Platelet 5-hydroxytryptamine transport, an electroneutral mechanism coupled to potassium

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Abstract
Transport of 5-hydroxytryptamine [5,HT, serotonin] into plasma membrane vesicles isolated from porcine blood platelets was stimulated when a K+ gradient (in > out) was imposed across the vesicle membrane. This stimulation occurred without measurable electrical potential across the membrane. Addition of valinomycin induced a membrane potential of approximately 50 mV (interior negative) as estimated by uptake of the lipophilic cation triphenylmethylphosphonium, but had surprisingly little effect on 5-HT transport. Addition of 2,4-dinitrophenol dissipates the valinomycin-induced membrane potential. Without inomycin, 2,4-dinitrophenol had no effect on 5-HT transport but valinomycin and 2,4-dinitrophenol together inhibited transport, probably by dissipation of the K+ gradient. These results are consistent with an electroneutral mechanism in which 5-HT influx is directly coupled to K+ efflux and argue against an electrogenic mechanism in which there is a net influx of positive charge with 5-HT.