The Cellular Transport of Magnesium in Rat Liver

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 (K_bs) and efflux (K_sb) 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 K_bs and a 31% increase in K_sb. 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 K_sb but had no effect on K_bs. 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.