Further characterization of adenosine transport in renal brush-border membranes

Abstract

Adenosine transport has been further characterized in rat renal brush-border membranes (BBM). The uptake shows two components, one sodium-independent and one sodium-dependent. Both components reflect, at least partly, translocation via a carrier mechanism, since the presence of adenosine inside the vesicles stimulates adenosine uptake in the presence as well as in the absence of sodium outside the vesicles. The sodium-dependent component is saturable ( K m adenosine = 2.9 μM, V max = 142 pmol/min per mg protein) and is abolished at low temperatures. The sodium-independent uptake has apparently two components: one saturable ( K m = 4–10 μM, V max = 174 pmol/min per mg protein) and one non-saturable ( V max = 3.4 pmol/min per mg protein, K m > 2000 μM). Inosine, guanosine, 2-chloroadenosine and 2′-deoxyadenosine inhibit the sodium-dependent and -independent transport, as shown by trans-stimulation experiments, probably because of translocation via the respective transporter. Uridine and dipyridamole inhibited only the sodium-dependent uptake. Other analogs of adenosine showed no inhibition. The kinetic parameters of the inhibitors of the sodium-dependent component were further investigated. Inosine was the most potent inhibitor with a K i (1.9 μM) less than the K m of adenosine. This suggests a physiological role for the BBM ecto-adenosine deaminase (enzyme which extracellularly converts adenosine to inosine), balancing the amount of nucleoside taken up as adenosine or inosine by the renal proximal tubule cell.