Heart and lung alterations in neonatal rats exposed to CO or high altitude

Carbon Monoxide: Endogenous Production, Physiological Functions, and Pharmacological Applications

Over the last decade, studies have unraveled many aspects of endogenous production and physiological functions of carbon monoxide (CO). The majority of endogenous CO is produced in a reaction catalyzed by the enzyme heme oxygenase (HO). Inducible HO (HO-1) and constitutive HO (HO-2) are mostly recognized for their roles in the oxidation of heme and production of CO and biliverdin, whereas the biological function of the third HO isoform, HO-3, is still unclear. The tissue type-specific distribution of these HO isoforms is largely linked to the specific biological actions of CO on different systems. CO functions as a signaling molecule in the neuronal system, involving the regulation of neurotransmitters and neuropeptide release, learning and memory, and odor response adaptation and many other neuronal activities. The vasorelaxant property and cardiac protection effect of CO have been documented. A plethora of studies have also shown the importance of the roles of CO in the immune, respiratory, reproductive, gastrointestinal, kidney, and liver systems. Our understanding of the cellular and molecular mechanisms that regulate the production and mediate the physiological actions of CO has greatly advanced. Many diseases, including neurodegenerations, hypertension, heart failure, and inflammation, have been linked to the abnormality in CO metabolism and function. Enhancement of endogenous CO production and direct delivery of exogenous CO have found their applications in many health research fields and clinical settings. Future studies will further clarify the gasotransmitter role of CO, provide insight into the pathogenic mechanisms of many CO abnormality-related diseases, and pave the way for innovative preventive and therapeutic strategies based on the physiologic effects of CO.

Cardiac morphology and function following long-term exposure to carbon monoxide at high altitude in rats

Although chronic exposure to high altitude or carbon monoxide (CO) induces several cardiovascular adaptations in mammals, very little information is available on the cardiovascular effects due to exposure to these concomitant stresses. In particular, cardiac functional responses to chronic CO inhalation at high altitude has never been investigated. Thirty-two male Dark Agouti rats were exposed for 10 wk in steel chambers to (1) normoxic conditions (N rats), (2) 4000 m simulated high altitude (chronic hypobaric hypoxia, CH rats), (3) 50 ppm CO (CO rats), or (4) 50 ppm CO at 4000 m simulated high altitude (CH + CO rats). Left ventricular (LV) morphology and function were evaluated in normoxic conditions (1) using transthoracic Doppler echocardiography, just before as well as after exposure to environments, and (2) using transparietal intraventricular catheterisms following a short ventilatory-controlled thoracotomy (10 ml/kg volume inflow, pO2 ~159 torr) after exposure. Right ventricular (RV) function was evaluated using the same catheterism process after exposure. Body weight increased in all groups during exposure, but to a lesser extent in CH and CH + CO than in N and CO rats. At the end of exposure, hematocrit ratio was higher in CH than in CO rats, and in CH + CO than in CH rats. Carboxyhemoglobin levels were higher in CH + CO than in CO, CH, or N rats. Although neither a simulated high altitude nor CO alone had an effect on LV morphology and function, combined environments increased LV + septum weight, interventricular and posterior wall thicknesses, relative wall thickness, and LV systolic function. LV diastolic function was not altered by environmental conditions in the four groups. CO intensified the altitude-induced RV hypertrophy as well as the altitude-induced decrease in RV diastolic function. RV systolic function increased in CH rats, but adding CO did not amplify this adaptation process. Moreover, significant polynomial relationships were obtained between hematocrit ratio and LV + septum weight or left ventricular systolic pressure. Our results indicate that CO at high altitude induced severe hematological responses that could be involved in morphological and functional cardiac adaptations of both ventricles. Data indicate that CO at high altitude may be more detrimental to cardiac function than CO inhaled at sea level.

Carbon monoxide promotes hypoxic pulmonary vascular remodeling

CO is a biologically active gas that produces cellular effects by multiple mechanisms. Because cellular binding of CO by heme proteins is increased in hypoxia, we tested the hypothesis that CO interferes with hypoxic pulmonary vascular remodeling in vivo. Rats were exposed to inspired CO (50 parts/million) at sea level or 18,000 ft of altitude [hypobaric hypoxia (HH)], and changes in vessel morphometry and pulmonary pressure-flow relationships were compared with controls. Vascular cell single strand DNA (ssDNA) and proliferating cell nuclear antigen (PCNA) were assessed, and changes in gene and protein expression of smooth muscle alpha-actin (sm-alpha-actin), beta-actin, and heme oxygenase-1 (HO-1) were evaluated by Western analysis, RT-PCR, and immunohistochemistry. After 21 days of HH, vascular pressure at constant flow and vessel wall thickness increased and lumen diameter of small arteries decreased significantly. The presence of CO, however, further increased both pulmonary vascular resistance (PVR) and the number of small muscular vessels compared with HH alone. CO + HH also increased vascular PCNA and nuclear ssDNA expression compared with hypoxia, suggesting accelerated cell turnover. CO in hypoxia downregulated sm-alpha-actin and strongly upregulated beta-actin. CO also increased lung HO activity and HO-1 mRNA and protein expression in small pulmonary arteries during hypoxia. These data indicate an overall propensity of CO in HH to promote vascular remodeling and increase PVR in vivo.

Biological chemistry of carbon monoxide

Carbon monoxide (CO) has many effects in biology due to its complex biochemical activities. These actions of CO depend primarily on its ability to bind heme proteins (Hp) and to inhibit or alter their biochemical functions. Whether CO is derived from exogenous or endogenous sources, its cellular activity is related to its concentration and the concentration of molecular O(2), as well as to the availability of reduced transition metals such as Fe(II). In this respect, the CO/O(2) ratio and O(2)-dependent changes in local oxidation-reduction state assume critical importance in determining the physiological effects of CO by affecting the functions of specific Hp. By interacting with Hp, CO influences electron-transport reactions in a variety of ways, which can produce either prooxidant or antioxidant effects. Similarly, Hp relationships also govern how changes in CO concentration influence the physiological and pathological effects of nitric oxide and the relationships of the two biologically active gases to metal-catalyzed oxidations. This article provides a brief update on the biochemistry of CO as it relates to Hp binding, chemical oxidative processes, and cellular function.

Ischemic tolerance of the heart by adaptation to chronic hypoxia is suppressed by high subchronic carbon monoxide exposure

This study was designed to investigate whether exposure to carbon monoxide (CO) could alter the ischemic tolerance induced by chronic hypoxia. We aimed to determine whether chronic hypoxia-induced cardiovascular adaptation was modified during the return to normoxia or by subchronic CO exposure. The degree of resistance to an in vitro transient ischemia was measured, using the Langendorff method, in hearts from rats previously exposed to chronic hypoxic hypoxia and/or subchronic CO exposure to 600 ppm. Chronic hypoxia decreased ischemic contracture (15.6 +/- 04.9 vs. 60.8 +/- 07.7%) and improved both contractile recovery (59.6 +/- 07.3 vs. 21.8 +/- 06.8%) and ventricular arrhythmia during reperfusion (0 vs. 45%) compared to a control normoxic group. However, in our chronic hypoxia regression model many parameters returned near to control values except for the persistence of cardiomegaly, a significant decrease in both ischemic contracture (22.0 +/- 04.9 vs. 60.8 +/- 07.7%), and ventricular tachycardia (25 vs. 45%). CO exposure alone increased the coronary flow and improved both contractile recovery (42.6 +/- 7.2 vs. 21.8 +/- 6.8%) and ventricular arrhythmia (16.7 vs. 45%) without altering the action potential shape. These two models causing tissue hypoxia induced the same degree of polycythemia or cardiomegaly and provided similar ischemic tolerance. CO exposure after chronic hypoxia exacerbated ischemic contracture (69.3 +/- 10.5 vs. 22.0 +/- 14.5%) and ventricular tachycardia incidence (100 vs. 50%) but with significant alteration in contractile recovery (12.7 +/- 10.5%) compared to the chronic hypoxia or CO exposure. Thus, CO exposure completely suppressed the chronic hypoxia-induced ischemic tolerance.

Expression of heme oxygenase-1 in the lung in chronic hypoxia

Heme oxygenase (HO)-1 is an oxygen-dependent enzyme that may regulate vascular tone and cell proliferation through the production of carbon monoxide (CO). We tested the hypothesis that HO-1 is upregulated in the lung in chronic hypoxia by exposing male Sprague-Dawley rats to 17,000 feet (395 Torr) for 0, 1, 3, 7, 14, or 21 days. After exposure, blood gases, carboxyhemoglobin (COHb) levels, and hematocrit were measured, and the lungs were either inflation fixed for immunohistochemistry or frozen for later measurement of HO enzyme activity, Western blot for HO-1 protein, and RT-PCR for HO-1 mRNA. The heart was excised and weighed, and the right-to-left heart weight ratio was determined. During hypoxia, the hematocrit increased progressively, reaching significantly higher values than the control value after 3 days. COHb levels increased above the control value after 1 day of hypoxia and increased progressively between 14 and 21 days, whereas arterial PO(2) and arterial PCO(2) did not vary significantly. HO-1 protein determined by Western blot increased for the first 7 days and declined thereafter; however, enzyme activity was elevated only after 1 day. Changes in HO-1 during hypoxia were localized by immunohistochemistry to inflammatory cells (early) and newly muscularized arterioles (later). Lung HO-1 mRNA normalized to glyceraldehyde-3-phosphate dehydrogenase was increased after 1 and 21 days. The data indicate that lung HO-1 protein and activity are upregulated only during early chronic hypoxia, whereas persistent COHb elevations indicate high endogenous CO production rates at nonpulmonary sites. If CO has antiproliferative properties, the lack of HO enzyme activity in the lung may be permissive for pulmonary vascular proliferation in hypoxia.

Long-term impairment in the neurochemical activity of the sympathoadrenal system after neonatal hypoxia in the rat

The study evaluates the long-term effect of neonatal hypoxia on the neurochemical activity of the sympathoadrenal system in the rat. One-day-old male pups were exposed to hypoxia (10% O2) for 6 d and thereafter reared under normoxia. Neonatal hypoxia reduced the body weight of 3- and 8-wk-old rats and did not change the blood pressure at 6 wk of age. In sympathetic ganglia, the content and/or turnover rates of norepinephrine were reduced in neonatal-hypoxic rats of 3 and 8 wk of age, but the content and turnover rates of dopamine were unaltered. The effect was not dependent on the type of ganglion. In the superior cervical ganglion, neonatal hypoxia had a selective effect on the type of catecholamine (dopamine versus norepinephrine), thus suggesting a selective-altered maturation of noradrenergic neurons, but presumably not of the dopaminergic small, intensely fluorescent cells. A long-term deficiency in adrenal activity was the consequence of neonatal hypoxia, as shown by the decrease in the content and turnover rate of dopamine. Neonatal hypoxia elicited a long-term decrease in the content and turnover rates of norepinephrine in heart and lungs but failed to induce a significant effect in kidneys. However, this effect was not tissue-specific. Data provide evidence that a hypoxic episode occurring during a critical period of development in the rat induces a long lasting decrease in the neurochemical activity of the sympathoadrenal system. These results are discussed in terms of their implications for human pathology.

Low-dose carbon monoxide does not reduce vasoconstriction in isolated rat lungs

Recent studies have demonstrated that nitric oxide (NO) acts as a pulmonary vasodilator when inhaled in low concentrations. Due to the physicochemical similarities between NO and carbon monoxide (CO), it was speculated that low concentrations of CO would have similar effects in the isolated rat lung. The purpose of this study was to determine the role of CO (200 and 1000 ppm) in modulating hypoxia- and angiotensin II (AII)-induced pulmonary vasoconstriction, using isolated salt-perfused lungs of normotensive (CON) or pulmonary hypertensive male rats. Pulmonary hypertensive rats (ALT), induced by simulated altitude exposure (4572 m; 430 mm Hg for 32-48 days), were studied to determine the actions of low-dose CO in a remodeled pulmonary vascular bed. Right ventricular hypertrophy and polycythemia were evident in the ALT rats, suggesting that simulated altitude exposure induced pulmonary hypertension and consequent pulmonary vascular remodeling. CO did not significantly affect pulmonary vascular responses to acute hypoxia (6% CO2, balance N2) in either CON or ALT rats. There were also no significant differences in pulmonary pressor responses to AII injections (0.2 or 0.4 micrograms) in CON or ALT lungs either immediately following or during an acute hypoxia + CO exposure. Therefore, acute low-dose CO exposure (< 1000 ppm) does not appear to attenuate pulmonary vasoconstriction in isolated rat lungs.

Pulmonary vascular responsiveness in rats following neonatal exposure to high altitude or carbon monoxide

Exposure of adult and neonatal rats to high altitude increases pulmonary vascular responsiveness during the exposure. A study was undertaken to determine if a short exposure of neonatal rats to either high-altitude or carbon monoxide (CO) hypoxia would cause persistent alterations in pulmonary vascular responsiveness postexposure. One-day-old male Sprague-Dawley rats were obtained as 16 litters of 10-12 pups each. At 2 days of age, 4 litters were exposed to CO (500 ppm) for 32 days, and 4 litters were exposed to ambient air (AIR) in Detroit (200 m). Another 4 litters were exposed to 3500 m altitude (ALT) in a chamber for 32 days, and 3 litters were exposed to ambient conditions in Fort Collins (CON, 1524 m). After the exposures, all rats were maintained at 1524 m. At 2, 40, 76, and 112 days postexposure, lungs were isolated and perfused with Earle's salt solution (+Ficoll, 4 g%). Pulmonary vascular responsiveness was assessed by dose responses to angiotensin II (AII, 0.025-0.40 micrograms) and acute hypoxia (3% O2 for 3 min). AII responses were higher in ALT vs CON rats at all ages, but no differences were noted between CO and AIR rats. Acute hypoxic responses were higher in ALT versus CON rats at 2 and 40 days postexposure, but no differences were noted between CO and AIR rats. Baseline pulmonary vascular resistance and pulmonary arterial pressure (in isolated lungs) were higher in ALT rats at all four ages compared to the other three groups. Both the ALT and CO rats displayed hypertrophy of the right ventricle (RV) and the left ventricle (LV) at the termination of treatment and elevated hematocrit. LV hypertrophy and polycythemia regressed with time, but RV hypertrophy remained significant in the ALT rats through 112 days postexposure. The results indicate that neonatal exposure to ALT, but not CO, causes a persistent increase in pulmonary vascular responsiveness and RV hypertrophy for at least 112 days after termination of the exposure.