All of the influxes that were clearly above baseline were inhibited when the radiolabeled amino acids were added using serum rather than a simple buffer suggesting competition for transport with the amino acids present in serum. Competition was investigated further and confirmed by measuring uptake of tracer in the presence of an excess of individual unlabelled amino acids [300].18 Quantitative estimates of the influxes of various amino acids in rats when plasma concentrations of tracers were held constant by controlled infusions [369] or during perfusion of isolated brains [43, 370, 371] have confirmed the pattern seen using BUI measurements [300, 372] (see Table 4).
Table 4 Influx and net flux of amino acids in the indicated species
Rat1 Rat2 Rat3 Rat4 Rat5 Rat6 Dog7 Dog7 Cat8 Sheep9 Human10 Human11 Human12 Human13 Human14
Influx* Influx Influx Influx Influx Net Net* Net* Net Net Net Net Net Net Net
Phenylalanine 6.5 7 8 13.2 − 0.6 0.12 0.01 2.2 0.06 0.3 − 0.5
Leucine 6.2 15 14.5  9.7 1.56 1.28 1.9 6 5.2 3.2 2.2 0.6
Isoleucine 1.8 4  5.7 0.78 0.46 1.5 3.1 1.2 1.4 0.7 0.3
Valine 1.3 1.8  2.9 1.26 0.37 2.7 11.0 1.7 0.8 − 1.7
Tyrosine 5.3 7 4.1 − 1.1 0 0.06 1.6 0.1 − 0.1 − 0.1
Methionine 1.6 1.7 − 1.1 0 − 0.12 1.0 0.6 0.2 0.2
Tryptophan − 1.1
Histidine 2.5 2.5 − 1.7 − 0.23 − 0.7 0.7 − 0.3 − 1.3
threonine 1.2 0.8 − 3.4 0.62 − 0.1 5.0 2.6 0.1
Arginine 1 − 7.4 − 0.2 − 0.05 2 0.7 − 0.5 − 0.1
Lysine 6.2 9 − 1.1 0.58 − 1.6 2.5 − 0.3 − 0.9
Glutamate  1.7 − 0.24 − 0.18 − 0.05 1.4 − 0.5 − 6
Glutaminea 11.6 1 − 15.4 − 6.6 − 4.3 − 1.94 − 7.4 11.0 − 20 − 20.4
nerv. stim.b glutamate − 1.26
nerv. stim.b glutamine − 6.69
Values are stated in nmol g−1 min−1. Influxes have been measured only in rats. The only available measurements of net fluxes in rats, shown in the italicized column, failed to reach statistical significance
* Data used by Pardridge [379] in his comparison of influxes and net fluxes
aIn some columns the values are for glutamine + asparagine as the assay used detected both
bRelease measured during nervous stimulation
1Banos et al. [369]
2Hawkins et al. [583]
3Mans et al. [584]
4Ennis et al. [415]
5Smith and Stoll [43]
6Brosnan et al. [378]
7Betz and Gilboe [365]
8Abdul-Ghani et al. [394]
9Pell and Bergman [585]
10Felig et al. [586]
11Lying-Tunell et al. [587]
12Eriksson et al. [588]
13Grill et al. [589]
14Strauss et al. [590]