Cyclic GMP relaxes the internal anal sphincter in Hirschsprung's disease

In Hirschsprung's disease (HD), the aganglionic colon and internal anal sphincter (IAS) fail to relax. Aganglionic colon of HD patients relaxes in response to exogenous nitric oxide (NO), whereas the IAS from HD patients does not relax. The authors hypothesized that the failure of IAS relaxation is caused by a local deficiency of cyclic guanosine monophosphate (cGMP), the final metabolite in NO-mediated smooth muscle relaxation. To test this hypothesis, the authors measured the isometric tension of smooth muscle strips taken from the IAS and aganglionic colon of patients with HD before and after exposure to cGMP and compared this with ganglionic colon and IAS from normal controls. In HD patients both the IAS and aganglionic colon relaxed in response to cGMP (P < .05). The amount of relaxation observed in both the aganglionic colon and IAS was comparable to that measured in the normal controls. The observation that exogenous cGMP relaxes the IAS, whereas exogenous NO does not, suggests that mechanisms for relaxation may be different than those in the aganglionic colon and may explain persistent IAS dysfunction after resection of aganglionic colon. The defect of the IAS in HD may be the inability of the NO/cGMP pathway to induce smooth muscle cell relaxation rather than a defect in the smooth muscle cell.