Urate and antioxidants inhibit Na +-adenosine transport in rat renal brush border membranes

Abstract

The effects of urate and antioxidants were evaluated on sodium-dependent [ 3H]adenosine transport (Na +/ADO) in rat renal brush border membrane vesicles (BBMV). Na +/ADO was estimated for a range of adenosine concentrations of 1–10 μmol/l in BBMV preincubated with urate (0.5–50 μmol/l). Michaelis-Menten kinetics showed a significant increase in K m values from 2.48±0.49 μmol/l in control to 20.58±4.56 μmol/l with 50 μmol/l urate; V max (243±15 pmol/mg protein×min) was not modified. Menadione (10 μmol/l) significantly increased the Na +/ADO activity, from 17.57±5.50 in the control, to 27.70±7.60 pmol/mg prot.×min (a 1.60 times increase, p<0.05). This stimulation was prevented when BBMV were preincubated with either 1 μmol/l α-tocopherol (trolox) or urate. Similarly conjugated dienes and malonaldehyde were stimulated in a dose-dependent fashion by menadione and the effect was inhibited with 10 μmol/l trolox. The antioxidants probucol, captopril and allopurinol inhibited in a concentration-dependent manner the Na +/ADO (IC 50 were 79±8, 100±9 and 89±9 nM, respectively). This effect might be specific on K m of the Na +/ADO, since 1 μmol/l trolox (IC 50=1000±20 nM), inhibited V max but not K m of the Na +/glucose transport. Our results suggest that the Na +/ADO in BBMV is modified by agents that affect the redox status of the membranes.