H
2 receptors are positively coupled to
adenylate cyclase via
Gs. It is a potent stimulant of
cAMP production, which leads to activation of
protein kinase A.
[ PKA functions to phosphorylate certain proteins, affecting their activity. The drug betazole is an example of a histamine H2 receptor agonist.
]
Function
Histamine is a ubiquitous messenger molecule released from mast cells, enterochromaffin-like cells, and neurons.[ Its various actions are mediated by histamine receptors H1, H2, H3 and H4. The histamine receptor H2 belongs to the ]rhodopsin
Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction ...
-like family of G protein-coupled receptors. It is an integral membrane protein and stimulates gastric acid secretion. It also regulates gastrointestinal motility and intestinal secretion and is thought to be involved in regulating cell growth and differentiation.
Tissue distribution
Histamine H2 receptors are expressed in the following tissues:
;Peripheral tissuesNeutrophils
Neutrophils (also known as neutrocytes or heterophils) are the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. They form an essential part of the innate immune system, with their functions varying in ...
* Mast cells
* Heart
* Uterus
; Central nervous system tissuescerebellum
The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...
Physiological responses
Activation of the H2 receptor results in the following physiological responses:
* Stimulation of gastric acid
Gastric acid, gastric juice, or stomach acid is a digestive fluid formed within the stomach lining. With a pH between 1 and 3, gastric acid plays a key role in digestion of proteins by activating digestive enzymes, which together break down the ...
secretion (Target of anti-histaminergics (H2 receptors) for peptic ulcer disease and GERD)
* Smooth muscle
Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...
relaxation (Experimental histamine H2 receptor agonist used for asthma and COPD)
* Inhibit antibody synthesis, T-cell proliferation and cytokine production
* Vasodilation – PKA
PKA may refer to:
* Professionally known as:
** Pen name
** Stage persona
* p''K''a, the symbol for the acid dissociation constant at logarithmic scale
* Protein kinase A, a class of cAMP-dependent enzymes
* Pi Kappa Alpha, the North-American so ...
activity causes phosphorylation of MLCK, decreasing its activity, resulting in MLC of myosin
Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility.
The first myosin ...
being dephosphorylated by MLCP and thus inhibiting contraction. The smooth muscle relaxation leads to vasodilation.[ Page 479]
* Inhibition of neutrophil activation and chemotaxis
See also
* H2-receptor antagonist
* Histamine H1-receptor
* Histamine H3-receptor
* Histamine H4-receptor
References
Further reading
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External links
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{{Histaminergics
Histamine receptors