CHARACTERISTICS OF GLUTAMINE TRANSPORT IN PRIMARY TISSUE-CULTURE OF RAT SKELETAL-MUSCLE

Glutamine transport was studied in preconfluent monolayered, mononucleated myoblasts (4 days old) and in fused, multinucleated, differentiated myotubes (10 days old), both prepared from neonatal rat skeletal muscle. The initial (60 s) rate of 50 muM glutamine uptake in myoblasts and myotubes was stereospecific, saturable, and largely (80%) Na+ dependent. At glutamine concentrations of 0.01-1 mM, Na+-dependent uptake showed saturation kinetics: in myoblasts, the Michaelis constant (K(m)) was 197 +/- 38 muM, maximum velocity (V(max)) was 1,165 +/- 60 pmol.min-1.mg protein-1; in myotubes, K(m) was 174 +/- 51 muM and V(max) was 1,435 +/- 47 pmol.min-1.mg protein-1. The Na+-dependent glutamine uptake was Li+ tolerant in both myoblasts and myotubes. The Na+-dependent uptake of 50 muM L-[3H]glutamine was investigated in the presence of various amino acids at 0.01-10 mM. Histidine and asparagine competitively inhibited glutamine uptake, but inhibition by serine was noncompetitive; glutamate, arginine, leucine, and 2-aminobicyclo(2,2,1)heptane-2-carboxylate (BCH) had no significant inhibitory effects; 2-(methylamino)isobutyrate (MeAIB) caused a small but significant inhibition. In parallel with a stimulation of glucose transport, addition of insulin stimulated Na+-dependent glutamine uptake within 1 h by a maximum of 27% in myoblasts and 42% in myotubes (half-maximal stimulation at 0.3 nM insulin). Glucagon had no effect. Kinetic analysis revealed that the insulin-stimulated increase in glutamine transport was due to a V(max) effect, which was cycloheximide inhibitable. The insulin-stimulated increase was Li+ tolerant and not inhibited by MeAIB or cysteine at 1 mM. The results indicate that the predominant glutamine transporter of neonatal rat skeletal muscle cells in primary tissue culture is System N(m). System N(m) also appears to be the major insulin-sensitive glutamine transport component in skeletal muscle. Primary muscle culture appears to be a useful preparation for studying glutamine transport and its regulation.