Cerebral alanine transport and alanine aminotransferase reaction: alanine as a source of neuronal glutamate.
Alanine transport and the role of alanine amino-transferase in the synthesis and consumption of glutamate were investigated in the preparation 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 electrical potential, which suggests a cotransport with Na+. Rapid accumulation of the Na(+)-alanine complex by synaptosomes stimulated activity of the Na+/K+ pump and increased energy utilization; this, in turn, activated the ATP-producing pathways, glycolysis and oxidative phosphorylation. Accumulation of Na+ also caused a small depolarization of the plasma membrane, a rise in [Ca2+]i, and a release of glutamate. Intra-synaptosomal metabolism of alanine via alanine amino-transferase, as estimated 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 [15N]alanine formation from [15N]glutamine was seven to eight times higher than the rate of [15N]-glutamate generation from [15N]alanine. It is concluded that (a) overloading of nerve endings with alanine could be deleterious to neuronal function because it increases release of glutamate; (b) the activity of synaptosomal alanine aminotransferase is much slower than that of glutaminase and hence unlikely to play a major role in maintaining [glutamate] during neuronal activity; and (c) alanine amino-transferase might serve as a source of glutamate during recovery from ischemia/hypoxia when the alanine concentration rises and that of glutamate falls.
Erecińska
M
M
Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104-6084.
Nelson
D
D
Nissim
I
I
Daikhin
Y
Y
Yudkoff
M
M
D000409:Alanine / Q000378:metabolism*; D000410:Alanine Transaminase / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D001921:Brain / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D016257:Fura-2; D005971:Glutamates / Q000096:biosynthesis / Q000378:metabolism*; D018698:Glutamic Acid; D005973:Glutamine / Q000378:metabolism; D007553:Isotope Labeling / Q000379:methods; D007700:Kinetics; D008297:Male; D009474:Neurons / Q000378:metabolism*; D009587:Nitrogen Isotopes; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000494:pharmacology; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*
D005971:Glutamates; D009587:Nitrogen Isotopes; D005973:Glutamine; D018698:Glutamic Acid; D012964:Sodium; D000410:Alanine Transaminase; D000409:Alanine; D011188:Potassium; D002118:Calcium; D016257:Fura-2