Mast cells are key effector cells that work at the interface of innate and adaptive immune responses and regulate diverse processes important in health and disease. Inappropriate mast cell activation is associated with a wide range of allergic diseases such as asthma, allergic rhinitis and coeliac disease. All mast cell functions are ultimately mediated by the secretion of pre-formed and newly synthesised mediators, which is controlled by Ca2+ signalling. Identifying the ion channels that regulate mast cell Ca2+ signalling is therefore important for human health and will provide insight for development of new mast cell stabilising drugs. It has recently been established that Orai1, the highly Ca2+-selective plasma membrane channel, and Stim1, the endoplasmic reticulum Ca2+ sensor, regulate Ca2+ influx following intracellular Ca2+ store depletion. These proteins are essential for degranulation, synthesis of lipid mediators and the secretion of some cytokines from mouse mast cells1. The identity and function of other Ca2+ influx channels in mast cells is still unclear. We are investigating the role of TRPC channels in human mast cells. Like Orai channels, TRPC channels are also activated following stimulation of receptors coupled to phospholipase C (PLC) but they are less Ca2+ selective, allowing passage of Ba2+ and Sr2+ ions. TRPC channels appear to localise to distinct submembrane compartments than Orai1/Stim12 and may therefore couple to distinct receptor complexes. TRPC3, 6 and 7 channels are activated by diacylglycerol (DAG); TRPC1, 4 and 5 channels are also activated downstream of PLC, possibly by Ca2+ store depletion2. Intracellular Ca2+ measurements in the LAD 2 human mast cell line and primary human lung mast cells (HLMCs) showed that both Ca2+ and Ba2+ ions enter the cells after passive store depletion with thapsigargin, or following stimulation of two classes of PLC-linked receptors, namely FcεRI and P2Y purinoceptors (n>100 cells, N=3 independent experiments). The observed permeability to Ba2+ indicates that channels other than Orai1 may contribute to Ca2+ signalling in human mast cells. β-hexosaminidase release assays in LAD 2 cells showed that Ba2+ and Sr2+ can support mast cell degranulation in the absence of extracellular calcium (N=3), suggesting that TRPC channels may contribute to mast cell function. Stimulation of cells with the DAG analogue, OAG, produced Ca2+ influx in 35% of LAD 2 cells (n=120 cells, N=3) and 79% of HLMCs (n=98 cells, N=3), suggesting the involvement of the TRPC3/6/7 subgroup. Whole-cell patch-clamp recordings using conditions designed to prevent activation of ICRAC and TRPM73 showed that OAG activated an outwardly-rectifying current in LAD 2 cells. RT-PCR confirmed the presence of mRNA for TRPC1, 3, 5 and 6 channels (N=3). Ongoing research is aimed at further characterising the properties and physiological function of endogenous TRPC channels in human mast cells.