Uridine adenosine tetraphosphate induces contraction of airway smooth muscle

Abstract

Contraction of airway smooth muscle (ASM) plays an important role in the regulation of air flow and is potentially involved in the pathophysiology of certain respiratory diseases. Extracellular nucleotides regulate ASM contraction via purinergic receptors, but the signaling mechanisms involved are not fully understood. Uridine adenosine tetraphosphate (Up4A) contains both pyrimidine and purine moieties, which are known to potentially activate P2X and P2Y receptors. Both P2X and P2Y receptors have been identified in the lung, including airway epithelial cells and ASM. We report here a study of purinergic signaling in the respiratory system, with a focus on the effect of Up4A on ASM contraction. Up4A induced contraction of rat isolated trachea and extrapulmonary bronchi as well as human intrapulmonary bronchioles. Up4A-induced contraction was blocked by di-inosine pentaphosphate, a P2X antagonist, but not by suramin, a nonselective P2 antagonist. Up4A-induced contraction was also attenuated by α,β-methylene-ATP-mediated P2X receptor desensitization. Several P2X receptors were detected at the mRNA level: P2X1, P2X4, P2X6, and P2X7, and to a lesser extent P2X3. Furthermore, the Up4A response was inhibited by removal of extracellular Ca2+ and by the presence of the L-type Ca2+ channel blocker, nifedipine, or the Rho-associated kinase inhibitor, H1152. We conclude that Up4A stimulates ASM contraction, and the underlying signaling mechanism appears to involve P2X (most likely P2X1) receptors, extracellular Ca2+ entry via L-type Ca2+ channels, and Ca2+ sensitization through the RhoA/Rho-associated kinase pathway. This study will add to our understanding of the pathophysiological roles of extracellular nucleotides in the lung.