Abstract

The beta-amino acid transport capabilities of rat renal epithelium were assessed using brushborder membrane vesicles (BBMV). Taurine, a metabolically inert sulfur-containing amino acid, was studied with emphasis on the renal adaptation to dietary sulfur amino acid alteration. Three isoproteinic diets were given to Sprague-Dawley rats: low-sulfur-amino-acid diet (LTD), normal-sulfur-amino-acid diet (NTD), and high-taurine diet (HTD). Our studies demonstrated that taurine is actively transported into membrane vesicles by a sodium-dependent transport system. This transport is enhanced by hyperpolarization with valinomycin and decreased by dissipation of the sodium gradient by gramicidin. On LTD (compared to NTD), plasma taurine, urinary taurine, and fractional excretion of taurine were reduced. On HTD (compared to NTD), plasma taurine, urinary taurine, and fractional excretion of taurine were elevated. In vitro studies in BBMV from NTD animals revealed a Km of 40 microM and Vmax of 102 pmoles/mg protein/30 sec. Other beta-amino acids significantly inhibited BBMV taurine accumulation. BBMV taurine uptake was enhanced after LTD (compared to NTD) and diminished after HTD (compared to NTD). These studies indicate that a renal adaptation to dietary alterations in sulfur-containing amino acids occurs and that the luminal brushborder membrane participates in the adaptation. Renal adaptative mechanisms to dietary change may serve to help conserve amino acids during deprivation and to excrete amino acids during periods of excess.