Extracellular magnesium-dependent sodium efflux in squid giant axons.
Experiments were designed to determine whether the putative Na(+)-Mg2+ exchanger previously demonstrated to mediate Mg2+ efflux (R. DiPolo and L. Beagué. Biochim. Biophys. Acta 946: 424-428, 1988) could also mediate the efflux of Na+ (presumably a Na+ efflux-Mg2+ influx exchange) in squid giant axons. The effects of external Mg2+ (Mg(o)) on 22Na efflux were measured in internally dialyzed, ATP-fueled axons in which the contribution to Na+ efflux by other pathways was inhibited. To facilitate measurement of Mg(o)-dependent Na+ efflux, the intracellular concentration of Na+ was increased. To prevent Na(+)-Na+ exchange, external Na+ was replaced by tris(hydroxymethyl)aminomethane. To assess the effect of Mg(o) on Na+ efflux without altering the total divalent cation concentrations, Mg(o) was replaced mole-for-mole by external Ba2+ (Ba(o)). This manipulation produced reversible reductions in Na+ efflux. These reductions were neither due to membrane hyperpolarization nor to a direct effect of Bao but were due instead to the reduction in Mg(o). The Mg(o)-dependent Na+ efflux was inhibited by external amiloride but was spared by bumetanide. In the absence of external Na+, the Mgo-dependent Na+ efflux increased as a function of external Mg2+ with Michaelis-Menten kinetics. These results indicate that the Na(+)-Mg2+ exchange can mediate the efflux of Na+ (operate in Na+ efflux-Mg2+ influx mode of exchange).
Gonzalez-Serratos
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H
Department of Biophysics, University of Maryland Medical School, Baltimore 21201.
Rasgado-Flores
H
H
D000255:Adenosine Triphosphate / Q000378:metabolism; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001369:Axons / Q000187:drug effects / Q000502:physiology*; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D049832:Decapodiformes; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology*; D010042:Ouabain / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*
D001464:Barium; D010042:Ouabain; D000584:Amiloride; D000255:Adenosine Triphosphate; D012964:Sodium; D008274:Magnesium; D002118:Calcium