MAGNESIUM TRANSPORT ACROSS THE BASOLATERAL PLASMA-MEMBRANE OF THE FISH ENTEROCYTE

In tilapia (Oreochromis mossambicus) intestine, Mg2+ transport across the epithelium involves a transcellular, Na+- and Na+/K+-ATPase depend ent pathway. In our search for the Mg2+ extrusion mechanism of the bas olateral compartment of the enterocyte, we could exclude Na+/Mg2+ anti port or ATP-driven transport. Evidence is provided, however, that Mg2 movement across the membrane is coupled to anion transport. In basola teral plasma membrane vesicles, an inwardly directed CT gradient stimu lated Mg2+ uptake (as followed with the radionuclide Mg-27) twofold. A s Cl--stimulated uptake was inhibited by the detergent saponin and by the ionophore A23187, Mg2+ may be accumulated intravesicularly above c hemical equilibrium. Valinomycin did not affect uptake, suggesting tha t electroneutral symport activity occurred. The involvement of anion c oupled transport was further indicated by the inhibition of Mg2+ uptak e by the stilbene derivative, 4,4'-diisothiocyanato-stilbene-2,2'-disu lfonic acid. Kinetic analyses of the Cl--stimulated Mg2+ uptake yielde d a K-m (Mg2+) of 6.08 +/- 1.29 mmol . l(-1) and a K-m (Cl-) of 26.5 /- 6.5 mmol . l(-1), compatible with transport activity at intracellul ar Mg2+- and Cl--levels. We propose that Mg2+ absorption in the tilapi a intestine involves an electrically neutral anion symport mechanism.