Gamete development inPlasmodium berghei regulated by ionic exchange mechanisms

Abstract

Ionic regulation in the induction of exflagellation ofPlasmodium berghei was investigated by culturing the parasites in various isotonic media. Of the salts tested, NaHCO₃ exhibited the highest activity in inducing exflagellation, whereas KHCO₃ showed no activity. In the absence of HCO ⁻₃ , media containing monovalent cation (Na⁺, K⁺, Cs⁺, Rd⁺, choline⁺, lysine⁺, arginine⁺) and Cl⁻ also induced exflagellation, but their activities were lower than that of NaHCO₃. Anions of Br⁻ or NO ⁻₃ could be substituted with Cl⁻, whereas other anions such as I⁻, NO ⁻₂ , SO ²⁻₄ , SCN⁻, H₂PO ⁻₄ , or HPO ²⁻₄ failed to induce exflagellation, as did tetramethylammonium-Cl, CaCl₂, MgSO₄, MgCl₂ and sucrose as well. These results suggest that the induction of exflagellation requires the presence of Na⁺ and HCO ⁻₃ or monovalent, membrane-permeable cation and Cl⁻ in the medium. Measurements of the efflux of H[¹⁴C]O ⁻₃ or Cl⁻ indicated that these anions were released from the cells into the NaCl or the NaHCO₃ medium, respectively, probably by exchange in HCO ⁻₃ /Cl⁻. Determination of intracellular ionic concentrations by electron microscopic X-ray microanalysis of cryopreserved specimens revealed that in the NaHCO₃ medium, external Na⁺ (and probably HCO ⁻₃ ) enters the gametocytes by exchange with internal Cl⁻ (and probably H⁺), whereas in Cl⁻-containing media, external unspecified cation and Cl⁻ influx by exchange, probably with H⁺ and HCO ⁻₃ . It is therefore suggested that two separate ion exchangers, i.e., Na⁺-dependent HCO ⁻₃ (in)/Cl⁻(out) and nonspecific monovalent-cation-dependent Cl⁻(in)/HCO ⁻₃ (out) exchangers, are involved in the induction of gametogenesis inP. berghei. Furthermore, the presence of both classes of anion in the medium enhanced exflagellation activity and increased Na⁺ uptake more than did the NaCl or NaHCO₃ medium alone. The apparent synergistic enhancement by two contraactive anion exchangers is consistent with a “recycling” model of pH_i regulation, in which HCO ⁻₃ and Cl⁻ are exchanged between the cells and the media, resulting in the acceleration of monovalent cation/H⁺ exchange.