Malaria studies in vitro. IV: Chloroquine resistance and the intracellular pH of erythrocytes parasitised with Plasmodium berghei.

The erythrocytic stages of strains of plasmodia sensitive to chloroquine (CQ) concentrate some three to six times more drug than CQ-resistant strains. A simple, but so far untested hypothesis is that the intracellular pH of drug resistant strains is higher than that for sensitive strains. This would result in a reduced accumulation of any weakly basic drug molecule (e.g. CQ) which is capable of passive diffusion through the cellular membran. The mean intracellular pH of erythrocytes parasitised with either CQ-sensitive or CQ-resistant strains of P. berghei was determined, therefore, from the distribution of 14C-5, 5-dimethyloxazolidine-2,4-dione between intra- and extracellular aqueous phases. The results obtained showed that over the range 6-8--7-2 the intracellular pH is governed directly by the extracellular pH and that the intracellular pH of CQ-sensitive parasites is slightly above that for resistant ones, neither being greatly different from that for uninfected erythrocytes. There was no evidence, therefore, to support, the suggested hypothesis. Elimination of this possibility adds weight to the opinion that changes in drug sensitivity of these plasmodia are the result of changes in the properties of drug-binding sites.