Tissue distribution of cadmium and metallothionein as a function of time of day and dosage

Cadmium-109 chloride (1 mg or 48 ng Cd2+/kg body wt) was administered intraperitoneally to rats at one of eight selected times of day. Exactly 48 hr later each animal was sacrificed, and the cadmium content of the blood, brain, heart, kidney, liver, and testes was determined. Metallothionein levels in the liver and kidney were also measured. Distribution and retention of cadmium was very different at the different dose levels. Approximately 60% of the higher dose of cadmium was retained in the six tissues examined, while only 11.5% of the lower dose could be accounted for in these six tissues. The liver retained the largest percentage of the administered cadmium at both dose levels, but the magnitude of the retention differed by a factor of 6 (57.3% of the higher dose and 9.6% of the lower dose). The pattern of cadmium distribution among the other tissues was also different. At the 1-mg Cd2+/kg body wt level, the kidneys accumulated the second largest fraction of cadmium, followed by the blood, heart, testes, and brain. In the 48-ng Cd2+/kg body wt groups the order was kidney, testes, blood, heart, and brain. Only in the testes of animals receiving the low dose of cadmium was there an effect of time of day, and here the effect was marked. When cadmium was administered during the dark phase of the daily cycle, the testes contained an average of six times more cadmium than when cadmium was given during the light phase. Similarly, levels of metallothionein in the kidney were significantly higher when cadmium was administered during the dark phase. A trend toward higher metallothionein levels in the liver during the dark phase was also observed, but this trend was not statistically significant.