Transport interactions of cystine and dibasic amino acids in isolated rat renal tubules

Abstract

Isolated renal cortical tubules prepared from adult male Sprague-Dawley rats were used to study the nature of cystine entry into tubule cells and its transport interactions with dibasic amino acids. The uptake of cystine over time was progressive, reaching a steady-state after 60 min of incubation. Analysis of the intracellular pool after incubation revealed that a significant fraction of the transported cystine was reduced to cysteine. A kinetic analysis of uptake demonstrated that two systems for cellular entry of cystine existed with a K m1 of 0.012 mM and K m2 of 0.55 mM. Cystine uptake was sodium dependent with an apparent K m for sodium of 36 mEq/liter. Lysine inhibited cystine uptake via the low K m system, but appeared not to inhibit cystine uptake via the high K m system. Ornithine, leucine, and isoleucine each inhibited cystine uptake via the low K m system. Arginine appeared to affect both systems for cystine uptake. Cystine inhibited the uptake of lysine by isolated renal tubules. The data suggest that cystine at physiologic concentrations is transported into renal tubule cells nearly equally by two systems, of which, the low K m system is shared with the dibasic amino acids. A defective low K m system could in part explain human cystinuria.