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CHAPTER FOUR
Learning Objectives:
After going through this unit, the student will be able to:
- Explain the role of histamine in anaphylactic reactions
- List some of the therapeutic uses and adverse effects of H1 antagonists
- Describe the major pharmacological actions of prostaglandins E and F
INTRODUCTION
“Autacoids” (Greek “self-remedy”) is a collective term for various endogenous peptides, prostaglandins, leukotrienes, and cytokines. These are sometimes also called local hormones. They play important roles in physiologic processes and also have several pharmacological significances.
Histamine
It is a potent tissue amine widely distributed in plant and animal tissues and in the venoms of bees. In man, it is formed by decarboxylation of histidine and major portion is stored in mast cells and basophils.
Mechanisms of Action: It acts on 2 major types of receptors
- Stimulation of H1 receptors results in smooth muscle contraction, increased vascular permeability, and mucus production. These effects are blocked competitively by H1 antagonists.
- Activation of H2 receptors increases gastric acid production, and this effect is blocked by H2 blockers such as cimetidine.
- Both types of receptors are involved in vascular dilatation and edema formation.
Pharmacological Actions:
Cardiovascular system
Histamine produces dilatation of capillaries and venules accompanied by a fall in blood pressure. The mechanism is direct relaxation of the smooth muscles of blood vessels. This effect cannot be adequately reversed by antihistaminic agents but by adrenaline.
