Absorption and metabolism of allopurinol and oxypurinol by rat jejunum in vitro: effects on uric acid transport.

The transport and metabolism of allopurinol and oxypurinol have been examined in a preparation of adult rat jejunum in vitro. When present in the lumen (50-500 mumol/l) allopurinol appeared in the serosal secretions at lower concentrations, together with its metabolite oxypurinol and an unidentified substance. Oxypurinol in the lumen (1.4-430 mumol/l) was transported into the serosal secretions and at luminal concentrations below approximately 150 mumol/l was concentrated in the secretions. The transport of oxypurinol into serosal secretions was greater than that of allopurinol. Allopurinol and oxypurinol decreased the appearance of urate, in the serosal secretions and the luminal fluid, by an amount which was greater than the concomitant increase in oxypurines; they increased the rate constant of the mono-exponential washout of endogenous urate and induced purine secretion into the lumen. The transport of uric acid from the lumen into the serosal secretions was inhibited by allopurinol and oxypurinol. With allopurinol the specific radioactivity (14C) of urate in the serosal secretions approached that in the lumen; with oxypurinol the specific radioactivity was approximately 70% that in the lumen. There was no evidence for the uphill movement of urate from the lumen.