Induction of nucleoside transport sites into the host cell membrane of Babesia bovis infected erythrocytes

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Normal bovine erythrocytes have negligible ability to transport adenosine and related nucleosides across their cell membrane. However, infection with the intraerythrocytic parasite Babesia bovis was found to induce a nucleoside permeation site into the host cell membrane. Transport experiments over periods of up to 30 s determined that the transport rate of 1 μM adenosine into the infected cell was 1.72 ± 1.2 pmol incorporated (μl cell water) −1s −1, a rate three times higher than for normal human erythrocytes. Incorporation studies over 6 h with labelled adenosine indicated that the purine moiety was incorporated into parasite nucleic acids. The mammalian nucleoside transport inhibitors, nitrobenzylthioinosine (NBMPR), nitrobenzylthioguanosine (NBTGR), dilazep and dipyridamole inhibited the induced nucleoside transport mechanism in the Babesia-infected erythrocytes, though at higher concentrations than those required to inhibit normal human erythrocyte transport. An ID 50 value for NBMPR of 0.36 μM was determined. Phloretin and 5′- p-fluorosulphonyl benzoyl adenosine-HCl (5FSBA) were also shown to be inhibitory, with ID 50 values of 0.11 and 0.18 μM, respectively, whilst phlorizin and verapamil at 1 μM had no effect. Binding studies with [ 3H]NBMPR indicated that high-affinity NBMPR binding sites could not be detected in either normal or B. bovis infected bovine erythrocytes. The results indicate that the induced nucleoside permeation site(s) in B. bovis infected erythrocytes has characteristics different from either human erythrocytes or erythrocytes infected with the malarial parasites Plasmodium falciparum or Plasmodium yoelii.