The molecular composition of receptor- or store-operated calciumchannels in vascular smooth muscle is not elucidated. TRPCproteins have been proposed as candidates to fulfil this role. Inorder to determine the role of TRPC1 in the calcium signalevoked by agonists in rat aorta smooth muscle cells (VSMC), itsexpression was inhibited by transfecting the cells with smallinterfering RNA directed against TRPC1 (siRNA-TRPC1).Functional evaluation was performed by measuring the calciumsignal by fluorescence microscopy in fura-2 loaded VSMC. Caentry was identified by Mn quenching of fura-2 fluorescence.Investigation of the gene expression of TRPC isoforms in VSMCby RT-PCR revealed that TrpC1 was the most abundant. In cellstransfected with siRNA-TRPC1, TRPC1 mRNA expression wasinhibited by 72 ± 3 % (n=4) and TRPC1 protein expression wasattenuated by 86 ± 8 % (n=39 cells). TrpC6 mRNA expressionwas unchanged. In VSMC, Ca entry evoked by endothelin-1 wasabolished by BQ-123, suggesting that ETA receptor occupation is