Zinc Uptake into Synaptosomes

Abstract

Zinc uptake was studied in synaptosomes, isolated by the Ficoll flotation technique, using the radiotracer 65Zn. True uptake of zinc could be discriminated from binding to the outside of the synaptosomes by the absence of accumulation at 0 degree C and the dependency of the rate of uptake on the medium osmolarity. The zinc uptake, studied in the presence of various zinc-complexing agents, showed saturation kinetics when analyzed in terms of [Zn]free, yielding Km = 0.25 microM. The zinc uptake was independent of both ATP and the Na+ gradient. No efflux of zinc could be demonstrated from preloaded synaptosomes due to the formation of insoluble zinc complexes inside the synaptosomes. The results are discussed in terms of the modulation of diverse neurochemical processes by zinc.

Subunit-Dependent High-Affinity Zinc Inhibition of Acid-Sensing Ion Channels

“…This would require the presence of a high-affinity Zn 2ϩ uptake system. Recent cloning of Zn 2ϩ transporters in mammalian brains (Palmiter and Findley, 1995) and the finding of both high-and low-affinity Zn 2ϩ uptake mechanisms at neuronal presynaptic terminals (Howell et al, 1984;Wensink et al, 1988;Palmiter et al, 1996a,b) strongly suggest that a dynamic change in Zn 2ϩ concentration in the nanomolar range might be possible.…”

Zinc Inhibition of cAMP Signaling

“…The fate of neuronally released Zn 2ϩ is not totally clear. It is likely that some of the metal is taken back up by the presynaptic neurons and reconcentrated into vesicles (8). The movement of Zn 2ϩ from presynaptic to postsynaptic cell has been documented.…”

“…It is generally accepted that the transfer of metals across the plasma membranes involves metal complexes that influence the efficiency with which the metal is delivered or transferred (8,(21)(22)(23) Fig. 4, Zn 2ϩ became a more potent inhibitor of forskolin-stimulated cAMP accumulation when ascorbate was present.…”

“…One condition was the use of the heavy metal ionophore, pyrithione, whereas another condition more apt to reflect a physiological relevant condition was the addition of ascorbate. It is not uncommon that the form of metal chelation influences the efficiency of metal transport (8,(21)(22)(23). Ascorbate is of particular interest, because it accumulates in the brain (being maintained at relatively high (1-2 mM) concentrations (29,30)).…”

The mechanism of zinc uptake by cultured rat liver cells.

“…65Zn uptake is stimulated by addition of histidine in human erythrocytes (Van Wouwe, Veldhuizen, de Goeij & Van den Hamer, 1990), rat renal proximal tubular cells (Gachot et al 1991) and rat erythrocytes (Aiken et al 1992), yet the resulting changes in Zn2+ concentration were not quantified. Where histidine has been added and free Zn2+ concentrations have been estimated, 65Zn uptake has been found to correlate quite well with the free Zn2+ concentration in rat fibroblasts (Ackland & McArdle, 1990) and in rat brain synaptosomes (Wensink, Molenaar, Woroniecka & Van den Hamer, 1988).…”