Regulation of cellular osmolarity and volume in Tetrahymena.

Freshwater Protozoa are hyperosmotic to their environment. Direct determinations of intracellular osmolarity on Amoeba proteus and Pelomyxa carolinensis have given values between 101 and 117 m-osmole/kg cells, when the external osmolarity was 2-8 m-osmole/1 (Lovtrup & Pigon, 1951; Riddick, 1968; Schmidt-Nielsen & Schrauger, 1963). No measurements were made of intracellular osmolarity in response to changes in the medium. There have been no reliable direct measurements of osmolarity on ciliates, but there is evidence as to what solutes are responsible for the intracellular osmolarity. Most of the evidence available is for the concentrations of sodium, potassium, chloride and phosphates (orthophosphates and phosphate esters) in Tetrahymena pyriformis, and these could account for c. 75 m-osmole/kg cells (Dunham, 1970; Cline & Conner, 1966). Attempts have been made to quantify the free amino acids of Tetrahymena, jnd the results suggest that they may be important osmotic constituents as well (Scherbaum et al. 1959; Wu & Hogg, 1956). In the marine ciliate Miamiensis avidus in sea water the intracellular free amino acids constituted 300 mM/ kg cells, a large fraction of the total intracellular osmolarity. The amount of free amino acids/cell was found to increase or decrease in response to corresponding changes in the external osmolarity (Kaneshiro, Holz & Dunham, 1969). The purpose of the present study was to determine the intracellular osmolarity of Tetrahymena pyriformis and the relationship between the internal and external osmolarities. The contribution of the free amino acids to the intracellular osmolarity was also determined. In the most dilute media the intracellular osmolarity of Tetrahymena is i n m-osmole/kg cells; determined from the freezing-point depression of cell extracts. About 90% of the intracellular osmolarity could be accounted for by sodium, potassium, chloride and the free amino acids. When the external osmolarity was increased or decreased there were corresponding changes in the amounts of the intracellular solutes, principally amino acids. Regulation of cell volume and the activity of the contractile vacuole could be correlated with the change in the amounts of intracellular free amino acids under conditions of osmotic stress.