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

Role of ion channels in coronary microcirculation: a review of the literature

In normal coronary arteries, several different mechanisms of blood flow regulation exist, acting at different levels of the coronary tree: endothelial, nervous, myogenic and metabolic regulation. In addition, physiologic blood flow regulation is also dependent on the activity of several coronary ion channels, including ATP-dependent K(+) channels, voltage-gated K(+) channels and others. In this context, ion channels contribute by matching demands for homeostatic maintenance. They play a primary role in rapid response of both endothelium and vascular smooth muscle cells of larger and smaller arterial vessels of the coronary bed, leading to coronary vasodilation. Consequently, an alteration in ion channel function or expression could be directly involved in coronary vasomotion dysfunction.

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.

Contribution of electromechanical coupling between Kv and Ca v1.2 channels to coronary dysfunction in obesity

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

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.

Nitric oxide synthase/K+ channel cascade triggers the adenosine A(2B) receptor-sensitive renal vasodilation in female rats

Adenosine A2B-receptors mediate the adenosine-evoked renal vasodilations in male rats. Here, we tested whether this finding could be replicated in female renal vasculature and whether K(+) hyperpolarization induced by nitric oxide synthase (NOS) and/or heme oxygenase (HO) accounts for adenosine A2B receptor-sensitive renal vasodilations. In phenylephrine-preconstricted perfused kidneys, vasodilations caused by the adenosine analog 5'-N-ethylcarboxamidoadenosine (NECA, 1.6-50 nmol) were attenuated after blockade of adenosine A2B (alloxazine) but not A2A [8-(3-Chlorostyryl) caffeine, CSC] or A3 receptors (N-(2-methoxyphenyl)-N'-[2-(3-pyridinyl)-4-quinazolinyl]-urea, VUF 5574), confirming the preferential involvement of A2B receptors in NECA responses. NOS activation mediated the A2B receptor-mediated NECA response because: (i) NOS inhibition (N(ω)-nitro-L-arginine-methyl ester, L-NAME) attenuated NECA vasodilations, (ii) concurrent L-NAME/alloxazine exposure caused more inhibition of NECA responses, and (iii) inhibition of NECA responses by alloxazine disappeared in L-arginine-supplemented preparations. Although HO inhibition (zinc protoporphyrin) failed to modify NECA responses, the attenuation of these responses by alloxazine disappeared in hemin (HO inducer)-treated preparations. NECA vasodilations were also attenuated after exposure to BaCl2, glibenclamide but not tetraethylammonium (blockers of inward rectifier, ATP-sensitive, and Ca(2+)-dependent K(+)-channels, respectively). The combined alloxazine/BaCl2/glibenclamide infusion caused no additional attenuation of NECA vasodilations. Vasodilations caused by minoxidil (K(+)-channel opener) were reduced by L-NAME or BaCl2/glibenclamide, supporting the importance of NOS signaling in K(+) hyperpolarization. NECA or minoxidil vasodilations were attenuated by ouabain, Na(+)/K(+)-ATPase inhibitor, and in KCl-preconstricted preparations. Overall, facilitation of adenosine A2B receptor/NOS/K(+) channel/Na(+)/K(+)-ATPase cascade underlies NECA vasodilations in female rats. Enhancing HO activity, albeit not causally related to NECA vasodilations, improves the pharmacologically compromised (alloxazine) NECA response.

Regulation of foam cells by adenosine

Macrophages rely on reverse cholesterol transport mechanisms to rid themselves of excess cholesterol. By reducing accumulation of cholesterol in the artery wall, reverse cholesterol transport slows or prevents development of atherosclerosis. In stable macrophages, efflux mechanisms balance influx mechanisms, and accumulating lipids do not overwhelm the cell. Under atherogenic conditions, inflow of cholesterol exceeds outflow, and the cell is ultimately transformed into a foam cell, the prototypical cell in the atherosclerotic plaque. Adenosine is an endogenous purine nucleoside released from metabolically active cells by facilitated diffusion and generated extracellularly from adenine nucleotides. Under stress conditions, such as hypoxia, a depressed cellular energy state leads to an acute increase in the extracellular concentration of adenosine. Extracellular adenosine interacts with 1 or more of a family of G protein-coupled receptors (A(1), A(2A), A(2B), and A(3)) to modulate the function of nearly all cells and tissues. Modulation of adenosine signaling participates in regulation of reverse cholesterol transport. Of particular note for the development of atherosclerosis, activation of A(2A) receptors dramatically inhibits inflammation and protects against tissue injury. Potent antiatherosclerotic effects of A(2A) receptor stimulation include inhibition of macrophage foam cell transformation and upregulation of the reverse cholesterol transport proteins cholesterol 27-hydroxylase and ATP binding cassette transporter A1. Thus, A(2A) receptor agonists may correct or prevent the adverse effects of inflammatory processes on cellular cholesterol homeostasis. This review focuses on the importance of extracellular adenosine acting at specific receptors as a regulatory mechanism to control the formation of foam cells under conditions of lipid loading.

A2 adenosine receptors and vascular pathologies

Cardiovascular disease, a leading cause of death and morbidity, is regulated, among various factors, by inflammation. The level of the metabolite adenosine is augmented under stress, including inflammatory, hypoxic, or injurious events. Adenosine has been shown to affect various physiological and pathological processes, largely through 1 or more of its 4 types of receptors: the A1 and A3 adenylyl cyclase inhibitory receptors and the A2A and A2B adenylyl cyclase stimulatory receptors. This article focuses on reviewing common and distinct effects of the 2 A2-type adenosine receptors on vascular disease and the mechanisms involved. Understanding the pathogenesis of vascular disease mediated by these receptors is important to the development of therapeutics and to the prevention and management of disease.

Contribution of voltage-dependent K+ and Ca2+ channels to coronary pressure-flow autoregulation

The mechanisms responsible for coronary pressure-flow autoregulation, a critical physiologic phenomenon that maintains coronary blood flow relatively constant in the presence of changes in perfusion pressure, remain poorly understood. This investigation tested the hypothesis that voltage-sensitive K(+) (K(V)) and Ca(2+) (Ca(V)1.2) channels play a critical role in coronary pressure-flow autoregulation in vivo. Experiments were performed in open-chest, anesthetized Ossabaw swine during step changes in coronary perfusion pressure (CPP) from 40 to 140 mmHg before and during inhibition of K(V) channels with 4-aminopyridine (4AP, 0.3 mM, ic) or Ca(V)1.2 channels with diltiazem (10 μg/min, ic). 4AP significantly decreased vasodilatory responses to H(2)O(2) (0.3-10 μM, ic) and coronary flow at CPPs = 60-140 mmHg. This decrease in coronary flow was associated with diminished ventricular contractile function (dP/dT) and myocardial oxygen consumption. However, the overall sensitivity to changes in CPP from 60 to 100 mmHg (i.e. autoregulatory gain; Gc) was unaltered by 4-AP administration (Gc = 0.46 ± 0.11 control vs. 0.46 ± 0.06 4-AP). In contrast, inhibition of Ca(V)1.2 channels progressively increased coronary blood flow at CPPs > 80 mmHg and substantially diminished coronary Gc to -0.20 ± 0.11 (P < 0.01), with no effect on contractile function or oxygen consumption. Taken together, these findings demonstrate that (1) K(V) channels tonically contribute to the control of microvascular resistance over a wide range of CPPs, but do not contribute to coronary responses to changes in pressure; (2) progressive activation of Ca(V)1.2 channels with increases in CPP represents a critical mechanism of coronary pressure-flow autoregulation.

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.

Functional contribution of P2Y1 receptors to the control of coronary blood flow

Activation of ADP-sensitive P2Y(1) receptors has been proposed as an integral step in the putative "nucleotide axis" regulating coronary blood flow. However, the specific mechanism(s) and overall contribution of P2Y(1) receptors to the control of coronary blood flow have not been clearly defined. Using vertically integrative studies in isolated coronary arterioles and open-chest anesthetized dogs, we examined the hypothesis that P2Y(1) receptors induce coronary vasodilation via an endothelium-dependent mechanism and contribute to coronary pressure-flow autoregulation and/or ischemic coronary vasodilation. Immunohistochemistry revealed P2Y(1) receptor expression in coronary arteriolar endothelial and vascular smooth muscle cells. The ADP analog 2-methylthio-ADP induced arteriolar dilation in vitro and in vivo that was abolished by the selective P2Y(1) antagonist MRS-2179 and the nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester. MRS-2179 did not alter baseline coronary flow in vivo but significantly attenuated coronary vasodilation to ATP in vitro and in vivo and the nonhydrolyzable ATP analog ATPγS in vitro. Coronary blood flow responses to alterations in coronary perfusion pressure (40-100 mmHg) or to a brief 15-s coronary artery occlusion were unaffected by MRS-2179. Our data reveal that P2Y(1) receptors are functionally expressed in the coronary circulation and that activation produces coronary vasodilation via an endothelium/nitric oxide-dependent mechanism. Although these receptors represent a critical component of purinergic coronary vasodilation, our findings indicate that P2Y(1) receptor activation is not required for coronary pressure-flow autoregulation or reactive hyperemia.

Contribution of voltage-dependent K⁺ channels to metabolic control of coronary blood flow

The purpose of this investigation was to test the hypothesis that K(V) channels contribute to metabolic control of coronary blood flow and that decreases in K(V) channel function and/or expression significantly attenuate myocardial oxygen supply-demand balance in the metabolic syndrome (MetS). Experiments were conducted in conscious, chronically instrumented Ossabaw swine fed either a normal maintenance diet or an excess calorie atherogenic diet that produces the clinical phenotype of early MetS. Data were obtained under resting conditions and during graded treadmill exercise before and after inhibition of K(V) channels with 4-aminopyridine (4-AP, 0.3mg/kg, iv). In lean-control swine, 4-AP reduced coronary blood flow ~15% at rest and ~20% during exercise. Inhibition of K(V) channels also increased aortic pressure (P<0.01) while reducing coronary venous PO(2) (P<0.01) at a given level of myocardial oxygen consumption (MVO(2)). Administration of 4-AP had no effect on coronary blood flow, aortic pressure, or coronary venous PO(2) in swine with MetS. The lack of response to 4-AP in MetS swine was associated with a ~20% reduction in coronary K(V) current (P<0.01) and decreased expression of K(V)1.5 channels in coronary arteries (P<0.01). Together, these data demonstrate that K(V) channels play an important role in balancing myocardial oxygen delivery with metabolism at rest and during exercise-induced increases in MVO(2). Our findings also indicate that decreases in K(V) channel current and expression contribute to impaired control of coronary blood flow in the MetS. This article is part of a Special Issue entitled "Coronary Blood Flow".