Induction of heme oxygenase produces load-independent cardioprotective effects in hypertensive rats

The contribution of carbon monoxide to vascular tonus

OBJECTIVE

The aim of this descriptive study was to examine the contribution of CO in the maintenance of vascular tonus in different organs and different vessel segments; the underlying mechanism of CO-induced vasodilation was investigated.

METHODS

Sixty Wistar albino rats, aged 6-8 months, were used in this study. Response to CO by isolated arteries from the thoracic and abdominal aorta and mesenteric, renal, gastrocnemius, and gracilis muscles as well as heart, lung, and brain vascular beds was endogenously and exogenously studied using organ baths or myograph. In addition, HO-2 protein expression was assessed using Western blot analysis in isolated vessel segments.

RESULTS

Although CO was shown to contribute to the regulation of vascular tonus in all feed arteries except those of the gracilis vascular bed, no effect was observed in the resistance arteries, with the sole exception of the pial artery. No relationship between HO-2 protein level and CO contribution to endogenous vascular tonus was observed.

CONCLUSIONS

While the vasodilator effect of CO in vessels smaller than 600 μm in diameter was found to be mediated via potassium channels, in vessels larger than 600 μm in diameter, the effect was through both the potassium channels and the cGMP pathway.

Potential therapeutic effects of natural heme oxygenase-1 inducers in cardiovascular diseases

SIGNIFICANCE

Many physiological effects of natural antioxidants, their extracts or their major active components, have been reported in recent decades. Most of these compounds are characterized by a phenolic structure, similar to that of α-tocopherol, and present antioxidant properties that have been demonstrated both in vitro and in vivo. Polyphenols may increase the capacity of endogenous antioxidant defenses and modulate the cellular redox state. Such effects may have wide-ranging consequences for cellular growth and differentiation.

CRITICAL ISSUES

The majority of in vitro and in vivo studies conducted so far have attributed the protective effect of bioactive polyphenols to their chemical reactivity toward free radicals and their capacity to prevent the oxidation of important intracellular components. One possible protective molecular mechanism of polyphenols is nuclear factor erythroid 2-related factor (Nrf2) activation, which in turn regulates a number of detoxification enzymes.

RECENT ADVANCES

Among the latter, the heme oxygenase-1 (HO-1) pathway is likely to contribute to the established and powerful antioxidant/anti-inflammatory properties of polyphenols. In this context, it is interesting to note that induction of HO-1 expression by means of natural compounds contributes to prevention of cardiovascular diseases in various experimental models.

FUTURE DIRECTIONS

The focus of this review is on the role of natural HO-1 inducers as a potential therapeutic strategy to protect the cardiovascular system against various stressors in several pathological conditions.

Role of haeme oxygenase-1 in resolution of oxidative stress-related pathologies: focus on cardiovascular, lung, neurological and kidney disorders

The present review examines the role of the cytoprotective enzyme haeme oxygenase-1 (HO-1) in adaptive responses to inflammatory disease and explores strategies for its clinical use, with particular emphasis on use of therapeutic use of the enzyme using phytochemical inducers of HO-1 such as extracts of Ginkgo biloba, curcumin, and flavonoids extracted from seeds of the sour cherry (Prunus cerasus). This laboratory has identified strategies by which combinations of dietary phytochemicals may be configured to synergistically strengthen immunoregulatory mechanisms that normally prevent inflammation from leading to disease. A major focus of this research initiative has been HO-1, which is capable of substantially reducing oxidative stress by several mechanisms. HO-1 metabolizes haeme that accumulates in tissues because of red blood cell turnover. Two products of this degradation - carbon monoxide (CO) and bilirubin - have potent capacity for reducing oxidative stress and for counteracting its effects. A description will be provided of how HO-1 products maintain healthy tissue function and remediate oxidative tissue damage. This will be explored in four major organ systems, including the cardiovascular system, the lungs, the central nervous system and the kidneys. Particular focus will be given to the physiological coordination of cardiovascular functions mediated by CO produced by HO-1 and to nitric oxide (NO), a gaseous second messenger expressed by nitric oxide synthetase. A major unifying theme of the present review is an exploration of the potential use of dietary phytochemical formulations as tools for the clinical application of HO-1 in therapeutic reduction of oxidative stressors, with resultant improved treatment of inflammatory pathologies.

Blood pressure and renal blow flow responses in heme oxygenase-2 knockout mice

Heme oxygenase (HO) is the enzyme responsible for the breakdown of heme-generating carbon monoxide (CO) and biliverdin in this process. HO-2 is the constitutively expressed isoform in most tissues, such as the kidney and vasculature. CO generated by HO is believed to be an important vasodilator in the renal circulation along with another gas, nitric oxide (NO). To determine the importance of HO-2 in the regulation of blood pressure and renal blood flow (RBF), we treated HO-2 knockout (KO) mice chronically with either ANG II or N(G)-nitroarginine methyl ester (l-NAME). Basal blood pressures were not different between wild-type (WT), heterozygous (HET), or KO mice and averaged 113 +/- 3 vs. 115 +/- 2 vs. 116 +/- 2 mmHg. Similar increases in blood pressure to chronic ANG II as well as l-NAME treatment were observed in all groups with blood pressures increasing an average of 30 mmHg in response to ANG II and 15 mmHg in response to l-NAME. Basal RBFs were not different between the groups averaging 6.0 +/- 0.5 (n = 6) vs. 4.8 +/- 0.6 (n = 10) vs. 5.8 +/- 0.7 (n = 6) ml*min(-1)*g(-1) kidney weight in WT, HET, and KO mice. HO-2 KO and HET mice exhibited an attenuated decrease in RBF in response to acute administration of ANG II, while no differences were observed with l-NAME. Our data indicate that blood pressure and RBF responses to increased ANG II or inhibition of nitric oxide are not significantly enhanced in HO-2 KO mice.

Exploiting cGMP-based therapies for the prevention of left ventricular hypertrophy: NO* and beyond

Left ventricular hypertrophy (LVH), an increased left ventricular (LV) mass, is common to many cardiovascular disorders, initially developing as an adaptive response to maintain myocardial function. In the longer term, this LV remodelling becomes maladaptive, with progressive decline in LV contractility and diastolic function. Indeed LVH is recognised as an important blood-pressure independent predictor of cardiovascular morbidity and mortality. The clinical efficacy of current treatments for LVH is reduced, however, by their tendency to slow disease progression rather than induce its reversal, and thus the development of new therapies for LVH is paramount. The signalling molecule cyclic guanosine-3',5'-monophosphate (cGMP), well-recognised for its role in regulating vascular tone, is now being increasingly identified as an important anti-hypertrophic mediator. This review is focused on the various means by which cGMP can be stimulated in the heart, such as via the natriuretic peptides, to exert anti-hypertrophic actions. In particular we address the limitations of traditional nitric oxide (NO*) donors in the face of the potential therapeutic advantages offered by novel alternatives; NO* siblings, ligands of the cGMP-generating enzymes, soluble (sGC) and particulate guanylyl cyclases (pGC), and phosphodiesterase inhibitors. Further impact of cGMP within the cardiovascular system is also discussed with a view to representing cGMP-based therapies as innovative pharmacotherapy, alone or concurrent with standard care, for the management of LVH.

Targeting heme oxygenase: therapeutic implications for diseases of the cardiovascular system

Heme oxygenase (HO) is important in attenuating the overall production of reactive oxygen species through its ability to degrade heme and to produce carbon monoxide, biliverdin/bilirubin, and release of free iron. Excess free heme catalyzes the formation of reactive oxygen species, which leads to endothelial cell (EC) dysfunction as seen in numerous pathologic vascular conditions including systemic hypertension and diabetes, as well as in ischemia/reperfusion injury.The up-regulation of HO-1 can be achieved through the use of pharmaceutical agents such as metalloporphyrins and statins. In addition, atrial natriuretic peptide and nitric oxide donors are important modulators of the heme-HO system, either through induction of HO-1 or the increased biologic activity of its products. Gene therapy and gene transfer, including site- and organ-specific targeted gene transfer have become powerful tools for studying the potential role of the 2 isoforms of HO, HO-1/HO-2, in the treatment of cardiovascular disease, as well as diabetes. HO-1 induction by pharmacological agents or the in vitro gene transfer of human HO-1 into ECs increases cell cycle progression and attenuates angiotensin II, tumor necrosis factor-alpha, and heme-mediated DNA damage; administration in vivo corrects blood pressure elevation after angiotensin II exposure. Delivery of human HO-1 to hyperglycemic rats significantly lowers superoxide levels and prevents EC damage and sloughing of vascular EC into the circulation. In addition, administration of human HO-1 to rats in advance of ischemia/reperfusion injury considerably reduces tissue damage.The ability to up-regulate HO-1 either through pharmacological means or through the use of gene therapy may offer therapeutic strategies for the prevention of cardiovascular disease in the future. This review discusses the implications of HO-1 delivery during the early stages of cardiovascular system injury or in early vascular pathology, and suggests that pharmacological agents that regulate HO activity or HO-1 gene delivery itself may become powerful tools for preventing the onset or progression of various cardiovascular diseases.

Pharmacological and clinical aspects of heme oxygenase

This review is intended to stimulate interest in the effect of increased expression of heme oxygenase-1 (HO-1) protein and increased levels of HO activity on normal and pathological states. The HO system includes the heme catabolic pathway, comprising HO and biliverdin reductase, and the products of heme degradation, carbon monoxide (CO), iron, and biliverdin/bilirubin. The role of the HO system in diabetes, inflammation, heart disease, hypertension, neurological disorders, transplantation, endotoxemia and other pathologies is a burgeoning area of research. This review focuses on the clinical potential of increased levels of HO-1 protein and HO activity to ameliorate tissue injury. The use of pharmacological and genetic probes to manipulate HO, leading to new insights into the complex relationship of the HO system with biological and pathological phenomena under investigation, is reviewed. This information is critical in both drug development and the implementation of clinical approaches to moderate and to alleviate the numerous chronic disorders in humans affected by perturbations in the HO system.

Inhibition of heme oxygenase?1 impairs cardiac muscle sensitivity to beta?adrenergic stimulation

Heme oxygenase-1 (HO-1) is the inducible isoform of heme oxygenase and plays a role in defense against cellular stress. The effects of HO-1 on cardiac muscle contractility, however, are unknown.

METHODS

HO-1 was induced by intraperitoneal injection of hemin in rabbits 24 and 48 h before isolating right ventricular papillary muscles for mechanical in vitro analysis at baseline and during stimulation with isoprenalin. Western blotting and activity measurement con.rmed upregulation of HO-1 in ventricular tissue, and immunohistochemical stainings showed localization in the cardiac endothelium.

RESULTS

Baseline mechanical performance of papillary muscles and maximal inotropic response to ISO was not significantly affected by HO-1 induction. Also, the log(EC50) of the ISO concentration-response curve was not affected by HO-1 induction. Inhibition of heme oxygenase with stanneous mesoporphyrin or chromium mesoporphyrin in muscles with induced HO-1, however, shifted the log(EC50) of the ISO concentration-response curve from -6.9 +/- 0.2 to -6.0 +/- 0.2 (p = 0.008).

CONCLUSION

These results indicate that induction of cardiac HO-1 has no direct effect on baseline contractility. Pharmacological inhibition of HO-1 upon induction, however, diminishes cardiac muscle sensitivity to beta-adrenergic stimulation. These results caution against pharmacologically targeting HO-1 when an activated adrenergic system is important for hemodynamic stability.

Heme oxygenase-1 inhibits angiotensin II-induced cardiac hypertrophy in vitro and in vivo

BACKGROUND

Heme oxygenase-1 (HO-1) is a stress-response enzyme implicated in cardioprotection. To explore whether HO-1 has a role in cardiac remodeling response, the effect of its overexpression on angiotensin II (Ang II)-induced cardiac hypertrophy was examined.

METHODS AND RESULTS

HO-1 was induced in cultured rat neonatal cardiomyocytes by treatment with cobalt protoporphyrin IX (CoPPIX) or a recombinant adenovirus carrying the human HO-1 gene. Ang II-induced myocyte hypertrophy assessed by increments in cell size, [3H]leucine uptake, and protein content was suppressed by HO-1 overexpression. Cotreatment of cells with tin protoporphyrin IX, a HO inhibitor, significantly reversed the suppressive effect of HO-1. Bilirubin, one of the byproducts of heme degradation by HO-1, mediated the suppressive effect through the inhibition of Ang II-induced production of reactive oxygen species, as detected by a 2',7'-dichlorofluorescein probe. The antihypertrophic effect of HO-1 was also demonstrated in rats receiving chronic Ang II infusions. Cotreatment of animals with CoPPIX significantly attenuated Ang II-induced left ventricular hypertrophy and hyperdynamic contractions, whereas concomitant treatment with tin protoporphyrin IX abolished CoPPIX-mediated cardioprotection in vivo.

CONCLUSIONS

HO-1 attenuates Ang II-induced cardiac hypertrophy both in vitro and in vivo, and bilirubin mediates, at least in part, the antihypertrophic effect of HO-1 via inhibition of reactive oxygen species production after Ang II stimulation.

Protective effect of SnCl2 on K2Cr2O7-induced nephrotoxicity in rats: the indispensability of HO-1 preinduction and lack of association with some antioxidant enzymes

We have shown that the ameliorative effect of stannous chloride (SnCl2) pretreatment on potassium dichromate (K2Cr2O7)-induced renal damage 24 h after K2Cr2O7 injection was associated with the induction of heme oxygenase-1 (HO-1). In this work we evaluated: (a) if the protective effect of SnCl2 (given 12 h before K2Cr2O7) is associated with changes in the renal activity of HO-1, superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione reductase (GR), and catalase (CAT) 24 and 48 h after K2Cr2O7 injection, and (b) if HO-1 induction is indispensable before K2Cr2O7 injection. It was found that the protective effect of SnCl2 on renal function was observed both at 24 and 48 h reaching its maximum at 24 h when HO-1 expression was higher. Cu,Zn-SOD, Mn-SOD, and GR activities remained unchanged whereas GPx and CAT activities decreased at 48 h in K2Cr2O7-treated rats. The activity of Cu,Zn-SOD, Mn-SOD, GPx, CAT, and GR was unchanged in the SnCl2-treated rats. To fulfill the objective (b) groups of rats treated with K2Cr2O7 and SnCl2 (given at the same time or 12 h after K2Cr2O7) were studied 24 h after K2Cr2O7-injection. The simultaneous injections of SnCl2 and K2Cr2O7 had no protective effect whereas the injection of SnCl2 12 h after K2Cr2O7 exacerbated renal damage. In conclusion, the protective effect of SnCl2 on K2Cr2O7-induced nephrotoxicity is associated with HO-1 induction and not with other antioxidant enzymes (Cu,Zn-SOD, Mn-SOD, GPx, GR, and CAT) and SnCl2 has a preventive and not a therapeutic effect on renal damage induced by K2Cr2O7.

Dysregulation of endogenous carbon monoxide and nitric oxide production in patients with advanced ischemic or nonischemic cardiomyopathy

Carbon monoxide (CO) and nitric oxide (NO) are endogenous vasoregulatory molecules whose role in heart failure is not fully known. Exhaled CO and NO measurement provide novel noninvasive assessment of their endogenous production. We compared exhaled CO and NO in 24 patients with advanced ischemic and nonischemic cardiomyopathy and in 13 control subjects without known cardiac disease at rest and at 1 and 5 minutes after exercise testing. Exhaled CO was lower in patients with cardiomyopathy at rest (1.66 +/- 0.2 vs 1.80 +/- 0.5 ppm, p = 0.02) and 1 minute after exercise (1.35 +/- 0.2 vs 1.81 +/- 0.5 ppm, p = 0.009), with a similar trend at 5 minutes after exercise (1.45 +/- 0.3 vs 1.81 +/- 0.5 ppm, p = 0.14). Exhaled CO decreased in patients with cardiomyopathy after exercise (p <0.001 and p = 0.02 at rest vs 1 and 5 minutes after exercise, respectively) but was maintained in controls. Exhaled NO did not differ between patients with cardiomyopathy and controls at rest (9.48 +/- 1.4 vs 9.68 +/- 1.5 ppb, p = NS) and after exercise (1 minute: 10.91 +/- 1.8 vs 9.19 +/- 1.2 ppb; 5 minutes: 10.52 +/- 1.5 vs 8.90 +/- 1.2 ppb, p = NS). Exhaled NO increased after exercise in patients with cardiomyopathy (p = 0.01 and p = 0.04 rest vs exercise at 1 and 5 minutes, respectively), but was maintained in controls. Exhaled CO and NO were not correlated with peak oxygen consumption in patients with cardiomyopathy. The differential responses in exhaled CO and NO at rest or with exercise between patients with cardiomyopathy and normal controls may point to dysregulation in endogenous CO and NO production.

Heme oxygenase-1 in growth control and its clinical application to vascular disease

Heme oxygenase-1 (HO-1) catalyzes the degradation of heme to carbon monoxide (CO), iron, and biliverdin. Biliverdin is subsequently metabolized to bilirubin by the enzyme biliverdin reductase. Although interest in HO-1 originally centered on its heme-degrading function, recent findings indicate that HO-1 exerts other biologically important actions. Emerging evidence suggests that HO-1 plays a critical role in growth regulation. Deletion of the HO-1 gene or inhibition of HO-1 activity results in growth retardation and impaired fetal development, whereas HO-1 overexpression increases body size. Although the mechanisms responsible for the growth promoting properties of HO-1 are not well established, HO-1 can indirectly influence growth by regulating the synthesis of growth factors and by modulating the delivery of oxygen or nutrients to specific target tissues. In addition, HO-1 exerts important effects on critical determinants of tissue size, including cell proliferation, apoptosis, and hypertrophy. However, the actions of HO-1 are highly variable and may reflect a role for HO-1 in maintaining tissue homeostasis. Considerable evidence supports a crucial role for HO-1 in blocking the growth of vascular smooth muscle cells (SMCs). This antiproliferative effect of HO-1 is mediated primarily via the release of CO, which inhibits vascular SMC growth via multiple pathways. Pharmacologic or genetic approaches targeting HO-1 or CO to the blood vessel wall may represent a promising, novel therapeutic approach in treating vascular proliferative disorders.

HO-1 induction attenuates renal damage and oxidative stress induced by K2Cr2O7

Heme oxygenase (HO) is the rate-limiting enzyme in the degradation of heme; its inducible isozyme HO-1 protects against some types of acute tissue injury. The expression and functional role of HO-1 in rats with renal injury induced by potassium dichromate (K(2)Cr(2)O(7)) was investigated in this work. Rats were studied 24 h after a single injection of K(2)Cr(2)O(7). To address the possible protective effect of HO-1 in this experimental model, this enzyme was induced by an injection of stannous chloride (SnCl(2)) 12 h before K(2)Cr(2)O(7) administration. The functional role of HO-1 in K(2)Cr(2)O(7) + SnCl(2)-treated animals was tested by inhibiting HO activity with an injection of zinc (II) protoporphyrin IX (ZnPP) 18 h before K(2)Cr(2)O(7). In K(2)Cr(2)O(7)-treated rats: (i) renal HO-1 content, measured by Western blot, increased 2.6-fold; and, (ii) renal nitrotyrosine and protein carbonyl content, markers of oxidative stress, increased 3.5- and 1.36-fold, respectively. Renal damage and oxidative stress were ameliorated and HO-1 content was increased in the K(2)Cr(2)O(7) + SnCl(2) group. The attenuation of renal injury and oxidative stress was lost by the inhibition of HO activity in K(2)Cr(2)O(7) + SnCl(2) + ZnPP-treated animals. Our data suggest that HO-1 overexpression induced by SnCl(2) is responsible for the attenuation of renal damage and oxidative stress induced by K(2)Cr(2)O(7).

Role of heme oxygenase-1 in the regulation of blood pressure and cardiac function

Heme oxygenase (HO) is a cytoprotective enzyme that degrades heme (a potent oxidant) to generate carbon monoxide (a vasodilatory gas that has anti-inflammatory properties), bilirubin (an antioxidant derived from biliverdin), and iron (sequestered by ferritin). Because of the properties of inducible HO (HO-1) and its products, we hypothesized that HO-1 would play an important role in the regulation of cardiovascular function. In this article, we will review the role of HO-1 in the regulation of blood pressure and cardiac function and highlight previous studies from our laboratory using gene deletion and gene overexpression transgenic approaches in mice. These studies will include the investigation of HO-1 in the setting of hypertension (renovascular), hypotension (endotoxemia), and ischemia/reperfusion injury (heart). In a chronic renovascular hypertension model, hypertension, cardiac hypertrophy, acute renal failure, and acute mortality induced by one kidney-one clip surgery were more severe in HO-1-null mice. In addition, HO-1-null mice with endotoxemia had earlier resolution of hypotension, yet the mortality and the incidence of end-organ damage were higher in the absence of HO-1. In contrast, mice with cardiac-specific overexpression of HO-1 had an improvement in cardiac function, smaller myocardial infarctions, and reduced inflammatory and oxidative damage after coronary artery ligation and reperfusion. Taken together, these studies suggest that an absence of HO-1 has detrimental consequences, whereas overexpression of HO-1 plays a protective role in hypoperfusion and ischemia/reperfusion injury.

Cardioselective overexpression of HO-1 prevents I/R-induced cardiac dysfunction and apoptosis

Heme oxygenase (HO)-1 converts heme to bilirubin, carbon monoxide, and iron. Our prior work has suggested a cardioprotective role for HO-1 in heart failure. To test whether HO-1 (heat shock protein 32) prevents cardiomyocyte apoptosis and cardiac dysfunction after ischemia-reperfusion (I/R), we generated transgenic mice overexpressing HO-1 in the heart under the control of the alpha-myosin heavy chain promoter. HO-1 transcript and protein increased markedly in the heart only. In an isolated heart preparation, we observed an enhanced functional recovery during reperfusion after ischemia in the transgenic hearts compared with nontransgenic controls. I/R injury was also performed in intact animals by coronary ligation and reperfusion to assess the protective role of HO-1 overexpression on heart apoptosis. HO-1 overexpression reduced cardiac apoptosis, as evidenced by fewer terminal deoxynucleodidyl transferase-mediated dUTP nick-end labeling-positive or in situ oligo ligation-positive myocytes, compared with nontransgenic mice. Our results indicate that cardioselective overexpression of HO-1 exerts a cardioprotective effect after myocardial I/R in mice, and this effect is probably mediated via an antiapoptotic action of HO-1.

Aldosterone breakthrough during angiotensin II receptor antagonist therapy in stroke-prone spontaneously hypertensive rats

Aldosterone breakthrough during ACE inhibitor therapy has been reported. This study investigates changes in plasma aldosterone concentration (PAC) and its mechanism and effects on target organ damage during long-term angiotensin II type 1 (AT1) receptor antagonist (AT1A) therapy in hypertensive rats. An AT1A (candesartan, 1 mg/kg per day PO) was administered in stroke-prone spontaneously hypertensive rats from 4 weeks of age for 34 weeks. PAC was significantly decreased during the first 4 weeks but showed aldosterone breakthrough after 8 weeks of AT1A administration. Plasma angiotensin II concentration was significantly elevated, whereas no change was seen in plasma ACTH or serum potassium. The mechanism(s) of aldosterone breakthrough were investigated by giving high doses of candesartan (3 mg/kg per day PO), dexamethasone (200 microg/kg per day IP), or the AT2 antagonist (PD123319, 10 mg/kg per day SC) during the last week of the 24-week AT1A treatment period. Dexamethasone and AT2 antagonist but not high-dose AT1A produced a significant decrease in PAC, with a larger decrease produced by the AT2 antagonist. To clarify the effects of the residual aldosterone, effects of coadministration of low-dose spironolactone (10 mg/kg per day SC), an aldosterone antagonist, on left ventricular hypertrophy and expression of brain natriuretic peptide mRNA were determined. Low-dose spironolactone further improved left ventricular hypertrophy and brain natriuretic peptide mRNA expression despite no additional depressor effect. These results suggest that aldosterone breakthrough occurs during long-term AT1A therapy, mainly by an AT2-dependent mechanism. Residual aldosterone may attenuate the cardioprotective effects of AT1A.

The role of heme oxygenase signaling in various disorders

Modern methods of cell and molecular biology, augmented by molecular technology, have great potential for helping to unravel the complex mechanisms of various diseases. They also have the potential to help us try to dissect the events which follow the altered physiological conditions. Thus, there is every reason to believe that some of the potential mechanisms will be translated sooner or later into the clinic. Heme oxygenase (HO)-related mechanisms play an important role in several aspects of different diseases. In the past several years, significant progress has been made in our understanding of the function and regulation of HO. The objective of this article is to review current knowledge relating to the importance of HO mechanism in various diseases including myocardial ischemia/reperfusion, hypertension, cardiomyopathy, organ transplantation, endotoxemia, lung diseases, and immunosuppression. The morbidity and mortality of these diseases remain high even with optimal medical management. Furthermore, in this review, we consider various factors influencing the HO system and finally assess current pharmacological approaches to their control.

Exacerbation of chronic renovascular hypertension and acute renal failure in heme oxygenase-1-deficient mice

Heme oxygenase (HO) is a cytoprotective enzyme that degrades heme (a potent oxidant) to generate carbon monoxide (a vasodilatory gas that has anti-inflammatory properties), bilirubin (an antioxidant derived from biliverdin), and iron (sequestered by ferritin). Because of properties of HO and its products, we hypothesized that HO would be important for the regulation of blood pressure and ischemic injury. We studied chronic renovascular hypertension in mice deficient in the inducible isoform of HO (HO-1) using a one kidney-one clip (1K1C) model of disease. Systolic blood pressure was not different between wild-type (HO-1(+/+)), heterozygous (HO-1(+/-)), and homozygous null (HO-1(-/-)) mice at baseline. After 1K1C surgery, HO-1(+/+) mice developed hypertension (140+/-2 mm Hg) and cardiac hypertrophy (cardiac weight index of 5.0+/-0.2 mg/g) compared with sham-operated HO-1(+/+) mice (108+/-5 mm Hg and 4.1+/-0.1 mg/g, respectively). However, 1K1C produced more severe hypertension (164+/-2 mm Hg) and cardiac hypertrophy (6.9+/-0.6 mg/g) in HO-1(-/-) mice. HO-1(-/-) mice also experienced a high rate of death (56%) within 72 hours after 1K1C surgery compared with HO-1(+/+) (25%) and HO-1(+/-) (28%) mice. Assessment of renal function showed a significantly higher plasma creatinine in HO-1(-/-) mice compared with HO-1(+/-) mice. Histological analysis of kidneys from 1K1C HO-1(-/-) mice revealed extensive ischemic injury at the corticomedullary junction, whereas kidneys from sham HO-1(-/-) and 1K1C HO-1(+/-) mice appeared normal. Taken together, these data suggest that chronic deficiency of HO-1 does not alter basal blood pressure; however, in the 1K1C model an absence of HO-1 leads to more severe renovascular hypertension and cardiac hypertrophy. Moreover, renal artery clipping leads to an acute increase in ischemic damage and death in the absence of HO-1.

Reduced vasorelaxant effect of carbon monoxide in diabetes and the underlying mechanisms

Carbon monoxide (CO) is an endogenous gaseous factor that relaxes vascular tissues by acting on both the cGMP pathway and calcium-activated K+ (K(Ca)) channels. Whether the vascular effect of CO is altered in diabetes had been unknown. It was found that the CO-induced relaxation of tail artery tissues from streptozotocin-induced diabetic rats was significantly decreased as compared with that of nondiabetic control rats. The blockade of the cGMP pathway with ODQ (1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one) completely abolished the CO-induced relaxation of diabetic tissues but only partially inhibited the CO effect in normal tissues. Single-channel conductance of K(Ca) channels in diabetic smooth muscle cells (SMCs) was not different from that of normal SMCs. However, the sensitivity of K(Ca) channels to CO in diabetic SMCs was significantly reduced. CO (10 micromol/l) induced an 81 +/- 24% increase in the mean open probability of single K(Ca) channels in normal SMCs but had no effect in diabetic SMCs. Longterm culture of normal vascular SMCs with 25 mmol/l glucose or 25 mmol/l 3-OMG (3-O-methylglucose) but not 25 mmol/l mannitol significantly reduced the sensitivity of K(Ca) channels to CO. On the other hand, the sensitivity of K(Ca) channels to CO was regained in diabetic SMCs that were cultured with 5 mmol/l glucose for a prolonged period. The decreased vasorelaxant effect of CO in diabetes represents a novel mechanism for the vascular complications of diabetes, which could be closely related to the glycation of K(Ca) channels in diabetic vascular SMCs.