Metabolic regulation of coronary vascular tone: role of endothelin-1

Class of hemorrhagic shock is associated with progressive diastolic coronary flow reversal and diminished left ventricular function

Introduction: The relationship between coronary artery flow and left ventricular (LV) function during hemorrhagic shock remains unknown. The aim of this study was to quantify coronary artery flow directionality alongside left ventricular function through the four classes of hemorrhage shock. Methods: Following baseline data collection, swine were exsanguinated into cardiac arrest via the femoral artery using a logarithmic bleed, taking each animal through the four classes of hemorrhagic shock based on percent bleed (class I: 15%; class II: 15%-30%; class III: 30%-40%; class IV: >40%). Telemetry data, left ventricular pressure-volume loops, and left anterior descending artery flow tracings over numerous cardiac cycles were collected and analyzed for each animal throughout. Results: Five male swine (mean 72 ± 12 kg) were successfully exsanguinated into cardiac arrest. Mean left ventricular end-diastolic volume, end-diastolic pressure, and stroke work decreased as the hemorrhagic shock class progressed (p < 0.001). The proportion of diastole spent with retrograde coronary flow was also associated with class of hemorrhagic shock (mean 5.6% of diastole in baseline, to 63.9% of diastole in class IV; p < 0.0001), worsening at each class from baseline through class IV. Preload recruitable stroke work (PRSW) decreased significantly in classes II through IV (p < 0.001). Systemic Vascular Resistance (SVR) is associated with class of hemorrhage shock (p < 0.001). Conclusion: With progressive classes of hemorrhagic shock left ventricular function progressively decreased, and the coronary arteries spent a greater proportion of diastole in retrograde flow, with progressively more negative total coronary flow. Preload recruitable stroke work, a load-independent measure of inotropy, also worsened in severe hemorrhagic shock, indicating the mechanism extends beyond the drop in preload and afterload alone.

Mitochondrial DNA integrity and function are critical for endothelium-dependent vasodilation in rats with metabolic syndrome

Endothelial dysfunction in diabetes is generally attributed to oxidative stress, but this view is challenged by observations showing antioxidants do not eliminate diabetic vasculopathy. As an alternative to oxidative stress-induced dysfunction, we interrogated if impaired mitochondrial function in endothelial cells is central to endothelial dysfunction in the metabolic syndrome. We observed reduced coronary arteriolar vasodilation to the endothelium-dependent dilator, acetylcholine (Ach), in Zucker Obese Fatty rats (ZOF, 34 ± 15% [mean ± standard deviation] 10^-3 M) compared to Zucker Lean rats (ZLN, 98 ± 11%). This reduction in dilation occurred concomitantly with mitochondrial DNA (mtDNA) strand lesions and reduced mitochondrial complex activities in the endothelium of ZOF versus ZLN. To demonstrate endothelial dysfunction is linked to impaired mitochondrial function, administration of a cell-permeable, mitochondria-directed endonuclease (mt-tat-EndoIII), to repair oxidatively modified DNA in ZOF, restored mitochondrial function and vasodilation to Ach (94 ± 13%). Conversely, administration of a cell-permeable, mitochondria-directed exonuclease (mt-tat-ExoIII) produced mtDNA strand breaks in ZLN, reduced mitochondrial complex activities and vasodilation to Ach in ZLN (42 ± 16%). To demonstrate that mitochondrial function is central to endothelium-dependent vasodilation, we introduced (via electroporation) liver mitochondria (from ZLN) into the endothelium of a mesenteric vessel from ZOF and restored endothelium-dependent dilation to vasoactive intestinal peptide (VIP at 10^-5 M, 4 ± 3% vasodilation before mitochondrial transfer and 48 ± 36% after transfer). Finally, to demonstrate mitochondrial function is key to endothelium-dependent dilation, we administered oligomycin (mitochondrial ATP synthase inhibitor) and observed a reduction in endothelium-dependent dilation. We conclude that mitochondrial function is critical for endothelium-dependent vasodilation.

Coronary microvascular adaptations distal to epicardial artery stenosis

Until recently, epicardial coronary stenosis has been considered the primary outcome of coronary heart disease, and clinical interventions have been dedicated primarily to the identification and removal of flow-limiting stenoses. However, a growing body of literature indicates that both epicardial stenosis and microvascular dysfunction contribute to damaging myocardial ischemia. In this review, we discuss the coexistence of macro- and microvascular disease, and how the structure and function of the distal microcirculation is impacted by the hemodynamic consequences of an epicardial, flow-limiting stenosis. Mechanisms of endothelial dysfunction as well as alterations of smooth muscle function in the coronary microcirculation distal to stenosis are discussed. Risk factors including diabetes, metabolic syndrome, and aging exacerbate microvascular dysfunction in the myocardium distal to a stenosis, and our current understanding of the role of these factors in limiting collateralization and angiogenesis of the ischemic myocardium is presented. Importantly, exercise training has been shown to promote collateral growth and improve microvascular function distal to stenosis; thus, the current literature reporting the mechanisms that underlie the beneficial effects of exercise training in the microcirculation distal to epicardial stenosis is reviewed. We also discuss recent studies of therapeutic interventions designed to improve microvascular function and stimulate angiogenesis in clinically relevant animal models of epicardial stenosis and microvascular disease. Finally, microvascular adaptation to removal of epicardial stenosis is considered.

Endothelin-1 Regulation of Exercise-Induced Changes in Flow: Dynamic Regulation of Vascular Tone

Although endothelin (ET)-1 is a highly potent vasoconstrictor with considerable efficacy in numerous vascular beds, the role of endogenous ET-1 in the regulation of vascular tone remains unclear. The perspective that ET-1 plays little role in the on-going regulation of vascular tone at least under physiologic conditions is supported by findings that potential ET-1 constriction is minimized by the release of the vasodilator and ET-1 synthesis inhibitor, nitric oxide (NO). Indeed, ET-1 release and constriction is self-limited by ET-1-induced, endothelial ET_B receptor-mediated release of NO. Moreover, even if the balance between ET-1 and NO were reversed as the result of lowered NO activity, as occurs in a number of pathophysiologies associated with endothelial dysfunction, the well-known resistance of ET-1 constriction to reversal (as determined with exogenous ET-1) precludes ET-1 in the dynamic, i.e., moment-to-moment, regulation of vascular tone. On the other hand, and as presently reviewed, findings of ET-1-dependent modulation of organ blood flow with exercise under physiologic conditions demonstrate the dynamic regulation of vascular tone by ET-1. We speculate that this regulation is mediated at least in part through changes in ET-1 synthesis/release caused by pulsatile flow-induced shear stress and NO.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

Impaired coronary metabolic dilation in the metabolic syndrome is linked to mitochondrial dysfunction and mitochondrial DNA damage

Mitochondrial dysfunction in obesity and diabetes can be caused by excessive production of free radicals, which can damage mitochondrial DNA. Because mitochondrial DNA plays a key role in the production of ATP necessary for cardiac work, we hypothesized that mitochondrial dysfunction, induced by mitochondrial DNA damage, uncouples coronary blood flow from cardiac work. Myocardial blood flow (contrast echocardiography) was measured in Zucker lean (ZLN) and obese fatty (ZOF) rats during increased cardiac metabolism (product of heart rate and arterial pressure, i.v. norepinephrine). In ZLN increased metabolism augmented coronary blood flow, but in ZOF metabolic hyperemia was attenuated. Mitochondrial respiration was impaired and ROS production was greater in ZOF than ZLN. These were associated with mitochondrial DNA (mtDNA) damage in ZOF. To determine if coronary metabolic dilation, the hyperemic response induced by heightened cardiac metabolism, is linked to mitochondrial function we introduced recombinant proteins (intravenously or intraperitoneally) in ZLN and ZOF to fragment or repair mtDNA, respectively. Repair of mtDNA damage restored mitochondrial function and metabolic dilation, and reduced ROS production in ZOF; whereas induction of mtDNA damage in ZLN reduced mitochondrial function, increased ROS production, and attenuated metabolic dilation. Adequate metabolic dilation was also associated with the extracellular release of ADP, ATP, and H2O2 by cardiac myocytes; whereas myocytes from rats with impaired dilation released only H2O2. In conclusion, our results suggest that mitochondrial function plays a seminal role in connecting myocardial blood flow to metabolism, and integrity of mtDNA is central to this process.

Adaptations of the endothelin system after exercise training in a porcine model of ischemic heart disease

OBJECTIVE

To the test the hypothesis that exercise training would increase endothelin-mediated vasoconstriction in collateral-dependent arteries via enhanced contribution of ET(A).

METHODS

An ameroid constrictor was surgically placed around the proximal LCX artery to induce gradual occlusion in Yucatan miniature swine. Eight weeks postoperatively, pigs were randomized into sedentary or exercise-training (treadmill; 5 days/week; 14 weeks) groups. Subsequently, arteries (~150 μm diameter) were isolated from collateral-dependent and nonoccluded myocardial regions and studied.

RESULTS

Following exercise training, ET-1-mediated contraction was significantly enhanced in collateral-dependent arteries. Exercise training induced a disproportionate increase in the ET(A) contribution to the ET-1 contractile response in collateral-dependent arteries, with negligible contributions by ET(B). In collateral-dependent arteries of sedentary pigs, inhibition of ET(A) or ET(B) did not significantly alter ET-1 contractile responses in collateral-dependent arteries, suggesting compensation by the functionally active receptor. These adaptations occurred without significant changes in ET(A), ET(B), or ECE mRNA levels but with significant exercise-training-induced elevations in endothelin levels in both nonoccluded and collateral-dependent myocardial regions.

CONCLUSIONS

Taken together, these data reveal differential adaptive responses in collateral-dependent arteries based upon physical activity level. ET(A) and ET(B) appear to compensate for one another to maintain contraction in sedentary pigs, whereas exercise-training favors enhanced contribution of ET(A).

Divergent effects of aging and sex on vasoconstriction to endothelin in coronary arterioles

OBJECTIVE

The risk for cardiovascular disease increases with advancing age; however, the chronological development of heart disease differs in males and females. The purpose of this study was to determine whether age-induced alterations in responses of coronary arterioles to the endogenous vasoconstrictor, endothelin, are sex-specific.

METHODS

Coronary arterioles were isolated from young and old male and female rats to assess vasoconstrictor responses to endothelin (ET), and ETa and ETb receptor inhibitors were used to assess receptor-specific signaling.

RESULTS

In intact arterioles from males, ET-induced vasoconstriction was reduced with age, whereas age increased vasoconstrictor responses to ET in intact arterioles from female rats. In intact arterioles from both sexes, blockade of either ETa or ETb eliminated age-related differences in responses to ET; however, denudation of arterioles from both sexes revealed age-related differences in ETa-mediated vasoconstriction. In arterioles from male rats, ETa receptor protein decreased, whereas ETb receptor protein increased with age. In coronary arterioles from females, neither ETa nor ETb receptor protein changed with age, suggesting age-related changes in ET signaling occur downstream of ET receptors.

CONCLUSIONS

Thus, aging-induced alterations in responsiveness of the coronary resistance vasculature to endothelin are sex-specific, possibly contributing to sexual dimorphism in the risk of cardiovascular disease with advancing age.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Heart of the matter: coronary dysfunction in metabolic syndrome

Metabolic syndrome (MetS) is a collection of risk factors including obesity, dyslipidemia, insulin resistance/impaired glucose tolerance, and/or hypertension. The incidence of obesity has reached pandemic levels, as ~20-30% of adults in most developed countries can be classified as having MetS. This increased prevalence of MetS is critical as it is associated with a two-fold elevated risk for cardiovascular disease. Although the pathophysiology underlying this increase in disease has not been clearly defined, recent evidence indicates that alterations in the control of coronary blood flow could play an important role. The purpose of this review is to highlight current understanding of the effects of MetS on regulation of coronary blood flow and to outline the potential mechanisms involved. In particular, the role of neurohumoral modulation via sympathetic α-adrenoceptors and the renin-angiotensin-aldosterone system (RAAS) are explored. Alterations in the contribution of end-effector K(+), Ca(2+), and transient receptor potential (TRP) channels are also addressed. Finally, future perspectives and potential therapeutic targeting of the microcirculation in MetS are discussed. This article is part of a Special Issue entitled "Coronary Blood Flow".

Exercise limits the production of endothelin in the coronary vasculature

We previously demonstrated that endothelin (ET)-mediated coronary vasoconstriction wanes with increasing exercise intensity via a nitric oxide- and prostacyclin-dependent mechanism (Ref. 23). Therefore, we hypothesized that the waning of ET coronary vasoconstriction during exercise is the result of decreased production of ET and/or decreased ET receptor sensitivity. We investigated coronary ET receptor sensitivity using intravenous infusion of ET and coronary ET production using intravenous infusion of the ET precursor Big ET, at rest and during continuous treadmill exercise at 3 km/h in 16 chronically instrumented swine. In the systemic vasculature, Big ET and ET induced similar changes in hemodynamic parameters at rest and during continuous exercise at 3 km/h, indicating that exercise does not alter ET production or receptor sensitivity in the systemic vasculature. In the coronary vasculature, infusion of ET resulted in similar dose-dependent decreases in coronary blood flow and coronary venous oxygen tension and saturation at rest and during exercise. In contrast, administration of Big ET resulted in dose-dependent decreases in coronary blood flow, as well as coronary venous oxygen tension and saturation at rest. These effects of Big ET were significantly reduced during exercise. Altogether, our data indicate that continuous exercise at 3 km/h attenuates ET-mediated coronary vasoconstriction through reduced production of ET from Big ET rather than through reduced ET sensitivity of the coronary vasculature. The decreased ET production during exercise likely contributes to metabolic coronary vasodilation.

Sodium-calcium exchanger and lipid rafts in pig coronary artery smooth muscle

Pig coronary artery smooth muscle expresses, among many other proteins, Na+-Ca²+-exchanger NCX1 and sarcoplasmic reticulum Ca²+ pump SERCA2. NCX1 has been proposed to play a role in refilling the sarcoplasmic reticulum Ca²+ pool suggesting a functional linkage between the two proteins. We hypothesized that this functional linkage may require close apposition of SERCA2 and NCX1 involving regions of plasma membrane like lipid rafts. Lipid rafts are specialized membrane microdomains that appear as platforms to co-localize proteins. To determine the distribution of NCX1, SERCA2 and lipid rafts, we isolated microsomes from the smooth muscle tissue, treated them with non-ionic detergent and obtained fractions of different densities by sucrose density gradient centrifugal flotation. We examined the distribution of NCX1; SERCA2; non-lipid raft plasma membrane marker transferrin receptor protein; lipid raft markers caveolin-1, flotillin-2, prion protein, GM1-gangliosides and cholesterol; and cytoskeletal markers clathrin, actin and myosin. Distribution of markers identified two subsets of lipid rafts that differ in their components. One subset is rich in caveolin-1 and flotillin-2 and the other in GM1-gangliosides, prion protein and cholesterol. NCX1 distribution correlated strongly with SERCA2, caveolin-1 and flotillin-2, less strongly with the other membrane markers and negatively with the cytoskeletal markers. These experiments were repeated with a non-detergent method of treating microsomes with sonication at high pH and similar results were obtained. These observations are consistent with the observed functional linkage between NCX1 and SERCA2 and suggest a role for NCX1 in supplying Ca²+ for refilling the sarcoplasmic reticulum.

Metabolic regulation of coronary vascular tone: role of hydrogen peroxide, purinergic components, and angiotensin

Metabolic regulation plays an important role in modifying coronary vascular tone. We hypothesized that hydrogen peroxide, purinergic components, and angiotensin, produced by cardiac myocytes control coronary vascular tone in proportion to metabolism. We measured changes in the diameter of isolated, pressurized coronary arterioles in response to supernatant from isolated cardiac myocytes in rats (stimulated for 20-, 60-, and 120-min at 400 bpm). Changes in the diameter of arterioles were determined under control conditions following treatment of arterioles with an adenosine receptor antagonist, 8-PSPT, a P2Y1 receptor antagonist, MRS-2179, or an angiotensin II receptor antagonist, olmesartan. A supernatant (500 microl to a 2 ml bath) from myocytes stimulated for 20-, 60- and 120-min caused graded vasodilation (14.1+/-0.4, 20.2+/-1.6, 53.8+/-6.2%, P<0.01 vs. non-stimulated, respectively). In 20-min stimulation, catalase with myocyte supernatants eliminated vasodilation. Following 60-min stimulation, catalase converted myocyte supernatant-induced vasodilation to a vasoconstriction (-15.1+/-1.0%), and this vasoconstriction was eliminated by olmesartan. Upon 120-min stimulation, catalase partially reduced the vasodilation by myocyte supernatants (37.2+/-3.8%). The remaining vasodilation was converted to a vasoconstriction with 8-PSPT and MRS-2179, and this vasoconstriction was completely eliminated with olmesartan. Cardiac myocytes modulate vascular tone through the net effects of hydrogen peroxide, purinergic components (adenosine and ADP), and angiotensin in proportion to ischemia.

Exercise increases the angiotensin II effects in isolated portal vein of trained rats

Training in rats adapts the portal vein to respond vigorously to sympathetic stimuli even when the animal is re-exposed to exercise. Moreover, changes in the exercise-induced effects of angiotensin II, a potent venoconstrictor agonist, in venous beds remain to be investigated. Therefore, the present study aimed to assess the effects of angiotensin II in the portal vein and vena cava from sedentary and trained rats at rest or submitted to an exercise session immediately before organ bath experiments. We found that training or exposure of sedentary animals to a single bout of running exercise does not significantly change the responses of the rat portal vein to angiotensin II. However, the exposure of trained animals to a single bout of running exercise enhanced the response of the rat portal vein to angiotensin II. This enhancement appeared to be territory-specific because it was not observed in the vena cava. Moreover, it was not observed in endothelium-disrupted preparations and in preparations treated with N(omega)-nitro-l-arginine methyl ester hydrochloride, indomethacin, BQ-123 or BQ-788. These data indicate that training causes adaptations in the rat portal vein that respond vigorously to angiotensin II even upon re-exposure to exercise. This increased response to angiotensin II requires an enhancement of the vasocontractile influence of endothelin beyond the influence of nitric oxide and vasodilator prostanoids.

Hydrogen peroxide generated from cardiac myocytes impacts metabolic dilation in coronary arterioles

During oxidative cardiac metabolism, the myocardium produces reactive oxygen species, such as superoxide and hydrogen peroxide (H(2)O(2)). We hypothesized H(2)O(2) is a coronary metabolic dilator linking regulation of coronary tone with myocardium metabolism. Dilation of isolated, pressurized coronary arterioles (76 +/- 10 microm, diameter) in reaction to supernatant collected from enzymatically isolated cardiac myocytes was measured. Isolated rat myocytes were stimulated electrically [unpaced or stimulated at 200, 400 beats/min (bpm)]. H(2)O(2) was significantly generated by pacing (400 bpm n = 11, 9.3 +/- 0.4 microM P < 0.01, versus unpaced) and the addition of this supernatant caused vasodilation (500 microL to 2 mL bath, 14.6 +/- 0.7%, P < 0.01 versus unpaced). Supernatant from unpaced myocytes was not vasoactive. To clarify the source of H(2)O(2), myocytes were also stimulated at 400 bpm following treatment with Mn-TBAP (25 microM), which mimics the action of Mn-SOD, and apocynin (3 mM), an NADPH oxidase inhibitor (n = 11, each). Mn-TBAP increased H(2)O(2) generation in myocyte supernatant stimulated at 400 bpm (12.2 +/- 0.8 microM, P < 0.01 versus 400 bpm stimulation only). Treatment of the myocytes with Mn-TBAP augmented vasodilation by the stimulated myocyte supernatant (19.6 +/- 1.1%, P < 0.01 versus untreated myocyte supernatant). Apocynin did not alter vasodilation to myocyte supernatant. These results suggest that the main source of superoxide by metabolic stimuli is cardiac myocytes and Mn-SOD is a scavenger from superoxide to H(2)O(2). We conclude that H(2)O(2) is a key metabolic vasodilator produced by myocardium.

Role of endogenous ET-1 in the regulation of myocardial blood flow in lean and obese humans

Endothelin is an important determinant of peripheral vascular tone, and increased endogenous endothelin activity contributes to peripheral vascular dysfunction in human obesity. The contributions of endothelin to the regulation of coronary vascular tone in health in humans have not been well studied. We hypothesized that the contribution of endothelin to the regulation of myocardial perfusion would be augmented in human obesity. Using [NH(3)]ammonia positron emission tomography (PET), we measured myocardial perfusion under resting and adenosine-stimulated conditions on two separate days, with and without concurrent exposure to BQ123, an antagonist of type A endothelin receptors (1 micromol/min IV beginning 90 min before measurement). We studied 10 lean and 9 obese subjects without hypertension, hyperlipidemia, or diabetes mellitus. We observed a BQ123-induced increase in resting myocardial perfusion of approximately 40%, not different between lean and obese subjects (BQ123-induced increase in flow: lean 0.12 +/- 0.20, obese 0.32 +/- 0.51 ml/g/min, P = 0.02 BQ123 effect, P = 0.27 comparing response across groups). Although basal flow rates varied by region of the myocardium, the BQ123 effect was seen in all regions. BMI and cholesterol were significantly related to BQ123-induced increases in basal tone in multivariable analysis. There was no baseline difference in the adenosine-stimulated increase in blood flow between lean and obese subjects, and BQ123 failed to augment these responses in either group. These observations suggest that endothelin is an important contributor to the regulation of myocardial perfusion under resting conditions in healthy lean and obese humans, with increased contributions in proportion to increasing obesity.

Sodium-calcium exchange mediated contraction in left anterior descending and left ventricular branch arteries

We tested the hypothesis that the de-endothelialized artery rings from the left anterior descending (LAD) coronary artery and its left ventricular branch (LVB) differ in their contractile responses to Na⁺–Ca²⁺-exchanger (NCX) mediated Ca²⁺-entry, muscarinic receptor activation with carbachol, and sarco/endoplasmic reticulum Ca²⁺ pump (SERCA) inhibition with thapsigargin. In LVB, the force of contraction (in N/g tissue) produced by the NCX mediated Ca²⁺-entry (17.5 ± 1.4) and carbachol (18 ± 1.5) was only slightly smaller than that due to membrane depolarization with KCl (24.0 ± 1.0). In contrast, in LAD the force of contraction produced with NCX (8.7 ± 0.7) and carbachol (6.1 ± 1.1) was much smaller than with KCl (15.7 ± 0.7). Thapsigargin also contracted LVB with greater force than LAD. When isolated microsomes were used, the binding to the muscarinic receptor antagonist quinuclidinyl benzilate was greater in LVB than in LAD. Microsomes were also used for Western blots. The intensities of signals for both SERCA and NCX were greater in LVB than in LAD. These biochemical observations were consistent with the contractile experiments. Thus, it appears that the differences between LAD and the resistance arteries may begin as early as LVB.

Regulation of coronary vascular tone via redox modulation in the alpha1-adrenergic-angiotensin-endothelin axis of the myocardium

We hypothesized that alpha(1)-adrenoceptor stimulation of cardiac myocytes results in the production of an endothelin (ET)-releasing factor that stimulates the coronary vasculature to release ET and, by manipulating the redox state of cardiac and vascular cells, may influence the extent of alpha(1)-adrenergic-ET-1 vasoconstriction. Dihydroethidium (DHE) and dichlorodihydrofluorescein (DCF) intensities were increased by phenylephrine stimulation in isolated rat cardiac myocytes, which were enhanced by the mitochondrial electron transport chain complex I inhibitor rotenone (DHE: 20.4 +/- 1.2-fold and DCF: 25.2 +/- 0.9-fold, n = 8, P < 0.01, respectively) but not by the NADPH oxidase inhibitor apocynin. Olmesartan, an angiotensin II type 1 receptor antagonist, and enalaprilate did not change DHE and DCF intensities by phenylephrine. Next, we measured the vasoconstriction of isolated, pressurized rat coronary arterioles (diameter: 74 +/- 8 microm) in response to supernatant collected from isolated cardiac myocytes. The addition of supernatant from phenylephrine-stimulated myocytes to a 2-ml vessel bath (n = 8 each) caused volume-dependent vasoconstriction (500 microl: -14.8 +/- 2.2%). Olmesartan and TA0201, an ET type A receptor antagonist, converted vasoconstriction into vasodilation (8.5 +/- 1.2% and 10.5 +/- 0.5%, P < 0.01, respectively) in response to supernatant from phenylephrine-stimulated myocytes, which was eliminated with catalase. Vasoconstriction was weakened using supernatant from phenylephrine with rotenone-treated myocytes. Treatment of arterioles with apocynin to myocyte supernatant converted vasoconstriction into vasodilation (7.8 +/- 0.8%, P < 0.01). These results suggest that alpha(1)-adrenergic stimulation in cardiac myocytes produces angiotensin I and H(2)O(2) and that angiotensin releases ET-1 through NADPH oxidase in coronary arterioles. Thus, coronary vasoconstriction via the alpha-adrenergic-angiotensin-ET axis appears to require redox-mediated signaling in cardiac and vascular cells.

Alterations in vasomotor control of coronary resistance vessels in remodelled myocardium of swine with a recent myocardial infarction

The mechanism underlying the progressive deterioration of left ventricular (LV) dysfunction after myocardial infarction (MI) towards overt heart failure remains incompletely understood, but may involve impairments in coronary blood flow regulation within remodelled myocardium leading to intermittent myocardial ischemia. Blood flow to the remodelled myocardium is hampered as the coronary vasculature does not grow commensurate with the increase in LV mass and because extravascular compression of the coronary vasculature is increased. In addition to these factors, an increase in coronary vasomotor tone, secondary to neurohumoral activation and endothelial dysfunction, could also contribute to the impaired myocardial oxygen supply. Consequently, we explored, in a series of studies, the alterations in regulation of coronary resistance vessel tone in remodelled myocardium of swine with a 2 to 3-week-old MI. These studies indicate that myocardial oxygen balance is perturbed in remodelled myocardium, thereby forcing the myocardium to increase its oxygen extraction. These perturbations do not appear to be the result of blunted β-adrenergic or endothelial NO-mediated coronary vasodilator influences, and are opposed by an increased vasodilator influence through opening of K_ATP channels. Unexpectedly, we observed that despite increased circulating levels of noradrenaline, angiotensin II and endothelin-1, α-adrenergic tone remained negligible, while the coronary vasoconstrictor influences of endogenous endothelin and angiotensin II were virtually abolished. We conclude that, early after MI, perturbations in myocardial oxygen balance are observed in remodelled myocardium. However, adaptive alterations in coronary resistance vessel control, consisting of increased vasodilator influences in conjunction with blunted vasoconstrictor influences, act to minimize the impairments of myocardial oxygen balance.

Coronary structure and perfusion in health and disease

Blood flow is distributed through the heart muscle via a system of vessels forming the coronary circulation. The perfusion of the myocardium can be hampered by atherosclerosis creating localized obstructions in the epicardial vessels or by microvascular disease. In early stages of the disease, these impediments to blood flow are offset by dilation of the resistance vessels, which normally compensates for a decrease in perfusion pressure or increased metabolism. However, this dilatory reserve can become exhausted, which in general occurs first at the deeper layers of the heart wall where intramural vessels are subjected to compressive forces related to heart contraction. In the catheterization laboratory, guide wires of 0.33 mm diameter are available that are equipped with a pressure and flow velocity sensor at the tip, which can be positioned distal to the stenosis. These signals provide information about the impediment of the stenosis on coronary flow and allow for the evaluation of the status of the microcirculation. However, the interpretation of these signals is strongly model-dependent and therefore it is of paramount importance to develop realistic models reflecting the anatomy and unique physiology of the coronary circulation.

Regulation of Coronary Blood Flow During Exercise

Exercise is the most important physiological stimulus for increased myocardial oxygen demand. The requirement of exercising muscle for increased blood flow necessitates an increase in cardiac output that results in increases in the three main determinants of myocardial oxygen demand: heart rate, myocardial contractility, and ventricular work. The approximately sixfold increase in oxygen demands of the left ventricle during heavy exercise is met principally by augmenting coronary blood flow (∼5-fold), as hemoglobin concentration and oxygen extraction (which is already 70–80% at rest) increase only modestly in most species. In contrast, in the right ventricle, oxygen extraction is lower at rest and increases substantially during exercise, similar to skeletal muscle, suggesting fundamental differences in blood flow regulation between these two cardiac chambers. The increase in heart rate also increases the relative time spent in systole, thereby increasing the net extravascular compressive forces acting on the microvasculature within the wall of the left ventricle, in particular in its subendocardial layers. Hence, appropriate adjustment of coronary vascular resistance is critical for the cardiac response to exercise. Coronary resistance vessel tone results from the culmination of myriad vasodilator and vasoconstrictors influences, including neurohormones and endothelial and myocardial factors. Unraveling of the integrative mechanisms controlling coronary vasodilation in response to exercise has been difficult, in part due to the redundancies in coronary vasomotor control and differences between animal species. Exercise training is associated with adaptations in the coronary microvasculature including increased arteriolar densities and/or diameters, which provide a morphometric basis for the observed increase in peak coronary blood flow rates in exercise-trained animals. In larger animals trained by treadmill exercise, the formation of new capillaries maintains capillary density at a level commensurate with the degree of exercise-induced physiological myocardial hypertrophy. Nevertheless, training alters the distribution of coronary vascular resistance so that more capillaries are recruited, resulting in an increase in the permeability-surface area product without a change in capillary numerical density. Maintenance of α- and ß-adrenergic tone in the presence of lower circulating catecholamine levels appears to be due to increased receptor responsiveness to adrenergic stimulation. Exercise training also alters local control of coronary resistance vessels. Thus arterioles exhibit increased myogenic tone, likely due to a calcium-dependent protein kinase C signaling-mediated alteration in voltage-gated calcium channel activity in response to stretch. Conversely, training augments endothelium-dependent vasodilation throughout the coronary microcirculation. This enhanced responsiveness appears to result principally from an increased expression of nitric oxide (NO) synthase. Finally, physical conditioning decreases extravascular compressive forces at rest and at comparable levels of exercise, mainly because of a decrease in heart rate. Impedance to coronary inflow due to an epicardial coronary artery stenosis results in marked redistribution of myocardial blood flow during exercise away from the subendocardium towards the subepicardium. However, in contrast to the traditional view that myocardial ischemia causes maximal microvascular dilation, more recent studies have shown that the coronary microvessels retain some degree of vasodilator reserve during exercise-induced ischemia and remain responsive to vasoconstrictor stimuli. These observations have required reassessment of the principal sites of resistance to blood flow in the microcirculation. A significant fraction of resistance is located in small arteries that are outside the metabolic control of the myocardium but are sensitive to shear and nitrovasodilators. The coronary collateral system embodies a dynamic network of interarterial vessels that can undergo both long- and short-term adjustments that can modulate blood flow to the dependent myocardium. Long-term adjustments including recruitment and growth of collateral vessels in response to arterial occlusion are time dependent and determine the maximum blood flow rates available to the collateral-dependent vascular bed during exercise. Rapid short-term adjustments result from active vasomotor activity of the collateral vessels. Mature coronary collateral vessels are responsive to vasodilators such as nitroglycerin and atrial natriuretic peptide, and to vasoconstrictors such as vasopressin, angiotensin II, and the platelet products serotonin and thromboxane A2. During exercise, ß-adrenergic activity and endothelium-derived NO and prostanoids exert vasodilator influences on coronary collateral vessels. Importantly, alterations in collateral vasomotor tone, e.g., by exogenous vasopressin, inhibition of endogenous NO or prostanoid production, or increasing local adenosine production can modify collateral conductance, thereby influencing the blood supply to the dependent myocardium. In addition, vasomotor activity in the resistance vessels of the collateral perfused vascular bed can influence the volume and distribution of blood flow within the collateral zone. Finally, there is evidence that vasomotor control of resistance vessels in the normally perfused regions of collateralized hearts is altered, indicating that the vascular adaptations in hearts with a flow-limiting coronary obstruction occur at a global as well as a regional level. Exercise training does not stimulate growth of coronary collateral vessels in the normal heart. However, if exercise produces ischemia, which would be absent or minimal under resting conditions, there is evidence that collateral growth can be enhanced. In addition to ischemia, the pressure gradient between vascular beds, which is a determinant of the flow rate and therefore the shear stress on the collateral vessel endothelium, may also be important in stimulating growth of collateral vessels.

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.

Redox-dependent coronary metabolic dilation

We have observed that hydrogen peroxide (H2O2), the dismutated product of superoxide, is a coronary metabolic dilator and couples myocardial oxygen consumption to coronary blood flow. Because the chemical activity of H2O2 favors its role as an oxidant, and thiol groups are susceptible to oxidation, we hypothesized that coronary metabolic dilation occurs via a redox mechanism involving thiol oxidation. To test this hypothesis, we studied the mechanisms of dilation of isolated coronary arterioles to metabolites released by metabolically active (paced at 400 min) isolated cardiac myocytes and directly compared these responses with authentic H2O2. Studies were performed under control conditions and using interventions designed to reduce oxidized thiols [0.1 microM dithiothreitol (DTT) and 10 mM N-acetyl-L-cysteine (NAC)]. Aliquots of the conditioned buffer from paced myocytes produced vasodilation of isolated arterioles (peak response, 71% +/- 6% of maximal dilation), whereas H2O2 produced complete dilation (92% +/- 7%). Dilation to either the conditioned buffer or to H2O2 was significantly reduced by the administration of either NAC or DTT. The location of the thiols oxidized by the conditioned buffer or of H2O2 was determined by the administration of the fluorochromes monochlorobimane (20 microM) or monobromotrimethylammoniobimane (20 microM), which covalently label the reduced total or extracellular-reduced thiols, respectively. H2O2 or the conditioned buffer predominantly oxidized intracellular thiols since the fluorescent signal from monochlorobimane was reduced more than that of monobromotrimethylammoniobimane. To determine whether one of the intracellular targets of thiol oxidation that leads to dilation is the redox-sensitive kinase p38 mitogen-activated protein (MAP) kinase, we evaluated dilation following the administration of the p38 inhibitor SB-203580 (10 microM). The inhibition of p38 attenuated dilation to either H2O2 or to the conditioned buffer from stimulated myocytes by a similar degree, but SB-203580 did not attenuate dilation to nitroprusside. Western blot analysis for the activated form of p38 (phospho-p38) in the isolated aortae revealed robust activation of this enzyme by H2O2. Taken together, our results show that an active component of cardiac metabolic dilation, like that of H2O2, produces dilation by the oxidation of thiols, which are predominantly intracellular and dependent activation on the p38 MAP kinase. Thus coronary metabolic dilation appears to be mediated by redox-dependent signals.

Exercise hyperaemia in the heart: the search for the dilator mechanism

Coronary blood flow is tightly coupled to myocardial oxygen consumption to maintain a consistently high level of myocardial oxygen extraction over a wide range of physical activity. This tight coupling has been proposed to depend on periarteriolar oxygen tension, signals released from cardiomyocytes (adenosine acting on K(ATP) channels) and the endothelium (prostanoids(,) nitric oxide, endothelin) as well as neurohumoral influences (catecholamines, endothelin), but the contribution of each of these regulatory pathways, and their interactions, to exercise hyperaemia in the human heart are still incompletely understood. Thus, in the human heart, nitric oxide, prostanoids, adenosine and K(ATP) channels each contribute to resting tone, but evidence for a critical contribution to exercise hyperaemia is lacking. In dogs K(ATP) channel activation together with adenosine and nitric oxide contribute to exercise hyperaemia in a non-linear redundant fashion. In contrast, in swine nitric oxide, adenosine and K(ATP) channels contribute to resting coronary resistance vessel tone control in a linear additive manner, but are not mandatory for exercise hyperaemia in the heart. Rather, exercise hyperaemia in swine appears to involve K(Ca) channel opening that is mediated, at least in part, by exercise-induced beta-adrenergic activation, possibly in conjunction with exercise-induced blunting of an endothelin-mediated vasoconstrictor influence. In view of these remarkable species differences in coronary vasomotor control during exercise, future studies are required to determine whether exercise hyperaemia in humans follows a canine or porcine control design.

Hydrogen Peroxide

OBJECTIVE

We tested the hypothesis that hydrogen peroxide (H2O2), the dismutated product of superoxide (O2*-), couples myocardial oxygen consumption to coronary blood flow. Accordingly, we measured O2*- and H2O2 production by isolated cardiac myocytes, determined the role of mitochondrial electron transport in the production of these species, and determined the vasoactive properties of the produced H2O2.

METHODS AND RESULTS

The production of O2*- is coupled to oxidative metabolism because inhibition of complex I (rotenone) or III (antimycin) enhanced the production of O2*- during pacing by about 50% and 400%, respectively; whereas uncoupling oxidative phosphorylation by decreasing the protonmotive force with carbonylcyanide-p-trifluoromethoxyphenyl-hydrazone (FCCP) decreased pacing-induced O2*- production. The inhibitor of cytosolic NAD(P)H oxidase assembly, apocynin, did not affect O2*- production by pacing. Aliquots of buffer from paced myocytes produced vasodilation of isolated arterioles (peak response 67+/-8% percent of maximal dilation) that was significantly reduced by catalase (5+/-0.5%, P<0.05) or the antagonist of Kv channels, 4-aminopyridine (18+/-4%, P<0.05). In intact animals, tissue concentrations of H2O2 are proportionate to myocardial oxygen consumption and directly correlated to coronary blood flow. Intracoronary infusion of catalase reduced tissue levels of H2O2 by 30%, and reduced coronary flow by 26%. Intracoronary administration of 4-aminopyridine also shifted the relationship between myocardial oxygen consumption and coronary blood flow or coronary sinus pO2.

CONCLUSIONS

Taken together, our results demonstrate that O2*- is produced in proportion to cardiac metabolism, which leads to the production of the vasoactive reactive oxygen species, H2O2. Our results further suggest that the production of H2O2 in proportion to metabolism couples coronary blood flow to myocardial oxygen consumption.

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.

Coronary vasoconstrictor influence of angiotensin II is reduced in remodeled myocardium after myocardial infarction

The renin-angiotensin system plays an important role in cardiovascular homeostasis by contributing to the regulation of blood volume, blood pressure, and vascular tone. Because AT(1) receptors have been described in the coronary microcirculation, we investigated whether ANG II contributes to the regulation of coronary vascular tone and whether its contribution is altered during exercise. Since the renin-angiotensin system is activated after myocardial infarction, resulting in an increase in circulating ANG II, we also investigated whether the contribution of ANG II to the regulation of vasomotor tone is altered after infarction. Twenty-six chronically instrumented swine were studied at rest and while running on a treadmill at 1-4 km/h. In 13 swine, myocardial infarction was induced by ligation of the left circumflex coronary artery. Blockade of AT(1) receptors (irbesartan, 1 mg/kg iv) had no effect on myocardial O(2) consumption but resulted in an increase in coronary venous O(2) tension and saturation both at rest and during exercise, reflecting coronary vasodilation. Despite increased plasma levels of ANG II after infarction and maintained coronary arteriolar AT(1) receptor levels, the vasodilation evoked by irbesartan was significantly reduced both at rest and during exercise. In conclusion, despite elevated plasma levels, the vasoconstrictor influence of ANG II on the coronary circulation in vivo is reduced after myocardial infarction. This reduction in ANG II-induced coronary vasoconstriction may serve to maintain perfusion of the remodeled myocardium.

NO and prostanoids blunt endothelin-mediated coronary vasoconstrictor influence in exercising swine

Withdrawal of the endothelin (ET)-mediated vasoconstrictor influence contributes to metabolic coronary vasodilation during exercise. Because production of nitric oxide (NO) and prostanoids increases with increasing shear stress and because NO and prostanoids are able to modify the release of ET, we hypothesized that the withdrawal of ET-mediated coronary vasoconstriction during exercise is mediated through NO and/or prostanoids. To test this hypothesis, 19 chronically instrumented swine were studied at rest and while running on a treadmill up to 85-90% of maximal heart rate. Blockade of ET(A)/ET(B) receptors with tezosentan resulted in an increase in coronary venous O(2) levels (i.e., in coronary vasodilation) at rest, which waned at increasing levels of exercise intensity. Inhibition of either NO synthase [N(omega)-nitro-l-arginine (l-NNA)] or cyclooxygenase (indomethacin) did not affect the response to tezosentan under resting conditions but unmasked a vasodilator response to tezosentan during exercise. The vasodilator response to tezosentan during exercise increased progressively after combined administration of l-NNA and indomethacin. These findings suggest that NO and prostanoids act synergistically to inhibit the vasoconstrictor influence of ET on the coronary circulation during exercise, thereby facilitating the exercise-induced vasodilation of coronary resistance vessels.

KCa+ channels contribute to exercise-induced coronary vasodilation in swine

Coronary blood flow is controlled via several vasoactive mediators that exert their effect on coronary resistance vessel tone through activation of K(+) channels in vascular smooth muscle. Because Ca(2+)-activated K(+) (K(Ca)(+)) channels are the predominant K(+) channels in the coronary vasculature, we hypothesized that K(Ca)(+) channel activation contributes to exercise-induced coronary vasodilation. In view of previous observations that ATP-sensitive K(+) (K(ATP)(+)) channels contribute, in particular, to resting coronary resistance vessel tone, we additionally investigated the integrated control of coronary tone by K(Ca)(+) and K(ATP)(+) channels. For this purpose, the effect of K(Ca)(+) blockade with tetraethylammonium (TEA, 20 mg/kg iv) on coronary vasomotor tone was assessed in the absence and presence of K(ATP)(+) channel blockade with glibenclamide (3 mg/kg iv) in chronically instrumented swine at rest and during treadmill exercise. During exercise, myocardial O(2) delivery increased commensurately with the increase in myocardial O(2) consumption, so that myocardial O(2) extraction and coronary venous Po(2) (Pcv(O(2))) were maintained constant. TEA (in a dose that had no effect on K(ATP)(+) channels) had a small effect on the myocardial O(2) balance at rest and blunted the exercise-induced increase in myocardial O(2) delivery, resulting in a progressive decrease of Pcv(O(2)) with increasing exercise intensity. Conversely, at rest glibenclamide caused a marked decrease in Pcv(O(2)) that waned at higher exercise levels. Combined K(Ca)(+) and K(ATP)(+) channel blockade resulted in coronary vasoconstriction at rest that was similar to that caused by glibenclamide alone and that was maintained during exercise, suggesting that K(Ca)(+) and K(ATP)(+) channels act in a linear additive fashion. In conclusion, K(Ca)(+) channel activation contributes to the metabolic coronary vasodilation that occurs during exercise. Furthermore, in swine K(Ca)(+) and K(ATP)(+) channels contribute to coronary resistance vessel control in a linear additive fashion.

Molecular genetics of human hypertension

EH (essential hypertension) is a major public health problem in many countries due to its high prevalence and its association with coronary heart disease, stroke, renal disease, peripheral vascular disease and other disorders. Epidemiological studies have demonstrated that EH is heritable. Owing to the fact that blood pressure is controlled by cardiac output and total peripheral resistance, many molecular pathways are believed to be involved in the disease. In this review, recent genetic studies investigating the molecular basis of EH, including different molecular pathways, will be highlighted.

Metabolic communication from cardiac myocytes to vascular endothelial cells

The endothelium releases substances that affect both vascular and cardiac myocytes. However, under conditions of augmented metabolic demands and cardiac work, signals from the cardiac myocytes may be critical for the endothelium to fulfill its secretory and regulatory function in the vascular bed. Therefore, we hypothesized that cardiac myocytes produce substances that alter the resting membrane potential of endothelial cells and thus vascular tone. Isolated rat cardiac myocytes were electrically stimulated at the rate of 0 and 400 beats/min (Po2 = 150 mmHg), and supernatants were collected from each group (Sup-0; control) and (Sup-400) and used within 6 mo. These supernatants were applied to human coronary endothelial cells that were subsequently analyzed by using the whole cell and cell-attached patch-clamp modes. Sup-0 had no effect on the whole cell current and the zero-current potential. The Sup-0 from myocytes treated with aprotinin, an inhibitor of kallikrein and serine protease, reduced whole cell current between −120 and −60 mV. Sup-400 depolarized endothelial cells from the resting membrane potential of −45 to −5 mV ( P < 0.05), increased the magnitude of an inward current, and activated an outward current. Moreover, Sup-400 cells assayed in cell-attached patches increased single channel amplitude and the probability of a channel being in the open state. These effects were reversed by the Sup-400 from aprotinin-treated cells. We conclude that under certain metabolic conditions, isolated cardiac myocytes produce and release vasoactive substances that alter the function of K+ channels in vascular endothelial cells. Thus cardiac myocytes seem to communicate metabolic information to the endothelium, which could potentially influence vascular tone.

Role of endothelin in α-adrenoceptor coronary vasoconstriction

It has been proposed that α-adrenoceptor vasoconstriction in coronary resistance vessels results not from α-adrenoceptors on coronary smooth muscle but from α-adrenoceptors on cardiac myocytes that stimulate endothelin (ET) release. The present experiments tested the hypothesis that the α-adrenoceptor-mediated coronary vasoconstriction that normally occurs during exercise is due to endothelin. In conscious dogs ( n = 10), the endothelin ETA/ETB receptor antagonist tezosentan (1 mg/kg iv) increased coronary venous oxygen tension at rest but not during treadmill exercise. This result indicates that basal endothelin levels produce a coronary vasoconstriction at rest that is not observed during the coronary vasodilation during exercise. In contrast, the α-adrenoceptor antagonist phentolamine increased coronary venous oxygen tension during exercise but not at rest. The difference between the endothelin blockade and α-adrenoceptor blockade results indicates that α-adrenoceptor coronary vasoconstriction during exercise is not due to endothelin. However, in anesthetized dogs, bolus intracoronary injections of the α-adrenoceptor agonist phenylephrine produced reductions in coronary blood flow that were partially antagonized by endothelin receptor blockade with tezosentan. These results are best explained if α-adrenoceptor-induced endothelin release requires high pharmacological concentrations of catecholamines that are not reached during exercise.

Cardiac myocytes control release of endothelin-1 in coronary vasculature

Alpha-adrenergic vasoconstriction in the coronary circulation is mediated through alpha-adrenoceptors on cardiac myocytes and subsequent release of endothelin, a very potent, long-lasting vasoconstrictor. Recent studies found that adult cardiac myocytes do not express the preproendothelin gene. Thus we hypothesized that alpha-adrenoceptor stimulation on the cardiac myocytes results in the production of an endothelin-releasing factor, which stimulates the coronary vasculature to produce endothelin. We tested this hypothesis by using an in vitro model in which isolated adult rat cardiac myocytes can be stimulated with an alpha-adrenoceptor agonist (phenylephrine). Their bathing fluid is then transferred to isolated coronary arterioles, and vasoactive responses are measured. To identify the source of endothelin, the endothelin-converting enzyme inhibitor phosphoramidon was added to either the myocytes or the isolated arterioles. Phenylephrine enhanced the vasoconstrictor properties of the myocyte bathing fluid. Administration of phosphoramidon (in either the presence or the absence of phenylephrine) to the myocytes had no effect on the vasoactive properties of the bathing fluid. In contrast, administration of phosphoramidon to the isolated arteriole before administration of the bathing fluid converted vasoconstriction to vasodilation, similar to the effect of the endothelin A receptor antagonist JKC-301, indicating that the endothelin is indeed produced by the coronary vasculature. Administration of the angiotensin type 1 receptor antagonist losartan to the vessel bath enhanced vasodilation to the bathing fluid of the phenylephrine-treated but not control myocytes. In conclusion, during alpha-adrenergic activation cardiac myocytes release a factor, probably angiotensin II, that stimulates the vascular production of endothelin. Although the physiological implications of this mechanism are not obvious, this may represent a protective mechanism that integrates neuronal vasoconstrictor mechanisms with myocardial metabolism, which minimizes periods of both coronary underperfusion and overperfusion.

Matching coronary blood flow to myocardial oxygen consumption

At rest the myocardium extracts approximately 75% of the oxygen delivered by coronary blood flow. Thus there is little extraction reserve when myocardial oxygen consumption is augmented severalfold during exercise. There are local metabolic feedback and sympathetic feedforward control mechanisms that match coronary blood flow to myocardial oxygen consumption. Despite intensive research the local feedback control mechanism remains unknown. Physiological local metabolic control is not due to adenosine, ATP-dependent K(+) channels, nitric oxide, prostaglandins, or inhibition of endothelin. Adenosine and ATP-dependent K(+) channels are involved in pathophysiological ischemic or hypoxic coronary dilation and myocardial protection during ischemia. Sympathetic beta-adrenoceptor-mediated feedforward arteriolar vasodilation contributes approximately 25% of the increase in coronary blood flow during exercise. Sympathetic alpha-adrenoceptor-mediated vasoconstriction in medium and large coronary arteries during exercise helps maintain blood flow to the vulnerable subendocardium when cardiac contractility, heart rate, and myocardial oxygen consumption are high. In conclusion, several potential mediators of local metabolic control of the coronary circulation have been evaluated without success. More research is needed.

Pathophysiological plasma ET-1 levels antagonize β-adrenergic dilation of coronary resistance vessels in conscious dogs

On the basis of in vitro experiments showing that endothelin (ET)-1 interferes with smooth muscle ATP-sensitive K+ (KATP) channel opening, which is pivotal in β-adrenergic coronary dilation, we hypothesized that pathophysiological plasma ET-1 levels impair β-adrenergic dilation of resistance coronary vessels. In conscious instrumented dogs, graded intravenous doses of dobutamine caused the expected inotropic responses. As myocardial O2 consumption (MV̇o2) increased, the disproportionate rise in coronary sinus (CS) Po2 indicates that increases in coronary blood flow (CBF) exceeded metabolic requirements, consistent with β-adrenergic dilation. ET-1 intravenous infusions, to reach pathophysiological plasma levels, reduced slopes of the Po2-MV̇o2 and CBF-MV̇o2 relations. In contrast, the first derivative of left ventricular pressure over time responses to dobutamine were not impaired during ET-1 delivery. Clazosentan, an ETA receptor blocker, prevented reduction of the slope of Po2-MV̇o2 and CBF-MV̇o2 relations. After ganglionic blockade to exclude reflex influences, ET-1 still reduced slopes of Po2-MV̇o2 and CBF-MV̇o2 relations. To assess effects of ET-1 on endothelium-dependent and -independent coronary vascular responses, intracoronary ACh and nitroglycerin were given to directly target coronary vessels. CBF responses to ACh and nitroglycerin were maintained during ET-1 delivery. In contrast, responses to intracoronary KATP channel-dependent dilators adenosine and lemakalim were impaired by ET-1. In conclusion, pathophysiological levels of ET-1 impaired β-adrenergic dilation of resistance coronary vessels through an ETA receptor-dependent process. In contrast, left ventricular inotropic responses to dobutamine were not impaired during ET-1 delivery. Our data suggest that ET-1 may interfere with smooth muscle KATP channels to impair β-adrenergic coronary dilation.

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.

Myocardial O2 consumption in porcine left ventricle is heterogeneously distributed in parallel to heterogeneous O2 delivery

Myocardial blood flow is unevenly distributed, but the cause of this heterogeneity is unknown. Heterogeneous blood flow may reflect heterogeneity of oxygen demand. The aim of the present study was to assess the relation between oxygen consumption and blood flow in small tissue regions in porcine left ventricle. In seven male, anesthetized, open-chest pigs, local oxygen consumption was quantitated by computational model analysis of the incorporation of 13C in glutamate via the tricarboxylic acid cycle during timed infusion of [13C]acetate into the left anterior descending coronary artery. Blood flow was measured with radioactive microspheres before and during acetate infusion. High-resolution nuclear magnetic resonance 13C spectra were obtained from extracts of tissue samples (159 mg mean dry wt) taken at the end of the acetate infusion. Mean regional myocardial blood flow was stable [5.0 ± 1.6 (SD) and 5.0 ± 1.4 ml·min−1·g dry wt−1 before and after 30 min of acetate infusion, respectively]. Mean left ventricular oxygen consumption measured with the NMR method was 18.6 ± 7.7 μmol·min−1·g dry wt−1 and correlated well ( r = 0.85, P = 0.02, n = 7) with oxygen consumption calculated from blood flow, hemoglobin, and blood gas measurements (mean 22.8 ± 4.7 μmol·min−1·g dry wt−1). Local blood flow and oxygen consumption were significantly correlated ( r = 0.63 for pooled normalized data, P < 0.0001, n = 60). We calculate that, in the heart at normal workload, the variance of left ventricular oxygen delivery at submilliliter resolution is explained for 43% by heterogeneity in oxygen demand.