Kinetics of and rate equations for the uptake of alpha-amino-isobutyric aicd and gamma-aminobutyric acid by mouse brain slices incubated in a glucose-free medium containing pyruvate as the energy source

Lactate-supported synaptic function in the rat hippocampal slice preparation

The present study was undertaken to examine the possibility that cerebral energy metabolism can be fueled by lactate. As a sole energy substrate, lactate supported normal synaptic function in rat hippocampal slices for hours without any sign of deterioration. Slices that were synaptically silent as a result of glucose depletion could be reactivated with lactate to show normal synaptic function. When slices were exposed to the glycolytic inhibitor iodoacetic acid, lactate-supported synaptic function was unaffected, whereas that supported by glucose was completely abolished. This indicated that lactate was metabolized directly via pyruvate to enter the tricarboxylic acid cycle. Thus, under conditions that lead to lactate accumulation (cerebral ischemia) this "end product" may be a useful alternative as a substrate for energy metabolism.

"Exchange diffusion": rate equations for the influx of alpha-aminoisobutyric acid into mouse cerebrum slices containing this amino acid

Rate equations for the gross influx of alpha-aminoisobutyric acid (AIB) into mouse cerebrum slices containing AIB have a first-order term for unsaturable concentrative influx, identical to the corresponding term for unloaded slices, and a modified Michaelis-Menten term, V'max/(1 + Kt/S), for saturable concentrative influx. [V'max identical to v'L(1 + Kt/S), where v'L = saturable component of influx, S = AIB concentration in medium, and Kt = Michaelis constant for unloaded slices.] Below a tissue AIB (T) of 19 mumol/g final wet weight, V'max increases linearly following V'max = V1 + m1T; above that value, Vmax is virtually constant. The transition is sharp. This equation is consistent with a carrier model for active transport. At the transition, intracellular AIB is about 1 molecule for every 70 amino acid residues of tissue protein, vastly more than could be accommodated by AIB-binding sites in cell membranes. The transition may come from a slow process that does not fill all sites when the tissue AIB is below the transition concentration, or from an AIB-induced phase transition in the membrane.

Rate equations and kinetics of uptake of alpha-aminoisobutyric acid and gamma-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

Influx of alpha-aminoisobutyric acid (AIB) and gamma-aminobutyric acid (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 two kinetically distinct, parallel components for concentrative uptake, one saturable, and one unsaturable. Rates are less with lactate than with pyruvate and still less with SMP (only GABA was studied), disproving the hypotheses 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 one 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 one-third. 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.