Entries Tagged 'Histamine Receptors' ↓

Termination of histamine action:Three principal ways exit to terminate the physiological effects of histamine.

• Cellular uptake : animal studies have documented the uptake of histamine by many cells. The histamine is metabolized once in the cell.
• Desensitization of cells: Some H1 receptors containing tissues exhibit a homogeneous loss of sensitivity to the  actions of histamine , perhaps as a result of receptors modification.
• Metabolism : the most common pathway for terminating histamine action involves enzymatic inactivation.

The enzyme histamine N- methyltransferase (HMT) is widely distributed among mammalian tissues and catalyzes the transfer of a methyl group from S- adenosyl L-Methionine (SAM) to the ring tele nitrogen of histamine producing N1- methyl histamine and  S- adenosyl- L- homocysteine. Histamine is also subject to oxidative deaminalion by diamine ozidase, yielding imidazole acetic acid, a physiologically inactive product excreted in the urine.  Similarly , NT – methyl  histamine is centered by both DAO and monoamine oxidase (MAO) to N-Methylimidazole acetic acid.

Receptors :Once released , the physiological effects of histamine are mediated by specific cell surface receptors. There are three different subtypes of histamine receptors , H₁, h₂, h₃

H₁ – Receptors: histamine h₁ receptors have been detected in wide variety of tissues including mammalian brain smooth muscle from airways, gastro intislinal GI tract, genitourinary system and the cardiovascular system, adrenal medulla and endothelial cells and lymphocytes.

H₂- receptors: H₂ –receptors have been detected in a wide variety of tissues(myocardial cells and cell membranes of acid secreting cells of the gastric mucosa) and mediate the gastric acid secretor actions of histamine. The H₂ receptors has the general characteristics of a G- protein coupled receptors.

H₃ – Receptors: the H₃ receptors is proposed to function as a neural auto receptors (presynaptic) serving to modulate histamine synthesis and release in the CNS.