Release of γ-[ 3H]aminobutyric acid from synaptosomes: effect of external cations and of ouabain

Abstract

In the present study we have investigated the effect of cations and ouabain on Ca 2+-independent and Ca 2+-dependent release of γ-[ 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 Ca 2+, ouabain of 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 Ca 2+ 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 Ca 2+, 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.