Acclimatization of newborn rats and guinea pigs to 3000 to 5000 m simulated altitudes

A simple and novel method to monitor breathing and heart rate in awake and urethane-anesthetized newborn rodents

Rodents are most useful models to study physiological and pathophysiological processes in early development, because they are born in a relatively immature state. However, only few techniques are available to monitor non-invasively heart frequency and respiratory rate in neonatal rodents without restraining or hindering access to the animal. Here we describe experimental procedures that allow monitoring of heart frequency by electrocardiography (ECG) and breathing rate with a piezoelectric transducer (PZT) element without hindering access to the animal. These techniques can be easily installed and are used in the present study in unrestrained awake and anesthetized neonatal C57/Bl6 mice and Wistar rats between postnatal day 0 and 7. In line with previous reports from awake rodents we demonstrate that heart rate in rats and mice increases during the first postnatal week. Respiratory frequency did not differ between both species, but heart rate was significantly higher in mice than in rats. Further our data indicate that urethane, an agent that is widely used for anesthesia, induces a hypoventilation in neonates whilst heart rate remains unaffected at a dose of 1 g per kg body weight. Of note, hypoventilation induced by urethane was not detected in rats at postnatal 0/1. To verify the detected hypoventilation we performed blood gas analyses. We detected a respiratory acidosis reflected by a lower pH and elevated level in CO2 tension (pCO2) in both species upon urethane treatment. Furthermore we found that metabolism of urethane is different in P0/1 mice and rats and between P0/1 and P6/7 in both species. Our findings underline the usefulness of monitoring basic cardio-respiratory parameters in neonates during anesthesia. In addition our study gives information on developmental changes in heart and breathing frequency in newborn mice and rats and the effects of urethane in both species during the first postnatal week.

Enhanced alveolar growth and remodeling in Guinea pigs raised at high altitude

To examine the effects of chronic high altitude (HA) exposure on lung structure during somatic maturation, we raised male weanling guinea pigs at HA (3800m) for 1, 3, or 6 months, while their respective male littermates were simultaneously raised at low altitude (LA, 1200m). Under anaesthesia, airway pressure was measured at different lung volumes. The right lung was fixed at a constant airway pressure for morphometric analysis under light and electron microscopy. In animals raised at HA for 1 month, lung volume, alveolar surface area and alveolar-capillary blood volume (V(c)) were elevated above LA control values. Following 3-6 months of HA exposure, increases in lung volume and alveolar surface area persisted while the initial increase in V(c) normalized. Additional adaptation occurred, including a higher epithelial cell volume, septal tissue volume and capillary surface area, a lower alveolar duct volume and lower harmonic mean diffusion barrier resulting in higher membrane and lung diffusing capacities. These data demonstrate enhanced alveolar septal growth and progressive acinar remodeling during chronic HA exposure with long-term augmentation of alveolar dimensions as well as functional compensation in lung compliance and diffusive gas transport.

Ventilatory acclimatization in awake guinea pigs raised at high altitude

To determine if laboratory strains of guinea pigs bred at sea level (SL) are "pre-adapted" to high altitude (HA), we raised litter-matched weanling Hartley guinea pigs for 4 months at SL, intermediate altitude (IA, 1250 m) or HA (3800 m) and exposed them acutely to 100, 21 and 12% inspired O2 at their respective altitude of residence. Control animals raised at SL were also exposed acutely to 10 and 8% inspired O2. In awake spontaneously breathing guinea pigs raised at SL, resting minute ventilation and tidal volume increased significantly when inspired O2 tension fell below about 60 mm Hg. In guinea pigs raised at IA or HA, ventilation was higher at any given inspired O2 tension in direct relationship to the altitude of residence. Resting hematocrit was also higher in animals raised at HA than at SL. We conclude that the pattern of ventilatory acclimatization to HA exposure in Hartley guinea pigs is similar to that in laboratory rats and human lowlanders; therefore laboratory guinea pigs are not pre-adapted and are suitable animals for the study of adaptation to high altitude.

Chronic hypoxia increases the NO contribution of acetylcholine vasodilation of the fetal guinea pig heart

To investigate the effect of chronic hypoxia (HPX) on vasodilation of the fetal heart, we exposed pregnant guinea pigs to room air or 12% O(2) for 4, 7, or 10 days. We excised hearts from anesthetized fetuses (60 +/- 3 days; 65-day gestation = term) and measured changes in both the coronary artery pressure of the isolated constant-flow preparation and endothelial nitric oxide synthase (eNOS) mRNA of fetal ventricles. Dilator responses to cumulative addition (10(-9)-10(-5) M) of acetylcholine and sodium nitroprusside in prostaglandin F(2alpha) (5 x 10(-6) M)-constricted hearts were similar among normoxia (NMX), 4-, 7-, and 10-day HPX (control). Nitro-L-arginine (L-NA, 10(-4)M), a NOS inhibitor, inhibited maximal acetylcholine dilation of hearts exposed to 10-day HPX greater than NMX, 4-, and 7-day HPX. Hypoxia (after 7 and 10 days) increased eNOS mRNA of fetal ventricles compared with NMX and 4-day HPX. 4-Aminopyridine (3 mM), a voltage-dependent K(+)-channel inhibitor, inhibited acetylcholine- but not sodium nitroprusside-induced dilation of NMX and 10-day HPX hearts to a similar magnitude. Glibenclamide (10(-5) M), an ATP-sensitive K(+)-channel inhibitor, had no effect on vasodilation. We conclude that chronic HPX increases the contribution of NO but does not alter K(+)-channel activation in response to acetylcholine-stimulated coronary dilation. Thus increases in NO production via upregulation of eNOS gene expression may be an adaptive response to chronic HPX in the fetal coronary circulation.

Dietary nucleotide supplementation raises erythrocyte 2, 3-diphosphoglycerate concentration in neonatal rats

The present study was designed to test if dietary intake of nucleotides increases erythrocyte 2,3-diphosphoglycerate (2,3-DPG) in neonatal rats. To this end, rat pups were fed a nucleotide-supplemented formula (S, n = 14) from d 9 until d 16 after birth. The results were compared with those obtained from a group of breast-fed pups (C, n = 14) and a group of pups artificially fed with nucleotide-free formula (NS, n = 14). Neonatal weight, 2,3-DPG concentration, hematocrit (Hct) and hemoglobin concentration (Hb) were determined before the experiment (d 9) and after 7 d of treatment (d 16). In all groups, 2,3-DPG concentration was greater at d 16 than d 9, and the increase was greater in the S group than in the NS group. Alterations in neonatal weight, Hct and Hb concentration did not differ among the groups. On d 16 the 2, 3-DPG/Hb ratio, reflecting the affinity of hemoglobin for oxygen, was significantly higher in the C and S groups than in the NS group. We conclude that in neonatal rats, dietary nucleotides increase erythrocyte 2,3-DPG concentration. Studies need to be conducted in humans to assess the effect of this increase on both neonatal peripheral hemodynamics and metabolism in this species.

Time course of lung growth following exposure to hypobaria and/or hypoxia in rats

Four-week-old rats were divided into five groups: general controls, weight-matched controls (weight matched to hypobaric hypoxia), hypobaric hypoxia, normobaric hypoxia, and hypobaric normoxia. Lung growth impairment in weight-matched animals occurred by reduction in cell number and size. In both hypoxic groups, lung weight, RNA and protein were significantly higher on day 3, and DNA on day 5 and remained higher thereafter. Maximum 3H-TdR incorporation occurred on day 3 in both hypoxic groups. Hypoxia increased RNA/DNA ratio on day 1 and protein/DNA on day 3. Following 3 days of recovery, DNA synthesis and RNA/DNA ratio of hypoxic groups and controls were identical. DNA synthesis also doubled on day 5 in hypobaric normoxia compared to general controls. Hypoxia up regulates lung growth despite down regulation by undernutrition. Maximum lung growth stimulation occurs during early exposure by cellular hypertrophy followed by hyperplasia. Low pressure by itself also stimulates lung growth. Cellular activity returns immediately to normal levels after removal of hypoxic stimulus.

The effects of prenatal exposure to hypoxia on the behavior of rats during their life span

The aim of this study was to investigate the influence of moderate prenatal damage on adaptability during the juvenile, adult, and senile phases. Pregnant rats were exposed to a 12% normobaric hypoxia from day 1 to 17 postconception. Pregnancy was normal in both the treated animals and the controls. Erythrocytes, hemoglobin, and hematocrit did not increase in the treated pregnant animals. During the first 3 weeks, the F1 generation showed developmental deviations in physiological characteristics. Throughout subsequent ontogeny, motor performance, cognitive ability, and adaptability to physical stress were determined with a test battery of varying demands. Some of the differences (e.g., locomotor activity, learning ability) between juvenile untreated and treated rats disappeared during the adult phase. Motor and coordinative abilities, however, remained partially impaired in the old rats, especially under high demands. This study, and previous findings with alcohol (37), indicate that prenatal exposure to a noxa may result in a highly differentiated brain injury pattern. Depending on the different functions, damage may intensify age-dependent adaptive disorders or provoke impairment without influencing the course of development.

Neonatal hypoxia: long term effects on pulmonary arterial muscle

The purpose of this study was to determine if neonatal hypoxia alters pulmonary arterial smooth muscle (PASM) function in young adult rats. One day old rats were made hypoxic (FIO2 = 0.1) for 5 days, then maintained under normoxic conditions until young adulthood (45-50 days). Age-matched rats were used as controls. Body weight, hematocrit, dry lung weight, and right to left heart ratios were measured. Reactivity and/or responsiveness of rings of main right and left pulmonary artery of the adult to various agonists, including high K+ (80 mM KCl), norepinephrine (NE), serotonin (5HT), adenosine (AD), and acute in vitro hypoxic vasoconstriction were assessed. Isometric force production was normalized to calculated tissue cross-sectional area (N/cm2). Maximum force production (PO) in response to 80 mM KCl for isolated rings from the hypoxic group was significantly less than for controls. Isometric force production in response to NE or to 5HT was also lower in the hypoxic group although the difference was significant for 5HT only when the endothelium was rendered non-functional. When the endothelium was intact, arterial rings from experimental animals relaxed at low doses of adenosine (10(-8) M to 10(-5) M), while control arterial muscle showed no response at these concentrations. The mean dose-response curve for NE from preparations with intact endothelium from experimental animals was significantly lower than that for the control animals, at least at doses greater than 10(-7) M. To mimic acute hypoxic pulmonary vasoconstriction, isolated rings of the main right and left pulmonary artery were precontracted with either 30 mM KCl or 2.5 x 10(-7) M NE and then made hypoxic by lowering muscle bath PO2 to 30-40 mmHg. In conclusion there was no difference in the hypoxic response per se between arterial rings from experimental animals and controls. However, maximum reactivity to high potassium stimulation and to norepinephrine stimulation is decreased in pulmonary arterial smooth muscle of adult animals that had been exposed to 5 days of hypoxia as neonates.

Hypoxia induced hearing loss in animal models of the fetus in-utero

The human fetus in-utero has low arterial oxygen tension. It has, therefore, been suggested that at greater than 28 weeks gestational age, the fetus may have a sensori-neural hearing loss comparable to that seen in adult cats exposed to similar degrees of hypoxia. This is due to hypoxia induced depression of the endocochlear potential. However, fetal blood is provided with compensatory mechanisms (elevated hematocrit and hemoglobin and special fetal hemoglobin) which enable pick up and transport of more oxygen from the placenta than adult blood under the same physiological conditions. Therefore, the hypothesis of a fetal sensori-neural hearing loss due to oxygen lack was tested in the following animal models: a) Adult cats to which feline red blood cells were infused thus causing a polycythemia similar to fetal conditions; b) Adult rats acclimated to altitude in a hypobaric chamber, inducing erythropoiesis with elevated hematocrit and hemoglobin; c) Neonatal guinea pigs and goats studied when they were less than 12 hours old so that the fetal compensatory mechanisms were still present. In each model, hypoxia (PaO2 20-30 mmHg) induced an ABR threshold elevation resembling that obtained in the uncompensated adult animal. Thus these experiments seem to have confirmed the hypothesis of a fetal, hypoxic induced sensori-neural hearing loss even though such experiments have not been conducted directly on fetal animals.

Effects of chronic hypoxia on behavioral and physiological parameters

Aging is a process accompanied by progressive susceptibility to disturbances and impairments in adaptability. The influence of age on the capacity to adapt to chronic normobaric hypoxia (10%) was studied in 4, 20 and 27 month old rats. Adaptation was measured for four days by food and fluid intake and spontaneous activity. In addition the values of the pO2, pCO2, pH-value, Na+ concentration and hematocrit were determined. The results show an initial marked decline for the three modalities of behavior in all age groups. While age-dependent differences were exhibited, for the adaptation course in nourishment and spontaneous activity, the values of the blood gases and hematocrit did not reveal age-related changes. Thus, physiological parameters and behavior are not consistent regarding the effects of age on adaptation capacity.

Cellular hyperplasia and hypertrophy, capillary proliferation and myoglobin concentration in the heart of newborn and adult rats at high altitude

Newborn rats and their mothers were subjected to a simulated altitude of 5000 m for 4-5 weeks. Weight, capillary density (CD), fiber cross-sectional area (AF) and capillary-to-fiber ratio (C/F) of right (RV) and left (LV) ventricles and myocardial myoglobin (Mb) concentration were measured weekly in the newborns and at the end of the high altitude sojourn in the adults. Results were compared to sea level controls. In the adults, adaptive changes were only observed in the right ventricle. In newborns both, RV and LV, exhibited significant alterations. After 2 weeks at 5000 m the ventricular weight increase was 223% (RV) and 40% (LV) in the newborns and 96% in the adults' RV. Whereas only fiber hypertrophy was detectable in the RV of the dams, cardiac weight increase of the acclimatized neonates resulted from both, hypertrophy and hyperplasia of the myocytes. Appropriate capillary proliferation kept CD constant. Cardiac Mb concentration did not change. We conclude, that capillary neoformation primarily counteracts the increase of the O2 diffusion distance due to fiber hypertrophy and/or hyperplasia.

Comparison of glycogen store in two strains of rat and guinea-pig under fed and fasted conditions

Glycogen content in the liver, skeletal muscle and heart has been determined in Sprague-Dawley (SD) and Wistar (W) rats and in tricoloured (T) and albino Dunkin Hartley (DH) guinea-pigs. The 12-week-old animals were studied under non-fasted or control conditions (N) and after 48 hr of fast (F48). Hepatic glycogen was higher in DH guinea-pigs (95.6 +/- 3.8 mg g-1) than in W (77.2 +/- 5.3 mg g-1) and SD (80.2 +/- 2.3 mg g-1) rats under N conditions. Mean values for the two strains were slightly higher in guinea-pigs than in rats. After fasting, hepatic glycogen was almost exhausted in the two species but was higher in W (1.5 +/- 0.08 mg g-1) and T (1.5 +/- 0.2 mg g-1) than in SD and DH (0.6 +/- 0.1 mg g-1). The content of glycogen in the anterior muscles of the thigh was comparable in the two strains of rat and guinea-pig, but was twice as high in the guinea-pigs (DH:15.1 +/- 0.6; T: 16.4 +/- 0.7 mg g-1) as in the rats (SD: 8.1 +/- 0.2; W: 7.1 +/- 0.5 mg g-1) under N conditions. In F48 animals, muscular glycogen decreased by 41-46% (rats) and 38-39% (guinea-pigs). Hepatic and extra-liver glycogen stores were calculated and found higher in the guinea-pigs than in the rats. The total utilization during fasting was larger in the guinea-pigs (6140 mg/kg body wt) than in the rats (4500 mg/kg body wt).(ABSTRACT TRUNCATED AT 250 WORDS)

Inter and intraspecies genetic differences in survival to an acute carbon monoxide challenge in mice, rats, guinea-pigs, quails and chickens

1. Groups of small laboratory vertebrates, of both sexes, have been submitted to an acute carbon monoxide challenge, close to a LD50. 2. An interspecific classification, from low to high CO sensitivity, gives: guinea-pigs-mice-chicks-rats-quails. 3. Interstrain statistical differences were observed in all species, with the exception of guinea-pigs. Furthermore, among two selected genotypes of Japanese quails, one was resistant to acute nitrogen hypoxia and to carbon monoxide intoxication and the other sensitive to both these types of hypoxia. 4. Interspecific comparisons show similarities of genetic differences to acute CO intoxication and acute nitrogen normobaric hypoxia.

2,3-diphosphoglycerate in high- and low-altitude populations of the deer mouse

Erythrocyte DPG concentrations were studied in populations of Peromyscus maniculatus native to different altitudes. Deer mice native to, and resident at, high altitude showed higher DPG/Hb ratios than did deer mice native to, and resident at, low altitude. However, after prolonged acclimation to low altitude, the high-altitude natives showed lower DPG/Hb ratios than did low-altitude natives. The differences in baseline DPG levels appear to be determined primarily by genetic factors. The heritability (h2) of DPG/Hb ratios was estimated by two independent methods to be 0.51 and 0.66, but those values are inflated to an unknown extent by environmental correlations between relatives. Mild cold stress did not elicit an increase in DPG/Hb ratios. DPG levels tended to decrease with age, but the aging effect was not consistently observed in all samples. Neither sex nor alpha-globin genotype influenced DPG levels. The adaptive significance of the genetic differences in DPG metabolism between high- and low-altitude populations remains problematic.

2,3-DPG levels in relation to red cell enzyme activities in rat fetuses and hypoxic newborns

We have measured in red cells from fetal and adult Sprague-Dawley and Wistar rats the activities of phosphofructokinase (PFK), pyruvate kinase (PK) and diphosphoglyceromutase (DPGM) as key enzymes in the regulation of 2,3-diphosphoglycerate (2,3-DPG) levels to gather information on the possible causes of the low concentration of 2,3-DPG in fetal red cells. The most striking differences were seen with regard to PK and DPGM activities. The activity of PK was ten times higher in fetal compared to adult red cells, whereas red cell DPGM activity was absent in fetuses and high in adults. In addition, we studied postnatal changes in red cell PK and DPGm activities as well as in the 2,3-DPG concentration in Sprague-Dawley rats. The concentration of 2,3-DPG and the activity of DPGM in red cells increased to almost the adult value within 2 and 4 weeks after birth, respectively, while the activity of PK decreased concomitantly. The postnatal changes occurred similarly, when newborn rats grew up under conditions of hypoxic hypoxia at 0.46 atm (pO2 = 9.2 kPa). Our studies support the hypothesis that postnatal changes in 2,3-DPG levels are due to changes in the activity of certain glycolytic enzymes and that the switch from fetal-type to adult-type red cells follows a genetically determined time course.