Reabsorption of inorganic sulfate in the rat kidney: evidence for an adaptive depression of TmSO4 during SO4 loading.

Clearance methods were employed to study reabsorption of inorganic sulfate (SO4) in the rat kidney. During stepwise elevation of the So4 concentration in plasma by sulfate infusions (SO4 titration), the reabsorption of S)4 increased and reached the maximal reabsorptive capacity for inorganic sulfate (TmSO4). During further SO4 loading TMSO4 decreased by about 50%. At the same time GFR was stable, and the decline of the TmSO4 was not due to volume loading during infusion, since in time-controls with Ringer infusion alone the reabsorption of inorganic sulfate was almost unchanged. The decrease of the TmSO4 was also observed in thyroparathyroidectomized animals. It is concluded that the decline of TmSO4 was a result of the SO4 load per se and may be mediated by an unknown adaptive process.