Tubular reabsorption delay of amino acids in the rabbit kidney

Estimates were obtained both with a rapid-transient technique and under steady-state conditions of the average time required for filtered solutes in the intact rabbit kidney to cross tubular epithelium from tubular lumen into the peritubular inulin space. This interval, defined as the transepithelial passage time, was relatively short (about 15 s) for glucose, in agreement with earlier results in the dog. In contrast, several amino acids required an average transepithelial passage time of about 40 s. Values for aspartate could only be obtained when its large basolateral uptake was inhibited by excess succinate. The long transepithelial passage time may be due to the slow extrusion of reabsorbed amino acids across basolateral cell membranes. This hypothesis was supported by the extent of tissue accumulation of amino acids during reabsorption. Even though basolateral transfer may thus limit the linear rate of movement of filtered amino acids across the epithelium, the step determining fractional absorption from the lumen is situated at the brush border.