Abstract
Amino acid transport was measured in rat sarcolemmal vesicles (approximately 0.5 microliters/mg protein). Initial (45 s) uptake of glutamine tracer was stereospecific and saturable [Km 90 +/- 14 microM; maximum velocity (Vmax) 60 +/- 3 pmol.mg protein-1.min-1], it was Na+ dependent (but tolerated Li+ instead), and was stimulated by inside negative membrane potential. Transport of glutamine (5 microM) was inhibited by asparagine, histidine, alanine, serine, and phenylalanine at 1 mM (25-74%), but leucine and N-methylaminoisobutyric acid (MeAIB) did not significantly inhibit glutamine uptake. Glutamine efflux was accelerated by an outwardly directed Na+ concentration gradient. L-[14C]asparagine uptake was Na+ dependent and strongly inhibited by glutamine. L-[3H]serine uptake was Na+ dependent but did not tolerate Li(+)-for-Na+ substitution. L-[3H]phenylalanine uptake was Na+ independent. Differences between the ion dependence of glutamine, serine, and phenylalanine uptake and the lack of glutamine transport inhibition by MeAIB indicated that glutamine is not transported by systems ASC, L, or A. The properties of the glutamine transporter in sarcolemmal vesicles resemble those of the system Nm previously characterized in perfused skeletal muscle.
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