Lipoproteins

Lipoproteins and Cholesterol Transport

  • Lipoproteins: Combine with lipids to transport cholesterol through the blood.

    • High-Density Lipoprotein (HDL): More proteins, less fats; removes excess cholesterol from tissues and transports it back to the liver for recycling.

    • Low-Density Lipoprotein (LDL): More fats, less proteins; transports cholesterol to cells, contributing to plaque buildup in arteries.

Cholesterol Homeostasis

  • Dietary Intake: Fats in food are broken down in the small intestine and formed into glycerides, which are transported to the liver.

  • Cholesterol Manufacture: The liver manufactures cholesterol and manages its homeostasis through various receptors.

  • Cell Functions of Cholesterol:

    • Produces steroid hormones.

    • Forms bile acids.

    • Contributes to cell membranes.

Negative Feedback Mechanism

  • Definition: A regulatory mechanism where an increase in a certain variable leads to a decrease in that variable through corrective processes.

  • Example:

    • Thermostat Analogy: Set at 68°F. If it drops to 65°F, the heater turns on until it returns to 68°F, then shuts off.

  • Liver Regulation of LDL:

    • When LDL levels in the blood are high, liver receptors decrease, and LDL uptake declines. Conversely, low LDL levels increase receptor activity in the liver.

  • Connection to Fatty Liver Disease: Excess LDL leads to fat deposits in the liver due to receptor downregulation.

Coronary Artery Disease

  • Definition: Disease characterized by plaque buildup in coronary arteries, which supply oxygen and nutrients to the heart.

    • Plaque Composition: Lipids, lipoproteins, fats, cholesterol, and various blood cells (e.g., macrophages).

    • Foam Cells: Formed when macrophages engulf cholesterol; lead to artery hardening and narrowing.

    • Consequences: Can result in ischemia (reduced blood flow) and cell death due to plaque buildup.

Risk Factors for High Cholesterol

  • Non-Modifiable: Age.

  • Modifiable:

    • Obesity.

    • Dietary choices.

    • Genetics.

    • Hormonal changes.

    • High blood pressure.

    • High cholesterol levels.

Preventive Measures

  • Lifestyle Changes:

    • Diet: Reduce saturated fats, increase omega-3 fatty acids, consider Mediterranean diet.

    • Exercise: Lowers LDL levels and increases HDL levels.

Medications for Cholesterol Management

  • Classes of Drugs:

    1. Statins (HMG CoA reductase inhibitors):

    • Mechanism: Inhibit cholesterol production in the liver, lowering LDL levels and raising HDL levels.

    • Common Statins: Simvastatin, Atorvastatin, Rosuvastatin.

    • Side Effects: Abdominal pain, rash, headache, myopathy, rhabdomyolysis.

    • Monitoring: Liver enzymes and CPK levels due to risk of muscle damage.

    1. Bile Acid Sequestrants:

    • Mechanism: Bind bile acids in the intestine, preventing their reabsorption, leading to lower cholesterol circulation.

    • Side Effects: Mainly gastrointestinal (e.g., constipation).

    • Medication Timing: Administer 1 hour before or 4-6 hours after other medications to mitigate absorption issues.

    • Example: Cholestyramine, which needs to be dissolved in juice before ingestion.

    1. Niacin (Vitamin B3):

    • Mechanism: Lowers triglycerides, total cholesterol, and LDL; may increase HDL.

    • Side Effects: Flushing, itching, GI distress.

    • Administration Tips: Low doses taken with meals, possible premedication with aspirin to reduce flushing.

    1. Fibrates:

    • Mechanism: Primarily lower triglycerides, may lower LDL and increase HDL.

    • Risks: Interaction with statins increases muscle-related side effects.

    1. Cholesterol Absorption Inhibitors:

    • Example: Zetia.

    • Mechanism: Inhibits absorption of cholesterol in the small intestine.

Adverse Effects of Cholesterol Medications

  • Common side effects include abdominal pain, rash, headache, myopathy, and rhabdomyolysis.

  • Specific Monitoring:

    • Elevated liver enzymes indicate potential liver damage.

    • Check for CPK (Creatine Phosphokinase) levels to monitor muscle damage, especially in patients on statins.

Clinical Implications and Test Preparation

  • Recognize normal ranges of triglycerides and cholesterol.

  • Educate patients on lifestyle modifications and potential drug side effects.

  • Anticipate drug interactions (e.g., with anticoagulants and grapefruit juice).

Summary of Learning Points

  • Understand the role of lipoproteins in cholesterol transport.

  • Recognize the importance of cholesterol homeostasis and the consequences of its disruption.

  • Familiarize with drug classes and their mechanisms of action, side effects, and monitoring requirements.

  • Recall lifestyle changes that can aid in the management of cholesterol levels and overall cardiovascular health.

Review Questions

  • What are the cell functions of cholesterol?

  • How does negative feedback work regarding LDL receptors in the liver?

  • What lifestyle changes should patients be educated on to lower cholesterol levels?