Smooth muscle cells isolated from cecal circular smooth muscle of the guinea pig were used to determine whether thyrotropin-releasing hormone (TRH) can inhibit the contractile response produced by 10-6M carbachol by exerting a direct action on muscle cells. In addition, the inhibitory effect of 2´,5´-dideoxy-adenosine (an inhibitor of adenylate cyclase), phorbol 12-myristate 13-acetate (an inhibitor of particulate guanylate cyclase), 6-anilinoquinoline-5,8-quinone (an inhibitor of soluble guanylate cyclase) and Nω-nitro-L-arginine methyl ester (an inhibitor of nitric oxide synthase) on the TRH-induced relaxation of cecal circular smooth muscle cells was examined. TRH inhibited the contractile response produced by 10-6M carbachol in a concentration-dependent manner, with an IC50 value of 4 nM. 2´,5´-Dideoxyadenosine and phorbol 12-myristate 13-acetate did not have any significant effect on the TRH-induced relaxation. On the other hand, 6-anilinoquinoline-5,8-quinone and Nω-nitro-L-arginine methyl ester significantly inhibited the relaxation produced by TRH. Our findings show that TRH has a direct inhibitory effect on the isolated cecal circular smooth muscle cells via activation of nitric oxide synthase and soluble guanylate cy