Mechanism of action of cholera toxin on the opossum internal anal sphincter smooth muscle

Cholera toxin (CTX), an activator of G(s) protein, is an important pharmaco logical tool in G protein research. The effect and the mechanism of action of CTX in the gastrointestinal smooth muscle, including the internal anal s phincter (IAS), are not known. The present investigation was carried out to , examine the effects of CTX on the signal transduction associated with the adenylate cyclase (AC) pathway on the basal tone of the LAS smooth muscle. CTX caused a prompt and dose;dependent fall in the basal tone of the LAS t hat was not affected by the neurotoxins TTX and omega-conotoxin or the nitr ic oxide synthase inhibitor N-G-nitro-L-arginine. The cyclooxygenase inhibi tor indomethacin, cAMP-dependent protein kinase inhibitor Rp-8-bromoadenosi ne 3',5' cyclic monophosphorothioate inhibited CTX-induced IAS smooth muscl e relaxation. Furthermore, CTX caused a concentration-dependent, relaxation of the isolated smooth muscle cells (SMC) of the LAS, which was blocked by G(s)alpha antibody (G(s)alpha-Ab). The IAS smooth muscle relaxation was ac companied with an increase in the GTPase activity that was also specificall y blocked by G(s)alpha-Ab. We conclude that a major part of the inhibitory action of CTX in the LAS is via the direct response of the SMC that is Link ed with G(s) protein to the AC pathway. A part of the inhibitory action of CTX on the smooth muscle occurs via the activation of cyclooxygenase pathwa y. The relative contribution of such actions of CTX in the smooth muscle in the gastrointestinal motility disturbances following cholera infection rem ains to be determined.