Intensified gradients for endogenous amino acid substrates for transport system L on injecting a specific competitor for that system

Leucine and tissue distribution of bulky and small neutral amino acids in rats: dissociation between transport and insulin-mediated effects

The mechanism of the observed decrease in the plasma concentration of several amino acids in the presence of high levels of Leu has remained unexplained. In the present study a decrease in the plasma concentration of Ile, Val, Phe, Tyr, Met, Ala, Pro and Gly was observed after the intraperitoneal injection of Leu to weanling rats. Decreases in net intracellular concentrations in muscle accompanied the decrease in plasma of all of these amino acids except Pro and Gly. An increase in the distribution ratio muscle/plasma was observed exclusively for Gly after administration of Leu or of a non-insulinogenic transport system L analogue. Diazoxide suppressed the Leu-induced decreases in plasma and muscle intracellular concentrations of Ile and Val as well as of Pro in plasma. An increase in the distribution ratio liver/plasma was observed for Pro and Gly in the absence but not in the presence of diazoxide. All the above changes were statistically significant. Hence insulin probably mediates Leu effects, promoting an increased utilization of Ile and Val in muscle and of Pro in liver. A more direct effect of Leu appears to be involved in the apparent increased utilization of Phe, Tyr and Ala in the same tissue. Gly depletion in plasma can be explained by its trapping by inhibitory action of Leu on the exodus of Gly through transport system L.

Evidence for inhibition of exodus of small neutral amino acids from non-brain tissues in hyperphenylalaninaemic rats

The mechanism of the depletion of several plasma amino acids in PKU has remained unexplained. In the present study, a statistically significant decrease in the plasma concentration of several amino acids was observed 2 h after the intraperitoneal injection of Phe to weanling rats. The pattern was very similar to the one observed in PKU patients. Statistically significant increases in the distribution ratios liver/plasma and, mainly, muscle/plasma ratios accompanied in most of the cases the corresponding decreases in plasma concentrations. Equimolar injection under the same conditions of the non-insulinogenic transport system L analogue, the a(+/-) isomer of the 2-aminonorbornane-2-carboxylic acid, produced, in a parallel effect to Phe, statistically significant increases in the distribution ratios of Ala and Gly, and probably of Pro in muscle, as well as of Ala in liver. These results seem to indicate that the high intracellular Phe attained inhibits the exodus of small neutral amino acids through system L, causing their depletion in plasma and ultimately in the brain. This effect may be additive to the inhibition by Phe of the entry of bulky neutral amino acids at the level of the blood-brain barrier. Further study is needed to assess the relevance of these effects to PKU.

Effect of hyperosmolarity on the activity of amino acid transport system L in avian fibroblasts

The transport of selected neutral amino acids known as good substrates of amino acid transport System L has been studied in chick embryo fibroblasts exposed for 4 hours to hyperosmolar culture medium. The activity of the L system, as measured by initial rates of L-phenylalanine uptake, increased in hyperosmolarity treated cells when determined before any cell depletion of intracellular amino acids. This effect was lost after depletion but reappeared after reloading the cells with pertinent substrates of System L. This transport activity appeared to be related to the internal level of amino acids capable of exchange through System L. In hyperosmolarity-treated chick embryo fibroblasts a higher level of System L substrates was obtained during the reloading phase in comparison to control cells. This expanded amino acid pool reflected an increased activity of transport System A, an agency of amino acid mediation known to enlarge its capacity following a hyperosmolar treatment of chick embryo fibroblasts (see Tramacere et al., 1984). L-Methionine, a preferred substrate of both A and L systems, appeared to be involved in the coupling between the activity of amino acid transport Systems A and L in these cells.