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Department of Physiology, University of Dundee.

1. We have employed a paired-tracer isotope dilution technique in a perfused rat hindlimb preparation to obtain information on the kinetics of transport across the sarcolemmal membranes of acidic, neutral and basic amino acids. 2. We have defined the characteristics of the saturable transport of amino acids normally regarded as paradigm substrates for the A, ASC, L, y+(basic) and the dicarboxylic amino acid transport systems. Their maximal transport capacities (Vmax, nmol min-1 (g muscle)-1 and substrate concentrations for half-maximal transport (Km, mM) of representative amino acid substrates are as follows: 2-aminoisobutyrate (AIB), Vmax = 15 +/- 7, Km = 1.26 +/- 0.6; alanine, Vmax = 332 +/- 53, Km = 3.9 +/- 0.9; serine, Vmax = 410 +/- 61, Km 3.4 +/- 0.5; leucine, Vmax = 2800 +/- 420, Km = 20 +/- 2; lysine, Vmax = 136 +/- 46, Km = 2.1 +/- 1.3; glutamate, Vmax = 86 +/- 6, Km = 1.05 +/- 0.05; proline, Vmax = 196 +/- 48, Km = 4.1 +/- 0.6. 3. Glycine uptake was faster than expected on the basis of diffusion but was not saturable and showed uptake that could be best described by a first-order rate constant of 0.07 +/- 0.003 min-1. 4. We have attempted to discriminate kinetically between possible routes of entry for an amino acid on the basis of competitive and non-competitive interaction between substrates potentially sharing common routes. On this basis, the major routes of alanine entry appear to be via the ASC and L systems with the A system playing a quantitatively minor role. Glutamate and aspartate appear to be transported exclusively by a dicarboxylate amino acid carrier. The branched-chain amino acids (BCAA) and the aromatic amino acid, phenylalanine, are almost equivalent substrates for an L-like system. 5. Insulin had no detectable effect on the uptake of paradigm substrates for ASC, L, y+, the dicarboxylic amino acid or glycine transport systems. 6. Transport of serine and lysine was Na+ dependent. Lysine transport apparently occurred with a stoichiometry of 2 Na+: 1 lysine. With the exception of alanine, whose transport was partially Na+ dependent, all other amino acids examined in the present study were transported in a Na+-independent manner.

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