Kinetics and localization of tubular resorption of "acidic" amino acids. A microperfusion and free flow micropuncture study in rat kidney.

The unidirectional resorption rates of L-glutamate (initial concentrations of 0.07, 0.66, 2.0 or 20.0 mmol X 1(-1)), D-glutamate (0.66 mmol X 1(-1) in the presence or absence of 20 mmol X 1(-1) L-glutamate), and of L-aspartate (0.073, 0.3, 0.66, 2.0 or 5.0 mmol X 1(-1)) were determined in the rat proximal convolution. L-Glutamate resorption was saturable. A permeability coefficient (P) of less than or equal to 20 microns2 X S-1, and a maximum resorption rate (Jmax) of 0.15 +/- 0.015 (SEM) nmol X S-1 X m-1 at a Km of 0.17 +/- 0.025 (SEM) mmol X 1(-1) was obtained for L-glutamate. For L-aspartate, Jmax was 0.13 +/- 0.005 at a Km 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 of L-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 X 1(-1) initial L-glutamate. It fell to 35 or 33% respectively if the initial L-glutamate concentration was 20 mmol X 1(-1) or if the resorption of 0.66 mmol X 1(-1) D-glutamate in presence of 20 mmol X 1(-1) L-glutamate was measured. The fractional excretion of endogenous L-glutamate in the final urine amounted to 0.13 +/- 0.012% of the filtered load. It is concluded that L-glutamate and L-aspartate are quickly resorbed in early parts of the proximal convolution (low Km). Saturation already occurs when there is a small increase in the filtered load (low Jmax). 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.