Histamine

Histamines: Histamine is a biogenic amine, it occurs in many tissues in almost all forms of life. Histamine plays a role in the regulation of gastric secretion, in the body’s protective mechanism at the site of injury (triple response, acute inflammation) and as a mediator in allergic and anaphylactic condition. Histamine is synthesized from the amino acid histidine by the action of histidine decarboxylase. In man about 5mg of histamine are formed daily.

Mechanism of Action: Histamine acts on two separate and distinct postsynaptic receptor, termed H1 and H2 receptor. Contraction of the smooth muscle of the bronchi and the intestine, and most of the depressor effect on blood pressure are mediated by H1 receptor, and antagonised by the antihistamines like mepyramine and diphenhydramine. The H2 receptors mediate the action of histamine on the gastric secretion and cardiac acceleration. These action are antagonized by cimetidine and ranitidine. A third H3 receptor is a presynaptic receptor, distributed predominantly in the brain, myenteric plexus, and certain other neurone. Activation of H3 receptor decreases transmitter release from histaminergic and other neurone.

Histamine Releasing Drug: Endogenous histamine may be released by certain drugs, chemicals and physical agents. The major source of the histamine liberated are the mast cells. Such a histamine release may be of magnitude to precipitate an anaphylactic reaction.

Clinical Uses:

Gastric Analysis:  Histamine is useful for the differential diagnosis of pernicious anaemia from other stomach disease on the basis of achlorhydria.

Diagnosis of Pheochromocytoma: Histamine 3 mcg/kg i.v. Glucagon (0.5 – 1.0mg i.v) or phentolamine (5mg, i.v) may be employed for the diagnosis of Pheochromocytoma.

To test bronchial hyperactivity in asthmatics.

Betahistine

It is an orally active, H1 selective histamine analogue. It has been used to control vertigo in patient of Meniere’s disease (Dizziness and Tinitis of Ear), possibly acts by causing vasodilatation in asthmatics and ulcer patient.

Dose: VERTIN 8mg tab 1/2 to 1 tab QID

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