The Ca++ transport mechanism in the red cell membrane was studied in resealed ghost cells. It was found that the red cell membrane can transport Ca++ from inside the cell into the medium against great concentration gradient ratios. Tracing the movement of 45Ca infused inside red cells indicated that over 95% of all Ca++ in the cells was transported into media in 20 min incubation under the optimum experimental conditions. The influence of temperature on the rate constant of transport indicated an activation energy of 13,500 cal per mole. The optimum pH range of media for the transport was between 7.5 and 8.5. As energy sources, ATP1, CTP, and UTP were about equally effective, GTP somewhat less effective, and ITP least effective among the nucleotides tested. The Ca++ transport does not appear to involve exchange of Ca++ with any monovalent or divalent cations. Also, it is not influenced by oligomycin, sodium azide, or ouabain in high concentrations, which inhibit the Ca++ transport in mitochondria or in sarcoplasmic reticulum. In these respects, the Ca++ transport mechanism in the red cell membrane is different from those of mitochondria and the sarcoplasmic reticulum.

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