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 P2X(4), ivermectin, augmented the current indicating the existence of P2X(4) receptors. Immunohistochemistry and reverse transcription/polymerase chain reaction analysis and Western blot analysis also showed the distribution of the P2X(4) receptors. The inward current was reduced by SKF-96365, an inhibitor of both voltage-dependent Ca(2+) channels (VDCCs) and voltage-independent Ca(2+) channels, although a VDCC antagonist, verapamil, did not affect the current. SKF-96365 caused complete suppression of both the increase in the intracellular Ca(2+) concentration and the contraction of ASM cells induced by ATP. Our results demonstrate that P2X(4) receptors exist on ASM and that the receptors are responsible for Ca(2+) influx. These findings suggest that the Ca(2+) influx regulated by P2X(4) receptors plays an important role in ASM contraction by a pathway distinct from VDCC.
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