Kinetics and localization of tubular resorption of ?acidic? amino acids

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Abstract
The unidirectional resorption rates ofl-glutamate (initial concentrations of 0.07, 0.66, 2.0 or 20.0 mmol·l−1,d-glutamate (0.66 mmol·l−1 in the presence or absence of 20 mmol·l−1 l-glutamate), and ofl-aspartate (0.073, 0.3, 0.66, 2.0 or 5.0 mmol·l−1) were determined in the rat proximal convolution.l-Glutamate resorption was saturable. A permeability coefficient (P) of <−20 μm2·s−1, and a maximum resorption rate (J max) of 0.15±0.015 (SEM) nmol ·s−1·m−1 at aK m of 0.17±0.025 (SEM) mmol·l−1 was obtained forl-glutamate. Forl-aspartate,J max was 0.13 ±0.005 at aK m of 0.1±0.013. A free flow glutamate concentration profile along the proximal convolution was (I) predicted from these constants and (II) actually measured by means of free flow micropuncture. The data agree very well and show that more than 90% of the filtered load is resorbed within the first third of the proximal convolution. The late proximal and early distal free flow recoveries ofl-glutamate amounted to 5.3±1.7% (SEM) and 6.6±1.4% of the filtered load, respectively. In contrast to this, unidirectional resorption during the microperfusion of the same tubule section was high: fractional resorption amounted to ca. 96% at 2 mmol·l−1 initiall-glutamate. It fell to 35 or 33% respectively if the initiall-glutamate concentration was 20 mmol·l−1 or if the resorption of 0.66 mmol·l−1 d-glutamate in presence of 20 mmol·l−1 l-glutamate was measured. The fractional excretion of endogenousl-glutamate in the final urine amounted to 0.13±0.012% of the filtered load. It is concluded thatl-glutamate andl-aspartate are quickly resorbed in early parts of the proximal convolution (lowK m). Saturation already occurs when there is a small increase in the filtered load (lowJ max) The nephron section between the late proximal and early distal nephron sites also reabsorbs “acidic” amino acids. Normally, however, the back leak cancels this out, and net flux becomes zero. Deep nephrons seem to handle amino acids somewhat differently than superficial nephrons do.

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