AXIAL HETEROGENEITY OF ADENOSINE TRANSPORT AND METABOLISM IN THE RABBIT PROXIMAL TUBULE

Transport and metabolism of adenosine were studied in the S1, S2, and S3 segments of the rabbit proximal renal tubule. Isolated segments were perfused in vitro with uniformly labelled C-14-adenosine to measure the lumen-to-bath flux of adenosine. This flux rate was measured by the disappearance of C-14 from the luminal fluid (J(D)) and simultaneously by the appearance of C-14 in the bathing solution (J(A)), expressed as femtomoles per minute per millimeter of tubule length (fmol.min-1.mm-1). At a perfused concentration of 83.3-mu-M adenosine, when corrected for metabolism, the J(D)S for adenosine in the S1, S2, and S3 segments were 735, 212, and 273, respectively. J(A)S, corrected for metabolism, were 0, 0, and 4.8 fmol.min-1.mm-1 for the S1, S2, and S3 segments, indicating that very little or no C-14-adenosine moved across the basolateral membrane. To correct for metabolism of C-14-adenosine, the perfusion fluid, collected fluid, tubular extract, and bathing fluid, from three tubules of each segment type, were analyzed by high-performance liquid chromatography to identify C-14-adenosine and its C-14-metabolites. At 83.3-mu-M, all segments metabolized adenosine extensively. Consequently, adenosine-5'-monophosphate (AMP) and inosine were found in tubule cells of all segments. Inosine also appeared in the collected fluid, but AMP did not. In S1 and S2 segments, none of the C-14 in the bathing solutions could be identified and no adenosine was found. Of the small amounts of C-14 found in bathing solutions from S3 segments, about 27% appeared to be adenosine, the rest were inosine and hypoxanthine or unidentified metabolites. Cell-to-lumen adenosine concentration ratios, corrected for metabolism but not for binding, were 7.3, 21.3, and 8.7 for the S1, S2, and S3 segments, respectively. We conclude that adenosine is avidly transported across the luminal membrane in all segments of the proximal tubule and then extensively metabolized. Therefore, any adenosine in the glomerular filtrate is not likely to appear in renal venous blood or the urine as adenosine. These data suggest that adenosine in the urine must come primarily from some point in the nephron distal to the proximal tubule.