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...