Platelet 5-HT content and uptake in essential hypertension: role of endogenous digitalis-like factors and plasma cholesterol

Amphetamine actions at the serotonin transporter rely on the availability of phosphatidylinositol-4,5-bisphosphate

Nerve functions require phosphatidylinositol-4,5-bisphosphate (PIP2) that binds to ion channels, thereby controlling their gating. Channel properties are also attributed to serotonin transporters (SERTs); however, SERT regulation by PIP2 has not been reported. SERTs control neurotransmission by removing serotonin from the extracellular space. An increase in extracellular serotonin results from transporter-mediated efflux triggered by amphetamine-like psychostimulants. Herein, we altered the abundance of PIP2 by activating phospholipase-C (PLC), using a scavenging peptide, and inhibiting PIP2-synthesis. We tested the effects of the verified scarcity of PIP2 on amphetamine-triggered SERT functions in human cells. We observed an interaction between SERT and PIP2 in pull-down assays. On decreased PIP2 availability, amphetamine-evoked currents were markedly reduced compared with controls, as was amphetamine-induced efflux. Signaling downstream of PLC was excluded as a cause for these effects. A reduction of substrate efflux due to PLC activation was also found with recombinant noradrenaline transporters and in rat hippocampal slices. Transmitter uptake was not affected by PIP2 reduction. Moreover, SERT was revealed to have a positively charged binding site for PIP2. Mutation of the latter resulted in a loss of amphetamine-induced SERT-mediated efflux and currents, as well as a lack of PIP2-dependent effects. Substrate uptake and surface expression were comparable between mutant and WT SERTs. These findings demonstrate that PIP2 binding to monoamine transporters is a prerequisite for amphetamine actions without being a requirement for neurotransmitter uptake. These results open the way to target amphetamine-induced SERT-dependent actions independently of normal SERT function and thus to treat psychostimulant addiction.

Endogenous serotonin in human blood platelets: factors that may influence reference values

The endogenous content of serotonin in human platelets has been used in many clinical studies to indicate platelet activation. A decrease in platelet serotonin compared with controls has been regarded as an indicator of platelet activation. However, the results published are difficult to compare, because of huge variations in endogenous serotonin between control groups in different investigations. This is likely to be because of lack of standardization. Several factors that influence the endogenous platelet serotonin content were studied in more than 200 blood donors. The most important factor was the total g force of the centrifugation used to isolate platelets. Also the age and sex of platelet donors, number of platelets in platelet-rich plasma, and mean platelet volume influenced normal serotonin values. Using a standardized centrifugation procedure (2700 g min) the mean endogenous serotonin was 2.80 nmol/10(9) platelets in young women and 2.58 in elderly women, and 2.67 in young men and 2.30 in elderly men. The differences both for age and sex were statistically significant. Endogenous platelet serotonin shows intrapersonal stability over time, since endogenous platelet serotonin did not change on repeated venepuncture for 9 weeks. Factors such as age, sex and isolation procedure must therefore be considered when endogenous platelet serotonin are studied in relation to disease and treatment.

The relation between serotonin levels and insufficient blood pressure decrease during night-time in hypertensive patients

AIM

The serotonin levels in thrombocytes are decreased in hypertensive patients. The aim of our study was to investigate the relationship between serotonin levels and insufficient nocturnal blood pressure (BP) decrease (non-dipper) in hypertensive patients.

PATIENTS AND METHODS

Fifty-six hypertensive patients and 27 healthy control subjects were included in the study. Of the hypertensive patients, 28 were classified as dippers and 28 as non-dippers based on nocturnal BP drops of >10 mmHg and <10 mmHg, respectively. Thrombocyte serotonin levels, serum uric acid, and C-reactive protein (CRP), and urinary albumin/creatinine ratios were analysed. Thrombocyte serotonin levels were measured using an enzyme immunoassay.

RESULTS

The thrombocyte serotonin level was 378.9 +/- 69.5 ng/10(9) platelet in the non-dipper group, 424.7 +/- 58.6 ng/10(9) platelet in the dipper group, and 518.1 +/- 35.9 ng/10(9) platelet in the control group. Serotonin levels in the non-dipper group were significantly lower than in the dipper group. Serotonin levels negatively correlated with blood pressure (r = -0.6, p<0.001). CRP concentration in the non-dipper group was higher than in the dipper (4.8 +/- 1.4 vs 3.6 +/- 1.6, p<0.01) and control (2.4 +/- 0.9, p<0.001) groups, and microalbuminuria was significantly higher in the non-dipper group compared with dipper (24.9 +/- 8.6 vs 13.4 +/- 8.8, p<0.001) and control (9.6 +/- 4.8, p<0.001) groups. Serotonin level was negatively correlated with microalbuminuria (p<0.001, r = -0.3), uric acid (p<0.01, r = -0.3), and CRP (p<0.01, r = -0.35).

CONCLUSION

In non-dipper hypertensive patients, thrombocyte serotonin levels were significantly lower than in dipper and control groups. Serotonin levels may be related to insufficient nocturnal blood pressure decrease in hypertensive patients.

Natural 1-O-alkylglycerols reduce platelet-activating factor-induced release of [3H]-serotonin in rabbit platelets

Natural 1-O-alkylglycerols have multiple biological activities with distinct mechanisms. In THP-1 monocytes, they amplify platelet-activating factor production. In endothelial cells, they participate in the production of 1-O-alkyl-2-acyl-sn-glycerol, a PKC inhibitor. Since PAF as well as PKC may interfere with platelet functions, we studied the effect of natural alkylglycerols purified from shark liver oil on [3H]-serotonin release from rabbit platelets in vitro. [3H]-alkylglycerols (1 microM) were consistently incorporated into platelet lipids and after a 2-h incubation, they were metabolised into phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol, which represented 53.5+/-1.7%, 36.3+/-1.8%, 5.3+/-0.5% of metabolised [3H]-alkylglycerols, respectively. Alkylglycerols (10 microM) had no effect on spontaneous [3H]-serotonin release. However, alkylglycerols partially inhibited PAF-induced [3H]-serotonin release while they did not modify thrombin-induced release. These data show that alkylglycerols inhibit partially and specifically PAF-induced platelet stimulation and suggest that this effect could result from interfering with PAF receptors.

Reduced platelet serotonin content in rabbits with dietary hypercholesterolemia

Serotonin [5-hydroxytryptamine (5-HT)] has been implicated in platelet activation and vasoconstriction, two processes that contribute to arterial thrombosis in atherosclerotic diseases. In the present study, Japanese White rabbits fed 1% cholesterol for 5 weeks were used to investigate the response of hypercholesterolemic vascular arteries and platelets to 5-HT. Contractions of the thoracic aorta induced by 5-HT were comparable between the cholesterol-fed group and the age-matched control group. However, acetylcholine-induced vasodilation in arteries preconstricted with 5-HT was moderately but significantly attenuated in the cholesterol-fed rabbits. Platelet aggregation responses to 5-HT (0.1-3 micromol/l) in combination with epinephrine (5 micromol/l), adenosine diphosphate (ADP) (0.3-10 micromol/l), 9,11-dideoxy-9alpha,11alpha-methanoepoxy-prostaglandin F2alpha (U-46619) (1-30 micromol/l) or collagen (3 microg/ml) were significantly enhanced in cholesterol-fed rabbits. In contrast, platelet 5-HT content determined with a high-performance liquid chromatography-electrochemical detector (HPLC-ECD) was significantly decreased in cholesterol-fed rabbits. These results suggest a possible association among the endothelial dysfunction, platelet aggregation and platelet 5-HT content in rabbits with dietary hypercholesterolemia.

Reduced platelet serotonin content and release in familial hypercholesterolaemia

Plasma and platelet serotonin (5-HT) concentrations, and resting and collagen-induced 5-HT release in platelet-rich plasma were studied in normal and familial hypercholesterolaemic (FH) subjects. Platelet 5-HT concentrations were significantly reduced (-37%, P < 0.01) in FH patients whilst mean plasma concentrations, although increased, were not significantly different from those in normal subjects. Platelet 5-HT correlated negatively with plasma cholesterol when the data for normal subjects and FH, patients were combined (r = -0.48, P = 0.005). It also correlated negatively with low-density lipoprotein (LDL) (FH data, r = -0.59, P = 0.03; normal and FH data, r = -0.49, P = 0.004) but positively with high-density lipoprotein (HDL) (FH r = 0.79, P = 0.001; normal and FH, r = 0.37, P = 0.03). Collagen (5-160 micrograms/ml) stimulated platelet 5-HT release occurred in a concentration-dependent manner. In FH patients stimulated 5-HT release was reduced (10 micrograms/ml collagen, -40%, P < 0.05) and accompanied by increased collagen EC50 values (P < 0.02). Resting 5-HT release was increased substantially in FH patients but not significantly. Our data provide evidence for a relationship between circulating cholesterol and platelet serotonergic mechanisms. It is proposed that abnormalities relating to platelet-plasma 5-HT dynamics, perhaps due to enhanced platelet activity or decreased platelet uptake, may contribute to the cardiovascular complications in FH.

Is there an association among low untreated serum lipid levels, anger, and hazardous driving?

Anger and hazardous driving were explored in two studies for their possible explanatory role in the low cholesterol-violent mortality association. In Study 1, we obtained fasting blood samples and indices of anger and driving habits from 102 healthy young adults. Among men (but not women), lower low-density lipoprotein (LDL) cholesterol levels were associated with significantly higher anger (r = 41, p < .01) and unsafe driving habits (r = -.30, p < .05), and although total cholesterol levels were correlated in the predicted direction, these were not significant. In Study 2, 78 healthy young men completed the same measures, and this time, both lower total and LDL-cholesterol were significantly related to increased anger and dangerous driving. Implications of these findings are discussed as are possible avenues for future research. Further scrutiny of the components of serum cholesterol and their relation to psychological factors may aid us in better understanding the reasons why men are at increased risk for hazardous behavior and violent death.

Evidence for separate serotonin and catecholamine compartments in human platelets

Serotonin (5-HT) and the catecholamines (CA), noradrenaline (NA) and adrenaline (Ad), are platelet dense-granule constituents which influence platelet activity and vessel tone. Platelets accumulate 5-HT via an active process whilst CA uptake occurs mainly by passive diffusion. The platelet contents and collagen-stimulated efflux of 5-HT, NA and Ad were examined in normal individuals to establish whether relationships exist between these monoamines. Regression analysis revealed that platelet 5-HT was not related to platelet NA or Ad levels. Platelet NA, however, correlated positively with Ad (r = 0.61, P < 0.01). Collagen-induced release of all three monoamines occurred in a dose-dependent manner. The collagen EC50 values for 5-HT and CA release, however, differed and were greater for 5-HT release: 9.6 +/- 0.8 vs. 3.8 +/- 0.2 microgram/ml collagen, 5-HT vs. NA, P < 0.001; 9.6 +/- 0.8 vs. 3.9 +/- 0.5 microgram/ml, 5-HT vs Ad, P < 0.001. These data may reflect differences regarding the triggering mechanisms for 5-HT and CA release and provide evidence for separate compartments of intra-platelet 5-HT and CA and possibly distinct populations of 5-HT and CA containing dense granules and/or platelets.

Brain serotonin (5-HT) neuroendocrine function in patients taking cholesterol-lowering drugs

Brain serotonin (5-HT) neuroendocrine function, plasma tryptophan, and platelet 5-HT content were examined in 20 patients treated in a lipid clinic for hypercholesterolaemia with combined drug and diet therapy and in 20 healthy matched controls. Treatment had produced a substantial decrease in total cholesterol concentrations in the patients, but they still had higher cholesterol and triglyceride levels than control subjects. The patients were somewhat more depressed than controls but did not differ from them in degree of hostility, free or total plasma tryptophan, or prolactin response to 30 mg of d-fenfluramine. This study does not reveal evidence of abnormal brain 5-HT neuroendocrine function in hypercholesterolaemic patients receiving cholesterol-lowering medications and diet.

Increases in depression after cholesterol-lowering drug treatment

To investigate the possibility that increases in depressive symptoms might occur in patients who have undergone cholesterol-lowering interventions, the authors administered the Center for Epidemiological Studies-Depression scale before and after cholesterol lowering to 6 men who were referred to a lipid clinic. All of the patients' cholesterol levels were reduced after the 6-week intervention, and 4 of the patients' depression scores increased; scores of 2 of the 4 met the criteria for mild clinical depression. Further study of possible links among low cholesterol, depressive symptoms, and serotonergic activity is needed.

The role of platelets in essential hypertension

Recent research is helping us understand the complex interactions that occur between platelets and their environment. The several intracellular events that occur during platelet activation are being identified as ar their effects on other platelets, the endothelium and coagulation factors. Heightened platelet activation is seen early in essential hypertension and probably plays an important role in the initiation and progression of atherosclerosis and the disorders associated with it. This review identifies some of the changes in platelet structure and function in essential hypertension and their role in the pathogenesis of hypertensive vascular disease.

Relationships between serum lipids, serotonin, platelet aggregation and some fibrinolytic parameters in humans

We have studied relationships between serum lipids, plasma serotonin (5-hydroxytryptamine, 5-HT), platelet aggregation in the platelet-rich plasma (PRP), and some fibrinolytic parameters in healthy volunteers. LDL was positively related to 5-HT-induced platelet aggregation. Higher serum HDL levels were related to higher fibrinolytic activity expressed by shorter euglobulin clot lysis time (ECLT). No correlation existed between serum lipids and plasma 5-HT. Plasma 5-HT was found to be related to tissue plasminogen activator (t-PA), suggesting some link between plasma serotonin and the release of t-PA from endothelial cells. There were mutual relations among fibrinolytic parameters such as tissue plasminogen activator (t-PA) antigens, total plasminogen activator inhibitor-1 (PAI-1), t-PA-PAI-1 complex, free PAI-1, and ECLT. In conclusion higher LDL may be more thrombogenic due to enhanced 5-HT induced platelet aggregation whereas high HDL may be more protective against thrombosis due to enhancement of fibrinolysis.

Low serum cholesterol and suicide

"Primary prevention trials which have shown that the lowering of serum cholesterol concentrations in middle-aged subjects by diet, drugs, or both leads to a decrease in coronary heart disease have also reported an increase in deaths due to suicide or violence. There has been no adequate explanation for this association. I have reviewed the relevant published work and describe a physiological mechanism that might account for this curious finding. One of the functions of serotonin in the central nervous system is the suppression of harmful behaviour impulses. When mouse brain synaptosomal membrane cholesterol is increased there is a pronounced increase in the number of serotonin receptors. Low membrane cholesterol decreases the number of serotonin receptors. Since membrane cholesterol exchanges freely with cholesterol in the surrounding medium, a lowered serum cholesterol concentration may contribute to a decrease in brain serotonin, with poorer suppression of aggressive behaviour".

Plasma cholesterol and depressive symptoms in older men

In several clinical trials of interventions designed to lower plasma cholesterol, reductions in coronary heart disease mortality have been offset by an unexplained rise in suicides and other violent deaths. We have tried to find out whether depressive illness is related to low plasma cholesterol concentrations in men of 50 years and older. In 1985-87, Beck depression inventories were obtained from 1020 white men, aged 50-89 years, in the Rancho Bernardo, California, cohort. Disease history and behaviours were assessed by standard questionnaires. Plasma cholesterol and weight were measured at this time, as they had been in 1972-74. Among men aged 70 years and older, categorically defined depression was three times more common in the group with low plasma cholesterol (< 4.14 mmol/L) than in those with higher concentrations (5/31 [16%] vs 22/363 [6%]; p = 0.033). Depressive symptom scores correlated significantly and inversely with plasma cholesterol concentrations, even after adjustment for age, health status, number of chronic illnesses, number of medications, and exercise, as well as measured weight loss and change in plasma cholesterol in the previous 13 years. Our finding that low plasma cholesterol is associated with depressive symptoms in elderly men is compatible with observations that a very low total cholesterol may be related to suicide and violent death. Since cholesterol lowering in the general population is widely recommended, this observation warrants further investigation.

Stimulation of the active transport of serotonin into human platelets by hydrogen peroxide

The effect of H2O2 on the active transport of serotonin (5-HT) by human platelets was investigated. Platelets were exposed to either a single dose of H2O2 or to H2O2 generated by the glucose/glucose oxidase or xanthine/xanthine oxidase enzyme systems. H2O2 (12.5 to 100 microM) produced a rapid, concentration-dependent and time-dependent increase in 5-HT transport which was maximal after a 2-min incubation and decreased with continued incubation. Catalase (1000 units) completely prevented H2O2-induced stimulation, and fluoxetine (1 microM) totally blocked 5-HT uptake into stimulated platelets. The glucose/glucose oxidase (3.12 to 100 milliunits) and the xanthine/xanthine oxidase system, superoxide dismutase (250 units) failed to alter the stimulation, whereas catalase (1000 units) effectively prevented the response. The kinetics of 5-HT transport indicated that H2O2 treatment did not alter the Km of 5-HT transport (Km control = 1.0 +/- 0.2 x 10(-6) M vs Km H2O2 = 1.1 +/- 0.1 x 10(-6) M) but markedly increased the maximal rate of 5-HT transport (Vmax control = 131.4 +/- 4.6 pmol/10(8) platelets/4 min vs Vmax H2O2 = 206.7 +/- 9.1 pmol/10(8) platelets/4 min). These data demonstrated that exposure of human platelets to H2O2 resulted in a stimulation of the active transport of 5-HT and suggested that H2O2 may function to regulate this process.

5-Hydroxytryptamine kinetics and activation of blood platelets in patients with essential hypertension

To investigate possible alterations in 5-hydroxytryptamine (5HT) kinetics and sensitivity of blood platelets in patients with essential hypertension, 45 essential hypertensive patients and 45 normotensive healthy subjects matched in pairs for age, sex, and smoking status were compared. There were 18 women and 27 men in each group, ranging from 30 to 73 years of age. Results of essential hypertensive patients differed in several ways from those of normotensive subjects. In essential hypertensive patients, maximal 5HT uptake velocity (Vmax) decreased with increasing blood pressure and age and was reduced the most in older men. Vmax was positively related to the EC50 of 5HT for inducing a shape change reaction. In essential hypertensive patients, both Vmax of 5HT uptake and the EC50 of 5HT for shape change showed positive correlations with the 5HT content in platelets; the former relation was different between the essential hypertensive and normotensive groups (F = 5.53; p = 0.02). These results indicate reduced uptake of 5HT by blood platelets and suggest enhanced 5HT plasma concentrations in local areas, especially vascular lesions in essential hypertensive patients. Increased periplatelet concentrations of 5HT may lead to preactivation of platelets and possibly stimulation of vascular smooth muscle via their 5HT2-receptors. These changes are likely to be involved in the pathogenesis of increased thromboembolic complications in essential hypertensive patients, particularly in older men.

Impaired uptake of 5 hydroxytryptamine platelet in essential hypertension: clinical relevance

Serotonin (5-hydroxytryptamine; 5HT) kinetics and platelet activation by 5HT were studied in patients with essential hypertension (n = 45), and in matched normotensive subjects (n = 45). Platelet response to 5HT and plasma beta-thromboglobulin increased with age in men, both normotensives and hypertensives. Beta-thromboglobulin and 5-hydroxyindoleacetic acid (5HIAA) excretion were higher in hypertensive men than in women. In women, no changes in platelet activity or 5HIAA excretion were found. 5HT plasma concentrations increased with blood pressure. Platelet 5HT uptake (Vmax and KM) were the lowest in hypertensive men greater than or equal to 60 years of age. This may indicate that 5HT uptake in vivo in normotensives is far below maximum (VNT much less than Vmax), whereas in hypertensive men it may be close to maximum (VHT approximately Vmax). This could reflect significantly higher 5HT plasma concentrations in vivo hypertensives than in normotensives. The reduced uptake (which was found only in hypertensive men) may indicate an insufficient compensation of the enhanced 5HT release from aggregating platelets in older men, in whom platelet activity is enhanced in vivo. It is concluded that the defect in platelet 5HT uptake in hypertensives--along with the enhanced platelet aggregation--may contribute to a critical increase in 5HT plasma concentrations locally. An increase in 5HT concentrations leads to biochemical changes (higher 5HIAA excretion) as well as to an enhanced stimulation by 5HT. This may be of clinical relevance especially in older men, in whom 5HT2-receptor mediated responses are enhanced.