High performance liquid chromatography (HPLC) determination of endogenous serotonin released from aggregating platelets

Regulation of cardiac afferent excitability in ischemia

The heart at the time of Sir William Harvey originally was thought to be an insensate organ. Today, however, we know that this organ is innervated by sensory nerves that course centrally though mixed nerve pathways that also contain parasympathetic or sympathetic motor nerves. Angina or cardiac pain is now well recognized as a pressure-like pain that occurs during myocardial ischemia when coronary artery blood flow is interrupted. Sympathetic (or spinal) afferent fibers that are either finely myelinated or unmyelinated are responsible for the transmission of information to the brain that ultimately allows the perception of angina as well as activation of the sympathetic nervous system, resulting in tachycardia, hypertension, and sometimes arrhythmias. Although early studies defined the importance of the vagal and sympathetic cardiac afferent systems in reflex autonomic control, until recently there has been little appreciation of the mechanisms of activation of the sensory endings. This review examines the role of a number of chemical mediators and their sources that are activated by the ischemic process. In this regard, patients with ischemic syndromes, particularly myocardial infarction and unstable angina, are known to have platelet activation, which leads to release of a number of chemical mediators, including serotonin, histamine, and thromboxane A(2), all of which stimulate ischemically sensitive cardiac spinal afferent endings in the ventricles through specific receptor-mediated processes. Furthermore, protons from lactic acid, bradykinin, and reactive oxygen species, especially hydroxyl radicals, individually and frequently in combination, stimulate these endings during ischemia. Cyclooxygenase products appear to sensitize the endings to the action of bradykinin and histamine. These studies of the chemical mechanisms of activation of cardiac sympathetic afferent endings during ischemia have the potential to provide targeted therapies that can modify the angina and the deleterious reflex responses that have the potential to exacerbate ischemia and myocardial cell death.

Activated platelets contribute to stimulation of cardiac afferents during ischaemia in cats: role of 5-HT(3) receptors

Myocardial ischaemia activates blood platelets and cardiac sympathetic afferents, which mediate chest pain and cardiovascular reflex responses. We have demonstrated that activated platelets stimulate ischaemically sensitive cardiac sympathetic afferents. Platelets absorb and release 5-hydroxytryptamine (5-HT) when they are activated. In the present study we hypothesized that, by releasing 5-HT, activated platelets stimulate cardiac afferents during ischaemia through a 5-HT(3) receptor mechanism. Platelet-rich plasma (PRP) and platelet-poor plasma (PPP) were obtained from cats. Activation of platelets in PRP was induced by thrombin (5 units ml(-1)) or collagen (2 mg kg(-1)). Using high-performance liquid chromatography, we observed that the concentration of 5-HT was increased significantly in suspensions of platelets activated with thrombin (PRP+thrombin, 28 +/- 1.7 microM) or collagen (PRP+collagen, 27 +/- 2.5 microM) compared with suspensions of unactivated platelets (PRP+saline, 2.3 +/- 0.8 microM) and PPP. During myocardial ischaemia and reperfusion, tirofiban, a specific inhibitor of platelet glycoprotein (GP) IIb-IIIa receptors (100 microg kg(-1), I.V., followed by 5 microg kg(-1) min(-1)), significantly reduced the increase in the concentration of 5-HT in cardiac venous plasma from ischaemic region. Nerve activity of single-unit cardiac afferents was recorded from the left sympathetic chain (T2-T5) in anaesthetized cats. Eighty ischaemically sensitive and seven ischaemically insensitive cardiac afferents were identified. Tirofiban reduced the ischaemia-related increase in activity of seven cardiac sympathetic afferents by 50 %. Injection of 1.5 ml of PRP+collagen or PRP+thrombin into the left atrium (LA) increased activity of 16 cardiac afferents. Tropisetron (300 microg kg(-1), I.V.), a selective 5-HT(3) receptor antagonist, eliminated the afferent's responses to platelets activated with collagen or thrombin. Moreover, LA injection of 5-HT (20-40 microg kg(-1)) and PBG (100 microg kg(-1)), a 5-HT(3) receptor agonist, stimulated nine ischaemically sensitive cardiac sympathetic afferents, significantly increasing the activity of these afferents. However, injection of alpha-M-5-HT (100 microg kg(-1), LA), a 5-HT(2) receptor agonist, stimulated only two of the nine ischaemically sensitive cardiac afferents, and thus did not significantly alter impulse activity of this group of afferents. Both the 5-HT(1) (5-CT, 100 microg kg(-1), LA) and 5-HT(4) receptor agonists (SC53116, 100 microg kg(-1), LA) did not stimulate any of the nine afferents tested. Tropisetron (300 microg kg(-1), I.V.) also eliminated the response of seven ischaemically sensitive cardiac afferents to exogenous 5-HT and attenuated the ischaemia-related increase in activity of nine cardiac sympathetic afferents by 41 %. Conversely, LA injection of 5-HT (40 microg kg(-1)) did not stimulate any of seven ischaemically insensitive cardiac afferents, although this group of afferents consistently responded to bradykinin (3 microg, LA). These data indicate that during myocardial ischaemia the activated platelets stimulate cardiac sympathetic afferents, at least in part, through a 5-HT(3) receptor mechanism.

A plasmatic factor may cause platelet activation in acute ischemic stroke

To study the pathogenesis of platelet activation in ischemic stroke, ionized calcium ([Cai2+]) was measured in aequorin-loaded gel-filtered platelets in the basal and stimulated state. Basal [Cai2+] was increased in stroke patients maximally 36-72 hours after onset. The increase in [Cai2+] after stimulation with thrombin, collagen, and platelet-activating factor were also greater in stroke patients, but the profiles of these [Cai2+] changes were parallel to control. Cross incubation of control platelets with plasma from stroke patients resulted in raised basal [Cai2+] and caused the release of serotonin from platelets. These results indicate that the higher platelet basal [Cai2+] in stroke patients represents a lowered threshold for activation and that this may be due to a plasmatic factor rather than a primary platelet defect.

Development and validation of an improved radioimmunoassay for serotonin in platelet-rich plasma

A radioimmunoassay (RIA) using a 125I-tracer is described for measurement of serotonin (5-hydroxytryptamine, 5-HT) in human platelet-rich plasma (PRP). Antisera were raised against 5-HT-succinamate conjugated to bovine albumin and, to improve assay sensitivity, the analyte was made chemically similar to the immunogen by conversion to N-acetylserotonin prior to assay, using the specific amino reagent N-acetoxysuccinimide. The assay shows good correlation with a high-pressure liquid chromatography (HPLC) reference method (5-HT RIA = 1.007 X 5-HT HPLC + 29.3, r = 0.936, p less than 0.001, n = 40), indicating that no significant cross-reactions were detected. Samples of PRP are diluted 1/20 to fall within the working range (80-15% B/B0) of the assay, which is 4.75-325 nmol/l, (0.95-65.0 pmol/tube), corresponding to 95-6500 nmol/l in PRP. Intra- and interassay coefficients of variation were 5.0-10.5% and 12.0-21.2% respectively for serotonin concentrations of 250-2,500 nmol/l added to platelet-poor plasma. With this improved assay, it is possible to analyse up to 100 samples/day, compared with 10-20 samples/day by HPLC.

Quantitative determination of serotonin released from human platelets

A high-performance liquid chromatographic technique has been used for measuring the amount of endogenous serotonin (5-hydroxytryptamine, 5-HT) released from human platelets stimulated by adenosine diphosphate (ADP) and collagen. Four minutes after challenge both agents induce maximal and similar 5-HT releases corresponding to 14% (ADP) and 17% (collagen) of the intraplatelet 5-HT content. ADP and collagen seem to have different mechanisms of inducing secretion, documented by the different relationship between the release patterns and the optical tracings for aggregation. Endogenous 5-HT release might be useful tool for monitoring states of platelet activation.

High-performance liquid chromatographic assay of serotonin in human plasma

A sensitive and reproducible reversed-phase high-performance liquid chromatography with electrochemical detection for determination of serotonin in human plasma is described. The method has an average coefficient of variation of 3.5%; the recovery of serotonin and 5-hydroxy-N-methyltryptamine (internal standard) accounts for 95 +/- 2%. Average serotonin concentrations in platelet-free plasma and in platelet-rich plasma from 21 normal subjects 22-63-year-old) ranged from 0.6 to 4.9 ng/ml (mean 2.8 +/- 1.3 SD) and from 38.3 to 106.5 ng/10(8) platelets (mean 66.8 +/- 18.9 SD), respectively. The release of endogenous serotonin from human aggregating platelets challenged by collagen or adenosine diphosphate has been quantitated.