UPTAKE AND PROCESSING OF FE-59-LABELED AND I-125 LABELED RAT TRANSFERRIN BY EARLY ORGANOGENESIS RAT CONCEPTUSES IN-VITRO

The delivery of iron to the early organogenesis rat embryo has been st udied, using Fe-59-and I-125-labelled rat transferrin. Rat conceptuses at 9.5 days postconception were cultured for 27 or 51 h in whole rat serum. Rat transferrin labelled with Fe-59 was added for the final 0.1 , 0.5, 6, 24 or 48 h of culture. Radioactivity accumulated progressive ly in both the embryo and the visceral yolk sac. Similar results were obtained when unconjugated Fe-59(3+) was added to the rat serum used a s culture medium. Both acid-soluble and acid-insoluble Fe-59 were subs tantially present in the embryo and yolk sac after all exposure period s. When conceptuses were cultured in the presence of I-125-labelled ra t transferrin, acid-soluble radioactivity was progressively released i nto the culture medium, but accumulation into the embryo and visceral yolk sac was slight and did not change with duration of exposure to th e labelled protein. Similar findings were obtained using I-125-labelle d bovine serum albumin. In these experiments, there was a close corres pondence between the amount of iron accumulated by the embryo and visc eral yolk sac in the final 24 h of a 51-h culture and the amount of tr ansferrin converted into acid-soluble products in the same period. Vis ceral yolk sacs from 17.5-day pregnant rats were explanted and culture d in the presence of Fe-59-labelled rat transferrin, I-125-labelled ra t transferrin or I-125-labelled bovine serum albumin, for periods up t o 3 h. Again uptake of Fe-59 increased with time of incubation, and th e I-125-labelled proteins were digested to acid-soluble products which were released into the culture medium. The results indicate that tran sferrin delivers iron for incorporation into both the embryo and the v isceral yell; sac, and are consistent with a mechanism involving recep tor-mediated endocytosis of iron-laden transferrin by the cells of the visceral yolk sac. The transferrin itself appears to be quantitativel y degraded, following delivery of iron to the yolk sac cells, a result that differs from findings in other cell types, in which the protein is not degraded but returns to the plasma membrane to participate in f urther cycles of iron acquisition and delivery. (C) 1997 W.B. Saunders Company Ltd.