Effect of solute concentration on intracellular water volume and hydraulic conductivity of human blood platelets.

The intracellular water volume of human blood platelets was determined using tritiated water. The cells responded as osmometers over an observed range of solute concentration from 0.292 to 2.180 osmol kg-1. Only 87% of intracellular water was apparently osmotically active (i.e. Ponder's R was 0.87). Changes in cell volume induced by small step changes in external osmolality were followed photometrically and the time constant for the exponential approach of cell volume to its new equilibrium value was determined. Hydraulic conductivity (LP) was calculated from the time constant and was 1.41 X 10(-6) cm atm-1 s-1 under isotonic conditions at 37 degrees C. LP was inversely dependent on extracellular solute concentration, but it was independent of the direction of movement of water across the plasma membrane.