Nitric oxide (NO) and carbon monoxide (CO) seem to be neurotransmitters in the brain. The colocalization of their respective biosynthetic enzymes, neuronal NO synthase (nNOS) and heme oxygenase-2 (HO2), in enteric neurons and altered intestinal function in mice with genomic deletion of the enzymes (nNOS ⌬͞⌬ and HO2 ⌬͞⌬ ) suggest neurotransmitter roles for NO and CO in the enteric nervous system. We now establish that NO and CO are both neurotransmitters that interact as cotransmitters. Small intestinal smooth muscle cells from nNOS ⌬͞⌬ and HO2 ⌬͞⌬ mice are depolarized, with apparent additive effects in the double knockouts (HO2 ⌬͞⌬ ͞nNOS ⌬͞⌬ ). Muscle relaxation and inhibitory neurotransmission are reduced in the mutant mice. In HO2 ⌬͞⌬ preparations, responses to electrical field stimulation are nearly abolished despite persistent nNOS expression, whereas exogenous CO restores normal responses, indicating that the NO system does not function in the absence of CO generation. N itric oxide (NO) is a neurotransmitter in the central, peripheral, and enteric nervous systems (ENS; refs. 1-3). In the ENS, NO is produced in enteric neurons by neuronal NO synthase (nNOS) and acts as a intercellular inhibitory neurotransmitter by diffusing from neurons to adjacent smooth muscle cells where it activates soluble guanylate cyclase, resulting in smooth muscle relaxation. Carbon monoxide (CO) has been postulated to be a second gaseous neurotransmitter (4, 5). CO is produced together with ferrous iron and bilirubin by the action of heme oxygenase, in collaboration with cytochrome P450 reductase and biliverdin reductase (6). Like NOS, heme oxygenase occurs in inducible and constitutive isoforms; heme oxygenase-1 (HO1) is highly inducible and is involved in cellular responses to stress (6, 7), whereas heme oxygenase-2 (HO2) is constitutively expressed (6) and discretely localized to neurons in the brain (4), enteric neurons (8-11), and interstitial cells of Cajal (ICC) in the mouse intestine (12). Because nNOS and HO2 are colocalized in some enteric neurons, NO and CO may function as coneurotransmitters (8,(13)(14)(15). Several studies show that CO can modulate important physiologic functions. Thus, exogenous CO modulates the release of hypothalamic hormones (16, 17), regulates vascular tone (18), and is a protective factor in hypoxia (19). In the intestine, CO relaxes the opossum internal anus sphincter (20) and hyperpolarizes isolated human and canine jejunal circular smooth muscle cells (10, 21). These previous investigations have relied on nonspecific inhibitors of heme oxygenase (22) or the application of exogenous CO. Thus, these studies do not provide direct evidence for CO as an endogenously produced neurotransmitter. Recently, we found attenuated intestinal smooth muscle relaxation and delayed intestinal transit in mice genetically deficient in HO2, suggesting that CO may be produced endogenously as an enteric neurotransmitter (8).In the present study, we employ mice with targeted genomic deletions of HO2 (HO2 Ϫ 15.5, H...

Objective: We aimed to investigate the relationship between clinical characteristics, outcomes and the severity of severe acute respiratory syndrome coronavirus 2 pneumonia. Methods: We performed a systematic review and meta-analysis using PubMed, Embase, and Cochrane Library databases to assess the clinical characteristics and outcomes of confirmed COVID-19 cases and compared severe (ICU) and nonsevere (non-ICU) groups. Results: We included 12 cohort studies including 2,445 patients with COVID-19. Compared with nonsevere (non-ICU) patients, severe (ICU) disease was associated with a smoking history (P = .003) and comorbidities including chronic obstructive pulmonary disease (OR = 5.08, P < .001), diabetes (OR = 3.17, P < .001), hypertension (OR = 2.40, P < .001), coronary heart disease (OR = 2.66, P < .001), cerebrovascular diseases (OR = 2.68, P = .008), and malignancy (OR=2.21, P = .040). We found significant differences between the 2 groups for fever, dyspnea, decreased lymphocyte and platelet counts, and increased leukocyte count, C-creative protein, procalcitonin, lactose dehydrogenase, aspartate aminotransferase, alanine aminotransferase, creatinine kinase, and creatinine levels (P < .05). Significant differences were also observed for multiple treatments (P < .05). Patients in the severe (ICU) group were more likely to have complications and had a much higher mortality rate and lower discharge rate than those with nonsevere (non-ICU) disease (P < .05). Conclusions: Investigation of clinical characteristics and outcomes of severe cases of COVID-19 will contribute to early prediction, accurate diagnosis, and treatment to improve the prognosis of patients with severe illness.

Carbon monoxide (CO) is proposed as a physiological messenger. CO activates cGMP and has a direct effect on potassium channels. Both actions of CO lead to hyperpolarization of a cell's resting membrane potential, suggesting that CO may function as a hyperpolarizing factor, although direct evidence is still lacking. Here we take advantage of the known membrane potential gradient that exists in the muscle layers of the gastrointestinal tract to determine whether CO is an endogenous hyperpolarizing factor. We find that heme oxygenase-2-null mice have depolarized smooth muscle cells and that the membrane potential gradient in the gut is abolished. Exogenous CO hyperpolarizes the membrane potential. Regions of the canine gastrointestinal tract that are more hyperpolarized generate more CO and have higher heme oxygenase activity than more depolarized regions. Our results suggest that CO is a critical hyperpolarizing factor required for the maintenance of intestinal smooth muscle membrane potential and gradient.A role for carbon monoxide (CO) is proposed in cell signaling and neurotransmission (1, 2), hormone release (3, 4), as a cytoprotective agent (5), and in regulation of vascular tone (6, 7). The major source of CO is from the breakdown of heme by heme oxygenase (HO); the absence of HO-2, the constitutive HO isoform, significantly impairs the production of CO in gut and other tissues (8). HO-2 is widely expressed in tissues containing smooth muscle, including blood vessels (6), the pulmonary system (9), and the gastrointestinal tract (10), suggesting that CO is continually produced in these tissues. Multiple mechanisms of action for CO have been proposed, including activation of guanylyl cyclase and direct activation of K ϩ channels (11). Both mechanisms can result in membrane hyperpolarization, raising the possibility that CO can act as an endogenous hyperpolarizing factor. To determine whether CO is an endogenous hyperpolarizing factor, we took advantage of the known membrane potential gradients that exist in the muscle layers of the gastrointestinal tract. We chose the gastrointestinal tract as our model because our previous studies in the HO-2-null mouse demonstrate that intestinal smooth muscle is depolarized in the HO-2-null mouse (2), and because of the presence of a measurable smooth muscle membrane potential gradient that has not been reported in vascular smooth muscle. We hypothesized that if CO acts as a physiologically relevant hyperpolarizing factor, then CO production should mirror the membrane potential gradient along the long axis of the stomach as well as across the gut wall and that these resting membrane potential gradients would be diminished or abolished in HO-2-null mice. In the gastrointestinal tract of all species studied, there is a large gradient (Ϸ30 mV) in membrane potential along the long axis of the stomach from the fundus (proximal stomach) to the pylorus (distal stomach) (12). There is also a membrane potential gradient across the thickness of circular muscle layer (12, 13). In the ...