Release of gamma-[3H]aminobutyric acid from synaptosomes: effect of external cations and of ouabain.
In the present study we have investigated the effect of cations and ouabain on Ca(2+)-independent and Ca(2+)-dependent release of gamma-[3H]aminobutyric acid ([3H]GABA) from sheep brain synaptosomes. The presence of Na+ in the external medium is essential for the Ca(2+)-independent release induced by K+ or ouabain. Thus, in the absence of Ca2+, ouabain or K+ causes the release of [3H]GABA provided that Na+ is present in the external medium. Under K(+)-depolarizing conditions, in a Na+ medium, either ouabain or Ca2+ further increases the [3H]GABA release induced by depolarization, but their effects are not additive. The presence of external Na+ is not required for the Ca(2+)-dependent release of [3H]GABA due to K+ depolarization, and this release, which occurs in a choline medium, is not modified by ouabain. Under these conditions (choline medium) K(+)-depolarization dependent release is absolutely dependent on external Ca2+, which suggests that this release of [3H]GABA occurs only by exocytosis, without the carrier-mediated efflux which normally co-exists with exocytosis due to K(+)-depolarization in a Na+ medium. It is likely that the release induced by ouabain or K+ involves the membrane carrier which responds to changes in membrane potential.
Santos
M S
MS
Center for Cell Biology, Department of Zoology, University of Coimbra, Portugal.
Gonçalves
P P
PP
Carvalho
A P
AP
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002412:Cations / Q000494:pharmacology*; D002794:Choline / Q000378:metabolism; D005089:Exocytosis / Q000187:drug effects; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D010042:Ouabain / Q000494:pharmacology*; D011188:Potassium / Q000494:pharmacology; D012756:Sheep; D012964:Sodium / Q000378:metabolism; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D002412:Cations; D005680:gamma-Aminobutyric Acid; D010042:Ouabain; D012964:Sodium; D002794:Choline; D011188:Potassium; D002118:Calcium