The role of chloride ion in platelet serotonin transport.

Chloride ion is required for platelet serotonin transport. This anion requirement is relatively specific for Cl-. Only Br- fully substitutes, while SCN-, NO2-, and NO3- are much less effective. Serotonin influx requires external Cl-, and efflux requires internal Cl-. Although internal Cl- is not required for transport and has little effect on the initial rate of influx, it decreases the steady state level of serotonin accumulation. Chloride ion does not merely fulfill a need for a permanent ion since it is required for exchange of internal with external serotonin. Moreover, serotonin accumulation still requires external Cl- when a K+ diffusion potential (interior negative) is imposed across the vesicle membrane with valinomycin. Furthermore, Cl- lowers the Km and raises the Vmax for transport, suggesting direct effects on the transporter. These results strongly suggest serotonin-Cl- co-transport across the platelet plasma membrane.