Epithelial transport of magnesium in the kidney of fish.

That the kidneys of marine fish have powerful renal mechanisms for the excretion of magnesium (Mg) from the body has been known since the early 1930s, but it took another 40 years before the first renal Mg transport model was suggested by Natochin and Gusev. Since rates of net renal sodium (Na) reabsorption were closely correlated with rates of net renal Mg secretion in scorpion fish, Natochin and Gusev proposed tubular Na/Mg exchange transport. However, confirmation of Na/Mg exchange in other fish kidneys has been elusive. Detailed renal clearance studies in sea water rainbow trout have shown that bladder reabsorption of Na and water, the process which concentrates Mg in the bladder, accounts for much of Natochin's original observation. Nevertheless, studies of isolated perfused proximal tubules of the flounder and killifish do show inverse relationships between the concentrations of Na and Mg in the tubule lumen, consistent with Na/Mg exchange. Unfortunately, large paracellular Na permeabilities in renal proximal tubules do not clarify whether paracellular Na fluxes of Na/Mg exchange transport across the brush border membrane are responsible for generating inverse concentrations of Na and Mg in the tubule lumen. These uncertainties have led the authors to their present use of brush border membrane vesicles to look for evidence of Na/Mg exchange transport.