Flashcards: Histamine

• Autacoids definition

Active substances in the body.

• Autacoids classification

Decarboxylated amino acids (histamine, serotonin), Polypeptides (angiotensin, kinins, vasopressin, ANP, VIP, substance P), Eicosanoids (prostaglandin, leukotrienes).

• Histamine release (hypersensitivity)

Type 1 (immediate hypersensitivity) involves Antigen + Antibody (IgE) on mast cell leading to histamine release.

• Chemical histamine releasers

Morphine and curare.

• Histamine mechanism of action

Its binding by histamine receptors (H1, H2, H3, H4) results in biological action.

• H1 receptor location

Smooth muscle, endothelium, brain (CNS).

• H1 receptor function

Vasodilation, spasmogenic on smooth muscle (bronchi, GIT, Uterus), Skin

• H2 receptor location

Gastric mucosa, cardiac muscle, brain (CNS).

• H2 receptor function

Positive ionotropic & chronotropic (cardiac stimulation), increase gastric HCL secretion.

• H3 receptor location

Presynaptic neurons, brain (CNS).

• H3 receptor function

Decreased neurotransmitter release

• H4 receptor location

Neutrophils, eosinophils.

• H4 receptor function

Play role in immunological reactions.

• Histamine antagonists (physiological)

Adrenaline, which has opposite action on H1 (bronchodilation).

• Inhibitors of histamine release (mast cell stabilizer)

Cromolyn, cromoglycate, ketotifen, which decrease Ag/Ab reaction.

• Mast cell stabilizers use

Used as prophylactics in bronchial asthma.

• Inhibitors of histamine release (beta-2 agonist)

Salbutamol.

• Inhibitors of histamine release (methyl-xanthines)

PDE inhibitors e.g. theophylline.

• Clinical use of H2 blocker

Peptic ulcer disease (PUD), Gastroesophageal reflux disease (GERD) (proton pump inhibitors more effective), prevention of stress ulcer.

• Cimetidine adverse effect

Inhibits the binding of dihydrotestosterone to androgen receptor (anti-androgenic effect) and increases serum prolactin, causing gynecomastia in male.