Peritubular uptake and brush border transport of 28Mg by flounder renal tubules

Abstract

The uptake of 28Mg by isolated renal tubules of winter flounder, Pseudopleuronectes americanus, was studied by compartmental analysis. Two phases of uptake were seen in steady-state conditions. The slow-exchanging compartment was 46% of the total Mg content, and uptake into this compartment was saturable (Km = 0.36 mM) and inhibited by 10 mM CaCl2, dinitrophenol, and ouabain but not by furosemide, Na-free medium, or cytochalasin B. The fast-exchanging compartment was 5% of the total Mg, and uptake into this compartment showed sigmoid saturation kinetics (K 1/2 = 1.62 mM). The fast uptake rate was inhibited by dinitrophenol, ouabain, Na-free medium, 10 mM CaCl2, and furosemide but stimulated by cytochalasin B. 28Mg efflux from brush border membrane vesicles (BBMV) was stimulated by both an inside positive electrical potential generated by 100 mM KCl (out greater than in, in the presence of valinomycin) and by 100 mM NaCl (out greater than in). The latter effect was still present when the membrane electrical potential was short-circuited with 100 mM KCl (in = out) in the presence of valinomycin. Uptake of Mg by BBMV was stimulated by reversal of the aforementioned gradients. Both uptake and efflux were stimulated by A23187. BBMV uptake was inhibited by 10 mM CaCl2 and unaffected by furosemide. The relationship of electrical gradient-driven and Na gradient-driven Mg transport to the Mg secretory function of these tubules is discussed.