ATP sensitive potassium channels are involved in adenosine A2 receptor mediated coronary vasodilatation in the dog

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

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.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

Purinergic transmission in blood vessels

There are nineteen different receptor proteins for adenosine, adenine and uridine nucleotides, and nucleotide sugars, belonging to three families of G protein-coupled adenosine and P2Y receptors, and ionotropic P2X receptors. The majority are functionally expressed in blood vessels, as purinergic receptors in perivascular nerves, smooth muscle and endothelial cells, and roles in regulation of vascular contractility, immune function and growth have been identified. The endogenous ligands for purine receptors, ATP, ADP, UTP, UDP and adenosine, can be released from different cell types within the vasculature, as well as from circulating blood cells, including erythrocytes and platelets. Many purine receptors can be activated by two or more of the endogenous ligands. Further complexity arises because of interconversion between ligands, notably adenosine formation from the metabolism of ATP, leading to complex integrated responses through activation of different subtypes of purine receptors. The enzymes responsible for this conversion, ectonucleotidases, are present on the surface of smooth muscle and endothelial cells, and may be coreleased with neurotransmitters from nerves. What selectivity there is for the actions of purines/pyrimidines comes from differential expression of their receptors within the vasculature. P2X1 receptors mediate the vasocontractile actions of ATP released as a neurotransmitter with noradrenaline (NA) from sympathetic perivascular nerves, and are located on the vascular smooth muscle adjacent to the nerve varicosities, the sites of neurotransmitter release. The relative contribution of ATP and NA as functional cotransmitters varies with species, type and size of blood vessel, neuronal firing pattern, the tone/pressure of the blood vessel, and in ageing and disease. ATP is also a neurotransmitter in non-adrenergic non-cholinergic perivascular nerves and mediates vasorelaxation via smooth muscle P2Y-like receptors. ATP and adenosine can act as neuromodulators, with the most robust evidence being for prejunctional inhibition of neurotransmission via A1 adenosine receptors, but also prejunctional excitation and inhibition of neurotransmission via P2X and P2Y receptors, respectively. P2Y2, P2Y4 and P2Y6 receptors expressed on the vascular smooth muscle are coupled to vasocontraction, and may have a role in pathophysiological conditions, when purines are released from damaged cells, or when there is damage to the protective barrier that is the endothelium. Adenosine is released during hypoxia to increase blood flow via vasodilator A2A and A2B receptors expressed on the endothelium and smooth muscle. ATP is released from endothelial cells during hypoxia and shear stress and can act at P2Y and P2X4 receptors expressed on the endothelium to increase local blood flow. Activation of endothelial purine receptors leads to the release of nitric oxide, hyperpolarising factors and prostacyclin, which inhibits platelet aggregation and thus ensures patent blood flow. Vascular purine receptors also regulate endothelial and smooth muscle growth, and inflammation, and thus are involved in the underlying processes of a number of cardiovascular diseases.

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.

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.

Mechanisms of metabolic coronary flow regulation

Coronary blood flow is tightly adjusted to the oxygen requirements of the myocardium. The underlying control mechanisms keep coronary venous pO(2) at a rather constant level around 20mm Hg under a variety of physiological conditions. Because coronary flow may increase more than 5-fold during exercise without any signs of under- or overperfusion, coronary flow must be controlled, at least in part, in a feed forward manner. Likely metabolic factors contributing to feed forward control are carbon dioxide and reactive oxygen species. Adaptation of coronary flow to exercise under physiological conditions involves in addition to metabolic control feed forward neuronal and endothelium-dependent control. Under pathological conditions, e.g. vessel stenosis or anemia, or specific environmental conditions, e.g. high altitude exposure, cardiac oxygenation may become critical, especially if oxygen demand is increased during physical exercise. Under such conditions the fall of coronary pO(2) may directly result in opening of oxygen sensitive potassium or closure of calcium channels. Furthermore the fall of pO(2) results in the production of vasoactive metabolites, e.g. adenosine, nitric oxide or prostaglandins, and in proton accumulation. All of these adaptations support a reduction of coronary vessel resistance. This article is part of a Special Issue entitled "Coronoray Blood Flow".

Adenosine A2 receptors are involved in the activation of ATP-sensitive K+ currents during metabolic inhibition in guinea pig ventricular myocytes

It has been hypothesized that an interaction among adenosine A1 receptors, protein kinase C (PKC) activation, and ATP-sensitive potassium channels (KATP) mediates ischemic preconditioning in experiments on different animal species. The purpose of this study was to determine if activation of KATP is functionally coupled to A1 receptors and (or) PKC activation during metabolic inhibition (MI) in guinea pig ventricular myocytes. Perforated-patch using nystatin and conventional whole-cell recording methods were used to observe the effects of adenosine and adenosine-receptor antagonists on the activation of KATP currents during MI induced by application of 2,4-dinitrophenol (DNP) and 2-deoxyglucose (2DG) without glucose, in the presence or absence of a PKC activator, phorbol 12-myristate 13-acetate (PMA). Adenosine accelerated the time course activation of KATP currents during MI under the intact intracellular condition or dialyzed condition with l mmol/L ATP in the pipette solution. The accelerated effect of adenosine activation of KATP under MI was not reversed by a nonselective Al adenosine receptor antagonist, 8-(p-sulfophenyl)theophylline (SPT), or a specific Al adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). However, the adenosine A2 receptor antagonist alloxazine reversed the time course activation of the KATP current under MI. An adenylate cyclase activator, forskolin, did not further abbreviate the time course activation of KATP with or without adenosine. Application of a PKC blocker, chelerythrine, reversed the time course activation of KATP by adenosine under MI. In addition, pretreatment with a PKC activator, PMA, had similar effects to adenosine, while adenosine did not further shorten the time required for activation of KATP currents during MI with PMA pretreatment. There is no direct evidence of activation of KATP currents by adenosine A1 receptor during metabolic inhibition under our experimental condition. However, adenosine A2 receptor activation is involved in the KATP channel activation in the guinea pig ventricular myocytes, of which effect is not mediated through the increase in intracellular cAMP. Adenosine seems to interact with PKC activation to open KATP during MI, but a possible link between the adenosine A2 receptor and PKC activation in this process needs further elucidation.

Role of potassium channels in coronary vasodilation

K+ channels in coronary arterial smooth muscle cells (CASMC) determine the resting membrane potential ( Em) and serve as targets of endogenous and therapeutic vasodilators. Em in CASMC is in the voltage range for activation of L-type Ca2+ channels; therefore, when K+ channel activity changes, Ca2+ influx and arterial tone change. This is why both Ca2+ channel blockers and K+ channel openers have such profound effects on coronary blood flow; the former directly inhibits Ca2+ influx through L-type Ca2+ channels, while the latter indirectly inhibits Ca2+ influx by hyperpolarizing Em and reducing Ca2+ channel activity. K+ channels in CASMC play important roles in vasodilation to endothelial, ischemic and metabolic stimuli. The purpose of this article is to review the types of K+ channels expressed in CASMC, discuss the regulation of their activity by physiological mechanisms and examine impairments related to cardiovascular disease.

THE ADENOSINE 2A RECEPTOR AGONIST ATL-146E ATTENUATES EXPERIMENTAL POSTHEMORRHAGIC VASOSPASM

OBJECTIVE

Selective adenosine 2A receptor agonists, such as ATL-146e, are known to be potent anti-inflammatory agents devoid of systemic side effects and have been used clinically in a number of disease states. However, adenosine 2A receptor agonists have not been studied in the treatment of cerebral vasospasm after subarachnoid hemorrhage. The present study investigated the efficacy of ATL-146e in the prevention of leukocyte infiltration and attenuation of posthemorrhagic vasospasm.

METHODS

The rodent femoral artery model of vasospasm was used. Forty male Sprague-Dawley rats were randomly assigned to four different groups (vehicle, 1 ng/kg/min, 10 ng/kg/min, or 100 ng/kg/min ATL-146e administered via subcutaneous osmotic minipump). Vasospasm was evaluated at posthemorrhage Day 8 (period of peak constriction) by calculating the lumen cross-sectional area (expressed as percent change in luminal area: ratio of blood-exposed vessel to normal saline-exposed vessel) and radial wall thickness. Immunostaining with anti-CD45 monoclonal antibody to detect leukocytes was used to evaluate localized inflammation.

RESULTS

Significant vasospasm was noted in the vehicle-treated (blood-exposed) control group (78.5%, P < 0.001; expressed as a ratio of luminal area of the saline [no blood] control), but not in the ATL-146e-treated groups (lumen ratio to control: 105.0, 83.4, and 91.3% for the 1, 10, and 100 ng/kg/min groups, respectively). Additionally, infiltration of inflammatory cells was reduced significantly and radial wall thickness was decreased in the ATL-146e-treated groups compared with the vehicle-treated control group.

CONCLUSION

Selective activation of the adenosine 2A receptor with ATL-146e prevented posthemorrhagic vasospasm and reduced leukocyte infiltration in this experimental model. This agent is worthy of further investigation and lends credence to the hypothesis supporting a role for inflammation in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage.

Effect of K+ATP channel and adenosine receptor blockade during rest and exercise in congestive heart failure

K(+)(ATP) channels are important metabolic regulators of coronary blood flow (CBF) that are activated in the setting of reduced levels of ATP or perfusion pressure. In the normal heart, blockade of K(+)(ATP) channels results in a approximately 20% reduction in resting CBF but does not impair the increase in CBF that occurs during exercise. In contrast, adenosine receptor blockade fails to alter CBF or myocardial oxygen consumption (MVO(2)) in the normal heart but contributes to the increase in CBF during exercise when vascular K(+)(ATP) channels are blocked. Congestive heart failure (CHF) is associated with a decrease in CBF that is matched to a decrease in MVO(2) suggesting downregulation of myocardial energy utilization. Because myocardial ATP levels and coronary perfusion pressure are reduced in CHF, this study was undertaken to examine the role of K(+)(ATP) channels and adenosine in dogs with pacing-induced CHF. Myocardial blood flow (MBF) and MVO(2) were measured during rest and treadmill exercise before and after K(+)(ATP) channel blockade with glibenclamide (50 microg/kg/min ic) or adenosine receptor blockade with 8-phenyltheophylline (8-PT; 5 mg/kg iv). Inhibition of K(+)(ATP) channels resulted in a decrease in CBF and MVO(2) at rest and during exercise without a change in the relationship between CBF and MVO(2). In contrast, adenosine receptor blockade caused a significant increase in CBF that occurred secondary to an increase of MVO(2). These findings demonstrate that coronary K(+)(ATP) channel activity contribute to the regulation of resting MBF in CHF, and that endogenous adenosine may act to inhibit MVO(2) in the failing heart.

Metabolic coronary flow regulation--current concepts

The concept of metabolic coronary flow control provides a rationale for the close relationship of coronary flow and myocardial metabolic rate of oxygen. The concept is based on the presence of an oxygen (metabolic) sensor coupled functionally to effector mechanisms, which control vascular tone. Four modes of metabolic control models have been proposed. 1) An oxygen sensor located in the wall of coronary vessels coupling to smooth muscle tension. Endothelial prostaglandin production may support this concept. 2) An oxygen sensing mechanism located in the myocardium and changing metabolism in response to changes of local pO(2). Adenosine is a metabolite produced at an accelerated rate when the supply-to-demand relationship for oxygen falls. 3) Sensing of oxygen turnover may be achieved by carbon dioxide production and, potentially, by mitochondrial production of reactive oxygen species. 4) The red blood cell might serve as an oxygen sensor in response to changes of haemoglobin oxygenation. A potential link to vessel relaxation may be red cell ATP release. A large body of experimental evidence supports the notion that K(ATP) channels play a significant role causing smooth muscle hyper-polarization. However, additional yet unknown effector mechanisms must exist, because block of K(ATP) channels does not lead to deterioration of coronary flow control under conditions of exercise. Thus, although several lines of evidence show that metabolic flow regulation is effective during hypoxic conditions,mechanisms mediating normoxic metabolic flow control still await further clarification.

Reduced coronary reserve in response to short-term ischaemia and vasoactive drugs in ex vivo hearts from diabetic mice

AIM

The aim of the present study was to compare the coronary flow (CF) reserve of ex vivo perfused hearts from type 2 diabetic (db/db) and non-diabetic (db/+) mice.

METHODS

The hearts were perfused in the Langendorff mode with Krebs-Henseleit bicarbonate buffer (37 degrees C, pH 7.4) containing 11 mmol L(-1) glucose as energy substrate. The coronary reserve was measured in response to three different interventions: (1) administration of nitroprusside (a nitric oxide donor), (2) administration of adenosine and (3) production of reactive hyperaemia by short-term ischaemia.

RESULTS

Basal CF was approximately 15% lower in diabetic when compared with non-diabetic hearts (2.1 +/- 0.1 vs. 2.6 +/- 0.2 mL min(-1)). The maximum increase in CF rate in response to sodium nitroprusside and adenosine was significantly lower in diabetic (0.6 +/- 0.1 and 0.9 +/- 0.1 mL min(-1) respectively) than in non-diabetic hearts (1.2 +/- 0.1 and 1.4 +/- 0.1 mL min(-1) respectively). Also, there was a clear difference in the rate of return to basal CF following short-term ischaemia between diabetic and non-diabetic hearts. Thus, basal tone was restored 1-2 min after the peak hyperaemic response in non-diabetic hearts, whereas it took approximately 5 min in diabetic hearts.

CONCLUSION

These results show that basal CF, as well as the CF reserve, is impaired in hearts from type 2 diabetic mice. As diabetic and non-diabetic hearts were exposed to the same (maximum) concentrations of NO or adenosine, it is suggested that the lower coronary reserve in type 2 diabetic hearts is, in part, because of a defect in the intracellular pathways mediating smooth muscle relaxation.

Protein kinase A-dependent activation of inward rectifier potassium channels by adenosine in rabbit coronary smooth muscle cells

We studied the effect of adenosine on the Ba(2+)-sensitive K(IR) channels in the smooth muscle cells isolated from the small-diameter (<100microm) coronary arteries of rabbit. Adenosine increased K(IR) currents in concentration-dependent manner (EC(50)=9.4+/-1.4microM, maximum increase of 153%). The adenosine-induced stimulation of K(IR) current was blocked by adenylyl cyclase inhibitor, SQ22536 and was mimicked by adenylyl cyclase activator, forskolin. The adenosine-induced increase of current was blocked by cyclic AMP-dependent protein kinase (PKA) inhibitors, KT 5720 and Rp-8-CPT-cAMPs. The adenosine-induced increase of K(IR) currents was blocked by an A(3)-selective antagonist MRS1334, while the antagonists of other subtypes (DPCPX for A(1), ZM241385 for A(2A), and alloxazine for A(2B)) were all ineffective. Furthermore, an A(3)-selective agonist, 2-Cl-IB-MECA induced increase of K(IR) currents. We also examined the effect of adenosine on coronary blood flow (CBF) rate by using the Langendorff-perfused heart. In the presence of glibenclamide to exclude the effects of ATP-sensitive K(+) (K(ATP)) channels, CBF was increased by adenosine (10microM), which was blocked by the addition of Ba(2+) (50microM). Above results suggest that adenosine increases K(IR) current via A(3) subtype through the activation of PKA in rabbit small-diameter coronary arterial smooth muscle cells.

2-(1-Hexyn-1-yl)adenosine-induced intraocular hypertension is mediated via K+ channel opening through adenosine A2A receptor in rabbits

The present study was performed to clarify the mechanism of change in intraocular pressure by 2-(1-hexyn-1-yl)adenosine (2-H-Ado), a selective adenosine A2 receptor agonist, in rabbits. 2-H-Ado (0.1%, 50 microl)-induced ocular hypertension (E(max): 7.7 mm Hg) was inhibited by an adenosine A2A receptor antagonist 1,3,7-trimethyl-8-(3-chlorostyryl)xanthine, ATP-sensitive K+ channel blocker glibenclamide or 5-hydroxydecanoic acid, but not by an adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine, an adenosine A2B receptor antagonist alloxazine or a cyclooxygenase inhibitor indomethacin. The outflow facility induced by 2-H-Ado seems to be independent of increase in intraocular pressure or ATP-sensitive K+ channel. In contrast, the recovery rate in intraocular pressure decreased by hypertonic saline was accelerated by 2-H-Ado, and this response was dependent on ATP-sensitive K+ channel. These results suggest that 2-H-Ado-induced ocular hypertension is mediated via K+ channel opening through adenosine A2A receptor, and this is probably due to aqueous formation, but independent of change in outflow facility or prostaglandin production.

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.

Comparison of combination therapy of adenosine and nitroprusside with adenosine alone in the treatment of angiographic no-reflow phenomenon

We sought to compare the combination therapy of adenosine and nitroprusside in no-reflow phenomenon during percutaneous coronary intervention. Improvement in coronary flow from no-reflow to postdrug state was evaluated. Patients who received adenosine (n = 21) were compared to ones who received the combination of adenosine and nitroprusside (n = 20) for treatment. Improvement of TIMI flow grades was higher in the group that received combined therapy (1.5 +/- 1.0 vs. 0.8 +/- 0.6; P < 0.05). Combination therapy of adenosine and nitroprusside is safe and provides better improvement in coronary flow compared to intracoronary adenosine alone in case of impaired flow during coronary interventions.

Selective A2A adenosine receptor agonist as a coronary vasodilator in conscious dogs: potential for use in myocardial perfusion imaging

The authors sought to demonstrate the advantages of a selective, potent, short-acting A adenosine receptor agonist, CVT-3146 (2-(N-pyrazolyl)Ado derivative), for potential clinical use as a coronary vasodilator during myocardial perfusion imaging. The use of adenosine in a pharmacological stress test during myocardial imaging is limited by side effects mediated by A1 and A2B adenosine receptors and by its ultrashort duration of action. CVT-3146 (0.1-5 microg/kg) and adenosine (13-267 microg/kg) were given as peripheral intravenous injections in 10 awake dogs instrumented for measurement of coronary blood flow (CBF). CVT-3146 caused a dose-dependent increase of CBF (ED50 = 0.34 +/- 0.08 microg/kg, maximal increase = 221 +/- 18%, n = 6). Adenosine was less potent (ED = 51 +/- 15 microg/kg, p < 0.05) but equieffective (maximal increase in CBF = 227 +/- 11%). The increase in CBF caused by 2.5 microg/kg CVT-3146 reached 84 +/- 5% of the maximal reactive hyperemia following 20 s of coronary occlusion (n = 4). After a 10-s injection of CVT-3146 (2.5 microg/kg), the increase in CBF remained at least twofold above baseline for 97 +/- 14 s, whereas for adenosine (267 microg/kg), the twofold increase in CBF lasted only 24 +/- 2 s (p < 0.01, n = 6). A 30-s injection of 2.5 microg/kg CVT-3146 prolonged the twofold increase in CBF up to 221 +/- 20 s. No atrioventricular block was noted. At 2.5 microg/kg, the peak effect of CVT-3146 on CBF was associated with a short-lasting (20 +/- 6 s) increase in heart rate (78 +/- 9 bpm) and decrease in mean arterial blood pressure (13 +/- 6 mm Hg, p < 0.05, n = 6). CVT-3146 is a potent coronary vasodilator. Its short duration of action, minimal and transient systemic hemodynamic effects, and ease of administration may make this agonist suitable for pharmacological coronary vasodilation during myocardial perfusion imaging for noninvasive detection of subcritical arterial stenosis.

Coronary metabolic adaptation restricted by endothelin in the dog heart

Endothelin elicits long-lasting vasoconstriction in the coronary bed. This remarkable spastic response raises the question whether or not the metabolic adaptive mechanisms of the coronaries are activated under endothelin effect. The role of the compensatory mediators adenosine and inosine was investigated before and after intracoronary (i.c.) administration of endothelin-1 (ET-1, 1.0 nmol) using 1-min reactive hyperemia (RH) tests on in situ dog hearts (n=15) with or without blocking the ATP-sensitive potassium (K+(ATP)) channels by glibenclamide (GLIB, 1.0 micromol min(-1), i.c.). The release of adenosine and inosine via the coronary sinus was measured by HPLC during the first minute of RH. Endothelin-1 reduced baseline coronary blood flow (CBF) and RH response (hyperemic excess flow (EF) control vs. ET-1: 81.7+/-13.6 vs. 43.4+/-10.9 ml, P<0.01), while it increased the net nucleoside release (adenosine, control vs. ET-1: 58.9+/-20.4 vs. 113.7+/-39.4 nmol, P<0.05; inosine: 242.1+/-81.8 vs. 786.9+/-190.8 nmol, P<0.05). GLIB treatment alone did not change baseline CBF but also reduced RH significantly and increased nucleoside release (EF control vs. GLIB: 72.1+/-11.7 vs. 31.9+/-5.5 ml, P<0.01; adenosine: 18.8+/-4.6 vs. 63.0+/-24.8 nmol, P<0.05; inosine: 113.0+/-37.2 vs. 328.2+/-127.5 nmol, P<0.05). Endothelin-1 on GLIB-treated coronaries further diminished RH and increased nucleoside release (EF: 21.5+/-8.0 ml, P<0.05 vs. GLIB; adenosine: 75.3+/-28.1 nmol, NS; inosine: 801.9+/-196.6 nmol, P<0.05 vs. GLIB). The data show that ET-1 reduces metabolic adaptive capacity of the coronaries, and this phenomenon is due to decreased vascular responsiveness and not to the blockade of ischemic mediator release from the myocardium. The coronary effect of ET-1 may partially be dependent on K+(ATP) channels.

Control of coronary blood flow during exercise

Under normal physiological conditions, coronary blood flow is closely matched with the rate of myocardial oxygen consumption. This matching of flow and metabolism is physiologically important due to the limited oxygen extraction reserve of the heart. Thus, when myocardial oxygen consumption is increased, as during exercise, coronary vasodilation and increased oxygen delivery are critical to preventing myocardial underperfusion and ischemia. Exercise coronary vasodilation is thought to be mediated primarily by the production of local metabolic vasodilators released from cardiomyocytes secondary to an increase in myocardial oxygen consumption. However, despite various investigations into this mechanism, the mediator(s) of metabolic coronary vasodilation remain unknown. As will be seen in this review, the adenosine, K(+)(ATP) channel and nitric oxide hypotheses have been found to be inadequate, either alone or in combination as multiple redundant compensatory mechanisms. Prostaglandins and potassium are also not important in steady-state coronary flow regulation. Other factors such as ATP and endothelium-derived hyperpolarizing factors have been proposed as potential local metabolic factors, but have not been examined during exercise coronary vasodilation. In contrast, norepinephrine released from sympathetic nerve endings mediates a feed-forward betaadrenoceptor coronary vasodilation that accounts for approximately 25% of coronary vasodilation observed during exercise. There is also a feed-forward alpha-adrenoceptor-mediated vasoconstriction that helps maintain blood flow to the vulnerable subendocardium when heart rate, myocardial contractility, and oxygen consumption are elevated during exercise. Control of coronary blood flow during pathophysiological conditions such as hypertension, diabetes mellitus, and heart failure is also addressed.

K(ATP)(+) channels, nitric oxide, and adenosine are not required for local metabolic coronary vasodilation

The role of ATP-sensitive K(+) (K(ATP)(+)) channels, nitric oxide, and adenosine in coronary exercise hyperemia was investigated. Dogs (n = 10) were chronically instrumented with catheters in the aorta and coronary sinus and instrumented with a flow transducer on the circumflex coronary artery. Cardiac interstitial adenosine concentration was estimated from arterial and coronary venous plasma concentrations using a previously tested mathematical model. Experiments were conducted at rest and during graded treadmill exercise with and without combined inhibition of K(ATP)(+) channels (glibenclamide, 1 mg/kg iv), nitric oxide synthesis (N(omega)-nitro-L-arginine, 35 mg/kg iv), and adenosine receptors (8-phenyltheophylline, 3 mg/kg iv). During control exercise, myocardial oxygen consumption increased ~2.9-fold, coronary blood flow increased ~2.6-fold, and coronary venous oxygen tension decreased from 19.9 +/- 0.4 to 13.7 +/- 0.6 mmHg. Triple blockade did not significantly change the myocardial oxygen consumption or coronary blood flow response during exercise but lowered the resting coronary venous oxygen tension to 10.0 +/- 0.4 mmHg and during exercise to 6.2 +/- 0.5 mmHg. Cardiac adenosine levels did not increase sufficiently to overcome the adenosine receptor blockade. These results indicate that combined inhibition of K(ATP)(+) channels, nitric oxide synthesis, and adenosine receptors lowers the balance between total oxygen supply and consumption at rest but that these factors are not required for local metabolic coronary vasodilation during exercise.

Functional and molecular characterization of receptor subtypes mediating coronary microvascular dilation to adenosine

Adenosine is a potent vasodilator of the coronary microvessels and is implicated in the regulation of coronary blood flow during metabolic stress. However, the receptor subtypes and the vasodilatory mechanism responsible for the dilation of coronary microvessels to adenosine remain unclear. In the present study, using an isolated-vessel preparation we demonstrated that porcine coronary arterioles (50-100 microm) dilated concentration-dependently to adenosine, CPA (adenosine A1 receptor agonist) and CGS21680 (adenosine A2A receptor agonist). These vasodilations were not altered by the A1 receptor antagonist CPX, but were abolished by the selective A2A receptor antagonist ZM241385, indicating that activation of A2A receptors mediates these vasodilatory responses. The protein kinase A inhibitor Rp-8-Br-cAMPS abolished coronary arteriolar dilations to adenylyl cyclase activator forskolin and cAMP analog 8-Br-cAMP, but failed to inhibit adenosine- and CGS21680-induced dilations. The calcium-activated potassium channel inhibitor iberiotoxin also did not affect vasodilations to adenosine and CGS21680. In contrast, the ATP-sensitive potassium (K(ATP)) channel inhibitor glibenclamide abolished vasodilations to adenosine and CGS21680 but did not affect vasodilations to forskolin and 8-Br-cAMP. In addition, the cAMP level in coronary microvessels was not increased by adenosine or CGS21680. The results from RT/PCR and in situ hybridization indicated that adenosine A2A receptor mRNA was encoded in coronary arterioles and the left anterior descending (LAD) artery but not in cardiomyocytes, whereas the A1 receptor transcript was detected in the LAD artery and cardiomyocytes but not in arterioles. Similarly, adenosine A1 and A2A proteins were expressed in the LAD artery, but only A2A receptors were expressed in coronary arterioles. Collectively, these functional data suggest that coronary arteriolar dilation to adenosine is primarily mediated by the opening of K(ATP) channels through activation of A2A receptors. This conclusion is corroborated by the molecular data showing that coronary arterioles only express adenosine A2A receptors. Furthermore, the dilation of coronary microvessels to adenosine A2A receptor activation appears to be independent of cAMP signaling.

Coronary vascular K+ATP channels contribute to the maintenance of myocardial perfusion in dogs with pacing-induced heart failure

The functional role of coronary vascular ATP-sensitive potassium (K+ATP) channels in the regulation of coronary blood flow (CBF) has not been determined in chronic heart failure (CHF). To test the hypothesis that K+ATP channels contribute to myocardial perfusion in HF, we examined the effects of intracoronary infusion of glibenclamide, an inhibitor of K+ATP channels, on basal CBF in control and CHF dogs. CHF was produced in mongrel dogs by pacing the right ventricle for 4 weeks. Under anesthesia, CBF in the left anterior descending coronary artery, other hemodynamic and metabolic parameters, or regional myocardial blood flow were measured. Basal CBF was less in CHF dogs than in controls. Glibenclamide at the graded doses (5, 15 and 50 microg x kg(-1) x min(-1) decreased CBF in both control and CHF dogs. The percentage decrease in CBF with glibenclamide at 50 microg x kg(-1) x min(-1) was greater (p<0.01) in CHF dogs than in controls. The greater decrease in CBF with glibenclamide at 50microg x kg(-1) x min(-1) was associated with myocardial ischemia. Glibenclamide decreased myocardial blood flow in each sublayer of the myocardium in the 2 groups. These results suggest that the basal activity of coronary vascular K+ATP channels is increased in CHF dogs but not in controls. This may contribute to the maintenance of myocardial perfusion in CHF.

Role of K(ATP)(+) channels and adenosine in the control of coronary blood flow during exercise

The present study was designed to examine the role of ATP-sensitive potassium (K(ATP)(+)) channels during exercise and to test the hypothesis that adenosine increases to compensate for the loss of K(ATP)(+) channel function and adenosine inhibition produced by glibenclamide. Graded treadmill exercise was used to increase myocardial O(2) consumption in dogs before and during K(ATP)(+) channel blockade with glibenclamide (1 mg/kg iv), which also blocks adenosine mediated coronary vasodilation. Cardiac interstitial adenosine concentration was estimated from arterial and coronary venous values by using a previously tested mathematical model (Kroll K and Stepp DW. Am J Physiol Heart Circ Physiol 270: H1469-H1483, 1996). Coronary venous O(2) tension was used as an index of the balance between O(2) delivery and myocardial O(2) consumption. During control exercise, myocardial O(2) consumption increased approximately 4-fold, and coronary venous O(2) tension fell from 19 to 14 Torr. After K(ATP)(+) channel blockade, coronary venous O(2) tension was decreased below control vehicle values at rest and during exercise. However, during exercise with glibenclamide, the slope of the line of coronary venous O(2) tension vs. myocardial O(2) consumption was the same as during control exercise. Estimated interstitial adenosine concentration with glibenclamide was not different from control vehicle and was well below the level necessary to overcome the 10-fold shift in the adenosine dose-response curve due to glibenclamide. In conclusion, K(ATP)(+) channel blockade decreases the balance between resting coronary O(2) delivery and myocardial O(2) consumption, but K(ATP)(+) channels are not required for the increase in coronary blood flow during exercise. Furthermore, interstitial adenosine concentration does not increase to compensate for the loss of K(ATP)(+) channel function.

Characterization of adenosine action in isolated rat renal artery. Possible role of adenosine A(2A) receptors

Adenosine (0.1-300 microM) induced concentration- and endothelium-dependent relaxation of rat renal artery (RRA). N(G)-Nitro-L-arginine (L-NOARG, 10 microM) significantly reduced adenosine-elicited dilatation, but not the application of indomethacin (10 microM), ouabain (100 microM) or tetraethylammonium (TEA, 500 microM). In the presence of high concentration of K(+) (100 mM) or glibenclamide (1 microM), adenosine-evoked relaxation was almost abolished. 8-(3-Chlorostyril)caffeine (CSC, 0.3-3 microM), a selective A(2A)-antagonist, significantly reduced adenosine-evoked dilatation in a concentration-dependent manner (pA(2)=7.29). Conversely, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 nM), an A(1)-antagonist, did not alter adenosine-induced relaxation. These results indicate that adenosine produces endothelium-dependent relaxation of isolated RRA. Dilatation evoked by adenosine is mediated by predominant releasing of endothelium-derived hiperpolarizing factor (EDHF) and also in one part of nitric oxide (NO) from endothelial cells. The obtained results also suggest that RRA response to adenosine is most likely initiated by activation of endothelial adenosine A(2A) receptors.

A(2A) adenosine receptor mediated potassium channel activation in rat epididymal smooth muscle

The effects of A(2) adenosine receptor agonists upon phenylephrine-stimulated contractility in preparations of rat epididymis were investigated. Preparations responded to phenylephrine (3 microM) with submaximal contractions. Adenosine and the stable agonists 5'-N-ethylcarboxamido-adenosine (NECA) and 2-p-(2-carboxyethyl) phenethylamino-N-ethylcarboxamide adenosine (CGS 21680) inhibited phenylephrine-induced contractions (potency order, NECA>CGS 21680>adenosine). The A(2A) receptor-selective antagonist, 4-(2-[7-amino-2-(2-furyl)[1,2,4]-triazolo-[2,3-a][1,3, 5]triazin-5-ylamino]ethyl)phenol (ZM 241385, 30 microM) blocked the response to NECA. The A(2A) adenosine receptor-mediated inhibitory responses to NECA were reduced by the K(ATP) channel blocker, glibenclamide (3 microM) and abolished by charybdotoxin (100 nM). The diterpene forskolin elicited a concentration-dependent inhibition of phenylephrine (3 microM)-stimulated contractility (by 62+/-8% of control at 100 microM). Charybdotoxin (100 nM), but not glibenclamide (3 microM) blocked the forskolin (10 microM) inhibition of phenylephrine-stimulated contractility. NECA elicited concentration-dependent increases in both cyclic AMP and cyclic GMP accumulation which were antagonized by ZM 241385 (30 nM). The protein kinase G activator, APT-cyclic GMP (8-(-Aminophenylthio) guanosine-3',5'-cyclic monophosphate) and the protein kinase A activator (Sp)-8-bromoadenosine-3',5'-cyclic monophosphorothioate (Sp-8-Br-cyclic AMPs), inhibited phenylephrine (3 microM) induced contractions of rat epididymis. Glibenclamide (3 microM), but not charybdotoxin (100 nM), inhibited ATP-cyclic GMP responses. Charybdotoxin (100 nM), but not glibenclamide (3 microM) reduced the effect of Sp-8-Br-cyclic AMPs. This study shows that the A(2A) adenosine receptor inhibition of epididymal contractility may be mediated through the activation of charybdotoxin- and glibenclamide-sensitive potassium channels and may involve the activation of both protein kinases A and G.

Coronary microcirculation: physiology and pharmacology

Coronary microvessels play a pivotal role in determining the supply of oxygen and nutrients to the myocardium by regulating the coronary flow conductance and substance transport. Direct approaches analyzing the coronary microvessels have provided a large body of knowledge concerning the physiological and pharmacological characteristics of the coronary circulation, as has the rapid accumulation of biochemical findings about the substances that mediate vascular functions. Myogenic and flow-induced intrinsic vascular controls that determine basal tone have been observed in coronary microvessels in vitro. Coronary microvascular responses during metabolic stimulation, autoregulation, and reactive hyperemia have been analyzed in vivo, and are known to be largely mediated by metabolic factors, although the involvement of other factors should also be taken into account. The importance of ATP-sensitive K(+) channels in the metabolic control has been increasingly recognized. Furthermore, many neurohumoral mediators significantly affect coronary microvascular control in endothelium-dependent and -independent manners. The striking size-dependent heterogeneity of microvascular responses to all of these intrinsic, metabolic, and neurohumoral factors is orchestrated for optimal perfusion of the myocardium by synergistic and competitive interactions. The regulation of coronary microvascular permeability is another important factor for the nutrient supply and for edema formation. Analyses of collateral microvessels and subendocardial microvessels are important for understanding the pathophysiology of ischemic hearts and hypertrophied hearts. Studies of the microvascular responses to drugs and of the impairment of coronary microvessels in diseased conditions provide useful information for treating microvascular dysfunctions. In this article, the endogenous regulatory system and pharmacological responses of the coronary circulation are reviewed from the microvascular point of view.

Role of K+ATP channels in local metabolic coronary vasodilation

ATP-sensitive potassium (K+ATP) channels have been shown to play a role in the maintenance of basal coronary vascular tone in vivo. K+ATP channels are also involved in the coronary vasodilator response to adenosine. The aim of this study was to determine the role of K+ATP channels in local metabolically mediated increases in coronary blood flow during cardiac electrical paired pacing without catecholamine effects. In 10 anesthetized closed-chest dogs, coronary blood flow was measured in the left circumflex coronary artery, and myocardial O2 consumption was calculated using the arteriovenous O2 difference. Cardiac interstitial adenosine concentration was estimated from coronary venous and arterial plasma adenosine measurements using a previously described, multicompartmental, axially distributed, mathematical model. Paired stimulation increased heart rate from 57 to 120 beats/min, myocardial O2 consumption 88%, and coronary blood flow 76%. During K+ATP channel blockade with glibenclamide, baseline coronary blood flow decreased in relation to myocardial O2 consumption and thus coronary sinus O2 tension fell. Paired-pulse pacing with glibenclamide resulted in increases in myocardial O2 consumption and coronary blood flow similar to those during control pacing. Coronary venous and estimated interstitial adenosine concentration did not increase sufficiently to overcome the glibenclamide blockade. In conclusion, K+ATP channels are not required for locally mediated metabolic increases in coronary blood flow that accompany myocardial O2 consumption during pacing tachycardia without catecholamines, and adenosine levels do not increase sufficiently to overcome the glibenclamide blockade.

Increase in the vasorelaxant potency of K(ATP) channel opening drugs by adenosine A(1) and A(2) receptors in the pig coronary artery

Myograph recording from ring segments of pig small coronary arteries was used to investigate the effects of adenosine receptor activation on the vasorelaxant potency of ATP-sensitive K(+) channel opening drugs. Receptor activation with 2-chloroadenosine (2-CA, 300 nM) increased the potency of both nicorandil and levcromakalim, shifting the pEC(50)s from 4.68+/-0.03 to 5.05+/-0.04 and from 6.34+/-0.06 to 6.72+/-0.06, respectively (P<0.05 in each case). Experiments with selective adenosine receptor agonists (2-chloro-N(6)-cyclopentyladenosine (CCPA), 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS 21680)) and antagonists (8-cyclopentyl-1, 3-dipropylxanthine (DPCPX), 4-(2-[7-amino-2-(2-furyl)[1,2, 4]triazolo[2,3-a] [1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385)) suggest that both A(1) and A(2a) receptors can increase the potency of nicorandil, while that of levcromakalim is increased only by A(2) receptors. Adenosine receptor activation did not affect the potency of pinacidil. Thus, adenosine receptor activation can increase the potency of some K(+) channel opening drugs to relax coronary arteries, but the details of the interaction with adenosine receptors depend on the particular drug.

Specificity of synergistic coronary flow enhancement by adenosine and pulsatile perfusion in the dog

1. Coronary flow elevation from enhanced perfusion pulsatility is synergistically amplified by adenosine. This study determined the specificity of this interaction and its potential mechanisms. 2. Mean and phasic coronary flow responses to increasing pulsatile perfusion were assessed in anaesthetized dogs, with the anterior descending coronary artery servoperfused to regulate real-time physiological flow pulsatility at constant mean pressure. Pulsatility was varied between 40 and 100 mmHg. Hearts ejected into the native aorta whilst maintaining stable loading. 3. Increasing pulsatility elevated mean coronary flow +11.5 +/- 1.7 % under basal conditions. Co-infusion of adenosine sufficient to raise baseline flow 66 % markedly amplified this pulsatile perfusion response (+82. 6 +/- 14.3 % increase in mean flow above adenosine baseline), due to a leftward shift of the adenosine-coronary flow response curve at higher pulsatility. Flow augmentation with pulsatility was not linked to higher regional oxygen consumption, supporting direct rather than metabolically driven mechanisms. 4. Neither bradykinin, acetylcholine nor verapamil reproduced the synergistic amplification of mean flow by adenosine and higher pulsatility, despite being administered at doses matching basal flow change with adenosine. 5. ATP-sensitive potassium (KATP) activation (pinacidil) amplified the pulse-flow response 3-fold, although this remained significantly less than with adenosine. Co-administration of the phospholipase A2 inhibitor quinacrine virtually eliminated adenosine-induced vasodilatation, yet synergistic interaction between adenosine and pulse perfusion persisted, albeit at a reduced level. 6. Thus, adenosine and perfusion pulsatility specifically interact to enhance coronary flow. This synergy is partially explained by KATP agonist action and additional non-flow-dependent mechanisms, and may be important for modulating flow reserve during exercise or other high output states where increased flow demand and higher perfusion pulsatility typically co-exist.

Effects of ischemia on cerebral arteriolar dilation to arterial hypoxia in piglets

BACKGROUND AND PURPOSE

Arterial hypoxia mediates cerebral arteriolar dilation primarily via mechanisms involving activation of ATP-sensitive K+ channels (K[ATP]), which we have shown to be sensitive to ischemic stress. In this study, we determined whether ischemia/reperfusion alters cerebral arteriolar responses to arterial hypoxia in anesthetized piglets. Since adenosine plays an important role in cerebrovascular responses to hypoxia, we also determined whether adenosine-induced arteriolar dilation is affected by ischemic stress. We tested the hypothesis that reductions in cerebral arteriolar dilator responses after ischemia would be proportional to the contribution of K(ATP) to hypoxia and adenosine.

METHODS

Pial arteriolar diameters were measured using a cranial window and intravital microscopy. We examined arteriolar responses to arterial hypoxia (inhalation of 8.5% and 7.5% O2), to topical adenosine (10[-5] and 10[-4] mol/L) and to arterial hypercapnia (inhalation of 5% and 10% CO2 in air) before and after 10 minutes of global ischemia. Ischemia was achieved by increasing intracranial pressure. Arterial hypercapnia was used as a positive control for the effectiveness of the ischemic insult. In addition, we evaluated cerebral arteriolar responses to 10(-5) and 10(-4) mol/L adenosine applied topically with or without glibenclamide, a selective inhibitor of K(ATP) (10[-5] and 10[-6] mol/L). Finally, we administered theophylline (20 mg/kg, i.v.) to assess the contribution of adenosine to cerebral arteriolar dilation to arterial hypoxia.

RESULTS

Before ischemia, cerebral arterioles dilated by 19+/-3% to moderate and 29+/-4% to severe hypoxia (n=7; P<.05); 13+/-2% to 10(-5) and 20+/-1% to 10(-4) mol/L adenosine (n=9; P<.05); and by 17+/-2% to moderate and 28+/-3% to severe hypercapnia (n=6; P<.05). After ischemia, cerebral arteriolar responses to hypoxia and adenosine were unchanged. In contrast, cerebral arteriolar dilation to hypercapnia was impaired by ischemia (1+/-1% and 2+/-1% at 1 hour; n=6). Glibenclamide reduced cerebral arteriolar dilation to adenosine by approximately one half (n= 7). In addition, blockade of adenosine receptors by theophylline (20 mg/kg, i.v.) almost totally suppressed cerebral arteriolar dilation to arterial hypoxia (n = 6).

CONCLUSIONS

Cerebrovascular responsiveness is selectively affected by anoxic stress. In addition, cerebral arteriolar dilation to hypoxia and adenosine is maintained after ischemia despite the expected impairment in K(ATP) function.

Adenosine-induced hyperpolarization in guinea pig coronary artery involves A2b receptors and KATP channels

The role of P1 purinoceptor subtypes, the adenylyl cyclase (AC) pathway, and ATP-sensitive K+ (KATP) channels in adenosine (Ado)-induced membrane hyperpolarization was investigated in isolated segments of the guinea pig coronary artery using conventional microelectrode techniques. Ado (1-100 microM) elicited concentration-dependent hyperpolarization (half-maximal effective concentration 7.5 +/- 0.5 microM) that averaged 28.6 +/- 2.9 mV at 100 microM Ado. The A1 selective agonist N6-cyclopentyladenosine (CPA), the A1/A2 agonist 2-chloroadenosine, and the A2a/A2b agonist 5-(N-ethylcarboxamido)adenosine (NECA) each induced glibenclamide (3 microM)-sensitive hyperpolarization at 10 microM. However, the selective A2a-receptor agonists CGS-21680 and N6-[2-(3,5-dimethoxyphenyl)-2-(2-methoxyphenyl]ethyladenosine (10 microM each) were without effect. Responses to CPA and NECA were significantly reduced by the AC inhibitor SQ-22,536 (100 microM). Activation of the AC-adenosine 3',5'-cyclic monophosphate (cAMP)-protein kinase A (PKA) pathway by four additional methods, i.e., 1) forskolin (0.3-1 microM), 2) isoproterenol (0.1-1 microM), 3) combined milrinone (0.4 microM) and rolipram (30 microM), and 4) combined N6-phenyladenosine 3',5'-monophosphate, 8-(6-aminohexyl)aminoadenosine 3',5'-cyclic monophosphate, and the Sp-isomer of 5,6-dichloro-1-D-ribofuranosylbenzimidazole-3', 5'-cyclic monophosphothioate (100 microM each), also gave rise to glibenclamide-sensitive hyperpolarization. These results suggest that stimulation of A2b receptors coupled to AC represents the predominant mechanism by which Ado elicits hyperpolarization in this vessel. The ensuing increase in cAMP activates PKA, which then increases the activity of KATP channels. Our results further suggest that KATP channels are an important target for numerous pathways that raise cAMP levels in the coronary artery.

Adenosine inhibition of neutrophil damage during reperfusion does not involve K(ATP)-channel activation

This study tests the hypothesis that cardioprotection exerted by adenosine A2-receptor activation and neutrophil-related events involves stimulation of ATP-sensitive potassium (K(ATP)) channels on neutrophils during reperfusion. The adenosine A2 agonist CGS-21680 (CGS) inhibited superoxide radical generation from isolated rabbit polymorphonuclear neutrophils (PMNs) in a dose-dependent manner from 17.7 +/- 2.1 to 7.4 +/- 1.3 nmol/5 x 10(6) PMNs (P < 0.05). Pinacidil, a K(ATP)-channel opener, partially inhibited superoxide radical production, which was completely reversed by glibenclamide (Glib). Incremental doses of Glib in combination with CGS (1 microM) did not alter CGS-induced inhibition of superoxide radical generation. CGS significantly reduced PMN adherence to the endothelial surface of aortic segments in a dose-dependent manner from 189 +/- 8 to 50 +/- 6 PMNs/mm2 (P < 0.05), which was also not altered by incremental doses of Glib. Infusion of CGS (0.025 mg/kg) before reperfusion reduced infarct size from 29 +/- 2% in the Vehicle group to 15 +/- 1% in rabbits undergoing 30 min of ischemia and 120 min of reperfusion (P < 0.05). Glib (0.3 mg/kg) did not change the infarct size (28 +/- 2%) vs. the Vehicle group and did not attenuate infarct size reduction by CGS (16 +/- 1%). Glib did not change blood glucose levels. Cardiac myeloperoxidase activity was decreased in the ischemic tissue of the CGS group (0.15 +/- 0.03 U/100 mg tissue) compared with the Vehicle group (0.37 +/- 0.05 U/100 mg tissue; P < 0.05). We conclude that adenosine A2 activation before reperfusion partially reduces infarct size by inhibiting neutrophil activity and that this effect does not involve K(ATP)-channel stimulation.

Myocardial reactive hyperemia in experimental chronic heart failure: evidence for the role of K+ adenosine triphosphate-dependent channels and cyclooxygenase activity

BACKGROUND

Several studies suggest that coronary perfusion is abnormal in heart failure. The fact that these deficits may results in an altered coronary reserve remains controversial. Therefore, coronary adaptability to short-duration ischemia and the resultant myocardial reactive hyperemia were investigated in a model of chronic heart failure.

METHODS AND RESULTS

Experiments were performed in normal and failing hamster hearts (UM-X7.1, aged > 225 days). Heart rate, left ventricular developed pressure, and coronary flow were recorded continuously before and after each 30-second ischemia in isolated perfused heart preparations. Studies were conducted under control conditions and in the presence of four inhibitors of potential mediators of the reactive hyperemia response: the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (30 microM), the adenosine antagonist 8-(p-sulfophenyl)theophylline (50 microM), the K+ cyclic adenosine triphosphate-dependent channel antagonist glibenclamide (10 microM), and the cyclooxygenase inhibitor indomethacin (10 microM). Baseline hemodynamic parameters were all significantly impaired in failing hearts. Under control conditions, failing hearts were able to respond adequately to a 30-second ischemia: repayment-to-debt ratio averaged 1.02 +/- 0.09 as compared with 1.10 +/- 0.09 in normal hearts (P = NS). All inhibitors significantly reduced basal coronary perfusion except for indomethacin. Of the four inhibitors of potential mediators of the myocardial reactive hyperemic response, only glibenclamide and indomethacin impaired the repayment-to-debt ratio. In their presence, repayment-to-debt ratio was reduced by 40% of the baseline response (P < .01) without significant difference between normal and failing hearts. On the contrary, NG-nitro-L-arginine methyl ester and 8-(p-sulfophenyl)theophylline did not alter the repayment-to-debt ratio.

CONCLUSIONS

These observations demonstrate the capacity of the failing heart to tolerate short-duration ischemia despite the presence of significant alterations in its basal coronary perfusion. In addition, results suggest that activation of K+ adenosine triphosphate-dependent channels and the presence of cyclooxygenase by-products are important determinants of coronary adaptation to short-duration ischemia in this model of chronic heart failure.

Beta 2-adrenergic dilation of resistance coronary vessels involves KATP channels and nitric oxide in conscious dogs

BACKGROUND

We considered that beta 2-adrenergic stimulation may dilate resistance coronary vessels by opening ATP-sensitive potassium (KATP) channels, thereby triggering NO formation.

METHODS AND RESULTS

In conscious instrumented dogs after beta 1-adrenergic blockade, intracoronary (IC) injections of acetylcholine (ACh), nitroglycerin (NTG), and pirbuterol (PIR), a selective beta 2-adrenergic agonist, were performed before and after blockade of NO formation with IC N omega-nitro-L-arginine methyl ester (L-NAME, 50 micrograms.kg-1.min-1 x 12 minutes) or blockade of KATP channels with IC glibenclamide (25 micrograms.kg-1.min-1 x 12 minutes followed by 2 micrograms.kg-1.min-1). PIR (50.0 ng/kg) increased coronary blood flow (CBF) by 32 +/- 6 from 43 +/- 7 mL/min and by only 11 +/- 2 (P < .01) from 40 +/- 7 mL/min after L-NAME. Increases in CBF to ACh were also reduced by L-NAME, but NTG responses were not. Before glibenclamide, PIR increased CBF by 33 +/- 5 from 45 +/- 7 mL/min and by only 14 +/- 3 (P < .01) from 36 +/- 5 mL/min thereafter. CBF responses to ACh and NTG were maintained after glibenclamide. Lemakalim, a selective opener of KATP channels, caused dose-dependent increases in CBF that were partially inhibited by L-NAME. In experiments in which CBF was controlled, the fall in distal coronary pressure caused by PIR was less after L-NAME or glibenclamide than before.

CONCLUSIONS

beta 2-Adrenergic dilation of resistance coronary vessels involves both the opening of KATP channels and NO formation. L-NAME antagonized lemakalim responses consistent with a link between the opening of KATP channels and NO formation in canine resistance coronary vessels.

ATP-sensitive potassium channels in isolated rat aorta during physiologic, hypoxic, and low-glucose conditions

In arterial smooth muscle, adenosine triphosphate (ATP)-sensitive potassium channels are the targets of a variety of synthetic and endogenous vasodilators. In this study, we evaluated the influence of glibenclamide, an ATP-sensitive K(+)-channel blocker, on various vasodilator responses, including those by levcromakalim under hypoxic and low-glucose conditions in isolated rat aortic rings. The concentration-response curves induced by methacholine and sodium nitroprusside (after precontraction with 1 microM phenylephrine) were not affected by glibenclamide. Glibenclamide influenced neither the adenosine- nor the iloprost- (a stable prostacyclin) induced vasodilator effects. Glibenclamide caused a concentration-dependent rightward shift of the concentration-response curves of levcromakalim. The vascular tone induced by phenylephrine was not affected under low-glucose conditions, whereas hypoxia caused a decrease in the phenylephrine-induced contraction when compared with that under normal circumstances. Under all conditions, glibenclamide did not influence the phenylephrine-induced increase in vascular tone. Under low-glucose and hypoxic conditions, the concentration-response curves for levcromakalim showed a significantly less steep slope than under normal conditions, and higher concentrations of glibenclamide were necessary to inhibit the vasodilator response induced by levcromakalim under these experimental conditions adopted to mimic pathologic conditions. In conclusion, methacholine, sodium nitroprusside, adenosine, and iloprost appear not to induce vasodilation in the rat aorta by glibenclamide-sensitive K+ channels, whereas hypoxia and low-glucose levels cause an impaired function of the glibenclamide-sensitive K+ channels.

Coronary effects of diadenosine tetraphosphate resemble those of adenosine in anesthetized pigs: involvement of ATP-sensitive potassium channels

Diadenosine tetraphosphate (Ap4A) is an adenine nucleotide with vasodilatory properties. We examined the effects of Ap4A on coronary circulation in comparison with those of adenosine, its metabolite, in anesthetized pigs. Left atrial (LA) infusion of Ap4A at increasing doses of 100, 200, and 300 micrograms/kg/min increased coronary blood flow (CBF) and decreased systemic blood pressure (BP) and coronary vascular resistance (CVR). Ap4A had no effect on large epicardial coronary artery diameter (CoD). Likewise, LA infusion of adenosine at doses of 150 and 300 micrograms/kg/min increased CBF and decreased BP and coronary vascular resistance (CVR) but did not affect CoD. Therefore, the vasodilatory effects of Ap4A and adenosine were predominant in small coronary resistance vessels and negligible in large coronary arteries. Pretreatment with glibenclamide (2 mg/kg, intravenously, i.v.), a specific blocker of ATP-sensitive potassium channels (KATP), attenuated alterations of CBF, BP, and CVR induced by Ap4A and by adenosine. In contrast, treatment with cromakalim (0.5 microgram/kg/min i.v.), an activator of KATP, enhanced the coronary effects of Ap4A and adenosine. Therefore, the opening of KATP in the pig coronary circulation is involved in the in vivo vasodilatory effects of Ap4A and adenosine. Treatment with 8-phenyltheophylline (8-PT, 4 mg/kg i.v.), an adenosine receptor antagonist, suppressed CBF increases induced by Ap4A (20 micrograms/kg/min, intracoronarily, i.c.) and adenosine (5 micrograms/kg/min i.c.) by 68 and 90%, respectively. These findings suggest that the in vivo coronary effects of Ap4A are largely caused by the opening of KATP through rapid degradation to adenosine to activate adenosine receptors.

Smooth muscle relaxant activity of A1- and A2-selective adenosine receptor agonists in guinea pig trachea: involvement of potassium channels

The relaxant activities of N6-cyclopentyladenosine (CPA), an A1-selective agonist, and of 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA), a potent A2-receptor agonist, in the carbachol-contracted guinea pig isolated trachea have been evaluated. Both CPA and CPCA induced concentration-dependent relaxations of the guinea pig trachea, CPCA demonstrating a more potent but less efficient activity. 8-Cyclopentyl-1,3-dimethylxanthine (CPT) and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (10 microM), both selective and potent A1-adenosine receptor antagonists, induced only a weak inhibition of CPA while 3,7-dimethyl-1-propargylxanthine (DMPX) (10 microM), a selective A2-adenosine receptor antagonist, failed to antagonize the relaxant activity of CPA. These results indicate that a major component of the tracheal relaxant activity of CPA occurred by a mechanism which is insensitive to the antagonist potency of A1- and A1-xanthine adenosine antagonists and therefore was not mediated by A1- or A1-adenosine receptors activation. The relaxant activity of CPCA was inhibited by DMPX, which supported the involvement of A2-adenosine receptors. Glibenclamide (10 microM), an inhibitor of KATP-channels, inhibited the relaxant activity of CPCA, whereas it was without effect on CPA. Iberiotoxin (180 nM), an inhibitor of the large-conductance CA2(+)-activated K+ channel, inhibited the relaxant action of CPA and CPCA. However, verapamil can offset the inhibition of CPA provided by iberiotoxin which suggests that such an antagonism does not represent an interaction between the toxin and CPA at the level of the large-conductance CA2(+)-activated K(+)-channel gating but rather functional antagonism attributable to the promotion of CA2+ influx by the toxin. In contrast, verapamil only partially reversed the inhibition of CPCA relaxant activity provided by iberiotoxin. Taken together, these results suggest that A2-adenosine receptor subtypes are coupled to KATP-channels and large-conductance CA2(+)-activated K(+)-channels in the guinea pig trachea whereas the unidentified adenosine receptor subtype, involved in CPA relaxant activity, is not.

Modulation of vasorelaxant responses to potassium channel openers by basal nitric oxide in the rat isolated superior mesenteric arterial bed

1. We have used the isolated buffer-perfused mesenteric arterial bed of the rat to assess the modulation of vasorelaxation to potassium channel openers (KCOs) by basal nitric oxide. 2. The dose-response curves to the KCOs, levcromakalim and pinacidil, in preconstricted preparations were significantly shifted to the left in the presence of the nitric oxide synthase inhibitor (100 microM) NG-nitro-L-arginine methyl ester (levcromakalim, ED50 = 4.47 +/- 0.70 nmol vs. 1.73 +/- 0.26 nmol, P < 0.001; pinacidil, ED50 = 16.1 +/- 4.8 nmol vs. 5.43 +/- 1.10 nmol, P < 0.001). The vasorelaxant responses to papaverine, a vasodilator which acts independently of potassium channels was unaffected by NG-nitro-L-arginine methyl ester (L-NAME). 3. Removal of the endothelium, by perfusion with the detergent CHAPS (0.3%), significantly (P < 0.001) increased the potency of levcromakalim as a vasodilator (ED50 4.47 +/- 0.70 nmol vs. 2.59 +/- 0.31 nmol). The subsequent administration of L-NAME following perfusion with CHAPS did not lead to any additional enhancement of responses to levcromakalim. 4. The presence of the non-selective adenosine antagonist, 8-phenyltheophylline (8-PT, 10 microM) significantly (P < 0.001) shifted the dose-response curve to levcromakalim to the left (ED50 4.47 +/- 0.70 nmol vs. 1.11 +/- 0.32 nmol). In the presence of both L-NAME and 8-PT, the dose-response curve to levcromakalim was also significantly (P < 0.01) shifted to the left compared with control (ED50 in the presence of both L-NAME and 8-PT was 0.42 +/- 0.08 nmol). 5. The presence of 8-bromo cyclic GMP (10 microM) reversed the increase potency of levcromakalim, observed following inhibition of nitric oxide synthase (ED50 in the presence of L-NAME was 0.59 +/- 0.01 nmol and in the presence of 8-bromo cyclic GMP plus L-NAME the ED50 was 3.17 +/- 0.80 nmol). However in the absence of L-NAME, the cell permeable analogue of cyclic GMP, 8-bromo cyclic GMP, did not affect the dose-response curve to levcromakalim compared with control (control ED50 value was 4.16 +/- 0.52 nmol vs. 3.85 +/- 1.13 nmol in the presence of 8-bromo cyclic GMP). 6. The present investigation demonstrates that both basal nitric oxide and adenosine modulate vasorelaxation to the KCOs levcromakalim and pinacidil. The modulatory effect of nitric oxide may be mediated via cyclic GMP.

Glibenclamide, a selective inhibitor of ATP-sensitive K+ channels, attenuates metabolic coronary vasodilatation induced by pacing tachycardia in dogs

BACKGROUND

We previously reported that glibenclamide (a selective inhibitor of ATP-sensitive K+ channels [K+ATP channels]) inhibited metabolic coronary vasodilatation induced by beta 1-adrenoceptor stimulation. However, the role of K+ATP channels in metabolic coronary vasodilatation induced by tachycardia is still unknown. This study aimed to determine whether glibenclamide attenuates metabolic coronary vasodilatation induced by pacing-induced tachycardia.

METHODS AND RESULTS

In anesthetized dogs, increasing heart rate from 103 +/- 1 to 160 beats per minute with atrial pacing increased coronary blood flow without altering arterial pressure and left ventricular pressure. Intracoronary infusion of glibenclamide at 1.5 and 5.0 micrograms.kg-1.min-1 did not alter basal coronary blood flow but significantly attenuated (P < .01) the tachycardia-induced coronary vasodilatation without altering the tachycardia-induced increase in myocardial oxygen consumption (MVO2). In conscious dogs, intracoronary glibenclamide at 5.0 micrograms.kg-1.min-1 attenuated (P < .05) coronary vasodilatation induced by ventricular pacing from 85 +/- 6 to 150 beats per minute. Glibenclamide markedly attenuated coronary vasodilation evoked with the K+ATP channel opener pinacidil.

CONCLUSIONS

These data indicate that blockade of coronary vascular K+ATP channels with glibenclamide inhibited metabolic coronary vasodilatation induced by pacing tachycardia in dogs, suggesting that K+ATP channels are involved in the mechanism mediating metabolic coronary vasodilatation associated with pacing tachycardia.