Nitric Oxide Not Carbon Monoxide Mediates Nonadrenergic Noncholinergic Relaxation in the Murine Internal Anal Sphincter

Background & Aims: Inhibitory reflexes in the internal anal sphincter (IAS) are controlled by inhibitory nonadrenergic, noncholinergic innervation (i-NANC). We investigated the roles of 3 different neurohumoral agonists as possible i-NANC neurotransmitters: carbon monoxide (CO), nitric oxide (NO), and vasoactive intestinal peptide (VIP). Methods: IAS smooth muscle strips were isolated from wild-type (WT), heme oxygenase (HO)-2 knockout (HO-2−/−) and neuronal NO synthase (nNOS) knockout (nNOS−/−) mice. Relaxation of IAS was induced by CO, NO, VIP, and electrical field stimulation (EFS) in the presence and absence of neurohumoral inhibitors (tin protoporphyrin IX [SnPP IX] for CO synthesis, Nω-nitro-L-arginine [L-NNA] for NO synthesis, and VIP10–28 for VIP receptor). Western blot and immunohistochemistry were used to test the presence and localization of HO (for CO synthesis) types 1 (HO-1) and 2 (HO-2), neuronal NO synthase (nNOS, for NO synthesis), and VIP. Results: All 3 neurohumoral agonists produced relaxation (with no difference between WT and HO-2−/− IAS), but CO was over 100 times less potent than NO and VIP. EFS produced relaxation in WT and HO-2−/− IAS with the same intensity. L-NNA and nNOS deletion (∼80%) and VIP10–28 (∼15%) significantly inhibited the relaxations, whereas SnPP IX had no effect. Positive immunoreactivities for HO-2, nNOS, and VIP were found in the myenteric plexus of WT IAS. HO-2−/− IAS did not express immunoreactivity for HO-2. Conclusions: i-NANC relaxations of mouse IAS are primarily mediated via NO (by nNOS activity) and partly via VIP. CO directly relaxes the mouse IAS but does not play any significant role in the i-NANC relaxation.