Regional Cerebrovascular Transport of Leucine as Measured by the in situ Brain Perfusion Technique

Leucine influx into six brain regions was determined in anesthetized rats with the in situ brain perfusion technique using either saline or plasma perfusate. This technique has several advantages over other methods such as the brain uptake index (BUI) technique. The concentration dependence of l-leucine influx was best described by a model with a saturable and a nonsaturable component. For the saturable component, V_max equaled 10.67 ± 0.21 × 10⁻⁴ μmol s⁻¹ g⁻¹ and K_m equaled 0.0257 ± 0.0009 μmol ml⁻¹, whereas the constant of nonsaturable diffusion (K_d) equaled 0.957 ± 0.067 × 10⁻⁴ s⁻¹ in the parietal lobe during saline perfusion. V_max was higher in the cortical lobes than in other brain areas, probably owing to a greater capillary surface area. There were no regional differences in K_m or K_d. The apparent K_m for l-leucine during plasma perfusion was 20 times greater than the K_m during saline perfusion, and 3 to 4 times greater than the plasma leucine concentration, owing to competitive inhibition of leucine transport by other large neutral amino acids in plasma. These results for V_max, K_m, and K_d differ by three- to fourfold from previous estimates obtained with the BUI technique. The high apparent K_m during plasma perfusion indicates that leucine influx is a linear function of plasma concentration up to 0.5 μmol ml⁻¹ when the plasma concentrations of other amino acids remain constant, whereas influx would be approximately constant when plasma concentrations of all large neutral amino acids increased or decreased by a constant fraction.