Adenosine:DiagnosticandTherapeuticUsesin Cardiovascular Medicine*

ceeds supply.@ Adenosine measurement is difficult in any given circumstance, as elements such as endothe hal cell and blood uptake and release, interstitial concentration, intracellular binding with SAH-hydro lase, and flow all influence venous adenosine concen trations. Adenosine is removed from tissues either by phos phorylation to AMP by adenosine kinase or by deam ination by adenosine deaminase. Although extracel lular adenosine deaminase exists, both of these enzymes are predominantly intracellular; therefore, adenosine must b@transported into the cell for break down. This is achieved by simple and facilitated diffusion via a bidirectional nucleoside transport mechanism. Transport into cells is also necessary for the rapid removal of adenosine from its site of action on the cell surface. Conversely, adenosine formed inside the cell must be transported outside in order to activate extracellular receptors. Medications such as dipyridamole block the transport of adenosine resulting in a net accumulation of extracellular adenosine@ (Fig 1).