Urate and antioxidants inhibit Na%2b-adenosine transport in rat renal brush border membranes

The effects of urate and antioxidants were evaluated on sodium-dependent [3H]adenosine transport (Na%2b/ADO) in rat renal brush border membrane vesicles (BBMV). Na%2b/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 x min) was not modified. Menadione (10 μmol/l) significantly increased the Na%2b/ADO activity, from 17.57 ± 5.50 in the control, to 27.70 ± 7.60 pmol/mg prot. x 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%2b/ADO (IC50 were 79 ± 8, 100 ± 9 and 89 ± 9 nM, respectively). This effect might be specific on K(m) of the Na%2b/ADO, since 1 μmol/l trolox (IC50 = 1000 ± 20 nM), inhibited V(max) but not K(m) of the Na%2b/glucose transport, Our results suggest that the Na%2b/ADO in BBMV is modified by agents that affect the redox status of the membranes.

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