AMINO-ACID-DEPENDENT MODULATION OF AMINO-ACID-TRANSPORT IN XENOPUS-LAEVIS OOCYTES

We have measured rates of uptake of arginine, glutamine, glutamate, se rine, phenylalanine and glycine in Xenopus laevis oocytes cultured for periods of up to 24 h in saline in the presence or absence of a mixtu re of 20 amino acids at concentrations approximating those in Xenopus plasma. Amino acid supplementation increased the total intracellular a mino acid concentration from 8.2 to 18.4 nmol per oocyte. Specific Na-dependent amino acid transporters (systems B-0,B-+, X(ag)(-)) exhibit 'adaptive regulation' (up-regulation during amino acid deprivation an d down-regulation during amino acid supplementation). Na+-independent transporters of glutamate, glutamine and glycine (including system asc ) display an opposite modulation in activity, which may help to combat aminoacid-induced oxidative stress by increasing the supply of glutat hione precursors. Single amino acids at physiological plasma concentra tions (0.47 mmol l(-1) L-alanine, 0.08 mmol l(-1) L-glutamate) mimicke d at least some effects of the amino acid mixture. The mechanisms of t ransport modulation do not appear to include trans-amino acid or membr ane potential effects and, in the case of Na+-independent transport, a re independent of protein or mRNA synthesis. Furthermore, activation o f protein kinase C by phorbol 12-myristate 13-acetate did not signific antly affect endogenous glutamine and glutamate transport. The Xenopus oocyte appears to possess endogenous signalling mechanisms for select ively modulating the activity of amino acid transport proteins express ed in its surface membranes, a factor for consideration when using ooc ytes as an expression system for structure-function studies of cloned amino acid transporters.