Influence of Dietary Protein Levels on the Fate of Methylmercury and on Amino Acid Transport at the Renal Brush Border Membrane in Rats

Abstract

We investigated the influence of dietary protein levels on the fate of methylmercury (MeHg) in rats, and the difference in its urinary excretion was discussed from the viewpoint of the dietary protein level-dependent alteration in activity of the neutral amino acid transport system, through which MeHg metabolites such as MeHg-L-cysteine are reabsorbed, at the renal brush border membrane. When MeHg was administered to rats fed either a 24.8% protein diet (normal protein diet, NPD) or a 7.5% protein diet (low protein diet, LPD), urinary Hg excretion was much lower in LPD-fed rats than in NPD-fed rats, whereas no difference was observed in fecal excretion 1 day after MeHg administration. At that time, Hg concentrations in the brain and plasma were similar in the two dietary groups, whereas the concentrations in liver and blood were higher, but the renal concentration was lower in LPD-fed rats than in NPD-fed rats. Regardless of the presence of Na⁺, uptake of ¹⁴C-L-phenylalanine for the first 20 sec was higher in the renal brush border vehicles from LPD-fed rats than in those from NPD-fed rats. These results suggest that dietary protein deficiency enhances the neutral amino acid transport at the renal brush border membrane, which could lead efficient reabsorption of MeHg metabolites from the proximal luminal space, and it might play a key role in the marked decrease in urinary excretion of MeHg.