Nonsaturable amino acid reabsorption in kidneys of normal and mercury-poisoned rabbits

At high plasma concentrations, a high-capacity, low-affinity or nonsaturable flux (Jhc) accounts for a residual fractional reabsorption of cycloleucine, aspartate, and AIB of approximately 50% of the filtered load in rabbits; Jhc in micromoles per milliliter glomerular filtrate is reduced in Hg-poisoned animals. The nonsaturable flux of cycloleucine is characterized by a transepithelial transit time (TET) of approximately 2 min in control animals; it was consistently much longer in Hg-poisoned animals. The clearance ratio of creatinine/inulin averaged 1.0, and no Jhc could be demonstrated for glucose. We conclude that Jhc is a high-capacity, low-affinity amino acid flux which passes through an intracellular solute pool, and which is sensitive to Hg at both the brush border and the basolateral cell membrane. If calculation of the saturation constants of aspartate reabsorption is restricted to experiments in which U/P less than 1.0, i.e. where Jhc is unlikely to contribute greatly to reabsorption, values some 20% lower than those previously reported are obtained; the Hg inhibition still is apparently uncompetitive in nature.