Elsevier

Experimental Parasitology

Volume 120, Issue 3, November 2008, Pages 280-285
Experimental Parasitology

Research brief
Plasmodium falciparum: Growth response to potassium channel blocking compounds

https://doi.org/10.1016/j.exppara.2008.07.010Get rights and content

Abstract

Potassium channels are essential for cell survival and regulate the cell membrane potential and electrochemical gradient. During its lifecycle, Plasmodium falciparum parasites must rapidly adapt to dramatically variant ionic conditions within the mosquito mid-gut, the hepatocyte and red blood cell (RBC) cytosols, and the human circulatory system. To probe the participation of K+ channels in parasite viability, growth response assays were performed in which asexual stage P. falciparum parasites were cultured in the presence of various Ca2+-activated K+ channel blocking compounds. These data describe the novel anti-malarial effects of bicuculline methiodide and tubocurarine chloride and the novel lack of effect of apamine and verruculogen. Taken together, the data herein imply the presence of K+ channels, or other parasite-specific targets, in P. falciparum-infected RBCs that are sensitive to blockade with Ca2+-activated K+ channel blocking compounds.

Introduction

In a persistent human infection, Plasmodium falciparum parasites must successfully traverse widely variant environmental milieus; from the intracellular locale of the hepatocyte and RBC1, to the mosquito mid-gut and human circulatory system. The parasite’s ability to adapt to variations in its environment is dependent on transporter pumps, exchangers and ion channels, which together function to maintain the intra-parasite environment in the face of the often rapidly changing external conditions. Many classes of putative P. falciparum channels and transporters have been identified (Martin et al., 2005). The proper functioning of these channels and transporters is thought to be central for maintaining parasite viability (recently reviewed by Kirk (2004)).

Potassium (K+) channels are transmembrane proteins that gate open and closed to control the flow of K+ ions across cell membranes. K+ channels play critical roles in the regulation of the transmembrane electrochemical gradient, cell membrane potential and intracellular osmolarity and are essential for the survival of all known cells types. The modes and rate of channel activation (including voltage-dependence, Ca2+-sensitivity, H+-sensitivity, phosphorylation-dependence), level of ion conductance and sensitivity to K+ channel blocking compounds varies depending on the type of K+ channel. The Ca2+-activated K+ channels can be broadly classified into three types; big (BK), intermediate (IK) and small (SK) conductance K+ channels (see Hille for review (2001)).

The P. falciparum genome encodes two putative K+ channels (PfK1 and PfK2) that possess greatest homology to Ca2+-activated K+ channels (Ellekvist et al., 2004, Waller et al., 2008). Since these K+ channels appear to be essential for P. falciparum survival (Waller et al., 2008), the effect of various K+ channel blocking compounds upon the in vitro growth of P. falciparum asexual stage parasites was investigated using 72 h [3H]-hypoxanthine incorporation assays. Here, we report the novel anti-malarial effects for each of bicuculline methiodide and tubocurarine chloride and the novel lack of anti-malarial effects of apamine and verruculogen.

Section snippets

Parasite culture

Plasmodium falciparum 3D7 parasites were maintained by in vitro culture using standard culture conditions with 0.5% Albumax II (Invitrogen) supplementation and sorbitol synchronized as previously described (Waller et al., 2008). Parasitaemias were determined by microscopic examination of Giemsa-stained thin blood smears.

[3H]-hypoxanthine incorporations assays

Seventy two hours [3H]-hypoxanthine incorporation assays were performed using previously described methods (Waller et al., 2003) to determine the parasite’s growth response to

Results

Seventy-two hour [3H]-hypoxanthine incorporation assays were performed to determine the parasite’s growth response to various K+ channel blockers. Graphical plots of the growth inhibition data derived from the [3H]-hypoxanthine incorporations assays for each compound are shown in Fig. 1. The calculated 50% and 90% Inhibitory Concentration (IC50 and IC90) ± Standard Error of the Mean (SEM) are shown in Table 1. The IC50 and IC90 data for the anti-malarial quinine and quinidine (Table 1) were in

Discussion

The putative P. falciparum K+ channels PfK1 and PfK2 possess greatest homology to Ca2+-activated K+ channels (Waller et al., 2008). The compounds tested were selected based upon their ability to selectively block the three types (BK, IK, or SK) of Ca2+-activated K+ channels, and included apamine, bicuculline methiodide, charybdotoxin, haloperidol, tubocurarine chloride and verruculogen. The established anti-malarial quinine and quinidine, which possess demonstrated K+ channel blocking

Acknowledgments

We thank Ji-Mee Hwang for technical assistance during the initial stages of this investigation. KLW is supported by an Australian NHMRC Howard Florey Centenary Research Fellowship. This research was funded by the USAMRMC grant #DAMD17-02-1-0290.

References (26)

  • C.G. Benishin et al.

    Phenothiazines and haloperidol block Ca-activated K channels in rat forebrain synaptosomes

    Molecular Pharmacology

    (1988)
  • D.C. Benton et al.

    Differences in the actions of some blockers of the calcium-activated potassium permeability in mammalian red cells

    British Journal of Pharmacology

    (1999)
  • C. Brugnara et al.

    Ca(2+) -activated K+ channels of human and rabbit erythrocytes display distinctive patterns of inhibition by venom peptide toxins

    Journal of Membrane Biology

    (1995)
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