Toxicity of methylmercury: effects on different ages of rats.

LD50 of methylmercury chloride has been shown to be dependent on the ages of the rats. As the age increases, the LD50 decreases, i.e. the younger rats could tolerate higher doses of methylmercury than the older one. The LD50 were 39.6 +/- 2.3, 33.1 +/- 2.1, 30.3 +/- 1.0, 27.1 +/- 1.0, 24.7 +/- 1.5 and 23.9 +/- 1.1 mg Hg/kg for the 200 g, 300 g, 350 g, 400 g, 450 g and 500 g rates respectively. The elimination of mercury from blood showed little correspondence to age during the 30 days duration. The onset of neurological symptoms after receiving 25 mg Hg/kg of methylmercury chloride occurred between 8 to 15 days post dosing in the surviving rats. Rats unaffected during the latency period did not show neurological signs if their blood-mercury levels decreased to below 100 ppm. Young and old rats showed marked differences in the distribution of mercury in the blood. In the erythrocyte membrane, the eight week old rats retained a higher concentration of the toxic metal than did the 19.5 week old rats. Also, there was significant differences in the ratios of mercury content in the red blood cells to that of plasma; young rats showing 115:1 and for the old ones being 5:1. The permeability of erythrocyte membrane to mercury might play an important role in the age factors on the suceptibility of methylmercury intoxication.