Red blood cells (RBCs) infected with the human malaria parasite Plasmodium falciparum have an increased permeability to a range of small, structurally unrelated solutes (including the osmolytes sorbitol and taurine) via a malaria-induced pathway (Kirk et al. 1994). This pathway has several similar characteristics to the volume-sensitive channels found in most mammalian cells (Kirk, 1997). However, such channels are not normally operative in human RBCs, which makes a direct comparison of their properties impossible. In contrast, chicken RBCs are susceptible to malaria and also possess a volume-activated osmolyte channel (VAOC), which has been reported previously (Porter & Martin, 1992). Here we report a characterisation of the sorbitol and taurine transport pathways present in the host cell membrane of chicken RBCs infected with the malaria parasite P. gallinaceum.

RBCs for experimentation were collected from uninfected and malaria-infected chickens via cardiac puncture into sodium heparin, in accordance with UK legislation. For this procedure 2-week-old chickens were anaesthetised by intramuscular injection (0.1 and 0.3 ml depending on size) of anaesthetic containing Rompan, Ketaset and phosphate-buffered saline in a ratio of 1:2:3. Following the procedure the chickens were killed humanely by cervical dislocation.

Parasitised RBCs in isotonic media showed a marked increase in the rate of influx of sorbitol (76 times higher) and, to a lesser degree, taurine (3 times higher) when compared with RBCs from uninfected chickens. The increased influx of sorbitol was via a non-saturable pathway, which was inhibited by 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). The IC₅₀ value (the concentration of inhibitor at which the total influx of a solute is reduced by half) was measured to be 6.2 ± 2.8 µM (mean ± S.D.; n = 3). Taurine was also transported into parasitised RBCs via an induced pathway, which was inhibited by NPPB. The IC₅₀ value for the effect of NPPB on taurine transport in malaria-infected chicken RBCs was measured to be 7.3 ± 0.6 µM (mean ± S.D.; n = 3). The IC₅₀ values were not found to be statistically different (P = 0.67; two-tailed, paired t test), which suggests both solutes use the same induced pathway.

Further sorbitol fluxes showed that the malaria-induced pathway had different sensitivities to a range of anion channel inhibitors (NPPB, niflumate > furosemide >> 1,9-dideoxyforskolin) when compared with the endogenous VAOC (NPPB, niflumate, 1,9-dideoxyforskolin >> furosemide). There were also differences in the selectivity for sorbitol over taurine for the two permeation routes. The permeability ratio for sorbitol relative to taurine was 0.6 for the VAOC and 17 for the malaria-induced pathway.

The results of this study show that the malaria parasite P. gallinaceum induces a new transport pathway, which is functionally distinct from the endogenous VAOC found in chicken RBCs and has several similar characteristics to those of the induced pathway in human malaria-infected RBCs.

We thank The Wellcome Trust (grant no. 058230) for financial support.