Expression of heme oxygenase-2 (HO-2)-like immunoreactivity in rat tissues

Heme oxygenase-1 and heme oxygenase-2 expression in bruises

The first step in catabolism of hemoglobin in a bruise is performed by the enzyme heme oxygenase, which produces biliverdin that is then reduced to bilirubin. The development of yellow coloration in bruises can be attributed to local accumulation of degradation products of hemoglobin, including bilirubin, but it is not clear why there is a delay before this color change is apparent. One explanation may be that time is required for the establishment of heme oxygenase activity at the bruise site. This study used immunohistochemistry to examine the time course of expression of heme oxygenase-1 and heme oxygenase-2 in a rat bruise model. Heme oxygenase-1 levels rose above background from 6 h to peak from days 1 to 3. There was strong expression by macrophages, but only occasional neutrophils expression of heme oxygenase-1. Heme oxygenase-2 did not change significantly from background levels. The results suggest that the delay in the development of yellow coloration of bruises may in part be attributed to the requirement for macrophages to be recruited to the site of injury.

CO and CO-releasing molecules (CO-RMs) in acute gastrointestinal inflammation

Carbon monoxide (CO) is enzymatically generated in mammalian cells alongside the liberation of iron and the production of biliverdin and bilirubin. This occurs during the degradation of haem by haem oxygenase (HO) enzymes, a class of ubiquitous proteins consisting of constitutive and inducible isoforms. The constitutive HO2 is present in the gastrointestinal tract in neurons and interstitial cells of Cajal and CO released from these cells might contribute to intestinal inhibitory neurotransmission and/or to the control of intestinal smooth muscle cell membrane potential. On the other hand, increased expression of the inducible HO1 is now recognized as a beneficial response to oxidative stress and inflammation. Among the products of haem metabolism, CO appears to contribute primarily to the antioxidant and anti-inflammatory effects of the HO1 pathway explaining the studies conducted to exploit CO as a possible therapeutic agent. This article reviews the effects and, as far as known today, the mechanism(s) of action of CO administered either as CO gas or via CO-releasing molecules in acute gastrointestinal inflammation. We provide here a comprehensive overview on the effect of CO in experimental in vivo models of post-operative ileus, intestinal injury during sepsis and necrotizing enterocolitis. In addition, we will analyse the in vitro data obtained so far on the effect of CO on intestinal epithelial cell lines exposed to cytokines, considering the important role of the intestinal mucosa in the pathology of gastrointestinal inflammation.

Review article: carbon monoxide in gastrointestinal physiology and its potential in therapeutics

BACKGROUND

While carbon monoxide (CO) is a known toxin, it is now recognised that CO is also an important signalling molecule involved in physiology and pathophysiology.

AIMS

To summarise our current understanding of the role of endogenous CO in the regulation of gastrointestinal physiology and pathophysiology, and to potential therapeutic applications of modulating CO.

METHODS

This review is based on a comprehensive search of the Ovid Medline comprehensive database and supplemented by our ongoing studies evaluating the role of CO in gastrointestinal physiology and pathophysiology.

RESULTS

Carbon monoxide derived from haem oxygenase (HO)-2 is predominantly involved in neuromodulation and in setting the smooth muscle membrane potential, while CO derived from HO-1 has anti-inflammatory and antioxidative properties, which protect gastrointestinal smooth muscle from damage caused by injury or inflammation. Exogenous CO is being explored as a therapeutic agent in a variety of gastrointestinal disorders, including diabetic gastroparesis, post-operative ileus, organ transplantation, inflammatory bowel disease and sepsis. However, identifying the appropriate mechanism for safely delivering CO in humans is a major challenge.

CONCLUSIONS

Carbon monoxide is an important regulator of gastrointestinal function and protects the gastrointestinal tract against noxious injury. CO is a promising therapeutic target in conditions associated with gastrointestinal injury and inflammation. Elucidating the mechanisms by which CO works and developing safe CO delivery mechanisms are necessary to refine therapeutic strategies.

Heme oxygenase-1 deficiency promotes the development of necrotizing enterocolitis-like intestinal injury in a newborn mouse model

Necrotizing enterocolitis (NEC) is typified by mucosal destruction, which subsequently can lead to intestinal necrosis. Prematurity, enteral feeding, and bacterial colonization are the main risk factors and, combined with other stressors, can cause increased intestinal permeability, injury, and an exaggerated inflammatory response. Heme oxygenase-1 (HO-1) mediates intestinal protection due to anti-inflammatory, antioxidative, and antiapoptotic effects of its products carbon monoxide, biliverdin, and bilirubin. This study investigates a possible role of HO-1 in the pathogenesis of NEC using a newborn mouse model. We induced NEC-like intestinal injury in 7-day-old HO-1 heterozygous (HO-1 Het, Hmox1(+/-)) and wild-type (Wt, Hmox1(+/+)) mice by gavage feeding and hypoxic exposures. Control (Con) pups of both genotypes were dam-fed. Intestines of HO-1 Het Con pups appeared predisposed to injury, with higher histological damage scores, more TUNEL-positive cells, and a significant reduction in muscularis externa thickness compared with Wt Con pups. The increase in HO activity after HO-1 induction by the substrate heme or by hypoxic stress was significantly impaired in HO-1 Het pups. After induction of intestinal injury, HO-1 Het pups displayed significantly higher NEC incidence (78 vs. 43%), mortality (83 vs. 54%), and median scores (2.5 vs. 1.5) than Wt NEC pups. PCR array analyses revealed increased expressions of IL-1β, P-selectin, matrix metallopeptidase 2, collagen type XVIII-α1, serpine 1, and others in NEC-induced HO-1 Het ileal and jejunal tissues. We conclude that a partial HO-1 deficiency promotes experimental NEC-like intestinal injury, possibly mediated by exaggerated inflammation and disruption in tissue repair.

H2S induces catecholamine secretion in rat adrenal chromaffin cells

Hydrogen sulfide (H(2)S) is recognized as an important gaseous signaling molecule in mammalian tissues and exerts its modulating functions of different systems via targeting different ion channels and receptors. H(2)S can be synthesized from l-cysteine by cystathionine β-synthetase (CBS) or cystathionine γ-lyase (CSE). It has been reported recently that H(2)S can be synthesized and released in rat adrenal medulla chromaffin cells (AMCs) which play a critical role in the regulation of stress response by releasing catecholamine (CA). In the present study, we combined amperometry and whole-cell patch-clamp recording to explore the direct effect of exogenous H(2)S on CA release in AMCs and the underlying ionic mechanism. Amperometry showed that local application of NaHS, the H(2)S donor, evoked CA release from AMCs. Furthermore, the CA secretory response to NaHS was totally blocked by removing extracellular Ca(2+). Whole-cell patch-clamp experiments showed that H(2)S-induced CA release is produced by membrane depolarization generated by an inhibition of Ca(2+)-activated K(+) current [I(K(Ca)) current]. We conclude that H(2)S is capable of directly inducing CA release by inhibiting the I(K(Ca)) current. This conclusion indicates that H(2)S may involve in the response of adrenal medulla to stress by modulating I(K(Ca)) current and CA release in mammalian animals.

The role of heme oxygenase and carbon monoxide in inflammatory bowel disease

Inflammatory bowel disease (IBD), including ulcerative colitis (UC) and Crohn's disease, is a chronic and recurrent inflammatory disorder of the intestinal tract. Since the precise pathogenesis of IBD remains unclear, it is important to investigate the pathogenesis of IBD and to evaluate new anti-inflammatory strategies. Recent evidence suggests that heme oxygenase-1 (HO-1) plays a critical protective role during the development of intestinal inflammation. In fact, it has been demonstrated that the activation of HO-1 may act as an endogenous defensive mechanism to reduce inflammation and tissue injury in various animal intestinal injury models induced by ischemia-reperfusion, indomethacin, lipopolysaccharide-associated sepsis, trinitrobenzene sulfonic acid or dextran sulfate sodium. In addition, carbon monoxide (CO) derived from HO-1 has been shown to be involved in the regulation of intestinal inflammation. Furthermore, administration of a low concentration of exogenous CO has a protective effect against intestinal inflammation. These data suggest that HO-1 and CO may be novel therapeutic molecules for patients with gastrointestinal inflammatory diseases. In this review, we present what is currently known regarding the role of HO-1 and CO in intestinal inflammation.

Inhibition of heme oxygenase augments tubular sodium reabsorption

Heme oxygenase (HO) catalyzes the degradation of heme to form iron, biliverdin, and carbon monoxide (CO). The vascular actions of CO include direct vasodilation of vascular smooth muscle and indirect vasoconstriction through inhibition of nitric oxide synthase (NOS). This study was performed to examine the effects in the kidney of inhibition of heme oxygenase alone or combined with NOS inhibition. Chromium mesoporphyrin (CrMP; 45 μmol/kg ip), a photostable HO inhibitor, was given to control rats and N(G)-nitro-l-arginine methyl ester (l-NAME)-treated hypertensive rats (50 mg·kg⁻¹·day⁻¹), 12 h, 4 days). In control animals, CrMP decreased CO levels, renal HO-1 levels, urine volume, and sodium excretion, but had no effect on arterial pressure, renal blood flow (RBF), plasma renin activity (PRA), or glomerular filtration rate (GFR). In l-NAME-treated hypertensive rats, CrMP decreased endogenous CO and renal HO-1 levels and had no effect on arterial pressure, RBF, or GFR but decreased sodium and water excretion in a similar manner to control animals. An increase in PRA was observed in untreated rats but not in l-NAME-infused rats, indicating that this effect is associated with an absent NO system. The results suggest that inhibition of HO promotes water and sodium excretion by a direct tubular action that is independent of renal hemodynamics or the NO system.

Carbon monoxide involved in modulating HCO3- secretion in rat duodenum

We examined the effect of the tricarbonyl-dichlororuthenium (II) dimer (CORM-2), a carbon monoxide (CO) donor, on duodenal HCO(3)(-) secretion in rats and investigated whether endogenous CO produced by heme oxygenase (HO) is involved in the regulation of this secretion. Under urethane anesthesia, a duodenal loop was perfused with saline, and HCO(3)(-) secretion was measured at pH 7.0 using a pH stat method. CORM-2, biliverdin, FeCl(2), or ruthenium (III) chloride hydrate (RuCl(3)) was applied to the loop for 5 min. The mucosal application of CORM-2 dose-dependently increased HCO(3)(-) secretion, whereas neither RuCl(3), FeCl(2), nor biliverdin had an effect. The stimulatory effect was significantly attenuated by indomethacin but not N(G)-nitro-L-arginine methyl ester. The application of CORM-2 increased the mucosal prostaglandin (PG) E(2) content of the duodenum. The acid-induced HCO(3)(-) response was markedly inhibited by indomethacin and Sn(IV) protoporphyrin IX dichloride (SnPP; an inhibitor of HO) but not Cu(II) protoporphyrin dichloride, and the inhibitory effect of SnPP was significantly reversed by pretreatment with hemin, a substrate of HO. Perfusion of the duodenal loop with 100 mM HCl for 2 h caused a few hemorrhagic lesions in the mucosa, and this response was significantly worsened by the prior administration of SnPP and indomethacin. The expression of HO-1 but not HO-2 protein was up-regulated in the duodenum after the acid treatment. These results suggest that CO, generated endogenously or exogenously, stimulates HCO(3)(-) secretion in the duodenum, and this effect is mediated by endogenous PGs. It is assumed that HO/CO plays a role in maintaining the integrity of the duodenal mucosa.

Mechanical ventilation-induced oxidative stress in the diaphragm: role of heme oxygenase-1

BACKGROUND

Prolonged mechanical ventilation (MV) results in a rapid onset of diaphragmatic atrophy that is primarily due to increased proteolysis. Although MV-induced protease activation can involve several factors, it is clear that oxidative stress is a required signal for protease activation in the diaphragm during prolonged MV. However, the oxidant-producing pathways in the diaphragm that contribute to MV-induced oxidative stress remain unknown. We have demonstrated that prolonged MV results in increased diaphragmatic expression of a key stress-sensitive enzyme, heme oxygenase (HO)-1. Paradoxically, HO-1 can function as either a pro-oxidant or an antioxidant, and the role that HO-1 plays in MV-induced diaphragmatic oxidative stress is unknown. We tested the hypothesis that HO-1 acts as a pro-oxidant in the diaphragm during prolonged MV.

METHODS

To determine whether HO-1 functions as a pro-oxidant or an antioxidant in the diaphragm during MV, we assigned rats into three experimental groups: (1) a control group, (2) a group that received 18 h of MV and saline solution, and (3) a group that received 18 h of MV and was treated with a selective HO-1 inhibitor. Indices of oxidative stress, protease activation, and fiber atrophy were measured in the diaphragm.

RESULTS

Inhibition of HO-1 activity did not prevent or exacerbate MV-induced diaphragmatic oxidative stress (as indicated by biomarkers of oxidative damage). Further, inhibition of HO-1 activity did not influence MV-induced protease activation or myofiber atrophy in the diaphragm.

CONCLUSIONS

Our results indicate that HO-1 is neither a pro-oxidant nor an antioxidant in the diaphragm during MV. Furthermore, our findings reveal that HO-1 does not play an important role in MV-induced protease activation and diaphragmatic atrophy.

Direct effect of carbon monoxide on relaxation induced by electrical field stimulation in rat corpus cavernosum

Purpose

Carbon monoxide (CO) may mediate smooth muscle relaxation in the rat corpus cavernosum smooth muscle (CCSM). We hypothesized that CO plays a role in neurally derived, frequency-dependent relaxation of rat CCSM.

Materials and Methods

To study the effect of CO on CCSM relaxation induced by electrical field stimulation (EFS), a CCSM bundle was mounted on a force transducer and perfused with Hanks' balanced salt solution at 37℃ with 95% O₂ and 5% CO₂. After 1 hour equilibration with -500 mg of passive tension, contraction of the CCSM bundle was elicited by 10^-5 M phenylephrine, which was continuously added with different concentrations of CO (1%, 2%, and 5%). Frequency-dependent relaxation was induced by EFS trains (0.2 ms at 0.5-32 Hz, for 10 s) repeated at 2 min intervals over 15 min in the presence of adrenergic and muscarinic receptor blocking agents (guanethidine and atropine, respectively). To study the distribution of heme oxygenase-2 (HO-2) in the rat CCSM, we performed immunohistochemical evaluation.

Results

CO produced a dose-dependent enhancement of EFS-induced relaxation. Pretreatment with N^G-nitro-L-arginine (a nitric oxide synthase blocker) greatly reduced the EFS-induced relaxation in the presence of CO (-45%). Pretreatment with zinc protoporphyrin-IX (ZnPP-9, a heme oxygenase inhibitor) had no significant effect on EFS-induced relaxation in the absence or the presence of CO. We found immunoreactivity for HO-2 in CCSM and immunoreactivity for protein gene product 9.5 (PGP 9.5) in nerve fibers.

Conclusions

We conclude that CO produced a dose-dependent enhancement of EFS-induced relaxation in rat CCSM bundles, but neurally derived, frequency-dependent relaxation in the rat CCSM depended mostly on nitric oxide in response to nonadrenergic noncholinergic neurotransmission. Immunoreactivity for HO-2 was found in rat CCSM but not nerve fibers.

HEME OXYGENASE-DERIVED ENDOGENOUS CARBON MONOXIDE IMPAIRS FLOW-INDUCED DILATION IN RESISTANCE VESSELS

Vascular tissues normally express heat shock protein 32 (heme oxygenase [HO] 1), which degrades heme. A product of this reaction, carbon monoxide (CO), has been shown to promote relaxation of vascular smooth muscle, but it also inhibits NOS. Because flow-induced dilation is dependent upon the formation of endothelium-derived NO, we conducted the current study to determine if HO-mediated formation of CO impairs flow-induced dilation. In isolated pressurized first-order gracilis muscle arterioles, proximal and distal pressures were manipulated to generate intraluminal flows of 0 to 50 microL/min at a constant vascular midline pressure of 80 +/- 1 mmHg. Vehicle-treated vessels displayed flow-related vasodilation, which was abolished by a NOS inhibitor, Nomega-nitro-L-arginine methyl ester. Acute intraluminal pretreatment with an inhibitor of HO, chromium mesoporphyrin (CrMP), enhanced flow-induced responses in similarly prepared vessels. In contrast, a substrate for heme formation that drives CO generation, delta-aminolevulinic acid, abolished flow-induced dilation in a manner which could be fully prevented and reversed by CrMP. In addition, the HO product biliverdin had no effect on flow-induced dilation, whereas the responses were abolished by exogenous CO. Furthermore, spontaneous generation of CO was measured in isolated vascular segments to confirm that delta-aminolevulinic acid increased carbon formation by 29%, whereas CrMP reduced it by 43%. These data show flow-induced dilation can be impaired by a HO product, and that the impairment was not produced by biliverdin but is mimicked by CO. These results suggest that the HO-generated CO attenuates flow-induced dilation in the vasculature and, accordingly, may contribute to vascular dysfunction after injury.

Dependence of acetylcholine and ADP dilation of pial arterioles on heme oxygenase after transfusion of cell-free polymeric hemoglobin

Polymers of cell-free hemoglobin have been designed for clinical use as oxygen carriers, but limited information is available regarding their effects on vascular regulation. We tested the hypothesis that the contribution of heme oxygenase (HO) to acetylcholine-evoked dilation of pial arterioles is upregulated 2 days after polymeric hemoglobin transfusion. Dilator responses to acetylcholine measured by intravital microscopy in anesthetized cats were blocked by superfusion of the HO inhibitor tin protoporphyrin-IX (SnPPIX) in a group that had undergone exchange transfusion with hemoglobin 2 days earlier but not in surgical sham and albumin-transfused groups. However, immunoblots from cortical brain homogenates did not reveal changes in expression of the inducible isoform HO1 or the constitutive isoform HO2 in the hemoglobin-transfused group. To test whether the inhibitory effect of SnPPIX was present acutely after hemoglobin transfusion, responses were measured within an hour of completion of the exchange transfusion. In control and albumin-transfused groups, acetylcholine responses were unaffected by SnPPIX but were blocked by addition of the nitric oxide synthase inhibitor N(omega)-nitro-l-arginine (l-NNA) to the superfusate. In hemoglobin-transfused groups, the acetylcholine response was blocked by either SnPPIX or l-NNA alone. The effect of another HO inhibitor, chromium mesoporphyrin (CrMP), was tested on ADP, another endothelial-dependent dilator, in anesthetized rats. Pial arteriolar dilation to ADP was unaffected by CrMP in controls but was attenuated 62% by CrMP in rats transfused with hemoglobin. It is concluded that 1) polymeric hemoglobin transfusion acutely upregulates the contribution of HO to acetylcholine-induced dilation of pial arterioles in cats, 2) this upregulation persists 2 days after transfusion when 95% of the hemoglobin is cleared from the circulation, and 3) this acute upregulation of HO signaling is ubiquitous in that similar effects were observed with a different endothelial-dependent agonist (i.e., ADP) in a another species (rat).

Metabolic syndrome increases endogenous carbon monoxide production to promote hypertension and endothelial dysfunction in obese Zucker rats

Vascular heme oxygenase (HO) metabolizes heme to form carbon monoxide (CO). Increased heme-derived CO inhibits nitric oxide synthase and can contribute to hypertension via endothelial dysfunction in Dahl salt-sensitive rats. Obese Zucker rats (ZR) are models of metabolic syndrome. This study tests the hypothesis that endogenous CO formation is increased and contributes to hypertension and endothelial dysfunction in obese ZR. Awake obese ZR showed increased respiratory CO excretion, which was lowered by HO inhibitor administration [zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG) 25 micromol.kg(-1).24 h(-1) ip]. In awake obese ZR, chronically instrumented with femoral arterial catheters, blood pressure was elevated but was decreased by the HO inhibitor ZnDPBG. Body weight, blood glucose, glycated hemoglobin, plasma insulin, total and LDL cholesterol, oxidized LDL, and triglyceride levels were elevated in obese ZR, and, except for LDL cholesterol, were unchanged by HO inhibition. Total HO-1 protein levels were not different between lean and obese ZR aortas. In vitro experiments used isolated skeletal muscle arterioles with constant pressure and no flow, or constant midpoint, but altered endpoint pressures to establish graded levels of luminal flow. In obese ZR arterioles, responses to ACh and flow were attenuated. Acute in vitro pretreatment with an HO inhibitor, chromium mesoporphyrin, enhanced ACh and flow-induced dilation and abolished the differences between groups. Furthermore, exogenous CO prevented the restoration of flow-induced dilation by the HO inhibitor in obese ZR arterioles. These results suggest that HO-derived CO production is increased and promotes hypertension and arteriolar endothelial dysfunction in obese ZR with metabolic syndrome independent of affecting metabolic parameters.

Heme oxygenase-derived carbon monoxide promotes arteriolar endothelial dysfunction and contributes to salt-induced hypertension in Dahl salt-sensitive rats

Vascular tissues express heme oxygenase (HO), which metabolizes heme to form carbon monoxide (CO). Heme-derived CO inhibits nitric oxide synthase and promotes endothelium-dependent vasoconstriction. After 4 wk of high-salt diet, Dahl salt-sensitive (Dahl-S) rats display hypertension, increased vascular HO-1 expression, and attenuated vasodilator responses to ACh that can be completely restored by acute treatment with an inhibitor of HO. In this study, we examined the temporal development of HO-mediated endothelial dysfunction in isolated pressurized first-order gracilis muscle arterioles, identified the HO product responsible, and studied the blood pressure effects of HO inhibition in Dahl-S rats on a high-salt diet. Male Dahl-S rats (5–6 wk) were placed on high-salt (8% NaCl) or low-salt (0.3% NaCl) diets for 0–4 wk. Blood pressure increased gradually, and responses to an endothelium-dependent vasodilator, ACh, decreased gradually with the length of high-salt diet. Flow-induced dilation was abolished in hypertensive Dahl-S rats. Acute in vitro pretreatment with an inhibitor of HO, chromium mesoporphyrin (CrMP), restored endothelium-dependent vasodilation and abolished the differences between groups. The HO product CO prevented the restoration of endothelium-dependent dilation by CrMP. Furthermore, administration of an HO inhibitor lowered blood pressure in Dahl-S rats with salt-induced hypertension but did not do so in low-salt control rats. These results suggest that hypertension and HO-mediated endothelial dysfunction develop gradually and simultaneously in Dahl-S rats on high-salt diets. They also suggest that HO-derived CO underlies the impaired endothelial dysfunction and contributes to hypertension in Dahl-S rats on high-salt diets.

Heme oxygenase-mediated endothelial dysfunction in DOCA-salt, but not in spontaneously hypertensive, rat arterioles

Vascular heme oxygenase (HO) metabolizes heme to form carbon monoxide. Carbon monoxide inhibits nitric oxide synthase and promotes endothelium-dependent vasoconstriction. We reported HO-1-mediated endothelial dysfunction in Dahl salt-sensitive hypertension. Previous studies suggested that salt-sensitive hypertensive rats, but not spontaneously hypertensive rats (SHR), display endothelial dysfunction. This study examines the hypothesis that HO-1-mediated arteriolar endothelial dysfunction develops in deoxycorticosterone acetate (DOCA)-salt hypertensive (DOCA) rats, but not in SHR. Uninephrectomized (isoflurane anesthesia) male Sprague-Dawley rats received DOCA injections and saline drinking solution for 4 wk. Rats subjected to sham surgery received vehicle injections and tap water. Blood pressure was elevated in DOCA rats and SHR compared with sham and Wistar-Kyoto (WKY) groups. Aortic HO-1 expression and blood carboxyhemoglobin levels were elevated in the DOCA group, but not in SHR. In isolated gracilis muscle arterioles, ACh caused concentration-related vasodilation in all groups, with attenuated maximum responses in DOCA, but not in SHR, arterioles. Acute pretreatment with an inhibitor of HO, chromium mesoporphyrin, restored ACh-induced responses in DOCA arterioles to sham levels. ACh responses remained the same in SHR and WKY arterioles after chromium mesoporphyrin treatment. These data show that HO-1 levels and activity are increased and arteriolar responses to ACh are decreased in DOCA rats, but not in SHR. Furthermore, in DOCA arterioles, an inhibitor of HO restores ACh-induced vasodilation to sham levels. These results suggest that elevated HO-1 levels and activity, not resulting from hypertension per se, contribute to endothelial dysfunction in DOCA rats.

The role of carbon monoxide in the gastrointestinal tract

Carbon monoxide (CO) is a biologically active product of haem metabolism that contributes to the normal physiology of the gastrointestinal tract. In this article, we review recent data showing that CO is an integral regulator of gastrointestinal motility and an important factor in the response to gastrointestinal injury. CO is generated by haem oxygenase-2 (HO-2), which is constitutively expressed in many inhibitory neurones of the vertebrate enteric nervous system. The membrane potential gradients along and across the muscle layers of the gastrointestinal tract require the generation of CO by haem oxygenase-2. The presence of CO is also necessary for normal inhibitory neurotransmission in circular smooth muscle and appears to permit nitric oxide-mediated inhibitory neurotransmission. Genetic deletion of the haem oxygenase-2 gene in mice slows gut transit. The other major CO synthetic enzyme, haem oxygenase-1 (HO-1) is induced under conditions of stress or injury. Recent studies have demonstrated that up-regulation of haem oxygenase-1 protects the gut from several types of gastrointestinal injury, suggesting that CO or induction of HO-1 may find therapeutic use in gastrointestinal diseases and injuries. Furthermore, it is anticipated that the understanding of CO-mediated signalling in the gastrointestinal tract will inform studies in other tissues that express haem oxygenases.

Role of cGMP in carbon monoxide-induced cerebral vasodilation in piglets

The hypothesis was addressed that CO-induced cerebral vasodilation requires a permissive cGMP signal that can be produced by nitric oxide (NO). Anesthetized piglets were implanted with cranial windows for measurement of pial arteriolar responses to stimuli. Pial arterioles dilated in response to isoproterenol (Iso), sodium nitroprusside (SNP), and CO or the CO-releasing molecule Mn2(CO)10 [dimanganese decacarbonyl (DMDC)]. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a soluble guanylyl cyclase inhibitor, decreased cerebrospinal fluid (CSF) cGMP and selectively inhibited dilations to SNP and DMDC without affecting the dilation to Iso. However, DMDC did not cause an increase in cortical periarachnoid CSF cGMP concentration. cGMP clamp with a threshold dilator level of 8-bromo-cGMP (10(-4) M) and ODQ restored the dilation to DMDC that had been blocked by ODQ alone. Under these conditions, cGMP was present but could not increase. Inhibition of the pial arteriolar dilation to glutamate by N-nitro-l-arginine, which blocks NO synthase, was similar to that by heme oxygenase inhibitors, which block endogenous CO production. The dilation to glutamate, similar to dilation to DMDC, was partially restored by 8-bromo-cGMP and completely restored by SNP (5 x 10(-7) M). These data suggest that the permissive role of NO in CO- and glutamate-induced vasodilation involves maintaining the minimum necessary cellular level of cGMP to allow CO to cause dilation independently of increasing cGMP.

Carbon monoxide as an attenuator of vasoconstriction in piglet cerebral arterioles

Carbon monoxide (CO) is an endogenous dilator in the newborn cerebral circulation. The present study addressed the hypothesis that endogenous CO attenuates pial arteriolar vasoconstriction caused by hypocapnia, platelet activating factor, and elevated blood pressure. Experiments used anesthetized piglets with implanted, closed cranial windows. Topical application of a metal porphyrin inhibitor of heme oxygenase was used to inhibit production of CO. Chromium mesopophyrin increased vasoconstriction in response to hypocapnia. The constrictor response to a topical stimulus, platelet activating factor, was also increased by application of chromium mesoporphyrin. Inhibition of heme oxygenase did not constrict pial arterioles in normotensive newborn pigs (mean arterial pressure of about 70 mmHg), but did constrict pial arterioles of piglets with experimentally induced increases in arterial pressure (mean arterial pressure greater than 90 mmHg). In fact, pial arterioles of normotensive piglets transiently dilated to chromium mesoporphyrin, whereas those of hypertensive piglets progressively constricted during 10 min of chromium mesoporphyrin treatment. Therefore, inhibition of heme oxygenase augments cerebral vasoconstriction in response to several very different constrictor stimuli. These data suggest endogenous CO attenuates vasoconstrictor responses in the newborn cerebral circulation.

Heme oxygenase inhibitor restores arteriolar nitric oxide function in dahl rats

Vascular tissues express heme oxygenase (HO), which metabolizes heme to form carbon monoxide (CO). CO relaxes vascular smooth muscle but inhibits nitric oxide (NO) formation. Decreased NO synthesis may contribute to salt-induced hypertension in Dahl salt-sensitive (DS) rats. The current study examines the hypothesis that elevated levels of endogenous CO contribute to NO dysfunction in salt-induced hypertensive DS rats. Male DS rats were placed on high-salt (8% NaCl, HS) or low-salt (0.3% NaCl, LS) diets for 4 weeks. With respect to the LS group, the HS group's blood pressure and carboxyhemoglobin levels were elevated, and abdominal aortas showed 6-fold higher HO-1 protein levels. Experiments used isolated pressurized first-order gracilis muscle arterioles superfused with oxygenated modified Krebs buffer. An inhibitor of NO synthase, Nomega-nitro-L-arginine methyl ester (L-NAME), caused concentration-dependent vasoconstriction in both groups, with attenuated responses in HS arterioles. HS arterioles also showed attenuated vasodilatory responses to an endothelium-dependent vasodilator, acetylcholine. Acute pretreatment with an inhibitor of HO, chromium mesoporphyrin, enhanced vascular responses to L-NAME and acetylcholine in both groups but abolished the differences between HS and LS arterioles. These data show that HO-1 protein levels and CO production are increased in HS rats. Arteriolar responses to L-NAME and acetylcholine are impaired in HS rats compared with LS animals, and this difference can be abolished by an inhibitor of endogenous CO production. These results suggest that elevated levels of endogenous CO contribute to arteriolar NO dysfunction in DS rats with salt-induced hypertension.

Interstitial Cells in the Musculature of the Gastrointestinal Tract: Cajal and Beyond

Expression of the receptor tyrosine kinase KIT on cells referred to as interstitial cells of Cajal (ICC) has been instrumental during the past decade in the tremendous interest in cells in the interstitium of the smooth muscle layers of the digestive tract. ICC generate the pacemaker component (electrical slow waves of depolarization) of the smooth musculature and are involved in neurotransmission. By integration of ICC functions, substantial progress has been made in our understanding of the neuromuscular control of gastrointestinal motility, opening novel therapeutic perspectives. In this article, the ultrastructure and light microscopic morphology, as well as the functions and the development of ICC and of neighboring fibroblast-like cells (FLC), are critically reviewed. Directions for future research are considered and a unifying concept of mesenchymal cells, either KIT positive (the "ICC") or KIT negative "non-Cajal" (including the FLC and possibly also other cell types) cell types in the interstitium of the smooth musculature of the gastrointestinal tract, is proposed. Furthermore, evidence is accumulating to suggest that, as postulated by Santiago Ramon y Cajal, the concept of interstitial cells is not likely to be restricted to the gastrointestinal musculature.

Role of heme oxygenases in sepsis-induced diaphragmatic contractile dysfunction and oxidative stress

Heme oxygenases (HOs), essential enzymes for heme metabolism, play an important role in the defense against oxidative stress. In this study, we evaluated the expression and functional significance of HO-1 and HO-2 in the ventilatory muscles of normal rats and rats injected with bacterial lipopolysaccharide (LPS). Both HO-1 and HO-2 proteins were detected inside ventilatory and limb muscle fibers of normal rats. Diaphragmatic HO-1 and HO-2 expressions rose significantly within 1 and 12 h of LPS injection, respectively. Inhibition of the activity of inducible nitric oxide synthase (iNOS) in rats and absence of this isoform in iNOS(-/-) mice did alter sepsis-induced regulation of muscle HOs. Systemic inhibition of HO activity with chromium mesoporphyrin IX enhanced muscle protein oxidation and hydroxynonenal formation in both normal and septic rats. Moreover, in vitro diaphragmatic force generation declined substantially in response to HO inhibition both in normal and septic rats. We conclude that both HO-1 and HO-2 proteins play an important role in the regulation of muscle contractility and in the defense against sepsis-induced oxidative stress.

Cerebral vascular endothelial heme oxygenase: expression, localization, and activation by glutamate

Endogenous carbon monoxide (CO) contributes to vasodilator responses of cerebral microvessels in newborn pigs. We investigated the expression, intracellular localization, and activity of heme oxygenase (HO), the key enzyme in CO production, in quiescent cerebral microvascular endothelial cells (CMVEC) from newborn pigs. HO-1 and HO-2 isoforms were detected by RT-PCR, immunoblotting, and immunofluorescence. HO-1 and HO-2 are membrane-bound proteins that have a strong preference for the nuclear envelope and perinuclear area of the cytoplasm. Betamethasone (10(-6) to 10(-4) M for 48 h) was associated with upregulation of HO-2 protein by approximately 50% and inhibition of Cox-2 but did not alter HO-1 or endothelial nitric oxide synthase expression in CMVEC. In vivo betamethasone treatment of newborn pigs (0.2 and 5.0 mg/kg im for 48 h) upregulated HO-2 in cerebral microvessels by 30-60%. HO activity as (14)CO production from [(14)C]glycine-labeled endogenous heme was inhibited by chromium mesoporphyrin (10(-6) to 10(-4) M). L-Glutamate (0.3-1.0 mM) stimulated HO activity 1.5-fold. High-affinity specific binding sites for L-[(3)H]glutamate suggestive of the glutamate receptors were detected in CMVEC. Altogether, these data suggest that, in cerebral circulation of newborn pigs, endothelium-derived CO may contribute to basal vascular tone and to responses that involve glutamate receptor activation.

Stereologic description of the changing expression of constitutive nitric oxide synthase and heme oxygenase in the enteric plexuses of the pig small intestine during development

The similarities between heme oxygenase-2 (HO-2) and nitric oxide synthase (nNOS) and the transient expression of nNOS during development led us to investigate whether both systems are similarly affected by changes that occur during development and by regional differences along the small intestine. By combining NADPH diaphorase histochemistry and HO-2 immunohistochemistry on whole-mount preparations and by using stereologic methods, a qualitative and quantitative description of HO-2 and nNOS expression was obtained. Examinations were carried out on the small intestine of fetal, 1-2-day and 5-6-week-old pigs. In all age groups, three enteric plexuses were distinguished. The presence of HO-2-immunoreactive (HO-2-IR) and NADPH diaphorase-positive neurons corresponded to earlier morphological and physiological reports. Nevertheless, the total number of nitrergic neurons remained constant or decreased in the enteric plexuses, whereas the total number of HO-2-IR neurons displayed an overall increase. Changing concentrations of glucocorticoids, target-derived signals, presynaptic input, and an effect of HO-2 activity on nNOS synthesis are likely to play roles in the observed developmental changes. The numerical density of HO-2-IR neurons remained relatively constant along the intestinal tract; in contrast, the nitrergic neurons were most numerous in the inner submucous and myenteric plexus in the duodenum and ileum, respectively. It is believed that the duodenal nitrergic neurons in the inner submucous plexus could be involved in the regulation of duodenal secretion processes, whereas the region-dependent density in the myenteric plexus possibly forms the morphological basis for a regionally different participation of NO in the relaxation of the small intestine.

Developmental changes in heme-oxygenase-2 and bNOS expression in enteric neurons in the pig duodenum

There exists much parallelism between carbon monoxide- and nitric oxide-generating systems. Therefore, we wondered whether developmental and functional differences along the duodenum similarly affect, part of them, namely, heme oxygenase-2-(HO-2) and neural isoform of nitric oxide synthase- (nNOS) expressing neurons. By applying NADPH diaphorase histochemistry and HO-2 immunohistochemistry on whole-mount preparations and by using stereologic methods, a qualitative and quantitative description of HO-2 and nNOS expression was obtained. Examinations were carried out on the duodenum of fetal, neonatal and weaned pigs. At all ages, three enteric plexuses were readily distinguished. The presence of both enzymes fits in with other morphological and physiological reports. However, the expression of both enzymes significantly changed during development. The number of HO-2-IR neurons increased approximately 20-fold in the inner submucous and almost doubled in the myenteric plexus. In addition, the number of nNOS-expressing neurons displayed a significant decrease in the outer submucous plexus after weaning. High levels of glucocorticoids may cause the perinatally increased HO-2 expression, whereas an influence on nNOS expression is doubtful. Therefore, it seems that notwithstanding the high similarity between both systems, their expression is regulated differently in the pig duodenum.

Association of soluble guanylate cyclase with the sarcolemma of mammalian skeletal muscle fibers

Previous investigations have shown that NO-producing nitric oxide synthase (NOS)-1 and CO-generating heme oxygenase (HO-2) are associated with the sarcolemma of skeletal muscle fibers in many mammalian species. Despite numerous roles ascribed to NO and possibly also CO in skeletal muscle, a specific receptor for both gases has hitherto not been found in myofibers. Therefore, in the present work the appearance of the alpha1, beta1 and beta2 subunits of soluble guanylate cyclase (sGC), the most commonly known receptor for NO and potentially also CO, was analysed in mammalian skeletal muscles using immunoblotting and immunohistochemistry. Immunoblotting with an antibody against the beta1 subunit of sGC revealed a band of 70 kDa corresponding to the molecular weight of this protein. Immunohistochemistry with antibodies against the alpha1, beta1 and beta2 sGC subunits showed that the larger part of positivity was present in the sarcolemma region of skeletal muscle fibers and colocalized with NOS-1 mainly in type II myofibers and with HO-2 in type I and type II myofibers. For the first time, sarcolemmal association of sGC and its colocalization with NOS-1 generating the sGC-activator NO and with HO-2 producing the potential sGC upregulator CO have been demonstrated in the present study. These results enable a better understanding of the role of NO and CO in myofibers and suggest a so far unknown molecular mechanism for the interaction of sGC with the sarcolemma.

Haem oxygenase in enteric nervous system of human stomach and jejunum and co-localization with nitric oxide synthase

Recent evidence suggests that carbon monoxide (CO) may be a neurotransmitter, similar to nitric oxide (NO) in the enteric nervous system. The distribution of haem oxygenase (HO), the biosynthetic enzyme for CO, has been determined in the enteric nervous system of animals, but little is known about the distribution of HO in human gastrointestinal tract. The present study investigated the expression of HO and its colocalization with NO synthase (NOS), the biosynthetic enzyme for NO, in human antrum and jejunum. HO isoforms were identified using immunohistochemistry and NOS was identified by immunohistochemistry or NADPH-d histochemistry. HO-2 immunoreactive (IR) cell bodies in enteric ganglia and nerve fibres in longitudinal and circular muscle were found in both antrum and jejunum. Co-localization of HO-2 and NOS was about 40% in HO-2 containing cell bodies of myenteric ganglia and only 10% or less in cell bodies of submucous ganglia. HO-1 immunoreactivity was not detected in antrum or jejunum. The results suggest that CO is produced in human enteric ganglion neurones and indicate a possible role of CO as a neurotransmitter and possible interaction between HO and NOS pathways in inhibitory neurotransmission in the human gastrointestinal tract.

Heme oxygenase-2 is present in the sarcolemma region of skeletal muscle fibers and is non-continuously co-localized with nitric oxide synthase-1

There is increasing evidence that the heme oxygenase-2 (HO-2)/carbon monoxide (CO) pathway and the nitric oxide synthase (NOS)/nitric oxide (NO) pathway functionally cross-talk. Therefore, we investigated the appearance of HO-2 in mammalian skeletal muscles where NOS-1 is known to be expressed in high quantities. Immunoblotting of rat hind limb extensor muscles extracts revealed a single 36 kDa band demonstrating the existence of HO-2 in skeletal muscle and indicating the monospecifity of the antibody that was applied. Immunohistochemistry on healthy rat extensor hind limb muscles showed that HO-2 is present in satellite cells, endothelial cells of the vascular system, fibrocytes/fibroblasts but also fiber type-independently in extrafusal myofibers either in association with the non-junctional sarcolemma region, or in a subsarcolemmal network or, less prominently, in cross-striated stripes connected to longitudinally running lines. Combined HO-2 immunohistochemistry and NOS-1 histochemistry revealed an apparent co-localization of both molecules only in the non-junctional sarcolemma region of extrafusal type II myofibers outside costameres. In diseased muscles of mdx mice, HO-2 expression was not changed. In patients suffering from Duchenne's muscular dystrophy, it was absent in the sarcolemma region. In conclusion, the HO-2/CO system is present in mammalian skeletal muscle where it is non-continuously co-localized with the NOS-1/NO-system. This finding implicates an optionally functional cross-talk between both gaseous signaling pathways.

Inhaled carbon monoxide does not cause pulmonary vasodilation in the late-gestation fetal lamb

As observed with nitric oxide (NO), carbon monoxide (CO) binds and may activate soluble guanylate cyclase and increase cGMP levels in smooth muscle cells in vitro. Because inhaled NO (I(NO)) causes potent and sustained pulmonary vasodilation, we hypothesized that inhaled CO (I(CO)) may have similar effects on the perinatal lung. To determine whether I(CO) can lower pulmonary vascular resistance (PVR) during the perinatal period, we studied the effects of I(CO) on late-gestation fetal lambs. Catheters were placed in the main pulmonary artery, left pulmonary artery (LPA), aorta, and left atrium to measure pressure. An ultrasonic flow transducer was placed on the LPA to measure blood flow to the left lung. After baseline measurements, fetal lambs were mechanically ventilated with a hypoxic gas mixture (inspired O(2) fraction < 0.10) to maintain a constant fetal arterial PO(2). After 60 min (baseline), the lambs were treated with I(CO) [5-2,500 parts/million (ppm)]. Comparisons were made with I(NO) (5 and 20 ppm) and combined I(NO) (5 ppm) and I(CO) (100 and 2,500 ppm). We found that I(CO) did not alter left lung blood flow or PVR at any of the study doses. In contrast, low-dose I(NO) decreased PVR by 47% (P < 0.005). The combination of I(NO) and I(CO) did not enhance the vasodilator response to I(NO). To determine whether endogenous CO contributes to vascular tone in the fetal lung, zinc protoporphyrin IX, an inhibitor of heme oxygenase, was infused into the LPA in three lambs. Zinc protoporphyrin IX had no effect on baseline PVR, aortic pressure, or the pressure gradient across the ductus arteriosus. We conclude that I(CO) does not cause vasodilation in the near-term ovine transitional circulation, and endogenous CO does not contribute significantly to baseline pulmonary vascular tone or ductus arteriosus tone in the late-gestation ovine fetus.

Immunohistochemical detection of heme oxygenase-2 in the periodontal Ruffini ending of the rat incisor

The present study was carried out to examine the occurrence of heme oxygenase-2 (HO-2) in the periodontal ligament of the rat incisor. HO-2-like immunoreactive (-IR) structures showed dendritic profiles, resembling the Ruffini endings, in the alveolar half of the ligament of rat incisor. Neither thin nerve fibers nor perivascular nerve fibers displayed HO-2-like immunoreactivity (-LI). No non-neural elements exhibited HO-2-LI. Electron microscopy revealed that immunoreactions were diffusely observed in the axon terminals of the Ruffini endings, but neither terminal Schwann cells nor Schwann sheaths contained immunoreactions for HO-2. Both most neurons in the trigeminal ganglion and trigeminal mesencephalic nucleus showed HO-2-LI. The presence of HO-2 in the periodontal Ruffini endings and its absence in the periodontal thin nerve fibers suggest the involvement of carbon monoxide produced by HO-2 in mechanoreception in the periodontal ligament.

Heme oxygenase-2 distribution in anorectum: colocalization with neuronal nitric oxide synthase

Recent investigations have suggested carbon monoxide (CO) as a putative messenger molecule. Although several studies have implicated the heme oxygenase (HO) pathway, responsible for the endogenous production of CO, in the neuromodulatory control of the internal anal sphincter (IAS), its exact role is not known. Nitric oxide, produced by neuronal nitric oxide synthase (nNOS) of myenteric neurons, is an important inhibitory neural messenger molecule mediating nonadrenergic noncholinergic (NANC) relaxation of the IAS. The present studies were undertaken to investigate in detail the presence and coexistence of heme oxygenase-2 (HO-2) with nNOS in the opossum anorectum. In perfusion-fixed, frozen-sectioned tissue, HO-2 immunoreactive (IR) and nNOS IR nerves were identified using immunocytochemistry. Ganglia containing HO-2 IR neuronal cell bodies were present in the myenteric and submucosal plexuses throughout the entire anorectum. Colocalization of HO-2 IR and nNOS IR was nearly 100% in the IAS and decreased proximally from the anal verge. In the rectum, colocalization of HO-2 IR and nNOS IR was approximately 70%. Additional confocal microscopy studies using c-Kit staining demonstrated the localization of HO-2 IR and nNOS IR in interstitial cells of Cajal (ICC) of the anorectum. From the high rate of colocalization of HO-2 IR and nNOS IR in the IAS as well as the localization of HO-2 IR and nNOS IR in ICC in conjunction with earlier studies of the HO pathway, we speculate an interaction between HO and NOS pathways in the NANC inhibitory neurotransmission of the IAS and rectum.

Comparative localization of heme oxygenase-2 and nitric oxide synthase in the autonomic innervation to the human ductus deferens and seminal vesicle

PURPOSE

The aim of present study was to determine the topographic relationship between heme oxygenase-2 (HO-2), which synthesizes carbon monoxide (CO), and neuronal nitric oxide synthase (nNOS), which generates nitric oxide (NO), in the autonomic nerves of the human ductus deferens and seminal vesicle.

MATERIALS AND METHODS

Specimens of the ductus deferens and seminal vesicle were obtained during cancer surgery or vasectomy. HO-2 and nNOS were localized by indirect immunofluorescence. Additionally, the histochemical NADPH-diaphorase (NADPH-d) activity of NOS was demonstrated using a standard staining method and some modifications.

RESULTS

Anti-HO-2 labeling stained virtually all nerve cell bodies in local ganglia of the pelvic plexus, which is composed of a mixed population of postganglionic sympathetic and parasympathetic neurons supplying the pelvic viscera. Furthermore, nerve cell bodies in the wall of the seminal vesicle, which are considered an extension of the pelvic plexus, were also found to stain positively for HO-2. Some of the HO-2-immunoreactive ganglion cells were also nNOS-positive, their proportion varying between individual ganglia but generally not exceeding 20%. Both enzymes were present in large adventitial nerve trunks. Only nNOS but no HO-2 was found in small intramuscular and mucosal nerve fibers. In both the ductus deferens and seminal vesicle, the highest density of nNOS-containing nerve fibers was in the lamina propria of the mucosa. A well-developed plexus of nNOS-positive nerve fibers was also observed in the muscular layer of the seminal vesicle. By contrast, there was a very sparse innervation by nNOS-positive nerve fibers in the muscle coat of the ductus deferens. In addition, a population of epithelial cells in the seminal vesicle may contain an isoform of NOS, as revealed by a resistant NADPH-d activity.

CONCLUSIONS

These findings set the scene for functional studies which will hopefully clarify the biological role of CO and NO in the control of the ductus deferens and seminal vesicle.

Heme oxygenase-2 expression at rat neuromuscular junctions

The neuromuscular junction is specialized for rapid transmission of electrical signals. Nitric oxide synthase (NOS) is concentrated at the junction, and NO modulates transmission and could influence signaling pathways. Increasing evidence suggests that carbon monoxide (CO) serves as a neurotransmitter, and heme oxygenase (HO), the enzyme that catalyzes the formation of CO, is often colocalized with NOS. Immunoreactivity for HO-2 was present at rat neuromuscular junctions of leg muscles and persisted in denervated muscle indicating the localization of the enzyme to the postsynaptic surface. In contrast, HO-2 immunoreactivity was absent from the en grappe and orbital en plaque endplates of extraocular muscle (EOM), while only the global en plaque endplates possessed HO-2 immunoreactivity. The difference between EOM and leg endplates may arise from EOM's unique physiology. The presence of HO-2 at neuromuscular junctions suggests CO could serve as a pre- and post-synaptic messenger.

Hemoxygenase and nitric oxide synthase do not maintain human uterine quiescence during pregnancy

The nitric oxide (NO)-cGMP pathway has been proposed as a mechanism for relaxation of myometrium during pregnancy and as a modulator of labor. Carbon monoxide (CO), produced by hemeoxygenases (HO-1 and HO-2), also activates soluble guanylate cyclase to increase cGMP. A recent study reported a large increase in HO-1 and HO-2 proteins during pregnancy, suggesting that the HO-CO pathway may be important in the maintenance of uterine quiescence during pregnancy. In this study we used Western blotting, reverse transcription-polymerase chain reaction, and immunohistochemistry to determine HO-1 and HO-2 expression in nonpregnant, pregnant, and laboring myometrium. Immunolocalization of HO was also compared with endothelial and inducible nitric oxide synthases (eNOS and iNOS). In contrast to HO-1 protein, which was not detected in myometrium, HO-2 protein and mRNA were constitutively expressed, although there were no differences in expression between the groups. eNOS was expressed in endothelial cells but not in myometrial smooth muscle. iNOS protein was not detected in myometrium. These data do not support an up-regulation of HO-1 and HO-2 during pregnancy and are not consistent with a role for NO or a major role for CO in human myometrial quiescence. Our results are also in keeping with HO-2 being an noninducible protein.

Heme oxygenase immunoreactive neurons in the rat intestine and their relationship to nitrergic neurons

BACKGROUND

Carbon monoxide (CO), like nitric oxide (NO), is a putative gaseous neurotransmitter. CO is produced by the enzyme heme oxygenase (HO) acting on a family of heme-containing compounds. Two isomers of HO have been characterized (HO-1, HO-2). In the CNS and in peripheral ganglia HO-2 occurs in a majority of neurons. NO and CO function as transmitters of enteric neurons but the relative distribution of enteric neurons utilizing these gaseous transmitters is unknown in rodent. We have studied the distribution of HO-2 immunoreactivity and NO synthase (NOS) activity within the rat ileum.

METHODS

Tissue sections and primary neuronal cell cultures were incubated with a HO-2 specific antibody, and then assessed or reprocessed for NOS activity using NADPH-dependent diaphorase staining.

RESULTS

HO-2 immunoreactivity was expressed in subpopulations of myenteric and submucosal neurons. Approximately 45% of the ganglion cells in tissue section were HO-2 positive. This was similar in proportion to those found to stain for NOS activity, and 10% of HO-2 positive neurons also contained NOS. HO-2 immunoreactivity was also found in epithelial cells within the villi, and in interstitial cells around the myenteric plexus and within the smooth muscle. In culture, the distribution and colocalisation of HO-2 and NOS positive neurons was similar to that in tissue sections. We identified labelled neurons as either Dogiel Type I or II; only Type II cells colocalized NOS and HO-2.

CONCLUSION

Neurons, endocrine-like cells and interstitial cells with the capacity for CO production are distributed throughout the ileum and some neurons have the capacity to synthesize both NO and CO as gaseous messengers.

Contribution of endogenous carbon monoxide to regulation of diameter in resistance vessels

Endogenous carbon monoxide was proposed to subserve vasodepressor functions. If so, inhibition of heme oxygenase may be expected to promote vascular contraction. This hypothesis was examined in large and small arteries and in isolated first-order gracilis muscle arterioles of rat. The heme oxygenase inhibitors chromium mesoporphyrin (CrMP) and cobalt protoporphyrin (0.175-102 micromol/l) decreased the diameter of pressurized (80 mmHg) gracilis muscle arterioles, whereas magnesium protoporphyrin, a weak heme oxygenase inhibitor, did not. CrMP also elicited development of isometric tension in the muscular branch of the femoral artery but not in the aorta or femoral artery. Arteriolar constrictor responses to CrMP varied in relation to the intravascular pressure, were blunted in preparations exposed to exogenous carbon monoxide (100 micromol/l), and were unaffected by an endothelin receptor antagonist. Importantly, CrMP amplified the constrictor response to increases of pressure in gracilis arterioles. Accordingly, the constrictor effect of heme oxygenase inhibitors is attributable to magnification of myogenic tone due to withdrawal of a vasodilatory mechanism mediated by endogenous carbon monoxide. The study suggests that the vascular carbon monoxide system plays a role in the regulation of basal tone in resistance vessels.

Carbon monoxide: from toxin to endogenous modulator of cardiovascular functions

Carbon monoxide (CO) is a pollutant commonly recognized for its toxicological attributes, including CNS and cardiovascular effects. But CO is also formed endogenously in mammalian tissues. Endogenously formed CO normally arises from heme degradation in a reaction catalyzed by heme oxygenase. While inhibitors of endogenous CO production can raise arterial pressure, heme loading can enhance CO production and lead to vasodepression. Both central and peripheral tissues possess heme oxygenases and generate CO from heme, but the inability of heme substrate to cross the blood brain barrier suggests the CNS heme-heme oxygenase-CO system may be independent of the periphery. In the CNS, CO apparently acts in the nucleus tractus solitarii (NTS) promoting changes in glutamatergic neurotransmission and lowering blood pressure. At the periphery, the heme-heme oxygenase-CO system can affect cardiovascular functions in a two-fold manner; specifically: 1) heme-derived CO generated within vascular smooth muscle (VSM) can promote vasodilation, but 2) its actions on the endothelium apparently can promote vasoconstriction. Thus, it seems reasonable that the CNS-, VSM- and endothelial-dependent actions of the heme-heme oxygenase-CO system may all affect cardiac output and vascular resistance, and subsequently blood pressure.

Regulation of heme oxygenase-2 by glucocorticoids in neonatal rat brain: characterization of a functional glucocorticoid response element

Heme oxygenase-2 (HO-2) is constitutively expressed in mammalian tissues; together with HO-1 (HSP32) it catalyzes the cleavage of heme to produce biliverdin IX alpha, CO and Fe. Detection of a consensus sequence of the glucocorticoid response element (GRE) in the promoter region of the HO-2 gene prompted the present study which has investigated the role of glucocorticoids (Gcs) in the regulation of HO-2 protein and transcript development in the newborn rat brain and has examined the promoter activity of the GRE in HeLa cells. Using in situ hybridization histochemistry, we noted a pronounced increase in signal for HO-2 mRNA in the brain of 14-day-old rats postnatally treated with corticosterone (5 microg/g, 4 x, starting 24-36 h after birth). And, using immunohistochemistry, a striking increase in neuronal HO-2 immunostaining in treated brains was detected. The HO-2 GRE was tested for responsiveness to dexamethasone (DX) using both a promoterless CAT expression vector, and a heterologous promoter containing luciferase expression vector in HeLa cells. The HO-2 promoter containing the GRE and transcription start site induced CAT reporter gene activity in response to DX, whereas mutation or deletion in the GRE abolished hormone responsiveness. Similarly, constructs containing the GRE conferred responsiveness to DX in an orientation-independent manner and increased relative luciferase activity. Further, specific binding of glucocorticoid receptor protein to the GRE was observed; binding could be competed out only by excess cold GRE and not by mutated HO-2 GRE, or AP1. HO-2 mRNAs (approximately 1.3 and approximately 1.9 kb) increased in HeLa cells treated with DX (5 microM), the level reached a maximum at 24 h. DX did not effect HO-1 mRNA level. The increase in the HO-2 transcript was accompanied by an increase in HO-2 protein, as assessed by Western blot analysis, and an increase in HO activity, as measured by bilirubin formation. Also, an increase in intensity of immunostaining was noted in DX-treated HeLa cells. We conclude that the GRE present in the HO-2 gene promoter region is functional, and propose the direct involvement of the adrenal glucocorticoids in modulation of HO-2 gene expression. In the context of biological functions of heme degradation products, we suggest that this regulation may be of significance, particularly to the neurons.

The heme oxygenase system: a regulator of second messenger gases

The heme oxygenase (HO) system consists of two forms identified to date: the oxidative stress-inducible protein HO-1 (HSP32) and the constitutive isozyme HO-2. These proteins, which are different gene products, have little in common in primary structure, regulation, or tissue distribution. Both, however, catalyze oxidation of heme to biologically active molecules: iron, a gene regulator; biliverdin, an antioxidant; and carbon monoxide, a heme ligand. Finding the impressive heme-degrading activity of brain led to the suggestion that "HO in brain has functions aside from heme degradation" and to subsequent exploration of carbon monoxide as a promising and potentially significant messenger molecule. There is much parallelism between the biological actions and functions of the CO- and NO-generating systems; and their regulation is intimately linked. This review highlights the current information on molecular and biochemical properties of HO-1 and HO-2 and addresses the possible mechanisms for mutual regulatory interactions between the CO- and NO-generating systems.