Vascular Physiology and Cardiovascular Pharmacology

Hypertension: Primary vs. Secondary

Hypertension (HTN) is a chronic elevation of systemic blood pressure that increases the workload on the heart and damages the vasculature. It is clinically categorized into two types:

• Primary (Essential) Hypertension: This accounts for approximately $95%$ of cases and is characterized by a complex interplay of genetic predisposition and environmental factors.
• Secondary Hypertension: This is caused by an identifiable underlying condition, such as renal artery stenosis or endocrine disorders, and is often potentially curable.

The Renin-Angiotensin-Aldosterone System (RAAS)

The RAAS is a critical hormonal cascade for long-term blood pressure regulation. The process begins with the release of renin from the juxtaglomerular apparatus in the kidneys. Renin converts angiotensinogen to Angiotensin I, which is then converted to the potent vasoconstrictor Angiotensin II by the Angiotensin-Converting Enzyme (ACE).

Pharmacological Blockade of RAAS

To manage hypertension and heart failure, several classes of drugs target this system:

• ACE Inhibitors (e.g., Enalapril): These prevent the formation of Angiotensin II. A common side effect is a dry cough, caused by the accumulation of bradykinin.
• Angiotensin II Receptor Blockers (ARBs) (e.g., Losartan): These block the $\text{AT}_1$ receptors directly, providing similar benefits to ACE inhibitors without the cough.
• Renin Inhibitors (e.g., Aliskiren): These act at the very top of the cascade to prevent the initial conversion of angiotensinogen.

Adrenergic Pharmacology: Alpha and Beta Receptors

The sympathetic nervous system regulates vascular tone through catecholamines (epinephrine, norepinephrine, and dopamine) acting on G-protein coupled receptors.

Adrenergic Receptor Profiles

Key Vasopressors and Inotropes

• Norepinephrine (Levophed): A potent $\alpha_1$ and $\beta_1$ agonist. It is the first-line treatment for septic shock due to its intense vasoconstriction, which increases MAP.
• Epinephrine (Adrenalin): A non-selective agonist ($\alpha$ and $\beta$). At low doses, $\beta_2$ effects predominate (vasodilation); at high doses, $\alpha$ effects predominate (vasoconstriction). It is the drug of choice for anaphylaxis.
• Dopamine: Exhibits dose-dependent effects. Low doses stimulate $\text{D}_1$ receptors (renal vasodilation), moderate doses stimulate $\beta_1$ (inotropy), and high doses stimulate $\alpha_1$ (vasoconstriction).
• Dobutamine: Primarily a $\beta_1$ agonist used to improve cardiac output in cardiogenic shock without significantly increasing afterload.
• Isoproterenol (Isuprel): A pure $\beta_1/\beta_2$ agonist with no $\alpha$ activity. It acts as a "liquid pacemaker" but can cause significant hypotension due to $\beta_2$ vasodilation.

Exam Tip: When comparing Epinephrine and Norepinephrine, remember that Norepinephrine is generally more potent at $\alpha$ receptors, while Epinephrine has stronger $\beta_1$ (inotropic) and $\beta_2$ (metabolic/bronchodilatory) effects.

Calcium Channel Blockers (CCBs)

CCBs inhibit the influx of calcium through L-type channels, leading to muscle relaxation. They are divided into two main groups:

1. Dihydropyridines (e.g., Nicardipine, Clevidipine): These are potent arterial vasodilators used primarily for hypertensive emergencies.
2. Non-dihydropyridines (e.g., Verapamil, Diltiazem): These act on both the vasculature and the myocardium, providing negative inotropic (contractility) and chronotropic (heart rate) effects.

Phosphodiesterase (PDE) Inhibitors

PDE inhibitors prevent the breakdown of second messengers, prolonging their physiological effects.

• PDE3 Inhibitors (e.g., Milrinone): Known as "inodilators," they increase $\text{cAMP}$ to improve cardiac contractility while simultaneously causing vasodilation to reduce preload and afterload.
• PDE5 Inhibitors (e.g., Sildenafil): These selectively inhibit the breakdown of $\text{cGMP}$, making them effective pulmonary vasodilators and treatments for erectile dysfunction.

Nitric Oxide and Nitrovasodilators

Nitric Oxide (NO) is a potent endogenous vasodilator synthesized by endothelial cells.

Sodium Nitroprusside (SNP)

SNP is a rapid-acting donor that relaxes both arterial and venous smooth muscle.

Critical Warning: SNP carries a high risk of cyanide toxicity, especially at doses $> 2 \text{ mcg/kg/min}$. Signs include metabolic acidosis, altered mental status, and increased mixed venous oxygen content.

Nitroglycerin (NTG)

Unlike SNP, Nitroglycerin is an organic nitrate that primarily acts as a venodilator. By increasing venous capacitance, it reduces preload, which lessens myocardial wall tension and oxygen demand, making it ideal for treating angina.

Hydralazine

Hydralazine is a direct arterial vasodilator used for hypertension and pre-eclampsia. It is an effective afterload reducer but can cause reflex tachycardia due to sympathetic stimulation.

Other Specialized Agents

• Endothelin Receptor Antagonists (e.g., Bosentan): These block the effects of endothelin, one of the body's most potent vasoconstrictors.
• Catecholamine-Depleting Agents (e.g., Reserpine): These inhibit the uptake of catecholamines into storage vesicles, leaving them exposed to metabolic degradation.
• Fenoldopam: A $\text{D}_1$ agonist that provides renal vasodilation and natriuresis, making it useful in patients with renal insufficiency.