A kinetic analysis of sodium dependent glutamic acid transport in peripheral nerve

Uptake of [N-Me-3H]-choline by synaptosomes from the central nervous system of Locusta migratoria

The accumulation of 3H-choline by isolated synaptosomes from the central nervous system of locust was studied at concentrations varying from 0.05 to 40 microM. Kinetic analysis of the saturable process revealed a high-affinity and a low-affinity system. The high-affinity uptake was competitively inhibited by hemicholinium-3 and was absolutely dependent on external sodium. Elevated potassium concentrations inhibited choline uptake. The choline uptake by insect synaptosomes was found to be remarkably resistant to a variety of metabolic inhibitors. The reduced choline uptake under depolarizing conditions (high potassium concentration or veratridine) in the absence of calcium implies that electrochemical gradients are important for high-affinity choline uptake. Depolarization of preloaded synaptosomes under appropriate conditions resulted in a significant release of newly accumulated choline radioactivity.

Cation dependence of hypotaurine uptake in mouse brain slices

Mouse brain slices take up hypotaurine (2-aminoethanesulphinic acid) from medium by means of two concentrative low- and high-affinity transport systems. [35S]Hypotaurine uptake by the slices was significantly reduced in the absence of external potassium, calcium, or magnesium ions. An excess of potassium ions also inhibited hypotaurine uptake by one-half. Uptake was almost completely abolished on removal of sodium ions. The Km constants for both low- and high-affinity transport components increased in a low-sodium medium, suggesting that sodium ions are required when hypotaurine is attached to its possible carrier sites in plasma membranes. Sodium ions also mimicked allosteric effectors of hypotaurine transport, showing positive cooperativity. More than two sodium ions may be involved in the transport of one hypotaurine molecule across the cell membrane. The calculated activation energies of transport were fairly similar in normal and sodium-deficient media and thus sodium ions may not participate in the activation mechanisms of the transport. With respect to cation dependence, hypotaurine transport in brain slices exhibits features characteristic of neurotransmitter amino acids.

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.

Effects of polarizing currents and repetitive stimulation on the uptake of amino acids by peripheral nerve

The effects of polarizing currents and repetitive stimulation on the uptake of representative amino acids have been studied in peripheral nerve. Glutamic acid, lysine, pheylalanine, and GABA were chosen for study as representatives of different carrier-mediated transport mechanisms. Depolarizing currents inhibited uptakes of glutamic acid, lysine, and GABA; maximum inhibitions were 56.1%, 20.0%, and 16.7%, respectively. Pheylalanine uptake was not inhibited by polarizing currents under any experimental conditions. Only glutamic acid uptake was inhibited by hyperpolarizing current, with a maximum inhibition of 43.9%. Stimulation had no significant effect on uptake of lysine, phenylalanine, or GABA but was found to potentiate uptake of glutamic acid by 12%.

A model of GABA transport by cortical synaptosomes from the Long-Evans rat

The initial velocity of uptake of GABA by cortical synaptosomes from the Long-Evans rat has been measured as a function of both sodium and GABA concentration. These data were then fitted to the rate equation for each of several possible models, and the models giving least error were identified. Although one cannot unequivocally distinguish between two of the models on the basis of the goodness of fit, deductions based on the fits of subsets of the models point to one of the two models, The major predictions from this model are as follows: 1) Uptake of GABA is totally sodium-dependent. 2) Although plots of 1/v versus 1/[Na]2 are nonlinear, the coupling ratio for transport (Na/GABA) is 2. 3) For transport to take place, the order of combination with carrier must be Na, Na, GABA. 4) Maximal velocity will occur only at infinite Na and GABA concentrations. 5) There is a sigmoidal relationship between apparent maximal velocity (Va) and [Na]. 6) Kt, the [GABA] that gives a velocity equal to Va/2, rises and then falls as [Na] is increased from zero, passing through a maximum at 33.52 mM [Na]. 7) The relationship between initial velocity and [Na] is sigmoidal. 8) Jm, rate of uptake with infinite [Na], is hyperbolically related to [GABA]; Jm approaches Vmax as [GABA] becomes very large. 9) kappa Na, the [Na] giving a velocity equal to Jm/2, declines rapidly from 10(-7) M to 10(-5)M GABA, but is essentially constant at 10(-4)M above. 10) One GABA molecule is translocated per carrier molecule. The model is consistent with observations in the literature concerning the effects of sodium on the kinetics of transport.