HIF-1. Upon stabilization, HIF-1 elicits necessary genetic response to cope with the diminished oxygen level. In view of such critical role of HIF-1 in cellular oxygen sensing, recently there has been a heightened interest in understanding the biology of HIF-1 in the context of cardiovascular system. The following review describes some of the recent advances in this regard.

Integrative control of coronary resistance vessel tone by endothelin and angiotensin II is altered in swine with a recent myocardial infarction

Several studies have indicated an interaction between the renin-angiotensin (ANG II) system and endothelin (ET) in the regulation of vascular tone. Previously, we have shown that both ET and ANG II exert a vasoconstrictor influence on the coronary resistance vessels of awake normal swine. Here, we investigated whether the interaction between ANG II and ET exists in the control of coronary resistance vessel tone at rest and during exercise using single and combined blockade of angiotensin type 1 (AT1) and ETA/ETB receptors. Since both circulating ANG II and ET levels are increased after myocardial infarction (MI), we investigated if the interaction between these systems is altered after MI. In awake healthy swine, coronary vasodilation in response to ETA/ETB receptor blockade in the presence of AT1 blockade was similar to vasodilation produced by ETA/ETB blockade under control conditions. In awake swine with a 2- to 3-wk-old MI, coronary vasodilator responses to individual AT1 and ETA/ETB receptor blockade were virtually abolished, despite similar coronary arteriolar AT1 and ETA receptor expression compared with normal swine. Unexpectedly, in the presence of AT1 blockade (which had no effect on circulating ET levels), ETA/ETB receptor blockade elicited a coronary vasodilator response. These findings suggest that in normal healthy swine the two vasoconstrictor systems contribute to coronary resistance vessel control in a linear additive manner, i.e., with negligible cross-talk. In contrast, in the remodeled myocardium, cross-talk between ANG II and ET emerges, resulting in nonlinear redundant control of coronary resistance vessel tone.

Cross-Talk Between Cardiac Muscle and Coronary Vasculature

The cardiac muscle and the coronary vasculature are in close proximity to each other, and a two-way interaction, called cross-talk, exists. Here we focus on the mechanical aspects of cross-talk including the role of the extracellular matrix. Cardiac muscle affects the coronary vasculature. In diastole, the effect of the cardiac muscle on the coronary vasculature depends on the (changes in) muscle length but appears to be small. In systole, coronary artery inflow is impeded, or even reversed, and venous outflow is augmented. These systolic effects are explained by two mechanisms. The waterfall model and the intramyocardial pump model are based on an intramyocardial pressure, assumed to be proportional to ventricular pressure. They explain the global effects of contraction on coronary flow and the effects of contraction in the layers of the heart wall. The varying elastance model, the muscle shortening and thickening model, and the vascular deformation model are based on direct contact between muscles and vessels. They predict global effects as well as differences on flow in layers and flow heterogeneity due to contraction. The relative contributions of these two mechanisms depend on the wall layer (epi- or endocardial) and type of contraction (isovolumic or shortening). Intramyocardial pressure results from (local) muscle contraction and to what extent the interstitial cavity contracts isovolumically. This explains why small arterioles and venules do not collapse in systole. Coronary vasculature affects the cardiac muscle. In diastole, at physiological ventricular volumes, an increase in coronary perfusion pressure increases ventricular stiffness, but the effect is small. In systole, there are two mechanisms by which coronary perfusion affects cardiac contractility. Increased perfusion pressure increases microvascular volume, thereby opening stretch-activated ion channels, resulting in an increased intracellular Ca2+transient, which is followed by an increase in Ca2+sensitivity and higher muscle contractility (Gregg effect). Thickening of the shortening cardiac muscle takes place at the expense of the vascular volume, which causes build-up of intracellular pressure. The intracellular pressure counteracts the tension generated by the contractile apparatus, leading to lower net force. Therefore, cardiac muscle contraction is augmented when vascular emptying is facilitated. During autoregulation, the microvasculature is protected against volume changes, and the Gregg effect is negligible. However, the effect is present in the right ventricle, as well as in pathological conditions with ineffective autoregulation. The beneficial effect of vascular emptying may be reduced in the presence of a stenosis. Thus cardiac contraction affects vascular diameters thereby reducing coronary inflow and enhancing venous outflow. Emptying of the vasculature, however, enhances muscle contraction. The extracellular matrix exerts its effect mainly on cardiac properties rather than on the cross-talk between cardiac muscle and coronary circulation.

Contribution of endothelin to coronary vasomotor tone is abolished after myocardial infarction

Left ventricular dysfunction in swine with a recent myocardial infarction (MI) is associated with neurohumoral activation, including increased catecholamines and endothelin (ET). Although the increase in ET may serve to maintain blood pressure and, hence, perfusion of essential organs such as the heart and brain, it could also compromise myocardial perfusion by evoking coronary vasoconstriction. In the present study, we tested the hypothesis that endogenous ET contributes to perturbations in myocardial O2 balance during exercise in remodeled myocardium of swine with a recent MI. For this purpose, 26 chronically instrumented swine (10 with and 16 without MI) were studied at rest and while running on a treadmill at 1-4 km/h. After MI, plasma ET increased from 3.2 +/- 0.4 to 4.9 +/- 0.3 pM (P < 0.05). In normal swine, blockade of ETA (by EMD-122946) or ETA-ETB (by tezosentan) receptors resulted in an increase in coronary venous PO2, i.e., coronary vasodilation at rest, which decreased during exercise. In contrast, neither ETA nor ETA-ETB receptor blockade resulted in coronary vasodilation in swine with MI. Coronary vasoconstriction to intravenous ET-1 infusion in awake resting swine was blunted after MI. To investigate whether factors released by cardiac myocytes contributed to decreased vascular responsiveness to ET, we performed ET-1 dose-response curves in isolated coronary arterioles (70-200 microm). Vasoconstriction to ET-1 in isolated arterioles from MI swine was enhanced. In conclusion, the vasoconstrictor influence of endogenous as well as exogenous ET on coronary circulation in vivo is reduced. Because the response of isolated coronary arterioles to ET is increased after MI, the reduced vasoconstrictor influence in vivo suggests modulation of ET receptor sensitivity by cardiac myocytes, which may serve to maintain adequate myocardial perfusion.

Prostacyclin-mediated compensatory mechanism in the coronary circulation during acute NO synthase blockade

Nitric oxide (NO) in contrast to most prostanoids, plays a major role in the maintenance of coronary arterial tone under physiological conditions. However, in case of endothelial damage or other NO-depleting situations the importance of other vasodilating mechanisms may be increased. The aim of the present study was to investigate the crosstalk between the L-arginine - NO and the prostanoid systems in isolated rat hearts. Coronary flow and cardiac dynamics were measured in a standard Langendorff perfusion system. Application of indomethacin in the perfusion media failed to change coronary flow. Administration of L-NA, however, significantly decreased coronary flow by 24.8 +/- 2.3% (p < 0.01 vs. untreated control). In the presence of indomethacin, L-NA decreased coronary flow to an even greater extent by 35.8 +/- 5.2% (p < 0.05 vs. L-NA alone). Treatment of the preparations with L-NA or indomethacin failed to change cardiac work, coapplication of both drugs together, however, decreased cardiac work by 45 +/- 11% (p < 0.05 vs. untreated control). Heart rate remained constant throughout the experimental period and did not differ significantly between the treatment groups. The prostacyclin content of the effluent from the L-NA treated hearts was significantly higher than that of controls. We conclude that in case of decreased NO levels in the coronary circulation, arterial tone is maintained by prostacyclin production.

Balance between myogenic, flow-dependent, and metabolic flow control in coronary arterial tree: a model study

Myogenic response, flow-dependent dilation, and direct metabolic control are important mechanisms controlling coronary flow. A model was developed to study how these control mechanisms interact at different locations in the arteriolar tree and to evaluate their contribution to autoregulatory and metabolic flow control. The model consists of 10 resistance compartments in series, each representing parallel vessel units, with their diameters determined by tone depending on either flow and pressure [flow-dependent tone reduction factor (TRF(flow)) x Tone(myo)] or directly on metabolic factors (Tone(meta)). The pressure-Tone(myo) and flow-TRF(flow) relations depend on the vessel size obtained from interpolation of data on isolated vessels. Flow-dependent dilation diminishes autoregulatory properties compared with pressure-flow lines obtained from vessels solely influenced by Tone(myo). By applying Tone(meta) to the four distal compartments, the autoregulatory properties are restored and tone is equally distributed over the compartments. Also, metabolic control and blockage of nitric oxide are simulated. We conclude that a balance is required between the flow-dependent properties upstream and the constrictive metabolic properties downstream. Myogenic response contributes significantly to flow regulation.