Effect of glycochenodeoxycholic acid on unidirectional transepithelial fluxes of electrolytes in the perfused human ileum

Metabolism, temperature, and ventilation

In mammals and birds, all oxygen used (VO2) must pass through the lungs; hence, some degree of coupling between VO2 and pulmonary ventilation (VE) is highly predictable. Nevertheless, VE is also involved with CO2 elimination, a task that is often in conflict with the convection of O2. In hot or cold conditions, the relationship between VE and VO2 includes the participation of the respiratory apparatus to the control of body temperature and water balance. Some compromise among these tasks is achieved through changes in breathing pattern, uncoupling changes in alveolar ventilation from VE. This article examines primarily the relationship between VE and VO2 under thermal stimuli. In the process, it considers how the relationship is influenced by hypoxia, hypercapnia or changes in metabolic level. The shuffling of tasks in emergency situations illustrates that the constraints on VE-VO2 for the protection of blood gases have ample room for flexibility. However, when other priorities do not interfere with the primary goal of gas exchange, VE follows metabolic rate quite closely. The fact that arterial CO2 remains stable when metabolism is changed by the most diverse circumstances (moderate exercise, cold, cold and exercise combined, variations in body size, caloric intake, age, time of the day, hormones, drugs, etc.) makes it unlikely that VE and metabolism are controlled in parallel by the condition responsible for the metabolic change. Rather, some observations support the view that the gaseous component of metabolic rate, probably CO2, may provide the link between the metabolic level and VE.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Response to novelty, social and self-control behaviors, in rats exposed to neonatal anoxia: modulatory effects of an enriched environment

Perinatal asphyxia is a concern for public health and may promote subtle and long-lasting neuropsychiatric disorders. In the present study, newborn Wistar rat pups underwent a repeated 20-min exposure to a 100% N2 atmosphere (or air) on postnatal days (pnd) 1, 3, 5, and 7. Half of the animals were housed during adolescence (pnd 21-35) in an enriched environment. The consequences on behavior were assessed throughout adolescence to adulthood. When scored for social performance, adolescent rats exposed to neonatal asphyxia exhibited exaggerated levels of anogenital sniffing behavior, which was normalized by enriched living. In air-exposed controls, enriched living increased the expression of affiliative and novelty-seeking behaviors, as compared to standard housing. However, this enrichment-induced behavioral plasticity was not found in rats neonatally exposed to asphyxia. At adulthood, levels of impulsivity and 5-HT2A receptors in the striatum were markedly increased in neonatal-asphyxia rats kept in standard-housing conditions. Interestingly, impulsivity and receptor density were normalized by enriched rearing during adolescence. These findings indicate profound long-lasting behavioral alterations as a consequence of repeated neonatal asphyxia in rats. Beneficial effects of stimulation by an enriched environment during the still-plastic window of adolescence are suggested in these animals.

Influence of temperature on metabolism and breathing during mammalian ontogenesis

A literature survey of the ventilatory responses to changes in ambient temperature (T) in neonatal mammals reveals that, as in adults, the metabolic response to T is the major determining factor. In fact, the newborn's metabolic response to changes in T determines not only the pulmonary ventilation and the breathing pattern, but also the magnitude of the ventilatory responses to chemical stimuli and the intensity of the pulmonary reflexes at different T. The important difference from the adult is that in many neonatal mammals the control of body temperature (T(b)) is poorly developed. Hence, the metabolic response can be more similar to that of an ectothermic, rather than endothermic, animal, and T(b) can vary substantially with T. When hypoxia occurs in cold, T(b) can decrease greatly, because of the hypoxic drop in the thermoregulatory set-point, and this lowers pulmonary ventilation. Hence, in addition to the metabolic response, also the changes in T(b) are a factor modulating the ventilatory responses to T. Artificial warming of the newborn during hypoxia causes heat-dissipation responses that can be counterproductive. During ontogenesis, with prolonged cold conditions, the sustained alterations in metabolic rate and body growth do not modify the postnatal development of the respiratory control mechanisms. Presumably, this indicates that respiratory regulation develops independently from the individual's metabolic history.

Implications of hypoxic hypometabolism during mammalian ontogenesis

During hypoxia, many newborn mammals, including the human infant, decrease metabolic rate, therefore adopting a strategy common to many living creatures of all classes, but usually not adopted by adult humans and other large mammals. In acute hypoxic conditions, hypometabolism largely consists in actively dropping mechanisms of thermoregulation. One implication is a decrease in body temperature. This is a safety mechanism, which favours hypoxic survival. Indeed, artificial warming during hypoxia can be counterproductive. Because carbon dioxide is an important stimulus for pulmonary ventilation, the drop in its metabolic production may tilt the balance of ventilatory control in favor of respiratory inhibition. Some experimental data support this view. In conditions of sustained hypoxia, the newborn's hypometabolism also results from a depression of tissue growth and differentiation. Some organs are affected more than others. To what extent the blunted organ growth will be compatible with survival depends not only on the severity and duration of hypoxia, but also on the timing of its occurrence during development. Upon termination of hypoxia, the newborn's metabolic rate recovers and growth resumes at higher rate. Even if body weight may be completely regained, alterations in the respiratory mechanical properties and in aspects of ventilatory control can persist into adulthood, a phenomenon not seen when the hypoxia was experienced at later stages of development. Some of the long-term respiratory effects of neonatal hypoxia are reminiscent of those observed in adult animals and humans native and living in high altitude regions.

Developmental plasticity in respiratory control

Development of the mammalian respiratory control system begins early in gestation and does not achieve mature form until weeks or months after birth. A relatively long gestation and period of postnatal maturation allows for prolonged pre- and postnatal interactions with the environment, including experiences such as episodic or chronic hypoxia, hyperoxia, and drug or toxin exposures. Developmental plasticity occurs when such experiences, during critical periods of maturation, result in long-term alterations in the structure or function of the respiratory control neural network. A critical period is a time window during development devoted to structural and/or functional shaping of the neural systems subserving respiratory control. Experience during the critical period can disrupt and alter developmental trajectory, whereas the same experience before or after has little or no effect. One of the clearest examples to date is blunting of the adult ventilatory response to acute hypoxia challenge by early postnatal hyperoxia exposure in the newborn. Developmental plasticity in neural respiratory control development can occur at multiple sites during formation of brain stem neuronal networks and chemoafferent pathways, at multiple times during development, by multiple mechanisms. Past concepts of respiratory control system maturation as rigidly predetermined by a genetic blueprint have now yielded to a different view in which extremely complex interactions between genes, transcriptional factors, growth factors, and other gene products shape the respiratory control system, and experience plays a key role in guiding normal respiratory control development. Early-life experiences may also lead to maladaptive changes in respiratory control. Pathological conditions as well as normal phenotypic diversity in mature respiratory control may have their roots, at least in part, in developmental plasticity.

Age-dependent metabolic effects of repeated hypoxemia in piglets

The aim of this study was to determine whether repeated exposure to hypoxemia would modify the response to hypoxemia during maturation. We exposed piglets to three 1-h cycles of hypoxemia (PaO2 = 30 to 35 mmHg; 1 mmHg = 133.3 Pa) at 1 week (n = 9), 2-3 weeks (n = 10), and 4-5 weeks of age (n = 10). O2 consumption (VO2) and CO2 production (VCO2) were measured, and alveolar ventilation (VA) was derived from VCO2 and PaCO2. Levels of lactic acid (lactate) and serum catecholamines were also measured. With hypoxemia, time had a significant effect on VO2 and body temperature in an age-dependent fashion: that is, whereas the 1 week group and the 4-5 week group showed both variables decreasing over time, the 2-3 week group showed no drop in VO2 and a small increase in body temperature over time. Lactate levels increased with hypoxemia in all animals during the first exposure. However, with repeated exposures to hypoxemia, only the 2-3 week group continued to increase its lactate levels. Furthermore, the changes in lactate levels paralleled the changes in epinephrine levels with hypoxemia. We found, too, that although VA increased significantly with hypoxemia in all animals, this change was not modified by age or repeated exposures. No significant effects of age or repeated exposures were found in the cardiovascular response to hypoxemia. We concluded that, from a metabolic viewpoint, after repeated exposures to hypoxemia the 2-3 week animals responded differently.Key words: metabolic rate, lactic acid, maturation, catecholamines.

Effect of neonatal anoxia on the ventilatory response to hypoxia in developing rats

The purpose of the study was to determine whether an episode of anoxia in the neonatal period affects the hypoxic ventilatory response in developing rats. Three to 4 day old rats (EXP rats) (day 0 = day of birth) were exposed to 100% N(2) for about 20 min. In anoxia, animals changed their respiratory movements to "gasping," and were successfully autoresuscitated by placing them back into air. None of the EXP rats died after the resuscitation, and no differences were found between the EXP and control (CONT) rats in the somatic growth. The CONT rats were not exposed to anoxia during the neonatal period. On day 9-10(mean day 9) and days 23-29(mean day 25), ventilatory and metabolic responses to hypoxia (inspiratory oxygen fraction F(i)O(2 )= 10% on day 9; 15% and 10% on day 25) were compared in the EXP and CONT groups. On day 9, there were no differences in ventilatory and metabolic responses to hypoxia between the EXP and CONT groups. On day 25, 10% O(2)hypoxic ventilatory response (by the barometric method) was significantly higher in the EXP group than in the CONT group, as indicated by increases in both tidal volume (V(T)) and mean inspiratory flow (V(T)/T(I)). On the other hand, the metabolic effects were small, because metabolic rates (V'O(2) and V'CO(2), by a flow-through method) were similar in both EXP and CONT groups under each gaseous condition. These results suggest that an episode of anoxia during the neonatal period has long-lasting effects on the control of ventilation, including chemoreflexes, in rats.

Hypoxia inhibits cold-induced huddling in rat pups

We asked to what extent hypoxia would modify the huddling behaviour of young rats during cold exposure. Sets of five animals (postnatal age 9+/-1 days) were placed at predetermined positions in a chamber maintained at approximately 33 degrees C (warm) or approximately 15 degrees C (cold), in normoxia or hypoxia (10% inspired O2), and their movements monitored for 30 min by a video camera. The surface areas (SA) of each individual pup (SAi) and of the whole set of pups (SAset) was measured every 5 min. In warm, the rats spread out, and both SAi and SAset were the greatest, whether in normoxia or hypoxia. In hypoxia, the total travelled distance (TTD) was much greater than in normoxia. In cold, during normoxia, SAi and SAset were decreased because of postural changes and huddling, and body temperature, measured at the end of the exposure, was also decreased. In hypoxic-cold, compared to normoxic-cold, fewer pups were in contact with one another, SAi and SAset did not decrease and the drop in body temperature was larger. Differently from hypoxia, hypercapnia (5% CO2) did not modify the responses observed during breathing air, whether in warm or cold conditions. We conclude that hypoxia, in addition to inhibiting shivering and non-shivering thermogenesis, can also limit behavioural thermogenesis, with the effect of further lowering body temperature.