Plasma serotonin levels and the platelet serotonin transporter

Abstract

Serotonin (5HT) is an intermediate product of tryptophan metabolism located primarily in the enterochromaffin cells of the intestines, serotonergic neurons of the brain, and blood platelets, and is a well established neurotransmitter in the central nervous system. Although altered concentrations of circulating 5HT are implicated in several pathologic conditions, including hypertension, the mechanism underlying involvement of 5HT remains elusive. 5HT has potent vasoconstrictor activity (Watts 2005) and also enhances the hypertensive effects of known vasoconstrictors (Golino et al. 1991;Gujrati et al. 1994; Azzadin et al. 1995; Yildiz et al. 1996; Watts 2005). In hypertension and atherosclerosis (Golino et al. 1991; Yildiz et al. 1996), the blood vessels become more sensitive to the vasoconstrictor effects of 5HT, and this effect is further amplified by subcontractile levels of other vasoconstrictors such as angiotensin and endothelin (Yildiz et al. 1998; Watts 2005). Furthermore, 5HT levels and turnover are increased in primary hypertension and certain types of secondary hypertension, such as pregnancy, erythropoietin, and cyclosporine-induced hypertension (Gujrati et al. 1994; Azzadin et al. 1995; Watts 2005). The actions of 5HT are mediated by different types of receptors, but are terminated by a single 5HT transporter (SERT) (Blakely et al. 1991; Hoffman et al. 1991; Lesch et al. 1993; Ramamoorthy et al. 1993b). The uptake mechanism of platelet SERT regulates plasma 5HT levels and secures stable blood flow by decreasing the possibility of platelet-activation, which produces clots in blood vessels, resulting in platelet aggregation, and subsequently, hypertension (Pettinger et al. 1973; Shulman et al. 1989; Lichtman et al. 2002; Blann et al. 2003; Stoddard et al. 2003; Pidgeon et al. 2004). The relation between the plasma 5HT level and the uptake ability of SERT has been demonstrated using SERT knock-out mice (Bengel et al. 1998), polymorphisms in the SERT-linked promoter region, and SERT inhibitors (Heils et al. 1996; Lesch et al. 1996; Serebruany et al. 2001; Johnson et al. 2003). Overall, these studies emphasize that the 5HT uptake efficiency of SERT depends on the plasma 5HT level and the number of transporters on the plasma membrane (Ramamoorthy and Blakely 1999). However, neither the contributions of these phenomena nor the order of events in the hypertensive state are known.

In the present study, we first measured 5HT levels in platelets and platelet-poor plasma (PPP). We then characterized the platelet 5HT uptake rates, the catalytic properties (V_max), and the substrate-dependence (K_m) of SERT in platelets from subjects with chronic hypertension or normotension. The platelet 5HT uptake rate during hypertension was lower due to a decrease in V_max with no change in K_m for the platelet SERT. To understand the mechanism for the reduction in 5HT uptake, the expressions of SERT on the plasma membrane and in the intracellular pools were measured. The findings indicated that the decrease in V_max was due to a decrease in the amount of SERT molecules expressed on the platelet membrane but not in whole cell expression. Furthermore, we studied the interaction between the plasma 5HT levels and the density of SERT molecules on the platelet membrane. The effects of exogenous 5HT on the uptake rates and plasma membrane densities of SERT were measured in isolated platelets after pre-treating them with various concentrations of 5HT.

Together, our findings demonstrated that exogenous 5HT affects the uptake capacity of SERT by controlling the density of transporter molecules on the plasma membrane in a biphasic manner. Therefore, we hypothesize that in a hypertensive state, the elevated plasma 5HT levels induce a loss in the uptake function of platelets via a decrease in the density of SERT molecules on the plasma membrane. Through the feedback effect, plasma 5HT controls its own concentration levels by modulating the kinetic properties of platelet SERT.

Abbreviations used