Gamete development in Plasmodium berghei regulated by ionic exchange mechanisms

Abstract

Ionic regulation in the induction of exflagellation of Plasmodium 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 HCO3-, 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 NO3- could be substituted with Cl-, whereas other anions such as I-, NO2-, SO4(2-), SCN-, H2PO4-, or HPO4(2-) 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 HCO3- or monovalent, membrane-permeable cation and Cl- in the medium. Measurements of the efflux of H[14C]O3- or Cl- indicated that these anions were released from the cells into the NaCl or the NaHCO3 medium, respectively, probably by exchange in HCO3-/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 HCO3-) 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 HCO3-. It is therefore suggested that two separate ion exchangers, i.e., Na(+)-dependent HCO3-(in)/Cl-(out) and nonspecific monovalent-cation-dependent Cl-(in)/HCO3-(out) exchangers, are involved in the induction of gametogenesis in P. berghei.(ABSTRACT TRUNCATED AT 250 WORDS)