UPTAKE, RELEASE, AND METABOLISM OF ALANINE IN NEURONS AND ASTROCYTES IN PRIMARY CULTURES

The uptake, release, and metabolism of alanine were studied in primary cultures of cerebral cortical neurons or astrocytes and cerebellar gr anule neurons. All three cell types exhibited a saturable, sodium-depe ndent uptake of alanine with K(m) values (muM) of 256 +/- 30, 463 +/- 39, and 292 +/- 39, respectively, and V(max) values (nmol/min/mg) of 1 5.9 +/- 0.7, 7.9 +/- 0.01, and 17.4 +/- 0.8, respectively. The corresp onding values (nmol/min/mg) for the specific activity of alanine amino transferase were 4.7 +/- 0.4, 17.1 +/- 2.5, and 4.5 +/- 0.9 (all value s represent the mean +/- SEM). Release of alanine from the cells was r ectilinear with time over a 10 hr period in case of astrocytes (40 nmo l/hr/mg) and cerebellar granule neurons (21 nmol/hr/mg). In cortical n eurons the release rate declined from an initial value of 19 nmol/hr/m g during the first 3 hr to a value of less than 3 nmol/hr/mg during th e subsequent 7 hr of incubation. Metabolism of [C-14]alanine to (CO2)- C-14 was found to have a lag period of 15 min and subsequently the rat e of CO2 production was constant over a 45 min period with a value of 0.5 nmol/min/mg in granule neurons and about 0.3 nmol/min/mg in the ot her two cell types. Altogether the results show that alanine is prefer entially produced in and released from astrocytes and accumulated into both GABAergic cortical neurons and glutamatergic cerebellar granule neurons. (C) 1993 Wiley-Liss, Inc.