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

An extracellular Na+ (Na-0)-dependent Mg2+ efflux process that require s intracellular ATP has been proposed as the sole mechanism responsibl e for Mg2+ extrusion in internally dialyzed squid axons (12). We have shown that this exchanger can also ''reverse'' and mediate an extracel lular Mg2+ (Mg-0)-dependent Na+ efflux (16). We have extended these st udies and found that, in the presence of ouabain, bumetanide, tetrodot oxin, and K+ channel blockers and in the absence of extracellular Na+, K+, and bicarbonate, intracellular K+ and Cl- are also involved in th e Mg,-dependent Na+ efflux process. Two main observations support this view: 1) operation of the Mg-0-dependent Na+ efflux requires the pres ence of intracellular K+ and Cl-, and 2) Mg-0 removal produces a rever sible and nearly identical reduction in the magnitude of the simultane ous efflux of the ionic pairs K+-Na+ and Cl--Na+. These results sugges t that the putative bunletanide-insensitive Na-Mg exchanger also trans ports K+ and Cl-.