Erythrocyte uptake kinetics and cell to plasma gradients of leucine and phenylalanine in fed and fasted rats

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Abstract
The kinetic parameters of the L-phenylalanine and L-leucine uptake by isolated erythrocytes in fed and 24 hour starved rats have been determined. In addition, the in vivo compartmentation between blood cells and plasma of the above amino acids in arterial and venous blood vessels has also been studied under the above physiological situations. Both the L-phenylalanine and L-leucine uptake by erythrocytes was saturable and non concentrative. Starvation increased the Km value for the leucine uptake and did not significantly affect that of phenylalanine uptake. The in vivo blood cell/plasma (C/P) concentration ratio of both amino acids was higher than the unit. The starvation-induced changes in the relative distribution of these amino acids between the blood cell and the plasma compartments were significant for the phenylalanine in the aortic artery but not in venous blood. The transport system capabilities measured in vitro can not account for the maintenance of both the leucine and phenylalanine gradient between blood cells and plasma, and the starvation-induced changes in the blood amino acids compartmentation are not directly related entirely to the transport system capabilities.
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