In vascular smooth muscle, activation of plasma membrane ion channels is an important mechanism allowing for the increase in cytosolic calcium concentration and the development of contraction. In rat aorta, agonist-stimulated calcium entry is only partly inhibited by blockade of voltage-operated calcium channels (1), suggesting that another calcium entry pathway is activated. Canonical transient receptor potential (TRPC) proteins have been proposed as candidates for receptor- or store-operated calcium channels in smooth muscle cells (2). In order to determine the role of TRPC1 in the calcium signal evoked by agonists in rat aorta smooth muscle cells (VSMC), its expression was inhibited by transfecting the cells with small interfering RNA directed against TRPC1 (siRNA-TRPC1). Functional evaluation was performed by measuring the calcium signal by fluorescence microscopy in fura-2 loaded VSMC. Rats were humanely killed; the aorta was quickly removed, cleaned from adherent tissue and endothelium was gently rubbed off. Primary culture of VSMC was performed by the explant technique. The gene expression of TRPC isoforms in VSMC was investigated by RT-PCR. mRNA encoding TRPC1, -3, -4, -5, -6 and -7, but not TRPC2 was detected in VSMC. The highest level of expression was found for TRPC1. In cells transfected with siRNA-TRPC1, TRPC1 mRNA expression was inhibited by 72 ± 3% (n=4). In contrast, transfection with nonsilencing control siRNA did not affect the level of expression of TRPC1 mRNA. Immunocytochemistry revealed that TRPC1 protein expression was markedly attenuated in siRNA-TRPC1 transfected VSMC but was not modified in cells transfected with nonsilencing siRNA. Measurement of calcium signal evoked by endothelin-1 in VSMC showed that, in the absence of external calcium, endothelin-1 induced a rapid but transient increase in cytosolic calcium, reflecting the release of intracellular calcium. Addition of calcium into the external solution in the presence of endothelin-1 evoked a sustained increase in cytosolic calcium, which was completely blocked by the ET-A receptor antagonist BQ-123 and by micromolar concentration of Gd³⁺. In VSMC transfected with siRNA-TRPC1, calcium release evoked by endothelin-1 in calcium free solution was not modified, but the increase in cytosolic calcium observed after the addition of calcium into the external solution was markedly blunted. These results indicate that TRPC1 channel protein is involved in receptor-activated calcium entry in aortic VSMC.