Absence of a sodium-dependent transport component for leucine in a vertebrate
It has recently been shown that Xenopus laevis embryos are permeable to amino acids and nucleic acid precursors. Characterization of amino acid transport by Xenopus embryos has led to the finding that at least two transport systems, the previously described A and L systems are operative. In the work described here, the effect of altering the incubation medium with respect to sodium concentration and choline concentration (sodium-free medium) on leucine uptake by Xenopus laevis embryos was studied. Xenopus laevis embryos (stage 23-25) were incubated with 14C-leucine in four concentrations of sodium. The concentrations used were 0.0025M, 0.005M, 0.02M, and 0.06M in Na+. Comparison was made using media where sodium was replaced with choline at equivalent molar concentrations. Leucine uptake was sharply reduced as the sodium concentration of the incubation medium was increased. This effect was not observed when sodium was replaced with choline. There was no increase in the rate of back-transport of endogenous leucine as the sodium con centration was increased. Leucine uptake was found to be mildly ouabain-sensitive in the presence of sodium, and highly ouabain-sensitive in the presence of choline. The observation that sodium inhibits leucine uptake in Xenopus laevis embryos indicates that leucine uptake has no sodium dependent component whatsoever, and qualifies leucine as a model amino can be more acid with which sodium-independent transport fully characterized.