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 granule neurons. All three cell types exhibited a saturable, sodium-dependent uptake of alanine with Km values (microM) of 256 +/- 30, 463 +/- 39, and 292 +/- 39, respectively, and Vmax values (nmol/min/mg) of 15.9 +/- 0.7, 7.9 +/- 0.01, and 17.4 +/- 0.8, respectively. The corresponding values (nmol/min/mg) for the specific activity of alanine aminotransferase were 4.7 +/- 0.4, 17.1 +/- 2.5, and 4.5 +/- 0.9 (all values represent the mean +/- SEM). Release of alanine from the cells was rectilinear with time over a 10 hr period in case of astrocytes (40 nmol/hr/mg) and cerebellar granule neurons (21 nmol/hr/mg). In cortical neurons the release rate declined from an initial value of 19 nmol/hr/mg during the first 3 hr to a value of less than 3 nmol/hr/mg during the subsequent 7 hr of incubation. Metabolism of [14C]alanine to 14CO2 was found to have a lag period of 15 min and subsequently the rate 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 other two cell types. Altogether the results show that alanine is preferentially produced in and released from astrocytes and accumulated into both GABAergic cortical neurons and glutamatergic cerebellar granule neurons.