Renal transport of lysine and arginine in cystinuria.

To study the defective transport mechanism of cystinuria, renal tubular reabsorption of lysine or arginine in normal and cystinuria subjects was investigated by increasing the filtered load employing intravenous amino acid infusion. In the normal group the amino acid reabsorption rose with increases of the filtered load and reached a maximum (Tm). In the cystinuria group the elevation of amino acid reabsorption was poor at low filtered loads and some of the reabsorption rates fell below zero, whereas the tubular transport proceeded at a normal rate with a great increase of the filtered load. This might be explained as follows: At low filtered loads the filtered amino acid in the tubular lumen in patients with cystinuria is not absorbed into the cell because of a transport defect of the luminal membrane of the tubular cell, causing a large amount of urinary amino acid excretion. At high filtered loads the accumulated intraluminal amino acid permeates the tubular cell by a passive diffusion and is transported to the capillary across the intact basolateral membrane, which in turn only brings about a small urinary loss of amino acid. The infusion of lysine or arginine depressed the percentage of tubular reabsorption of other dibasic amino acids in both groups. In the cystinuria group the percentage of the dibasic amino acid reabsorption dropped sharply with an initial load of the inhibitor, but no more depression of the percentage of reabsorption occurred with further loads of the inhibitor.