Nigericin-induced Na+/H+ and K+/H+ exchange in synaptosomes: effect on [3H]GABA release.
The effect of the putative K+/H+ ionophore, nigericin on the internal Na+ concentration ([Nai]), the internal pH (pHi), the internal Ca2+ concentration ([Cai]) and the baseline release of the neurotransmitter, GABA was investigated in Na+-binding benzofuran isophtalate acetoxymethyl ester (SBFI-AM), 2',7'-bis(carboxyethyl)-5(6) carboxyfluorescein acetoxymethyl ester (BCECF-AM, fura-2 and [3H]GABA loaded synaptosomes, respectively. In the presence of Na+ at a physiological concentration (147 mM), nigericin (0.5 microM) elevates [Nai] from 20 to 50 mM, increases the pHi, 0.16 pH units, elevates four fold the [Cai] at expense of external Ca2+ and markedly increases (more than five fold) the release of [3H]GABA. In the absence of a Na+ concentration gradient (i.e. when the external Na+ concentration equals the [Nai]), the same concentration (0.5 microM) of nigericin causes the opposite effect on the pHi (acidifies the synaptosomal interior), does not modify the [Nai] and is practically unable to elevate the [Cai] or to increase [3H]GABA release. Only with higher concentrations of nigericin than 0.5 microM the ionophore is able to elevate the [Cai] and to increase the release of [3H]GABA under the conditions in which the net Na+ movements are eliminated. These results clearly show that under physiological conditions (147 mM external Na+) nigericin behaves as a Na+/H+ ionophore, and all its effects are triggered by the entrance of Na+ in exchange for H+ through the ionophore itself. Nigericin behaves as a K+/H+ ionophore in synaptosomes just when the net Na+ movements are eliminated (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 synaptosomes, as under standard conditions (i.e. in the presence of high external Na+) nigericin behaves as a Na+/H+ ionophore.
Depto. de Biología Celular, UNAM, Ciudad Universitaria, México, D.F., México.
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Rodríguez
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Sitges
D000818:Animals; D017920:Antiporters / Q000378:metabolism*; D001572:Benzofurans; D001921:Brain / Q000378:metabolism*; D005452:Fluoresceins; D005456:Fluorescent Dyes; D016257:Fura-2; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D051379:Mice; D009550:Nigericin / Q000494:pharmacology*; D010795:Phthalic Acids; D011188:Potassium / Q000378:metabolism*; D027961:Potassium-Hydrogen Antiporters; D012964:Sodium / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D017920:Antiporters; D001572:Benzofurans; D005452:Fluoresceins; D005456:Fluorescent Dyes; D010795:Phthalic Acids; D027961:Potassium-Hydrogen Antiporters; D017923:Sodium-Hydrogen Exchangers; C103805:sodium binding benzofuran isophthalate acetoxymethyl ester; C057433:2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester; D005680:gamma-Aminobutyric Acid; D012964:Sodium; D009550:Nigericin; D011188:Potassium; D016257:Fura-2