ELECTRODIFFUSIVE TRANSPORT OF MG ACROSS RENAL MEMBRANE-VESICLES OF THE RAINBOW-TROUT ONCORHYNCHUS-MYKISS

The mechanism of tubular Mg transport was investigated in membrane ves icles (MV) of trout kidneys prepared by differential centrifugation wi th sucrose. MV consisted largely of brush-border membranes, as indicat ed by high enrichments of brush-border membrane enzymes. Although meas ured transport of Mg-28 included a binding component, most membrane tr ansport was into or out of an osmotically active space. There was no e vidence for amiloride-sensitive Na/Mg exchange, nor was Mg uptake affe cted by the carboxyl group reagents trimethyloxonium tetrafluoroborate , glycine methyl ester . Cl-1-ethyl-3-(3-dimethyl-aminopropyl)carbodii mide, and N,N'-dicyclohexyl carbodiimide or the Ca channel modulators D-600, verapamil, diltiazem, and BAY K 8644. However, Mg uptake increa sed in the presence of inside-negative voltages generated by inward gr adients of the permeant anions NO3, SCN, and Cl or by outward gradient s of K (plus valinomycin). Alkaline-earth cations displayed the select ivity sequence VII (Mg > Ca > Sr > Ba) for cis-inhibition of Mg-28 upt ake. Mg efflux was trans-inhibited by La and Gd, and Mg uptake was cis -inhibited by Mn. The sulfhydryl group reagents p-chloromercuribenzoic acid and p-chloromercuriphenylsulfonate stimulated Mg uptake and effl ux. These results reveal an electrodiffusive pathway for Mg transport in trout renal MV.