HPLC-radiodetector analysis of standard, which represented the uptake buffer from (A) the donor chamber containing [14C] adenosine prepared as described in Methods section and (B) of the uptake buffer from acceptor chamber collected after 10 min of incubation with CPEC monolayers. Y-axis shows DPM, and retention time in minutes is on the X-axis. Under these conditions, peak elution of radioactivity in the standard occurred at 9.28 min. However, a negligible amount of radioactivity was eluted at this time in the sample from the acceptor chamber (B), where about 2/3 DPM appeared in the hypoxanthine peak and the rest in the adenine peak. These peaks were identified by retention times and spectral analysis.