Serotonin and the cardiovascular system

Abstract

Serotonin (5-HT) is a biogenic amine with diverse effects in the central nervous system as well as in the periphery. It regulates vascular biology, ranging from the control of vascular resistance and blood pressure to hemostasis and platelet function. 5-HT also plays an important role in the pathogenesis of cardiovascular diseases being involved in the development of valvulopathy, atherosclerosis and pulmonary hypertension. Outside the central nervous system 5-HT is synthesized in enterochromaffin cells of the intestine and is stored in dense granules in platelets and released during platelet activation. Most circulating 5-HT in man is stored in platelets, but only free 5-HT (not sequestered in granules in the platelets) has biological activity. Due to release of 5-HT from platelets during blood sampling and plasma preparation, there have been problems in establishing a reliable method for the determination of free 5-HT in blood. In paper I, we aimed to establish a method to assess the influence of platelet release upon platelet-poor plasma 5- HT measurement by concomitant determination of 5-HT, β-thromboglobulin (β-TG) and chromogranin A (CgA).We established our own methods of blood sampling and handling to avoid activating or destroying platelets and thereby minimize the component of platelet derived 5-HT measured in our samples. 5-HT and β-TG levels were measured using ELISA techniques according to manufactures instructions. The influence of platelets to blood 5-HT concentrations in our samples was evaluated by measuring both parameters in blood from patients with thrombocytopenia as well as thrombocytosis compared to healthy volunteers. Moreover, the optimal concentration of EDTA in the sampling tubes to prevent platelet 5-HT release was examined. Furthermore, 5-HT and β-TG were determined together with CgA in blood from patients with small intestinal carcinoids in order to separate between platelet and tumor derived 5-HT. Our assessment of the role of 5-HT platelet release shows that platelet numbers did not affect plasma 5-HT concentrations in contrast to β-TG.

Because of the role of 5-HT in the pathogenesis of pulmonary arterial hypertension (PAH) and acute coronary syndrome (ACS), we aimed in paper II and III to test if venous plasma 5-HT is a potential biomarker of PAH and ACS. We also measured venous blood β-TG in all participants to ensure that any increase in 5-HT levels measured is due to platelet release. Our study shows that the venous plasma 5-HT concentrations in patients with PAH were almost similar to those of healthy controls. Therefore, the measurement of venous plasma 5-HT concentrations is not suitable to distinguish between patients and healthy controls in a clinical setting. Indeed, the concentration in peripheral venous blood may not necessarily be the same as that in the pulmonary circulation, which is the concentration of interest with respect to the potential role of 5-HT in the pathogenesis of PAH. In the lungs, 5-HT may be released from pulmonary endothelial cells and from platelets resulting in higher local concentrations than those found in plasma and sufficient to induce a vasoconstrictor effect.

In paper III the venous plasma 5-HT concentrations in patients with ACS were almost similar to those of healthy controls. Patients with ACS did not either have significantly elevated venous plasma β-TG levels compared to controls. Our negative findings indicate that the amount of β-TG and 5-HT released from a platelet thrombus occluding a coronary artery is not sufficient to cause a measurable increase in the systemic circulation.

In paper IV, we aimed to develop a rather non-invasive model of aortic stenosis in rabbits using radiofrequency ablation technique (RFA). Considering that hemodynamic disturbances with accelerated and turbulent flow probably activate platelets and may cause release of 5-HT from dense granules, our hypothesis is that platelet-derived 5-HT may be a crucial factor in development of valvular heart disease. Therefore a suitable animal model of aortic stenosis is required to investigate disease mechanisms and potential therapies. In this study, by placing the heparinized RF catheter at the level of the aortic valve, we applied various strengths of RFA to induce aortic stenosis. The peak velocity of the aortic valve blood flow was measured as the mean of 5 cycles of continuous Doppler recordings, before and immediately after the procedure and six weeks after RFA. No increase in the PSV was observed immediately after radiofrequency ablation. PSV immediately after the procedure and at the six week follow-up were almost similar. We did not succeed in inducing aortic valve stenosis using different RFA energies. Inadequate RFA power or inappropriate positioning of the RFA catheter could be limitations of our study.