Coupling between platelet 5-hydroxytryptamine and potassium transport.

Platelet plasma membrane vesicles accumulate 5-hy- droxytryptamine in response to a gradient of NaCl (out > in). Although potassium ion is not required for this process, a K’ concentration gradient (in > out) elevates the steady state level of 5-hydroxytryptamine accu- mulation 5-fold. Transport is independent of the gen- eration of an electrical potential (interior negative) across the vesicle membrane when K’ is present inter- nally, but in its absence 5-hydroxytryptamine trans- port is apparently electrogenic, with net positive charge crossing the membrane with 5-hydroxytrypt- amine. Potassium ion directly interacts with the 5-hy- droxytryptamine transporter as evidenced by its ef- fects on the initial rate of 5-hydroxytryptamine trans- port. Internal K+ stimulates influx and external K’ inhibits. Even in the absence of a K’ gradient, K’ affects the kinetics of 5-hydroxytryptamine transport, 30 meq of K’/liter increasing the V,,,,, for 5-hydroxy- tryptamine transport 3-fold and increasing K,,, by 2- fold. The unidirectional efflux of [3H]5-hydroxytrypt- amine is stimulated by both K’ and unlabeled 5-hy- droxytryptamine in the external medium. Although 5- hydroxytryptamine-stimulated efflux requires external Na+, K’-induced efflux is independent of external Na’. A mechanism consistent with the above observations is proposed in which internal K+ accelerates the influx of 5-hydroxytryptamine by increasing the rate at which the 5-hydroxytryptamine binding site of the trans- porter “moves” from the internal surface of the mem- brane to the external surface. Concomitant with this movement, one potassium ion is transported from the vesicle lumen to the external medium. Blood platelets accumulate 5-hydroxytryptamine by a proc- ess which is believed to be similar, if not identical, to the reuptake of Shydroxytryptamine by serotonergic neurons (1, 2). In both cases, transport is an energy-dependent carrier- mediated process with an absolute requirement for Na’ and Cl- in the external medium (3-10). Plasma membrane vesicles isolated from platelets provide a useful model system for the