Calcium transport of Plasmodium chabaudi-infected erythrocytes.

Abstract

The calcium content and transport processes of Plasmodium chabaudi-infected rat erythrocytes were analyzed by atomic absorption spectroscopy and "Ca" flux measurements . Infected erythrocytes, after fractionation on metrizamide gradients according to stage of parasite development, exhibited progressively increasing levels of Ca 2+ with schizont and gametocytes containing 10-to 20-fold greater calcium levels than normal cells (0.54 ± 0.25 nmol/108 cells) . 45Ca 2 + flux experiments showed both increased influx and decreased efflux in infected erythrocytes . Tris/NH 4C1 lysis of normal erythrocytes preloaded with 45 Ca 2+ with the Ca 2+ ionophore A23187 released >90% of cell calcium after incubation in ethyleneglycol bis(aminoethylether) N, N'-tetraacetic acid containing buffer, whereas lysis of the infected erythrocyte membrane resulted in release of 10-20% cell Cat+ , with the remaining portion associated with the isolated parasite fraction . This information together with the effects of various metabolic inhibitors indicates the presence of a parasite Ca' compartment in P. chabaudi-infected erythrocytes .Dicyclohexylcarbodiimide (DCCD) an inhibitor of proton ATPases of chloroplasts, bacteria, yeast, and mitochondria, and the proton ionophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP), inhibited Ca 2+ influx and stimulated efflux from infected cells . These results combined with evidence for a DCCD-and CCCP-sensitive membrane potential in P. chabaudiinfected cells (Mikkelsen et al ., accompanying manuscript) suggest that Ca 2+ transport of intraerythrocytic parasites is coupled to a proton-motive force across the Plasmodia plasma membrane .The calcium ion is an essential component of many cellular regulatory processes, and cells expend considerable energy to maintain low levels of free cytoplasmic Cat+ . In the case of enucleated erythrocytes, low cytoplasmic Ca' concentrations are achieved by a combination of plasma membrane Ca" binding, low membrane Ca' permeability, and active extrusion by Ca 2+' Mg t +-ATPase pump (1, 2). Elevation of intracellular Ca 2+ results in decreased membrane deformability, altered monovalent cation transport, and modification of membrane protein structure, e.g . spectrin degradation (1, 3) . Malaria is a blood-borne disease in which the asexual development of the parasite Plasmodium occurs within the host erythrocyte. Few investigators have addressed the general issue of ion transport in Plasmodium-infected erythrocytes, although increased osmotic fragility is a common observation of Plasmodium-infected erythrocytes . Dunn (4) has shown increased sodium and decreased potassium intracellular levels resulting in part from decreased Na', K+-ATPase activity of parasitized erythrocytes . No information is available concerning Ca" levels and intracellular Ca" distribution in infected cells or the relative contributions of erythrocyte and parasite Ca' transport processes of infected cells. Such information may indicate mechanisms by which the parasite modifies its intracellu...

Membrane potential of Plasmodium-infected erythrocytes.

“…Metrizamide treatment does not alter parasite viability, transport of ionic constituents such as Ca" (8), or the Em of normal erythrocytes .…”

“…For further experiments a single-step metrizamide gradient (16 .5%, P = 1 .076) is used to separate "late" from "early" stages and uninfected cells (8). The Em estimates are: "late" fraction (-27 .8 to -45.6 mV) and "early" fraction (-10.1 to -17 .7 mV) .…”

“…Em and Stage of Parasite Development (Table 1) Parasite-infected erythrocytes are fractionated according to stage of parasite development on metrizamide gradients (8) . Metrizamide treatment does not alter parasite viability, transport of ionic constituents such as Ca" (8), or the Em of normal erythrocytes .…”

“…Maintenance of Sprague-Dawley rats, infection with P. chabaudi, andisolation of erythrocytes are described in the accompanying article (8).…”

Antimalarial drugs disrupt ion homeostasis in malarial parasites

“…We have reported the existence of Ca 2+ pools in the acidic compartment of rodent as well as human malaria parasites as a possible mechanism for the regulation of calcium concentration in these parasites , Passos & Garcia 1998. Calcium homeostasis in malaria parasites has been extensively studied by several authors (Tanabe et al 1982, Krungkrai & Yuthavong 1983, Scheibel et al 1987, Wasserman et al 1990, Adovelande et al 1993, Garcia et al 1997, Gazarini et al 2003, Gazarini & Garcia 2004, Beraldo et al, 2005, Budu et al 2007). Interestingly, in P. chabaudi, the second messenger IP 3 , mobilizes Ca 2+ not only from the classical ER-like pool but also from an acidic compartment, implying that these parasites utilize a calcium-mediated cell signaling mechanism similar to higher organisms (Passos & Garcia 1998).…”