Cytochemical demonstration of mercury deposits in trout liver and kidney following methyl mercury intoxication: differentiation of two mercury pools by selenium

The amount and the ultrastructural distribution of mercury was studied in seven different organs of rainbow trout (Salmo gairdneri) fingerlings following exposure to methyl mercury (MeHg)-contaminated fodder for periods of 2 and 7 weeks. The amounts of mercury retained by the whole fish and the selected organs were determined by measuring the uptake of 203Hg-labeled MeHg. Spleen, liver, and kidney had the highest concentrations after both experimental periods, while the largest relative increases were found in brain, muscle, and kidney. The subcellular distribution of mercury accumulations was demonstrated cytochemically in liver and kidney using the silver enhancement method by which accumulations of mercury-sulfides and/or mercury-selenides are made visible for light and electron microscopy. When sections prepared from the liver and kidney from fish, injected with selenium 2 hr prior to being killed, were compared with those of fish not treated with selenium, two distinct pools of mercury could be demonstrated, the HgS pool, and the HgSe pool. The HgS pool, supposed to represent inorganic mercury, was found exclusively within lysosomes. The increase of this pool from 2 to 7 weeks was most pronounced in the kidney. The HgSe pool, supposed to represent methyl mercury, was shown by the presence of silver deposits at new locations as well as by an increase in the amount of deposits within lysosomes. The new locations included (1) secretory-like vesicles and the bile canaliculi of the liver, suggesting a biliary excretion of this mercury pool; (2) microvilli and endosomes of kidney tubular cells, suggesting a glomerular filtration and subsequent reabsorption; and (3) mitochondria of proximal tubule cells.