ISOTOPE-DILUTION TECHNIQUE FOR DETERMINATION OF ENDOGENOUS FECAL EXCRETION AND TRUE ABSORPTION OF SELENIUM IN SE-75 LABELED RATS

In a balance study with 30 growing Sprague-Dawley rats the isotope-dil ution technique for determination of endogenous faecal excretion and t rue absorption was established for the trace element selenium. The ani mals were injected intramuscularly with Se-75 and killed in groups of 6 animals at day 2, 4, 6, 9, and 14 after the injection. During the ex periment one semisynthetic diet containing 150 ng Se/g (selenite) was fed restrictively to all animals. Within 2 days the administered trace r was almost completely released from the injection area. After an ini tial peak in Se-75 excretion at day 1 (urine) and day 2 (faeces) the S e-75 elimination was steadily decreasing. Three days after the Se-75 i njection the distribution of daily Se-75 excretion between urine and f aeces remained constant (83% vs. 17%). The specific Se-75 activities o f tissues decreased with increasing time elapsed after the Se-75 injec tion. The decrease was proportional to that of faeces only in the case of blood plasma. Thus, all other tissues and also the urine had to be excluded from use as a reference source for the isotope-dilution meth od. The estimates of endogenous faecal Se based on blood plasma as a r eference source were constant from dap 4 to the end of the experiment (endogenous Se: 89% of total faecal Se; true Se absorption: 99% of Se intake). In conclusion, the endogenous faecal Se can be estimated by t he isotope-dilution method on the basis of blood plasma as the referen ce source. The lag rime between 1 parenteral Se-75 injection and the o nset of Se-75 monitoring for isotope-dilution calculations is about 3 days.