Influence of membrane potential on the sodium-dependent uptake of gamma-aminobutyric acid by presynaptic nerve terminals: Experimental observations and theoretical considerations

Summary

Sodium, potassium and veratridine were tested for their effects on the uptake of gamma-aminobutyric acid (GABA) by pinched-off presynaptic nerve terminals (synaptosomes). As noted by previous investigators, the uptake from media containing 1 μm GABA (“high-affinity” uptake) is markedly Na-dependent; the uptake averaged 65 pmoles/mg synaptosome protein × min, with [Na]₀=145mm and [K]₀=5mm, and declined by about 90% when the external Na concentration ([Na]₀) was reduced to 13mm (Na replaced by Li). The relationship between [Na]₀ and GABA uptake was sigmoid, suggesting that two or more Na⁺ ions may be required to activate the uptake of one GABA molecule. Thermodynamic considerations indicate that with a Na⁺/GABA stoichiometry of 2∶1, the Na electrochemical gradient, alone, could provide sufficient energy to maintain a maximum steady-state GABA gradient ([GABA] _i /[GABA]₀) of about 10⁴ across the plasma membrane of GABA-nergic terminals.

In Ca-free media with constant [Na]₀, GABA uptake was inhibited, without delay, by increasing [K]₀ or by introducing 75 μm veratridine; the effect of veratridine was blocked by 200nm tetrodotoxin. The rapid onset (within 10 sec) of the veratridine and elevated-K effects implies that alterations in intra-terminal ion concentrations are not responsible for the inhibition. The uptake of GABA was inversely proportional to log [K]₀. These observations are consistent with the idea that the inhibitory effects of both veratridine and elevated [K]₀ may be a consequence of their depolarizing action. The data are discussed in terms of a barrier model (Hall, J. E., Mead, C.A., Szabo, G. 1973.J. Membrane Biol. 11:75) which relates carrier-mediated ionic flux to membrane potential.