Uptake and binding of cadmium and mercury to metallothionein in rat hepatocyte primary cultures

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Abstract
The administration of inorganic Cd and Hg in vivo resulted in markedly different metal concentrations in rat liver. Primary cultures of rat hepatocytes were utilized to gain insight into the dispositional differences between these chemically similar metals. Hepatocyte monolayer cultures were exposed to several concentrations of Cd or Hg (3, 10 and 30 .mu.M) in serum-containing medium for 30 min. The cells were washed and incubated in fresh medium for the remainder of the experiment. Hepatocytes exposed to Cd accumulated significantly more metal than hepatocytes exposed to equimolar concentrations of Hg. In cells exposed to 3 .mu.M Cd there was an initial loss of Cd from the hepatocytes when placed in fresh medium, followed by a gradual re-uptake of metal, concomitant with increased binding to metallothionein. In hepatocytes exposed to 3 and 10 .mu.M Cd, 87 and 77% of the intracellular Cd was bound to metallothionein within 24 h. Loss of Hg from hepatocytes pulsed with 30 .mu.M Hg was also observed upon the addition of fresh medium and continued for the duration of the experiment. No time-dependent increase in Hg binding to metallothionein was observed. A maximum of .apprx. 10% of the intracellular Hg was found associated with methallothionein in hepatocytes exposed to 30 .mu.M Hg. Studies utilizing [35S]Cys incorporation indicated significant increases in the amount of metallothionein synthesized in hepatocytes exposed to 3 and 10 .mu.M Cd (300% of control value) and 30 .mu.M Hg (150% of control value) 24 h after metal pulsing. Time-course studies revealed a 6-12 h lag in metallothionein synthesis, followed by a significant elevation in [35]Cys incorporation into metallothionein between 12-24 h. Isolated hepatocytes differentiated between Cd and Hg and preferentially accumulated the former and Cd strongly stimulated the induction of, and preferentially bound to, metallothionein; Hg induced weakly, and did not preferentially bind to, metallothionein.