Ca2+ sensitivity of synaptic vesicle dopamine, gamma-aminobutyric acid, and glutamate transport systems.
The effect of Ca2+ on the uptake of neurotransmitters by synaptic vesicles was investigated in a synaptic vesicle enriched fraction isolated from sheep brain cortex. We observed that dopamine uptake, which is driven at expenses of the proton concentration gradient generated across the membrane by the H+-ATPase activity, is strongly inhibited (70%) by 500 microM Ca2+. Conversely, glutamate uptake, which essentially requires the electrical potential in the presence of low Cl- concentrations, is not affected by Ca2+, even when the proton concentration gradient greatly contributes for the proton electrochemical gradient. These observations were checked by adding Ca2+ to dopamine or glutamate loaded vesicles, which promoted dopamine release, whereas glutamate remained inside the vesicles. Furthermore, similar effects were obtained by adding 150 microM Zn2+ that, like Ca2+, dissipates the proton concentration gradient by exchanging with H+. With respect to gamma-aminobutyric acid transport, which utilizes either the proton concentration gradient or the electrical potential as energy sources, we observed that Ca2+ or Zn2+ do not induce great alterations in the gamma-aminobutyric acid accumulation by synaptic vesicles. These results clarify the nature of the energy source for accumulation of main neurotransmitters and suggest that stressing concentrations of Ca2+ or Zn2+ inhibit the proton concentration gradient-dependent neurotransmitter accumulation by inducing H+ pump uncoupling rather than by interacting with the neurotransmitter transporter molecules.
Gonçalves
P P
PP
Centro de Biologia Celular, Departamento de Biologia, Universidade de Aveiro, Portugal. pgoncalves@bio.ua.pt
Meireles
S M
SM
Neves
P
P
Vale
M G
MG
D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D027322:Amino Acid Transport System X-AG; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000494:pharmacology*; D002540:Cerebral Cortex / Q000378:metabolism*; D004298:Dopamine / Q000378:metabolism*; D012756:Sheep; D013572:Synaptic Vesicles / Q000378:metabolism*; D015032:Zinc / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D018528:ATP-Binding Cassette Transporters; D027322:Amino Acid Transport System X-AG; D005680:gamma-Aminobutyric Acid; D015032:Zinc; D002118:Calcium; D004298:Dopamine