Sodium-dependent glutamate transport in cultured rat myotubes increases after glutamine deprivation.

Glutamine produced and stored in skeletal muscle is an important source of nitrogen and energy for the whole body in health and disease and, unsurprisingly, glutamine turnover in muscle is subject to substantial metabolic control. L-Glutamate, a necessary substrate for glutamine synthetase, is transported into muscle cells by Na(+)-dependent and -independent transport systems. In primary cultures of rat skeletal muscle myotubes (a useful model system for studies of muscle metabolism and membrane transport), Na(+)-dependent glutamate transport (Km approximately 0.7 mM glutamate) shows adaptive upregulation (65% increase in transport Vmax from 2.7 to 4.4 nmol.min-1 x mg protein-1) in cells within 24 h of glutamine depletion (t1/2 for increase of approximately 4 h), whereas Na(+)-independent glutamate uptake remains unaltered. Up-regulation of transport is suppressed by inhibitors of gene transcription (actinomycin-D) and translation (cycloheximide) and is reversed by glutamine supplementation. Increased glutamate transport capacity should provide extra substrate for glutamine synthesis in muscle cells. Thus, in concert with previously discovered increases in cell glutamine transport capacity and glutamine synthetase activity, it may represent part of a co-ordinated response to decreased glutamine availability (e.g., under circumstances of increased glutamine utilization by other tissues in vivo.