Transport of Iron and Transferrin Synthesis by the Seminiferous Epithelium of the Rat in Vivo1

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Abstract
The transport of radioactive iron across the seminiferous tubules was analyzed in vivo by light-microscope quantitative radiography. At 5 min after a single intratesticular injection of 55Fe-transferrin, a strong labeling of the basal aspect of the seminiferous epithelium was observed. Between 30 min and 2 h, the labeling on the basal aspect of the seminiferous epithelium decreased. This decrease was accompanied by a substantial increase of the radioautographic reaction over the cellular elements in the adluminal compartment. These results were consistent with the demonstration of 59Fe-labeled associated with meiotic spermatocytes and differentiating spermatids isolated by velocity sedimentation from testes injected with 59Fe-labeled human transferrin, radiolabeled rat transferrin was immunoprecipitated from homogenates of isolated tubules with a specific antibody and appeared as a single radioactive band on fluorographs of urea/polyacrylamide gels. Similarly, 59Fe-labeled rat transferrin but not 125I-transferrin was immunoprecipitated from rete testis fluids of testes infused with either 59Fe- or 125I-labeled human transferrin. Finally, the synthesis of testicular transferrin in vivo was demonstrated in fluorographs of immunoprecipitated transferrin after an intratesticular injection of 35S-methionine in rats whose livers were excluded from the general circulation by ligation of both the hepatic artery and the portal vein. Thus, our results demonstrated a unidirectional system of iron transport from the basal compartment of the seminiferious epithelium to the germ cells in the adluminal compartment involving two distinct transferrins, i.e., a serum transferrin and a testicular transferrin synthesized by the seminiferious epithelium.
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