PLATELET 5-HYDROXYTRYPTAMINE TRANSPORT VOL. 17, NO. 22, 1978 4739 be said that microtubules apparently play aa important role in determining the mobility and topography of receptor sites on cell membranes (Yahara and Edelman, 1975; Nicholson, 1976). Refer mces Adler, A. J., Greenfield, N. J., and Fasman, G. D. (1973), Attwood, D., Florence, A. T., and Gillan, J. M. N . (1974), zyxwvutsr J. Cann, J. R., and Hinman, N. D. (1975), Mol. Pharmacol. zyxwvut 11, Cam, J. R., and Hinman, N. D. (1976), Biochemistry zyxwvu 15, Chen, Y.-H., Yang, J. T., and Martinez, H. M. (1972), Bio- Eipper, B. A. (1972), Proc. Natl. Acad. Sci. U.S.A. 69, Eipper, B. zyxwvutsrqpo A. (1974), J. Biol. Chem. 249, 1407. Florence, zyxwvutsrqp A. T., and Parfitt, R. T. (1971), zyxwvutsrq J. Phys. Chem. 75, Frigon, R. P., and Lee, J. D. (1972), Arch. Biochem. Biophys. Frigon, R. P., and Timasheff, S. N. (1975), Biochemistry 14, Methods Enzymol. 27, 675. Pharm. Sei. 63, 988. 256. 46 14. chemistry 11, 4120. 2283. 3554. 153, 587. 4567. Greenfield, N., and Fasman, G. D. (1969), Biochemistry 8, Hinman, N. D., and Cann, J. R. (1976), Mol. Pharmacol. 12, Hinman, N. D., Morgan, J. L., Seeds, N. W., and Cam, J. R. Lee, J. C., Corfman, D., Frigon, R. P., and Timasheff, S. N. Lee, J. C., Corfman, D., Frigon, R. P., and Timasheff, S. N. Lowry, 0. H., Rosebrough, N. J., Farr, A. L., and Randall, R. Ludeiia, R. F., and Woodward, D. 0. (1 979, Ann. N. Y. Acad. Nicholson, G. L. (1976), Biochim. Biophys. Acta 457, 57. Seeman, J. L., Tedesco, J. L., Lee, T., Chan-Wong, M., Muller, P., Bowles, J., Whitaker, P. M., McManus, C., Tittler, M., Weinreich, P., Friend, W. C., and Brown, G. M. (1978), Fed. Proc., Fed. Am. SOC. Exp. Biol. 37, 130. Ventilla, M., Cantor, C. R., and Shelanski, M. (1972), Bio- chemistry I I, 1554. Weisenberg, R. C., Borisy, G. G., and Taylor, E. (1968), Biochemistry 7, 4466. Wilson, L., and Meza, I. (1972), J. Cell Biol. 55, 285a. Yahara, I., and Edelman, G. M. (1975), Ann. N.Y. Acad. Sci. 4108. 769. (1 973), Biochem. Biophys. Res. Commun. 52, 752. (1975), Ann. N.Y. Acad. Sci. 253, 284. (1978), Arch. Biochem. Biophys. 185, 4. J. (1951),J. Biol. Chern. 193, 265. Sci. 253, 272. 253, 455. Platelet 5-Hydroxytryptamine Transport, an Electroneutral Mechanism Coupled to Potassium? Gary Rudnick* and Pamlea J. Nelson ABSTRACT: Transport of 5-hydroxytryptamine into plasma membrane vesicles isolated from porcine blood platelets is stimulated when a potassium gradient (in zyxwvutsr > out) is imposed across the vesicle membrane. This stimulation occurs in the absence of measurable electrical potential across the mem- brane. Addition of valinomycin induces a membrane potential of approximately 50 mV (interior negative) as estimated by uptake of the lipophilic cation triphenylmethylphosphonium, but has surprisingly little effect on 5-hydroxytryptamine transport. Addition of 2,4-dinitrophenol dissipates the vali- T r a n s p o r t of 5-hydroxytryptamine (5-HT)' is an energy- dependent, carrier-mediated process with an absolute re- quirement for Na+ and CI- in the external medium (Sneddon, 1969; Lingjaerde, 197 1; Rudnick, 1977). Similarities between 5-HT transport into platelets and synaptosomes and brain t From the Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06510. Recieved April 1 I, 1978; recised manuscript receioed July 18, 1978. This work was supported by a Grant-in-Aid from the American Heart Association and with funds contributed in part by the Hartford, Connecticut Chapter. I Abbreviations used: 5-HT, 5-hydroxytryptamine; TPMP+, triphen- ylmethylphosphonium. nomycin-induced membrane potential. In the absence of val- inomycin, 2,4-dinitrophenol has no effect on 5-hydroxytryp- tamine transport but valinomycin and 2,4-dinitrophenol to- gether inhibit transport, probably by dissipation of the K+ gradient. These results are consistent with an electroneutral mechanism in which 5-hydroxytryptamine influx is directly coupled to potassium ion efflux and argue against an electro- genic mechanism in which there is a net influx of positive charge with 5-hydroxytryptamine. slices have led to the proposal that the same transport system is present in both tissues (Sneddon, 1973; Paasonen, 1968). A previous report from this laboratory demonstrated that plasma membrane vesicles isolated from human blood platelets ac- cumulate 5-HT to concentrations approximately 100 times greater than in the external medium (Rudnick, 1977). Accu- mulation is stimulated by imposition of a potassium ion gra- dient (in > out) across the vesicle membrane. This observation can be interpreted in one of two ways: (1) K+ efflux stimulates transport by creating an electrical potential (interior negative) across the membrane, or (2) K+ interacts directly with the 5-HT carrier, which couples K+ efflux to 5-HT uptake. The 0006-2960/78/0417-4739$01.00/0 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0 1978 American Chemical Society