Membrane potential of Plasmodium-infected erythrocytes.

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Abstract
The membrane potential (Em) of normal and P. chabaudi-infected rat erythrocytes was determined from the transmembrane distributions of the lipophilic anion, thiocyanate (SCN), and cation, triphenylmethylphosphnium (TPMP). The SCN- and TPMP-measured Em of normal erythrocytes are -6.5 .+-. 3 mV and -10 .+-. 4 mV, respectively. The TPMP-measured Em of infected cells depended on parasite developmental stage; late stages (schizonts and gametocytes) were characterized by a Em = -35 mV and early stages (ring and copurifying noninfected) by a low Em (-16 mV). The SCN-determined Em of infected cells was -7 mV regardless of parasite stage. Studies with different metabolic inhibitors including antimycin A, a proton ionophore (carbonylcyanide m-chlorophenylhydrazone [CCCP]), and a H+-ATPase inhibitor (N,N''-dicyclohexylcarbodiimide, [DCCD]) indicate that SCN monitors the Em across the erythrocyte membrane of infected and normal cells whereas TPMP accumulation reflects the Em across the plasma membranes of both erythrocyte and parasite. These inhibitor studies implicated proton fluxes in Em-generation of parasitized cells. Experiments with weak acids and bases to measure intracellular pH further support this proposal. Methylamine distribution and direct pH measurement after saponin lysis of erythrocyte membranes demonstrated an acidic pH for the erythrocyte matrix of infected cells. The transmembrane distributions of weak acids (acetate and 5,5-dimethyloxazolidine-2,4-dione) indicated a DCCD-sensitive alkaline compartment. The intraerythrocyte parasite Em and .DELTA.pH are in part the consequence of an electrogenic proton pump localized to the parasite plasma membrane.