Adenosine and Myocardial Ischemia <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

李成辉  主任医师  医学博士
中国中日友好医院麻醉科, 北京 100029
E-mail地址：chenghui_li@sina.com 
Chenghui Li, MD，PhD
Department of Anesthesiology, China Japan Friendship Hospital, Beijing 100029
E-mail address: chenghui_li@sina.com

ABSTRACT

  The cardiovascular effects of adenosine have been described in this article. Briefly adenosine shows the cardiac effects on the following aspects: (1)Adenosine can cause does-related, potent coronary vasodilatation, and this function is especially evident during myocardial ischemia. (2)Adenosine contributes to preconditioning via adenosine A1 and A3 receptor-mediated mechanism. (3)As an endogenous protective agent, adenosine plays an important role in attenuation of myocardial ischemia and reperfusion injury. (4)Adenosine is an endogenous myocardial anti-arrhythmic substance. (5)Adenosine can inhibit neutrophil activation, chemotaxis, and the expression of adhesion molecule, thereby reduces neutrophil adherence to endothelial cells. Adenosine, as a low molecular weight and diffusible metabolites accumulated pathologically during myocardial ischemia, could easily release into the coronary sinus blood, therefore adenosine could give insight into the metabolic state of the heart, and may be regarded as a sensitive predictor for myocardial ischemia.  
   Key words: Adenosine; Myocardial ischemia; Reperfusion injury

Cardiac adenosine production is closely related to adequate tissue oxygenation. The imbalance in the supply-to-demand ratio of oxygen is the major trigger mechanism for enhanced formation of adenosine by the heart.

1. The formation, degradation and transportation of adenosine
  The production of adenosine within tissues occurs via two pathways from independent precursors, adenosine monophosphate (AMP) and S-adenosylhomocysteine.
  (1) Pathway of dephosphorylation of AMP: Adenosine can be formed intracellularly and extracellularly by the dephosphorylation of AMP. AMP is catalyzed intracellularly to adenosine by a cytosolic 5'-nucleotidase. During periods of increased metabolic activity and high turnover of ATP, greater amounts of adenosine will be formed.
  Adenosine may also be produced from the dephosphorylation of AMP by an extracellular 5'-nucleotidase released by exocytosis with neurotransmitters from nerve endings, or derived from vascular endothelium and platelets.
  (2) Pathway of S -adenosylhomocysteine. The second major pathway involved in the production of adenosine is through the breakdown of S-adenosylhomocysteine to adenosine and homocysteine, which is catalyzed by the enzyme S-adenosylhomocysteine hydrolase.
  The contribution of each pathway to the total adenosine pool is variable, depending on tissue type and condition of the tissue. For example, under normal conditions, the greatest contribution to the adenosine pool is by the action of S - adenosylhomocysteine hydrolase, only a small amount of adenosine is formed by the catalytic degradation of AMP. However, in the condition of increased metabolism larger amounts of adenosine will be produced through the pathway of AMP degradation.
  The metabolic degradation of adenosine involves intracellular enzymatic pathways. Extracellular adenosine must first be taken up by the cells. Adenosine may then enter the nucleotide pool by phosphorylation via adenosine kinase to AMP. Another pathway of removal is through the action of the enzyme adenosine deaminase, which catalyzes the deamination of adenosine to inosine. Inosine is then further metabolized to hypoxanthine, xanthine and ultimately, uric acid.
  Adenosine uptake is achieved by simple and facilitated diffusion via a bi-directional nucleoside transportation mechanism. Once adenosine is released by the heart, it is rapidly removed from the vascular space by endothelial uptake and is incorporated by adenosine kinase into the adenine nucleotides, or it is deaminated by adenosine deaminase to inosine.  In addition, red blood cells are also a major cause of adenosine removal from circulating blood.

2. Cardiac adenosine receptors
The cardiovascular effects of adenosine are mediated by specific receptors located on the surface of cell membranes. On the basis of physiological, pharmacological studies and molecular cloning, different adenosine receptors have been identified and classified as A1, A2 (may be subdivided as A2A and A2B), and A3. These adenosine receptors are members of the G-protein-coupled receptor family, and are responsible for multiple cardiovascular effects and inflammation processes ^[1].
Adenosine  A1 receptors are major found in the cardiomyocytes mediating the sinus slowing and atrioventricular blocking action of adenosine. Via A1-receptor, adenosine directly decreases the atrial action potential duration and the automaticity of the sinoatrial node, Other cardiac pacemakers are also suppressed. Furthermore, adenosine also depresses conduction through the AV node and prolongs the refraction period. By these mechanisms, adenosine transiently interrupts cardiac impulse propagation.
　　Adenosine  A1 receptors are major found in the cardiomyocytes mediating the sinus slowing and atrioventricular blocking action of adenosine. Via A1-receptor, adenosine directly decreases the atrial action potential duration and the automaticity of the sinoatrial node, Other cardiac pacemakers are also suppressed. Furthermore, adenosine also depresses conduction through the AV node and prolongs the refraction period. By these mechanisms, adenosine transiently interrupts cardiac impulse propagation.
　　Adenosine A2receptors are major present in endothelial and vascular smooth muscle cells and cause coronary vasodilatation. Both A1 and A2 receptor subtypes are responsible for the inhibition or stimulation of adenylate cyclase ^[1]. The production of intracellular cyclic AMP (cAMP) is also regulated by A1 and A2 receptor subtypes, respectively.
　　Adenosine A3 receptors have been found major in cardiomyocytes ^[2], and the activation of A3 receptors protects the heart cells against injury during a subsequent exposure to ischemia. The protection mediated by A3 receptors exhibits a significantly longer duration than that produced by A1 receptor. 
 　　Activation of A3 receptors also causes the release of inflammatory mediators such as histamine from mast cells. Liang and Jacobson^[3] have demonstrated that activation of both A1 and A3 receptors is required to mediate the cardioprotective effect of brief ischemia, and cardiac adenosine A3 receptor mediates a sustained cardioprotective function. Based on these evidences, highly selective A3 adenosine receptor agonists have been investigated and shown to possess cardioprotective effects. It is undoubtedly that adenosine A3 receptors represent a new cardiac therapeutic target^[4].<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

3. Adenosine receptor-mediated cardiac effects
　　(1) Effects on vasodilatation
　　Adenosine causes does-related and potent coronary vasodilatation. This function is mediated initially by A1 receptors. It is shown that the activation of A2 or A2A receptors plays a major role in coronary vasodilatation^[5,6].
　　Meanwhile, adenosine can also cause vasodilatation indirectly via release of endothelium-derived relaxing factor or prostaglandin, or by endothelial release of NO.
　　Adenosine can also directly stimulate afferent nerves, including chemoreceptors, to cause an increase in sympathetic tone, and such mechanism of vascular regulation is especially evident during myocardial ischemia.
　　(2) Effects on preconditioning
　　The cardiac preconditioning is adenosine receptor-mediated and exhibits significant beneficial effects on myocardium^[7]. Studies in vitro suggest that the subtypes of adenosine receptors relevant to preconditioning against infarction are A1 and A3^[8,9]. In a rabbit Langendorff model of myocardial ischemia-reperfusion injury, selective A1 or A3 receptor agonists can both pharmacologically mimic ischemic preconditioning. An important step for adenosine receptor activation is protein kinase C (PKC), which facilitates the opening of ATP-sensitive potassium (K-ATP) channels^[10], probably leading to the enhancement of myocardial tolerance. The increase in interstitial adenosine during preconditioning is thought to be derived primarily from hydrolysis of 5'-AMP in the myocytes by cytosolic 5'-nucleotidase.

    (3) Effects on myocardial ischemia, infarction and reperfusion injury
    There are strong evidences^[11,12,] that adenosine serves as a protective agent endogenously formed during ischemia and reperfusion injury. Adenosine has direct actions to attenuate the degree of ischemia, to reduce the size of infarction, and to enhance the functional recovery of postischemic and reperfused myocardium. Both exogenously administered adenosine and endogenously produced adenosine can reduce infarct size by A1, A2 or A3 receptor activation. ^[8,13].
    Increased intracellular adenosine is considered as an endogenous protective agent against reperfusion injury and stunning heart. Adenosine, given to the cardioplegic solution, can reduce myocardial ischemic injury and ameliorate the postischemic recovery of cardiac function. It has been demonstrated that intra-coronary adenosine administration during reperfusion can reduce infarct size, increase regional myocardial blood flow and ventricular function. Thourani et al^[13] have compared the effects in reducing postischemic dysfunction between adenosine-supplemented blood cardioplegia and adenosine administered during reperfusion. They found that adenosine-supplemented blood cardioplegia reduces infarct size and preserves postischemic regional systolic and diastolic functions. If adenosine was administered during reperfusion period, it could even attenuate the coronary artery endothelial dysfunction^[13]. Furthermore, Cain et al^[14] have found in an experimental study that adenosine pretreatment can decrease myocardial tumor necrosis factor alpha (TNF- α) production, thereby improving human myocardial functions. In a phase II clinical trial with 253 patients undergoing CABG surgery, it was confirmed that adenosine treatment is safe and well tolerated, and may be associated with fewer postoperative complications^[15]. These effects of adenosine are based on the following mechanisms: a. decrease of neutrophil infiltration and capillary plugging. Adenosine can reduce polymorph nuclear leucocytes (PMNL) - induced coronary endothelial injury by A2 receptor mediated mechanism. b. Attenuation of free radical production by neutrophils concomitant with a decrease in intracellular calcium ion; c. inhibition of leukocyte adherence to vascular endothelium during reperfusion, and d. preservation of vascular endothelial function and structure.
    (4) Effects on anti-arrhythmic properties
    Adenosine is an endogenous myocardial anti-arrhythmic substance and can be used for the treatment of ischemia-induced arrhythmia. This effect is generally mediated via A1 receptors. Schreieck and Richardt^[12] have investigated the anti-arrhythmic effect of endogenous adenosine on ischemia-induced ventricular fibrillation in isolated rat hearts, and found that endogenous adenosine in acute ischemic myocardium is accumulated to a level which effectively decreases the occurrence of ventricular fibrillation via an A2 receptor activation. Furthermore, when adenosine accumulates in catecholamine-stimulated myocardium to a high level, it can effectively suppress the occurrence of ventricular arrhythmia^[16].
    (5) Effects on neutrophil action and adhesion molecule expression
    Adenosine can inhibit neutrophil activation via A1 and A2 receptor stimulation, thereby reducing neutrophil adherence to endothelial cells. Adenosine can reduce post-ischemic leukocyte-endothelium interaction via A2 receptor. In a canine model of ischemia and reperfusion, it was found that adenosine A2 receptor activation reduces reperfusion injury by inhibiting neutrophils accumulation, superoxide generation and coronary endothelial adherence^[17], suggesting a protective role of endogenous adenosine during ischemia / reperfusion process. Forman et al^[18] have demonstrated that the blockade of A1 adenosine receptors attenuates myocardial ischemic/reperfusion injury, possibly in part by decreasing the chemo-attractant response of neutrophils.
    Adenosine can dose-dependently inhibit the expressions of E-selectin and vascular cell adhesion molecule 1 (VCAM-1), implying that the vascular endothelium constitutes an important target for the anti-inflammatory actions of adenosine. Furthermore, adenosine also interferes with the L-selectin independent carbohydrate binding and the adhesion function of adhesion molecule CD11a / CD18. Seligmann et al^[19] have found in an experimental study that endogenous adenosine can prevent platelet adhesion and improve myocardial function during reperfusion period. This action is mediated by both A1 and A2 receptor subtypes. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

4. Adenosine release as an indicator for myocardial ischemia
    During myocardial ischemia, ADP, phosphate, and lactate level increase and also release into the coronary venous blood. However, the release of these large molecular weight intracellular components is a relatively late phenomenon that implies irreversible cell membrane damage. So these substauces cannot be taken as a sensitive ischemic predictor. Earlier evidences of ischemia can be provided only by leakage of diffusible metabolites, such as adenosine, that accumulate in cells and then enter the extracellular spaces. Because adenosine can easily pass through the cell membrane, so the pathologic increase in plasma adenosine could give insight into the metabolic state of the heart. Sollevi et al^[20]  found that patients with ischemic heart disease usually demonstrated higher arterial levels of adenosine. Bardenheuer et a^l[21]  found that adenosine concentration of coronary sinus blood increased in patients with stable angina when brief periods of cardiac ischemia from 30 to 90 sec were induced during coronary angioplasty, and could be used as a sensitive indicator for myocardial ischemia.
    Experimental studies with radioactive adenosine have shown that majority of the nucleoside released by the ischemic heart primarily originates from the myocytes^[22]. This conclusion is supported by the fact that cardiac myocytes contain 95% of the total myocardial adenine nucleotides and is 10-fold more sensitive towards oxygen deficiency than endothelial cells^[23].
    In summary, the multiple actions of adenosine play an important role for the regulation of cardiovascular system, especially during the period of myocardial ischemia and reperfusion injury. Because of the strategic location between the cardiac vascular and interstitial space the adenosine possesses an important regulatory function for the local homeostasis of plasma adenosine, and may be regarded as a myocardial predictor.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

References
1. Baraldi PG, et al. Med Res Rev, 2000; 20: 103-128
2. Dougherty C et al. FASEB J, 1998; 12: 1785-1792
3. Liang BT, et al. Proc Natl Acad Sci USA, 1998; 95: 6995-6999
4. Von Lubitz DK, et al. Eur J Pharmacol, 1999; 369: 313-317
5. Shryock JC, et al.  Circulation, 1998; 98: 711-7188
6. Belardinelli L, et al. J Pharmacol Exp Ther, 1998; 284: 1066-1073
7. Miura T, et al. Clin Exp Pharmacol Physiol, 1999; 26: 92-99
8. Hill RJ, et al. J Mol Cell Cardiol, 1998; 30: 579-585
9. Pagliaro P, et al. Life Sci, 1999; 64: 1071-1078
10. Mei DA, et al. J Mol Cell Cardiol, 1998; 30: 1225-1236
11. McCully JD, et al. Ann Thorac Surg, 1999; 67: 699-704
12. Schreieck J, et al. J Mol Cell Cardiol, 1999; 31: 123-134
13. Thourani VH, et al. Circulation, 1999; 100: II376-383
14. Cain BS, et al. J Surg Res, 1998; 76: 117-123
15. Mentzer RM Jr, et al. Ann Surg, 1999; 229: 643-649
16. Gorge B, et al. Basic Res Cardiol, 1998; 93: 264-268
17. Jordan JE, et al. J Pharmacol Exp Ther, 1997; 280: 301-309
18. Forman MB, et al. J Pharmacol Exp Ther, 2000; 292: 929-938
19. Seligmann C, et al. J Cardiovasc Pharmacol, 1998; 32: 156-163
20. Sollevi A, et al. Acta Physiol Scand, 1984; 121: 165-171
21. Schrader J, et al. Pflugers Arch, 1976; 367: 129-135
22. Bardenheuer HJ, et al. Cardiovasc Res, 1994; 28: 656-662
23. Mertens S, et al. Z Kakdiol, 1989; 78 (suppl 1): 227

    李成辉，卫生部中日友好医院麻醉科主任医师, 科副主任。先后获得同济医科大学（现为华中科技大学同济医学院）和德国海德堡大学两个医学博士学位。在临床麻醉中，尤其关注新的发展方向和动态，近年曾专程赴德学习心脏移植和肝移植的麻醉。对科研始终保持浓厚兴趣，先后承担德国巴登-符腾堡州青年科学家基金课题、教育部留学回国人员启动基金课题、卫生部青年基金课题、国家自然科学基金课题等，研究重点为缺血与再灌注损伤。已发表论著二十多篇。现为本刊编委。