Blood-Brain Transfer of D-Glucose, L-Leucine, and L-Tryptophan in the Rat

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Abstract
Recently Oldendorf developed a method for measurement of the early uptake of substances from the capillary bed into brain tissue after a single capillary passage of a bolus injected rapidly into the common carotid artery of the rat. The uptake of a test substance is expressed relative to the uptake of a highly diffusible reference substance, tritiated water. In this study experimental data are presented allowing to correct the uptake values for the unknown loss of tritiated water, so that the fractional unidirectional uptake (Extraction, E) can be calculated. This method is used to investigate the uptake kinetics for D-glucose, L-leucine, and L-tryptophan. For the three substances investigated the uptake kinetics involved both saturation and linear kinetics. Km values of 11 mM for D-glucose, 0.16 mM for D-glucose, 0.16 mM for L-leucine, and 0.19 mM for L-tryptophan were found. The uptake capacity, Vm, was calculated using a blood flow value of 0.85 ml/(g x min); Vm was for D-glucose 1.6, for L-leucine 0.027, and for L-trytophan 0.024 mumol/(g x min). The D-glucose Vm in the present study is comparable with Vm values in the literature, and indicates that the method may be employed for quantitative analyses of the blood-brain transfer of solutes.