The classical type of transient receptor potential channel (TRPC) is a molecular candidate for Ca2+-permeable cation channels in mammalian cells. Especially, TRPC4 has the similar properties to Ca2+-permeable nonselective cation channels (NSCCs) activated by muscarinic stimulation in visceral smooth muscles. In visceral smooth muscles, NSCCs activated by muscarinic stimulation were blocked by anti-G alpha i/o antibodies. However, there is still no report which G alpha proteins are involved in the activation process of TRPC4. Among G alpha proteins, only G alpha i protein can activate TRPC4 channel. The activation effect of G alpha i was specific for TRPC4 because God has no activation effect on TRPC5, TRPC6 and TRPV6. Coexpression with muscarinic receptor M2 induced TRPC4 current activation by muscarinic stimulation with carbachol, which was inhibited by pertussis toxin. These results suggest that Gai is involved specifically in the activation of TRPC4. (C) 2008 Elsevier Inc. All rights reserved.