Effects of Osmotic Pressure on the Oxidative Metabolism of Leishmania major Promastigotes

Leishmania major promastigotes were washed and resuspended in an iso-osmotic buffer. The rate of oxidation of ¹⁴C-labeled substrates was then measured as a function of osmolality. An acute decrease in osmolality (achieved by adding H₂O to the cell suspension) caused an increase in the rates of ¹⁴CO₂ production from 6-¹⁴C]glucose and, to a lesser extent, from 1, (3)-¹⁴C]glycerol. An acute increase in osmolality (achieved by adding NaCl, KCl, or mannitol) strongly inhibited the rates of ¹⁴CO₂ production from 1-: ¹⁴C]alanine, 1-¹⁴C]glutamate, and 1, (3)-¹⁴C]glycerol. The rates of ¹⁴CO₂ formation from 1-¹⁴C]laurate, 1-¹⁴C]acetate, and 2-¹⁴C]glucose (all of which form 1-¹⁴C]acetyl CoA prior to oxidation) were also inhibited, but less strongly, by increasing osmolality. These data suggest that with increasing osmolality there is an inhibition of mitochondrial oxidative capacity, which could facilitate the increase in alanine pool size that occurs in response to hyper-osmotic stress. Similarly, an increase in oxidative capacity would help prevent a rebuild up of the alanine pool after its rapid loss to the medium in response to hypo-osmotic stress.