Intracellular cystine loading inhibits transport in the rabbit proximal convoluted tubule

Cystinosis is an autosomal recessive disorder characterized by a high intracellular cystine concentration. To establish an in vitro model of this disorder and examine the mechanism of the proximal tubule transport defect seen with elevated intracellular cystine concentrations, rabbit proximal convoluted tubules (PCT) were perfused in vitro. PCTs were loaded with cystine using cystine dimethyl ester, a permeative methyl ester derivative. Bath cystine dimethyl ester (0.5 mM) reduced volume absorption (J(v)) (0.67 ± 0.07 to 0.15 ± 0.09 nl/mm · min, P < 0.01), bicarbonate transport (J(TCO₂)) (47.2 ± 4.9 to 11.1 ± 2.8 pmol/mm · min, P < 0.001) and glucose transport ((J)GLU) (34.1 ± 1.5 to 19.7 ± 1.5 pmol/mm·min, P < 0.001). The methyl esters of leucine (0.5 mM), and tryptophan (0.5 and 2.0 mM) had no effect on these parameters. To examine if intracellular reduction of cystine to cysteine could contribute to the inhibition in transport, the effect of bath cysteine methyl ester on proximal tubular transport was examined. Bath cysteine methyl ester (2 but not 0.5 mM) resulted in an inhibition in J(v), J(GLU), and J(TCO₂). Cystine dimethyl ester had no effect on mannitol or bicarbonate permeability. These data are consistent with intracellular proximal tubular cystine accumulation resulting in an inhibition of active transport.