Signalling and receptor crosstalk mediated by full and partial agonists of the muscarinic M3 receptor

This study shows that stimulation of endogenously expressed Galpha q/11-coupled muscarinic M3 receptors in HEK 293 cells allows a subsequent robust Ca2+ response to stimulation of beta 2-adrenoceptors. This study provided the first demonstration that despite the apparent inability of some partial agonists of the muscarinic receptor to mediate elevations of intracellular Ca2+, in its continued presence facilitate a Ca2+ response to stimulation of beta 2-adrenoceptors with noradrenaline, of greater magnitude than that facilitated by full agonists. This response was dependent on the concentration of both muscarinic receptor agonist and noradrenaline. Facilitation of noradrenaline-mediated Ca2+ responses occur only in the presence of activated muscarinic receptors, as treatment with atropine abolished responses to noradrenaline. Crosstalk was independent of the presence of extracellular Ca 2+, but dependent on thapsigargin-sensitive intracellular stores. Investigation of potential mechanisms revealed a lack of enhanced PLC activity and ruled out a potential role for either PKA or PKC. An alternate mechanism for crosstalk was suggested based on the ability of adenylyl cyclase inhibitors to attenuate noradrenaline-mediated Ca2+ responses and the ability of forskolin or dbcAMP to mimic crosstalk. A role for Epac in contributing to the mechanism of crosstalk was identified using the Epac specific activator, 8-pCPT-2 '-O-Me-cAMP. It is possible that the ability of muscarinic receptor agonists to reveal Ca2+ signalling by noradrenaline is by Epac-cAMP signalling complex-mediated sensitisation of intracellular Ca2+ channels. Furthermore, the ability of noradrenaline to reveal Ca2+ signalling in the continued presence of muscarinic receptor agonists has been demonstrated in rat tracheal smooth muscle cells and the human lung epithelial NCI H292 cell line