H2 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.
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-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.
Histamine H2 receptors are expressed in the following tissues:
- Peripheral tissues
- Central nervous system tissues
Activation of the H2 receptor results in the following physiological responses:
- Stimulation of gastric acid secretion (Target of anti-histaminergics (H2 receptors) for peptic ulcer disease and GERD)
- Smooth muscle relaxation (Experimental histamine H2 receptor agonist used for asthma and COPD)
- Inhibit antibody synthesis, T-cell proliferation and cytokine production
- Vasodilation – PKA activity causes phosphorylation of MLCK, decreasing its activity, resulting in MLC of myosin being dephosphorylated by MLCP and thus inhibiting contraction. The smooth muscle relaxation leads to vasodilation.
- Inhibition of neutrophil activation and chemotaxis