Abstract

In the present study the effect of phosphatidylinositol 4,5-bisphosphate (PIP2) was studied on a native TRPC1 store-operated channel (SOC) in freshly dispersed rabbit portal vein myocytes. Application of diC8-PIP2, a water soluble form of PIP2, to quiescent inside-out patches evoked single channel currents with a unitary conductance of 1.9 pS. DiC8-PIP2-evoked channel currentswere inhibited by anti-TRPC1 antibodies and these characteristics are identical to SOCs evoked by cyclopiazonic acid (CPA) and BAPTA-AM. SOCs stimulated by CPA, BAPTA-AM and the phorbol ester phorbol 12,13-dibutyrate (PDBu) were reduced by anti-PIP2 antibodies and by depletion of tissue PIP2 levels by pre-treatment of preparations with wortmannin and LY294002. However, these reagents did not alter the ability of PIP2 to activate SOCs in inside-out patches. Co-immunoprecipitation techniques demonstrated association between TRPC1 and PIP2 at rest, which was greatly decreased by wortmannin and LY294002. Pre-treatment of cells with PDBu, which activates protein kinase C (PKC), augmented SOC activation by PIP2 whereas the PKC inhibitor chelerythrine decreased SOC stimulation by PIP2. Co-immunoprecipitation experiments provide evidence that PKC-dependent phosphorylation of TRPC1 occurs constitutively and was increased by CPA and PDBu but decreased by chelerythrine. These novel results show that PIP2 can activate TRPC1 SOCs in native vascular myocytes and plays an important role in SOC activation by CPA, BAPTA-AM and PDBu. Moreover, the permissive role of PIP2 in SOC activation requires PKC-dependent phosphorylation of TRPC1.