Comparison of the circadian rhythm in cell proliferation in corneal epithelium of male rats studied under normal and hypobaric (hypoxic) conditions

Groups of 20-45-day-old rats maintained on a light (0600-1800)/dark (1800-0600) regimen with food and water available ad libitum were studied for the effect of hypoxic hypoxia on the circadian rhythm of corneal epithelial mitosis and thymidine incorporation. In experiments conducted during the months of September and November, hypoxic hypoxia was accomplished by the exposure of rats to a simulated altitude of 7500 m in one series of experiments, or to a gaseous mixture of 8% oxygen and 92% nitrogen at sea level atmospheric pressure (760 mmHg) in another series of experiments. Controls were included as well. Statistically significant (P less than 0.05) circadian rhythmicity in the corneal mitotic index was substantiated in the control animals with mesor (M) = 12.4%, amplitude (A) = 9.6% and acrophase (phi) of 0911. In the hypoxic hypoxia situation, the mesor and amplitude were depressed to 8.6 and 5.9%, respectively. In control groups, thymidine incorporation was circadian rhythmic with M = 38.5 and A = 11.3 cpm/microns DNA and acrophase of 2255. In the hypoxic hypoxia situation, the mesor was similar to the controls; whereas the amplitude was suppressed to 6.1% and acrophase was phase advanced by about 7 hr. Changes in the circadian rhythm of corneal mitosis and in thymidine incorporation under hypoxic hypoxia can be explained by programmed-in-time energy requirements during the corneal cell regeneration cycle.     

Perinatal history of hypoxia leads to lower vascular pressures and hyporeactivity to angiotensin II in isolated lungs of adult rats

The most dramatic changes in pulmonary circulation occur at the time of birth. We hypothesized that some of the effects of perinatal hypoxia on pulmonary vessels are permanent. We studied the consequences of perinatal exposure to hypoxia (12 % O2 one week before and one week after birth) in isolated lungs of adult male rats (approximately 12 weeks old) perfused with homologous blood. Perfusion pressure-flow relationship was tilted towards lower pressures in the perinatally hypoxic as compared to the control, perinatally normoxic rats. A non-linear, distensible vessel model analysis revealed that this was due to increased vascular distensibility in perinatally hypoxic rats (4.1 +/- 0.6 %/mm Hg vs. 2.3 +/- 0.4 %/mm Hg in controls, P = 0.03). Vascular occlusion techniques showed that lungs of the perinatally hypoxic rats had lower pressures at both the pre-capillary and post-capillary level. To assess its role, basal vascular tone was eliminated by a high dose of sodium nitroprusside (20 microM). This reduced perfusion pressures only in the lungs of rats born in hypoxia, indicating that perinatal hypoxia leads to a permanent increase in the basal tone of the pulmonary vessels. Pulmonary vasoconstrictor reactivity to angiotensin II (0.1-0.5 microg) was reduced in rats with the history of perinatal-hypoxia. These data show that perinatal hypoxia has permanent effects on the pulmonary circulation that may be beneficial and perhaps serve to offset the previously described adverse consequences.     

Population differences in response to hypoxic stress in Atlantic salmon

Understanding whether populations can adapt to new environmental conditions is a major issue in conservation and evolutionary biology. Aquatic organisms are increasingly exposed to environmental changes linked with human activities in river catchments. For instance, the clogging of bottom substratum by fine sediments is observed in many rivers and usually leads to a decrease in dissolved oxygen concentrations in gravel beds. Such hypoxic stress can alter the development and even be lethal for Atlantic salmon (Salmo salar) embryos that spend their early life into gravel beds. In this study, we used a common garden experiment to compare the responses to hypoxic stress of four genetically differentiated and environmentally contrasted populations. We used factorial crossing designs to measure additive genetic variation of early life‐history traits in each population. Embryos were reared under normoxic and hypoxic conditions, and we measured their survival, incubation time and length at the end of embryonic development. Under hypoxic conditions, embryos had a lower survival and hatched later than in normoxic conditions. We found different hypoxia reaction norms among populations, but almost no population effect in both treatments. We also detected significant sire × treatment interactions in most populations and a tendency for heritability values to be lower under stressful conditions. Overall, these results reveal a high degree of phenotypic plasticity in salmon populations that nevertheless differ in their adaptive potential to hypoxia given the distinct reaction norms observed between and within populations.     

Hypoxia Differentially Reduces GABAA Receptor Density During Embryonic Chick Optic Lobe Development

It has been demonstrated that the CNS is severely affected by hypoxic-ischemic insults during the prenatal-perinatal period, including imbalance in excitatory and inhibitory neurotransmitter release. Using a previously developed model of acute normobaric hypoxic hypoxia on chick embryos, we studied alterations observed both on [3H]GABA binding saturation parameters and on lactate concentration on successive embryonic days (ED). While maximal density of GABA binding sites (Bmax) from the low-affinity site was significantly reduced in an age-dependent manner, earlier stages of development (ED12 and 16) proving more vulnerable (ED12: control = 5.48 +/- 0.20, hypoxia = 3.90 +/- 0.39 pmol/mg prot, P < .05; ED16: control = 3.89 +/- 0.26, hypoxia = 2.80 +/- 0.28 pmol/mg prot, P < .05), ligand affinity (Kd) values and kinetic constants of the high-affinity site remained unaltered. Not unlikely, a physiological hypoxic state prevailing from ED17 up to hatching time rendered the whole embryo less sensitive to an externally induced hypoxic state (ED17: control = 2.93 +/- 0.06, hypoxia = 2.38 +/- 0.04 pmol/mg prot, P < .05; ED18: control = 2.97 +/- 0.12, hypoxia = 2.87 +/- 0.27 pmol/mg prot). Lactate levels in chick optic lobe homogenates were constant during development. The increase observed after hypoxic treatment compared to control value was significant at all stages studied, but increased percentage changes proved similar, indicating that all days of development equally perceive externally induced hypoxia. In conclusion, the present work demonstrates that after normobaric hypoxic hypoxia at different embryonic days, the embryo senses the externally induced hypoxic state as from ED12, but the GABA(A) receptor is differentially affected. It may be speculated that a different subunit composition of GABA(A) receptor is assembled in order to build a more stable receptor capable of resisting the physiological hypoxic state observed during the last few days before hatching.     

Time-course of the polycythemic response in normoxic and hypoxic mice with high blood oxygen affinity induced by cyanate administration

The Hb-O₂ affinity and the erythropoietic response as a function of time were studied in mice treated with sodium cyanate for up to 2 months. Cyanate increased the Hb-O₂ affinity in normoxic mice more than in chronically hypoxic mice. The hemoglobin concentration rose as a function of time both in normoxic and hypoxic conditions but reached higher levels in hypoxia. After 42 days of study (21 days of hypoxia) hemoglobin reached maximum levels and thereafter showed a plateau in both cyanate and control animals. It is concluded that a chronic left-shifted oxygen dissociation curve does not avoid the development of hypoxic polycythemia in mice. Moreover, prolonged cyanate administration potentiates the crythropoietic response to chronic hypoxia. Since polycythemia is an index of tissue hypoxia, the results show that the high hemoglobin affinity did not prevent tissue hypoxia in low PO₂ conditions. Results showing beneficial effects of high hemoglobin oxygen affinity induced by cyanate based on acute hypoxic expositions should be cautiously interpreted with regard to their adaptive value in animals chronically exposed to natural or simulated hypoxia.Abbreviations Hb hemoglobin - NaOCN sodium cyanate - ODC oxygen dissociation curve - P ₅₀ PO₂ at which hemoglobin is half saturated with O₂     

Effect of hypoxia on protein composition of synaptic plasma membranes from cerebral cortex during aging

The effect of hypoxia on the protein composition of synaptic plasma membranes (SPM) isolated from cerebral cortex of rats at 4, 12, and 24 months of age was investigated. The proteins were separated by SDS polyacrilamide gel electrophoresis and the percent content was evaluated by measuring the optical density of the stained gels. After hypoxic treatment various proteins showed significant changes. Some proteins were only affected at 4 and 12 months of age and not at 24 months. The various modified porteins may be identified according to their molecular weight, as follows: the 18 kDa protein with calmodulin; the 23 kDa protein with D₃ subunits; the 28 kDa protein could contain the subunit of the Ca²⁺ channel. The changes in the amount of some SPM proteins during hypoxia is consistent with the alteration in membrane polarization and neurotransmission observed in this condition. The effect of aging at the synaptosomal level seems to be a selective process; after hypoxia the age-related changes of many proteins are more pronounced.Special issue dedicated to Dr. Santiago Grisolia     

Criteria of individual variations in the reactivity of the respiratory system

Individual variations of the respiratory system reactivity have been studied in experiments on rats. It is shown expedient to estimate reactivity of the respiratory system to hypoxic hypoxia by the pattern of changes in the total oxygen uptake. Animals demonstrating no essential changes in the oxygen uptake in response to hypoxia (11% O2) are referred to individuals with high reactivity of the respiratory system; those responding by a drastic decrease in the oxygen uptake--to animals with low reactivity of the respiratory system. A strong correlation is determined between the respiratory system reactivity and individual resistance of organism to acute hypoxic hypoxia.     

Redox enzymes in the tissues of the large intestine in the postnatal ontogeny of rats]

Using histochemical methods, age-related changes in activity of some redox enzymes in muscular and superficial layers of the mucous membrane, as well as in neurons of the myenteric nervous plexus of the large intestine have been studied in albino rats 5-day-old, 1-, 5-, 13-, 24-month-old. In young animals (1-5-month-old) an essential increase of the enzymatic activity of the energy metabolism takes place, in mature animals--stabilization of these processes, senescence brings about multidirectional changes in them. Manifestation degree of the changes in energy metabolism, occurring in old age are determined by certain metabolic and functional peculiarities in the organ tissue. In old animals certain strain of the energy metabolism develops, resulting from discoordination of energetic cycles in tissue of the large intestine wall.     

Effects of acute hypoxia and hyperoxia on respiratory rate in albino rats chronically exposed to hypoxia

Changes in respiratory frequencies with hypoxic or hyperoxic exposure were studied in: 12 normoxic control rats (N) born and raised in normoxic environment at sea level; 12 rats (A) born and raised in normoxic environment at sea level exposed to normobaric hypoxia (10% O2 in N2) as adults; 12 rats of first generation (G1) raised in the above mentioned hypoxic environment since a few hours after birth; 12 rats of third generation (G3) conceived and born in the hypoxic environment of hypoxic parents of second generation and maintained continuously under hypoxic conditions until their utilization. The response of A rats to 10% O2 and 7% O2 breathing was elevated (57% and 86% over air breathing). The mean respiratory frequency of A rats exposed to 7% O2 rose to a greater extent than did that of N rats. The G1 and G3 rats were less responsive to 7% O2 (64% and 37% over air breathing, respectively) than N and A rats; however, in G1 rats the exposure to 7% O2 produced a greater rise of frequency than in G3 rats. Furthermore A rats, G1 rats and G3 rats were less responsive to 97% O2 breathing (19%, 19% and 11% below air breathing, respectively). Comparing these data with previous findings we suggest that, with chronic exposure to hypoxia, changes in ventilatory response to hypoxia and hyperoxia occur in the following manner: I) loss of response to hypoxia if chronic exposure is begun in the immediate postnatal period; 2) degree of response to hypoxia or hyperoxia influenced by duration of chronic exposure.     

Cellular and transcriptomic analysis of NS0 cell response during exposure to hypoxia

Mammalian cells have the ability to alter their gene expression in order to survive or adapt to a variety of environment stresses including hypoxic stress. Maintaining oxygen supply has been accepted as essential for cell survival and growth. To determine the cellular and molecular changes which take place under oxygen deprivation, an NS0 cell line producing a human-mouse chimeric antibody was cultured under hypoxic conditions (<1%). Various cellular parameters such as viability, productivity, metabolism, apoptosis and cell cycle were studied and notable changes were shown to be accompanied by changes in metabolic rates. When the cells where exposed to hypoxia for 48 h, cell growth was suppressed and cell death was detected. To better understand and explore the mechanisms underpinning these biological alterations and to identify the genes involved in the genetic reprogramming, genome-wide analyses were performed using GeneChip Mouse Genome arrays. The gene expression profiling generated by the microarray technique revealed that hypoxia, even in the early stages (12h), induces significant changes in gene expression in NS0 cells. The primary responses to hypoxia within the cells were: (1) the up-regulation of pathways such as glycolysis that ultimately lead to alternative routes of ATP generation and increased oxygen availability; and (2) the down-regulation of genes involved in purine/pyrimidine and one carbon pool metabolisms required for DNA and RNA synthesis. By combining gene expression and physiological changes under hypoxia, it was possible to explore the mechanisms of hypoxia-induced alterations in more depth.     

Attenuation of pulmonary arterial smooth muscle cell proliferation following hypoxic pulmonary hypertension by the Na+/H+ exchange inhibitor amiloride

Chronic hypoxia results in pulmonary hypertension. To investigate the role of Na+/H+ exchange in this process, we determined the effect of amiloride, a Na+/H+ exchange inhibitor, on hypoxic pulmonary hypertension and pulmonary arterial smooth muscle cell proliferation, both in vivo and in vitro. Sprague-Dawley rats were placed either in a hypobaric, hypoxic chamber (10.5% 02) or under normal 21% O2 atmosphere for 8 h each day for 3 weeks. Rats under hypoxic conditions received 1, 3, or 10 mg/kg/d amiloride or the vehicle alone. Hematologic indices, including red blood cells, hemoglobin, hematocrit and mean corpuscular hemoglobin increased in hypoxic rats, but these changes were prevented by treatment with amiloride. In the hypoxic rats, the right ventricular systolic pressure and right ventricular hypertension index (weight ratio of right ventricular to left and septum together) were increased by 88% and 129%, respectively. Arteriolar wall thickness and area in the hypoxia-treated animals increased 3- and 2-fold, respectively, over normoxic controls; the increase in each of these indices was attenuated by amiloride in a dose-dependent manner. In cultured pulmonary arterial smooth muscle cells, hypoxia greatly increased cellular proliferation, and this similarly showed a dose-dependent attenuation in the presence of amiloride. Amiloride did not affect blood pressure in vivo or cause cell damage in vitro. These data suggest that the Na+/H+ exchange inhibitor amiloride may represent an effective adjunctive therapy in pulmonary hypertension induced by chronic hypoxia.     

Chronic hypoxia abolishes posthypoxia frequency decline in the anesthetized rat

In anesthetized rats, increases in phrenic nerve amplitude and frequency during brief periods of hypoxia are followed by a reduction in phrenic nerve burst frequency [posthypoxia frequency decline (PHFD)]. We investigated the effects of chronic exposure to hypoxia on PHFD and on peripheral and central O2-sensing mechanisms. In Inactin-anesthetized (100 mg/kg) Sprague-Dawley rats, phrenic nerve discharge and arterial pressure responses to 10 s N2 inhalation were recorded after exposure to hypoxia (10 +/- 0.5% O2) for 6-14 days. Compared with rats maintained at normoxia, PHFD was abolished in chronic hypoxic rats. Because of inhibition of PHFD, the increased phrenic burst frequency and amplitude after N2 inhalation persisted for 1.8-2.8 times longer in chronic hypoxic (70 s) compared with normoxic (25-40 s) rats (P < 0.05). After acute bilateral carotid body denervation, N2 inhalation produced a short depression of phrenic nerve discharge in both chronic hypoxic and normoxic rats. However, the degree and duration of depression of phrenic nerve discharge was smaller in chronic hypoxic compared with normoxic rats (P < 0.05). We conclude that after exposure to chronic hypoxia, a reduction in PHFD contributes to an increased duration of the acute hypoxic ventilatory response in anesthetized rats. Furthermore, after exposure to chronic hypoxia, the central network responsible for respiration is more resistant to the depressant effects of acute hypoxia in anesthetized rats.     

Region-specific effects on brain metabolites of hypoxia and hyperoxia overlaid on cerebral ischemia in young and old rats: a quantitative proton magnetic resonance spectroscopy study

Background  

Both hypoxia and hyperoxia, deregulating the oxidative balance, may play a role in the pathology of neurodegenerative disorders underlain by cerebral ischemia. In the present study, quantitative proton magnetic resonance spectroscopy was used to evaluate regional metabolic alterations, following a 24-hour hypoxic or hyperoxic exposure on the background of ischemic brain insult, in two contrasting age-groups of rats: young - 3 months old and aged - 24 months old.     

Age-dependent effects of chronic hypoxia on pulmonary vascular reactivity

Effects of age on the pulmonary vascular responses to histamine (HIST), norepinephrine (NE), 5-hydroxytryptamine (5-HT), and KCl were studied in isolated, perfused lungs from juvenile (7-wk-old), adult (14-wk-old), and mature adult (28-wk-old) normoxic rats and compared with age-matched rats exposed to chronic hypoxia for either 14 or 28 days. Chronic hypoxia changed vasoconstriction to HIST and NE to vasodilation in lungs from juvenile and adult rats. Mature adult lungs only vasoconstricted to these amines in both control and hypoxic animals. Pressor responses to 5-HT were not affected by chronic hypoxia regardless of age group. Pressor responses to KCl were also not altered by hypoxia, but lungs from older rats showed greater control responsiveness to KCl compared with lungs from juveniles. Only lungs from juvenile animals developed significant elevations of base-line resistance as a result of hypoxic exposure. To investigate the contribution of H1-, H2-, and beta-receptors in these changes, we employed chlorpheniramine, metiamide, and propranolol, respectively, as blocking agents in another series of experiments. Chlorpheniramine either reduced vasoconstriction or increased vasodilation to HIST in lungs from both control and hypoxic animals, whereas metiamide was without effect. Propranolol either increased vasoconstriction or reversed vasodilation to HIST and NE in all lungs studied. The present data demonstrate the important interaction between chronic hypoxia and age that can alter pulmonary vascular tone and reactivity. The inverse relationship between age and elevation of pulmonary vascular resistance after chronic hypoxic exposure may be the key element that changes pulmonary vascular reactivity observed during hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)