Adenosine kinase (AK) expression and function during hypoxia. (A) Microarray analysis of AK in response to hypoxia. Confluent HMEC-1s were exposed to normoxia (21% oxygen) or hypoxia (2% oxygen, 24 hours) and relative AK expression was quantified from total RNA by microarray analysis. *Decreased fluorescence, P < .005. (B) Confluent HMEC-1/HUVEC monolayers were exposed to normoxia or hypoxia (2% oxygen) for the indicated time. Total RNA was isolated, and AK mRNA levels were determined by real-time RT-PCR. Data are expressed as fold change in transcript over normoxia plus or minus SD, and have been calculated relative to internal housekeeping gene (β-actin). Results are derived from 3 experiments in each condition (*P < .001). (C) Human saphenous vein was obtained from patients undergoing aorto-coronary bypass surgery and exposed ex vivo to normoxia (24 hours) or hypoxia. Real-time PCR was used to define AK mRNA levels. Results are derived from 3 experiments (*P < .001). (D) Confluent epithelial CaCo-2 cells were exposed to normoxia or hypoxia, and AK mRNA levels were determined (*P < .005). (E) Expression of AK protein during hypoxia in HMEC or CaCo-2 cells exposed to indicated periods of hypoxia. (F) Endothelial monolayers were exposed to indicated periods of hypoxia, and total AK activity was determined by HPLC (■, *P < .01). To determine specificity, cells were preincubated with 10 μM ITU (, #P < .01). Data are derived from 5 monolayers in each condition, and results are expressed as mean percent E-adenosine conversion plus or minus SD.