Uptake of isoleucine by P falciparum–infected and uninfected human erythrocytes at 37°C. The extracellular medium contained all of the common, naturally occurring amino acids at concentrations that fall within the normal plasma range ([ile] = 70 μM). Uninfected cells were from the same donors as the erythrocytes used for parasite culture. (A) Time courses for the uptake of isoleucine by P falciparum–infected (•) and uninfected (○) cells. The left axis shows the actual amount of isoleucine taken up [and present in the acid-soluble (ie, non-protein) fraction], whereas the right axis shows the [¹⁴C]isoleucine distribution ratio (ie, the concentration of acid-soluble [¹⁴C]isoleucine within the cell relative to that in the extracellular medium). The data are averaged from 10 separate experiments performed on different days, each on cells from different donors, and are shown ± SEM. (B) Initial rates of isoleucine influx in P falciparum–infected (▪) and uninfected (⊡) cells. Isoleucine influx was determined from the uptake of [¹⁴C]isoleucine measured over 15 seconds and in the presence of the protein synthesis inhibitors cycloheximide (40 μM) and anisomycin (150 μM). Where indicated, furosemide [an inhibitor of the parasite-induced new permeability pathways (NPPs)] was added at a concentration of 200 μM, and BCH (an inhibitor of the endogenous erythrocyte L system) was added at a concentration of 20 mM. The data are averaged from 4 separate experiments performed on different days, each on cells from different donors, and are shown ± SEM. (C) Time courses for the incorporation of isoleucine into protein by P falciparum–infected erythrocytes in the presence (○) or absence (•) of furosemide (200 μM) and by isolated P falciparum trophozoites in the presence (▵) or absence (⊡) of furosemide (200 μM). The data are averaged from 10 separate experiments performed on different days, each on cells from different donors and are shown ± SEM. The data obtained with infected erythrocytes in the absence of furosemide and with isolated parasites both in the presence and absence of furosemide, overlay one another.