Adenosine is an endogenous regulator of vascular tone. This activity of
adenosine is terminated by its uptake and metabolism by microvascular endothelial cells (MVEC). The predominant transporter involved is ENT1 (equilibrative
nucleoside transporter subtype 1). MVEC also express the nucleobase transporter (ENBT1) which is involved in the cellular flux of
adenosine metabolites such as
hypoxanthine. Changes in either of these transport systems would impact the bioactivity of
adenosine and its metabolism, including the formation of
oxygen free radicals. MVEC isolated from skeletal muscle of ENT1(+/+) and ENT1(-/-) mice were subjected to oxidative stress induced by simulated
ischemia/reperfusion or
menadione. The functional activities of ENT1 and ENBT1 were assessed based on zero-trans influx kinetics of radiolabeled substrates. There was a reduction in the rate of ENBT1-mediated
hypoxanthine uptake by ENT1(+/+) MVEC treated with
menadione or after exposure to conditions that simulate
ischemia/reperfusion. In both cases, the
superoxide dismutase mimetic
MnTMPyP attenuated the loss of ENBT1 activity, implicating
superoxide radicals in the response. In contrast, MVEC isolated from ENT1(-/-) mice showed no reduction in ENBT1 activity upon treatment with
menadione or simulated
ischemia/reperfusion, but they did have a significantly higher level of
catalase activity relative to ENT1(+/+) MVEC. These data suggest that ENBT1 activity is decreased in MVEC in response to the increased
superoxide radical that is associated with
ischemia/reperfusion injury. MVEC isolated from ENT1(-/-) mice do not show this reduction in ENBT1, possibly due to increased
catalase activity.