Neuronal efflux of noradrenaline induced by tris or lithium as substitutes for extracellular sodium

Summary

Vasa deferentia of either untreated or reserpine (R) and/or pargyline (P) pretreated rats were incubated with³H-noradrenaline and then washed with amine- and Ca²⁺-free solution until (after 100 min) the efflux of radioactivity largely originated from adrenergic nerve endings; COMT was inhibited by U-0521 (U).

After 110 min of wash out, the sodium chloride in the wash-out solution was replaced by an equimolar concentration of either Tris-HCl or LiCl. This caused a despramine-sensitive (i.e., carrier-mediated) efflux of tritiated noradrenaline. The initial increase of the “low Na⁺”-induced efflux dependent on the experimental conditions: it was most pronounced when the axoplasmic concentration of noradrenaline was high (RPU) and relatively small when MAO and vesicular storage were intact (U). The effects of Li⁺ and Tris⁺ differed with regard to the time course of the efflux of tritium: under all three experimental conditions (RPU, PU, U), Tris⁺ caused the rate of efflux of tritium to increase gradually within the 30 min period of observation, while Li⁺⁺ either had a “peak-effect” (RPU, PU) or a “plateau-effect” (U). Under “U-conditions” Tris⁺ caused a slowly increasing, pronounced increase with time of the efflux of both,³H-noradrenaline and³H-DOPEG; whereas Li⁺ caused only a small and sustained increase of the efflux of³H-noradrenaline and a decrease in the efflux of³H-DOPEG.

Conclusions: 1) The results are compatible with the view that the buffering agent Tris can diffuse into nerve endings and then also into storage vesicles, and, thus, increases the intravesicular pH; as a consequence of the elevated pH, the leakage of noradrenaline from the vesicles increases and, thus, more noradrenaline becomes available for deamination in and outward transport from the axoplasm. 2) A decrease of the sodium-gradient (brought about by e.g. low extracellular sodium) increases the availability of carrier sites on the internal face of the axonal membrane. This results in outward transport only when the axoplasmic concentration of noradrenaline is elevated (either due to inhibition of MAO or to increased vesicular efflux of noradrenaline).