Antihypertensive Drug Classification and Mechanisms

3. Adrenoblockers (Adrenoceptor Antagonists)

Adrenoblockers work by competing with catecholamines (like adrenaline and noradrenaline) for binding sites on the adrenoreceptors located on target tissues.

Alpha ($\alpha$)-Adrenoblockers

These target the $\alpha$-receptors, which are primarily responsible for vasoconstriction.

• Non-selective $\alpha$-blockers: These block both pre- and postsynaptic receptors, such as Phenalamino and Tropaphenum.
• Selective $\alpha_1$-blockers: These target the postsynaptic $\alpha_1$-receptors specifically to induce vasodilation, such as Prazosin.

Beta ($\beta$)-Adrenoblockers

These target the $\beta$-receptors, which influence heart rate and contractility.

• Non-selective $\beta$-blockers: These block both $\beta_1$ (heart) and $\beta_2$ (bronchi/vessels) receptors, such as Propranolol (anaprilinum).
• Cardioselective $\beta_1$-blockers: These are designed to target the heart more specifically to minimize side effects in the lungs, such as Atenolol, Talinolol, and Metoprolol.
• Combined $\alpha, \beta$-blockers: Drugs like Labetalol provide blockade of both receptor types.

Exam Tip: When distinguishing between $\beta$-blockers, remember that "cardioselectivity" refers to the preference for $\beta_1$ receptors. This is clinically important because non-selective blockers (like Propranolol) can cause bronchospasm in patients with asthma by blocking $\beta_2$ receptors in the lungs.

Drugs Affecting Systemic Humoral Regulation

Beyond the nervous system, the body uses hormones to regulate blood pressure. The most significant of these is the Renin-Angiotensin System (RAS), which controls blood volume and vascular tone.

The Renin-Angiotensin System (RAS)

Drugs targeting this system are essential for long-term blood pressure management:

1. Angiotensin II Synthesis Inhibitors: These are also known as ACE (Angiotensin-Converting Enzyme) inhibitors. They prevent the conversion of Angiotensin I to the potent vasoconstrictor Angiotensin II. Examples include Captopril and Enalapril.
2. Angiotensin Receptor Blockers (ARBs): Instead of stopping the production of Angiotensin II, these drugs block its ability to bind to the $AT_1$ receptors. Losartan is a primary example.
3. Vasopeptidase Inhibitors: These act on the enzymes involved in the breakdown of vasoactive peptides, such as Omapratilate.

Drugs of Myotropic Action

Myotropic drugs do not act on the nerves or hormones, but rather directly on the muscle cells (myocytes) of the heart or blood vessels.

Ion Channel Blockers

One of the most important ways to influence muscle contraction is by altering the flow of ions across the cell membrane.

• Calcium Channel Blockers (CCBs): By blocking the entry of calcium ions into the smooth muscle cells of the arteries and the cardiac muscle cells, these drugs cause vasodilation and reduce the force of heart contraction.

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Summary Table of Antihypertensive Mechanisms