GLUTAMINE TRANSPORT IN CEREBELLAR GRANULE CELLS IN CULTURE

In the present study, uptake of glutamine by rat cerebellar granule ce lls, a predominantly glutamatergic nerve cell population, has been inv estigated. Glutamine is taken up by granule cells via at least three t ransport systems, A, ASC and L. The L-type low affinity system (K-m = 2.6 mM) is the major transport system in the absence of Na+. The syste ms A and ASC represent the Na+-dependent transport routes, both with a lmost identical high affinity for glutamine (K-m = 0.26 mM). Similar t ransport systems for glutamine are also found in cerebral cortical neu rons, a predominantly GABAergic nerve cell population, and cerebral co rtical astrocytes. The glutamine transport properties in granule cells , however, show a series of differences from that of cortical neurons and astrocytes: (1) uptake of glutamine by granule cells is primarily mediated by system A (54%), while contributions by system A in cortica l neurons and astrocytes are less than 30%; (2) granule cells exhibit strikingly higher transport efficiency for glutamine (V-max/K-m = 20 m in(-1) for system A as compared to the V-max/K-m ratio of 5 min(-1) in cortical neurons and astrocytes), and (3) the initial uptake rates an d the steady-state accumulation levels of glutamine are two- to threef old higher in granule cells than that of cortical neurons and astrocyt es. These results taken together suggest that in accordance with the i mportant need to replenish the neurotransmitter pool of glutamate, glu tamatergic neurons exhibit highly efficient transport systems to accum ulate glutamine, one of the major precursors of glutamate.