Heme Oxygenase, a Likely Regulator of cGMP Production in the Brain: Induction in Vivo of HO-1 Compensates for Depression in NO Synthase Activity.

To examine the role of carbon monoxide (CO) as a putative neuronal messenger and regulator of cGMP level in vivo, we exploited an animal model to increase brain capability to generate CO. The sole source of CO in mammalian systems is the alpha-meso carbon bridge of the heme molecule cleaved by heme oxygenase isozymes, HO-1 and HO-2. In adult animals, the noninducible isozyme HO-2 is the predominant form in the brain. We chose to increase, rather than inhibit, brain heme oxygenase activity because synthetic metalloporphyrins, such as Zn-protoporphyrin, which are the only known effective inhibitors of the isozymes, are also potent inhibitors of soluble guanylate cyclase, the enzyme that generates cGMP. In newborn rats both heme oxygenase isozymes were found expressed at low levels, and in the cerebellum heme oxygenase activity could be induced by treatment of 2-day-old animals with a selective depletor of glutathione, buthionine-SR-sulfoximine. The increase in activity was accompanied by marked increases in HO-1 protein and the 1.8 kb HO-1 mRNA in the cerebellum. Despite a pronounced decrease in activity of the hemoprotein nitric oxide synthase, no change in cGMP level was observed. The decrease in the synthase could not be explained by an inhibited heme biosynthesis activity. This unchanged level of cGMP suggests that NO is not the only gaseous heme ligand that can activate guanylate cyclase resulting in the generation of cGMP, but rather that CO may also function in this capacity. Increased capability of select cerebellar cell populations to generate CO, as indicated by an increase in their HO-1 protein content, points to the active role of this isozyme in maintenance of cGMP level under stress conditions, when nitric oxide production is compromised. The cell populations expressing HO-1 protein included those in pia matter and glia, such as astrocytes.