The permeability of human platelets to glycerol was determined at 37OC, 25OC, and OΒ°C from the rate of change of cell volume after abrupt addition of 0.5 mol/liter glycerol in phosphate-buffered saline. lntracellular water volume was measured employing both tritiated water and a photometric method. lntracellular glycerol was measured employing tritiated glycerol. The glycerol permeabilit coefficient derived from the tracer cell volume data was 4.0 k 0.7 x 10-cm/s at 37OC, and 1.1 0.4 x lo-' cmls at 25OC, and the photometric data gave a permeability coefficient of 5.4 k 0.4 x lo-' cm/s at 37OC. The activation energy between 23OC and 37OC for glycerol permeation was 19.8 kcal/mol. The cells were virtually impermeable to glycerol at OOC. The minimum intracellular water volume attained after the addition of 0.5 mol/liter glycerol at 37OC determined by the photometric method was 47.8% of normal water volume, whereas the minimum water volume calculated assuming that glycerol exerted its full osmotic effect (i.e., u = 1) was 45.6%. The reflexion coefficient was therefore assumed to be unity. Neither method of cell volume determination could be used with 1 or 2 mol/liter glycerol: adequate separation of the cells from the labeled medium could not be achieved in the tracer method; in the photometric method, it was apparent that transmittance (660 nm) was influenced by one or more variables in addition to cell volume.
METHODS
Platelet suspensions
Human blood was drawn from apparently healthy donors as 450-ml units into bags containing the anticoagulant citrate-phosphate-dextrose. Platelet-rich plasma (PRP) was prepared by centrifuging the blood within 1 hour of collection at 250 g for 20 min. The concentration of platelets in the supernatant PRP was 3.4 5 0.1 x lo8