[Receptor mediated effects of adenosine and caffeine]

Adenosine consists of one ribose and one purine moiety and binds to specific receptors on cell membranes. The receptors are coupled to G-proteins and additionally to various effector-systems. When a mismatch occurs between energy supply and energy demand, adenosine is produced by the catabolism of adenosine triphosphate. The metabolism of an organ is thereby coupled to the local blood supply (metabolic vasodilation). In addition to vasodilation, adenosine has several electrophysiological, cardioprotective, metabolic, and antiinflammatory properties. Adenosine is rapidly metabolized in blood and interstitial fluid, through cell absorption and degradation by adenosine deaminase. The short half-life of adenosine limits its clinical value. However, there are several ways of increasing the interstitial concentration of adenosine. At present, adenosine or adenosine-potentiating substances are used clinically to terminate supraventricular tachycardias, to induce myocardial ischemia in patients who are unable to exercise, and to reduce myocardial ischemia or reperfusion injury. Caffeine and other methylxanthines are adenosine receptor antagonists, and several of the pharmacodynamic properties of these substances are caused by adenosine receptor antagonism.