NIGERICIN-INDUCED NA+ H+ AND K+/H+ EXCHANGE IN SYNAPTOSOMES - EFFECT ON [H-3] GABA RELEASE/

The effect of the putative K+/H+ ionophore, nigericin on the internal Na+ concentration ([Na-i]) the internal pH (pH(i)), the internal Ca2concentration ([Ca-i]) and the baseline release of the neurotransmitte r, GABA was investigated in Na+-binding benzofuran isophtalate acetoxy methyl ester (SBFI-AM), 2',7'-bis(carboxyethyl)-5(6) carboxyfluorescei n acetoxymethyl ester (BCECF-AM), fura-2 and [H-3]GABA loaded synaptos omes, respectively. In the presence of Na+ at a physiological concentr ation (147 mM), nigericin (0.5 mu M) elevates [Na-i] from 20 to 50 mM, increases the pH(i), 0.16 pH units, elevates four fold the [Ca-i] at expense of external Ca2+ and markedly increases (more than five fold) the release of [H-3]GABA. In the absence of a Na+ concentration gradie nt (i.e. when the external Na+ concentration equals the [Na-i]), the s ame concentration (0.5 mu M) of nigericin causes the opposite effect o n the pH(i) (acidifies the synaptosomal interior), does not modify the [Na-i] and is practically unable to elevate the [Ca-i] or to increase [H-3]GABA release. Only with higher concentrations of nigericin than 0.5 mu M the ionophore is able to elevate the [Ca-i] and to increase t he release of [H-3]GABA under the conditions in which the net Na+ move ments are eliminated. These results clearly show that under physiologi cal conditions (147 mM external Na+) nigericin behaves as a Na+/H+ ion ophore, and all its effects are triggered by the entrance of Na+ in ex change for H+ through the ionophore itself. Nigericin behaves as a K+/ H+ ionophore in synaptosomes just when the net Na+ movements are elimi nated (i.e. under conditions in which the external and the internal Na + concentrations are equal). In summary care must be taken when using the putative K+/H+ ionophore nigericin as an experimental tool in syna ptosomes, as under standard conditions (i.e. in the presence of high e xternal Na+) nigericin behaves as a Na+/H+ ionophore.