Practical Pharmacology

The response of human skin to histamine and adrenaline

Lab 18

What is histamine?

Amino acid derivative produced by decarboxylation of a.a histidine.

Where it is present?

In mast cells and basophills, bounded to granules in inactive form (oxidated or methylated)






Role of histamine in the body.
1-in allergic reactions .
2- in gastric acid secretion.
3- as neurotransmitter in the brain.

Release of histamine

1-In allergic reactions.
2-Trauma : mechanical or chemical.
3-drugs as morphin or tubocurrarine.
4-proteolytic enzymes and snake venome.
5-compound 48/80 release histamine specifically from mast cells.
Note: release of histamine show –ve feed back mechanism

Action

H1 receptors: endothelium & sm.m.
H2 receptors: gastric mucosa and cardiac m.
H3 receptors: in CNS.


Effects
1-CVS:- tachy cardia ( both stimulatory effect on heart & reflex tachy cardia).
-hypotention: both systolic & diastolic (vasodilation of arteriols & precapillary sphinctors)
-Odema: due toH1 receptor stimulation on endothelium of bl.v and increase permeability)

2-GIT: sm,m contraction & increase acid and pepsin secretion.

3-Res:bronchoconstriction.
4-nerve endings: stimulate sensory nerve endings (itching and urticaria).
5-Skin: flush:dilation of capillary.
wheal: odema due to increase permeability.
flare: axonal reflex, dilation of arteriols.

Histamine antagonists

1-physiological antagonists: Adrenaline, dilate blood vessles and release bronchoconstriction
2-release inhibitors: reduce degranulation by immunological reactions.( steroids & Na cromoglycate)
3-receptors antagonist: mainly H1 blockers blocks effect on bronchial and gastric sm.m
1st generation: enter CNS ( strong sedative effect)
2nd generation: less lipid soluble, less entry to CNS.
H2 blockers : mainly to reduce gastric acid secretion.

dermographism

procedure

1- clean forarm.
2- draw 4 circles.
3-place adrop of the following solutions in each circle: histamine 1: 100, histamine one drop+ 2 drops of antihistamine ( diphenhydramine), adrenaline1:1000, normal saline0.9%.
4-prick the skin through the drops of drugs with sharp needle and the prick should be intradermal (no blood).
5- monitor and explain the results.