CEREBRAL ALANINE TRANSPORT AND ALANINE AMINOTRANSFERASE REACTION - ALANINE AS A SOURCE OF NEURONAL GLUTAMATE

Alanine transport and the role of alanine aminotransferase in the synt hesis and consumption of glutamate were investigated in the preparatio n of rat brain synaptosomes. Alanine was accumulated rapidly via both the high- and low-affinity uptake systems. The high-affinity transport was dependent on the sodium concentration gradient and membrane elect rical potential, which suggests a cotransport with Na+. Rapid accumula tion of the Na+-alanine complex by synaptosomes stimulated activity of the Na+/K+ pump and increased energy utilization; this, in turn, acti vated the ATP-producing pathways, glycolysis and oxidative phosphoryla tion. Accumulation of Na+ also caused a small depolarization of the pl asma membrane, a rise in [Ca2+](i), and a release of glutamate. Intras ynaptosomal metabolism of alanine via alanine aminotransferase, as est imated from measurements of N fluxes from labeled precursors, was much slower than the rate of alanine uptake, even in the presence of added oxoacids. The velocity of [N-15]alanine formation from [N-15]glutamin e was seven to eight times higher than the rate of [N-15]glutamate gen eration from [N-15]alanine. It is concluded that (a) overloading of ne rve endings with alanine could be deleterious to neuronal function bec ause it increases release of glutamate; (b) the activity of synaptosom al alanine aminotransferase is much slower than that of glutaminase an d hence unlikely to play a major role in maintaining [glutamate] durin g neuronal activity; and (c) alanine aminotransferase might serve as a source of glutamate during recovery from ischemia/hypoxia when the al anine concentration rises and that of glutamate falls.