Inositol Trisphosphate and Smooth Muscle Function

This chapter reviews selected areas in the field of inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] signal transduction in airway smooth muscle, smooth muscle, and other cell types. It discusses some unresolved issues regarding roles of the inositol phospholipid transduction system in smooth muscle and in airway smooth muscle. G proteins have been linked to β isomers of inositol-phospholipid-specific phospholipase C (PLC). Multiple different receptor subtypes can activate PLC-mediated phosphatidylinositol 4,5-bisphosphate (PIP₂) hydrolysis using different G proteins. The principal inositol phospholipids involved in the "PI cycle" are phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), and PIP₂. Ins(1,4,5)P₃ can be degraded via two pathways: 3-kinases forming Ins(1,3,4,5)P₄ and 5-phosphatases degrading the compound to Ins(1,4)P₂. In addition to Ins(1,4,5)P₃-sensitive stores, two other types of intracellular Ca²⁺ stores have been identified: ryanodine-sensitive stores and GTP-sensitive stores. High-affinity, specific Ins(1,4,5)P₃ binding to membranes or plasma membranes has been reported in a number of different cells.