Regulation of adenosine 5′-triphosphate (ATP)-gated P2X4 receptors on tracheal smooth muscle cells

We examined the effects of extracellular adenosine 5′-triphosphate (ATP) on single airway smooth muscle (ASM) cells from porcine trachea using a patch-clamp technique. ATP induced a sustained inward current. Phospholipase C inhibitor U-73122 failed to inhibit the current, suggesting the involvement of P2X receptor. A specific effecter of P2X4, ivermectin, augmented the current indicating the existence of P2X4 receptors. Immunohistochemistry and reverse transcription/polymerase chain reaction analysis and Western blot analysis also showed the distribution of the P2X4 receptors. The inward current was reduced by SKF-96365, an inhibitor of both voltage-dependent Ca2+ channels (VDCCs) and voltage-independent Ca2+ channels, although a VDCC antagonist, verapamil, did not affect the current. SKF-96365 caused complete suppression of both the increase in the intracellular Ca2+ concentration and the contraction of ASM cells induced by ATP. Our results demonstrate that P2X4 receptors exist on ASM and that the receptors are responsible for Ca2+ influx. These findings suggest that the Ca2+ influx regulated by P2X4 receptors plays an important role in ASM contraction by a pathway distinct from VDCC.