RESUMO
The transport of ions and solutes at the epithelial surface of the renal proximal tubule increases during periods of reduced dietary intake and decreases with dietary excess. We have used the sulfur-containing beta-amino acid, taurine, as a probe of this renal adaptive response to altered dietary sulfur amino acid intake to better understand the mechanisms of renal amino acid reabsorption. There exists an age-related precession of taurine uptake values by brush border membrane vesicles prepared from nursing rats from youngest to oldest. However, despite the immaturity of this transport mechanism, epithelial membranes become able to display a full renal adaptive response to altered sulfur amino acid intake sometime between the 7th and 14th day of life. This adaptive response is expressed in both "up regulation" and "down regulation" by means of a change in the initial rate of Na(+)-taurine cotransport. No changes in the lipid microenvironment of the membrane, as assessed by measurements of membrane fluidity, are evident. The lack of adaptation observed in 7-day-old pups may be due to immaturity of the Na+ transporting mechanism which energizes the uptake of amino acids.
