The effects of endogenous adenosine and adenosine receptor agonists were examined on hypoxia-induced myocardial stunning of guinea-pig isolated paced left atria and papillary muscles. Hypoxia (30 minutes) reduced developed tension and increased diastolic tension (contracture) of left atria (41.8 ± 11.5%) and papillary muscles (17.7 ± 6.2%). Developed tension recovered to 80.8 ± 3.15 and 77.2 ± 5.3% 15 minutes after reoxygenation (stunning). Recovery of left atria was unaffected by adenosine deaminase (1 IU mL−1) but was depressed in papillary muscles (15 minutes, 48.6 ± 4.3%) and contracture (46.1 ± 7.5%) increased. Endogenous adenosine therefore protects from ventricular but not atrial stunning. Adenosine receptor agonists were introduced at 10 minutes into hypoxia. CPA (A1 selective, 3 × 10−8 M) impaired left atrial recovery (5 minutes, 38.1 ± 5.0%), through direct negative inotropy, but did not affect papillary muscles. CGS21680 (A2A selective, 3 × 10−7 M) did not affect recovery. APNEA (A1/A3 receptor agonist, 10−8 M), increased recovery rate of left atria. Improved rate and extent of recovery of papillary muscles by APNEA (15 minutes, 94.8 ± 3.1%) was prevented by the A3 receptor antagonist, MRS-1220 (10−7 M). IB-MECA (A3 selective, 3 × 10−7 M) increased atrial recovery rate but not the maximum developed tension reached in either tissue. However, when added at reoxygenation, IB-MECA caused complete recovery of both tissues, in the absence or presence of adenosine deaminase. Thus, A3 receptor stimulation reverses myocardial stunning of isolated atria and papillary muscles.
Myocardial ischemia and subsequent restoration of normal coronary blood flow causes profound changes in contractile function and electrical activity of the heart. 1,2 Although hypoxia plays a role in the changes produced by ischemia, its effects are compounded by acidosis, lactate accumulation, hyperkalemia and substrate deprivation, which also occur during ischemia. 3 Most studies of electrical and contractile changes of the heart in ischemia and reperfusion have used either whole heart or multicellular preparations of cardiac tissue. 4,5 Although these isolated heart preparations accurately reproduce the physiological situation of ischemia and reperfusion, the interpretation of changes in contractile function is complicated by the coronary vasculature. 6,7 In an attempt to overcome these difficulties, nonperfused isolated cardiac muscle models of ischemia and hypoxia have been developed. 8–10 In these, hypoxia or simulated ischemia cause a decrease in cardiac contractile function. On reoxygenation, cardiac contractility remains impaired—a phenomenon termed myocardial stunning. 11
The degree of stunning can be attenuated by activation of adenosine receptors during ischemia via endogenous adenosine, released during ischemia, 12,13 or through exogenous adenosine added during ischemia. 14 Adenosine receptor activation is also implicated in mediating the protective effect of myocardial preconditioning, 15–17 whereby a brief period of ischemia followed by reperfusion protects the heart from a subsequent more prolonged period of ischemia. 18
Hence, adenosine receptor activation, by adenosine or synthetic analogues, has been shown to be protective in many models of ischemia/reperfusion and preconditioning. In most cases, the cardioprotection has been attributed to activation of A115–17,19,20 or A2A receptors. 21,22 More recently, the A3-adenosine receptor has also been implicated in mediating cardioprotection against experimental infarction or contractile dysfunction in isolated hearts pretreated with an A3 receptor agonist. 23–25 The A3 receptor agonist, IB-MECA, has been shown to attenuate stunning and infarction after coronary artery occlusion in conscious rabbits. 7 Preconditioning has been mimicked by pretreatment of human isolated atrial trabeculae with IB-MECA 26 or rabbit isolated hearts with CB-MECA. 27
The purpose of this study was to establish a model of hypoxia-induced stunning in guinea-pig isolated left atria and papillary muscles and to assess the role, if any, played by endogenous adenosine release in this model. Secondly, the possible cardioprotection against myocardial stunning by exogenously applied adenosine receptor agonists was examined. Since all of the previous cardioprotection studies from other laboratories have used pretreatment with the adenosine agonists, in the present study they are introduced during or at the end of hypoxia. This mode of administration is regarded as being more clinically relevant than pretreatment because myocardial infarction and ischemia are rarely foreseen.
From the Division of Pharmacology, Welsh School of Pharmacy, Cardiff University, King Edward VII Avenue, Cathays Park, Cardiff, UK.
Received for publication November 20; 2002; accepted October 30, 2003.
Supported by the Royal Pharmaceutical Society of Great Britain and the British Heart Foundation.
Reprints: Kenneth J. Broadley, DSc, Division of Pharmacology, Welsh School of Pharmacy, Cardiff University, King Edward VII Avenue, Cathays Park, Cardiff CF10 3XF, UK. (e-mail: BroadleyKJ@Cardiff.ac.uk).