conclusions: In vascular smooth muscle cells (VSMCs), activation of Ca -permeable store-operated channels (SOCs) composed of canonical transient receptor potential channel 1 (TRPC1) subunits mediate Ca entry pathways which regulate contraction, proliferation and migration that are processes associated with vascular disease. Activation of TRPC1-based SOCs requires protein kinase C (PKC) activity, which is proposed to phosphorylate TRPC1 proteins to promote channel opening by phosphatidylinositol 4,5-bisphosphate (PIP ). We investigated the identity of the PKC isoform involved in activating TRPC1-based SOCs in rat mesenteric artery VSMCs. TRPC1-based SOCs were reduced by PKCδ inhibitors and knockdown of PKCδ expression. Store depletion induced interactions between TRPC1 and PKCδ and PKCδ-dependent phosphorylation of TRPC1. Furthermore, generation of store-operated interactions between PIP and TRPC1 and activation of TRPC1-based SOCs by PIP required PKCδ. These findings reveal that PKCδ activity has an obligatory role in activating TRPC1-based SOCs, through regulating PIP -mediated channel opening. unassigned: In vascular smooth muscle cells (VMSCs), stimulation of Ca -permeable canonical transient receptor potential channel 1 (TRPC1)-based store-operated channels (SOCs) mediate Ca entry pathways which regulate cell contraction, proliferation and migration that are processes associated with vascular disease. It is therefore important to understand how TRPC1-based SOCs are activated. Stimulation of TRPC1-based SOCs requires protein kinase C (PKC) activity, with store-operated PKC-dependent phosphorylation of TRPC1 essential for channel opening by phosphatidylinositol 4,5-bisphosphate (PIP ). Experimental protocols used to activate TRPC1-based SOCs suggest that the PKC isoform involved requires diacylglycerol (DAG) but is Ca -insensitive, which are characteristics of the novel group of PKC isoforms (δ, ε, η, θ). Hence the present study examines if a novel PKC isoform(s) is involved in activating TRPC1-based SOCs in contractile rat mesenteric artery VSMCs. Store-operated whole-cell cation currents were blocked by Pico145, a highly selective and potent TRPC1/4/5 channel blocker and T1E3, a TRPC1 blocking antibody. PKCδ was expressed in VSMCs, and selective PKCδ inhibitory peptides and knockdown of PKCδ expression with morpholinos oligomers inhibited TRPC1-based SOCs. TRPC1 and PKCδ interactions and phosphorylation of TRPC1 induced by store depletion were both reduced by pharmacological inhibition and PKCδ knockdown. In addition, store-operated PIP and TRPC1 interactions were blocked by PKCδ inhibition, and PKCδ was required for PIP -mediated activation of TRPC1 currents. These results identify involvement of PKCδ in stimulation of TRPC1-based SOCs and highlights that store-operated PKCδ activity is obligatory for channel opening by PIP , the likely activating ligand. This article is protected by copyright. All rights reserved.