How do serotonin (5‑hydroxy‑tryptamine) and norepinephrine affect the cardiovascular system?

Cardiovascular Effects of Serotonin and Norepinephrine

Norepinephrine Effects on the Cardiovascular System

Norepinephrine acts as a potent sympathetic neurotransmitter that increases cardiac output, heart rate, myocardial oxygen demand, and peripheral vascular resistance through direct stimulation of cardiac adrenergic receptors and systemic vasoconstriction. 1

Direct Cardiac Effects

• Increases ventricular contractility by stimulating β1-adrenergic receptors on myocytes, leading to enhanced cardiac output and blood pressure 1
• Elevates heart rate through chronotropic effects on cardiac pacemaker cells 1
• Increases myocardial oxygen demand, which can precipitate ischemia and oxidative stress in vulnerable myocardium 1

Vascular Effects

• Causes peripheral vasoconstriction, increasing both preload and afterload on the heart 1
• Raises systemic vascular resistance, contributing to elevated blood pressure 1

Chronic Activation Consequences

• Prolonged norepinephrine exposure leads to maladaptive cardiac remodeling, including ventricular dilation, decreased contractility, and progressive heart failure 1
• Down-regulates β1-adrenergic receptors and uncouples β2-adrenergic receptors, making the myocardium less responsive to further adrenergic stimulation 1
• Promotes cardiac fibrosis and myocyte necrosis, contributing to structural deterioration of the heart 1
• This chronic adrenergic activation portends poor prognosis in heart failure patients, which is why beta-blockers have become standard therapy to interrupt this deleterious cycle 1

Serotonin Effects on the Cardiovascular System

Serotonin produces complex and often opposing cardiovascular effects depending on receptor subtype activation, with 5-HT4 receptors mediating positive inotropic and chronotropic effects in human hearts, while other receptor subtypes can cause vasodilation, vasoconstriction, bradycardia, or tachycardia. 2, 3

Cardiac Effects

• 5-HT4 receptors are the primary mediators of serotonin's direct cardiac effects in humans, producing positive inotropic (increased contractility) and chronotropic (increased heart rate) effects 3, 4
• Increases phosphorylation of phospholamban via 5-HT4 receptor activation, enhancing calcium handling and contractility in human atrial tissue 4
• Can cause arrhythmias through 5-HT4 receptor stimulation in human hearts 3
• 5-HT3 receptors on vagal nerve endings trigger the von Bezold-Jarisch reflex, causing initial bradycardia and hypotension 2, 5

Vascular Effects

• Produces triphasic blood pressure response: initial hypotension (vagal reflex via 5-HT3), middle pressor phase (vasoconstriction via 5-HT2), and prolonged hypotension (vasodilation via 5-HT1-like receptors) 2
• 5-HT2 receptors mediate vasoconstriction in most vascular beds, increasing blood pressure 2, 5
• 5-HT1-like receptors cause vasodilation through four mechanisms: decreased sympathetic outflow centrally, reduced transmitter release at nerve terminals, direct smooth muscle relaxation, and endothelial nitric oxide release 2
• Can be stored in and released from vascular adrenergic nerve endings, contributing to local vascular tone regulation 6

Central Nervous System Regulation

• Central 5-HT1A receptors decrease sympathetic activity and increase vagal drive, lowering blood pressure and heart rate 5
• Central 5-HT2 receptors increase sympathetic activity, raising blood pressure and potentially contributing to hypertension development 5
• 5-HT3 and 5-HT7 receptors centrally regulate cardiovascular reflexes, particularly parasympathetic control of heart rate 5

Clinical Implications

Drug-Induced Cardiovascular Effects

• SNRIs (serotonin-norepinephrine reuptake inhibitors) increase blood pressure by approximately 2/1 mmHg through enhanced norepinephrine activity 1
• Venlafaxine can cause dose-dependent blood pressure elevation, particularly at doses above 225 mg daily, due to norepinephrine reuptake inhibition 7
• Duloxetine and milnacipran may cause hypertension, tachycardia, and increased pulse as common adverse effects related to their norepinephrine reuptake inhibition 7
• SSRIs do not increase blood pressure as they lack norepinephrine reuptake inhibition 1

Pathophysiological Roles

• Serotonin may play roles in sepsis, ischemia, and reperfusion injury through 5-HT4 receptor mechanisms 3
• The physiological role of platelet-stored serotonin in vascular tone regulation remains unclear, though its role in coagulation is well-established 5
• Dysregulated serotonin signaling is associated with various cardiovascular pathologies, including functional disorders and metabolic conditions 8