The TRPC1 (transient receptor potential canonical 1) protein, which is thought to encode a non-selective cation channel activated by store depletion and/or an intracellular messenger, is expressed in a number of non-excitable cells. However, the physiological functions of TRPC1 are not well understood. The aim of these studies was to investigate the function of TRPC1 in liver cells using small interfering RNA (siRNA) to ablate the TRPC1 protein. Treatment of H4-IIE liver cells with siRNA targeted against TRPC1 caused an approx. 50% decrease in expression of the human TRPC1 protein in cells transfected with cDNA encoding human TRPC1, and a 50% decrease in expression of the endogenous TRPC1 protein (assessed by Western blot and immunofluorescence). The decrease in endogenous TRPC1 protein in cells transfected with TRPC1 siRNA was associated with a greater increase in cell volume (compared with the increase observed in control cells) immediately after cells were placed in a hypotonic medium, and an enhanced regulatory cell volume decrease after exposure to hypotonic medium. Treatment with siRNA targeted against TRPC1 also led to a 25% inhibition of thapsigargin-stimulated Ca(2+) inflow, a 40% inhibition of ATP and maitotoxin-stimulated Ca(2+) inflow, and a 50% inhibition of maitotoxin-stimulated Mn(2+) inflow. The idea that, in liver cells, TRPC1 encodes a non-selective cation channel involved directly or indirectly in the regulation of cell volume is consistent with the results obtained.