The effects of chloroquine and verapamil on digestive vacuolar pH of P-falciparum either sensitive or resistant to chloroquine

In the preceding paper, we present a novel method for measuring the digesti ve vacuolar pH (pH(vac)) of the malarial parasite Plasmodium falciparum, an d show that, surprisingly, pH(vac) is lower for chloroquine resistant (CQR) Dd2 parasites relative to chloroquine sensitive (CQS) HB3. These data. may have important consequences for elucidating mechanisms of antimalarial dru g resistance and for developing new antimalarial therapy. Additional issues central to a better understanding of antimalarial pharmacology and antimal arial drug resistance require detailed comparative data on the effects of k ey drugs and other compounds on parasite biophysical parameters such as pH( vac), measured under close-to-physiologic conditions. Since the methods we develop in the previous paper allow us to record fluorescence signals from spatially well-defined regions of the living parasite while they are under continuous perfusion, it is relatively straightforward for us to test how a ntimalarial drugs (e.g. chloroquine, CQ) and other compounds (e.g. the chem oreversal agent verapamil [VPL]) affect pH(vac). In this paper, we measure both short term (i.e. initial perfusion conditions) and longer-term effects of CQ and VPL for living, intraerythrocytic CQS (HB3) and CQR (Dd2) malari al parasites under constant perfusion with physiologically relevant buffers . We find that VPL normalizes pH(vac) for Dd2 to a value near that measured for HB3, but has no effect on pH(vac) for HB3. Longer term CQ exposure is found to alter pH(vac) for HB3 but not Dd2, and short-term exposure to the drug has no significant effect in either strain. The results may help resol ve longstanding debate regarding the effects of CQ and VPL on parasite phys iology, and further support our evolving hypothesis for the mechanism of CQ resistance. (C) 2000 Elsevier Science B.V. All rights reserved.