Angiotensin-Converting Enzyme Inhibitors

Angiotensin-converting enzyme (ACE) inhibitors lower blood pressure by blocking the enzyme that converts angiotensin I to angiotensin II, the principal effector of the renin-angiotensin system. By reducing angiotensin II, they decrease vasoconstriction and aldosterone-driven sodium retention, and because ACE also degrades bradykinin, they raise bradykinin levels, which contributes both to their effects and to a characteristic cough.

Definition

ACE inhibitors are drugs that competitively inhibit angiotensin-converting enzyme (kininase II), reducing the formation of angiotensin II and the breakdown of bradykinin, thereby lowering vascular resistance and blood pressure.

Scope

This entry covers the mechanism of renin-angiotensin system inhibition at the converting-enzyme step, the bradykinin pathway that distinguishes ACE inhibitors from angiotensin receptor blockers, and the trial evidence establishing cardiovascular and renal benefit. It is reference material, not prescribing guidance.

Core questions

• How does blocking the converting-enzyme step alter the renin-angiotensin cascade?
• Why does bradykinin accumulation distinguish ACE inhibitors from angiotensin receptor blockers?
• What does outcome trial evidence show about cardiovascular and renal protection?

Key concepts

• Renin-angiotensin-aldosterone system
• Angiotensin-converting enzyme (kininase II)
• Angiotensin II as effector peptide
• Bradykinin accumulation and ACE-inhibitor cough
• Efferent arteriolar tone and renal protection
• Combination renin-angiotensin blockade

Mechanisms

Angiotensin-converting enzyme catalyses the conversion of angiotensin I to the active vasoconstrictor angiotensin II and also degrades the vasodilator bradykinin. By inhibiting this enzyme, ACE inhibitors lower circulating and tissue angiotensin II, reducing arteriolar constriction and aldosterone-mediated sodium and water retention; they also raise bradykinin, augmenting vasodilation but contributing to a dry cough and, rarely, angioedema. In the kidney, reduced angiotensin II preferentially relaxes the efferent glomerular arteriole, lowering intraglomerular pressure, a haemodynamic effect linked to renal protection in proteinuric disease.

Clinical relevance

ACE inhibitors are a core class for understanding renin-angiotensin system pharmacology and are studied across hypertension, heart failure, and chronic kidney disease. This entry describes their mechanism and the evidence behind their use as educational reference; it does not provide dosing or individualised treatment advice.

Evidence & guidelines

Major guidelines list ACE inhibitors among first-line antihypertensive classes and emphasise their role where renin-angiotensin blockade offers additional benefit, such as in chronic kidney disease with albuminuria and in heart failure with reduced ejection fraction. Trials including HOPE supported broad cardiovascular benefit, and ALLHAT compared an ACE inhibitor against a calcium channel blocker and a diuretic; ONTARGET examined combining an ACE inhibitor with an angiotensin receptor blocker and found no added benefit over either alone with more adverse effects, informing the recommendation against routine dual blockade.

History

ACE inhibition emerged from study of peptides in pit-viper venom that potentiated bradykinin and inhibited the converting enzyme, leading to the design of captopril as the first orally active agent and subsequently to longer-acting prodrugs such as enalapril and ramipril. Large outcome trials over the following decades extended their use beyond blood-pressure lowering to heart failure and renal protection.

Debates

Should an ACE inhibitor and an angiotensin receptor blocker be combined?

Dual renin-angiotensin blockade was once explored for additive benefit, but the ONTARGET trial found combining telmisartan and ramipril produced no outcome advantage over either drug alone while increasing adverse events, and guidelines now advise against routine combination.

Seminal works

• hope-2000
• ontarget-2008
• allhat-2002

Frequently asked questions

Why do ACE inhibitors sometimes cause a dry cough?

Angiotensin-converting enzyme also breaks down bradykinin; inhibiting the enzyme raises bradykinin levels, which is thought to provoke the characteristic dry cough in some people.

How do ACE inhibitors differ from angiotensin receptor blockers?

ACE inhibitors block formation of angiotensin II and raise bradykinin, whereas angiotensin receptor blockers block the angiotensin II type 1 receptor directly and do not raise bradykinin, so they rarely cause cough.