Histamine is one of the most studied biomolecules in medicine and is most notably known for its effects on smooth muscle contraction, vascular permeability and regulation of stomach acid. H2 receptors are positively coupled to adenylate cyclase via Gs. It increases the intracellular Ca2+ concentrations and release Ca2+ from intracellular stores by coupling to Gq. H2 receptors have been found to be located in a variety of tissues, including the brain, gastric cells, and cardiac tissue. Histamine H2 receptors have a potent effect on gastric acid secretion, and the inhibition of this secretory process by H2 receptor antagonists has provided evidence for an important physiological role of histamine in the regulation of gastric secretion. It also regulates gastrointestinal motility and intestinal secretion and is thought to be involved in regulating cell growth and differentiation. It has also been demonstrated to control the relaxation of smooth muscles.