ADAPTATION RESPONSES IN RATS TO LONG-TERM FEEDING OF HIGH-ZINC DIETS - EMPHASIS ON INTESTINAL METALLOTHIONEIN

When faced with an abundance of heavy metals in the diet, higher anima ls produce metallothionein (MT) in the intestinal mucosa to sequester the metals in an attempt to reduce their absorption into the body. Thi s paper shows that this strategy may only be used for short-term expos ure, and a more effective, sustainable strategy is adopted during long -term exposure. The concentration of dietary zinc was abruptly elevate d and offered to rats continually for a specified period. The results showed that an abrupt change in dietary zinc caused an immediate eleva tion of intestinal MT concentration, which remained elevated for about 2 weeks and then began to decline. After about 5 weeks, MT concentrat ions in the intestines of rats fed high-zinc diets were not different from controls fed normal-zinc diets for the entire period. The concent rations of zinc in serum, liver, and kidney followed the same course a s mucosal MT, elevated during the initial phase of feeding high-zinc a nd near control values during the latter phase of feeding. Although th e rats were consuming diets with zinc concentrations about 7 fold high er than controls, and intestinal MT concentrations were not elevated, the serum and tissue concentrations of zinc were near control values. This suggests that the induction of intestinal MT may be an immediate and short-term strategy for coping with high intakes of certain metals , and that some other, more efficient mechanism is adopted during long -term exposure.