MECHANISMS OF MANGANESE TRANSPORT IN RABBIT ERYTHROID-CELLS

1. The mechanisms of manganese transport into erythroid cells were inv estigated using rabbit reticulocytes and mature erythrocytes and Mn-54 -labelled MnCl2 and Mn-2-transferrin. In some experiments iron uptake was also studied. 2. Three saturable manganese transport mechanisms we re identified, two for Mn2+ (high and low affinity processes) and one for transferrin-bound manganese (Mn-Tf). 3. High affinity Mn2+ transpo rt occurred in reticulocytes but not erythrocytes, was active only in low ionic strength media such as isotonic sucrose and had a K-m of 0.4 mu M. It was inhibited by metabolic inhibitors and several metal ions . 4. Low affinity Mn2+ transport occurred in erythrocytes as well as i n reticulocytes and had K-m values of approximately 20 and 50 mu M for the two types of cells, respectively. The rate of Mn2+ transport was maximal in isotonic KCl, RbCl or CsCl, and was inhibited by NaCl and b y amiloride, valinomycin, diethylstilboestrol and other ion transport inhibitors. The direction of Mn2+ transport was reversible, resulting in Mn2+ efflux from the cells. 5. The uptake of transferrin-bound mang anese occurred only with reticulocytes and depended on receptor-mediat ed endocytosis of Mn-Tf. 6. The characteristics of the three saturable manganese transport mechanisms were similar to corresponding mechanis ms of iron uptake by erythroid cells, suggesting that the two metals a re transported by the same mechanisms. 7. It is proposed that high aff inity manganese transport is a surface representation of the process r esponsible for the transport of manganese across the endosomal membran e after its release from transferrin. Low affinity transport probably occurs by the previously described Na+ - Mg2+ antiport, and may functi on in the regulation of intracellular manganese concentration by expor ting manganese from the cells.