Absorption and Excretion

PCBs are readily absorbed through many tissue membranes, including by microorganisms, the gills of fish, the skin of many animals, and the gastrointestinal tract. In plants PCBs may enter the cellular structure, but are more likely to be contained within the cuticular layers. PCBs accumulate in lipid-rich tissues, and in animals it is usual to express concentrations on a lipid weight basis. Lipid-rich tissues are the main storage sites in animals, and the liver (being the first organ chemicals absorbed into the blood from the gastrointestinal tract encounter, highly blood-perfused, and the main metabolic site) is generally relatively enriched in PCBs on a lipid-weight basis. The release ofPCBs occurs when fat is redistributed due to food shortage, poor health, hibernation, lactation, incubation, and migration. However, PCBs are not necessarily released pro rata, and adipose PCB concentrations tend to increase at these times. PCB concentrations found in tissues of a dairy cow in the UK in 1996 are shown in Table 1. It can clearly be seen that in the subcutaneous fat and internal organs PCBs have been distributed relatively evenly, on a lipid-weight basis, and that concentrations in

Table 1 Concentrations of PCBs in tissues from a dairy cow

Tissue

[PCB] (pgg1 fresh)

[PCB] (pgg1 lipid)

Liver

210

14000

Kidney fat

4600

Kidney

65

4200

Diaphragm

140

4100

Subcutaneous fat

5100

Adapted from Thomas GO, Sweetman AJ, and Jones KC (1999) Metabolism and body-burden of PCBs in lactating dairy cows. Chemosphere 39: 1533-44.

Adapted from Thomas GO, Sweetman AJ, and Jones KC (1999) Metabolism and body-burden of PCBs in lactating dairy cows. Chemosphere 39: 1533-44.

liver are much higher than the other tissues, again on a lipid-weight basis.

Excretion of PCBs is generally poor, and highly dependent on the structure of individual PCB congeners. Plants can lose PCBs by volatilization to air (or dissolution to water) (due to a change in a parameter controlling equilibrium - e.g., PCBs may partition back to the air after an increase in temperature, or after a change to low PCB concentrations in air relative to the plant), by shedding of surface cuticular fragments and waxes, and by senescence of leaves. PCBs are generally not accessible to the sites of metabolism in plants. Animals do not excrete PCBs effectively in feces (except those unabsorbed from the diet) unless partitioning to feces across the gastrointestinal tract is favored (e.g., if the diet has very low PCB concentrations compared to those accumulated in the body). Excretion of metabolized PCBs is the dominant elimination route for animals, but there is very wide variation in the ability of organisms to metabolize PCBs, and wide variation in the efficiency of metabolism of different PCB congeners. The mechanisms of metabolism of PCBs by animals is dealt with in more detail below. Female animals can excrete PCBs to their offspring in the lipid-rich tissues of eggs, and female mammals can, additionally, pass PCBs to their offspring during gestation and lactation.

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