EXTRACELLULAR MG2+-DEPENDENT NA+, K+, AND CL- EFFLUX IN SQUID GIANT-AXONS

An extracellular Na+ (Na-0)-dependent Mg2+ efflux process that requires intracellular ATP has been proposed as the sole mechanism responsible for Mg2+ extrusion in internally dialyzed squid axons (12). We have shown that this exchanger can also ''reverse'' and mediate an extracellular Mg2+ (Mg-0)-dependent Na+ efflux (16). We have extended these studies and found that, in the presence of ouabain, bumetanide, tetrodotoxin, and K+ channel blockers and in the absence of extracellular Na+, K+, and bicarbonate, intracellular K+ and Cl- are also involved in the Mg,-dependent Na+ efflux process. Two main observations support this view: 1) operation of the Mg-0-dependent Na+ efflux requires the presence of intracellular K+ and Cl-, and 2) Mg-0 removal produces a reversible and nearly identical reduction in the magnitude of the simultaneous efflux of the ionic pairs K+-Na+ and Cl--Na+. These results suggest that the putative bunletanide-insensitive Na-Mg exchanger also transports K+ and Cl-.