A comparison of the rate equations, kinetic parameters, and activation energies for the initial uptake of L-lysine, L-valine, gamma-aminobutyric acid, and alpha-aminoisobutyric acid by mouse brain slices

The role of excitatory amino acids in hypothermic injury to mammalian spinal cord neurons

Hypothermia has been reported to be beneficial in CNS physical injury and ischemia. We previously reported that posttraumatic cooling to 17 degrees C for 2 h increased survival of mouse spinal cord (SC) neurons subjected to physical injury (dendrite transection) but that cooling below 17 degrees C caused a lethal NMDA receptor-linked stress to both lesioned and uninjured neurons. The present study tested whether cooling below 17 degrees C increases extracellular levels of excitatory amino acids (EAA). SC cultures were placed at 10 degrees C or 37 degrees C. Glutamate (Glu) and aspartate (Asp) levels were higher in the medium of the cooled cultures after 0.5 h (23 +/- 4 nM/microgram vs. 4 +/- 1 nM/microgram and 4 +/- 1 nM/microgram vs. 1 +/- 0 nM/microgram, respectively). The concentration of each EAA then declined and reached a plateau at 2-4 h that was still significantly higher than control levels (p < 0.0001, two-factor ANOVA, three cultures per group). Other amino acids (glycine, asparagine, glutamine, serine) showed an opposite pattern, with higher levels in the 37 degrees C group. Both NMDA and non-NMDA antagonists prevented the lethal cold injury. Survival of SC neurons cooled at 10 degrees C for 2 h and rewarmed for 22 h was 58% +/- 25% in the control group, 94% +/- 5% in the CNQX-treated group, 97% +/- 5% in the DAPV-treated group, and 99% +/- 2% in the group treated with both antagonists [p < 0.0006, one factor ANOVA, five cultures (> 120 neurons) per group]. These results show that death of neurons cooled to 10 degrees C is caused by elevated extracellular Glu and Asp and requires activation of both the NMDA and non-NMDA receptor subtypes.

Different effects of hypothermia on amino acid incorporation and on amino acid uptake in the brain in vivo

The temperature dependence of the incorporation of amino acids into cerebral proteins and that of the transport of amino acids through the blood-brain barrier were studied. We measured the protein synthesis rate in vivo over a wide temperature range (14 degrees C-38 degrees C) in male Sprague-Dawley rats using a flooding dose of labeled valine. There was a linear dependence of the protein synthesis rate on temperature. The temperature quotient expressed as per cent decrease per 1 degree C was somewhat lower at the lower temperatures, a decrease from 7.8% in the 37.7-32.5 degrees C range to 6.7% in the 25.5-14 degrees C range. The transport of the three amino acids phenylalanine, lysine, and alanine, representing three transport systems, through the blood-brain barrier showed no temperature dependence in vivo. The results show that in hypothermia cerebral metabolic rates are lowered to a great extent, while some aspects of metabolic transport are not affected.

Amino acid uptake systems in lizard and chick brain cells

The uptake of seven amino acids, alpha-aminoisobutyric acid (AIB), cyclo-leucine (cyclo-Leu), gamma-aminobutyric acid (GABA), glycine (Gly), glutamic acid (Glu), lysine (Lys), and taurine (Tau), representatives of different amino acid transport systems, was studied in slices of brain from Tokay lizards and White Leghorn chicks. In descending order, the rate of the initial uptake of the amino acids in both species was Glu greater than Gly greater than GABA greater than Cyclo-Leu greater than AIB greater than Lys greater than Tau. The substrate specificities and the differences in sodium and temperature dependence of the uptake of the amino acids indicate the presence of several distinct amino acid transport systems, some sodium-dependent and some sodium-independent. The structural specificity of amino acid transport classes in the brain of these species is similar to that in other vertebrate brain preparations.

Comparative aspects of the apparent Michaelis constant for neutral amino acid transport in several animal tissues

The apparent Michaelis constant, Km, for transport of a number of neutral amino acids has been compared between intestine, heart, brain and erythrocytes among a variety of animals using values available in the literature. Neutral amino acids with side chains containing 3, 4, 7 and 9 carbon atoms had approximately equal mean Km values when tested for intestinal transport among a variety of species; alanine appeared to have a mean Km value that was larger than those found for the first group, and glycine had a significantly greater mean Km than all of the other compounds tested. Km values for phenylalanine and tryptophan measured in rat heart were found to be close to the means measured for these substrates in intestine. The mean Km values measured in mammalian brain for each of the neutral amino acid substrates were found not be significantly different from each other. When the means of Km values for the neutral amino acids tested were compared between intestine and brain, only the glycine means were shown to differ significantly between the organs. Based on data for several mammalian species, brain appears to have a greater average apparent affinity for glycine than does intestine. In the human erythrocytes and in a few other mammalian species, Km values for all neutral amino acids tested with exception of glycine were found to be similar in magnitude to each other and to the Km averages of neutral amino acids found in intestine for the series containing 3-9 carbon atoms. The Km value for glycine in the human erythrocyte was noted to be substantially lower in value than the averages for glycine in brain or intestine. Avian red blood cells appear to have high apparent affinity for neutral amino acid transport when compared with red cells of several mammalian species.

"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.

L-DOPA (L-3,4-dihydroxyphenylalanine) uptake by human red blood cells

The uptake of L-DOPA (L-3,4-dihydroxyphenylalanine) was studied in normal human red blood cells in vitro using L-[3-14C]DOPA. Uptake was slow, tending towards a distribution ratio close to unity with a half-time to equilibrium of one hour. Uptake was not Na+-dependent. Concentration dependence studies showed both saturable and non-saturable components of uptake, and inhibition studies using L-leucine and L-tryptophan suggest that the L and T systems of red cell amino acid uptake are involved. A powerful inhibitor of both systems, 3,4-dihydroxy-2-methylpropriophenone (U-0521), is described. It is concluded that uptake is by carrier-mediated facilitated diffusion via the L and T systems for which L-DOPA has low affinity.

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

Mouse cerebrum slices were incubated in a medium containing pyruvate instead of glucose as the energy source. After a preincubation period alpha-aminoisobutyric acid (AIB) or gamma-aminobutyric acid (GABA) was added, and the rate of uptake by the slices was measured. AIB and GABA are taken up to above their concentration in the medium. Although influx is slower, the rate equation is the same as that for influx from a glucose medium; namely v = Vmax/1 x Kt/S) + kuS, where v = rate of uptake, S = concentration of AIB or GABA in the medium, and Vmax, Kt, and Ku are constants. The equation shows two parallel pathways for concentrative uptake, one saturable and one unsaturable. The uptake systems for AIB and GABA are qualitatively different. The maximum rate of uptake of AIB by the saturable component is the same in both media even though, in the pyruvate medium, AIB is bound less strongly to the 'carriers'. The maximum rate of uptake of GABA by the saturable component is less in the pyruvate medium although GABA is bound somewhat more strongly to the 'carriers'. The temperature coefficients of the kinetic parameters and their corresponding energies were determined for GABA. Going from glucose to pyruvate medium has little effect on the Arrhenius activation energy (Ea) associated with Vmax and the heat of reaction (delta H) associated with Kt but increases Ea associated with Ku by 1140%.

Influx of gamma-aminobutyric acid and alpha-aminoisobutyric acid into mouse cerebrum slices incubated in a pyruvate medium with or without added glucose or glucose analogues compared with influx from a glucose medium

Concentrative influx of gamma-aminobutyric acid (GABA) and alpha-amino-isobutyric acid (AIB) into incubated mouse cerebrum slices is decreased when pyruvate is substituted for glucose. Influx of GABA from pyruvate medium is not increased by presence of glucose, 2-deoxy-D-glucose (2-DOG), or 3-O-methyl-D-glucose (3-O-MeG). Influx of AIB is restored to the rate from glucose medium if 2-DOG is present initially, but is not restored if 2-DOG is added with AIB. Influx is not restored if 3-O-MeG is present initially, but is restored if 3-O-MeG is added with AIB. Influx is restored if glucose is present initially or is added with AIB.

The complete rate equation, including the explicit dependence on Na+ ions, for the influx of alpha-aminoisobutyric acid into mouse brain slices

The rate equation, including dependence on Na+-ion concentration for the influx of alpha-aminoisobutyric acid into mouse brain slices incubated in isotonic glucose medium at 37 degrees C, is v = 0.402 S/(1.02(1 + 788/[Na+]2)+S)+0.0477S, where v = influx in mu mol/min, g final wet wt of slices; [Na+] = concentbutyric acid in medium, in mM. This equation shows two kinetically independent, parallel pathways of concentrative uptake: one, saturable and dependent on Na+; the other, unsaturable and independent of Na+. Influx is independent of ionic strength, Cl- ion per se, and a moderate increase in tonicity. The binding of substrate to the saturable carrier depends on the Na+ concentration; the maximum capacity of this carrier does not. For transport, 2 Na+ ions must interact with each saturable transport site. This does not imply coupling between the flux of Na+ and the flux of alpha-aminoisobutyric acid.

Taurine transport by rat hepatocytes in primary culture

Hepatic taurine concentration is 30--100 times that of plasma, suggesting an efficient taurine uptake mechanism in the hepatocyte. The characteristics of hepatic taurine transport were studied in primary cultures of adult rat hepatocytes. Taurine uptake was concentrative and linear for over 4 h. At taurine concentrations 2.5--100 microM, uptake was saturable with constants Km = 44 microM, V = 0.28 nmol/mg protein per min and EA = 13.2 kcal/mol. Uptake was inhibited 41% by incubation under N2 and was competitively inhibited by beta-alanine (Ki = 94 microM) and hypotaurine (Ki = 14 microM). Uptake was linearly dependent upon Na+ concentration from 0 to 140 mM. A second nonsaturable uptake process was identifiable only at taurine concentrations greater than 1 mM. This process was presumed to represent passive diffusion. At taurine concentrations existing in plasma, taurine enters the hepatocyte primarily by a single, Na+-dependent, carrier-mediated, oxygen-requiring transport process.