Rate Equations and Kinetics of Uptake of α‐Aminoisobutyric Acid and γ‐Aminobutyric Acid by Mouse Cerebrum Slices Incubated in Media Containing L(+)‐Lactate or a Mixture of Succinate, L‐Malate, and Pyruvate as the Energy Source

Abstract

Influx of .alpha.-aminoisobutyric acid (AIB) and GABA by mouse cerebrum slices incubated with L-lactate or a mixture of succinate, L-malate and pyruvate (SMP) as the energy source, follows the phenomenological rate equation for influx from pyruvate and glucose media: v = Vmax/(1 + Kt/S) + kuS, where v is rate and S is concentration of amino acid. There are 2 kinetically distinct, parallel components for concentrative uptake, 1 saturable and 1 unsaturable. Rates are less with lactate than with pyruvate, and are less still with SMP (only GABA was studied), which disproves the hypothesis that lower rates with pyruvate compared to glucose are due to an abnormal redox state in the tissue or to a Krebs cycle unbalanced by input at only 1 point. The carriers for AIB and GABA are qualitatively different. In lactate medium, the capacity of each AIB carrier is unchanged, but its affinity is reduced to 1/3. In lactate and SMP media, the capacity of the saturable GABA carrier is diminished, although its affinity is increased. Rates from these media with added glucose or a glucose analog confirm that amino acid and glucose fluxes are not coupled.