Inhibitory Effect of Rhodamine 123 on the Growth of the Rodent Malaria Parasite, Plasmodium yoelii1

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Abstract
The effect of the cationic permeant fluorescent dye, rhodamine 123 (R123), on the in vivo growth of P. yoelii was examined. P. yoelii-infected mouse erythrocytes were incubated in vitro with R123 and injected i.v. into mice. Examination of daily parasitemias showed that R123 delayed parasite growth whereas rhodamine 110, a neutral compound, and fluorescein, a negatively charged fluorescent dye, did not. Infected erythrocytes treated with R123 were not cleared from the circulation even 7 h after injection. Quantitation of cell-associated R123 by spectrophotometry revealed that infected cells with increased levels of R123 considerably prolonged the 2% prepatent period, the time required for the parasite to develop a 2% parasitemia. Degenerating parasites within and outside the host erythrocytes were observed on day 1 of infection in the mice. R123, which accumulated in infected erythrocytes, inhibits the growth of P. yoelii; when R123-labeled infected erythrocytes were treated with 1-10 .mu.M carbonylcyanide m-chlorophenylhydrazone (CCCP), a proton ionophore, to release R123 from the cells, the inhibitory effect on the growth rate of P. yoelii was partially reversed.