Mediators of coronary reactive hyperaemia in isolated mouse heart

Unraveling the Gordian knot of coronary pressure-flow autoregulation

The coronary circulation has the inherent ability to maintain myocardial perfusion constant over a wide range of perfusion pressures. The phenomenon of pressure-flow autoregulation is crucial in response to flow-limiting atherosclerotic lesions which diminish coronary driving pressure and increase risk of myocardial ischemia and infarction. Despite well over half a century of devoted research, understanding of the mechanisms responsible for autoregulation remains one of the most fundamental and contested questions in the field today. The purpose of this review is to highlight current knowledge regarding the complex interrelationship between the pathways and mechanisms proposed to dictate the degree of coronary pressure-flow autoregulation. Our group recently likened the intertwined nature of the essential determinants of coronary flow control to the symbolically unsolvable "Gordian knot". To further efforts to unravel the autoregulatory "knot", we consider recent challenges to the local metabolic and myogenic hypotheses and the complicated dynamic structural and functional heterogeneity unique to the heart and coronary circulation. Additional consideration is given to interrogation of putative mediators, role of K+ and Ca2+ channels, and recent insights from computational modeling studies. Improved understanding of how specific vasoactive mediators, pathways, and underlying disease states influence coronary pressure-flow relations stands to significantly reduce morbidity and mortality for what remains the leading cause of death worldwide.

Periconceptional alcohol alters in vivo heart function in ageing female rat offspring: Possible involvement of oestrogen receptor signalling

NEW FINDINGS

What is the central question of this study? What are the cardiovascular consequences of periconceptual ethanol on offspring throughout the lifespan? What is the main finding and its importance? It is shown for the first time that periconceptional alcohol has sex-specific effects on heart growth, with ageing female offspring exhibiting decreased cardiac output. Altered in vivo cardiac function in ageing female offspring may be linked to changes in cardiac oestrogen receptor expression.

ABSTRACT

Alcohol exposure throughout gestation is detrimental to cardiac development and function. Although many women decrease alcohol consumption once aware of a pregnancy, exposure prior to recognition is common. We, therefore, examined the effects of periconceptional alcohol exposure (PC:EtOH) on heart function, and explored mechanisms that may contribute. Female Sprague-Dawley rats received a liquid diet ±12.5% v/v ethanol from 4 days prior to mating until 4 days after mating (PC:EtOH). Cardiac function was assessed via echocardiography, and offspring were culled at multiple time points for assessment of morphometry, isolated heart and aortic ring function, protein and transcriptional changes. PC:EtOH-exposed embryonic day 20 fetuses (but not postnatal offspring) had larger hearts relative to body weight. Ex vivo analysis of hearts at 5-7 months old (mo) indicated no changes in coronary function or cardiac ischaemic tolerance, and apparently improved ventricular compliance in PC:EtOH females (compared to controls). At 12 mo, vascular responses in isolated aortic rings were unaltered by PC:EtOH, whilst echocardiography revealed reduced cardiac output in female but not male PC:EtOH offspring. At 19 mo, left ventricular transcript and protein for type 1 oestrogen receptor (ESR1), HSP90 transcript and plasma oestradiol levels were all elevated in female PC:EtOH exposed offspring. Summarising, PC:EtOH adversely impacts in vivo heart function in mature female offspring, associated with increased ventricular oestrogen-related genes. PC:EtOH may thus influence age-related heart dysfunction in females through modulation of oestrogen signalling.

Early cardiac aging linked to impaired stress-resistance and transcriptional control of stress response, quality control and mitochondrial pathways

Phenotypic and transcriptomic evidence of early cardiac aging, and associated mechanisms, were investigated in young to middle-aged male mice (C57Bl/6; ages 8, 16, 32, 48 wks). Left ventricular gene expression (profiled via Illumina MouseWG-6 BeadChips), contractile and coronary function, and stress-resistance were assessed in Langendorff perfused hearts under normoxic conditions and following ischemic insult (20 min global ischemia-45 min reperfusion; I-R). Baseline or normoxic contractile function was unaltered by age, while cardiac and coronary 'reserves' (during β-adrenoceptor stimulation; 1 μM isoproterenol) declined by 48 wks. Resistance to I-R injury fell from 16 to 32 wks. Age-dependent transcriptional changes In un-stressed hearts were limited to 104 genes (>1.3-fold; 0.05 FDR), supporting: up-regulated innate defenses (glutathione and xenobiotic metabolism, chemotaxis, interleukins) and catecholamine secretion; and down-regulated extracellular matrix (ECM), growth factor and survival (PI3K/Akt) signaling. In stressed (post-ischemic) myocardium, ∼15-times as many genes (1528) were age-dependent, grouped into 6 clusters (>1.3-fold change; 0.05 FDR): most changing from 16 wks (45 % up/44 % down), a further 5 % declining from 32 wks. Major age-dependent Biological Processes in I-R hearts reveal: declining ATP metabolism, oxidative phosphorylation, cardiac contraction and morphogenesis, phospholipid metabolism and calcineurin signaling; increasing proteolysis and negative control of MAPK; and mixed changes in nuclear transport and angiogenic genes. Pathway analysis supports reductions in: autophagy, stress response, ER protein processing, mRNA surveillance and ribosome/translation genes; with later falls in mitochondrial biogenesis, oxidative phosphorylation and proteasome genes in I-R hearts. Summarizing, early cardiac aging is evident from 16 to 32 wks in male mice, characterized by: declining cardiovascular reserve and stress-resistance, transcriptomic evidence of constitutive stress and altered catecholamine and survival/growth signaling in healthy hearts; and declining stress response, quality control, mitochondrial energy metabolism and cardiac modeling processes in stressed hearts. These very early changes, potentially key substrate for advanced aging, may inform approaches to healthy aging and cardioprotection in the adult heart.

Adenosine and adenosine receptor-mediated action in coronary microcirculation

Adenosine is an ubiquitous extracellular signaling molecule and plays a fundamental role in the regulation of coronary microcirculation through activation of adenosine receptors (ARs). Adenosine is regulated by various enzymes and nucleoside transporters for its balance between intra- and extracellular compartments. Adenosine-mediated coronary microvascular tone and reactive hyperemia are through receptors mainly involving A_2AR activation on both endothelial and smooth muscle cells, but also involving interaction among other ARs. Activation of ARs further stimulates downstream targets of H₂O₂, K_ATP, K_V and K_Ca2+ channels leading to coronary vasodilation. An altered adenosine-ARs signaling in coronary microcirculation has been observed in several cardiovascular diseases including hypertension, diabetes, atherosclerosis and ischemic heart disease. Adenosine as a metabolite and its receptors have been studied for its both therapeutic and diagnostic abilities. The present review summarizes important aspects of adenosine metabolism and AR-mediated actions in the coronary microcirculation.

GTP-cyclohydrolase deficiency induced peripheral and deep microcirculation dysfunction with age

The present study assessed the impact of impaired tetrahydrobiopterin (BH4) production on vasoreactivity from conduit and small arteries along the vascular tree as seen during aging. For this purpose, the mutant hyperphenylalaninemic mouse (hph-1) was used. This model is reported to be deficient in GTP cyclohydrolase I, a rate limiting enzyme in BH4 biosynthesis. BH4 is a key regulator of vascular homeostasis by regulating the nitric oxide synthase 3 (NOS3) activity. In GTP-CH deficient mice, the aortic BH4 levels were decreased, by -77% in 12 week-middle-aged mice (young) and by -83% in 35-45 week-middle-aged mice (middle-aged). In young hph-1, the mesenteric artery ability to respond to flow was slightly reduced by 9%. Aging induced huge modification in many vascular functions. In middle-aged hph-1, we observed a decrease in aortic cGMP levels, biomarker of NO availability (-46%), in flow-mediated vasodilation of mesenteric artery (-31%), in coronary hyperemia response measured in isolated heart following transient ischemia (-27%) and in cutaneous microcirculation dilation in response to acetylcholine assessed in vivo by laser-doppler technic (-69%). In parallel, the endothelium-dependent relaxation in response to acetylcholine in conduit blood vessel, measured on isolated aorta rings, was unchanged in hph-1 mice whatever the age. Our findings demonstrate that in middle-aged GTP-CH depleted mice, the reduction of BH4 was characterized by an alteration of microcirculation dilatory properties observed in various parts of the vascular tree. Large conduit blood vessels vasoreactivity, ie aorta, was unaltered even in middle-aged mice emphasizing the main BH4-deletion impact on the microcirculation.

Assessing Renal Microvascular Reactivity by Laser Speckle-Contrast Imaging in Angiotensin-II-Treated Mice

Introduction

The kidney is one of the main organs affected by microvascular damage wrought by hypertension. We developed an approach to investigate renal microcirculatory disturbance in live mice by measuring post-occlusive reactive hyperemia (PORH), a reactivity test exploring endothelial and neuro-microvascular functioning. Laser speckle-contrast analysis (LASCA) assesses microvascular blood flow; it provides real-time images of spatial and temporal blood flow dynamics. We compared basal blood flow and PORH test between control and angiotensin-II-treated mice (Ang-II) to validate the model.

Objective

The study objective was to develop an approach to investigate renal microcirculation, and then to compare microvascular reactivity assessed on LASCA in control versus Ang-II mice.

Methods

Thirty 7-week-old wild-type C57BL/6J mice were allocated into two groups. One received angiotensin-II via osmotic minipumps (Ang-II; n=15); the other served as control (n=15). Basal blood flow was measured on LASCA. The PORH test was then performed in the two groups.

Results

Control mice had significantly lower basal renal microcirculatory flow, expressed in perfusion units (PU), than Ang-II-treated mice (1448 ± 96 vs 1703 ± 185 PU, respectively; P < 0.05). Peak flow was lower in controls than in Ang-II mice (1617±104 vs.1724 ± 205 PU, respectively; P=0.21). Control mice had significantly higher kidney PORH than Ang-II mice (8±3 vs 1±4%, respectively; P < 0.05).

Conclusion

We developed an innovative technique to study renal microcirculation in mice. Ang-II-treated mice showed significantly higher basal blood flow than controls, while PORH was significantly higher in controls than in Ang-II mice.

Reduced coronary reactive hyperemia in mice was reversed by the soluble epoxide hydrolase inhibitor (t-AUCB): Role of adenosine A2A receptor and plasma oxylipins

Coronary reactive hyperemia (CRH) protects the heart against ischemia. Adenosine A_2AAR-deficient (A_2AAR^-/-) mice have increased expression of soluble epoxide hydrolase (sEH); the enzyme responsible for breaking down the cardioprotective epoxyeicosatrienoic acids (EETs) to dihydroxyeicosatrienoic acids (DHETs). sEH-inhibition enhances CRH, increases EETs, and modulates oxylipin profiles. We investigated the changes of oxylipins and their impact on CRH in A_2AAR^-/- and wild type (WT) mice. We hypothesized that the attenuated CRH in A_2AAR^-/- mice is mediated by changes in oxylipin profiles, and that it can be reversed by either sEH- or ω-hydroxylases-inhibition. Compared to WT mice, A_2AAR^-/- mice had attenuated CRH and changed oxylipin profiles, which were consistent between plasma and heart perfusate samples, including decreased EET/DHET ratios, and increased hydroxyeicosatetraenoic acids (HETEs). Plasma oxylipns in A_2AAR^-/- mice indicated an increased proinflammatory state including increased ω-terminal HETEs, decreased epoxyoctadecaenoic/dihydroxyoctadecaenoic acids (EpOMEs/DiHOMEs) ratios, increased 9-hydroxyoctadecadienoic acid, and increased prostanoids. Inhibition of either sEH or ω-hydroxylases reversed the reduced CRH in A_2AAR^-/- mice. In WT and sEH^-/- mice, blocking A_2AAR decreased CRH. These data demonstrate that A_2AAR-deletion was associated with changes in oxylipin profiles, which may contribute to the attenuated CRH. Also, inhibition of sEH and ω-hydroxylases reversed the reduction in CRH.

Cavin-1 deficiency modifies myocardial and coronary function, stretch responses and ischaemic tolerance: roles of NOS over-activity

Caveolae and associated cavin and caveolins may govern myocardial function, together with responses to mechanical and ischaemic stresses. Abnormalities in these proteins are also implicated in different cardiovascular disorders. However, specific roles of the cavin-1 protein in cardiac and coronary responses to mechanical/metabolic perturbation remain unclear. We characterised cardiovascular impacts of cavin-1 deficiency, comparing myocardial and coronary phenotypes and responses to stretch and ischaemia-reperfusion in hearts from cavin-1 ^+/+ and cavin-1 ^-/- mice. Caveolae and caveolins 1 and 3 were depleted in cavin-1 ^-/- hearts. Cardiac ejection properties in situ were modestly reduced in cavin-1 ^-/- mice. While peak contractile performance in ex vivo myocardium from cavin-1 ^-/- and cavin-1 ^+/+ mice was comparable, intrinsic beating rate, diastolic stiffness and Frank-Starling behaviour (stretch-dependent diastolic and systolic forces) were exaggerated in cavin-1 ^-/- hearts. Increases in stretch-dependent forces were countered by NOS inhibition (100 µM L-NAME), which exposed negative inotropy in cavin-1 ^-/- hearts, and were mimicked by 100 µM nitroprusside. In contrast, chronotropic differences appeared largely NOS-independent. Cavin-1 deletion also induced NOS-dependent coronary dilatation, ≥3-fold prolongation of reactive hyperaemic responses, and exaggerated pressure-dependence of coronary flow. Stretch-dependent efflux of lactate dehydrogenase and cardiac troponin I was increased and induction of brain natriuretic peptide and c-Fos inhibited in cavin-1 ^-/- hearts, while ERK1/2 phospho-activation was preserved. Post-ischaemic dysfunction and damage was also exaggerated in cavin-1 ^-/- hearts. Diverse effects of cavin-1 deletion reveal important roles in both NOS-dependent and -independent control of cardiac and coronary functions, together with governing sarcolemmal fragility and myocardial responses to stretch and ischaemia.

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.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Adenosine relaxation in isolated rat aortic rings and possible roles of smooth muscle Kv channels, KATP channels and A2a receptors

BACKGROUND

An area of ongoing controversy is the role adenosine to regulate vascular tone in conduit vessels that regulate compliance, and the role of nitric oxide (NO), potassium channels and receptor subtypes involved. The aim of our study was to investigate adenosine relaxation in rat thoracic aortic rings, and the effect of inhibitors of NO, prostanoids, Kv, KATP channels, and A2a and A2b receptors.

METHODS

Aortic rings were freshly harvested from adult male Sprague Dawley rats and equilibrated in an organ bath containing oxygenated, modified Krebs-Henseleit solution, 11 mM glucose, pH 7.4, 37 °C. Isolated rings were pre-contracted sub-maximally with 0.3 μM norepinephrine (NE), and the effect of increasing concentrations of adenosine (1 to 1000 μM) were examined. The drugs L-NAME, indomethacin, 4-aminopyridine (4-AP), glibenclamide, 5-hydroxydecanoate, ouabain, 8-(3-chlorostyryl) caffeine and PSB-0788 were examined in intact and denuded rings. Rings were tested for viability after each experiment.

RESULTS

Adenosine induced a dose-dependent, triphasic relaxation response, and the mechanical removal of the endothelium significantly deceased adenosine relaxation above 10 μM. Interestingly, endothelial removal significantly decreased the responsiveness (defined as % relaxation per μM adenosine) by two-thirds between 10 and 100 μM, but not in the lower (1-10 μM) or higher (>100 μM) ranges. In intact rings, L-NAME significantly reduced relaxation, but not indomethacin. Antagonists of voltage-dependent Kv (4-AP), sarcolemma KATP (glibenclamide) and mitochondrial KATP channels (5-HD) led to significant reductions in relaxation in both intact and denuded rings, with ouabain having little or no effect. Adenosine-induced relaxation appeared to involve the A2a receptor, but not the A2b subtype.

CONCLUSIONS

It was concluded that adenosine relaxation in NE-precontracted rat aortic rings was triphasic and endothelium-dependent above 10 μM, and relaxation involved endothelial nitric oxide (not prostanoids) and a complex interplay between smooth muscle A2a subtype and voltage-dependent Kv, SarcKATP and MitoKATP channels. The possible in vivo significance of the regulation of arterial compliance to left ventricular function coupling is discussed.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Requisite Role of Kv1.5 Channels in Coronary Metabolic Dilation

RATIONALE

In the working heart, coronary blood flow is linked to the production of metabolites, which modulate tone of smooth muscle in a redox-dependent manner. Voltage-gated potassium channels (Kv), which play a role in controlling membrane potential in vascular smooth muscle, have certain members that are redox-sensitive.

OBJECTIVE

To determine the role of redox-sensitive Kv1.5 channels in coronary metabolic flow regulation.

METHODS AND RESULTS

In mice (wild-type [WT], Kv1.5 null [Kv1.5(-/-)], and Kv1.5(-/-) and WT with inducible, smooth muscle-specific expression of Kv1.5 channels), we measured mean arterial pressure, myocardial blood flow, myocardial tissue oxygen tension, and ejection fraction before and after inducing cardiac stress with norepinephrine. Cardiac work was estimated as the product of mean arterial pressure and heart rate. Isolated arteries were studied to establish whether genetic alterations modified vascular reactivity. Despite higher levels of cardiac work in the Kv1.5(-/-) mice (versus WT mice at baseline and all doses of norepinephrine), myocardial blood flow was lower in Kv1.5(-/-) mice than in WT mice. At high levels of cardiac work, tissue oxygen tension dropped significantly along with ejection fraction. Expression of Kv1.5 channels in smooth muscle in the null background rescued this phenotype of impaired metabolic dilation. In isolated vessels from Kv1.5(-/-) mice, relaxation to H2O2 was impaired, but responses to adenosine and acetylcholine were normal compared with those from WT mice.

CONCLUSIONS

Kv1.5 channels in vascular smooth muscle play a critical role in coupling myocardial blood flow to cardiac metabolism. Absence of these channels disassociates metabolism from flow, resulting in cardiac pump dysfunction and tissue hypoxia.

Metabolic hyperemia requires ATP-sensitive K+ channels and H2O2 but not adenosine in isolated mouse hearts

We have previously demonstrated that adenosine-mediated H2O2 production and opening of ATP-sensitive K(+) (KATP) channels contributes to coronary reactive hyperemia. The present study aimed to investigate the roles of adenosine, H2O2, and KATP channels in coronary metabolic hyperemia (MH). Experiments were conducted on isolated Langendorff-perfused mouse hearts using combined pharmacological approaches with adenosine receptor (AR) knockout mice. MH was induced by electrical pacing at graded frequencies. Coronary flow increased linearly from 14.4 ± 1.2 to 20.6 ± 1.2 ml·min(-1)·g(-1) with an increase in heart rate from 400 to 650 beats/min in wild-type mice. Neither non-selective blockade of ARs by 8-(p-sulfophenyl)theophylline (8-SPT; 50 μM) nor selective A2AAR blockade by SCH-58261 (1 μM) or deletion affected MH, although resting flow and left ventricular developed pressure were reduced. Combined A2AAR and A2BAR blockade or deletion showed similar effects as 8-SPT. Inhibition of nitric oxide synthesis by N-nitro-l-arginine methyl ester (100 μM) or combined 8-SPT administration failed to reduce MH, although resting flows were reduced (by ∼20%). However, glibenclamide (KATP channel blocker, 5 μM) decreased not only resting flow (by ∼45%) and left ventricular developed pressure (by ∼36%) but also markedly reduced MH by ∼94%, resulting in cardiac contractile dysfunction. Scavenging of H2O2 by catalase (2,500 U/min) also decreased resting flow (by ∼16%) and MH (by ∼24%) but to a lesser extent than glibenclamide. Our results suggest that while adenosine modulates coronary flow under both resting and ischemic conditions, it is not required for MH. However, H2O2 and KATP channels are important local control mechanisms responsible for both coronary ischemic and metabolic vasodilation.

Contribution of Kv7 channels to basal coronary flow and active response to ischemia

The goal of the present study was to determine the role of KCNQ-encoded Kv channels (Kv7 channels) in the passive and active regulation of coronary flow in normotensive and hypertensive rats. In left anterior descending coronary arteries from normotensive rats, structurally different Kv7.2 to 7.5 activators produced relaxations, which were considerably less in arteries from hypertensive rats and were not mimicked by the Kv7.1-specific activator R-L3. In isolated, perfused heart preparations, coronary flow rate increased in response to the Kv7.2 to 7.5 activator (S)-1 and was diminished in the presence of a Kv7 inhibitor. The expression levels of KCNQ1-5 and their known accessory KCNE1-5 subunits in coronary arteries were similar in normotensive and hypertensive rats as measured by quantitative polymerase chain reaction. However, Kv7.4 protein expression was reduced in hypertensive rats. Application of adenosine or A2A receptor agonist CGS-21680 produced concentration-dependent relaxations of coronary arteries from normotensive rats, which were attenuated by application of Kv7 inhibitors. Kv7 blockers also attenuated the ischemia-induced increase in coronary perfusion in Langendorff studies. Overall, these data establish Kv7 channels as crucial regulators of coronary flow at resting and after hypoxic insult.

A1 adenosine receptor negatively modulates coronary reactive hyperemia via counteracting A2A-mediated H2O2 production and KATP opening in isolated mouse hearts

We previously demonstrated that A2A, but not A2B, adenosine receptors (ARs) mediate coronary reactive hyperemia (RH), possibly by producing H2O2 and, subsequently, opening ATP-dependent K(+) (KATP) channels in coronary smooth muscle cells. In this study, A1 AR knockout (KO), A3 AR KO, and A1 and A3 AR double-KO (A1/A3 DKO) mice were used to investigate the roles and mechanisms of A1 and A3 ARs in modulation of coronary RH. Coronary flow of isolated hearts was measured using the Langendorff system. A1 KO and A1/A3 DKO, but not A3 KO, mice showed a higher flow debt repayment [~30% more than wild-type (WT) mice, P < 0.05] following a 15-s occlusion. SCH-58261 (a selective A2A AR antagonist, 1 μM) eliminated the augmented RH, suggesting the involvement of enhanced A2A AR-mediated signaling in A1 KO mice. In isolated coronary arteries, immunohistochemistry showed an upregulation of A2A AR (1.6 ± 0.2 times that of WT mice, P < 0.05) and a higher magnitude of adenosine-induced H2O2 production in A1 KO mice (1.8 ± 0.3 times that of WT mice, P < 0.05), which was blocked by SCH-58261. Catalase (2,500 U/ml) and glibenclamide (a KATP channel blocker, 5 μM), but not N(G)-nitro-l-arginine methyl ester, also abolished the enhanced RH in A1 KO mice. Our data suggest that A1, but not A3, AR counteracts the A2A AR-mediated CF increase and that deletion of A1 AR results in upregulation of A2A AR and/or removal of the negative modulatory effect of A1 AR, thus leading to an enhanced A2A AR-mediated H2O2 production, KATP channel opening, and coronary vasodilation during RH. This is the first report implying that A1 AR has a role in coronary RH.

Evidence for the involvement of NADPH oxidase in adenosine receptors-mediated control of coronary flow using A1 and A3 knockout mice

The NADPH oxidase (Nox) subunits 1, 2 (gp91 phox) and 4 are the major sources for reactive oxygen species (ROS) in cardiovascular system. In conditions such as ischemia-reperfusion injury and hypoxia, both ROS and adenosine are released suggesting a possible interaction. We hypothesized that ROS generated through Nox is involved in adenosine-induced coronary flow (CF) responses. Adenosine (10^-8-10^-5.5 M) increased CF in isolated hearts from wild type (WT; C57/BL6), A₁ adenosine receptor (AR) knockout (A₁KO), A₃AR KO (A₃KO) and A₁ and A₃AR double KO (A₁/A₃DKO) mice. The Nox inhibitors apocynin (10^-5 M) and gp91 ds-tat (10^-6 M) or the SOD and catalase-mimicking agent EUK134 (50 μM) decreased the adenosine-enhanced CF in the WT and all the KOs. Additionally, adenosine increased phosphorylation of p47-phox subunit and ERK 1/2 without changing protein expression of Nox isoforms in WT. Moreover, intracellular superoxide production was increased by adenosine and CGS-21680 (a selective A_2A agonist), but not BAY 60-6583 (a selective A_2B agonist), in mouse coronary artery smooth muscle cells (CASMCs) and endothelial cells (CAECs). This superoxide increase was inhibited by the gp91 ds-tat and ERK 1/2 inhibitor (PD98059). In conclusion, adenosine-induced increase in CF in isolated heart involves Nox2-generated superoxide, possibly through ERK 1/2 phosphorylation with subsequent p47-phox subunit phosphorylation. This adenosine/Nox/ROS interaction occurs in both CASMCs and CAECs, and involves neither A₁ nor A₃ ARs, but possibly A_2A ARs in mouse.

Effect of human 15-lipoxygenase-1 metabolites on vascular function in mouse mesenteric arteries and hearts

Lipoxygenases regulate vascular function by metabolizing arachidonic acid (AA) to dilator eicosanoids. Previously, we showed that endothelium-targeted adenoviral vector-mediated gene transfer of the human 15-lipoxygenase-1 (h15-LO-1) enhances arterial relaxation through the production of vasodilatory hydroxyepoxyeicosatrienoic acid (HEETA) and trihydroxyeicosatrienoic acid (THETA) metabolites. To further define this function, a transgenic (Tg) mouse line that overexpresses h15-LO-1 was studied. Western blot, immunohistochemistry and RT-PCR results confirmed expression of 15-LO-1 transgene in tissues, especially high quantity in coronary arterial wall, of Tg mice. Reverse-phase HPLC analysis of [(14)C]-AA metabolites in heart tissues revealed enhanced 15-HETE synthesis in Tg vs. WT mice. Among the 15-LO-1 metabolites, 15-HETE, erythro-13-H-14,15-EETA, and 11(R),12(S),15(S)-THETA relaxed the mouse mesenteric arteries to the greatest extent. The presence of h15-LO-1 increased acetylcholine- and AA-mediated relaxation in mesenteric arteries of Tg mice compared to WT mice. 15-LO-1 was most abundant in the heart; therefore, we used the Langendorff heart model to test the hypothesis that elevated 15-LO-1 levels would increase coronary flow following a short ischemia episode. Both peak flow and excess flow of reperfused hearts were significantly elevated in hearts from Tg compared to WT mice being 2.03 and 3.22 times greater, respectively. These results indicate that h15-LO-1-derived metabolites are highly vasoactive and may play a critical role in regulating coronary blood flow.

Interactions between A(2A) adenosine receptors, hydrogen peroxide, and KATP channels in coronary reactive hyperemia

Myocardial metabolites such as adenosine mediate reactive hyperemia, in part, by activating ATP-dependent K(+) (K(ATP)) channels in coronary smooth muscle. In this study, we investigated the role of adenosine A(2A) and A(2B) receptors and their signaling mechanisms in reactive hyperemia. We hypothesized that coronary reactive hyperemia involves A(2A) receptors, hydrogen peroxide (H(2)O(2)), and KATP channels. We used A(2A) and A(2B) knockout (KO) and A(2A/2B) double KO (DKO) mouse hearts for Langendorff experiments. Flow debt for a 15-s occlusion was repaid 128 ± 8% in hearts from wild-type (WT) mice; this was reduced in hearts from A(2A) KO and A(2A)/(2B) DKO mice (98 ± 9 and 105 ± 6%; P < 0.05), but not A(2B) KO mice (123 ± 13%). Patch-clamp experiments demonstrated that adenosine activated glibenclamide-sensitive KATP current in smooth muscle cells from WT and A(2B) KO mice (90 ± 23% of WT) but not A(2A) KO or A(2A)/A(2B) DKO mice (30 ± 4 and 35 ± 8% of WT; P < 0.05). Additionally, H(2)O(2) activated KATP current in smooth muscle cells (358 ± 99%; P < 0.05). Catalase, an enzyme that breaks down H(2)O(2), attenuated adenosine-induced coronary vasodilation, reducing the percent increase in flow from 284 ± 53 to 89 ± 13% (P < 0.05). Catalase reduced the repayment of flow debt in hearts from WT mice (84 ± 9%; P < 0.05) but had no effect on the already diminished repayment in hearts from A(2A) KO mice (98 ± 7%). Our findings suggest that adenosine A(2A) receptors are coupled to smooth muscle KATP channels in reactive hyperemia via the production of H(2)O(2) as a signaling intermediate.

Left ventricular output and aortic blood flow in response to changes in preload and afterload in the preterm piglet heart

Low systemic blood flow occurs in up to 30% of infants born at less than 30 wk gestation. It is associated with increased morbidity and mortality, and current treatments are ineffective in 40% of cases. The aim of this study was to assess the ability of the preterm heart to respond to the acute shifts in preload and afterload that occur at the time of birth. Myocardial and coronary vascular function was assessed using an isolated working heart model in term (115 days) and preterm (92 days) piglets. Cardiac output/kg body wt in preterm hearts was ∼50% lower than that of term hearts (P = 0.001). Pressure development was similar in term and preterm hearts. Elevations in preload increased cardiac output and aortic flow similarly in term and preterm hearts, demonstrating significant preload "reserve". By contrast, elevations in afterload markedly depressed aortic flow, with a greater proportion of cardiac output being distributed to coronary flow in preterm hearts at high afterloads. The demands of increased workload were associated with greater increases in coronary flow in preterm hearts compared with term hearts. In preterm hearts, exposure to maternal glucocorticoids resulted in increased aortic flow when afterload was below 25 mmHg. These data suggest the preterm heart lacks the functional capacity to acutely adapt to postnatal afterload. To maximize systemic blood flow in preterm infants, treatments limiting afterload, while harnessing significant preload reserve, should be further explored.

The adenosine A₂A receptor - myocardial protectant and coronary target in endotoxemia

BACKGROUND

Cardiac injury and dysfunction are contributors to disease progression and mortality in sepsis. This study evaluated the cardiovascular role of intrinsic A₂A adenosine receptor (A₂AAR) activity during lipopolysaccharide (LPS)-induced inflammation.

METHODS

We assessed the impact of 24 h of LPS challenge (20 mg/kg, IP) on cardiac injury, coronary function and inflammatory mediator levels in Wild-Type (WT) mice and mice lacking functional A₂AARs (A₂AAR KO).

RESULTS

Cardiac injury was evident in LPS-treated WTs, with ~7-fold elevation in serum cardiac troponin I (cTnI), and significant ventricular and coronary dysfunction. Absence of A₂AARs increased LPS-provoked cTnI release at 24 h by 3-fold without additional demise of contraction function. Importantly, A₂AAR deletion per se emulated detrimental effects of LPS on coronary function, and LPS was without effect in coronary vessels lacking A₂AARs. Effects of A₂AAR KO were independent of major shifts in circulating C-reactive protein (CRP) and haptoglobin. Cytokine responses were largely insensitive to A₂AAR deletion; substantial LPS-induced elevations (up to 100-fold) in IFN-γ and IL-10 were unaltered in A₂AAR KO mice, as were levels of IL-4 and TNF-α. However, late elevations in IL-2 and IL-5 were differentially modulated by A₂AAR KO (IL-2 reduced, IL-5 increased). Data demonstrate that in the context of LPS-triggered cardiac and coronary injury, A₂AAR activity protects myocardial viability without modifying contractile dysfunction, and selectively modulates cytokine (IL-2, IL-5) release. A₂AARs also appear to be targeted by LPS in the coronary vasculature.

CONCLUSIONS

These experimental data suggest that preservation of A₂AAR functionality might provide therapeutic benefit in human sepsis.

Contributions of A2A and A2B adenosine receptors in coronary flow responses in relation to the KATP channel using A2B and A2A/2B double-knockout mice

Adenosine plays a role in physiological and pathological conditions, and A(2) adenosine receptor (AR) expression is modified in many cardiovascular disorders. In this study, we elucidated the role of the A(2B)AR and its relationship to the A(2A)AR in coronary flow (CF) changes using A(2B) single-knockout (KO) and A(2A/2B) double-KO (DKO) mice in a Langendorff setup. We used two approaches: 1) selective and nonselective AR agonists and antagonists and 2) A(2A)KO and A(2B)KO and A(2A/2B)DKO mice. BAY 60-6583 (a selective A(2B) agonist) had no effect on CF in A(2B)KO mice, whereas it significantly increased CF in wild-type (WT) mice (maximum of 23.3 ± 9 ml·min(-1)·g(-1)). 5'-N-ethylcarboxamido adenosine (NECA; a nonselective AR agonist) increased CF in A(2B)KO mice (maximum of 34.6 ± 4.7 ml·min(-1)·g(-1)) to a significantly higher degree compared with WT mice (maximum of 23.1 ± 2.1 ml·min(-1)·g(-1)). Also, CGS-21680 (a selective A(2A) agonist) increased CF in A(2B)KO mice (maximum of 29 ± 1.9 ml·min(-1)·g(-1)) to a significantly higher degree compared with WT mice (maximum of 25.1 ± 2.3 ml·min(-1)·g(-1)). SCH-58261 (an A(2A)-selective antagonist) inhibited the NECA-induced increase in CF to a significantly higher degree in A(2B)KO mice (19.3 ± 1.6 vs. 0.5 ± 0.4 ml·min(-1)·g(-1)) compared with WT mice (19 ± 3.5 vs. 3.6 ± 0.5 ml·min(-1)·g(-1)). NECA did not induce any increase in CF in A(2A/2B)DKO mice, whereas a significant increase was observed in WT mice (maximum of 23.1 ± 2.1 ml·min(-1)·g(-1)). Furthermore, the mitochondrial ATP-sensitive K(+) (K(ATP)) channel blocker 5-hydroxydecanoate had no effect on the NECA-induced increase in CF in WT mice, whereas the NECA-induced increase in CF in WT (17.6 ± 2 ml·min(-1)·g(-1)), A(2A)KO (12.5 ± 2.3 ml·min(-1)·g(-1)), and A(2B)KO (16.2 ± 0.8 ml·min(-1)·g(-1)) mice was significantly blunted by the K(ATP) channel blocker glibenclamide (to 0.7 ± 0.7, 2.3 ± 1.1, and 0.9 ± 0.4 ml·min(-1)·g(-1), respectively). Also, the CGS-21680-induced (22 ± 2.3 ml·min(-1)·g(-1)) and BAY 60-6583-induced (16.4 ± 1.60 ml·min(-1)·g(-1)) increase in CF in WT mice was significantly blunted by glibenclamide (to 1.2 ± 0.4 and 1.8 ± 1.2 ml·min(-1)·g(-1), respectively). In conclusion, this is the first evidence supporting the compensatory upregulation of A(2A)ARs in A(2B)KO mice and demonstrates that both A(2A)ARs and A(2B)ARs induce CF changes through K(ATP) channels. These results identify AR-mediated CF responses that may lead to better therapeutic approaches for the treatment of cardiovascular disorders.

Mechanisms involved in the adenosine-induced vasorelaxation to the pig prostatic small arteries

Benign prostatic hypertrophy has been related with glandular ischemia processes and adenosine is a potent vasodilator agent. This study investigates the mechanisms underlying the adenosine-induced vasorelaxation in pig prostatic small arteries. Adenosine receptors expression was determined by Western blot and immunohistochemistry, and rings were mounted in myographs for isometric force recording. A(2A) and A(3) receptor expression was observed in the arterial wall and A(2A)-immunoreactivity was identified in the adventitia-media junction and endothelium. A(1) and A(2B) receptor expression was not obtained. On noradrenaline-precontracted rings, P1 receptor agonists produced concentration-dependent relaxations with the following order of potency: 5'-N-ethylcarboxamidoadenosine (NECA) = CGS21680 > 2-Cl-IB-MECA = 2-Cl-cyclopentyladenosine = adenosine. Adenosine reuptake inhibition potentiated both NECA and adenosine relaxations. Endothelium removal and ZM241385, an A(2A) antagonist, reduced NECA relaxations that were not modified by A(1), A(2B), and A(3) receptor antagonists. Neuronal voltage-gated Ca(2+) channels and nitric oxide (NO) synthase blockade, and adenylyl cyclase activation enhanced these responses, which were reduced by protein kinase A inhibition and by blockade of the intermediate (IK(Ca))- and small (SK(Ca))-conductance Ca(2+)-activated K(+) channels. Inhibition of cyclooxygenase (COX), large-conductance Ca(2+)-activated-, ATP-dependent-, and voltage-gated-K(+) channel failed to modify these responses. These results suggest that adenosine induces endothelium-dependent relaxations in the pig prostatic arteries via A(2A) purinoceptors. The adenosine vasorelaxation, which is prejunctionally modulated, is produced via NO- and COX-independent mechanisms that involve activation of IK(Ca) and SK(Ca) channels and stimulation of adenylyl cyclase. Endothelium-derived NO playing a regulatory role under conditions in which EDHF is non-functional is also suggested. Adenosine-induced vasodilatation could be useful to prevent prostatic ischemia.

Contribution of adenosine A(2A) and A(2B) receptors to ischemic coronary dilation: role of K(V) and K(ATP) channels

This study was designed to elucidate the contribution of adenosine A(2A) and A(2B) receptors to coronary reactive hyperemia and downstream K(+) channels involved. Coronary blood flow was measured in open-chest anesthetized dogs. Adenosine dose-dependently increased coronary flow from 0.72 ± 0.1 to 2.6 ± 0.5 mL/minute/g under control conditions. Inhibition of A(2A) receptors with SCH58261 (1 μm) attenuated adenosine-induced dilation by ∼50%, while combined administration with the A(2B) receptor antagonist alloxazine (3 μm) produced no additional effect. SCH58261 significantly reduced reactive hyperemia in response to a transient 15 second occlusion; debt/repayment ratio decreased from 343 ± 63 to 232 ± 44%. Alloxazine alone attenuated adenosine-induced increases in coronary blood flow by ∼30% but failed to alter reactive hyperemia. A(2A) receptor agonist CGS21680 (10 μg bolus) increased coronary blood flow by 3.08 ± 0.31 mL/minute/g. This dilator response was attenuated to 0.76 ± 0.14 mL/minute/g by inhibition of K(V) channels with 4-aminopyridine (0.3mm) and to 0.11 ± 0.31 mL/minute/g by inhibition of K(ATP) channels with glibenclamide (3 mg/kg). Combined administration abolished vasodilation to CGS21680. These data indicate that A(2A) receptors contribute to coronary vasodilation in response to cardiac ischemia via activation of K(V) and K(ATP) channels.

Adenosine and its receptors in the heart: regulation, retaliation and adaptation

The purine nucleoside adenosine is an important regulator within the cardiovascular system, and throughout the body. Released in response to perturbations in energy state, among other stimuli, local adenosine interacts with 4 adenosine receptor sub-types on constituent cardiac and vascular cells: A(1), A(2A), A(2B), and A(3)ARs. These G-protein coupled receptors mediate varied responses, from modulation of coronary flow, heart rate and contraction, to cardioprotection, inflammatory regulation, and control of cell growth and tissue remodeling. Research also unveils an increasingly complex interplay between members of the adenosine receptor family, and with other receptor groups. Given generally favorable effects of adenosine receptor activity (e.g. improving the balance between myocardial energy utilization and supply, limiting injury and adverse remodeling, suppressing inflammation), the adenosine receptor system is an attractive target for therapeutic manipulation. Cardiovascular adenosine receptor-based therapies are already in place, and trials of new treatments underway. Although the complex interplay between adenosine receptors and other receptors, and their wide distribution and functions, pose challenges to implementation of site/target specific cardiovascular therapy, the potential of adenosinergic pharmacotherapy can be more fully realized with greater understanding of the roles of adenosine receptors under physiological and pathological conditions. This review addresses some of the major known and proposed actions of adenosine and adenosine receptors in the heart and vessels, focusing on the ability of the adenosine receptor system to regulate cell function, retaliate against injurious stressors, and mediate longer-term adaptive responses.

Metabolic syndrome reduces the contribution of K+ channels to ischemic coronary vasodilation

This investigation tested the hypothesis that metabolic syndrome decreases the relative contribution of specific K(+) channels to coronary reactive hyperemia. Ca(2+)-activated (BK(Ca)), voltage-activated (K(V)), and ATP-dependent (K(ATP)) K(+) channels were investigated. Studies were conducted in anesthetized miniature Ossabaw swine fed a normal maintenance diet (11% kcal from fat) or an excess calorie atherogenic diet (43% kcal from fat, 2% cholesterol, 20% kcal from fructose) for 20 wk. The latter diet induces metabolic syndrome, increasing body weight, fasting glucose, total cholesterol, and triglyceride levels. Ischemic vasodilation was determined by the coronary flow response to a 15-s occlusion before and after cumulative administration of antagonists for BK(Ca) (penitrem A; 10 microg/kg iv), K(V) (4-aminopyridine; 0.3 mg/kg iv) and K(ATP) (glibenclamide; 1 mg/kg iv) channels. Coronary reactive hyperemia was diminished by metabolic syndrome as the repayment of flow debt was reduced approximately 30% compared with lean swine. Inhibition of BK(Ca) channels had no effect on reactive hyperemia in either lean or metabolic syndrome swine. Subsequent inhibition of K(V) channels significantly reduced the repayment of flow debt ( approximately 25%) in both lean and metabolic syndrome swine. Additional blockade of K(ATP) channels further diminished ( approximately 45%) the repayment of flow debt in lean but not metabolic syndrome swine. These data indicate that the metabolic syndrome impairs coronary vasodilation in response to cardiac ischemia via reductions in the contribution of K(+) channels to reactive hyperemia.

Apyrase treatment of myocardial infarction according to a clinically applicable protocol fails to reduce myocardial injury in a porcine model

Background

Ectonucleotidase dependent adenosine generation has been implicated in preconditioning related cardioprotection against ischemia-reperfusion injury, and treatment with a soluble ectonucleotidase has been shown to reduce myocardial infarct size (IS) when applied prior to induction of ischemia. However, ectonucleotidase treatment according to a clinically applicable protocol, with administration only after induction of ischemia, has not previously been evaluated. We therefore investigated if treatment with the ectonucleotidase apyrase, according to a clinically applicable protocol, would reduce IS and microvascular obstruction (MO) in a large animal model.

Methods

A percutaneous coronary intervention balloon was inflated in the left anterior descending artery for 40 min, in 16 anesthetized pigs (40-50 kg). The pigs were randomized to 40 min of 1 ml/min intracoronary infusion of apyrase (10 U/ml, n = 8) or saline (0.9 mg/ml, n = 8), twenty minutes after balloon inflation. Area at risk (AAR) was evaluated by ex vivo SPECT. IS and MO were evaluated by ex vivo MRI.

Results

No differences were observed between the apyrase group and saline group with respect to IS/AAR (75.7 ± 4.2% vs 69.4 ± 5.0%, p = NS) or MO (10.7 ± 4.8% vs 11.4 ± 4.8%, p = NS), but apyrase prolonged the post-ischemic reactive hyperemia.

Conclusion

Apyrase treatment according to a clinically applicable protocol, with administration of apyrase after induction of ischemia, does not reduce myocardial infarct size or microvascular obstruction.

Adenosine receptors and the heart: role in regulation of coronary blood flow and cardiac electrophysiology

Adenosine is an autacoid that plays a critical role in regulating cardiac function, including heart rate, contractility, and coronary flow. In this chapter, current knowledge of the functions and mechanisms of action of coronary flow regulation and electrophysiology will be discussed. Currently, there are four known adenosine receptor (AR) subtypes, namely A(1), A(2A), A(2B), and A(3). All four subtypes are known to regulate coronary flow. In general, A(2A)AR is the predominant receptor subtype responsible for coronary blood flow regulation, which dilates coronary arteries in both an endothelial-dependent and -independent manner. The roles of other ARs and their mechanisms of action will also be discussed. The increasing popularity of gene-modified models with targeted deletion or overexpression of a single AR subtype has helped to elucidate the roles of each receptor subtype. Combining pharmacologic tools with targeted gene deletion of individual AR subtypes has proven invaluable for discriminating the vascular effects unique to the activation of each AR subtype. Adenosine exerts its cardiac electrophysiologic effects mainly through the activation of A(1)AR. This receptor mediates direct as well as indirect effects of adenosine (i.e., anti-beta-adrenergic effects). In supraventricular tissues (atrial myocytes, sinuatrial node and atriovetricular node), adenosine exerts both direct and indirect effects, while it exerts only indirect effects in the ventricle. Adenosine exerts a negative chronotropic effect by suppressing the automaticity of cardiac pacemakers, and a negative dromotropic effect through inhibition of AV-nodal conduction. These effects of adenosine constitute the rationale for its use as a diagnostic and therapeutic agent. In recent years, efforts have been made to develop A(1)R-selective agonists as drug candidates that do not induce vasodilation, which is considered an undesirable effect in the clinical setting.

Up-regulation of A 2B adenosine receptor in A 2A adenosine receptor knockout mouse coronary artery

In this study, we looked into possible compensatory changes of other adenosine receptors (ARs) in A(2A) genetic knockout mice (A2AKO) as well as the functional role of nitric oxide (NO) in A(2A) AR-mediated vasodilation. Gene expression of ARs from coronary arteries of A(2A) AR wild type mice (A2AWT) and A2AKO was studied using real-time PCR. Functional studies were carried out in isolated heart and isolated coronary artery preparations. A(2B) AR was found to be 4.5 fold higher in A2AKO than in A2AWT, while A(2A) AR expression was absent in A2AKO. There was no difference in A(1) and A(3) ARs between WT and KO animals. The concentration-relaxation curve for adenosine-5'-N-ethylcarboxamide (NECA, non-selective AR agonist) in isolated coronary arterial rings in A2AKO was shifted to the left when compared to A2AWT. The concentration-response curve for A(2B) selective agonist (BAY 60-6583) was also shifted to the left in A2AKO hearts. L-NAME, a non-specific NO synthase inhibitor, did not affect baseline coronary flow (CF) until the concentration reached 10 microM in A2AWT (76.32+/-11.35% from baseline, n=5). In A2AKO, the CF decreased significantly by L-NAME only at a higher concentration (100 microM, 93.32+/-5.8% from baseline, n=5). L-NMA (1 microM, n=4), another non-specific NO synthase inhibitor, also demonstrated similar results in decreasing CF (59.66+/-3.23% from baseline in A2AWT, while 81.76+/-8.91% in A2AKO). It was further demonstrated that the increase in CF by 100 microM NECA was significantly blunted with 10 microM L-NAME (377.08+/-25.23% to 305.41+/-30.73%, n=9) in A2AWT but not in A2AKO (153.66+/-22.7% to 143.88+/-36.65%, n=5). Similar results were also found using 50 nM of CGS-21680 instead of NECA in A2AWT (346+/-22.85 to 277+/-31.39, n=6). No change in CF to CGS-21680 was noted in A(2A)AKO. Our data demonstrate, for the first time, that coronary A(2B) AR was up-regulated in mice deficient in A(2A) AR. We also provide direct evidence supporting a role for NO in A(2A) AR-mediated coronary vasodilation. The data further support the role for A(2A) AR in the regulation of basal coronary tone through the release of NO.

P2 receptors in cardiovascular regulation and disease

The role of ATP as an extracellular signalling molecule is now well established and evidence is accumulating that ATP and other nucleotides (ADP, UTP and UDP) play important roles in cardiovascular physiology and pathophysiology, acting via P2X (ion channel) and P2Y (G protein-coupled) receptors. In this article we consider the dual role of ATP in regulation of vascular tone, released as a cotransmitter from sympathetic nerves or released in the vascular lumen in response to changes in blood flow and hypoxia. Further, purinergic long-term trophic and inflammatory signalling is described in cell proliferation, differentiation, migration and death in angiogenesis, vascular remodelling, restenosis and atherosclerosis. The effects on haemostasis and cardiac regulation is reviewed. The involvement of ATP in vascular diseases such as thrombosis, hypertension and diabetes will also be discussed, as well as various heart conditions. The purinergic system may be of similar importance as the sympathetic and renin-angiotensin-aldosterone systems in cardiovascular regulation and pathophysiology. The extracellular nucleotides and their cardiovascular P2 receptors are now entering the phase of clinical development.

Cyclooxygenase and nitric oxide synthase dependence of cutaneous reactive hyperemia in humans

We tested the hypothesis that cyclooxygenases (COXs) or COX products inhibit nitric oxide (NO) synthesis and thereby mask potential effects of NO on reactive hyperemia in the cutaneous circulation. We performed laser-Doppler flowmetry (LDF) with intradermal microdialysis in 12 healthy volunteers aged 19-25 yr. LDF was expressed as the percent cutaneous vascular conduction (%CVC) or as the maximum %CVC (%CVC(max)) where CVC is LDF/mean arterial pressure. We tested the effects of the nonisoform-specific NO synthase inhibitor nitro-L-arginine (NLA, 10 mM), the nonspecific COX inhibitor ketorolac (Keto, 10 mM), combined NLA + Keto, and NLA + sodium nitroprusside (SNP, 28 mM) on baseline and reactive hyperemia flow parameters. We also examined the effects of isoproterenol, a beta-adrenergic agonist that causes prostaglandin-independent vasodilation to correct for the increase in baseline flow caused by Keto. When delivered directly into the intradermal space, Keto greatly augments all aspects of the laser-Doppler flow response to reactive hyperemia: peak reactive hyperemic flow increased from 41 +/- 5 to 77 +/- 7%CVC(max), time to peak flow increased from 17 +/- 3 to 56 +/- 24 s, the area under the reactive hyperemic curve increased from 1,417 +/- 326 to 3,376 +/- 876%CVC(max).s, and the time constant for the decay of peak flow increased from 100 +/- 23 to 821 +/- 311 s. NLA greatly attenuates the Keto response despite exerting no effects on baseline LDF or on reactive hyperemia when given alone. Low-dose NLA + SNP duplicates the Keto response. Isoproterenol increased baseline and peak reactive flow. These results suggest that COX inhibition unmasks NO dependence of reactive hyperemia in human cutaneous circulation.

Mild hypothermia reduces cardiac post-ischemic reactive hyperemia

Background

In experimentally induced myocardial infarction, mild hypothermia (33–35°C) is beneficial if applied prior to ischemia or reperfusion. Hypothermia, when applied after reperfusion seems to confer little or no benefit. The mechanism by which hypothermia exerts its cell-protective effect during cardiac ischemia remains unclear. It has been hypothesized that hypothermia reduces the reperfusion damage; the additional damage incurred upon the myocardium during reperfusion. Reperfusion results in a massive increase in blood flow, reactive hyperemia, which may contribute to reperfusion damage. We postulated that hypothermia could attenuate the post-ischemic reactive hyperemia.

Methods

Sixteen 25–30 kg pigs, in a closed chest model, were anesthetized and temperature was established in all pigs at 37°C using an intravascular cooling catheter. The 16 pigs were then randomized to hypothermia (34°C) or control (37°C). The left main coronary artery was then catheterized with a PCI guiding catheter. A Doppler flow wire was placed in the mid part of the LAD and a PCI balloon was then positioned proximal to the Doppler wire but distal to the first diagonal branch. The LAD was then occluded for ten minutes in all pigs. Coronary blood flow was measured before, during and after ischemia/reperfusion.

Results

The peak flow seen during post-ischemic reactive hyperemia (during the first minutes of reperfusion) was significantly reduced by 43 % (p < 0.01) in hypothermic pigs compared to controls.

Conclusion

Mild hypothermia significantly reduces post-ischemic hyperemia in a closed chest pig model. The reduction of reactive hyperemia during reperfusion may have an impact on cardiac reperfusion injury.

Contribution of anaerobic metabolism to reactive hyperemia in skeletal muscle

Elevated blood flow (reactive hyperemia) is seen in many organs after a period of blood flow stoppage. This hyperemia is often considered to be due in part to a shift to anaerobic metabolism during tissue hypoxia. The aim of our study was to test this hypothesis in skeletal muscle. For this purpose we measured NADH fluorescence at localized tissue areas in cat sartorius muscle during and after arterial occlusions of 5-300 s. In parallel studies, red blood cell (RBC) velocity was measured in venules. Tissue NADH fluorescence rose significantly with occlusions of 45 s or greater, reaching a maximum of 44% above control at 180 s. Peak RBC velocity rose to four times control as occlusion duration was increased from 5 to 45 s, but hyperemia duration was stable at approximately 70 s. With occlusions of 45-240 s, hyperemia duration increased progressively to 210 s while peak flow was unchanged. However, after 300-s occlusions, peak flow rose to six times above control and hyperemia duration fell to 140 s. With occlusions of 45-300 s the time integral both of increased NADH fluorescence and of reduced fluorescence following occlusion release showed a high degree of correlation with the additional hyperemia. We conclude that in this muscle anaerobic vasodilator metabolites are responsible for the increase in reactive hyperemia with arterial occlusions longer than 45 s. Since the durations of reactive hyperemia and reduced fluorescence are substantially different, vasodilator metabolite removal may be due to washout by the bloodstream rather than metabolic uptake.

Myocardial protection evoked by hyperoxic exposure involves signaling through nitric oxide and mitogen activated protein kinases

BACKGROUND

Hyperoxic exposure in vivo (> 95% oxygen) attenuates ischemia-reperfusion injury, but the signaling mechanisms of this cardioprotection are not fully determined. We studied a possible role of nitric oxide (NO) and mitogen activated protein kinases (MAPK) in hyperoxic protection.

METHODS

Mice (n = 7-9 in each group) were kept in normoxic or hyperoxic environments for 15 min prior to harvesting the heart and Langendorff perfusion with global ischemia (45 min) and reperfusion (60 min). Endpoints were cardiac function and infarct size. Additional hearts were collected to evaluate MAPK phosphorylation (immunoblot). The nitric oxide synthase inhibitor L-NAME, the ERK1/2 inhibitor PD98059 and the p38 MAPK inhibitor FR167653 were injected intraperitoneally before hyperoxia or normoxia.

RESULTS

Hyperoxia improved postischemic functional recovery and reduced infarct size (p < 0.05). Hyperoxic exposure caused cardiac phosphorylation of the MAPK family members p38 and ERK1/2, but not JNK. L-NAME, PD98059 and FR167653 all reduced the protection afforded by hyperoxic exposure, but did not influence performance or infarction in hearts of normoxic mice. The hyperoxia-induced phosphorylation of ERK1/2 and p38 was reduced by L-NAME and both MAPK inhibitors.

CONCLUSION

Nitric oxide triggers hyperoxic protection, and ERK1/2 and p38 MAPK are involved in signaling of protection against ischemia-reperfusion injury.

Genetic modulation of adenosine receptor function and adenosine handling in murine hearts: insights and issues

The adenosine receptor system has been attributed with a broad range of both physiological and so-called 'retaliatory' functions in the heart and vessels. Despite many years of research, the precise roles of adenosine within the cardiovascular system continue to be debated, and new functions are continually emerging. Adenosine acts via 4 known G-protein-coupled receptor (GPCR) sub-types: A(1), A(2A), A(2B), and A(3) adenosine receptors (ARs). In addition to roles in cardiovascular control, these receptors may represent therapeutic targets, having been attributed with roles in modifying cell death and injury, inflammatory processes, and cardiac and vascular remodeling during/after ischemic or hypoxic insult. A number of models have been developed in which AR sub-types and adenosine handling enzymes have been genetically deleted or transgenically overexpressed in an attempt to more equivocally identify the regulatory functions of these proteins, to identify their potential value as therapeutic targets, and to uncover new regulatory functions of this receptor family. Findings generally support current dogma regarding cardioprotection via A(1) and A(3)ARs, and coronary vasoregulation via A(2)AR sub-types. However, some outcomes are both novel and controversial. This review outlines AR-modified murine models currently under study from the perspective of cardiovascular phenotype.

Coronary reactive hyperaemia and arterial pressure in anaesthetized goats

To study the effects of arterial pressure on coronary reactive hyperaemia, left circumflex coronary artery flow was measured, and reactive hyperaemia was determined after 5, 10 or 20 s of occlusion of this artery in anaesthetized goats during normotension, hypertension and hypotension. During hypertension induced by aortic constriction (mean arterial pressure, MAP = 140 +/- 6 mmHg) coronary vascular resistance (CVR), reactive hyperaemia (ratio of peak in hyperaemic flow to control flow and ratio of repayment to debt) and the decrease in CVR during the peak in hyperaemic flow were comparable to those during normotension. During hypertension induced by noradrenaline (MAP = 144 +/- 6 mmHg) CVR was 16% lower (P < 0.05), reactive hyperaemia was reduced by 14-25% (P < 0.05) and the decrease in CVR during the peak in hyperaemic flow was lower than the values of these parameters during normotension. During hypotension induced by constriction of the caudal vena cava (MAP = 40 +/- 4 mmHg) CVR was 22% lower (P < 0.05), reactive hyperaemia was reduced by 25-65% (P < 0.05) and the decrease in CVR during the peak in hyperaemic flow was less compared to the values of these parameters during normotension. During hypotension induced by isoprenaline (MAP = 45 +/- 4 mmHg) CVR was 59% lower, reactive hyperaemia was reduced by 55-100% (P < 0.01) and the decrease in CVR during the peak in hyperaemic flow was less compared to the values of these parameters during normotension. Arterial pressure is a main determinant of coronary reactive hyperaemia after brief periods of ischaemia, and the relationship between arterial pressure and reactive hyperaemia may depend in part on changes in CVR after variations in arterial pressure. These changes in CVR may be related to the action on coronary vessels of myocardial factors and vascular myogenic mechanisms.

A3 adenosine receptor-mediated protection of the ischemic heart

The A3 adenosine receptor (A3AR) is attributed with multiple beneficial actions in ischemic-reperfused myocardium, including modulation of oncotic and apoptotic cell death and enhancement of contractile function. Additionally, the A3AR may attenuate vascular dysfunction and improve long-term outcome from myocardial insult (modulating hypertrophy and angiogenesis). Available evidence indicates that this receptor sub-type is minimally activated by endogenous adenosine during ischemia (A3AR antagonists exerting no effects on ischemic outcome), and is thus amenable to activation with exogenous agonists. Protected phenotypes arise with both pre- and post-ischemic treatment with A3AR agonists, and transient A3AR agonism also triggers early and delayed preconditioned states. The molecular basis for the varied protective actions of the A3AR remains poorly defined, and may well vary between species (e.g. rodent vs. human) and protective responses (e.g. acute vs. delayed protection). Nonetheless, A3ARs may be more promising as therapeutic "anti-ischemic" targets compared with other adenosine receptor subtypes, since A3AR agonists elicit fewer and less significant side-effects. This review addresses current knowledge and controversy regarding the protective actions (and associated signaling) of A3ARs in ischemic-reperfused heart.