Cystine loading induces Fanconi's syndrome in rats: In vivo and vesicle studies

To better understand the link between lysosomal cystine accumulation and the renal impairment seen in cystinosis, we have studied the effect of cystine loading in vivo, on renal function of rats, and in brush-border membrane vesicles (BBMV) prepared from the kidney cortex of the treated rats. Intraperitoneal injection of cystine dimethyl ester (CDME) (400 μmol, twice a day, for 5 days) led to an increased urine volume and excretion of glucose, phosphate, and protein. Kinetic analysis of α-methylglucoside initial flux in BBMV showed reduction in maximal transport capacity (V_max, from 10.1 ± 1.3 to 8.5 ± 0.7 nmol·min^-1·mg protein^-1; P < 0.01) with no change in Michaelis constant (K_m, 4.80 ± 0.08 and 4.90 ± 0.05 mM). The number of phlorizin binding sites declined (from 6.5 ± 0.7 to 4.1 ± 0.4 pmol/mg protein; P < 0.01) with no significant change in the affinity for phlorizin (0.64 ± 0.08 and 0.59 ± 0.06 μM). In the cortex homogenate, cystine concentration, which was undetectable in controls, increased to 0.97 ± 0.09 nmol 1/2 cystine/mg protein. Two hours after CDME administration, ATP content declined to ∼50% of control values. This decline was transient, and ATP content was recovered to control values 5 h after CDME administration. The treatment did not affect ouabain-sensitive adenosinetriphosphatase activity (40.0 ± 3.9 and 38.6 ± 4.7 nmol P_i·mg protein^-1·min^-1) or the number and affinity of ouabain binding sites (B_max = 1.48 ± 0.25 and 1.44 ± 0.18 pmol/mg, and K_d = 0.68 ± 0.09 and 0.72 ± 0.12 μM, respectively). We postulate that the reduction in ATP content and sodium-coupled glucose absorption may, at least partially, be the basis for the Fanconi's syndrome seen in the CDME-treated rats and is probably one of the mechanisms for the proximal tubular impairment seen in cystinotic patients.