Hepatic cytochrome P-450 in rats submitted to chronic hypobaric hypoxia

Cardioprotection after acute exposure to simulated high altitude in rats. Role of nitric oxide

AIM

In previous studies, upregulation of NOS during acclimatization of rats to sustained hypobaric hypoxia was associated to cardioprotection, evaluated as an increased tolerance of myocardium to hypoxia/reoxygenation. The objective of the present work was to investigate the effect of acute hypobaric hypoxia and the role of endogenous NO concerning cardiac tolerance to hypoxia/reoxygenation under β-adrenergic stimulation.

METHODS

Rats were submitted to 58.7 kPa in a hypopressure chamber for 48 h whereas their normoxic controls remained at 101.3 kPa. By adding NOS substrate L-arg, or blocker L-NNA, isometric mechanical activity of papillary muscles isolated from left ventricle was evaluated at maximal or minimal production of NO, respectively, under β-adrenergic stimulation by isoproterenol, followed by 60/30 min of hypoxia/reoxygenation. Activities of NOS and cytochrome oxidase were evaluated by spectrophotometric methods and expression of HIF1-α and NOS isoforms by western blot. Eosin and hematoxiline staining were used for histological studies.

RESULTS

Cytosolic expression of HIF1-α, nNOS and eNOS, and NO production were higher in left ventricle of hypoxic rats. Mitochondrial cytochrome oxidase activity was decreased by hypobaric hypoxia and this effect was reversed by L-NNA. After H/R, recovery of developed tension in papillary muscles from normoxic rats was 51-60% (regardless NO modulation) while in hypobaric hypoxia was 70% ± 3 (L-arg) and 54% ± 1 (L-NNA). Other mechanical parameters showed similar results. Preserved histological architecture was observed only in L-arg papillary muscles of hypoxic rats.

CONCLUSION

Exposure of rats to hypobaric hypoxia for only 2 days increased NO synthesis leading to cardioprotection.

Time course of regression of the protection conferred by simulated high altitude to rat myocardium: correlation with mtNOS

During acclimatization to sustained hypobaric hypoxia, retardation of age-associated decline in left ventricle mechanical activity and improved posthypoxic recovery were accompanied by upregulation of mitochondrial nitric oxide synthase (mtNOS). To evaluate the time course of regression of these effects on deacclimatization, rats exposed to 53.8 kPa in a hypopressure chamber for 5 mo were returned to 101.3 kPa, whereas controls remained at 101.3 kPa throughout the study. At three time points, contractile function in response to calcium and to hypoxia-reoxygenation (H/R) were determined in papillary muscle, and NOS activity and expression were determined in mitochondria isolated from left ventricle. Developed tension was, before H/R, 65, 58, and 40%, and, after H/R, 129, 107, and 71% higher than in controls at 0.4, 2, and 5 mo of normoxia, respectively. Maximal rates of contraction and relaxation followed a similar pattern. All three parameters showed a linear decline during deacclimatization, with mean half-time ( t1/2) of 5.9 mo for basal mechanical activity and 5.3 mo for posthypoxic recovery. Left ventricle mtNOS activity was 42, 27, and 20% higher than in controls at 0.4, 2, and 5 mo, respectively ( t1/2 = 5.0 mo). The expression of mtNOS showed similar behavior. The correlation of mtNOS activity with muscle contractility sustained a biphasic modulation, suggesting an optimal mtNOS activity. This experimental model would provide the most persistent effect known at present on preservation of myocardial mechanical activity and improved tolerance to O2 deprivation. Results support the putative role of mtNOS in the mechanism involved.

Effect of sustained hypobaric hypoxia during maturation and aging on rat myocardium. I. Mechanical activity

Long-lasting cardioprotection may be attained by chronic hypoxia. The basal parameters of contractile function and their response to hypoxia/reoxygenation were measured under isometric conditions, in papillary muscles isolated from left ventricle of rats that were submitted to 53.8 kPa in a hypobaric chamber from 7 wk of age and for their lifetime and of their siblings kept at 101.3 kPa. During acclimatization, hematocrit increased, body weight gain decreased, and heart weight increased with right ventricle hypertrophy. Papillary muscle cross-sectional area was similar in both control and hypoxic groups up to 45 wk of exposure. Developed tension (DT) was 34–64% higher in rats exposed to hypoxia for 10, 26, and 45 wk than in their age-matched controls, whereas resting tension was unchanged. Maximal rates of contraction and relaxation showed a similar pattern of changes as DT. Recovery of DT and maximal rates of contraction and relaxation after 60-min hypoxia and 30-min reoxygenation was also improved in adult hypoxic rats to values similar to those of young rats. Heart acclimatization was lost after 74 wk of exposure. Results are consistent with the development of cardioprotection during high-altitude acclimatization and provide an experimental model to study the mechanisms involved, which are addressed in the accompanying paper.

Cytochrome P450 enzyme-mediated drug metabolism at exposure to acute hypoxia (corresponding to an altitude of 4,500�m)

OBJECTIVE

To investigate the effect of acute hypoxia and concomitant changes in portal blood flow on the disposition of drugs mainly metabolized by the cytochrome P(450) enzymes (CYP) 3A4 (verapamil) and CYP1A2 (theophylline).

METHODS

Twenty healthy male participants were studied on two 14-h study days in a normobaric hypoxic chamber and were allocated randomly to one of two groups receiving short infusions of either theophylline (6 mg kg (-1) body weight) or verapamil (5 mg) intravenously. According to a randomized, cross-over design, participants were once exposed to normoxia and once to hypoxia (12% oxygen corresponding to the ambient( P)O(2) at an altitude of 4,500 m above sea level). The concentrations of theophylline, 1,3-dimethyluric acid, verapamil, and norverapamil were determined in serial blood samples by means of liquid chromatography-mass spectrometry (LC/MS/MS). Portal blood flow was assessed by transabdominal duplex ultrasonography.

RESULTS

Acute hypoxia did not alter the pharmacokinetics of theophylline [half-life+/-SD: 9.29+/-1.77 versus 9.39+/-1.40 (hypoxia)], 1,3-dimethyluric acid (12.9+/-4.72 versus 15.1+/-8.59), verapamil (2.00+/-0.98 versus 1.79+/-0.58), or norverapamil (7.98+/-2.94 versus 9.91+/-6.40). Individual changes of elimination half-life and changes in capillary oxygen saturation,( P)O(2), or portal vein flow were not correlated. Portal vein flow was unaffected by hypoxia.

CONCLUSIONS

Acute hypoxia corresponding to hypoxia at altitudes of 4,500 m does not impair the metabolism mediated by CYP1A2 or CYP3A4. At rapid ascent to and short-term stay at altitudes up to 4,500 m, the doses of drugs metabolized by these CYPs do therefore not require dose modification, and major changes in the disposition of already administered drugs are not to be expected.

Effects of hypobaric hypoxia on antioxidant enzymes in rats

1. The present study was undertaken to investigate the effects of hypobaric hypoxia, equivalent to an altitude of 5500 m, on antioxidant enzymes in rats. 2. Malondialdehyde levels in serum, heart, lung, liver and kidney of hypobaric-hypoxic rats were all significantly higher than in control rats by day 21 of exposure (P < 0.05), indicating increased oxidative stress. 3. Superoxide dismutase (SOD) catalyses the conversion of the superoxide anion to H2O2 and O2. The concentration of immunoreactive Mn-SOD in the serum of hypobaric-hypoxic rats was raised significantly from day 5 onwards, whereas in liver and lung, it had decreased significantly by day 21 (P < 0.05). 4. Glutathione peroxidase (GSH-Px) catalyses H2O2 and certain lipid peroxides. By day 21, GSH-Px activity had increased significantly in the heart and lungs, but decreased significantly in the liver (P < 0.05). 5. Catalase catalyses H2O2. Catalase activity in the liver and kidney of hypobaric-hypoxic rats was significantly decreased on day 1 (P < 0.05) though levels then recovered. 6. Mn-SOD mRNA in the liver of hypobaric-hypoxic rats was induced during the experiment, the effect being exceptionally marked, especially during the first 3 days of exposure to hypobaric hypoxia. 7. These results suggest that the liver may be more vulnerable than the other organs tested to oxidative stress under hypobaric hypoxia.

Acute moderate hypoxia in conscious rabbits: effect on hepatic cytochrome P450 and on reactive oxygen species

This study aimed to document whether acute moderate hypoxia modifies the amount and activity of hepatic cytochrome P450 and in addition, induces changes in the production or the ability to neutralize oxygen reactive species (ORS). Rabbits were exposed to a low partial pressure of oxygen (12%) for 8 or 24 h, killed, and the amount and activity of cytochrome P450, lipid peroxidation, microsomal chemiluminescence and enzymatic scavenger activity were assessed in the liver. After 8 h of hypoxia, total amount but not the activity of cytochrome P450 was decreased, although after 24 h of hypoxia, both the amount and the activity of cytochrome P450 were decreased. Hypoxia for 8 h increased the activity of glutathione peroxidase. However, after 24 h of hypoxia, lipid peroxidation, microsomal chemiluminescence and superoxide dismutase activity were increased, while hepatic glutathione and glutathione peroxidase activity were reduced, modifications that suggest an enhanced presence of ORS. In in-vitro studies, an ORS generating system reduced the activity of cytochrome P450 and enhanced lipid peroxidation of hepatic microsomal membranes, supporting the view that ORS can impair cytochrome P450. The results of the present study show that hypoxia induces changes in the amount and activity of cytochrome P450, as well as in the production or the ability to neutralize ORS, and that these changes are time-dependent.

Oxidative stress induced by intermittent exposure at a simulated altitude of 4000 m decreases mitochondrial superoxide dismutase content in soleus muscle of rats

The effects were examined of 6-month intermittent hypobaric (4000 m) exposure on the antioxidant enzyme systems in soleus and tibialis muscles of rats. At the end of the 6-month experimental exposure, the six rats in both the exposed group and the control group were sacrificed. Immunoreactive mitochondrial superoxide dismutase (Mn-SOD) contents were measured as well as the activities of antioxidant enzymes [Mn-SOD, cytosolic SOD (Cu,Zn-SOD), catalase (CAT), and glutathione peroxidase (GPX)]. Thiobarbituric acid-reactive substances (TBARS) were also determined as an indicator of lipid peroxidation. The high altitude exposure resulted in a marked increase in TBARS content in soleus muscle, suggesting increased levels of oxygen free radicals. Conversely, significant decreases in both Mn-SOD content and activity in soleus muscle were noted after exposure. Such trends were not noticed in tibialis muscle. On the other hand, no significant changes in Cu,Zn-SOD, CAT, or GPX were observed in either muscle. These results suggested that the increases in lipid peroxidation were most probably a result of decreased Mn-SOD function which was more depressed in oxidative than in glycolytic muscle.

Glucocorticoid response and adrenal lipid peroxidation in rats submitted to chronic hypobaric hypoxia

The well known physiological changes that occur in adaptation to chronic hypobaric hypoxia (CHH) prevent the fall in arterial blood oxygen pressure below normal limits but neither the ventilatory nor the circulatory adaptation are able to maintain a normal capillary blood pO2. We report here our findings on adrenal histology, function, and lipid peroxidation in intact and castrated rats of both sexes submitted to twelve weeks of adaptation to 4,400 m simulated altitude. In the CHH rats hematocrit was increased; adrenal weight was increased in males and decreased in castrated females. Adrenal histology and histochemistry were normal. Corticosterone level and uric acid were unchanged except in castrated animals: corticosterone was decreased in males and increased in females. Uric acid was increased in castrated males, the only group in which adrenal lipid peroxidation was unaffected by castration. In the remaining groups, it was diminished compared to their intact controls. Intact normoxic males showed a significant direct correlation between plasmatic lipoperoxidation products (TBA-reactive substances) and uric acid (r = 0.961), which disappeared under hypoxia or with castration. Sex related differences were found in hematocrit and adrenal weight responses. The presence of androgens was involved in the adaptative response of cortical and reticular histometry, lipid peroxidation, and corticosterone level. These results do not support Selye's hypothesis of exhausted adrenal glands in chronic stages of adaptation and suggest that a proper relation between adrenal and gonadal steroids hormones is essential for systemic and tissular responses to diminished oxygen supply.

Effect of chronic hypoxia on detoxication enzymes in rat liver

Studies were performed to determine the effects of chronic hypoxia on enzymes that catalyze various detoxication reactions. Rats were exposed to room air or 10.5% O2 for 10 days, and microsomes and postmicrosomal supernatants were isolated from liver. Detoxication enzyme activities were measured by radiochemical and spectrophotometric assays, and immunoreactive protein amounts were measured by Western blot analysis. Total cytochrome P450, as measured by the CO-difference spectrum, and activities of superoxide dismutase (EC 1.15.1.1), epoxide hydrolase (EC 4.2.1.63), catalase (EC 1.11.1.6), glutathione disulfide reductase (EC 1.6.4.2), and glutathione (GSH) S-transferase (EC 2.5.1.18) were not affected by this extent of hypoxia. In contrast, 10 days of hypoxia decreased activities or immunoreactivities (% of aerobic) of GSH peroxidase (EC 1.11.1.9) (54%), cytochrome P450EtOH2 (42%), CYP3A1 (53%), sulfotransferase (EC 2.8.2.1) (77%) and UDP-glucuronosyltransferase (EC 2.4.1.17) (65%). Activity of glucose-6-phosphate dehydrogenase (EC 1.1.1.49), an important enzyme in NADPH production was also decreased to 56% of the aerobic value, but Western blot analysis showed that the amount of protein reactive with antibodies to glucose-6-phosphate dehydrogenase was not affected by hypoxia. Thus, hypoxia may decrease activity of enzymes by regulatory mechanisms even though the amount of immuno-detectable enzyme is unchanged. Liver cells isolated from rats exposed to hypoxia also gave lower GSH synthetic rates than cells from normoxic rats. This result, together with the effect of hypoxia on glucose-6-phosphate dehydrogenase, indicates that the GSH supply for GSH-dependent detoxication reactions may be limited due to chronic hypoxia. To test directly whether chronic hypoxia increased sensitivity to a compound normally detoxified by a GSH-dependent reaction, sensitivity to tert-butyl hydroperoxide (t-BuOOH) of hepatocytes from rats exposed to in vivo hypoxia was compared to that from normoxic rats. The results showed that the cells from the hypoxic rats were much more sensitive to injury. Taken together, these results suggest that decreases in amounts and/or activities of detoxication enzymes during chronic hypoxia may result in increased susceptibility of cells to chemical injury.