Transport measurements in platelet plasma membrane vesicles. (A) Time course of [³H]-5-HT accumulation. Intracellular buffer contained K-phosphate (circles) or NaCl (triangles). External buffer contained NaCl. Gramicidin was added at 5 min (filled circles). From Rudnick (1977), reprinted with permission from the Journal of Biological Chemistry. (B) Increasing intravesicular [Na⁺] (with constant [Na⁺] out) decreased intravesicular 5-HT accumulated in response to transmembrane Na⁺ and K⁺ gradients. Error bars indicate SD. (C) Replot of data from B as the chemical potential of the accumulated 5-HT gradient vs. the chemical potential of the imposed Na⁺ gradient. From (Talvenheimo et al., 1983), reprinted with permission from the Journal of Biological Chemistry. (D) Accumulation of intravesicular 5-HT vs. the imposed K⁺ gradient (both expressed as the ratio of solute in/out). From Rudnick (1977), reprinted with permission from the Journal of Biological Chemistry. (E) Imposition of a K⁺ gradient (in > out) does not generate a membrane potential (measured by uptake of the lipophilic cation TPMP⁺) in the absence of valinomycin (VAL). Addition on the proton ionophore 2,4-dinitrophenol (DNP) allowed H⁺ influx to dissipate the K⁺ diffusion potential established by valinomycin. (F) Accumulation of 5-HT was driven by imposition of a K⁺ gradient and addition of valinomycin did not significantly enhance accumulation. However, 2,4-dinitrophenol addition dramatically inhibited accumulation in the presence of valinomycin, probably due to acidification of the vesicle interior. In both E and F, the K⁺ gradient was generated by dilution of vesicles equilibrated in NaCl + KCl into NaCl + LiCl. From (Rudnick and Nelson, 1978), reprinted with permission from Biochemistry.