The vasoactive properties of circulating adenosine and ATP were first recogн nized half a century ago (22, 23), although it is only in the last decade that the physiological importance of these properties and the classification of puriн noceptor-mediated vasoregulation have been investigated in detail (for reviews see 10, 13). As a result of these studies, it became clear that it is also important to understand the mechanisms by which purines are released to the extracellular medium, and the manner in which their effects may be regulated by inactivation or removal from the extracellular milieu. The potentially significant role played by the vascular endothelium in these processes can be inferred from several earlier observations of the rapid catabolism of adenine nucleotides on perfusion through vascular beds and from the demonstration that endothelial cells possess ectoenzymic activities capable of degrading adenine nucleotides to adenosine (see subsequent sections on nucleotide catabolism). More recently it has also been found that adenosine is efficiently removed from the circulation by uptake into endothelial cells (see next section).

The detailed characterization of these processes has been greatly assisted by the availability of cultured endothelial cells: the first publication, in 1973, clearly setting out methods for the isolation, identification, and growth of pure populations of endothelial cells in vitro (38), has led to the culture of endotheн lium from various species and sites by many research groups, which in tum has produced the realization that endothelial cells play active roles in many aspects of vascular homeostasis (for reviews see 37, 54). This review focuses on the ability of endothelial cells to catabolize exн tracellular nucleotides and take up nucleosides. In addition, we address the