Brain, kidney and liver 203Hg-methyl mercury uptake in the rat: relationship to the neutral amino acid carrier.

To investigate the effect of L-neutral amino acids on tissue levels of methyl mercury in the adult animal, rats were infused into the external jugular vein with solutions containing a) 0.05 mM 203Hg-MeHgCl and saline, b) 0.05 mM 203Hg-MgHgCl-0.1 mM L-cysteine, c) 0.05 mM 203Hg-MeHgCl-0.1 mM L-cysteine-0.1 mM L-cysteine-0.1 mM L-methionine, d) 0.05 mM 203Hg-MeHgCl-0.1 mM L-leucine, or e) 0.05 mM 203Hg-MeHgCl-0.1 mM L-cysteine-0.1 mM L-leucine, Groups of animals were sacrificed at 3 min. 7 hr, and 96 hr. Brain, kidney, and liver 203Hg radioactivity was measured by means of gamma-scintillation spectrometry. Brain 203Hg concentrations L-cysteine treated animals were significantly higher compared with saline treated animals (P less than 0.05) at 3 min., 7 hr and 96 hr. The coinjection or coinfusion of methyl mercury with L-cysteine and L-methionine abolished the L-cysteine-mediated brain 203Hg uptake (P less than 0.05), at each sacrifice time. Kidney and liver 203Hg concentrations were not significantly different in any of the treatment groups compared with controls, irrespective of the sacrifice time. Furthermore, the percentage of diffusible 203Hg (non-protein bound) at each sacrifice time was not statistically different irrespective of the treatment assigned. These results suggest that methyl mercury L-cysteine conjugates in the plasma may share a common transport step with the L-neutral amino acid carrier transport system and indicate the presence in brain capillaries of a transport system capable of selectively mediating methyl mercury uptake across the capillary endothelial cell membrane.