The bidirectional transport of Mg in rat liver was studied using slices labeled with 28Mg in a closed two-compartment system under steady-state conditions. The influx (Kbs) and efflux (Ksb) transfer coefficients governing transport between the extracellular phase and a rapidly exchanging cell fraction were 0.074 and 0.019 per min, respectively. An increased extracellular concentration of Mg++ caused a 30% decrease in Kbs and a 31% increase in Ksb. A decreased extracellular Mg++ had an opposite effect. At 0°C, both transfer coefficients were reduced by 65%. Increased pH and NaCN increased transport, whereas Ca++ reduced transport. Reduced pH, altered Na+:K+ ratio, Sr++, glucose deletion, iodoacetate, ethanol, and lactate had no significant influence. Dinitrophenol reduced Ksb but had no effect on Kbs. These data support the thesis that the intracellular concentration of Mg is in part regulated by a reciprocal change in the influx transfer coefficient and a parallel change in the efflux transfer coefficient in response to altered extracellular concentrations of Mg++. The qualitative and quantitative similarities of Mg and Ca transport in this system suggest that Mg and Ca share a common transport mechanism which is primarily dependent upon the binding of these divalent cations to macromolecular ligands within the cell membrane or within the cell.

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