5.1: Mechanisms of Action: Cardiovascular & Renal Agents

Mechanisms of Action: Cardiovascular & Renal Agents

In the OPRA exam, a deep understanding of cardiovascular and renal pharmacology is vital. Questions regularly test drug class distinctions, physiological mechanisms, metabolic pathways, and critical safety parameters established in Australian clinical guidelines (such as the Australian Medicines Handbook and Therapeutic Guidelines).

Beta-Adrenoceptor Antagonists (Beta-Blockers)

Beta-blockers competitively inhibit the effects of catecholamines (epinephrine and norepinephrine) at beta-adrenergic receptors. They are classified based on receptor selectivity:

Critical Safety and Counselling Points:

• Bronchospasm Risk: Non-selective beta-blockers are contraindicated in patients with a history of asthma. In patients with stable Chronic Obstructive Pulmonary Disease (COPD), selective beta-1 blockers are generally considered safe under close monitoring when indicated for compelling conditions (e.g., post-myocardial infarction or heart failure).
• Glycaemic Masking: In patients with diabetes, beta-blockers can mask the autonomic symptoms of hypoglycaemia, such as palpitations, tremor, and tachycardia. However, sweating (a cholinergic response) is not masked and remains a reliable sign.
• Withdrawal Syndrome: Abrupt cessation of long-term beta-blocker therapy can precipitate rebound hypertension, severe angina, arrhythmias, or myocardial infarction due to the upregulation of beta-adrenergic receptors. Doses must be tapered gradually over 1 to 2 weeks.

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Renin-Angiotensin-Aldosterone System (RAAS) Inhibitors

RAAS inhibitors are essential in the management of hypertension, heart failure, and chronic kidney disease.

Angiotensin-Converting Enzyme (ACE) Inhibitors vs. Angiotensin II Receptor Blockers (ARBs)

• ACE Inhibitors (e.g., Perindopril, Ramipril, Enalapril): Prevent the conversion of Angiotensin I to Angiotensin II by inhibiting ACE. Additionally, ACE is responsible for the degradation of bradykinin. Consequently, ACE inhibition leads to an accumulation of bradykinin in the respiratory tract, causing a dry, persistent cough (occurring in 5-20% of patients) and, rarely, life-threatening angioedema.
• ARBs (e.g., Candesartan, Irbesartan, Telmisartan): Selectively block the binding of Angiotensin II to the Angiotensin II Type 1 (AT1) receptor. Because they do not inhibit ACE, they do not affect bradykinin levels. Thus, ARBs are the primary alternative for patients who develop an ACEi-induced cough.

Renal and Systemic Haemodynamic Effects

Angiotensin II is a potent vasoconstrictor that preferentially acts on the efferent arteriole of the glomerulus.

• Renal Protection: In patients with diabetic nephropathy, RAAS inhibitors dilate the efferent arteriole, lowering intraglomerular capillary pressure. This reduces hyperfiltration, limits glomerular damage, and decreases proteinuria.
• Acute Kidney Injury Risk: Glomerular filtration pressure is maintained by efferent arteriole constriction. In states of severe renal hypoperfusion (e.g., bilateral renal artery stenosis, severe volume depletion, or stenosis of a solitary kidney), filtration pressure is highly dependent on Angiotensin II. Initiating an ACEi or ARB in these patients can cause a precipitous drop in glomerular filtration rate (GFR) and acute renal failure.

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Calcium Channel Blockers (CCBs)

CCBs block the entry of calcium ions through L-type calcium channels in vascular smooth muscle and cardiac myocytes. They are divided into two distinct classes:

1. Dihydropyridines (e.g., Amlodipine, Felodipine, Lercanidipine)

• Mechanism: Act selectively on vascular smooth muscle, causing peripheral and coronary vasodilation.
• Side Effects: Peripheral oedema (due to precapillary vasodilation causing fluid shift), headache, flushing, and reflex tachycardia (due to baroreceptor activation in response to sudden blood pressure drops, particularly with short-acting formulations). Lercanidipine is associated with a lower incidence of peripheral oedema compared to amlodipine.

2. Non-Dihydropyridines (e.g., Verapamil, Diltiazem)

• Mechanism: Act on both vascular smooth muscle and cardiac tissue (sinoatrial and atrioventricular nodes). They exert negative inotropic (reduced contractility), negative chronotropic (reduced heart rate), and negative dromotropic (reduced conduction velocity) effects.
• Contraindications: Due to negative inotropic effects, they are contraindicated in patients with heart failure with reduced ejection fraction (HFrEF).
• Drug Combinations: Combining non-dihydropyridines with beta-blockers is contraindicated or requires extreme caution due to the synergistic risk of severe bradycardia, AV block, and complete heart block.
• Key Side Effect: Verapamil commonly causes severe constipation by blocking calcium channels in gastrointestinal smooth muscle, slowing peristalsis.

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Diuretics: Sites of Action and Electrolyte Profiles

Diuretics promote the excretion of water and electrolytes by targeting specific transporters along the nephron.

Nephron Segment & Transporter Targets:
Thick Ascending Limb (Na+/K+/2Cl-)  --> Loop Diuretics (Furosemide)
Distal Convoluted Tubule (Na+/Cl-)   --> Thiazides (Indapamide, HCTZ)
Late Distal / Collecting (ENaC)     --> Amiloride
Collecting Duct (Mineralocorticoid)  --> Aldosterone Antagonists (Spironolactone)

1. Loop Diuretics (e.g., Furosemide)

• Site of Action: Inhibit the Na+/K+/2Cl- cotransporter in the thick ascending limb of the loop of Henle.
• Clinical Use: High-potency ('high-ceiling') diuretics used primarily for fluid overload in heart failure and renal impairment.
• Electrolyte Profile: Causes hypokalaemia, hyponatraemia, hypomagnesaemia, and hypocalcaemia (increases calcium excretion).
• Adverse Effects: Dehydration, pre-renal azotaemia, and dose-related ototoxicity (especially when administered intravenously at high speeds or combined with other ototoxic drugs like aminoglycosides).

2. Thiazide and Thiazide-like Diuretics (e.g., Hydrochlorothiazide, Indapamide, Chlorthalidone)

• Site of Action: Inhibit the Na+/Cl- cotransporter in the distal convoluted tubule.
• Clinical Use: Moderate-potency diuretics used first-line for hypertension. Less effective when eGFR falls below 30 mL/min (except chlorthalidone/metolazone).
• Electrolyte Profile: Causes hypokalaemia, hyponatraemia, hypomagnesaemia, but hypercalcemia (promotes calcium reabsorption).
• Metabolic Side Effects: Hyperuricaemia (can precipitate acute gouty arthritis by competing with uric acid for renal excretion) and hyperglycemia.

3. Potassium-Sparing Diuretics

• ENaC Inhibitors (e.g., Amiloride): Block epithelial sodium channels in the late distal tubule and collecting duct, indirectly reducing potassium excretion.
• Aldosterone Antagonists (e.g., Spironolactone, Eplerenone): Competitively block mineralocorticoid receptors in the collecting duct.
  • Spironolactone: Non-selective; binds to progesterone and androgen receptors, frequently causing endocrine side effects like gynaecomastia, breast tenderness, and menstrual irregularities.
  • Eplerenone: Highly selective for mineralocorticoid receptors; avoids endocrine side effects.
  • Risk: High risk of hyperkalaemia, especially when combined with ACEis, ARBs, or NSAIDs.

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Lipid-Lowering Agents

HMG-CoA Reductase Inhibitors (Statins)

• Mechanism: Competitively inhibit HMG-CoA reductase, the rate-limiting enzyme in hepatic cholesterol synthesis. The resulting depletion of intracellular cholesterol upregulates LDL receptors on the hepatocyte surface, increasing the clearance of circulating LDL-cholesterol.
• Adverse Effects: Myalgia (muscle pain without CK elevation) and myopathy (muscle pain with CK elevation). Rhabdomyolysis is a rare, severe condition involving muscle breakdown, CK elevation (>10 times Upper Limit of Normal), and myoglobinuria, which can lead to acute renal failure.
• Metabolism & Interactions: Simvastatin and atorvastatin are primary substrates of the CYP3A4 enzyme. Concomitant use of strong CYP3A4 inhibitors (e.g., clarithromycin, itraconazole, diltiazem) increases plasma concentrations, elevating myopathy risk. Rosuvastatin and pravastatin are not metabolized by CYP3A4 and are preferred in these clinical scenarios.

Other Agents

• Ezetimibe: Inhibits the NPC1L1 transporter in the brush border of the enterocytes, reducing dietary and biliary cholesterol absorption in the small intestine.
• PCSK9 Inhibitors (e.g., Evolocumab): Monoclonal antibodies that bind to PCSK9, preventing it from degrading LDL receptors, thus increasing LDL clearance from the blood.
• Fibrates (e.g., Fenofibrate): Agonists of PPAR-alpha, upregulating lipoprotein lipase to reduce triglycerides. High risk of myopathy when co-administered with statins.