Figure 1. Regulation of signal transduction and intracellular Ca²⁺in airway smooth muscle. When the muscarinic receptor is stimulated, voltage-dependent Ca²⁺channels (VDC) and Ca²⁺depletion-activated Ca²⁺channels (CDAC) are activated. Ca²⁺enters cytosol through these channels. Similarly, phospholipase C (PLC) is activated via the G proteins (Gq) linked to it, resulting in the rapid breakdown of phosphatidylinositol 4,5-bisphosphate (PIP²) to inositol 1,4,5-triphosphate (IP³) and diacylglycerol (DAG), both of which act as second messengers. Whereas DAG activates Ca^{2+/phospholipid-dependent}protein kinase (PKC). IP³mobilizes Ca²⁺from sarcoplasmic reticulum (SR) through IP^3-inducedCa²⁺release (IICR) channels, [9]which are also regulated by Ca²⁺. The SR can be functionally separated into two components: SR-[Greek small letter alpha] and SR-[Greek small letter beta]. SR-[Greek small letter alpha] involves two types of channels, IICR and Ca^2+-inducedCa²⁺release (CICR) channels, [8]whereas SR-[Greek small letter beta] involves only IICR channels. CICR channels can be activated by caffeine [9]and Ca²⁺.