Histamine

Histamines are naturally occurring in our body.  We find histamines in our skin, intestinal mucosa and respiratory tract.  Our body produces these histamines and they are stored in mast cells which release the histamines.  When the histamines are released, they go to receptors.  We are going to break them down into two receptors: H1 and H2.

• Naturally occurring in human tissue
  • Skin
  • Intestinal Mucosa
  • Respiratory tract
• Stored in mast cells
• Attach to H1 and H2 receptors

H1 Receptor Pharmacology

When we stimulate the H1 receptor, we have cardiovascular effects.  The cerebral blood vessels around the brain dilate and we get a histamine headache.  H1 receptors are on the capillaries and they dilate when stimulated.  That causes an increased amount of blood going to the capillaries, decreased amount going to the arteries causing decreased blood pressure and this is known as histamine shock.  Also on the capillaries, histamine causes decreased permeability which allows fluid to pass through easier (so that WBC’s can get to pathogens easier).  If the capillaries are next to the nasal mucosa, this will cause a runny nose.  If the capillaries surrounding tissue is skin, they’ll have edema or hives that are fluid filled.

H1 receptors are also on the bronchioles and the muscle surrounding them may constrict causing the person to have a hard time breathing due to bronchoconstriction.  H1 receptors are on the nerve endings and when they’re stimulated a person may feel itchy.

Recap

• Cerebral vessel dilation causes histamine headaches (aka cluster headache)
• Capillary dilation
  • Increased vascular volume, decreased arterial volume
  • Decreased BP
  • Histamine Shock
• Increased capillary permeability (fluids can pass out of the capillaries into surrounding tissues)
  • Allows free passage of plasma and protein
  • Leads to edema
• Bronchoconstriction
• Sensory Nerve Ending: H1 receptors are also on the nerve endings and when they’re stimulated they’ll generate itchy feelings.

H2 Receptor Pharmacology

Histamines can also go on an H2 receptors. The H2 receptors are found specifically in the stomach and histamine acts as a secretagogue which causes the secretion of acid.  A secretagogue is a substance that causes another substance to be secreted. The parietal cells are what release acid normally for digestion.  Histamine has a direct effect on these parietal cells and makes them secrete more gastric acid.  Acid is important for digestion, a natural function of histamine and related to digestion.

Recap

• Secretagogue
• Direct effect on parietal cells
• Increases gastric acid section

Mechanism of Action

Histamines will go to two different receptors, H1 and H2. These receptors can be blocked and the drug class that blocks the H1 receptors are called antihistamines. The drugs that block the H2 receptors are the anti-ulcer drugs. They decrease acid secretion and some are found OTC as well (e.g. Pepcid, Tagamet, Zantac, etc).

• Histamines stimulate H1 and H2 histamine receptors
• H1 Receptors are blocked by antihistamines
• H2 Receptors are blocked by anti-ulcer H2 antagonists

Functions of Histamines

Histamines are involved with allergic reactions and analphylactic shocks. So we reviewed all the bad things of histamines but they have good uses too. Histamines are involved in normal regulatory functions.  It regulates our bodies microcirculation such as in the capillaries.  It’s also used in tissue growth, repair and digestion since it promotes gastric acid secretion.

• Normal Physiology
  • Regulate microcirculation
  • Tissue Growth and Repair
  • Gastric acid secretion
• Hypersensitivity Reaction
  • Allergy
  • Anaphylactic Shock