Compartmental responses to acute osmotic stress in Leishmania major resultin rapid loss of Na+ and Cl-

The elemental composition of the cytoplasm, electron dense vacuoles, and he terochromatin and euchromatin regions of the nucleus of Leishmania major pr omastigotes was measured by electron probe X-ray microanalysis under iso-os motic conditions (305 mOsM) and shortly after a sudden increase (to 615 mOs M) or decrease (to 153 mOsM) in the osmolality of the buffer in which they were suspended. In response to acute hypotonicity a complete loss of Na fro m the electron dense vacuoles and an approximately threefold decrease in th e Na concentrations in the cytoplasm and the nuclear regions occurred, toge ther with an approximately threefold decrease in Cl content in each compart ment and a smaller (approx. 1.2-fold) decrease in K content. Thus, in addit ion to the rapid change in shape and release of amino acids known to occur in response to acute hypo-osmotic stress, a major efflux of Na and Cl, and, to a lesser extent, of K, also occurs. In response to acute hypertonicity Na in the acidocalcisomes did not change but Na content of the cytoplasm de creased by 33%. A small increase in the S content of the cytoplasm and the electron dense vacuolar compartments occurred. No changes were detectable i n Ca or Zn content in any of the compartments examined in response to hypot onicity or hypertonicity, (C) 2001 Elsevier Science Inc. All rights reserve d.