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Am J Physiol Gastrointest Liver Physiol 277: G152-G160, 1999;
0193-1857/99 $5.00
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Vol. 277, Issue 1, G152-G160, July 1999

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

Ya-Ping Fan, Sushanta Chakder, and Satish Rattan

Department of Medicine, Division of Gastroenterology and Hepatology, Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania 19107

Cholera toxin (CTX), an activator of Gs protein, is an important pharmacological tool in G protein research. The effect and the mechanism of action of CTX in the gastrointestinal smooth muscle, including the internal anal sphincter (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 IAS smooth muscle. CTX caused a prompt and dose-dependent fall in the basal tone of the IAS that was not affected by the neurotoxins TTX and omega -conotoxin or the nitric oxide synthase inhibitor NG-nitro-L-arginine. The cyclooxygenase inhibitor indomethacin, cAMP-dependent protein kinase inhibitor Rp-8-bromoadenosine 3',5' cyclic monophosphorothioate inhibited CTX-induced IAS smooth muscle relaxation. Furthermore, CTX caused a concentration-dependent relaxation of the isolated smooth muscle cells (SMC) of the IAS, which was blocked by Gsalpha antibody (Gsalpha -Ab). The IAS smooth muscle relaxation was accompanied with an increase in the GTPase activity that was also specifically blocked by Gsalpha -Ab. We conclude that a major part of the inhibitory action of CTX in the IAS is via the direct response of the SMC that is linked with Gs protein to the AC pathway. A part of the inhibitory action of CTX on the smooth muscle occurs via the activation of cyclooxygenase pathway. The relative contribution of such actions of CTX in the smooth muscle in the gastrointestinal motility disturbances following cholera infection remains to be determined.

nonadrenergic noncholinergic; guanine nucleotide-binding protein; G protein; guanosine triphosphatase; adenylate cyclase; nitric oxide synthase


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