Pharmacology: Antihistamines and Histamine

Introduction to Antihistamines

• Presenter introduces himself as Jumper Arias, a member of the Peruvian University Cayetano Heredia's Clinical Intelligence Society
• Main objectives of the session: understand antihistamines, their functions in the body, and their effects

Histamine: Definition and Functions

• Histamine: a chemical compound produced by the body, synthesized by mast cells
• Functions:
  • Stimulates nerve endings, causing pain and itching
  • Regulates sleep-wake cycle
  • Involved in appetite control
  • Modulates neurotransmitter release
  • Vasodilates blood vessels, decreasing blood pressure
  • Increases heart rate
  • Constricts smooth muscle, causing bronchoconstriction

Histamine Receptors

• H1 receptors: found in smooth muscle, endothelium, and nervous system
• H2 receptors: found in stomach, responsible for gastric acid secretion

Anaphylaxis

• A severe allergic reaction caused by histamine release
• Symptoms: skin rash, itching, swelling, nausea, vomiting, diarrhea, abdominal cramps, rapid heartbeat, and fall in blood pressure
• Triggers: food, insect bites, medications, latex, and other allergens

Antihistamines

• Classification:
  • H1 antihistamines:
    • 1st generation: crosses blood-brain barrier, causing sedation
    • 2nd generation: does not cross blood-brain barrier, non-sedating
  • H2 antihistamines: blocks H2 receptors in stomach, reducing gastric acid secretion

H1 Antihistamines

• Uses:
  • Allergic reactions (rash, itching, hives)
  • Insect bites
  • Motion sickness
  • Insomnia
• Examples of H1 antihistamines:
  • 1st generation: chlorpheniramine, diphenhydramine
  • 2nd generation: loratadine, cetirizine

H2 Antihistamines

• Uses:
  • Gastroesophageal reflux disease (GERD)
  • Peptic ulcers
  • Zollinger-Ellison syndrome
• Examples of H2 antihistamines:
  • Ranitidine, cimetidine, nizatidine, famotidine

Pharmacokinetics and Pharmacodynamics

• H1 antihistamines:
  • Well-absorbed orally
  • Metabolized by the liver
  • Excreted by the kidneys
• H2 antihistamines:
  • Well-absorbed orally
  • Metabolized by the liver
  • Excreted by the kidneys and liver

Introduction to Antihistamines

• Jumper Arias, a member of the Peruvian University Cayetano Heredia's Clinical Intelligence Society, leads the session on antihistamines
• The session aims to understand antihistamines, their functions in the body, and their effects

Histamine: Definition and Functions

• Histamine is a chemical compound produced by the body, synthesized by mast cells
• Histamine stimulates nerve endings, causing pain and itching
• It regulates sleep-wake cycle, appetite control, and modulates neurotransmitter release
• It vasodilates blood vessels, decreasing blood pressure, and increases heart rate
• Additionally, it constricts smooth muscle, causing bronchoconstriction

Histamine Receptors

• H1 receptors are found in smooth muscle, endothelium, and nervous system
• H2 receptors are found in the stomach, responsible for gastric acid secretion

Anaphylaxis

• Anaphylaxis is a severe allergic reaction caused by histamine release
• Symptoms include skin rash, itching, swelling, nausea, vomiting, diarrhea, abdominal cramps, rapid heartbeat, and fall in blood pressure
• Triggers include food, insect bites, medications, latex, and other allergens

Antihistamines

• Antihistamines can be classified into H1 and H2 antihistamines
• H1 antihistamines can be further divided into 1st generation (crosses blood-brain barrier, causing sedation) and 2nd generation (does not cross blood-brain barrier, non-sedating)
• H2 antihistamines block H2 receptors in the stomach, reducing gastric acid secretion

H1 Antihistamines

• H1 antihistamines are used to treat allergic reactions (rash, itching, hives), insect bites, motion sickness, and insomnia
• Examples of H1 antihistamines include 1st generation (chlorpheniramine, diphenhydramine) and 2nd generation (loratadine, cetirizine)

H2 Antihistamines

• H2 antihistamines are used to treat gastroesophageal reflux disease (GERD), peptic ulcers, and Zollinger-Ellison syndrome
• Examples of H2 antihistamines include ranitidine, cimetidine, nizatidine, and famotidine

Pharmacokinetics and Pharmacodynamics

• H1 antihistamines are well-absorbed orally, metabolized by the liver, and excreted by the kidneys
• H2 antihistamines are well-absorbed orally, metabolized by the liver, and excreted by the kidneys and liver

Introduction to Antihistamines

• The session aims to understand antihistamine medications, their functions in the body, side effects, and resolution of clinical context questions.

Histamine

• Histamine is a chemical compound produced by the body, synthesized mainly by mast cells.
• Mast cells are part of the immune system, mediating allergic and inflammatory responses.
• Histamine is found in tissues such as lungs, skin, blood vessels, and gastrointestinal tract.
• Histamine is formed by decarboxylation of the amino acid histidine.

Functions of Histamine

• Stimulates sensory nerve endings, mediating pain and itch perception.
• Regulates sleep-wake cycle.
• Modulates appetite and satiety.
• Regulates neurotransmitter release in the nervous system.
• Causes vasodilation of arterioles, decreasing blood pressure.
• Increases heart rate.
• Regulates smooth muscle contraction in internal organs.
• Stimulates gastric secretion.

Anaphylaxis

• An allergic reaction that induces histamine release.
• Causes: food allergy, insect bites, medication reactions, etc.
• Symptoms: erythema, urticaria, edema, hypotension, respiratory distress, tachycardia, nasal congestion.

H1 Antihistamines

• Divided into two groups: first and second generation.
• First-generation antihistamines cross the blood-brain barrier, causing sedation and somnolence.
• Second-generation antihistamines do not cross the blood-brain barrier, without sedation and somnolence.
• Clinical uses: treatment of allergies, insomnia, nausea, and vomiting.

H2 Antihistamines

• Block H2 receptors in stomach parietal cells, decreasing gastric acid secretion.
• Clinical uses: treatment of peptic ulcers, gastroesophageal reflux, gastritis, Zollinger-Ellison syndrome.

Pharmacokinetics of Antihistamines

• Well-absorbed orally, reaching maximum plasma concentration in 1-2 hours.
• First-generation antihistamines have a half-life of 4-6 hours, while second-generation antihistamines have a half-life of 12-24 hours.
• Metabolized in the liver and excreted in urine and feces.