Functional role of intravascular coronary endothelial adenosine receptors

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

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.

Targeting adenosine receptors in the development of cardiovascular therapeutics

Adenosine receptor stimulation has negative inotropic and dromotropic actions, reduces cardiac ischemia-reperfusion injury and remodeling, and prevents cardiac arrhythmias. In the vasculature, adenosine modulates vascular tone, reduces infiltration of inflammatory cells and generation of foam cells, and may prevent the development of atherosclerosis as a result. Modulation of insulin sensitivity may further add to the anti-atherosclerotic properties of adenosine signaling. In the kidney, adenosine plays an important role in tubuloglomerular feedback and modulates tubular sodium reabsorption. The challenge is to take advantage of the beneficial actions of adenosine signaling while preventing its potential adverse effects, such as salt retention and sympathoexcitation. Drugs that interfere with adenosine formation and elimination or drugs that allosterically enhance specific adenosine receptors seem to be most promising to meet this challenge.

Adenosine receptor-mediated coronary vascular protection in post-ischemic mouse heart

This study evaluated the ability of A1 and A3 adenosine receptor (AR) agonism, and A1, A2A, A2B and A3AR antagonism (revealing "intrinsic" responses), to modify post-ischemic coronary dysfunction in mouse heart. Vascular function was assessed before and after 20 min global ischemia and 30-45 min reperfusion in Langendorff perfused C57/Bl6 mouse hearts. Ischemic insult impaired coronary sensitivity to the endothelial-dependent dilators ADP (pEC50=6.8+/-0.1 vs. 7.6+/-0.1, non-ischemic) and acetylcholine (pEC50=6.1+/-0.1 vs. 7.3+/-0.1 in non-ischemic), and for the mixed endothelial-dependent/independent dilator 2-chloroadenosine (pEC50=7.5+/-0.1 vs. 8.4+/-0.1, non-ischemic). Endothelium-independent dilation in response to nitroprusside was unaltered (pEC50=7.0+/-0.1 vs. 7.1+/-0.1 in non-ischemic). Pre-treatment with a selective A1AR agonist (50 nM CHA) failed to modify coronary dysfunction, whereas A1AR antagonism (200 nM DPCPX) worsened the effects of I/R (2-chloroadenosine pEC50=6.9+/-0.1). Conversely, A3AR agonism (100 nM Cl-IB-MECA) did reduce effects of I/R (pEC50s=8.0+/-0.1 and 7.3+/-0.1 for 2-chloroadenosine and ADP, respectively), whereas antagonism (100 nM MRS1220) was without effect. While A2AAR agonism could not be assessed (due to pronounced vasodilatation), A2AAR antagonism (100 nM SCH58261) was found to exert no effect, and antagonism of A2BARs (50 nM MRS1754) was also ineffective. The protective actions of A3AR agonism were also manifest as improved reactive hyperemic responses. Interestingly, post-ischemic coronary dysfunction was also limited by: Na+-H+ exchange (NHE) inhibition with 10 or 50 microM BIIB-513 (2-chloroadenosine pEC50s=7.8+/-0.1, either dose), an effect not additive with A3AR agonism; Ca2+ antagonism with 0.3 microM verapamil (2-chloroadenosine pEC50=7.9+/-0.1); and Ca2+ desensitization with 5 mM BDM (2-chloroadenosine pEC50=7.8+/-0.1). In contrast, endothelin antagonism (200 nM PD142893) and anti-oxidant therapy (300 microM MPG+150 U/ml SOD+600 U/ml catalase) were ineffective. Our data collectively confirm that ischemia selectively impairs endothelial function and reactive hyperemia independently of blood cells. Vascular injury is intrinsically limited by endogenous (but not exogenous) activation of A1ARs, whereas exogenous A3AR activation further limits dysfunction (improving post-ischemic vasoregulation). Finally, findings suggest this form of post-ischemic coronary injury is unrelated to endothelin or oxidant stress, but may involve modulation of Ca2+ overload and/or related ionic perturbations.

Role of nitric oxide in the control of coronary resistance in teleosts

In mammals, the in vivo coronary blood flow and myocardial oxygen consumption are closely related via changes in coronary resistance in response to the metabolic demands of the myocardium. A fine neurohumoral regulation of coronary resistance holds true also in fish, and particularly in teleosts, where several vasoconstrictive and vasodilative mechanisms have been described, with numerous putative effectors, including prostanoids, acetylcholine, adrenaline, serotonin, adenosine, steroid hormones. Here, a resume is reported of the available evidence on the involvement of nitric oxide (NO) in the control of coronary resistance in teleosts and particularly in salmonids. Most of the evidence reported is from a comprehensive study performed on a Langedorff-type preparation of the isolated trout heart. Using a physio-pharmacological approach, the experiments performed on this preparation have demonstrated that trout coronary resistance is reduced by l-arginine (NOS substrate), nitroprusside and SNAP (NO donors) and is increased by the NOS inhibitors l-NNA and l-NAME. The vasodilation induced by nitroprusside is blocked by the guanylate cyclase inhibitor methylene blue. l-arginine increases NO release in the perfusate, while l-NNA reduces the release. NO release is inversely related with the coronary resistance. l-NNA inhibits the vasodilatory effects of acetylcholine, serotonin and adenosine. The vasodilation induced by adenosine is accompanied by NO release and involves stretch receptors. Hypoxia induces vasodilation and both adenosine and NO release in the preparation; the NO release under hypoxia is blocked by theophylline. On the whole these data indicate that NO plays a central role in the control of coronary resistance in trout. In particular, a main role for NO as an amplifier of the adenosine-mediated vasodilation under hypoxia can be hypothesized.

A3 adenosine receptor-mediated protection of the ischemic heart

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

Acute adenosinergic cardioprotection in ischemic-reperfused hearts

Cells of the cardiovascular system generate and release purine nucleoside adenosine in increasing quantities when constituent cells are "stressed" or subjected to injurious stimuli. This increased adenosine can interact with surface receptors in myocardial, vascular, fibroblast, and inflammatory cells to modulate cellular function and phenotype. Additionally, adenosine is rapidly reincorporated back into 5'-AMP to maintain the adenine nucleotide pool. Via these receptor-dependent and independent (metabolic) paths, adenosine can substantially modify the acute response to ischemic insult, in addition to generating a more sustained ischemia-tolerant phenotype (preconditioning). However, the molecular basis for acute adenosinergic cardioprotection remains incompletely understood and may well differ from more widely studied preconditioning. Here we review current knowledge and some controversies regarding acute cardioprotection via adenosine and adenosine receptor activation.

Sole activation of three luminal adenosine receptor subtypes in different parts of coronary vasculature

In isolated guinea pig hearts saline perfused at constant flow, adenosine A(1), A(2A), and A(3) (A(x)) agonists covalently bound to a large polymer (Pol; 2,000 kDa) were intracoronarily administered, and three effects were studied: dromotropic, vascular and inotropic. The rank order of potencies were the following: dromotropic (Pol-A(2A)Pol-A(1)>Pol-A(3)) and vascular and inotropic (Pol-A(2A)> or =Pol-A(1)Pol-A(3)), where the rank order of potency for Pol-A(x) depends on the part of the coronary vascular network involved; i.e., there is a vascular heterogeneity. The large size of Pol-A(x) prevents extravascular diffusion and causes it to act solely in the endothelial luminal surface. This implies their cardiac effects are due to endothelial mediators. Inhibition of nitric oxide (NO) and prostaglandin (PG) synthesis with N(G)-nitro-l-arginine methyl ester and indomethacin, respectively, show that the three cardiac effects of Pol-A(1) were mediated by NO and PG, whereas for Pol-A(2A) and Pol-A(3) the mediator was mainly NO but not PG. These results suggest that if Pol-A(x) activated the corresponding endothelial A(x)-receptor subtype, a different mediator would be produced.

The effect of systemic hypoxia on interstitial and blood adenosine, AMP, ADP and ATP in dog skeletal muscle

1. We investigated the effect of moderate systemic hypoxia on the arterial, venous and interstitial concentration of adenosine and adenine nucleotides in the neurally and vascularly isolated, constant-flow perfused gracilis muscles of anaesthetized dogs. 2. Systemic hypoxia reduced arterial PO2 from 129 to 28 mmHg, venous PO2 from 63 to 23 mmHg, arterial pH from 7.43 to 7.36 and venous pH from 7.38 to 7.32. Neither arterial nor venous PCO2 were changed. Arterial perfusion pressure remained at 109 +/- 8 mmHg for the first 5 min of hypoxia, then increased to 131 +/- 11 mmHg by 9 min, and then decreased again throughout the rest of the hypoxic period. 3. Arterial adenosine (427 +/- 98 nM) did not change during hypoxia, but venous adenosine increased from 350 +/- 52 to 518 +/- 107 nM. Interstitial adenosine concentration did not increase (339 +/- 154 nM in normoxia and 262 +/- 97 nM in hypoxia). Neither arterial nor venous nor interstitial concentrations of adenine nucleotides changed significantly in hypoxia. 4. Interstitial adenosine, AMP, ADP and ATP increased from 194 +/- 40, 351 +/- 19, 52 +/- 7 and 113 +/- 36 to 764 +/- 140, 793 +/- 119, 403 +/- 67 and 574 +/- 122 nM, respectively, during 2 Hz muscle contractions. 5. Adenosine, AMP, ADP and ATP infused into the arterial blood did not elevate the interstitial concentration until the arterial concentration exceeded 10 microM. 6. We conclude that the increased adenosine in skeletal muscle during systemic hypoxia is formed by the vascular tissue or the blood cells, and that adenosine is formed intracellularly by these tissues. On the other hand, adenosine formation takes place extracellularly in the interstitial space during muscle contractions.

Effects of arginine vasopressin in the heart are mediated by specific intravascular endothelial receptors

Arginine vasopressin induces vascular, inotropic and arrhythmogenic effects in the heart. Existing evidence, obtained indirectly, suggests that these effects occur through paracrine endothelial mechanisms. To demonstrate this, vasopressin was confined to the intravascular space by covalent coupling to high molecular weight (2x10(6) Da, vasopresin-dextran) dextran. Isolated guinea pig hearts were infused with equivalent concentrations of vasopressin and vasopressin-dextran. The negative inotropic and coronary vasopressor effects of vasopressin-dextran were similar to those evoked by vasopressin; in both cases effects were reversible. Free dextran had no effect on vascular resistance nor in ventricular developed pressure. The inotropic and vascular effects of both vasopressin and vasopressin-dextran were blocked by the vasopressin receptor antagonist [Adamantaneacetyl(1), o-Et-D-Tyr(2), Val(4), Aminobutyryl(6), Arg(8,9)]vasopressin (Adam-vasopressin), indicating that the effects of the two agonists were vasopressin receptor-mediated. To elucidate possible endothelial intermediaries of these effects, isolated guinea pig hearts were infused simultaneously with vasopressin or vasopressin-dextran and several inhibitors either of synthesis or blockers of receptors of possible endothelial mediators. Only reactive blue 2, a P(2y) purinoceptor antagonist, and suramin, a P(2y) and a P(2x) purinoceptor antagonist, caused a total reversal of vascular and inotropic effects of vasopressin and vasopressin-dextran. Pyridoxalphosphate-6-Azophenyl-2'-4'disulphonic acid, a P(2x) purinoceptor antagonist, was without effect. Our results provide direct evidence that the short-term cardiac effects of vasopressin are due to selective activation of intravascular purinoceptors and suggest that an intermediary of these effects is ATP.

Acute and nongenomic effects of testosterone on isolated and perfused rat heart

Gonadal steroid hormones influence vascular tone and the development of hypertension. There are sex differences in the incidence of cardiovascular diseases, and great attention has been placed on the study of estrogen cardiovascular effects. However, there are only a few reports on the effects of testosterone on the vasculature. It is commonly accepted that the mechanism of the action of steroid hormones on target tissues is mediated through the binding of hormones to cytoplasmic or nuclear receptors. However, some studies indicate that steroid action can be extremely rapid and therefore unlikely to be through a genomic mechanism. The purpose of this study was to assess the effect of intravascularly confined testosterone on an isolated rat heart to demonstrate acute and possibly nongenomic effects of the steroid. Our results show that testosterone blocked the adenosine vasodilator effect and increased vascular resistance, even when its presence was restricted to the coronary vascular lumen. These effects were exerted rapidly and possibly through nongenomic mechanisms.

Intravascular adenosine: the endothelial mediators of its negative dromotropic effects

Intravascular adenosine may exert its negative dromotropic effect via activation of luminal coronary endothelial receptors, which suggests the presence of transcellular dromotropic mediators of endothelial origin, perhaps nitric oxide (NO) and prostaglandins. We decided to test this hypothesis in isolated guinea pig hearts retrogradely perfused with Krebs-Henseleit solution. A pair of stimulating electrodes were placed in the right atria and the auricular-ventricular (A-V) delay recorded by means of a recording electrode placed on the left atria and an electrode placed on the tip of the ventricle. Hearts were paced at a rate of 3.8 +/- 0.2 Hz and perfused at a coronary flow of 9 +/- 0.25 ml/min. To obtain dose-response curves, single doses (as boluses) of different concentrations of adenosine were infused and the maximal increase in A-V delay induced by each dose was determined. Agents that inhibit NO accumulation, such as N(G)-nitro-L-arginine methyl ester (L-NAME) and oxyhemoglobin, diminished the effect of adenosine while NO-sparing agents, such as superoxide dismutase and dithiotreitol, enhanced the adenosine effect. Infusion of NO and the NO donor morpholinosydnonimine increased the A-V delay in a dose-dependent manner. In addition, the dose-response curve for adenosine was displaced downward and to the right by indomethacin, indicating also the involvement of prostaglandins. Infusion of L-NAME in addition to indomethacin further diminished the effects of adenosine, indicating that NO and prostaglandins acted simultaneously. To selectively activate intravascular endothelial adenosine receptors, adenosine amino congener (ADAC), an adenosine A1 receptor agonist, was covalently coupled to 2 X 10(6) Da dextran. When intracoronarily infused, the dextran-ADAC complex remains in the blood vessel lumen because it is too large to diffuse to the interstitium. On intracoronary administration, the dextran-ADAC complex caused a negative dromotropic effect which was diminished by L-NAME and indomethacin. Our data indicate that the dromotropic effect caused by intracoronarily administered adenosine is the result solely of activation of intravascular endothelial adenosine receptors, possibly type A , and that NO and prostaglandins are synergistic endothelial mediators of this effect.

Endothelium-mediated negative dromotropic effects of intravascular acetylcholine

Acetylcholine acting through specific muscarinic membrane receptors causes a negative dromotropic effect and, in blood vessels, causes a vasodilation which results from its action on the endothelial cells via release of nitric oxide (NO). We decided to study this effect in isolated Krebs-Henseleit retrogradely perfused guinea pig hearts. A pair of stimulating electrodes was placed in the right atrium and to record the auricular-ventricular interval (A-V delay) one recording electrode was placed on the left atrium and the other on the tip of the ventricle. Hearts were paced at a rate of 3.8+/-0.1 Hz and perfused at a coronary flow rate of 9+/-0.25 ml/min. To obtain dose-response curves, single doses (as boluses) of acetylcholine were infused and the maximal A-V delay induced by each dose was determined. Perfusion of agents that inhibit NO accumulation (L-Arginine methyl ester (L-NAME) (0.5 mM)) or oxyhemoglobin (6 microM) caused displacement of the acetylcholine dose-response curve downward and to the right. Perfusion of NO-sparing agents like superoxide dismutase and dithiothreitol caused an upward and leftward displacement. Infusion of NO solutions or a NO donor (diethylamine-nitric oxide [DEA-NO]) caused a dose-dependent negative dromotropic effect. In contrast, inhibition of the prostaglandin metabolic pathway by Indomethacin (0.01 mM) caused potentiation of acetylcholine effects which were reversed when it was co-perfused with L-NAME. When endothelial intravascular muscarinic receptors were selectively blocked by perfusion of a non-permeable macromolecule: dextran ( > 2000 kDa) covalently complexed to the receptor blocker (3-(2'-aminobenzhydryloxy) tropane)), the negative dromotropic effect of intravascular acetylcholine was diminished in a concentration-dependent manner up to complete blockade. Our data indicate that the dromotropic effect caused by intracoronary administration of acetylcholine is the result solely of activation of intravascular endothelial muscarinic receptors, that nitric oxide and prostaglandins are non-synergistic endothelial mediators of this effect and that there may be an interaction between NO and prostaglandin metabolic pathways.

Cardioprotection with a novel adenosine regulating agent mediated by intravascular adenosine

Adenosine is cardioprotective in models of myocardial stunning and infarction, but the precise compartment within the heart in which adenosine elicits its cardioprotective effects has not been determined. The goals of the present study were to (i) investigate the effects of a novel adenosine regulating agent, GP531 (5-amino-1-beta-n-(5-benzylamino-5-deoxyribofuranosyl) imidazole-4-carboxamide), on post-ischemic myocardial function, and (ii) examine the contribution of endogenous adenosine in the intravascular and interstitial compartments in mediating the beneficial effects. Pigs were instrumented for measurement of myocardial segment shortening, and for sampling of coronary venous blood and myocardial interstitial fluid for determination of adenosine concentration. Myocardial dysfunction was induced by 4 x 8 min coronary occlusions, and recovery of regional function was monitored for 2 h. In control pigs, function recovered to 24 +/- 2% of baseline after 2 h. Treatment with GP531 improved functional recovery to 55 +/- 3%. GP531-mediated cardioprotection was prevented by adenosine receptor blockade with 8-sulfophenyltheophylline (23 +/- 2%). GP531 did not affect basal adenosine levels, but caused a 2-fold greater increase in vascular adenosine concentration with ischemia (54.6 +/- 10.6 vs. 28.1 +/- 8.0 microM in controls. P < 0.05). In contrast, the interstitial adenosine concentration was not significantly different in treated vs. untreated control pigs (9.4 +/- 3.9 vs. 15.0 +/- 1.8 microM in controls). These data indicate that (1) GP531 improves recovery of myocardial function following ischemia reperfusion injury via an adenosine receptor-dependent mechanism, and (2) the cardioprotection is associated with increased intravascular, but not interstitial, adenosine concentration during ischemia. Therefore, we conclude that cardioprotection elicited by GP531-enhanced endogenous adenosine is dependent on an intravascular site of action.

Endothelial release of nitric oxide contributes to the vasodilator effect of adenosine in humans

BACKGROUND

The endogenous nucleoside adenosine plays an important role in the regulation of vascular tone, especially during ischemia. Experimental data derived from animal models suggest that nitric oxide (NO) contributes to the vasodilator effect of adenosine. The primary purpose of this investigation was to determine whether the endothelial release of NO contributes to adenosine-induced vasodilation in humans.

METHODS AND RESULTS

Venous occlusion plethysmography was used to assess the forearm blood flow (FBF) responses to graded intra-arterial infusions of adenosine (1.5 to 500 micrograms/min). Dose-response curves were constructed before and during intra-arterial infusion of the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) (2 mg/min, n = 6) or vehicle (n = 6). Before infusion of L-NMMA, adenosine caused a dose-dependent increase in FBF from 2.3 to 15.9 mL.min-1.dL-1. During concurrent infusion of L-NMMA, adenosine increased FBF from 1.7 to 10.0 mL.min-1.dL-1, and this change from baseline was significantly reduced compared with that before L-NMMA (P < .05). L-NMMA also attenuated the FBF response to adenosine when the basal constrictor effect of L-NMMA was prevented by coinfusion of the NO donor sodium nitroprusside (n = 6, P < .01). In contrast, L-NMMA did not affect the FBF response to intra-arterial infusion of the endothelium-independent vasodilator verapamil (from 2.0 to 13.9 mL.min-1.dL-1 before L-NMMA and from 1.3 to 13.6 mL.min-1.dL-1 during L-NMMA; n = 6, P = NS). The second objective of this study was to determine whether the adenosine-induced release of NO is mediated by activation of endothelial potassium channels, putatively coupled to adenosine receptors. Thus, the FBF response to adenosine was measured before and during infusion of the ATP-dependent potassium channel blocker tolbutamide (1 mg/min, n = 6), or the potassium channel blocker quinidine (0.5 mg/min, n = 6). The adenosine-mediated increments in FBF were not attenuated by either potassium channel blocker.

CONCLUSIONS

Adenosine-induced vasodilation in humans is mediated, at least in part, by endothelial release of NO. The transducing mechanism of this phenomenon is not known, but it does not appear to involve the activation of either ATP-dependent or quinidine-sensitive potassium channels.

Harnessing nature's own cardiac defense mechanism with acadesine, an adenosine regulating agent: importance of the endothelium

Although the effects of adenosine on the heart, including the clinical suppression of cardiac arrhythmias, have been recognized for more than half a century, it is only in the last decade that the therapeutic potential of adenosine has been recognized. Research related to the clinical application of adenosine has concentrated on two areas. The first came directly from early observations about the use of adenosine in treating cardiac arrhythmias, in particular supraventricular tachycardias. The second relates to the use of adenosine to protect the heart from the deleterious consequences of myocardial ischemia and reperfusion. This review will focus on the latter cardioprotective properties of adenosine, particularly those shown by a novel group of drugs termed adenosine regulating agents, the prototype of which is acadesine (Protara).

Rapid turnover of the AMP-adenosine metabolic cycle in the guinea pig heart

The intracellular flux rate through adenosine kinase (adenosine-->AMP) in the well-oxygenated heart was investigated, and the relation of the AMP-adenosine metabolic cycle (AMP<-->adenosine) to transmethylation (S-adenosylhomocysteine [SAH]-->adenosine) and coronary flow was determined. Adenosine kinase was blocked in isolated guinea pig hearts by infusion of iodotubercidin in the presence of the adenosine deaminase blocker erythro-9-(2-hydroxy-3-nonyl)adenine (5 mumol/L). Iodotubercidin (1 nmol/L to 4 mumol/L) caused graded increases in venous effluent concentrations of adenosine, from 8 +/- 3 to 145 +/- 32 nmol/L (mean +/- SEM, n = 3), and in coronary flow, which increased to maximal levels. Flow increases were completely abolished by adenosine deaminase (5 to 10 U/mL). Interstitial adenosine concentrations, estimated using a mathematical model, increased from 22 nmol/L during control conditions to 420 nmol/L during maximal vasodilation. The possibility that iodotubercidin caused increased venous adenosine by interfering with myocardial energy metabolism was ruled out in separate 31P nuclear magnetic resonance experiments. To estimate total normoxic myocardial production of adenosine (AMP-->adenosine<--SAH), the time course of coronary venous adenosine release was measured during maximal inhibition of adenosine kinase with 30 mumol/L iodotubercidin. Adenosine release increased more than 15-fold over baseline, reaching a new steady-state value of 3.4 +/- 0.3 nmol.min-1 x g-1 (n = 5) after 4 minutes. In parallel experiments, the relative roles of AMP hydrolysis and transmethylation (SAH hydrolysis) were determined, using adenosine dialdehyde (10 mumol/L) to block SAH hydrolase. In these experiments, adenosine release increased to similar levels of 3.4 +/- 0.5 nmol.min-1 x g-1 (n = 6) during inhibition of adenosine deaminase and adenosine kinase. It is concluded that (1) maximal increases in coronary flow are elicited by increases in interstitial adenosine concentration to approximately 400 nmol/L, (2) more than 90% of the adenosine produced in the heart is normally rephosphorylated to AMP without escaping into the venous effluent, (3) AMP hydrolysis is the dominant pathway for cardiac adenosine production under normoxic conditions, and (4) the high rate of adenosine salvage is due to rapid turnover of a metabolic cycle between AMP and adenosine. Rapid cycling may serve to amplify the relative importance of AMP hydrolysis over transmethylation in controlling cytosolic adenosine concentrations.

A2-purinoceptor-mediated relaxation in the guinea-pig coronary vasculature: a role for nitric oxide

1. The Langendorff heart preparation was used to investigate the mechanism of action of the endothelium-dependent vasodilatation evoked by adenosine and its analogues in the guinea-pig coronary vasculature. 2. The relative order of potency of adenosine and its analogues in causing a reduction in perfusion pressure was D-5'-(N-ethylcarboxamide)adenosine (NECA) = 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N- ethylcarboxamidoadenosine (CGS 21680)> R-N6-(2-phenylisopropyl)adenosine (R-PIA) = adenosine = 2-chloroadenosine (2-CA) > S-N6-(2-phenylisopropyl)adenosine (S-PIA) = N6-cyclopentyl-adenosine (CPA); thus suggesting the presence of A2-purinoceptors in this preparation. 3. 8-(p-Sulphophenyl)theophylline (8-PSPT; 3 x 10(-5) M) significantly reduced both the maximum amplitude and area of the vasodilatation produced in response to adenosine (5 x 10(-10) -5 x 10(-8) mol) without having any effect on the response to the P2-purinoceptor agonist, 2-methylthioATP. The relaxation induced by adenosine (5 x 10(-12) -5 x 10(-8) mol) was unaffected by the selective A1-purinoceptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 10(-8) M). This antagonist profile suggests that only A2-purinoceptors are present in the guinea-pig coronary vasculature. 4. The areas of the vasodilator response to adenosine (5 x 10(-10) -5 x 10(-7 mol), NECA (5 x 10(-12) -5 x 10(-7) mol) and CGS 21680 (5 x 10(-12) -5 x 10(-10) mol) were significantly reduced by NG-nitro-L-arginine methyl ester (L-NAME; 3 x 10(-5) M). The amplitude of the responses to low concentrations of adenosine (5 x 10-10-5 x 10-9mol), NECA (5 x 1011 mol) and CGS 21680 (5 x 1011-5 x 10-9mol)were significantly reduced by L-NAME (3 x 10-5 M).5. L-Arginine (1.5 x 10-3 M) significantly reversed the inhibition, by L-NAME (3 x 10-5 M), of the relaxant response to adenosine (5 x 10-8 mol), NECA (5 x I0- mol) and CGS 21680 (5 x 10-11 mol).6. Indomethacin (10-6 M) did not inhibit the response to adenosine, except at low doses (5 x 10-11-5 x 10-10 mol).7. It is concluded that in the guinea-pig coronary vasculature, while a major part of the vasodilator action of adenosine is probably directly via A2-receptors on the smooth muscle, activation of a subpopulation of A2-purinoceptors on endothelial cells by adenosine and its analogues induces relaxation via production of nitric oxide; prostanoids appear to play a minimal role in the relaxation induced by adenosine as in most other preparations.

Myocardial adenosine stimulates release of cyclic adenosine monophosphate from capillary endothelial cells in guinea pig heart

Activation of coronary endothelial cell adenylate cyclase was studied in the isolated guinea pig heart by prelabelling endothelial adenine nucleotides using intracoronary infusion of [3H]-adenosine, and measuring the coronary efflux of [3H]-cyclic adenosine monophosphate (cAMP). Hypoxia (30% O2) caused a 4-fold increase in coronary release of [3H]-cAMP, which was decreased by 63% by infusion of the adenosine receptor antagonist, theophylline (50 microM). During normoxic control conditions, degrading adenosine to non-vasoactive inosine by intracoronary infusion of adenosine deaminase (1.7 U/ml) caused a 20% decrease in the release of [3H]-cAMP. The effect of adenosine deaminase was reversed by a specific enzyme inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride. Coronary efflux of [3H]-cAMP during intracoronary infusion of 1 microM adenosine triphosphate (ATP), adenosine diphosphate or adenosine monophosphate (AMP) (plus adenosine deaminase 8 U/ml) was only 13% of that due to 1 microM adenosine. Adenosine receptor blockers theophylline and CGS 15943A caused equivalent inhibition of the coronary vasodilator actions of adenosine and ATP. Intracoronary infusion of prostaglandin E1 and the beta 2-adrenergic agonist procaterol caused parallel, dose-dependent increases in coronary conductance and the venous release of [3H]-cAMP. It is concluded that (1) under both normoxic and hypoxic conditions, adenosine formed by the heart may activate endothelial cell adenylate cyclase via membrane adenosine receptors, (2) coronary receptors for adenosine and ATP share common ligand affinities but ATP receptors are not coupled to adenylate cyclase, and (3) other vasodilators known to activate endothelial adenylate cyclase in vitro cause parallel increases in coronary conductance and adenylate cyclase activity in the beating heart.