To elucidate the mechanisms involved in Mg2+ transport at the apical and basolateral poles of the renal tubular epithelium, apical and basolateral plasma membrane vesicle preparations were derived from kidney tissue of freshwater- and seawater-adapted Mozambique tilapia Oreochromis mossambicus. Brush-border preparations were enriched 15.8-fold in alkaline phosphatase activity and consisted almost exclusively of right-side-out membrane vesicles. Basolateral membrane preparations were enriched 7.5-fold in Na+/K+-ATPase activity and contained resealed vesicles and leaky membrane fragments. Mg2+ association with brush-border and basolateral plasma membranes, traced using radioactive 27Mg, occurred in an osmotically active space. In all instances, Mg2+ binding to the vesicular membrane was low compared with the vesicular uptake. Mg2+ equilibration across the vesicular membrane of brush-border preparations was rapid and sensitive to the presence of extravesicular Ca2+, suggesting that the apical membrane of the renal epithelium contains a transport pathway for divalent cations. Application of various ionic gradients did not affect vesicular Mg2+ transport in apical and basolateral membrane preparations, suggesting the presence of an ion-coupled transport mechanism. ATP or ATP--S did not stimulate Mg2+ fluxes, indicating that Mg2+ transport does not proceed via an ATP-driven or activated transporter. In these aspects, vesicular Mg2+ transport was similar in seawater and freshwater preparations. These results suggest that the apical membrane of renal epithelial cells lacks an active secretory Mg2+ transport mechanism. We propose that the Mg2+ conductivity of the apical membrane reflects a route for downhill Mg2+ entry and is involved in renal Mg2+ reabsorption.

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