Organic Nitrates and Angina Treatment

Organic Nitrates: Mechanism of Action

Organic nitrates are a class of drugs primarily used to treat angina. Their mechanism of action involves several steps:

  1. Metabolism to Nitric Oxide (NO): Organic nitrates are metabolized into nitric oxide, involving sulfhydryl groups in the process.

  2. Activation of Guanylyl Cyclase: Nitric oxide activates guanylyl cyclase, an enzyme that converts guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP).

    GTP→Guanylyl CyclasecGMPGTP \xrightarrow{\text{Guanylyl Cyclase}} cGMP

  3. Increase in cGMP Levels: The activation of guanylyl cyclase leads to an increase in the levels of cGMP.

  4. Activation of Protein Kinase G: Elevated cGMP levels activate protein kinase G.

  5. Dephosphorylation of Myosin: Activated protein kinase G acts on the myosin-myosin phosphatase system, resulting in the dephosphorylation of myosin, reducing calcium concentrations.

  6. Smooth Muscle Relaxation: Ultimately, these steps lead to the relaxation of smooth muscle, including vascular smooth muscle.

Effects on Vascular Smooth Muscle

The primary effect of organic nitrates is the relaxation of smooth muscle, particularly vascular smooth muscle. While they can relax smooth muscle in other areas like the gastrointestinal tract and bronchioles, their therapeutic use is focused on the cardiovascular system.

Vasodilation

  • Organic nitrates induce vasodilation by reducing vascular smooth muscle contraction.
  • This vasodilation has significant implications for treating conditions like angina.

Angina and the Role of Organic Nitrates

Understanding Angina

  • Angina occurs when the heart's workload exceeds the blood supply it receives, leading to ischemia.
  • This results in anoxic conditions, a buildup of metabolites like lactate, and cardiac pain.

Cardiovascular Effects

To understand how organic nitrates help with angina, consider the following:

  • Afterload Reduction: Nitrates cause vasodilation on the arterial side, reducing resistance and afterload. This makes it easier for the heart to pump blood.

  • Preload Reduction: Organic nitrates uniquely vasodilate veins, decreasing venous return to the heart, thus reducing preload.

  • Coronary Artery Dilation: Nitrates dilate coronary arteries, improving blood flow to the heart muscle.

Mechanism Summarized

  1. Reduced afterload makes it easier for the heart to pump.
  2. Reduced preload decreases the heart's workload.
  3. Improved blood flow to the heart muscle ensures adequate oxygen supply.

These three mechanisms are key to the effectiveness of nitrates in treating angina.

Clinical Use in Angina

  • Organic nitrates are used to alleviate anginal attacks by causing vasodilation through the nitric oxide pathway.
  • They reduce both preload and afterload, making it easier for the heart to function, and improve blood supply to the heart.

Hospital Use

  • In a hospital setting, nitrates can be used in heart failure cases.
  • Reducing preload and afterload can improve the condition, especially in systolic heart failure cases.

Tolerance to Organic Nitrates

Development of Tolerance

  • Continuous use of organic nitrates can lead to tolerance, where the drug's effectiveness diminishes over time.
  • Tolerance manifests as a reduced response to the same dose.

Nitrate-Free Period

  • To prevent tolerance, a nitrate-free period is necessary.
  • A common method is using nitrate patches applied for 12 hours and removed for the subsequent 12 hours (e.g., applied at 8 a.m. and removed at 8 p.m.).

Reasons for Tolerance

  • The exact mechanism of tolerance isn't fully understood, but potential reasons include:
    • Depletion of sulfhydryl groups.
    • Reduced response in vascular smooth muscle.
    • Decreased conversion of nitrates to nitric oxide.

Glyceryl Trinitrate

Administration

  • Glyceryl trinitrate, also known as anginine, is administered sublingually (under the tongue) in small white tablets.
  • This method is commonly seen in movies when individuals experiencing angina place a tablet under their tongue for rapid relief.

First-Pass Metabolism

  • Glyceryl trinitrate has a very high first-pass metabolism.
  • When swallowed, drugs are absorbed into the portal system and transported to the liver, where they are metabolized. Some drugs are so effectively metabolized that only metabolites reach systemic circulation.
  • Sublingual administration bypasses this first-pass metabolism.

Bypassing First-Pass Metabolism

  • Sublingual administration allows the drug to enter the venous system and reach the heart before passing through the liver.
  • This bypasses the extensive metabolism that would occur if the drug were swallowed, ensuring that a therapeutic dose reaches its target quickly.

Side Effects of Organic Nitrates

  • Common side effects include dizziness, postural hypotension, and headaches.
  • These are often tolerable and can be managed through dose adjustments.

Drug Interactions: Organic Nitrates and PDE5 Inhibitors

PDE5 Inhibitors

  • PDE5 inhibitors, such as sildenafil (Viagra), are used to treat erectile dysfunction.

  • These drugs inhibit phosphodiesterase type 5 (PDE5), an enzyme that breaks down cyclic GMP.

Mechanism of Action of Sildenafil

  • Sildenafil increases cyclic GMP concentrations, leading to smooth muscle relaxation and increased blood flow to the penis.

Potential Catastrophic Interaction

  • Combining organic nitrates with PDE5 inhibitors can lead to a significant drop in blood pressure, potentially causing cardiovascular collapse and even death.
  • Both drug types increase cyclic GMP levels – nitrates by forming it, and PDE5 inhibitors by preventing its breakdown.

Recommendation

  • The combination of organic nitrates and PDE5 inhibitors should be avoided due to the risk of severe hypotension and cardiovascular collapse.