P2 Receptors in Airway Smooth Muscle

The hypertrophic airway smooth muscle (ASM) of asthmatic airways is hyperresponsive compared to that of healthy airways, although the underlying causes for this are unknown. P2 receptors, receptors for extracellular nucleotides including ATP, contribute to the regulation of smooth muscle contraction and proliferation. However, the roles of P2 receptors in ASM have not been fully elucidated. As the extracellular ATP concentration is increased in asthmatic airways, this could provide a mechanism by which P2 receptors contribute to the pathogenesis of ASM dysfunction in asthma. In this thesis, I have demonstrated using qPCR the presence of P2 receptor transcripts in murine airways, and in ASM cells cultured from murine airways and those of both healthy and asthmatic donors. Biochemical and immunohistochemical approaches have revealed the expression of P2X1, P2X4 and P2X7 receptor proteins in murine airways which localises to the ASM specifically, and in ASM cells cultured from murine airways and those of healthy and asthmatic individuals. Using Ca2+ imaging, ASM cells cultured from murine airways were shown to express functional P2X7, P2Y1 and P2Y2 receptors. Additionally, ASM cells cultured from healthy airways expressed only functional P2Y2 receptors whilst those from asthmatic airways expressed both functional P2X4 and P2Y2 receptors. The application of extracellular nucleotides did not contract ASM of either murine or human airways. Yet, the pre-treatment of ASM from mouse, but not humans, with extracellular nucleotides appeared to enhance its contraction. In addition, extracellular nucleotides did not stimulate human ASM cell proliferation, although UTP (100 μM) induced the proliferation of murine ASM cells. These data suggest that functional P2 receptors are expressed in ASM from mice and humans, but that P2 receptor function is comparable between healthy and asthmatic human ASM. The functional role of P2 receptors expressed by the ASM in asthma therefore remains unclear.