Cardiovascular disease including heart disease, arrhythmias and hypertension, is the leading cause of morbidity and mortality in the Western world. There are numerous devastating conditions affecting the heart and/or the vasculature, leading to high demand for cardiovascular drugs. This chapter focuses on some key therapeutic targets within the cardiovascular system and the drugs used to combat cardiovascular disease.

Renin-Angiotensin-Aldosterone System

The renin-angiotensin-aldosterone system (RAAS) is an important hormone-based pathway within the body that regulates fluid balance and thus systemic blood pressure. The system is activated by decreases in blood volume or pressure detected in two ways: a drop in blood pressure detected by baroreceptors (pressure sensors) located in the carotid sinus or a drop in flow rate through the kidneys, detected by the juxtaglomerular apparatus. The body responds to these stimuli to effect a restoration in blood pressure via the actions of three hormones; renin, angiotensin and aldosterone. Following the detected drop in blood pressure, the enzyme renin is released from specialised cells within the kidney. The substrate of renin is the inactive precursor of angiotensin I, angiotensinogen. Angiotensin I is then enzymatically converted by angiotensin converting enzyme (ACE) into angiotensin II, a hormone with various actions throughout the body that ultimately increase blood pressure, restoring fluid balance within the body.The adrenergic nervous system is a vital component of many processes throughout the body, including the cardiovascular system. Circulating catecholamines (e.g. adrenaline and noradrenaline) bind to and activate adrenergic receptors on cell membranes. Adrenergic receptors are a class of G-protein coupled receptors that elicit a variety of tissue-specific effects and exist in several subtypes.The predominant receptor subtype present in blood vessels is the a1-adrenergic receptor, activation of which by catecholamine binding causes activation of the phospholipase-C (PLC), inositol triphosphate (IP3), diacylglycerol (DAG) intracellular signalling pathway. This ultimately results in myocyte contraction, vasoconstriction and consequent increases in systemic blood pressure. Although the heart is myogenic, that is the impetus for contraction is self-initiated, the output of the heart is influenced by the central nervous system. The net effect of the sympathetic system on the heart is to increase cardiac output. The adrenergic receptors found in the heart belong to the ß-receptor subfamily and include ß1 and ß3 receptors. Catecholamine binding to ß1-receptors in the heart causes increases in cardiac output via a number of mechanisms: positive chronotropic effects, positive inotropic effects increased automaticity and conduction in both ventricular myocytes and the atrioventricular (AV) node. However ß3-receptor activation antagonises these actions, producing a negative inotropic effect and providing an inbuilt control system within the heart.Prolonged increase catecholamine levels in the circulation (e.g. when secreted from adrenal tumours or times of stress) can lead to chronic cardiovascular problems such as hypertension and arrhythmias. The parasympathetic system relies on the binding of the neurotransmitter acetylcholine (Ach) to muscarinic receptors, and has various roles throughout the body. Although blood vessels do express muscarinic receptors, they do not receive cholinergic innervation; however application of exogenous Ach results in a swift and profound vasodilation. Activation of muscarinic receptors (M2-subtype) in the heart by Ach released from the vagus nerve causes a reduction in cardiac output via opposite effects to adrenergic stimulation: negative chronotropic effects and decreases in AV node conduction as well as decreasing the force of atrial contractions.

Platelet activation and inhibition operates through surface receptors on platelets. Feedback loops enhance platelet activation (e.g. ADP released by platelets increases platelet activation, through the ADP receptor). Platelets (also known as thrombocytes) are small cells lacking nuclei that are responsible for haemostasis, or blood clotting. Damage or injury leading to blood loss and exposure of extracellular collagen fibres is detected, activating platelets. Once activated, platelets become adhesive, sticking to both the damaged vessel wall and each other, forming a clump of cells, or ‘clot’, helping to dam the vessel leak. They then begin to secrete cytokines that encourage invasion of fibroblasts present in the surrounding tissue which form a more permanent patch, either by creating healthy tissue, or depositing extracellular matrix to form a scar.

There are several conditions in which abnormal clotting can be damaging to the body; excess clotting can lead to vascular blockage and ischaemia or stroke; less commonly, deficient clotting can lead to excess blood loss, for example in haemophilia. To combat these diseases, there are drugs that modulate the clotting process.Drugs that prevent clotting (anti-coagulants) are important in those with an increased risk of clotting-mediated damage such as a stroke or ischaemia. As well being an analgesic and anti-pyretic, Aspirin is an anti-thrombotic agent given in low doses to those at risk of damage from clotting (e.g. following a heart attack). Aspirin’s anti-coagulant actions come from its suppression of key pro-clotting factors such as prostaglanding and thromboxanes via irreversible inactivation of the PTGS cyclooxygenase enzyme. This suppression of factors such as thromboxane A2 reduces platelet aggregation and thus prevents clot formation.P2Y12 inhibitors such as clopidogrel exert their anti-coagulant effect via inhibition of the P2Y12 subtype of the platelet ADP receptor. By blocking P2Y12, these drugs prevent activation of platelets and the formation of the fibrin network needed for clotting. Drugs such as abciximab and tirofiban prevent clotting via inhibition of the glycoprotein IIb/IIIa receptor preventing both platelet activation.

Submit manuscript at https://www.imedpub.com/submissions/american-pharmacology-pharmacotherapeutics.html

Regards,

John Mathews

American Journal of Pharmacology and Pharmacotherapeutics