What is the Difference Between H1 and H2 Receptors

The main difference between H1 and H2 receptors is that H1 receptors couple with Gq/11 and stimulate phospholipase C, while H2 receptors interact with Gs to activate adenylyl cyclase.

Histamines are a type of chemical the immune system of the body produces. Histamine is an amine, based on the structure of ammonia. It forms when the carboxyl group of histamine is removed from the amino acid histidine. Moreover, mast cells and basophils of the body store histamines. Histamines also cause allergic reactions in the body. When a person is allergic to a certain substance, such as pollen or dust, a series of chemical reactions happen in the body, initiating the release of histamines into the blood. Histamines interact with four types of histamine receptors. They are H1, H2, H3 and H4 receptors. All of these receptors belong to the G protein-coupled receptor family. Additionally, they are transmembrane receptors.

Key Areas Covered

1. What are H1 Receptors
     – Definition, Function, Location
2. What are H2 Receptors
     – Definition, Function, Location
3. Relationship Between H1 and H2 Receptors
     – Outline of Common Features
4. Difference Between H1 and H2 Receptors
     – Comparison of Key Differences

Key Terms

H1 Receptors, H2 Receptors, Histamine

What are H1 Receptors

H1 receptors are histamine-binding receptors. They are also G protein-coupled receptors. H1 receptors mediate histamine-induced modulation of many bodily functions, such as thermoregulation and sleep and feeding, which are physiological functions controlled by the brain. They also take part in type 1 hypersensitivity reactions. H1 receptors also play a role in functions like vasodilation, bronchoconstriction, nociception, inflammations, and allergies.

H1 receptors are present in endothelial cells, dermal cells, and keratinocytes in the skin, lymphocytes, dendric cells, respiratory epithelium, hepatic cells, and nerves. Furthermore, H1 is activated by histamine through Gαq/11. To increase the intracellular Ca++ levels, phospholipase C is also activated. Additionally, the activation through H1 receptors enhances Th1 and Th2 types of immune responses.

What are H2 Receptors

H2 receptors are Gs protein-coupled receptors that bind histamine. H2 receptors are present in smooth muscle cells, dendric cells, gastric parietal cells, T cells, B cells, the brain, and cardiac tissues. The activation of H2 receptors induces the secretion of gastric acids, stimulation of suppressed T cells, decrease in neutrophil and basophil chemotaxis and activation, the proliferation of lymphocytes and natural killer cell activity, vascular permeability, and production of mucus in the airway.

H2 receptor also plays a role in immune system activation. The activation includes the reduction of basophil degranulation, T cell proliferation, and antibody synthesis. H2 cells located in the parietal cells of the stomach regulate gastric acid levels. Furthermore, blocking the binding of histamine to the receptor reduces the amount of acid produced in the stomach.

Relationship Between H1 and H2 Receptors

• Both H1 and H2 receptors are histamine-binding receptors.
• They belong to the G protein-coupled receptor family.
• Moreover, a combination of H1 and H2 receptor activation contributes to the swelling of the nasal airway and rhinorrhea.

Difference Between H1 and H2 Receptors

Definition

An H1 receptor is a receptor that couples with Gq/11 and stimulates phospholipase C, whereas an H2 receptor is a receptor that interacts with Gs to activate adenylyl cyclase.

Function

H1 receptors are involved in cellular migration, vasodilation, and nociception, whereas H2 receptors are involved in vascular permeability, gastric acid secretion, and airway mucous production.

Location

Moreover, H1 receptors are present in endothelial cells, dermal cells and keratinocytes in the skin, lymphocytes, dendric cells, respiratory epithelium, hepatic cells, and nerves, whereas H2 receptors are present in smooth muscle cells, dendric cells, gastric parietal cells, T cells, B cells, brain and cardiac tissues.

Conclusion

In brief, H1 and H2 receptors are two histamine-binding receptors. The main difference between H1 and H2 receptors is that H1 receptors couple with Gq/11 and stimulate phospholipase C, while H2 receptors interact with Gs to activate adenylyl cyclase.

Reference:

1. “H1 Receptors.” The Journal of Allergy and Clinical Immunology.
2. “Histamine H2 receptor.” InterPro. EBI

Image Courtesy:

1. “H1 Receptor with Doxepin” By Nakane – From PDB entry 3RZE.Reference: Tatsuro Shimamura, Mitsunori Shiroishi, Simone Weyand, Hirokazu Tsujimoto, Graeme Winter, Vsevolod Katritch, Ruben Abagyan, Vadim Cherezov, Wei Liu, Gye Won Han, Takuya Kobayashi, Raymond C. Stevens  (July 2011). “Structure of the human histamine H1 receptor complex with doxepin”. Nature 475 (7354): 65–70. DOI:10.1038/nature10236. PMID 21697825. (Public Domain) via Commons Wikimedia