Histamine mediates a diverse group of processes ranging from vasodilation to gastric acid secretion. It produces its effects by binding to and activating receptors on the surface of cardiac, smooth muscle, endothelial, neuronal, and other cells. There are at least four receptor populations, H1, H2, H3, and H4. All four receptor subtypes have been cloned and belong to the G protein-coupled receptor superfamily. The histamine receptors can be distinguished on the basis of their post-receptor signal transduction mechanisms, tissue distribution, and sensitivities to various agonists and antagonists (Table 38.1). Currently, only the H1- and H2-receptors are targets of clinical drug therapy.
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