Glucose transport characteristics of quiescent thymocytes.

Rat thymocyte populations contain "active" cells which equilibrate quickly with 3-O-methylglucose and "quiescent" cells which equilibrate slowly. Glucose transport stimuli make quiescent cells behave like active cells as regards glucose transport. Glucose transport in quiescent cells was inhibited by phloretin and cytochalasin B. Active cells were heterogeneous in their susceptibility to inhibition by 0.1 micron cytochalasin B, half of them being inhibited about 95%, while half were inhibited modestly. The methylglucose entry Km of quiescent cells was 3 times that of active and stimulated cells. The methylglucose entry Vmax of quiescent cells was one-eighth that of stimulated cells. A fixed internal methylglucose concentration increased the influx Km of quiescent cells but not that of active or stimulated cells. These findings and others are compatible with a model in which the carrier of quiescent cells is masked by an alteration (such as ligand binding) which is relatively specific for empty as opposed to sugar-loaded carrier and rather indifferent as to in versus out carrier orientation. The masking alteration stabilizes inward-facing and outward-facing unloaded carrier against both translocation and loading. The data fail to show evidence of a masking alteration of loaded carrier.