Heme oxygenase 2 (HO-2), which synthesizes carbon monoxide (CO), has been localized by immunohistochemistry to endothelial cells and adventitial nerves of blood vessels. HO-2 is also localized to neurons in autonomic ganglia, including the petrosal, superior cervical, and nodose ganglia, as well as ganglia in the myenteric plexus of the intestine. Enzyme studies demonstrated that tin protoporphyrin-9 is a selective inhibitor of HO with -10-fold selectivity for HO over endothelial nitric oxide synthase (NOS) and soluble guanylyl cyclase. Inhibition of HO activity by tin protoporphyrin 9 reverses the component of endothelial-derived relaxation of porcine distal pulmonary arteries not reversed by an inhibitor of NOS. Thus, CO, like NO, may have endothelialderived relaxing activity. The similarity of NOS and HO-2 localizations and functions in blood vessels and the autonomic nervous system implies complementary and possibly coordinated physiologic roles for these two mediators.Carbon monoxide (CO) has been implicated as a biological messenger molecule analogous to nitric oxide (NO) (1, 2). Two forms of heme oxygenase (HO-1 and HO-2) convert heme to biliverdin and CO (3, 4). HO-1 is highly expressed in liver and spleen and is easily induced by heme and by oxidative stress (5,6). A noninducible form, HO-2, is widely expressed with high concentrations in the brain (3). In the brain, the distribution of HO-2 closely parallels that of soluble guanylyl cyclase (SGC) (1). The ability of CO to directly activate SGC (7,8) and the depletion of cGMP levels in olfactory neurons treated with HO inhibitors (1) supports a role for CO as a modulator of cGMP. Like nitric oxide synthase (NOS), HO-2 occurs in discrete neuronal populations throughout the brain (1, 9) and both NO and CO have been implicated in long-term potentiation (10, 11), although the specificity of HO inhibitors used in these experiments has been questioned (12, 13).Although NO is well established as a mediator in the periphery, in both neurons and blood vessels (14-17) roles for CO have been less well characterized. HO-2 is localized to the glomus cells of the carotid body, where CO may mediate carotid body reactivity to hypoxia (18). HO inhibitors diminish esophageal motility (19), while hypoxia augments HO-1 in smooth muscle cells (20), and HO inhibitors decrease Na,KATPase activity in cerebellar Purkinje cells (21).We now report discrete localizations of HO-2 in vascular endothelium, nerves in the adventitia of blood vessels, and peripheral autonomic ganglia. Inhibition of HO-2 attenuates a component of endothelium-dependent vasodilation, implicating CO in vascular regulation. MATERIALS AND METHODSMaterials. Anti-neurofilament antibody, acetylcholine (ACh), phenylephrine, and NADPH were from Sigma, metalThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.Abbreviations: ACh, acetylcholine; HO, heme o...

Neuronal nitric oxide synthase (nNOS) generates NO in neurons, and heme-oxygenase-2 (HO-2) synthesizes carbon monoxide (CO). We have evaluated the roles of NO and CO in intestinal neurotransmission using mice with targeted deletions of nNOS or HO-2. Immunohistochemical analysis demonstrated colocalization of nNOS and HO-2 in myenteric ganglia. Nonadrenergic noncholinergic relaxation and cyclic guanosine 3,5 monophosphate elevations evoked by electrical field stimulation were diminished markedly in both nNOS ⌬/⌬ and HO-2 ⌬/⌬ mice. In wild-type mice, NOS inhibitors and HO inhibitors partially inhibited nonadrenergic noncholinergic relaxation. In nNOS ⌬/⌬ animals, NOS inhibitors selectively lost their efficacy, and HO inhibitors were inactive in HO-2 ⌬/⌬ animals.Nitric oxide synthase (NOS) and heme oxygenase (HO) display numerous similarities. Inducible and constitutive isoforms reflect multiple distinct genes encoding either NOS or HO (1). Both endothelial NOS and HO-2, which is most concentrated in brain and testes (1), occur in the endothelial layers of blood vessels and mediate vasorelaxation (1, 2). Neuronal NOS (nNOS) and HO-2 are colocalized within adventitial neurons of blood vessels (1) and in autonomic ganglia (1) with NO being a likely transmitter in the autonomic nervous system (3). Both enzymes give rise to more than one product: NO and citrulline from NOS and carbon monoxide and biliverdin from HO (1, 2). NO and CO stimulate soluble guanylyl cyclase activity (1), and inhibitors of NOS (1, 2) or HO (4) lower cyclic guanosine 3Ј,5Ј monophosphate (cGMP) levels in certain tissues (1, 4).Intestinal myenteric plexus neurons express nNOS (5) and HO-2 (6, 7), the respective biosynthetic enzymes for NO and CO in the nervous system (1). NOS inhibitors (8) and HO inhibitors (9) partially reverse nonadrenergic noncholinergic (NANC) relaxation of various portions of the gastrointestinal pathway. Studies investigating NO and CO functions with inhibitors of NOS or HO-2 are confounded by potential nonspecificity of these agents. For example, concentrations of metalloporphyrins that inhibit HO also can inhibit soluble guanylyl cyclase (7,10, 11) and NOS (7,12).To elucidate a potential neural role for HO-2 products, we have used mice with targeted deletions of HO-2 (13) or nNOS (14). In the present study, we report diminished neurally evoked intestinal relaxation and depressed cGMP levels in nNOS ⌬/⌬ and HO-2 ⌬/⌬ mice. Furthermore, HO-2 and nNOS were colocalized within neurons associated with myenteric ganglia of wild-type mice. MATERIALS AND METHODSImmunohistochemistry. HO-2 antibody was prepared and used as described (7). Double-label immunofluorescence. Intestinal segments from male Sprague-Dawley rats were placed into an oxygenated organ chamber containing Krebs buffer (see organ bath methods) and held at 37°C. Colchicine was added directly into the organ chamber, and tissue was fixed in 4% paraformaldehyde after a 12-h incubation in 95% O 2 /5% CO 2 at 37°C. Tissue was fixed and sectioned as described (7)....