Regulation of Cellular Osmolarity and Volume in Tetrahymena

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Abstract
Tetrahymena pyrifor mis are hyperosmotic to external media of osmolarities from 2 to 171 m-osmole/1. The intracellular osmolarity, determined by freezing-point depression, is 111 m-osmole/kg cells in dilute media, and increases linearly with increasing external osmolarities. Over 80% of the intracellular osmolarity can be attributed to the concentration of sodium, potassium, chloride and the free amino acids. In response to an increase in the external osmolarity, Tetrahymena regulates its intracellular osmolarity by increasing the concentrations of free amino acids. The regulation of cellular volume under conditions of osmotic stress is achieved by an increase in the amount of osmotically active solutes and the regulation of the rate of elimination of fluid by the contractile vacuole.