Control Of Coronary Blood Flow Research Articles

Protection by L-arginine against Epinephrine-induced Arrhythmia and Cardiotoxicity

Nitric oxide is important in control of coronary blood flow, cardiomyocyte contractility, cardiac rhythm, and thrombogenicity. We aimed to investigate the effect of the nitric oxide precursor L-arginine on cardiac arrhythmia and myocardial injury induced by epinephrine in male Sprague-Dawley rats. The possible modulation of L-arginine effects by methylene blue (MethyB), a nitric oxide synthase inhibitor was also examined. Cardiac arrhythmia was induced with 10μg/kg epinephrine given intravenously (i.v.). L-arginine (200mg/kg) or L-arginine and MethyB (100mg/kg) were intraperitoneally (i.p.) administered prior to i.v. epinephrine. Other groups received i.p. saline, only L-arginine or only MethyB (100mg/kg). Results showed that compared with the saline control, epinephrine caused a significant bradycardia (188.7±24.4 vs. 405.6±1.19beats/min), increased QRS duration (0.039±0.006 vs. 0.0185±0.0001s), decreased R wave amplitude (0.18±0.01 vs. 0.23±0.011mv), and ventricular extrasystoles. L-arginine alone showed no significant effect on heart rate (380.8±10.0 vs. 405.6±1.19beats/min) but increased QRS duration (0.029±0.0002 vs. 0.0185±0.0001s), and R wave amplitude (0.47±0.01 vs. 0.23±0.011mv). L-arginine given prior to epinephrine prevented bradycardia, shortened the duration of arrhythmia and decreased the number of extrasystoles. The administration of L-arginine and MethyB almost completely suppressed epinephrine-induced ventricular arrhythmias. Epinephrine caused severe degeneration of muscle fibres, widening of intercellular spaces, cellular infiltrations and interstitial haemorrhage. L-arginine markedly attenuated, while L-arginine/MethyB completely prevented these changes. It is concluded that L-arginine protects against cardiac arrhythmias and cardiac tissue damage caused by epinephrine.

A Novel Postconditioning Approach Attenuates Myocardial Ischaemia-Reperfusion Injury in Rats.

Ischaemia-reperfusion injury (IRI) is the damage that occurs when blood flow is restored to a tissue or organ after a period of ischaemia. Postconditioning is a therapeutic strategy aimed at reducing the tissue damage caused by IRI. Postconditioning in rodents is a useful tool to investigate the potential mechanisms of postconditioning. Currently, there is no convenient approach for postconditioning rodents. Rats were subjected to a balloon postconditioning procedure. A balloon was used to control the flow in the vessel. This allowed for easy and precise manipulation of perfusion. Evans blue and triphenyltetrazolium chloride (TTC) double staining were used to determine the infarct size. Apoptosis in the myocardium was visualised and quantified by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL). Western blotting was performed to assess the expression of key apoptotic proteins, i.e., B-cell lymphoma 2 (Bcl-2), Bcl-2 Associated X (Bax), and cleaved caspase-3. The balloon control approach to postconditioning provided accurate control of coronary blood flow and simplified the postconditioning manipulation. Infarct size reduction was observed in IRI rats after post-conditioning. There was a decrease in cardiac apoptosis in IRI rats after conditioning, as detected by TUNEL staining. IRI rats showed increased Bcl-2 levels and decreased Bax and cleaved caspase-3 levels in the myocardium. Postconditioning was successfully applied in rats using this novel approach. Postconditioning with this approach reduced infarct size and apoptosis in the area at risk.

Integrated control of coronary blood flow in exercising swine by adenosine, nitric oxide, and KATP channels.

The interplay of mechanisms regulating coronary blood flow (CBF) remains incompletely understood. Previous studies in dogs indicated that CBF regulation by KATP channels, adenosine, and nitric oxide (NO) follows a nonlinear redundancy design and fully accounted for exercise-induced coronary vasodilation. Conversely, in swine, these mechanisms appear to regulate CBF in a linear additive fashion with considerable exercise-induced vasodilation remaining when all three mechanisms are inhibited. A direct comparison between these studies is hampered by the different doses and administration routes (intravenous vs. intracoronary) of drugs inhibiting these mechanisms. Here, we investigated the role of KATP channels, adenosine, and NO in CBF regulation in swine using identical drug regimen as previously employed in dogs. Instrumented swine were exercised on a motor-driven treadmill, before and after blockade of KATP channels (glibenclamide, 50 µg/kg/min ic) and combination of inhibition of NO synthase (Nω-nitro-l-arginine, NLA, 1.5 mg/kg ic) and adenosine receptors (8-phenyltheophylline, 8PT, 5 mg/kg iv) or their combination NLA + 8PT + glibenclamide. Glibenclamide and NLA + 8PT each produced coronary vasoconstriction both at rest and during exercise, whereas the combination of NLA + 8PT + glibenclamide resulted in a small further coronary vasoconstriction compared with NLA + 8PT that was, however, less than the sum of the vasoconstriction produced by NLA + 8PT and glibenclamide, each. Thus, in contrast to previous observations in the dog, 1) the coronary vasoconstrictor effect of glibenclamide was not enhanced in the presence of NLA + 8PT and 2) the exercise-induced increase in CBF was largely maintained. These findings show profound species differences in the mechanisms controlling CBF at rest and during exercise.NEW & NOTEWORTHY The present study demonstrates important species differences in the regulation of coronary blood flow by adenosine, NO, and KATP channels at rest and during exercise. In swine, these mechanisms follow a linear additive design, as opposed to dogs which follow a nonlinear redundant design. Simultaneous blockade of all three mechanisms virtually abolished exercise-induced coronary vasodilation in dogs, whereas a substantial vasodilator reserve could still be recruited during exercise in swine.

Sex differences in the coronary vascular response to combined chemoreflex and metaboreflex stimulation in healthy humans.

What is the central question of this study? Coronary blood flow in healthy humans is controlled by both local metabolic signalling and adrenergic activity: does the integration of these signals during acute hypoxia and adrenergic activation differ between sexes? What are the main findings and its importance? Both males and females exhibit an increase in coronary blood velocity in response to acute hypoxia, a response that is constrained by adrenergic stimulation in males but not females. These findings suggest that coronary blood flow control differs between males and females. Coronary hyperaemia is mediated through multiple signalling pathways, including local metabolic messengers and adrenergic stimulation. This study aimed to determine whether the coronary vascular response to adrenergic stressors is different between sexes in normoxia and hypoxia. Young, healthy participants (n=32; 16F) underwent three randomized trials of isometric handgrip exercise followed by post-exercise circulatory occlusion (PECO) to activate the muscle metaboreflex. End-tidal was controlled at (1) normoxic levels throughout the trial, (2) 50mmHg for the duration of the trial (hypoxia trial), or (3) 50mmHg only during PECO (mixed trial). Mean left anterior descending coronary artery velocity (LADVmean ; transthoracic Doppler echocardiography), heart rate and blood pressure were assessed at baseline and during PECO. In normoxia, there was no change in LADVmean or cardiac workload induced by PECO in males and females. Acute hypoxia increased baseline LADVmean to a greater extent in males compared with females (P<0.05), despite a similar increase in cardiac workload. The change in LADVmean induced by PECO was similar between sexes in normoxia (P=0.31), greater in males during the mixed trial (male: 12.8 (7.7) cm/s vs. female: 8.1 (6.3) cm/s; P=0.02) and reduced in males but not females in acute hypoxia (male: -4.8 (4.5) cm/s vs. female: 0.8 (6.2) cm/s; P=0.006). In summary, sex differences in the coronary vasodilatory response to hypoxia were observed, and metaboreflex activation during hypoxia caused a paradoxical reduction in coronary blood velocity in males but not females.

Disentangling the Gordian knot of local metabolic control of coronary blood flow.

Recognition that coronary blood flow is tightly coupled with myocardial metabolism has been appreciated for well over half a century. However, exactly how coronary microvascular resistance is tightly coupled with myocardial oxygen consumption (MV̇o2) remains one of the most highly contested mysteries of the coronary circulation to this day. Understanding the mechanisms responsible for local metabolic control of coronary blood flow has been confounded by continued debate regarding both anticipated experimental outcomes and data interpretation. For a number of years, coronary venous Po2 has been generally accepted as a measure of myocardial tissue oxygenation and thus the classically proposed error signal for the generation of vasodilator metabolites in the heart. However, interpretation of changes in coronary venous Po2 relative to MV̇o2 are quite nuanced, inherently circular in nature, and subject to confounding influences that remain largely unaccounted for. The purpose of this review is to highlight difficulties in interpreting the complex interrelationship between key coronary outcome variables and the arguments that emerge from prior studies performed during exercise, hemodilution, hypoxemia, and alterations in perfusion pressure. Furthermore, potential paths forward are proposed to help to facilitate further dialogue and study to ultimately unravel what has become the Gordian knot of the coronary circulation.

Obesity decreases the contribution of Kv channels to hypoxic coronary vasodilation

Background and Hypothesis: Our group previously demonstrated that reductions in the functional expression of voltage-dependent K+ (Kv) channels contribute to impaired metabolic control of coronary blood flow in the setting of obesity. This study tested the hypothesis that obesity diminishes the contribution of Kv channels to coronary vasodilation in response to hypoxemia.&#x0D; Experimental Design or Project Methods: Control lean (n = 7) and obese (n = 5) swine were anesthetized and the heart exposed via left lateral thoracotomy. Coronary blood flow was measured in response to hypoxemia, before and after inhibition of Kv channels by 4-aminopyridine (4-AP; 0.3 mg/kg, iv), by a flow probe placed about the left anterior descending coronary artery. Hypoxemia was induced by progressive increases in the amount of nitrogen introduced into the ventilator. Arterial blood samples were obtained at each reduction in arterial oxygenation via a catheter placed in the femoral artery.&#x0D; Results: Blood pressure decreased from ~88 ± 5 mmHg to ~68 ± 6 mmHg (P = 0.01) as arterial PO2 was reduced below 50 mmHg in both lean and obese swine (P = 0.51). In lean swine, coronary flow progressively increased from ~0.6 to &gt;3.0 ml/min/g as arterial PO2 was reduced. This response was decreased by ~40% in obese swine and by ~30% in lean swine treated with 4-AP. Administration of 4AP had no effect on coronary flow in obese swine.&#x0D; Conclusion and Potential Impact: These data support that Kv channels contribute to increases in coronary flow in response to hypoxemia in lean swine and that reductions in Kv channel function contribute to impaired hypoxic coronary vasodilation in obese swine. We propose that therapeutic targeting of obesity associated pathways (angiotensin-aldosterone system) known to influence K+ channel expression could improve coronary microvascular function and cardiovascular outcomes in subjects with obesity. Supported by R01 HL136386; T35 HL 110854.

The many faces of myocardial ischaemia and angina

Obstructive disease of the epicardial coronary arteries is the main cause of angina. However, a number of patients with anginal symptoms have normal coronaries or non-obstructive coronary artery disease (CAD) despite electrocardiographic evidence of ischaemia during stress testing. In addition to limited microvascular vasodilator capacity, the coronary microcirculation of these patients is particularly sensitive to vasoconstrictor stimuli, in a condition known as microvascular angina. This review briefly summarizes the determinants and control of coronary blood flow (CBF) and myocardial perfusion. It subsequently analyses the mechanisms responsible for transient myocardial ischaemia: obstructive CAD, coronary spasm and coronary microvascular dysfunction in the absence of epicardial coronary lesions, and variable combinations of structural anomalies, impaired endothelium-dependent and/or -independent vasodilation, and enhanced perception of pain. Lastly, we exemplify mechanism of angina during tachycardia. Distal to a coronary stenosis, coronary dilator reserve is already recruited and can be nearly exhausted at rest distal to a severe stenosis. Increased heart rate reduces the duration of diastole and thus CBF when metabolic vasodilation is no longer able to increase CBF. The increase in myocardial oxygen consumption and resulting metabolic vasodilation in adjacent myocardium without stenotic coronary arteries further acts to divert blood flow away from the post-stenotic coronary vascular bed through collaterals.

Beta-1 vs. beta-2 adrenergic control of coronary blood flow during isometric handgrip exercise in humans.

During exercise, β-adrenergic receptors are activated throughout the body. In healthy humans, the net effect of β-adrenergic stimulation is an increase in coronary blood flow. However, the role of vascular β1 vs. β2 receptors in coronary exercise hyperemia is not clear. In this study, we simultaneously measured noninvasive indexes of myocardial oxygen supply (i.e., blood velocity in the left anterior descending coronary artery; Doppler echocardiography) and demand [i.e., rate pressure product (RPP) = heart rate × systolic blood pressure) and tested the hypothesis that β1 blockade with esmolol improves coronary exercise hyperemia compared with nonselective β-blockade with propranolol. Eight healthy young men received intravenous infusions of esmolol, propranolol, and saline on three separate days in a single-blind, randomized, crossover design. During each infusion, subjects performed isometric handgrip exercise until fatigue. Blood pressure, heart rate, and coronary blood velocity (CBV) were measured continuously, and RPP was calculated. Changes in parameters from baseline were compared with paired t-tests. Esmolol (Δ = 3296 ± 1204) and propranolol (Δ = 2997 ± 699) caused similar reductions in peak RPP compared with saline (Δ = 5384 ± 1865). In support of our hypothesis, ΔCBV with esmolol was significantly greater than with propranolol (7.3 ± 2.4 vs. 4.5 ± 1.6 cm/s; P = 0.002). This effect was also evident when normalizing ΔCBV to ΔRPP. In summary, not only does selective β1 blockade reduce myocardial oxygen demand during exercise, but it also unveils β2-receptor-mediated coronary exercise hyperemia.NEW & NOTEWORTHY In this study, we evaluated the role of vascular β1 vs. β2 receptors in coronary exercise hyperemia in a single-blind, randomized, crossover study in healthy men. In response to isometric handgrip exercise, blood flow velocity in the left anterior descending coronary artery was significantly greater with esmolol compared with propranolol. These findings increase our understanding of the individual and combined roles of coronary β1 and β2 adrenergic receptors in humans.

Effect of adrenergic agonists on coronary blood flow: a laboratory study in healthy volunteers.

Myocardial oxygen supply and demand mismatch is fundamental to the pathophysiology of ischemia and infarction. The sympathetic nervous system, through α‐adrenergic receptors and β‐adrenergic receptors, influences both myocardial oxygen supply and demand. In animal models, mechanistic studies have established that adrenergic receptors contribute to coronary vascular tone. The purpose of this laboratory study was to noninvasively quantify coronary responses to adrenergic receptor stimulation in humans. Fourteen healthy volunteers (11 men and 3 women) performed isometric handgrip exercise to fatigue followed by intravenous infusion of isoproterenol. A subset of individuals also received infusions of phenylephrine (n = 6), terbutaline (n = 10), and epinephrine (n = 4); all dosages were based on fat‐free mass and were infused slowly to achieve steady‐state. The left anterior descending coronary artery was visualized using Doppler echocardiography. Beat‐by‐beat heart rate (HR), blood pressure (BP), peak diastolic coronary velocity (CBVpeak), and coronary velocity time integral were calculated. Data are presented as M ± SD. Isometric handgrip elicited significant increases in BP, HR, and CBVpeak (from 23.3 ± 5.3 to 34.5 ± 9.9 cm/sec). Isoproterenol raised HR and CBVpeak (from 22.6 ± 4.8 to 43.9 ± 12.4 cm/sec). Terbutaline and epinephrine evoked coronary hyperemia whereas phenylephrine did not significantly alter CBVpeak. Different indices of coronary hyperemia (changes in CBVpeak and velocity time integral) were significantly correlated (R = 0.803). The current data indicate that coronary hyperemia occurs in healthy humans in response to isometric handgrip exercise and low‐dose, steady‐state infusions of isoproterenol, terbutaline, and epinephrine. The contribution of β1 versus β2 receptors to coronary hyperemia remains to be determined. In this echocardiographic study, we demonstrate that coronary blood flow increases when β‐adrenergic receptors are stimulated (i.e., during exercise and different intravenous infusions). Our infusion paradigms and beat‐by‐beat imaging methodologies can be used in future studies to evaluate age‐, sex‐, and disease‐ differences in adrenergic control of coronary blood flow.

Open-loop (feed-forward) and feedback control of coronary blood flow during exercise, cardiac pacing, and pressure changes.

A control system model was developed to analyze data on in vivo coronary blood flow regulation and to probe how different mechanisms work together to control coronary flow from rest to exercise, and under a variety of experimental conditions, including cardiac pacing and with changes in coronary arterial pressure (autoregulation). In the model coronary flow is determined by the combined action of a feedback pathway signal that is determined by the level of plasma ATP in coronary venous blood, an adrenergic open-loop (feed-forward) signal that increases with exercise, and a contribution of pressure-mediated myogenic control. The model was identified based on data from exercise experiments where myocardial oxygen extraction, coronary flow, cardiac interstitial norepinephrine concentration, and arterial and coronary venous plasma ATP concentrations were measured during control and during adrenergic and purinergic receptor blockade conditions. The identified model was used to quantify the relative contributions of open-loop and feedback pathways and to illustrate the degree of redundancy in the control of coronary flow. The results indicate that the adrenergic open-loop control component is responsible for most of the increase in coronary blood flow that occurs during high levels of exercise. However, the adenine nucleotide-mediated metabolic feedback control component is essential. The model was evaluated by predicting coronary flow in cardiac pacing and autoregulation experiments with reasonable fits to the data. The analysis shows that a model in which coronary venous plasma adenine nucleotides are a signal in local metabolic feedback control of coronary flow is consistent with the available data.

A mathematical model of coronary blood flow control: simulation of patient-specific three-dimensional hemodynamics during exercise.

This work presents a mathematical model of the metabolic feedback and adrenergic feedforward control of coronary blood flow that occur during variations in the cardiac workload. It is based on the physiological observations that coronary blood flow closely follows myocardial oxygen demand, that myocardial oxygen debts are repaid, and that control oscillations occur when the system is perturbed and so are phenomenological in nature. Using clinical data, we demonstrate that the model can provide patient-specific estimates of coronary blood flow changes between rest and exercise, requiring only the patient's heart rate and peak aortic pressure as input. The model can be used in zero-dimensional lumped parameter network studies or as a boundary condition for three-dimensional multidomain Navier-Stokes blood flow simulations. For the first time, this model provides feedback control of the coronary vascular resistance, which can be used to enhance the physiological accuracy of any hemodynamic simulation, which includes both a heart model and coronary arteries. This has particular relevance to patient-specific simulation for which heart rate and aortic pressure recordings are available. In addition to providing a simulation tool, under our assumptions, the derivation of our model shows that β-feedforward control of the coronary microvascular resistance is a mathematical necessity and that the metabolic feedback control must be dependent on two error signals: the historical myocardial oxygen debt, and the instantaneous myocardial oxygen deficit.

Carbon Dioxide and the Heart: Physiology and Clinical Implications.

Carbon dioxide (CO2) is an end product of aerobic cellular respiration. In healthy persons, PaCO2 is maintained by physiologic mechanisms within a narrow range (35-45 mm Hg). Both hypercapnia and hypocapnia are encountered in myriad clinical situations. In recent years, the number of hypercapnic patients has increased by the use of smaller tidal volumes to limit lung stretch and injury during mechanical ventilation, so-called permissive hypercapnia. A knowledge and appreciation of the effects of CO2 in the heart are necessary for optimal clinical management in the perioperative and critical care settings. This article reviews, from a historical perspective: (1) the effects of CO2 on coronary blood flow and the mechanisms underlying these effects; (2) the role of endogenously produced CO2 in metabolic control of coronary blood flow and the matching of myocardial oxygen supply to demand; and (3) the direct and reflexogenic actions of CO2 on myocardial contractile function. Clinically relevant issues are addressed, including the role of increased myocardial tissue PCO2 (PmCO2) in the decline in myocardial contractility during coronary hypoperfusion and the increased vulnerability to CO2-induced cardiac depression in patients receiving a β-adrenergic receptor antagonist or with otherwise compromised inotropic reserve. The potential use of real-time measurements of PmO2 to monitor the adequacy of myocardial perfusion in the perioperative period is discussed.

Quantitative Analysis of Vasodilatory Action of Quercetin on Intramural Coronary Resistance Arteries of the Rat In Vitro

BackgroundDietary quercetin improves cardiovascular health, relaxes some vascular smooth muscle and has been demonstrated to serve as a substrate for the cyclooxygenase enzyme.Aims1. To test quantitatively a potential direct vasodilatory effect on intramural coronary resistance artery segments, in different concentrations. 2. To scale vasorelaxation at different intraluminal pressure loads on such vessels of different size. 3. To test the potential role of prostanoids in vasodilatation induced by quercetin.MethodsCoronary arterioles (70–240 µm) were prepared from 24 rats and pressurized in PSS, using a pressure microangiometer.ResultsThe spontaneous tone that developed at 50 mmHg was relaxed by quercetin in the 10−9 moles/lit concentration (p<0.05), while 10−5 moles/lit caused full relaxation. Significant relaxation was observed at all pressure levels (10–100 mmHg) at 10−7 moles/lit concentration of quercetin. The cyclooxygenase blocker indomethacin (10−5moles/lit) induced no relaxation but contraction when physiological concentrations of quercetin were present in the tissue bath (p<0.02 with Anova), this contraction being more prominent in smaller vessels and in the higher pressure range (p<0.05, Pearson correlation). A further 2–8% contraction could be elicited by the NO blocker L-NAME (10−4 moles/lit).ConclusionThese results demonstrate that circulating levels of quercetin (10−7 moles/lit) exhibit a substantial coronary vasodilatory effect. The extent of it is commeasurable with that of several other physiological mechanisms of coronary blood flow control. At least part of this relaxation is the result of an altered balance toward the production of endogenous vasodilatory prostanoids in the coronary arteriole wall.

Coronary responses to cold air inhalation following afferent and efferent blockade.

Cardiac ischemia and angina pectoris are commonly experienced during exertion in a cold environment. In the current study we tested the hypotheses that oropharyngeal afferent blockade (i.e., local anesthesia of the upper airway with lidocaine) as well as systemic β-adrenergic receptor blockade (i.e., intravenous propranolol) would improve the balance between myocardial oxygen supply and demand in response to the combined stimulus of cold air inhalation (-15 to -30°C) and isometric handgrip exercise (Cold + Grip). Young healthy subjects underwent Cold + Grip following lidocaine, propranolol, and control (no drug). Heart rate, blood pressure, and coronary blood flow velocity (CBV, from Doppler echocardiography) were continuously measured. Rate-pressure product (RPP) was calculated, and changes from baseline were compared between treatments. The change in RPP at the end of Cold + Grip was not different between lidocaine (2,441 ± 376) and control conditions (3,159 ± 626); CBV responses were also not different between treatments. With propranolol, heart rate (8 ± 1 vs. 14 ± 3 beats/min) and RPP responses to Cold + Grip were significantly attenuated. However, at peak exercise propranolol also resulted in a smaller ΔCBV (1.4 ± 0.8 vs. 5.3 ± 1.4 cm/s, P = 0.035), such that the relationship between coronary flow and cardiac metabolism was impaired under propranolol (0.43 ± 0.37 vs. 2.1 ± 0.63 arbitrary units). These data suggest that cold air breathing and isometric exercise significantly influence efferent control of coronary blood flow. Additionally, β-adrenergic vasodilation may play a significant role in coronary regulation during exercise.

Contribution of Hydrogen Sulfide to the Control of Coronary Blood Flow

This study examined the mechanisms by which H2 S modulates coronary microvascular resistance and myocardial perfusion at rest and in response to cardiac ischemia. Experiments were conducted in isolated coronary arteries and in open-chest anesthetized dogs. We found that the H2 S substrate l-cysteine (1-10mM) did not alter coronary tone of isolated arteries in vitro or coronary blood flow in vivo. In contrast, intracoronary (ic) H2 S (0.1-3mM) increased coronary flow from 0.49±0.08 to 2.65±0.13mL/min/g (p<0.001). This increase in flow was unaffected by inhibition of Kv channels with 4-aminopyridine (p=0.127) but was attenuated (0.23±0.02-1.13±0.13mL/min/g) by the KATP channel antagonist glibenclamide (p<0.001). Inhibition of NO synthesis (l-NAME) did not attenuate coronary responses to H2 S. Immunohistochemistry revealed expression of CSE, an endogenous H2 S enzyme, in myocardium. Inhibition of CSE with β-cyano-l-alanine (10μM) had no effect on baseline coronary flow or responses to a 15-second coronary occlusion (p=0.82). These findings demonstrate that exogenous H2 S induces potent, endothelial-independent dilation of the coronary microcirculation predominantly through the activation of KATP channels, however, our data do not support a functional role for endogenous H2 S in the regulation of coronary microvascular resistance.

Contribution of electromechanical coupling between KV and CaV1.2 channels to coronary dysfunction in obesity

Previous investigations indicate that diminished functional expression of voltage-dependent K(+) (KV) channels impairs control of coronary blood flow in obesity/metabolic syndrome. The goal of this investigation was to test the hypothesis that KV channels are electromechanically coupled to CaV1.2 channels and that coronary microvascular dysfunction in obesity is related to subsequent increases in CaV1.2 channel activity. Initial studies revealed that inhibition of KV channels with 4-aminopyridine (4AP, 0.3mM) increased intracellular [Ca(2+)], contracted isolated coronary arterioles and decreased coronary reactive hyperemia. These effects were reversed by blockade of CaV1.2 channels. Further studies in chronically instrumented Ossabaw swine showed that inhibition of CaV1.2 channels with nifedipine (10μg/kg, iv) had no effect on coronary blood flow at rest or during exercise in lean swine. However, inhibition of CaV1.2 channels significantly increased coronary blood flow, conductance, and the balance between coronary flow and metabolism in obese swine (P<0.05). These changes were associated with a ~50% increase in inward CaV1.2 current and elevations in expression of the pore-forming subunit (α1c) of CaV1.2 channels in coronary smooth muscle cells from obese swine. Taken together, these findings indicate that electromechanical coupling between KV and CaV1.2 channels is involved in the regulation of coronary vasomotor tone and that increases in CaV1.2 channel activity contribute to coronary microvascular dysfunction in the setting of obesity.

The Coronary Microcirculation in Health and Disease

Coronary blood flow is closely regulated to meet the changing metabolic demands of the working myocardium. Resistance of the coronary vasculature is determined by metabolic, myogenic, endothelial, and neural mechanisms. The influence of these control mechanisms varies throughout the coronary circulation, as they have dominant sites of action in vessels of different caliber. Coronary vascular resistance depends upon the coordinated response to these influences. Within a segment of the coronary circulation, resistance may be determined, for example, by competitive interaction between neural vasoconstriction and metabolic vasodilation. Such a system in which control occurs through multiple mechanisms with varying effects allows for precise control of coronary blood flow. This system also provides protection against dysfunction of a single control mechanism. If one fails, other control mechanisms can compensate for that loss of function. Thus, adequate delivery of oxygen and nutrients can be maintained despite potential dysfunction and large fluctuations in metabolic demands of the myocardium. In disease states, these regulatory mechanisms may also fail, and endothelial dysfunction is commonly seen in the setting of cardiac disease. Optimal cardioprotective therapies must target the coronary microcirculation and cardiac myocytes in tandem. Similarly, reversal of cardiac dysfunction requires concomitant amelioration of coronary microvascular dysfunction.

Role of Voltage‐dependent Kv7 Channels in the Regulation of Coronary Blood Flow

Previous studies have demonstrated a key role for voltage‐dependent K (KV) channels in the regulation of coronary metabolic and ischemic vasodilation. However, the KV channel subtypes responsible for the control of coronary blood flow have not been clearly defined. This investigation examined the hypothesis that KV7 channels are present in coronary vascular smooth muscle and regulate perfusion of the myocardium at rest and during pacing‐induced increases in metabolism. In anesthetized, open‐chest swine, we found that intracoronary administration of the KV7 agonist flupirtine (1 nM‐10 μM) or the KV7 antagonist linopiridine (1–100 μM) did not significantly affect baseline coronary blood flow (Δ = −0.002 ± 0.005 vs. −0.010 ± 0.004 ml/min/g, respectively). H2O2 increased coronary flow ~2‐fold and this response was unaffected by linopiridine. Whole‐cell recording of isolated coronary smooth muscle cells revealed that flupirtine and linopirdine both inhibited KV current by ~25 ± 3% (P &lt; 0.05). Cardiac pacing (57 + 5 to 102 + 12 bpm) increased myocardial oxygen consumption (~35%) and coronary blood flow (~38%). However, linopridine (10 μM) did not significantly alter this increase in coronary flow or the slope of the relationship between coronary blood flow and myocardial oxygen consumption (P = 0.61). These data do not support a prominent role for KV7 channels in the regulation of coronary blood flow. (Support: HL092245)

Contribution of Cav1.2 Channels to Coronary Microvascular Dysfunction in Metabolic Syndrome

Previous investigations by our group indicate that diminished functional expression of KV channels impairs control of coronary blood flow in obesity/metabolic syndrome (MetS). The goal of this investigation was to test the hypothesis that coronary microvascular dysfunction in MetS is also related to subsequent increases in CaV1.2 channel activity. Initial studies revealed that inhibition of KV channels with 4‐aminopyridine (4AP, 0.3 mM) increased intracellular [Ca2+], contracted isolated coronary arterioles, and decreased coronary reactive hyperemia, and that these effects were reversed by blockade of CaV1.2 channels. Further studies in chronically instrumented Ossabaw swine showed that inhibition of CaV1.2 channels with nifedipine (10 μg/kg, iv) had no effect on coronary blood flow at rest or during exercise in lean swine. However, blockade of CaV1.2 channels significantly elevated coronary blood flow, conductance, and the balance between flow and metabolism in swine with the MetS (P &lt; 0.05). These changes were associated with a ~50% increase in inward CaV1.2 current and elevations in α1c and α2δ1 channel subunit expression in coronary smooth muscle cells from MetS swine. Taken together, these data indicate that increased functional expression of coronary CaV1.2 channels contributes to coronary microvascular dysfunction in the MetS.

Control of Coronary Blood Flow During Exercise

During exercise, coronary blood flow increases to match the augmented myocardial oxygen demand because of tachycardia. Coronary vasodilation during exercise is via a combination of feedforward and feedback control mechanisms. Feedforward control is mediated by sympathetic β-adrenoceptor vasodilation. Feedback vasodilator control is via a novel hypothesis where adenine nucleotides released from red blood cells act on endothelial purinergic receptors.