Post-absorptive excretion

As mentioned previously BjSrn-Rasmussen and Lykken both reported a relationship between post-absorptive excretion of iron and serum ferritin The higher the subject s serum ferritin the greater the body iron stores and the more post-absorptive excretion which was observed For uniformity we have defined FAE-9 as the percentage of the dose of isotope which was excreted more than 9 days after the dose Nine days comprise three 3-day stool collection periods Bjorn-Rasmussen used a 10-day cutoff point (29) ... 

Figure 6 Relationship between serum Iron and post-absorptive excretion (PAE-9) of iron.

Fost-ahsorptive excretion of zinc and copper has not previously been reported Presumably the same or similar mechanisms are involved in the excretion of iron, zinc, and copper, since the excretion of the three metals appears to be synchronous in most subjects Post- absorptive excretion of iron goes on for four to five weeks after an oral dose of (29). Using stable isotopes of Fe, Zn, and Cu, we have observed post-absorptive excretion of all three metals for up to six weeks after the isotope dose. 

Post-absorptive excretion of zinc may involve zinc which has entered the enteropancreatic circulation, since not all of the zinc in pancreatic secretions is reabsor d (33). 

All of our absorption calculations, such as those presented above, have been based on the assumption that the first pulse of excretion represented unabsorbed isotope and subsequent excretion was post-absorptive These absorption values agree with metal absorption determined by other means, such as whole-body counting ... 

Some of these questions probably cannot be resolved with stable isotope studies However stable isotopes may well be of use in identifying conditions which affect post-absorptive metal excretion ... 

Atrazine, used as a selective pre- and post-emergence herbicide to control annual weeds in several crops, is the most representative compound of this group. It is also used as a non-selective herbicide in non-crop areas. After absorption, the compound is metabolized to dealkylated and deisopropy-lated derivatives. The unchanged compound and its metabolites are excreted in urine, where they can be detected by chromatography or enzyme-linked immunosorbent assay (Lucas et al., 1993). A mercapturic acid conjugate of atrazine has also been found in urine samples of workers spraying this herbicide (Lucas et al., 1993) (Table 6). 

Mean pooled urinary concentrations during the dosing periods were 2.4 pg chromium/g creatinine from exposure to chromium(VI) and 0.4 pg chromium/g creatinine from exposure to chromium(III) as compared to 0.23 pg chromium/g creatinine during the post-dosing time periods. The lower urinary excretion of chromium(III) after exposure to chromic oxide reflects the poorer absorption of inorganic chromium(III) compounds compared to inorganic chromium(VI) compounds. 

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