Transport of 5-hydroxytryptamine by human platelets incubated in glycerol

The active transport of 5-hydroxytryptamine was used to assess the effect of glycerol on human platelets. The amine is transported by a membrane-based process that requires energy. The initial rate of uptake was measured using low concentrations of tritium-labeled 5-hydroxytryptamine (0.25 to 2.0 mumol/l) and a short incubation time following exposure of platelets to 0.05 or 1.0 mol/l glycerol at 37 degrees C for 2, 15, or 30 minutes. Control values for Km and Vmax were 0.7 +/- 0.05 mumol/l and 24.0 +/- 1.2 pmol/10(8) cells/10 seconds, respectively. The kinetics of 5-hydroxytryptamine uptake were not affected by incubation of platelets in 0.5 mol/l glycerol. However, 1.0 mol/l glycerol had a time-dependent effect on the uptake process. Initially, 5-hydroxytryptamine transport was noncompetitively inhibited, but this effect was reversed after 15-minute incubation in 1.0 mol/l glycerol. Continued exposure to 1.0 mol/l glycerol, however, increased Km. It was concluded that concentrations of glycerol above 0.5 mol/l might not be suitable as a cryoprotectant for platelet cryopreservation.