Figure 11

Mechanisms of action of extracellular ATP on airway myocytes. ATP opens P2X receptors, which triggers external Na⁺ entry that depolarizes the plasma membrane and activates L-type voltage-operated Ca²⁺ channels. The subsequent [Ca²⁺]_i rises activates the contractile apparatus. In addition to these mechanisms, ATP acts on P2Y receptors and induces Ca²⁺ release from SR via protein Gq and PLC activation, mainly in IPB. The progressive return to baseline following the initial contraction is due to desensitization of the purinergic receptors associated, in extrapulmonary airways, with epithelium-independent PG. PG binds to EP receptor coupled to protein Gs and AC and hence induces the production of cAMP, which inhibits the contractile apparatus via PKA activation.