Cystinuria: reduced lysine permeability at the brush border of intestinal membrane cells.

Na-dependent L-lysine epithelial transport was assessed in vitro by measuring the intracellular accumulation and unidirectional influx across the enterocyte brush border membrane. Pieces of jejunum were obtained by peroral biopsies from 27 "control" children and 2 cystinuric patients. In these patients, the following observations are recorded: 1) Na-dependent lysine intracellular accumulation is eliminated; 2) Na-independent accumulation persists (the accumulation ratio is 1.69 +/- 0.34); 3) Na-intracellular concentration is significantly higher in cystinuric patients (60.2 +/- 4.2 mEq/liter) than in controls (42.0 +/- 4.2 mEq/liter), and 4) lysine influx at the luminal membrane is measurable only in tracer amounts, both in the presence of Na (0.17 +/- 0.03 mumole/h cm2) and in it absence (0.22 +/- 0.09 mumole/h cm2). These results suggest a specific loss of Na-dependent L-lysine transport at the luminal membrane of the enterocyte in both the patients with cystinuria. The basolateral membrane is probably permeable to L-lysine.