A simple method for continuous measurement of the volume of cells in suspension

A simple electrical method for continuous monitoring of the volume of cells in suspension is described. The procedure consists of placing cells in a medium containing an impermeant quaternary ammonium ion, such as tetramethylammonium (TMA), which behaves as an extracellular volume marker, and monitoring the activity of this ion with a quaternary ammonium ion-specific minielectrode. Changes in the activity of the cell impermeant TMA ion in the extracellular medium quantitatively reflect movements of water into, or out of, cells and, therefore, provide a direct measure of alterations in cell volume. A direct linear relation between the fractional decrease in rat red blood cell volume and the fractional increase in medium osmolality was observed using this method. The osmotically active water volume of these red blood cells, calculated from the slope of this relationship, was 73.6 +/- 0.7% of total cell volume, a value similar to that defined by other techniques. This method should be easily adaptable for use with virtually any type of separated cell, including epithelial cells.