II. Physiology

  1. Amine is synthesized in vivo by enzymatic decarboxylation of Histidine
    1. Histidine is stored in the secretory granules of Mast Cells
    2. Histamine is released from many human cells including Mast Cells and Basophils
      1. Examples: Allergic Reaction (e.g. ragweed, Bee sting allergy)
  2. Histamine activity is specific to three Histamine receptors
    1. H1 Receptors (cyclic GMP mediated)
      1. Bronchoconstriction (e.g. Asthma)
      2. Adrenal Catecholamine release
      3. Venoconstriction and increased capillary permeability
      4. Arteriole and capillary dilation (also H2 receptor mediated)
      5. Increased cardiac contractility (also H2 receptor mediated)
      6. Gastrointestinal Muscle Contraction
    2. H2 Receptors (cyclic AMP mediated)
      1. Tachycardia
      2. Increased pepsin and HCl acid secretion from the Stomach
    3. H3 Receptors
      1. Inhibits Histamine release from Mast Cells

III. References

  1. Olson (2020) Clinical Pharmacology, Medmaster Miami, p. 146-7

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