Heme oxygenase activity in the internal anal sphincter: Effects of nonadrenergic, noncholinergic nerve stimulation

Background & Aims: To date, the exact role of carbon monoxide (CO) in the n onadrenergic, noncholinergic (NANC) relaxation is not known. This is partly related to the: lack of. an appropriate method to measure heme oxygenase ( HO) activity in the gastrointestinal tissues. Methods: HO activity of the o possum internal anal sphincter (IAS) smooth muscle was determined using a n ewly developed assay system that used radiolabeled hemin as a substrate, En zyme activity of the IAS tissues was measured in the basal state, after ele ctric field stimulation (EFS), ganglionic stimulant dimethyl diphenyl piper azinium iodide (DMPP), and neuropeptide vasoactive intestinal polypeptide ( VIP). The presence and localization of HO was examined by Western blot anal ysis and immunocytochemistry, Results: NANC nerve stimulation of the IAS sm ooth muscle by EFS (0.25-5 Hz), DMPP, and VIP caused a significant increase in the HO activity of the IAS, The increase in HO activity by EFS was inhi bited by the HO inhibitor Tin protoporphyrin (1 x 10(-4) mol/L), Both HO-1 and HO-2 were present in the IAS tissue extracts, and both enzymes were loc alized in the neurons of the myenteric plexus, The method for HO activity d etermination used in the present study was found to be reliable and reprodu cible. Conclusions: The data suggest that the HO pathway may have a role in neurally mediated relaxation of the IAS. The exact site of involvement and the source of HO activity, however, remains to be determined.