Activation of two sites by adenosine receptor agonists to cause relaxation in rat isolated mesenteric artery

Differences in vasomotor function of mesenteric arteries between Ossabaw minipigs with predisposition to metabolic syndrome and Göttingen minipigs

Metabolic syndrome predisposes and contributes to the development and progression of atherosclerosis. The minipig strain "Ossabaw" is characterized by a predisposition to develop metabolic syndrome. We compared vasomotor function in Ossabaw minipigs before they developed their diseased phenotype to that of Göttingen minipigs without such genetic predisposition. Mesenteric arteries of adult Ossabaw and Göttingen minipigs were dissected postmortem and mounted on a myograph for isometric force measurements. Maximal vasoconstriction to potassium chloride (KClmax) was induced. Cumulative concentration-response curves were determined in response to norepinephrine. Endothelium-dependent (with carbachol) and endothelium-independent (with nitroprusside) vasodilation were analyzed after preconstriction by norepinephrine. In a bioinformatic analysis, variants/altered base pairs within genes associated with cardiovascular disease were analyzed. KClmax was similar between the minipig strains (15.6 ± 6.7 vs. 14.1 ± 3.4 ΔmN). Vasoconstriction in response to norepinephrine was more pronounced in Ossabaw than in Göttingen minipigs (increase of force to 143 ± 48 vs. 108 ± 38% of KClmax). Endothelium-dependent and endothelium-independent vasodilation were less pronounced in Ossabaw than in Göttingen minipigs (decrease of force to 46.4 ± 29.6 vs. 16.0 ± 18.4% and to 36.7 ± 25.2 vs. 2.3 ± 3.7% of norepinephrine-induced preconstriction). Vasomotor function was not different between the sexes. More altered base pairs/variants were identified in Ossabaw than in Göttingen minipigs for the exon encoding adrenoceptor-α1A. Vasomotor function in lean Ossabaw minipigs is shifted toward vasoconstriction and away from vasodilation in comparison with Göttingen minipigs, suggesting a genetic predisposition for vascular dysfunction and atherosclerosis in Ossabaw minipigs. Thus, Ossabaw minipigs may be a better model for human cardiovascular disease than Göttingen minipigs.NEW & NOTEWORTHY Animal models with a predisposition to metabolic syndrome and atherosclerosis are attracting growing interest for translational research, as they may better mimic the variability of patients with cardiovascular disease. In Ossabaw minipigs, with a polygenic predisposition to metabolic syndrome, but without the diseased phenotype, vasoconstriction is more and vasodilation is less pronounced in mesenteric arteries than in Göttingen minipigs. Ossabaw minipigs may be a more suitable model of human cardiovascular disease.

Increased ATP and ADO Overflow From Sympathetic Nerve Endings and Mesentery Endothelial Cells Plus Reduced Nitric Oxide Are Involved in Diabetic Neurovascular Dysfunction

Since the mechanism of human diabetic peripheral neuropathy and vascular disease in type 1 diabetes mellitus remains unknown, we assessed whether sympathetic transmitter overflow is altered by this disease and associated to vascular dysfunction. Diabetes was induced by streptozotocin (STZ)-treatment and compared to vehicle-treated rats. Aliquots of the ex vivo perfused rat arterial mesenteric preparation, denuded of the endothelial layer, were collected to quantify analytically sympathetic nerve co-transmitters overflow secreted by the isolated mesenteries of both groups of rats. Noradrenaline (NA), neuropeptide tyrosine (NPY), and ATP/metabolites were detected before, during, and after electrical field stimulation (EFS, 20 Hz) of the nerve terminals surrounding the mesenteric artery. NA overflow was comparable in both groups; however, basal or EFS-secreted ir-NPY was 26% reduced (p < 0.05) in diabetics. Basal and EFS-evoked ATP and adenosine (ADO) overflow to the arterial mesentery perfusate increased twofold and was longer lasting in diabetics; purine tissue content was 37.8% increased (p < 0.05) in the mesenteries from STZ-treated group of rats. Perfusion of the arterial mesentery vascular territory with 100 μM ATP, 100 nM 2-MeSADP, or 1 μM UTP elicited vasodilator responses of the same magnitude in controls or diabetics, but the increase in luminally accessible NO was 60-70% lower in diabetics (p < 0.05). Moreover, the concentration-response curve elicited by two NO donors was displaced downwards (p < 0.01) in diabetic rats. Parallel studies using primary cultures of endothelial cells from the arterial mesentery vasculature revealed that mechanical stimulation induced a rise in extracellular nucleotides, which in the cells from diabetic rats was larger and longer-lasting when comparing the extracellular release of ATP and ADO values to those of vehicle-treated controls. A 5 min challenge with purinergic agonists elicited a cell media NO rise, which was reduced in the endothelial cells from diabetic rats. Present findings provide neurochemical support for the diabetes-induced neuropathy and show that mesenteric endothelial cells alterations in response to mechanical stimulation are compatible with the endothelial dysfunction related to vascular disease progress.

Sympatholytic effect of intravascular ATP is independent of nitric oxide, prostaglandins, Na+ /K+ -ATPase and KIR channels in humans

KEY POINTS

Intravascular ATP attenuates sympathetic vasoconstriction (sympatholysis) similar to what is observed in contracting skeletal muscle of humans, and may be an important contributor to exercise hyperaemia. Similar to exercise, ATP-mediated vasodilatation occurs via activation of inwardly rectifying potassium channels (KIR ), and synthesis of nitric oxide (NO) and prostaglandins (PG). However, recent evidence suggests that these dilatatory pathways are not obligatory for sympatholysis during exercise; therefore, we tested the hypothesis that the ability of ATP to blunt α1 -adrenergic vasoconstriction in resting skeletal muscle would be independent of KIR , NO, PGs and Na+ /K+ -ATPase activity. Blockade of KIR channels alone or in combination with NO, PGs and Na+ /K+ -ATPase significantly reduced the vasodilatatory response to ATP, although intravascular ATP maintained the ability to attenuate α1 -adrenergic vasoconstriction. This study highlights similarities in the vascular response to ATP and exercise, and further supports a potential role of intravascular ATP in blood flow regulation during exercise in humans.

ABSTRACT

Exercise and intravascular ATP elicit vasodilatation that is dependent on activation of inwardly rectifying potassium (KIR ) channels, with a modest reliance on nitric oxide (NO) and prostaglandin (PG) synthesis. Both exercise and intravascular ATP attenuate sympathetic α-adrenergic vasoconstriction (sympatholysis). However, KIR channels, NO, PGs and Na+ /K+ -ATPase activity are not obligatory to observe sympatholysis during exercise. To further determine similarities between exercise and intravascular ATP, we tested the hypothesis that inhibition of KIR channels, NO and PG synthesis, and Na+ /K+ -ATPase would not alter the ability of ATP to blunt α1 -adrenergic vasoconstriction. In healthy subjects, we measured forearm blood flow (Doppler ultrasound) and calculated changes in vascular conductance (FVC) to intra-arterial infusion of phenylephrine (PE; α1 -agonist) during ATP or control vasodilatator infusion, before and after KIR channel inhibition alone (barium chloride; n = 7; Protocol 1); NO (l-NMMA) and PG (ketorolac) inhibition alone, or combined NO, PGs, Na+ /K+ -ATPase (ouabain) and KIR channel inhibition (n = 6; Protocol 2). ATP attenuated PE-mediated vasoconstriction relative to adenosine (ADO) and sodium nitroprusside (SNP) (PE-mediated ΔFVC: ATP: -16 ± 2; ADO: -38 ± 6; SNP: -59 ± 6%; P < 0.05 vs. ADO and SNP). Blockade of KIR channels alone or combined with NO, PGs and Na+ /K+ -ATPase, attenuated ATP-mediated vasodilatation (∼35 and ∼60% respectively; P < 0.05 vs. control). However, ATP maintained the ability to blunt PE-mediated vasoconstriction (PE-mediated ΔFVC: KIR blockade alone: -6 ± 5%; combined blockade:-4 ± 14%; P > 0.05 vs. control). These findings demonstrate that intravascular ATP modulates α1 -adrenergic vasoconstriction via pathways independent of KIR channels, NO, PGs and Na+ /K+ -ATPase in humans, consistent with a role for endothelium-derived hyperpolarization in functional sympatholysis.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Functional and RNA expression profile of adenosine receptor subtypes in mouse mesenteric arteries

Concentration-response curves (CRCs) of adenosine receptor (AR) agonists, NECA (nonspecific), CCPA (A1 specific), CGS-216870 (A2A specific), BAY 60-6583 (A2B specific), and Cl-IB-MECA (A3 specific) for mesenteric arteries (MAs) from 4 AR knockout (KO) mice (A1, A2A, A2B, and A3) and their wild type (WT) were constructed. The messenger RNA expression of MAs from KO mice and WT were also studied. Adenosine (10 to 10 M) and NECA (10 to 10 M) induced relaxation in all mice except A2B KO mice, which only showed constriction by adenosine at 10 to 10 and NECA at 10 to 10 M. The CCPA induced a significant constriction at 10 and 10 M in all mice, except A1KO. BAY 60-6583 induced relaxation (10 to 10 M) in WT and no response in A2BKO except at 10 M. The CRCs for BAY 60-6583 in A1, A2A, and A3 KO mice shifted to the left when compared with WT mice, suggesting an upregulation of A2B AR. No responses were noted to CGS-21680 in all mice. Cl-IB-MECA only induced relaxation at concentration greater than 10 M, and no differences were found between different KO mice. The CRC for Bay 60-6583 was not significantly changed in the presence of 10 M of L-NAME, 10 M of indomethacin, or both. Our data suggest that A2B AR is the predominant AR subtype and the effect may be endothelial independent, whereas A1 AR plays a significant modulatory role in mouse MAs.

Reciprocal modulation of anti-IgE induced histamine release from human mast cells by A₁ and A(2B) adenosine receptors

BACKGROUND AND PURPOSE

Adenosine is believed to participate in the pathological development of asthma through a mast cell-dependent mechanism. Our study aimed to pharmacologically characterize the functions of adenosine receptor (AR) subtypes (A₁, A(2A) , A(2B) and A₃) in primary human cultured mast cells (HCMC).

EXPERIMENTAL APPROACH

HCMC were derived from progenitor stem cells in buffy coat and the effects of adenosine receptor ligands on basal and IgE-dependent histamine release were evaluated.

KEY RESULTS

Adenosine and analogues alone did not induce HCMC degranulation. When HCMC were activated by anti-IgE after 10 min pre-incubation with adenosine, a biphasic effect on histamine release was observed with enhancement of HCMC activation at low concentrations of adenosine (10⁻⁹-10⁻⁷ mol·L⁻¹) and inhibition at higher concentrations (10⁻⁶-10⁻⁴ mol·L⁻¹). The potentiating action was mimicked by A₁ AR agonists CCPA and 2'MeCCPA, and inhibited by the A₁ AR antagonist PSB36. In contrast, the inhibitory action of adenosine was mimicked by the non-specific A₂ AR agonist CV1808 and attenuated by A(2B) AR antagonists PSB1115 and MRS1760. The non-selective AR antagonist CGS15943 attenuated both the potentiating and inhibitory actions.

CONCLUSIONS AND IMPLICATIONS

We have defined for the first time the contribution of A₁ and A(2B) ARs, respectively, to the potentiating and inhibitory action of adenosine on human mast cell activation. With reference to the current trend of developing novel anti-asthmatic agents from AR ligands, our results suggest that inhibition of human mast cell activation would be a mechanism for A₁ AR antagonists, but not A(2B) AR antagonists.

Lessons from human coronary aspirate

The interventional implantation of a stent into an atherosclerotic coronary artery is a unique and paradigmatic scenario of plaque rupture in humans. The use of protection devices not only prevents the released plaque particles and the superimposed thrombotic material from being washed and embolized into the coronary microcirculation of the individual patient, but permits also the retrieval and ex vivo analysis of particulate plaque debris and soluble substances. The particulate debris comprises typical cholesterol crystals, foam cells, hyalin material and calcium deposits from the atheroma as well as platelets and coagulation material; soluble substances include vasoconstrictors, such as serotonin and thromboxane, as well as inflammatory mediators, such as TNFα which amplifies vasoconstriction by inducing endothelial dysfunction. The vasoconstriction observed in a bioassay ex vivo correlates to clinical symptoms, angiographic stenosis and plaque burden, as assessed by intravascular ultrasound. The release of TNFα into the aspirate correlates to restenosis. Detailed analysis of the human coronary aspirate may promote a better understanding of the pathophysiology of the vulnerable atherosclerotic plaque and help to better antagonize the microvascular consequences of coronary microembolization, including the no reflow phenomenon. This article is part of a Special Issue entitled "Coronary Blood Flow."

Vasoconstrictor Potential of Coronary Aspirate From Patients Undergoing Stenting of Saphenous Vein Aortocoronary Bypass Grafts and Its Pharmacological Attenuation

Rationale:

Stent implantation into atherosclerotic plaques releases, apart from particulate debris, soluble substances that contribute to impaired microvascular perfusion.

Objective:

To quantify the release of vasoconstrictors and to determine the efficacy of coronary dilators to attenuate their action.

Methods and Results:

Using a distal protection/aspiration device, coronary arterial blood was retrieved before and during stenting in 22 patients with severe saphenous vein aorto-coronary bypass stenoses. The release of catecholamines, endothelin, serotonin, thromboxane B 2 , and tumor necrosis factor (TNF)α was measured. The response of rat mesenteric arteries with intact (+E) and denuded (−E) endothelium to aspirate plasma was normalized to that by KCl. Responses to selective receptor blockade, adenosine, nitroprusside, and verapamil against the aspirate-induced constriction were determined. The coronary arterial plasma withdrawn before stenting induced 21±5% and the aspirate plasma after stenting induced 95±8% of maximum KCl-induced vasoconstriction. Serotonin, thromboxane B 2 , and TNFα release into aspirate plasma increased by 1.9±0.2 μmol/L, 25.6±3.1 pg/mL, and 19.7±6.1 pg/mL, respectively, during stenting. The aspirate-induced vasoconstriction was largely antagonized by selective serotonin receptor blockade, with little further antagonism by additional thromboxane receptor blockade. TNFα did not induce constriction per se but potentiated the constriction with serotonin and the thromboxane-analog U-46619 in arteries +E. The concentrations to induce half-maximal vasodilation were comparable for nitroprusside (+E, 3.3×10 −8 ; −E, 1.9×10 −8 mol/L) and verapamil (+E, 8.3×10 −8 ; −E, 7.8×10 −8 mol/L), and the vasoconstriction was eventually eliminated. The vasodilator response to adenosine was dependent on functional endothelium and weaker.

Conclusion:

Serotonin is the main coronary vasoconstrictor after stenting, and thromboxane and TNFα somewhat potentiate the serotonin response. Nitroprusside and verapamil are more potent than adenosine to attenuate the aspirate plasma-induced vasoconstriction, and they are not dependent on functional endothelium.

Coordination of vasomotor responses by the endothelium

Increases in the diameter of small resistance arteries and arterioles occur secondary to processes that can be dependent or independent of changes in membrane potential. Hyperpolarization reduces the opening of voltage-gated calcium channels and thereby the stimulus for contraction of these resistance vessels. The stimulus for smooth muscle cell (SMC) hyperpolarization can occur directly via opening K(+)-channels expressed within those cells, but can also occur in response to stimulation of endothelial cells (ECs). This endothelium-dependent hyperpolarization (EDH) of smooth muscle often occurs in response to agonists that stimulate a rise in the Ca(2+) concentration of ECs, which in turn can open Ca(2+)-activated K-channels to hyperpolarize the ECs, and if present, patent gap junctions connecting ECs to SMCs (myoendothelial gap junctions) can potentially enable direct electrical coupling. There is also evidence to suggest a diffusible factor or factors hyperpolarizes SMCs (EDHF pathways). Furthermore, whether evoked in ECs or SMCs, hyperpolarization can spread a considerable distance to neighboring cells via gap junctions, causing remote dilatation termed ;spreading' or ;conducted' dilatation. This process is endothelium-dependent and likely relies on both homo- and heterocellular gap junctions. This review will focus on the cross-talk between ECs and SMCs that coordinates the spread of hyperpolarization and thus modulates smooth muscle tone.

Multiple receptor subtypes and multiple mechanisms of dilation are involved in vascular network dilation caused by adenosine

We previously demonstrated a vascular network response initiated by elevated tissue concentrations of adenosine that is distinct from the dilation caused when adenosine is applied directly to the arteriole. The purpose of this study was to elucidate the potential mechanism(s) for the different responses. In the cheek pouch of anesthetized hamster, arteriolar responses were measured when adenosine (10(-4)M) was applied with micropipette into the tissue 500 microm from the arteriole (n=67, baseline diameter 22+/-0.6 microm) or onto the arteriole itself. Application of adenosine to the vessel or into the tissue caused arteriolar dilation with similar concentration profiles. In stark contrast, the concentration profiles were significantly different for vessel and tissue initiated dilation when either sodium nitroprusside or methacholine was tested. Arteriolar dilation was not enhanced when adenosine was simultaneously applied with two pipettes at along a single arteriole; however, the dilation doubled when adenosine was applied simultaneously at arteriole and tissue. Control dilations caused by tissue adenosine (5+/-0.4 microm) were not altered by superfusion of the A(1) receptor antagonist DPCPX (10(-6)M; 4.6+/-0.3 microm), A(2B) receptor antagonist alloxazine (10(-6)M; 6+/-0.8 microm), or A(3) receptor antagonist MRS1220 (5 x 10(-9)M; 6+/-0.8 microm) but were abolished by the selective A(2A) receptor antagonist ZM241385 (10(-7)M; 1+/-0.2 microm), suggesting that activation of A(2A) receptors mediates these network responses. Disruption of arteriolar endothelium and direct arteriolar application of ZM241385 (10(-7)M; 5+/-0.4 microm) did not alter the dilation caused by tissue adenosine. However, local application of ZM241385 into the tissue inhibited adenosine-induced network responses (2+/-0.3 microm). Furthermore, application into the tissue of A(2A) receptor agonist CGS21680 (10(-5)M), but not A(1) (CPA; 10(-4)M), A2b (NECA, 10(-4)M) or A3 (IB-MECA; 10(-4)M) receptor agonists mimicked the adenosine network response. These data demonstrate dual, complimentary, yet distinct pathways for network dilations induced by increases in tissue adenosine.

The endogenous brain constituent N-arachidonoyl L-serine is an activator of large conductance Ca2+-activated K+ channels

The novel endocannabinoid-like lipid N-arachidonoyl L-serine (ARA-S) causes vasodilation through both endothelium-dependent and -independent mechanisms. We have analyzed the vasorelaxant effect of ARA-S in isolated vascular preparations and its effects on Ca(2+)-activated K(+) currents in human embryonic kidney cells stably transfected with the alpha-subunit of the human, large conductance Ca(+)-activated K(+) (BK(Ca)) channel [human embryonic kidney (HEK) 293hSlo cells]. ARA-S caused relaxation of rat isolated, intact and denuded, small mesenteric arteries preconstricted with (R)-(-)-1-(3-hydroxyphenyl)-2-methylaminoethanol hydrochloride (pEC(50), 5.49 and 5.14, respectively), whereas it caused further contraction of vessels preconstricted with KCl (pEC(50), 5.48 and 4.82, respectively). Vasorelaxation by ARA-S was inhibited by 100 nM iberiotoxin. In human embryonic kidney cells stably transfected with the alpha-subunit of the human BK(Ca) channel cells, ARA-S and its enantiomer, N-arachidonoyl-D-serine, enhanced the whole-cell outward K(+) current with similar potency (pEC(50), 5.63 and 5.32, respectively). The potentiation was not altered by the beta(1) subunit or mediated by ARA-S metabolites, stimulation of known cannabinoid receptors, G proteins, protein kinases, or Ca(2+)-dependent processes; it was lost after patch excision or after membrane cholesterol depletion but was restored after cholesterol reconstitution. BK(Ca) currents were also enhanced by N-arachidonoyl ethanolamide (pEC(50), 5.27) but inhibited by another endocannabinoid, O-arachidonoyl ethanolamine (pIC(50), 6.35), or by the synthetic cannabinoid O-1918 [(-)-1,3-dimethoxy-2-(3-3,4-trans-p-menthadien-(1,8)-yl)-orcinol] (pIC(50), 6.59), which blocks ARA-S-induced vasodilation. We conclude the following. 1) ARA-S directly activates BK(Ca) channels. 2) This interaction does not involve cannabinoid receptors or cytosolic factors but is dependent on the presence of membrane cholesterol. 3) Direct BK(Ca) channel activation probably contributes to the endothelium-independent component of ARA-S-induced mesenteric vasorelaxation. 4) O-1918 is a BK(Ca) channel inhibitor.

Purinergic receptors in the splanchnic circulation

There is considerable evidence that purines are vasoactive molecules involved in the regulation of blood flow. Adenosine is a well known vasodilator that also acts as a modulator of the response to other vasoactive substances. Adenosine exerts its effects by interacting with adenosine receptors. These are metabotropic G-protein coupled receptors and include four subtypes, A(1), A(2A), A(2B) and A(3). Adenosine triphosphate (ATP) is a co-transmitter in vascular neuroeffector junctions and is known to activate two distinct types of P2 receptors, P2X (ionotropic) and P2Y (metabotropic). ATP can exert either vasoconstrictive or vasorelaxant effects, depending on the P2 receptor subtype involved. Splanchnic vascular beds are of particular interest, as they receive a large fraction of the cardiac output. This review focus on purinergic receptors role in the splanchnic vasomotor control. Here, we give an overview on the distribution and diversity of effects of purinergic receptors in splanchnic vessels. Pre- and post-junctional receptormediated responses are summarized. Attention is also given to the interactions between purinergic receptors and other receptors in the splanchnic circulation.

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

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

ZM241385, DPCPX, MRS1706 are inverse agonists with different relative intrinsic efficacies on constitutively active mutants of the human adenosine A2B receptor

The human adenosine A(2B) receptor belongs to class A G protein-coupled receptors (GPCRs). In our previous work, constitutively active mutant (CAM) human adenosine A(2B) receptors were identified from a random mutation bank. In the current study, three known A(2B) receptor antagonists, 4-{2-[7-amino-2-(2-furyl)[1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-yl-amino]ethyl}phenol (ZM241385), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and N-(4-acetylphenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]acetamide (MRS1706) were tested on wild-type and nine CAM A(2B) receptors with different levels of constitutive activity in a yeast growth assay. All three compounds turned out to be inverse agonists for the adenosine A(2B) receptor because they were able to fully reverse the basal activity of four low-level constitutively active A(2B) receptor mutants and to partially reverse the basal activity of three medium-level constitutively active A(2B) receptor mutants. We also discovered two highly constitutively active mutants whose basal activity could not be reversed by any of the three compounds. A two-state receptor model was used to explain the experimental observations; fitting these yielded the following relative intrinsic efficacies for the three inverse agonists ZM241385, DPCPX, and MRS1706: 0.14 +/- 0.03, 0.35 +/- 0.03, and 0.31 +/- 0.02, respectively. Moreover, varying L, the ratio of active versus inactive receptors in this model, from 0.11 for mutant F84L to 999 for two highly constitutively active mutants yielded simulated dose-response curves that mimicked the experimental curves. This study is the first description of inverse agonists for the human adenosine A(2B) receptor. Moreover, the use of receptor mutants with varying levels of constitutive activity enabled us to determine the relative intrinsic efficacy of these inverse agonists.

Pharmacological characterisation of the adenosine receptor mediating increased ion transport in the mouse isolated trachea and the effect of allergen challenge

The effect of adenosine on transepithelial ion transport was investigated in isolated preparations of murine trachea mounted in Ussing chambers. The possible regulation of adenosine receptors in an established model of allergic airway inflammation was also investigated. Mucosally applied adenosine caused increases in short-circuit current (I(SC)) that corresponded to approximately 50% of the response to the most efficacious secretogogue, ATP (delta I(SC) 69.5 +/- 6.7 microA cm2). In contrast, submucosally applied adenosine caused only small (<20%) increases in I(SC), which were not investigated further. The A1-selective (N6-cyclopentyladenosine, CPA, 1 nM-10 microM), A2A-selective (2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxoamido adenosine; CGS 21680; 0.1-100 microM) and A3-selective (1-deoxy-1-[6-[[(3-iodophenyl)-methyl]amino]-9H-purin-9-yl]-N-methyl-beta-D-ribofuranuronamide; IB-MECA; 30 nM-100 microM) adenosine receptor agonists were either equipotent or less potent than adenosine, suggesting that these receptors do not mediate the response to adenosine. The A1 receptor selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 nM-1 microM) caused a rightward shift of the adenosine concentration-effect curve only at 1 microM. The mixed A2A/A2B receptor 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) also caused rightward shift of the adenosine concentration-effect curve, again only at micromolar concentrations, suggestive of the involvement of A2B receptors. In preparations from animals sensitised to ovalbumin and challenged over 3 days with aerosol ovalbumin, a decrease in baseline I(SC) was observed and responses to ATP were diminished. Similarly, the amplitude of responses to adenosine were attenuated although there was no change in potency. These results suggest that the A2B receptor mediates the I(SC) response to adenosine in the mouse trachea. This receptor does not appear to be regulated in a standard asthma model.

A1 and A2A adenosine receptor modulation of alpha 1-adrenoceptor-mediated contractility in human cultured prostatic stromal cells

1. This study investigated the possibility that adenosine receptors modulate the alpha(1)-adrenoceptor-mediated contractility of human cultured prostatic stromal cells (HCPSC). 2. The nonselective adenosine receptor agonist, 5'-N-ethylcarboxamido-adenosine (NECA; 10 nm-10 microm), and the A(1) adenosine receptor selective agonist, cyclopentyladenosine (CPA; 10 nm-10 microm), elicited significant contractions in HCPSC, with maximum contractile responses of 18+/-3% and 17+/-2% reduction in initial cell length, respectively. 3. In the presence of a threshold concentration of phenylephrine (PE) (100 nm), CPA (1 nm-10 microm) caused contractions, with an EC(50) of 124+/-12 nm and maximum contractile response of 37+/-4%. The A(1) adenosine receptor-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX 100 nm) blocked this effect. In the presence of DPCPX (100 nm), NECA (1 nm-10 microm) inhibited contractions elicited by a submaximal concentration of PE (10 microm), with an IC(50) of 48+/-2 nm. The A(2A) adenosine receptor-selective antagonist 4-(2-[7-amino-2-[furyl][1,2,4]triazolo[2,3-alpha][1,3,5,]triazin-5-yl amino]ethyl)phenol (Zm241385 100 nm) blocked this effect. 4. In BCECF-AM (10 microm)-loaded cells, both CPA (100 pM-1 microm) and NECA (100 pm-10 microm) elicited concentration-dependent decreases in intracellular pH (pH(i)), with EC(50) values of 3.1+/-0.3 and 6.0+/-0.3 nm, respectively. The response to NECA was blocked by Zm241385 (100 nm; apparent pK(B) of 9.4+/-0.4), but not by DPCPX (100 nm). The maximum response to CPA was blocked by DPCPX (100 nm), and unaffected by Zm241385 (100 nm). 5. NECA (10 nm-10 microm) alone did not increase [(3)H]-cAMP in HCPSC. In the presence of DPCPX (100 nm), NECA (10 nm-10 microm) caused a concentration dependent increase in [(3)H]-cAMP, with an EC(50) of 1.2+/-0.1 microm. This response was inhibited by Zm241385 (100 nm). CPA (10 nm-10 microm) had no effect on cAMP, in the presence or absence of forskolin (1 microm). 6. These findings are consistent with a role for adenosine receptors in the modulation of adrenoceptor-mediated contractility in human prostate-derived cells.

Anandamide is able to inhibit trigeminal neurons using an in vivo model of trigeminovascular-mediated nociception

Arachidonylethanolamide (anandamide, AEA) is believed to be the endogenous ligand of the cannabinoid CB(1) and CB(2) receptors. CB(1) receptors have been found localized on fibers in the spinal trigeminal tract and spinal trigeminal nucleus caudalis. Known behavioral effects of anandamide are antinociception, catalepsy, hypothermia, and depression of motor activity, similar to Delta(9)-tetrahydocannanbinol, the psychoactive constituent of cannabis. It may be a possible therapeutic target for migraine. In this study, we looked at the possible role of the CB(1) receptor in the trigeminovascular system, using intravital microscopy to study the effects of anandamide against various vasodilator agents. Anandamide was able to inhibit dural blood vessel dilation brought about by electrical stimulation by 50%, calcitonin gene-related peptide (CGRP) by 30%, capsaicin by 45%, and nitric oxide by 40%. CGRP(8-37) was also able to attenuate nitric oxide (NO)-induced dilation by 50%. The anandamide inhibition was reversed by the CB(1) receptor antagonist AM251. Anandamide also reduced the blood pressure changes caused by CGRP injection, this effect was not reversed by AM251. It would seem that anandamide acts both presynaptically, to prevent CGRP release from trigeminal sensory fibers, and postsynaptically to inhibit the CGRP-induced NO release in the smooth muscle of dural arteries. CB(1) receptors seem to be involved in the NO/CGRP relationship that exists in causing headache and dural blood vessel dilation. It also seems that some of the blood pressure changes caused by anandamide are mediated by a noncannabinoid receptor, as AM251 was unable to reverse these effects. It can be suggested that anandamide is tonically released to play some form of modulatory role in the trigeminovascular system.

Analysis of adenosine vascular effect in isolated rat aorta: possible role of Na+/K+-ATPase

The present experiments were undertaken in order to examine the effect of adenosine in isolated rat aorta, to investigate the possible role of intact endothelium and endothelial relaxing factors in this action and to determine which population of adenosine receptors is involved in rat aorta response to adenosine. Adenosine (0.1-300 microM) produced concentration-dependent (intact rings: pD2=4.39+/-0.09) and endothelium-independent (denuded rings: pD2=4.52+/-0.12) relaxation of isolated rat aorta. In the presence of high concentration of K+ (100 mM) adenosine-evoked relaxation was significantly reduced (maximal relaxation in denuded rings: control - 92.1+/-9.8 versus K+- 54.4+/-5.0). Similar results were obtained after incubation of ouabain (100 microM) or glibenclamide (1 microM). In K+-free solution, K+ (1-10 mM)-induced rat aorta relaxant response was significantly inhibited by ouabain (100 microM). Application of indomethacin (10 microM), NG-nitro-L-arginine (10 microM) or tetraethylammonium (500 microM) did not alter the adenosine-elicited effect in rat aorta. 8-(3-Chlorostyril)-caffeine (0.3-3 microM), a selective A2A-receptor antagonist, significantly reduced adenosine-induced relaxation of rat aorta in a concentration-dependent manner (pKB=6.57). Conversely, 1,3-dipropyl-8-cyclopentylxanthine (10 nM), an A1-receptor antagonist, did not affect adenosine-evoked dilatation. These results indicate that in isolated rat aorta, adenosine produces endothelium-independent relaxation, which is most probably dependent upon activation of smooth muscle Na+/K+-ATPase, and opening of ATP-sensitive K+ channels, to a smaller extent. According to receptor analysis, vasorelaxant action of adenosine in rat aorta is partly induced by activation of smooth muscle adenosine A2A receptors.

Adenosine receptor subtypes mediating coronary vasodilation in rat hearts

Adenosine receptor-mediated coronary vasodilation was studied in isolated hearts from young (1-2 months) and mature (12-18 months) Wistar rats. The nonselective agonist 5'-N-ethylcarboxamidoadenosine (NECA) induced biphasic concentration-dependant dilation with similar potencies in both age groups (p < 0.05). Despite similar potencies, responses to NECA were significantly depressed by 50% with age. NECA-mediated dilation was unaltered by selective A adenosine receptor (A1AR) antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 100 nM ) or A adenosine receptor (A2AAR) antagonist 5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5- ]pyrimidine (SCH 58261, 100 nM ). However, the A2B adenosine receptor (A2B AR) selective antagonist alloxazine (10 microM ) significantly reduced response magnitude to NECA in both age groups. Concentration-response curves to N -2-(4-aminophenyl) ethyladenosine (APNEA) induced biphasic concentration-dependent dilation in hearts from young animals. In the presence of the three combined antagonists, 1 microM DPCPX, 100 nM SCH 58261, and 1 microM alloxazine, the response magnitude was significantly attenuated (p < 0.05). The addition of the A3 adenosine receptor (A3AR) antagonist 3-ethyl-5-benzyl-2-methyl-4-phenylethyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS1191, 100 nM ) to the combined antagonists further attenuated vasodilator responses to APNEA. The results suggest that multiple adenosine receptor subtypes mediate dilation in the rat coronary circulation. NECA mediates vasodilation via the A2BAR subtype, while dilator responses to APNEA in the presence and absence of A1, A2, and A3 ARs antagonists provide evidence for a vasodilator role for A3 ARs in rat coronary circulation. The magnitude of the coronary dilator response is reduced with age and does not involve A2A or A1 ARs.

Vasoactivity of diadenosine polyphosphates in human small mesenteric resistance arteries

Diadenosine polyphosphates (ApnA) (n = 3-6) induced vasoconstrictions in isolated human mesenteric resistance arteries (hMRAs) mounted in a microvessel myograph (rank order of potency: Ap5A > Ap6A > Ap4A > Ap3A). The contractile effects of ApnA in hMRA were similar to their effects in rat MRA investigated previously. ATP, ADP, AMP, and adenosine had less contractile potency than ApnA, suggesting that the observed effects were not induced by the degradation products of ApnA. Ap4A- and Ap5A-induced vasoconstriction was inhibited by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (P2X purinoceptor antagonist) but not by ADP3'5' (P2Y purinoceptor antagonist). Thus, this purinergic vasoconstriction of hMRA seems to be P2X but not P2Y purinoceptor-mediated. In precontracted hMRA all ApnA caused vasorelaxations but (in contrast to rat MRA) the potencies of the ApnA did not differ significantly from each other. The ApnA degradation products had less vasorelaxing potency than ApnA, demonstrating that the vasorelaxations can be ascribed to the ApnA themselves. Ap5A-induced vasorelaxation of hMRA could neither be inhibited with ADP3'5' nor with PPADS, which reveals a decisive difference to the rat MRA where the inhibitory profile demonstrated the importance of the P2Y purinoceptor for Ap5A-induced vasorelaxation. However, Ap4A-induced vasorelaxation in hMRA could be inhibited by ADP3'5'. These findings show that Ap4A-induced vasorelaxation in hMRA is due to P2Y purinoceptor activation, that Ap5A evokes vasorelaxation in hMRA via another mechanism than Ap4A, and that data derived from the animal model cannot be simply transferred to human conditions.

Characterization of adenosine receptors mediating the vasodilator effects of adenosine receptor agonists in the microvasculature of the hamster cheek pouch in vivo

1 The aim of this study was to characterize the adenosine receptor mediating vasodilation in the microvasculature of the hamster cheek pouch in vivo. A range of adenosine agonists was used including N6-cyclopentyladenosine (CPA) (A1 agonist), 5'-N-ethylcarboxamidoadenosine (NECA) (non-selective), 2-chloroadenosine (2CADO) (non-selective), 2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680) (A2A agonist), N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IBMECA) (A3 agonist) and adenosine, as well as the adenosine antagonists 8-sulphophenyltheophylline (8-SPT) (A1/A2 antagonist), 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (A1 antagonist) and 4-(2-[7-amino-2-(2-furyl)[1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) (A2A antagonist). 2 All the adenosine analogues used induced vasodilation at concentrations between 10 nm and 1 microm, and the potency order was NECA > CGS 21680 > 2CADO > CPA=IBMECA >> adenosine, indicating an action at A2A receptors. 8-SPT (50 microm) antagonized vasodilator responses to NECA with an apparent pKB of 5.4, consistent with an action at A1 or A2 receptors and confirming that A3 receptors are not involved in this response. 3 DPCPX (10 nm) had no effect on vasodilation evoked by NECA, suggesting that this response was not mediated via A1 receptors, while ZM 241385 (10 nm) antagonized dilator responses to NECA with an apparent pKB of 8.9 consistent with an action via A2A receptors. 4 Overall these results suggest that adenosine A2A receptors mediate vasodilation in the hamster cheek pouch in vivo.

Adenosine A(2A) receptors in portal hypertension: their role in the abnormal response to adenosine of the cranial mesenteric artery in rabbits

1. Adenosine is a regulator of mesenteric vasodilation involved in auto-regulation and post-prandial hyperemia, but the adenosine receptor subtype involved in this relaxant effect is poorly characterized. We have now pharmacologically characterized this receptor in rabbit mesenteric arteries and investigated how this adenosine receptor response changes in portal hypertensive animals since the adenosine response is decreased. 2. The closest non-metabolisable adenosine analogue, 2-chloroadenosine (CADO), the mixed A(1)/A(2) receptor agonist, 5'-ethylcarboxamidoadenosine (NECA), and the selective A(2A) receptor agonist, 2-[4-(2-p-carbonyethyl)phenylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680) (1 pM -- 1 mM) relaxed noradrenaline pre-contracted arteries with a rank order of potency of CGS 21680 (EC(50)=20 nM) > or = NECA (60 nM)>>CADO (640 nM). 3. The selective A(2A) receptor 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, 100 nM), shifted to the right the CADO concentration-response curve. 4. In portal hypertensive animals, there was mainly a decreased potency but also a decreased efficacy of all tested adenosine agonists compared to normal animals. Concomitantly, there was a decreased adenosine plasma level and a decreased binding density of [(3)H]-CGS 21680 and [(3)H]-ZM 241385 to mesenteric artery membranes from portal hypertensive compared to normal rabbits. 5. These results indicate that A(2A) receptor activation is required for the adenosine-induced mesenteric relaxation and that the decreased density of A(2A) receptors may contribute to the decreased relaxation induced by adenosine of mesenteric arteries in portal hypertensive animals.

Pharmacology of adenosine receptors in the vasculature

Adenosine is widely distributed in mammals. One of the primary roles of adenosine within the cardiovascular system is to directly control the functions of both cardiac and vascular tissues. Recently, there has been considerable interest in the subclassification of adenosine receptors. Characterization of a heterogeneous population of receptors for adenosine could provide an opportunity for the development of novel compounds of therapeutic value. Adenosine is released from cells as a result of metabolism, and its release can be increased dramatically from cells that are metabolically stressed. This implies that adenosine can be released from a variety of cells throughout the body, as a result of increased metabolic rates, in concentrations that can have a profound impact on blood vessel function and, consequently, blood flow. It is recognized that the actions of this nucleoside on the vasculature are most prominent when oxygen demand is high and there is a reduction in oxygen tension at the site in question. Therefore, it is not surprising that adenosine has been shown to be an important regulator of blood vessel tone under hypoxic conditions. Furthermore, the activation of adenosine receptors on blood vessels can result in relaxation and/or contractions. The nature of the response subsequent to the activation of adenosine receptors is primarily dependent on the type of blood vessel involved and basal tone. This review will focus on the characterization of subtypes of adenosine receptors in blood vessels, as well as the effect of the stimulation of adenosine receptors on the peripheral circulation.

Functional characterization of coronary vascular adenosine receptors in the mouse

Coronary responses to adenosine agonists were assessed in perfused mouse and rat hearts. The roles of nitric oxide (NO) and ATP-dependent K(+) channels (K(ATP)) were studied in the mouse. Resting coronary resistance was lower in mouse vs rat, as was minimal resistance (2.2+/-0.1 vs 3.8+/-0.2 mmHg ml(-1) min(-1) g(-1)). Peak hyperaemic flow after 20 - 60 s occlusion was greater in mouse. Adenosine agonists induced coronary dilation in mouse, with pEC(50)s of 9.4+/-0.1 for 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethyl carboxamidoadenosine (CGS21680, A(2A)-selective agonist), 9.3+/-0.1 for 5'-N-ethylcarboxamidoadenosine (NECA, A(1)/A(2) agonist), 8.4+/-0.1 for 2-chloroadenosine (A(1)/A(2) agonist), 7.7+/-0.1 for N(6)-(R)-(phenylisopropyl)adenosine (R-PIA, A(1)/A(2B) selective), and 6.8+/-0.2 for adenosine. The potency order (CGS21680=NECA>2-chloroadenosine>R-PIA>adenosine) supports A(2A) adenosine receptor-mediated dilation in mouse coronary vessels. 0.2 - 2 microM of the A(2B)-selective antagonist alloxazine failed to alter CGS21680 or 2-chloroadenosine responses. pEC(50)s in rat were 6.7+/-0.2 for CGS21680, 7.3+/-0.1 for NECA, 7.6+/-0.1 for 2-chloroadenosine, 7.2+/-0.1 for R-PIA, and 6.2+/-0.1 for adenosine (2-chloroadenosine>NECA=R-PIA>CGS21680> adenosine), supporting an A(2B) adenosine receptor response. NO-synthase antagonism with 50 microM N(G)-nitro L-arginine (L-NOARG) increased resistance by approximately 25%, and inhibited responses to CGS21680 (pEC(50)=9.0+/-0.1), 2-chloroadenosine (pEC(50)=7.3+/-0.2) and endothelial-dependent ADP, but not sodium nitroprusside (SNP). K(ATP) channel blockade with 5 microM glibenclamide increased resistance by approximately 80% and inhibited responses to CGS21680 in control (pEC(50)=8.3+/-0.1) and L-NOARG-treated hearts (pEC(50)=7.3+/-0.1), and to 2-chloroadenosine in control (pEC(50)=6.7+/-0.1) and L-NOARG-treated hearts (pEC(50)=5.9+/-0.2). In summary, mouse coronary vessels are more sensitive to adenosine than rat vessels. A(2A) adenosine receptors mediate dilation in mouse coronary vessels vs A(2B) receptors in rat. Responses in the mouse involve a sensitive NO-dependent response and K(ATP)-dependent dilation.

Role of cyclic nucleotides in vasodilations of the rat thoracic aorta induced by adenosine analogues

Although adenosine analogues such as 5'-N-ethylcarboxamidoadenosine (NECA) relax the rat thoracic aorta in a partially endothelium-dependent manner via adenosine A(2A) receptors, others such as N(6)-R-phenylisopropyladenosine (R-PIA) act via an endothelium-independent, antagonist-insensitive mechanism. The role of cyclic nucleotides in these relaxations was investigated in isolated aortic rings using inhibitors of adenylate and guanylate cyclases as well as subtype-selective phosphodiesterase inhibitors. The adenylate cyclase inhibitor 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ 22536; 100 microM) significantly inhibited responses to NECA, but not responses to R-PIA. The type IV (cyclic AMP-selective) phosphodiesterase inhibitor 4-[(3-butoxy-4-methoxyphenyl)methyl]-2-imidazolidinone (RO 20-1724; 30 microM) significantly enhanced responses to NECA and to a lesser extent those to R-PIA. The guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3a]quinoxalin-1-one (ODQ; 100 microM) significantly inhibited responses to NECA and acetylcholine but not responses to R-PIA. The selective phosphodiesterase V (cyclic GMP-selective) inhibitors, zaprinast (10 microM) and 4-[[3',4'-(methylenedioxy)benzyl]amino]-6-methoxyquinazoline (MMQ; 1 microM), had no significant effect on responses to either NECA or R-PIA, but enhanced responses to acetylcholine. These results are consistent with the effects of NECA being via activation of endothelial receptors to release NO which stimulates guanylate cyclase, as well as smooth muscle receptors coupled to stimulation of adenylate cyclase. The lack of effect of zaprinast and MMQ on responses to NECA are likely to be due to simultaneous activation of both adenylate and guanylate cyclases in the smooth muscle, as cyclic AMP reduces the sensitivity of phosphodiesterase V to inhibitors. These results also suggest that the effects of R-PIA are via neither of these mechanisms.

Age-related changes in adenosine-mediated relaxation of coronary and aortic smooth muscle

We tested whether adenosine mediates nitric oxide (NO)-dependent and NO-independent dilation in coronary and aortic smooth muscle and whether age selectively impairs NO-dependent adenosine relaxation. Responses to adenosine and the relatively nonselective analog 5'-N-ethylcarboxamidoadenosine (NECA) were studied in coronary vessels and aortas from immature (1-2 mo), mature (3-4 mo), and moderately aged (12-18 mo) Wistar and Sprague-Dawley rats. Adenosine and NECA induced biphasic concentration-dependent coronary vasodilation, with data supporting high-sensitivity (pEC(50) = 5.2-5.8) and low-sensitivity (pEC(50) = 2.3-2.4) adenosine sites. Although sensitivity to adenosine and NECA was unaltered by age, response magnitude declined significantly. Treatment with 50 microM N(G)-nitro-L-arginine methyl ester (L-NAME) markedly inhibited the high-sensitivity site, although response magnitude still declined with age. Aortic sensitivity to adenosine declined with age (pEC(50) = 4.7 +/- 0.2, 3.5 +/- 0.2, and 2.9 +/- 0.1 in immature, mature, and moderately aged aortas, respectively), and the adenosine receptor transduction maximum also decreased (16.1 +/- 0.8, 12.9 +/- 0.7, and 9.6 +/- 0.7 mN/mm(2) in immature, mature, and moderately aged aortas, respectively). L-NAME decreased aortic sensitivity to adenosine in immature and mature tissues but was ineffective in the moderately aged aorta. Data collectively indicate that 1) adenosine mediates NO-dependent and NO-independent coronary and aortic relaxation, 2) maturation and aging reduce NO-independent and NO-dependent adenosine responses, and 3) the age-related decline in aortic response also involves a reduction in the adenosine receptor transduction maximum.

Low pH modulation of recombinant vanilloid receptors and perivascular capsaicin-sensitive sensory neurotransmission

The effect of low pH on capsaicin-sensitive sensory neurotransmission in the rat isolated mesenteric arterial bed and at recombinant (rVR1) vanilloid receptors was investigated. Mesenteric sensory neurogenic vasorelaxation elicited by electrical field stimulation was reversibly inhibited by lowering pH from 7.4 to 6.9 and 6.3. Capsaicin-induced vasorelaxation was not different at pH 6.9, but was attenuated at pH 6.3. Vasorelaxation to calcitonin gene-related peptide, the principal sensory motor neurotransmitter in rat mesenteric arteries, was not different at pH 6.9 or pH 6.3. In rVR1-transfected HEK293 cells, acidic conditions enhanced the affinities of capsaicin and capsazepine at rVR1, but did not affect the potency of carbachol at endogenous muscarinic receptors. Following inactivation of endogenous acid-sensitive ion channels, lowering pH (6.0-4.5) directly increased [Ca2+]i in rVR1-HEK293 cells (EC50 5.5). This response was abolished by 1 microM capsazepine. In conclusion, a decrease in pH (to 6.9 and 6.3) enhances the affinity of capsaicin at rVR1, but inhibits sensory neurotransmission in the rat mesenteric arterial bed. This likely explains why there is no evidence of an enhancement of sensitivity to capsaicin at endogenous vanilloid receptors, as observed with rVR1. When pH is reduced still further (6.0-5.5) there is direct activation of rVR1.

Relaxation of mouse isolated aorta to adenosine and its analogues does not involve adenosine A(1), A(2) or A(3) receptors

Relaxations to adenosine and analogues were investigated in the mouse aorta in the presence of the adenosine A(1) receptor-selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 30 nM), which did not affect relaxations to adenosine or its analogue N(6)-R-phenylisopropyladenosine (R-PIA) but abolished contractile adenosine A(1) receptor-mediated responses to these agonists. Relaxations to adenosine, 5'-N-ethylcarboxamidoadenosine, R-PIA, 2-[p-(2-carbonylethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680), and N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IB-MECA) were unaffected by the adenosine A(1)/A(2) receptor antagonist 8-sulphophenyltheophylline (100 microM). IB-MECA relaxations were unaffected by the adenosine A(3) receptor-selective antagonist 3-ethyl-5-benzyl-2-methyl-6-phenyl-4-phenylethynyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS1191, 30 microM) and R-PIA relaxations were unaffected by N(G)-nitro-L-arginine methyl ester (100 microM) and endothelium removal. In conclusion, relaxant responses to adenosine and analogues do not involve adenosine A(1), A(2) or A(3) receptors and are endothelium- and nitric oxide-independent.

Interaction of cyclic AMP modulating agents with levcromakalim in the relaxation of rat isolated mesenteric artery

The effect of cyclic AMP modulating agents on levcromakalim-induced relaxation was investigated in myograph-mounted rat mesenteric arteries. Forskolin (adenylyl cyclase activator), dibutyryl cyclic AMP (protein kinase A activator) and 5'-N-ethylcarboxamidoadenosine (NECA; adenosine receptor agonist) all potentiated the vasorelaxant effects of levcromakalim. The modulatory and relaxant effects of dibutyryl cyclic AMP, NECA and forskolin were sensitive to the protein kinase A inhibitor, Rp-cAMPS. However, relaxation to these three agents was unaffected by the K(ATP) inhibitor, glibenclamide. Dibutyryl cyclic AMP and NECA also caused levcromakalim to induce relaxation in the sub-nanomolar concentration range, however, this effect was Rp-cAMPS- and glibenclamide-insensitive. These results suggest that cyclic AMP modulating agents modulate K(ATP), even though this channel does not contribute to their relaxant effects.

Sympathoinhibition by adenosine A(1) receptors, but not P2 receptors, in the hamster mesenteric arterial bed

The aim of the present study was to determine whether there are prejunctional inhibitory P2 purine receptors on sympathetic nerves in the hamster isolated perfused mesenteric arterial bed. Adenosine 5'-O-(3-thiotriphosphate (ATPgammaS; 10 microM), adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS; 100 microM) and AMP (10 microM) had no significant effect on neurogenic contractions to electrical field stimulation. In contrast, P1 receptor agonists attenuated sympathetic vasoconstriction with a potency order of N(6)5'-(Nadenosine. The pEC(50) value for CPA was 7.5+/-0.1 (n=7). The concentration-inhibitory effect curve to CPA was shifted to the right by the adenosine A(1) receptor antagonist, 8-cyclopentyl-1, 3-dipropyl-xanthine (DPCPX; 10 nM; apparent pK(B) 9.6; n=6-7). In methoxamine raised-tone mesenteries CPA (0.001-10 microM) did not elicit vasorelaxation, and NECA and adenosine were only weak vasorelaxants. These results indicate that adenosine A(1) receptors, but not P2 receptors, inhibit prejunctionally sympathetic neurotransmission in the hamster mesenteric arterial bed.

Sensitization of visceral afferents to bradykinin in rat jejunum in vitro

1. We have investigated the effects of inflammatory mediators on visceral afferent discharge and afferent responses to bradykinin (BK) in rat jejunum using a novel in vitro technique. 2. Prostaglandin E2 (1 microM) augmented responses to BK without affecting basal firing, while histamine (100 microM) and adenosine (100 microM) activated basal discharge and enhanced BK responses. In contrast, 5-HT (100 microM) increased basal discharge without influencing responses to BK. 3. Afferent discharge induced by histamine was inhibited by both H1 (pyrilamine) and H3 (thioperamide) but not H2 (ranitidine) receptor antagonists at 10 microM. In contrast, sensitization to BK induced by histamine was inhibited by ranitidine (10 microM). 4. Afferent discharge induced by adenosine was blocked by the A1 receptor antagonist DPCPX (10 microM) but remained unaffected by A2A receptor blockade with ZM241385 (10 microM). In contrast, sensitization of BK responses by adenosine was unaffected by both antagonists. Basal discharge and BK-induced responses were unaffected by the A3 receptor agonist IB-MECA (1 microM). While involvement of A2B receptors is not excluded, adenosine may activate afferent discharge through A1 receptors, while sensitization to BK could involve a receptor other than A1, A2A or A3, possibly the A2B receptor. 5. Inhibition of cyclo-oxygenase with naproxen (10 microM) prevented sensitization after histamine but not adenosine. 6. Sensitization was mimicked by dibutyryl cAMP. This occurred without changes in basal firing and was unaffected by naproxen. 7. In conclusion, afferent discharge induced by BK is augmented by histamine, adenosine and PGE2, but not by 5-HT. Evidence suggests that sensitization involves separate mechanisms from afferent activation. Sensitization may be mediated by increases in cAMP following direct activation by mediators at the nerve terminal or through indirect pathways such as the release of prostaglandins.

Differences between the vasorelaxant activity of adenosine-receptor agonists on guinea-pig isolated aorta precontracted with noradrenaline or phenylephrine

The relaxant effect of adenosine and 5'-(N-ethylcarboxamido)adenosine (NECA) against alpha-adrenoceptor-mediated contractile tone in guinea-pig isolated aortic rings has been examined to determine if this A2B-receptor-mediated relaxation was dependent upon the contracting agent, and whether the contractions were dependent upon intracellular or extracellular calcium. Relaxation responses were consistently greater for aortic rings pre-contracted with phenylephrine (3x10(-6) M) than for rings pre-contracted with noradrenaline (3x10(-6) M). Maximum inhibition by NECA was significantly greater for phenylephrine-contracted aortae than for noradrenaline-contracted (81.9+/-2.8% compared with 25.0+/-1.5%). These differences persisted in the presence of beta- and alpha2-adrenoceptor blockade and could not, therefore, be attributed to stimulation of these receptors by noradrenaline. The ratio of the contractions obtained before and in the presence of adenosine or NECA was compared with the control ratio obtained before and after vehicle. Experiments were performed both in the presence of normal calcium levels and under calcium-free conditions. In normal-calcium medium, NECA inhibited phenylephrine-induced contractions (test ratio, 76.7+/-3.9%; control ratio, 133.1+/-9.8%) to a greater extent than noradrenaline-induced contractions (108.4+/-4.1 and 123.4+/-4.9%); adenosine similarly inhibited phenylephrine-induced contractions more than those induced by noradrenaline. Under calcium-free conditions, adenosine (36.7+/-11.9 and 110.7+/-26.6%) and NECA (55.2+/-9.1 and 87.1+/-14.9%) were only effective against phenylephrine-induced contractions. This suggests that activation of the A2B-receptor by these agonists inhibited intracellular mobilization of calcium for phenylephrine-induced contractions only. The effects on extracellular calcium influx were examined for phenylephrine- and noradrenaline-induced contractions in normal-calcium medium but in the presence of ryanodine to prevent intracellular calcium mobilization. NECA inhibited phenylephrine-induced contractions (77.3+/-12.4 and 111.4+/-9.3%), presumably by interfering with influx of calcium through receptor-operated calcium channels. In contrast, NECA failed to reduce noradrenaline-induced contractions (121.5+/-10.7 and 122.4+/-11.6%), suggesting that the effect on noradrenaline is predominantly via interaction with intracellular calcium. Adenosine was consistently a more effective relaxant than NECA, possibly because of an additional intracellular component of the response. We conclude that adenosine receptor agonists inhibit phenylephrine-induced contractions of guinea-pig aorta more selectively than noradrenaline-induced contractions. A2B-receptor stimulation might reveal a fundamental difference between the modes of contraction elicited by these two alpha-adrenoceptor agonists.

Effects of adenosine receptor agonists on induction of contractions to phenylephrine of guinea-pig aorta mediated via intra- or extracellular calcium

The vasorelaxant actions of adenosine and its analogue, 5'-(N-ethylcarboxamido)-adenosine (NECA), were investigated in guinea-pig isolated aortic rings by addition to the tissue prior to induction of a contraction by the alpha1-adrenoceptor agonist phenylephrine (PE, 3x10(-6) M). The effect was calculated from the ratio (C2/C1) of the contraction to PE before (C1) and in the presence of adenosine or NECA (C2). This was compared with a control ratio obtained at the same time in which no vasorelaxant was present during C2. Experiments were performed in either "normal" or "Ca2+ -free" bathing medium. Both adenosine and NECA caused inhibition of contractions in "normal" and "Ca2+ -free" conditions, the latter indicating that the vasorelaxant action was due in part to inhibition of intracellular Ca2+ mobilization. To determine whether inhibition of influx of extracellular Ca2+ is a target for the vasorelaxation, contractions to PE were obtained in "normal" Ca2+ and in the presence of ryanodine (10(-5) M), which prevents the release of intracellular Ca2+. These contractions were inhibited by NECA indicating that stimulation of A2-receptors by NECA interferes with the influx of Ca2+ via the opening of receptor-operated Ca2+ channels (ROCs). This study has demonstrated that cell surface A2-receptor stimulation in the guinea-pig aorta inhibits phenylephrine-induced contractions by interfering with both the release of intracellular Ca2+ and the influx of extracellular Ca2+, presumably via ROCs.

Species-dependent hemodynamic effects of adenosine A3-receptor agonists IB-MECA and Cl-IB-MECA

The purpose of this study was to compare the hemodynamic effects of the adenosine A3-receptor agonists N6-(3-iodobenzyl)-9-[5-(methylcarbamoyl)-beta-D-ribofuranosyl]aden ine (IB-MECA) and 2-chloro-N6-(3-iodobenzyl)-9-[5-(methylcarbamoyl)-beta-D-ribofu ranosy l]adenine (Cl-IB-MECA) in isolated rat and rabbit hearts and in the intact, open-chest pig. Isolated hearts perfused with Krebs-Henseleit buffer at a constant pressure (70 mmHg) were treated with 50 nM of either IB-MECA or Cl-IB-MECA. Neither IB-MECA nor Cl-IB-MECA altered ventricular function or heart rate in the isolated rat and rabbit hearts, and neither agent altered coronary flow in the rabbit. However, 2 min of IB-MECA treatment in the isolated rat heart increased coronary flow by 25%, an effect that did not exhibit tachyphylaxis. The IB-MECA-induced coronary dilation was only partially attenuated by the adenosine A3-receptor antagonist MRS-1191 (50 nM). IB-MECA-induced coronary dilation was completely blocked by the adenosine A2a-receptor antagonist 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2, 4-triazolo[1,5-c]pyrimidine (Sch-58261, 50 nM). Cl-IB-MECA (50 nM) did not increase coronary flow in the rat, but 100 nM did increase flow by 18%. In pentobarbital sodium-anesthetized pigs IB-MECA (5 micrograms/kg iv) decreased systemic blood pressure and increased pulmonary artery pressure, effects that did exhibit tachyphylaxis. These results illustrate that adenosine A3-receptor agonists produce species-dependent effects, which in the rat heart appear to be caused by adenosine A2a-receptor activation.

A2B adenosine receptors mediate relaxation of the pig intravesical ureter: adenosine modulation of non adrenergic non cholinergic excitatory neurotransmission

1. The present study was designed to characterize the adenosine receptors involved in the relaxation of the pig intravesical ureter, and to investigate the action of adenosine on the non adrenergic non cholinergic (NANC) excitatory ureteral neurotransmission. 2. In U46619 (10(-7) M)-contracted strips treated with the adenosine uptake inhibitor, nitrobenzylthioinosine (NBTI, 10(-6) M), adenosine and related analogues induced relaxations with the following potency order: 5'-N-ethylcarboxamidoadenosine (NECA) = 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA) = 2-chloroadenosine (2-CA) > adenosine > cyclopentyladenosine (CPA) = N6-(3-iodobenzyl)-adenosine-5'-N-methylcarboxamide (IB-MECA) = 2-[p-(carboxyethyl)-phenylethylamino]-5'-N-ethylcarboxamidoaden os ine (CGS21680). 3. Epithelium removal or incubation with indomethacin (3 x 10(-6) M) and L-N(G)-nitroarginine (L-NOARG, 3 x 10(-5) M), inhibitors of prostanoids and nitric oxide (NO) synthase, respectively, failed to modify the relaxations to adenosine. 4. 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10(-8) M) and 4-(2-[7-amino-2-(2-furyl) [1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385, 3 x 10(-8) M and 10(-7) M), A1 and A2A receptor selective antagonists, respectively, did not modify the relaxations to adenosine or NECA. 8-phenyltheophylline (8-PT, 10(-5) M) and DPCPX (10(-6) M), which block A1/A2-receptors, reduced such relaxations. 5. In strips treated with guanethidine (10(-5) M), atropine (10(-7) M), L-NOARG (3 x 10(-5) M) and indomethacin (3 x 10(-6) M), both electrical field stimulation (EFS, 5 Hz) and exogenous ATP (10(-4) M) induced contractions of preparations. 8-PT (10(-5) M) increased both contractions. DPCPX (10(-8) M), NECA (10(-4) M), CPCA, (10(-4) M) and 2-CA (10(-4) M) did not alter the contractions to EFS. 6. The present results suggest that adenosine relaxes the pig intravesical ureter, independently of prostanoids or NO, through activation of A2B-receptors located in the smooth muscle. This relaxation may modulate the ureteral NANC excitatory neurotransmission through a postsynaptic mechanism.

Tissue-specific effects of in vivo adenosine receptor blockade on glucose uptake in Zucker rats

Previous studies have shown that treatment of obese Zucker rats with the adenosine receptor antagonist 1,3-dipropyl-8-(p-acrylic) phenyl xanthine (BWA1433) improves intraperitoneal glucose tolerance. In this study, a euglycemic hyperinsulinemic clamp was performed on obese (fa/fa) and lean (Fa/fa) Zucker rats that had been treated orally with BWA1433 or vehicle for 1 wk. A constant infusion of [3H]glucose was initiated in fasted animals to measure basal whole body glucose kinetics. No differences in glucose concentration or rates of glucose production/disappearance were observed between lean or obese animals with or without BWA1433. During the euglycemic hyperinsulinemic clamp, whole body glucose disposal in obese Zucker rats was only 22% of that observed in lean animals. BWA1433 treatment increased glucose disposal by 88% in obese Zucker rats. At the end of the clamp, [14C]-2-deoxyglucose was injected to determine tissue-specific differences in glucose uptake. Gastrocnemius, soleus, heart, and liver of untreated obese animals had significantly lower glucose uptake than lean controls under hyperinsulinemic conditions. BWA1433 treatment of obese animals increased glucose uptake in gastrocnemius and soleus muscles by 44 and 47%, respectively. Conversely, BWA1433 treatment decreased glucose uptake in adipose tissue by 54 and 49% in obese and lean Zucker rats, respectively. In summary, BWA1433 improves glucose tolerance by increasing glucose uptake in skeletal muscle while decreasing glucose uptake by adipose tissue. This study suggests that insulin resistance in obese Zucker rats is tissue specific and that signaling from adenosine receptors may be a factor contributing to tissue-specific insulin resistance.