The conversion of AMP to adenosine via ecto-5'nucleotidase (CD73) has recently been shown to be of great importance for the development of allograft vasculopathy in a murine model. In human vessels the expression and function of ecto-nucleotidases is only sparsely documented. In the present study we assessed ecto-nucleotidase activities in human internal mammary artery (HIMA), a vessel frequently used in autologous bypass surgery. Within 3 hours after vessel graft isolation the specimen stored in cold preservation solution were transferred to the laboratory, cut longitudinally and mounted open face in a flow-through chamber (1.94 ml) and superfused with HEPES buffer (37°C) at 0.2 ml/min (chamber transit time 9.7 min). Activity of ecto-nucleotidases was assessed by application of etheno(e)-labelled AMP and measurement of (e)adenosine in the column effluent perfusate using fluorescence HPLC. Studies were approved by the institutional ethics committee.

Under baseline conditions (e)AMP was degraded to (e)adenosine by 22% during single column passage. Application of the CD73 inhibitor alpha-beta-methylene-adenosine-diphosphate (AOPCP, 50 mM) blocked (e)AMP degradation by 45%. In contrast, infusion of an inhibitor of alkaline phosphatase (levamisole, 500 mM) did not significantly attenuated (e)AMP degradation. Estimates of V_max- and K_M-values of CD73 were 2 nmol/min and 25 mM and of alkaline phosphatase were 10 nmol/min and 500 mM. After mechanical removal of the endothelial layer CD73 activity was no longer present, whereas a small alkaline phosphatase activity was unmasked. These studies demonstrate that (1) CD73 is the major catalytic activity converting AMP to adenosine in HIMA, (2) CD73 activity is largely restricted to the intimal layer, (3) in HIMA CD73 may provide a valuable target for treating graft vasculopathy.