Pharmacokinetics of methylmercury in sheep

A preliminary study of the pharmacokinetics of methylmercury was performed in sheep after a single intravenous dose of ²⁰³ Hg‐labeled methylmercury. Blood samples were taken periodically, and plasma and whole blood ²⁰³Hg were determined. Blood and plasma ²⁰³Hg concentrations exhibited a biphasic exponential decline. The half‐time for the major component was 14.6 days for plasma ²⁰³Hg and 14.1 days for whole blood. There was a uniform distribution of ²⁰³ Hg between red cells and plasma throughout the study. A red cell to plasma concentration ratio of 14.9:1 was obtained, with approximately 88% of the ²⁰³ Hg in blood associated with the red cell fraction. Both the feces and the wool represented major routes of excretion. Approximately 18% of the dose was present in the wool taken on day 8. This is compared with a total excretion in the feces of 11.5% of the dose over the same time course. The total of 29.5% compares favorably with a reduction of 32.5% in the terminal component of the whole blood curve. Sequential analysis of wool from the root end revealed increasing concentrations of ²⁰³ Hg to a peak. Using the first appearance of the ²⁰³ Hg label in the wool as a marker to determine the wool growth rate, the ²⁰³ Hg in wool shewed an exponential decline with a half‐time similar to that seen for whole blood and plasma. The wool to blood ratio was estimated to be approximately 120:1. Tissue analysis at sacrifice revealed a rather uniform distribution of ²⁰³ Hg label in the brain. Somewhat lower levels were observed in the spinal cord and ganglia. Kidney had the highest ²⁰³ Hg concentration, and edible muscle had approximately six times the concentration measured in blood. The concentration of ²⁰³ Hg was three times higher in bile than in plasma, consistent with the substantial fecal excretion observed. The sheep may be a useful animal model for studying regional deposition of methylmercury in the CNS and mechanisms of methylmercury deposition into hair.