Hypoxic hypometabolism in chicken embryos: conformism and downregulation.

Postnatally, during hypoxia the decrease in oxygen consumption ( [Formula: see text] ) can exceed what expected from the limitation in O2 availability, meaning that [Formula: see text] -downregulation exceeds O2-conformism. We questioned whether a similar phenomenon could occur prenatally, in chicken embryos at mid- (E11, out of 20.5 days) or near end- (E18) incubation. [Formula: see text] was measured with an open-flow system in the sequence of normoxia-normothermia (21% O2, 37 °C, 30 min), hypoxia in normothermia (Hx-NT, either 18, 15, 12 or 9% O2, 37 °C, 1 hour), hypoxia in hyperthermia (Hx-HT, up to 43 °C, 1 hour) and return to normoxia-normothermia (30 min). During Hx-NT [Formula: see text] invariably decreased in a [O2]-dependent fashion. The hypoxic drop in [Formula: see text] did not require a post-hypoxic payment of the O2-debt, implying that the decrease in [Formula: see text] reflected hypometabolism. [Formula: see text] did not differ significantly between Hx-HT and Hx-NT for [O2] = 15% or less, as expected by O2-conformism. Differently, with milder hypoxia (18% O2), [Formula: see text] during Hx-HT significantly exceeded that in Hx-NT, meaning that the value of [Formula: see text] in Hx-NT was not limited by O2 supply. We conclude that a phenomenon of hypoxic [Formula: see text] downregulation like that observed in postnatal mammals can occur also prenatally, in the chicken embryos. The mechanisms at the basis of the downregulation remain unresolved and could combine physiological and cellular processes.

Respiratory sinus arrhythmia in the immediate post-exercise period: correlation with breathing-specific heart rate.

BACKGROUND: Although the absolute values of pulmonary ventilation and cardiac output are similar, the designs of the respiratory and cardiovascular systems imply major differences in flow patterns, airflow being intermittent by comparison to the quasi-continuous pulmonary blood flow. PURPOSE: We hypothesized that respiratory sinus arrhythmia (RSA, difference in heart rate (fH) between inspiration and expiration, as percent of mean fH) ameliorates the inevitable differences between air- and blood-flow patterns. Specifically, we hypothesized RSA to correlate more closely to the ratio between fH and breathing frequency (fR) (fH/fR "breathing-specific heart rate", a proxy for cardio-respiratory coupling) than to either fH or fR alone. Hence, we designed protocols to change independently fH or fR. METHODS: We measured RSA breath-by-breath in 145 young men and women during spontaneous breathing, breathing under cues at different fR (to modify the denominator of fH/fR) and immediately post-exercise while breathing freely or by keeping fR as at rest (to modify the nominator of fH/fR). RESULTS: RSA had no significant correlation with fH, and a better correlation with fH/fR (r2 = 0.92) than with fR alone (r2 = 0.75); the variance of the Y values of the fH/fR-RSA correlation was ~ half that of the fR/RSA correlation (P < 0.002). CONCLUSIONS: We propose that the fH/fR-RSA relationship reflects a central process that ameliorates gas exchange against the difference between air- and blood-flow patterns. The neurological mechanisms are still conjectural. Measurements of RSA could offer a glimpse of the degree of cardio-respiratory central compensation in face of the inequality between blood flow and airflow.

Thermal tachypnea in avian embryos.

Many adult mammals and birds respond to high surrounding temperatures with thermal tachypnea - an increase in breathing frequency accompanied by shallow tidal volume, with minimal increase in oxygen consumption (V̇O2 ). This pattern favors heat dissipation by evaporative water loss (EWL) through the respiratory tract. We asked to what extent this response was apparent at the earliest stages of development, when pulmonary ventilation initiates. Measurements of pulmonary ventilation (V̇E; barometric technique), V̇O2  (open-flow methodology) and EWL (water scrubbers) were performed on chicken embryos at the earliest appearance of pulmonary ventilation, during the internal pipping stage. Data were collected, first, at the normal incubation temperature (37.5°C); then, ambient and egg temperatures were increased to approximately 44°C over a 2 h period. Other embryos of the same developmental stage (controls) were maintained in normothermia for the whole duration of the experiment. During heat exposure, the embryo's V̇O2  and carbon dioxide production increased little. In contrast, V̇E more than doubled (∼128% increase), entirely because of the large rise in breathing frequency (∼132% increase), with no change in tidal volume. EWL did not change significantly, probably because, within the egg, the thermal and water vapor gradients are almost nonexistent. We conclude that chicken embryos respond to a major heat load with tachypnea, like many adult mammals and birds do. Its appearance so early in development, although ineffective for heat loss, signifies that thermal tachypnea represents an important breathing response necessary to be functional from hatching.

The contribution of heart rate to the oxygen consumption of the chicken embryo during cold- or hypoxia-hypometabolism.

In embryos, cooling and hypoxia cause a decrease in oxygen consumption ( [Formula: see text] ); we asked what was the relative contribution of heart rate (HR) and of the 'not-HR' factor (the product of stroke volume and arterial-venous O2 difference) to the drop in [Formula: see text] . Data of HR (with subcutaneous electrodes) and [Formula: see text] (by an open-flow methodology) were collected simultaneously on chicken embryos close to end-incubation. Over the last four days of incubation (E16-E20) differences in HR contributed about 30% of the differences in resting [Formula: see text] among embryos. At E20, progressive cooling from 38 to 8°C decreased [Formula: see text] entirely because of the decrease in HR, with minimal compensation of the 'not-HR' component. The same pattern during cooling occurred in younger embryos (age E16), in E20 embryos simultaneously exposed to hypoxia (15% O2) and in E20 normoxic embryos which were incubated in hypoxia (15% O2). Differently, in E20 embryos in normothermia, progressive hypoxia (15%, 10% or 5% O2) lowered [Formula: see text] largely because of the reduction in the 'not-HR' component. We conclude that at end incubation during hypometabolism the changes in HR contribute very differently to the decrease in [Formula: see text] , from about the totality of it during cold to only about 10-20% during hypoxia, depending on its severity. It follows that during cold-hypometabolism, but not during hypoxic hypometabolism, the changes in HR are a good index of the changes in [Formula: see text] . The close relationship between [Formula: see text] and HR during cold-hypometabolism may permit estimates of the changes in [Formula: see text] from the changes in HR in infants undergoing therapeutic hypothermia.

Aerobic scope in chicken embryos.

We investigated the aerobic scope of chicken embryos, that is, the margin of increase of oxygen consumption ( [Formula: see text] ) above its normal value. [Formula: see text] was measured by an open-flow methodology at embryonic ages E3, E7, E11, E15, E19 and at E20 at the internal (IP) and external pipping (EP) phases, at the normal incubation temperature (Ta=38°C), in hypothermia (Ta=30°C) and in hyperthermia (Ta=41 and 44°C). In the cold, Q10 averaged ~2 at all ages, except in IP and EP when lower values (~1.5) indicated some degree of thermogenesis. In hyperthermia (38-44°C) Q10 was between 1 and 1.4. Hyperthermia had no significant effects on [Formula: see text] whether the results combined all ages or considered individual age groups, except in IP (in which [Formula: see text] increased 8% with 44°C) and EP embryos (+13%). After opening the air cell, which exposed the embryo to a higher O2 pressure, hyperthermic [Formula: see text] was significantly higher than in normothermia in E19 (+13%), IP (+22%) and EP embryos (+22%). We conclude that in chicken embryos throughout most of incubation neither heat nor oxygen availability limits the normal (normoxic-normothermic) values of [Formula: see text] . Only close to hatching O2-diffusion represents a limiting factor to the embryo's [Formula: see text] . Hence, embryos differ from postnatal animals for a nearly absent aerobic scope, presumably because their major sources of energy expenditure (growth and tissue maintenance) are constantly maximized.

The cooling time of fertile chicken eggs at different stages of incubation.

We asked whether or not the thermal characteristics of fertile avian eggs changed throughout incubation. The cooling and warming times, expressed by the time constant τ of the egg temperature response to a rapid change in ambient temperature, were measured in fertile chicken eggs at early (E7), intermediate (E11) and late (E20) stages of embryonic development. Same measurements were conducted on eggs emptied of their content and refilled with water by various amounts. The results indicated that (1) the τ of a freshly laid egg was ~50 min; (2) τ decreased linearly with the drop in egg water volume; (3) the dry eggshell had almost no thermal resistance but its wet inner membrane contributed about one-third to the stability of egg temperature; (4) the egg constituents (yolk, albumen and embryonic tissues) and the chorioallantoic circulation had no measurable effect on τ; (5) the presence of an air pocket equivalent in volume to the air cell of fertile eggs reduced τ by about 3 min (E7), 5 min (E11) and 11 min (E20). Hence, in response to warming the egg τ at E20 was slightly shorter than at E7. In response to cooling, the egg τ at E20 was similar to, or longer than, E7 because embryonic thermogenesis (evaluated by measurements of oxygen consumption during cold) offset the reduction in τ introduced by the air cell. In conclusion, until the onset of thermogenesis the thermal behavior of a fertile egg is closely approximated by that of a water-filled egg with an air volume equivalent to the air cell. It is possible to estimate the cooling τ of avian eggs of different species from their weight and incubation time.

Respiratory sinus arrhythmia in young men and women at different chest wall configurations.

Respiratory sinus arrhythmia (RSA) is an acceleration of heart rate during inspiration and deceleration with expiration. We asked whether or not in humans some of the volume-related information necessary for RSA originated from the chest wall. Men and women, 19-20 years old, were breathing supine. Rib cage and abdomen displacement provided an index of tidal volume (VT) and RSA was computed breath-by-breath from the peak and trough of instantaneous heart rate. First, measurements were taken during breathing at rest (protocol a, 129 male and 164 female). Then, in subgroups of the original subject population, measurements were collected for the first five breaths immediately following a brief breath-hold period (protocol b), predominantly with the rib cage or predominantly with the abdomen (protocol c), above functional residual capacity or below it (protocol d). As long as VT was constant, severe chest wall distortion (protocol c) did not modify RSA. A drop in absolute lung volume (protocol d) or an increase in VT (protocol b) respectively decreased and increased RSA. The results, globally taken, are compatible with the notion that in humans changes in lung volume are detected by lung mechanoreceptors, whereas chest wall reflexes play no role in RSA. No difference in RSA emerged between genders during resting breathing or modest breath-hold hyperventilation.

Thermographic analysis of the radiant heat of chicken and duck eggs in relation to the embryo's oxygen consumption.

In eggs, the metabolic activities of the developing embryo produce heat (H) that is dissipated in various forms, including radiation. Given that much of the total heat radiated by an egg (Htot) is heat acquired passively, we asked whether it was possible to detect the fraction produced metabolically (Hmetab) and the extent of its correlation with the embryo's metabolic rate. In chicken and duck eggs at various incubation ages, under standardized experimental conditions of heat conduction and convection, Hmetab was measured by thermography as the difference in Htot between fertile and sterile eggs. Then, Hmetab was correlated to the embryo's oxygen consumption ( [Formula: see text] ), measured by an open-circuit methodology. Heat loss by water evaporation was found to be less than 3% of the total. During the first half of incubation Hmetab was too small to be significantly separated from Htot. In the second half of incubation Hmetab was significant, represented 30-50% of the total energy consumed and correlated linearly with [Formula: see text] for a good fraction of incubation. We conclude that under standardized conditions of heat conduction and convection, in the second half of incubation thermography offers a simple tool not only to verify the progression of the embryo's incubation but also to estimate its metabolic rate.

Breathing pattern and ventilatory chemosensitivity of the 1-day old Muscovy duck (Cairina moschata) in relation to its metabolic demands.

Adult birds have a ventilatory equivalent (pulmonary ventilation-oxygen consumption ratio, V˙ E/ [Formula: see text] ) lower than mammals because of the superior gas exchange efficiency of their respiratory apparatus. In particular, adult Muscovy ducks (Cairina moschata) have been reported to have an extraordinary low ventilatory equivalent (~14mL STPD·mL BTPS(-1)). We asked if similar high efficiency was already apparent in duck hatchlings. Breathing pattern and V˙E were measured by the barometric technique and [Formula: see text] by an open-flow methodology in 1-day old Muscovy duck hatchlings (N=21); same measurements were performed on chicken hatchlings (N=21) for purpose of comparison. During air breathing V˙E/ [Formula: see text] was slightly, yet significantly, lower in ducklings (20.8) than in chicks (25.3), mostly because of a lower breathing frequency (f). The hatchlings of both species (N=14 per group) responded to inspired hypoxia (15 or 10% O2) or hypercapnia (2 or 4% CO2) with a clear hyperventilation; however, in ducklings the hypercapnic hyperventilation was smaller than in chicks because of a smaller increase in tidal volume and lower f. We conclude that duck and chicken hatchlings just a few hours old have the high ventilatory efficiency typical of birds, although possibly not as high as their adults. The low f and blunted V˙E response to hypercapnia of the newborn duck could be related to the aquatic habitat of the species. In such a case, it would mean that these characteristics are genetic traits, the phenotypic expression of which does not require diving experience.

Chicken hatchlings prefer ambient temperatures lower than their thermoneutral zone.

We investigated whether or not the preferred ambient temperature (Tapref) of the 1-day old chicken hatchling, a precocial neonate with excellent locomotory capacity, clearly identifiable thermogenesis and independence from maternal care, coincides with the lower critical temperature (LCT) of thermoneutrality and minimal oxygen consumption (V̇(O(2))). Tapref of single chicks measured in a thermocline (N=16) averaged 33.5±0.3 °C (mode, 33.3±0.4 °C). The same value was obtained in hatchlings studied in pairs. LCT was computed from the ambient temperature (Ta)-V̇(O(2)) relationship, constructed by slowly decreasing the Ta of a respirometer from 38 to 29 °C over 2.5h, while continuously measuring V̇(O(2)) by an open-flow methodology; LCT averaged 36.4 °C±0.3 or 36.8 °C±0.4, depending on the method of computation. In all hatchlings Tapref was lower than LCT (P<0.001), by a magnitude that depended on the method of computation of the two variables, 2.8 °C±0.3 (P<0.001) or 3.9 °C±0.5. The Tapref-LCT difference implied that, at Tapref, V̇(O(2)) was higher than at thermoneutrality. We conclude that in the chicken hatchling thermal preference does not coincide with thermoneutrality, probably because during development what seems optimal from a thermoregulatory viewpoint may not necessarily be so for other regulatory functions.

Thermographic analysis of body surface temperature of mammals.

Among mammals, the similarity in body temperature indicates that body size differences in heat loss must match the body size differences in heat production. This study tested the possibility that body surface temperature (Tbs), responsible for heat loss through radiation and convection, may vary systematically with the animal's body mass (M). Tbs was measured by whole body thermography in 53 specimens from 37 eutherian mammals ranging in M from a few grams to several tons. Numerous thermographs were taken from all angles, indoor, with the animals standing still in absence of air convection and of external radiant sources, at the ambient temperature of 20-22°C, 22-25°C, or 25-27°C. Data were analysed as whole body surface average, as average of the "effective" body surface area (those regions with temperatures exceeding ambient temperature by > 1.5°C or by > 5°C), as the peak histogram distribution and as average of the regions with the top 20% temperature values. For all modes of data analysis and at all ambient temperatures Tbs was independent of the animal's M. From these data, the heat loss by radiation and natural convection combined was estimated to vary to the 2/3 power of M. It is concluded that, for the same ambient conditions, the surface temperature responsible for radiation and convection is essentially body-size independent among mammals.

The Mouse-To-Elephant Metabolic Curve: Historical Overview.

Although it is intuitive that large mammals need more food than smaller ones, it is not so obvious that, relative to their body mass, larger mammals consume less than smaller ones. In fact, on a per kg basis, the resting metabolic rate of a mouse is some 50 times higher than that of an elephant. The fact that metabolism could not be proportional to the mass of the animal was suggested by Sarrus and Rameaux in 1838. The first indication that oxygen consumption (or other indices of metabolic rate, Y) related to the animal body mass (M) according to an exponential of the type Y = a · Mb , where b was about 0.75, was presented by Max Kleiber in 1932. Two years later Samuel Brody had collected sufficient data to construct the first "mouse-to-elephant" metabolic curve. The physiological basis of the relationship has been the object of many hypotheses, often accompanied by a great deal of controversy. This historical essay traces the origin of the mouse-to-elephant metabolic function, recalling the earliest concepts of metabolism and its measurements to understand the body size dependency, which is still one of the most elusive phenomena in comparative physiology. A brief look at the metabolic scaling of nonmammalian organisms will be included to frame the mouse-to-elephant curve into a broader context and to introduce some interesting interpretations of the mammalian function. © 2023 American Physiological Society. Compr Physiol 13:4513-4558, 2023.

Social interaction and the thermogenic response of chicken hatchlings.

The aggregation of two or more individuals of the same species (huddling) is common in mammals and birds, especially in the cold. The physical contact reduces the weight-specific body surface exposed to the environment, thus lowering heat loss and the thermogenic needs. This study investigated the possibility that the mere presence of a conspecific, in absence of physical contact, may by itself influence metabolic rate during cold. The oxygen consumption (Vo2) of pairs of chicken hatchlings was measured when the hatchlings were in isolation (individuals), together in the respirometer but kept separated by a grid (separated) or together in the respirometer free to huddle (together), in random order, in warm (ambient normothermia, 37.5 °C) and cold conditions (26 °C, 1 h). In warm, Vo2 did not differ significantly among individuals, separated and together (~ 1.03 ± 0.04 ml O2/min). During the whole cold period, Vo2 of individuals exceeded the value by 23.3 ± 3.1 ml of O2, significantly more than in separated (15.3 ± 2.0 ml O2, P<0.01) and together (13.9 ± 3.3 ml O2; P<0.001). Separated and together did not differ significantly. Vo2 in the cold averaged 149 ± 7% of the value measured in normothermia in isolated, 132 ± 5% in separated and 128 ± 7% in together. By the end of the cold-exposure, Vo2 averaged 166 ± 8% of normothermia in isolated, 146 ± 8% in separated and 140 ± 9% in together. In all cases, values of isolated significantly exceeded those of separated (P<0.01) and together (P<0.0001), while separated and together did not differ from each other (P>0.05; Two-way RM ANOVA). Hence, in this experimental model, social interaction without physical contact decreased the thermogenic response to cold as much as huddling did. Presumably, during the cold exposure, social interaction lowered the additional energetic cost of the stress of isolation.

Breath-by-breath analysis of respiratory sinus arrhythmia in dogs.

Dogs differ greatly in size, heart (HR) and breathing rates (BR). In addition, they have a clear Respiratory Sinus Arrhythmia (RSA) at rest. Therefore, better than any other mammalian species, dogs offer an opportunity to test whether resting RSA varies with body weight, HR or BR. Sequences of inter-beat-intervals (IBI, ms) a few-minutes long were collected in twenty-three resting dogs of different sizes, together with pneumograms. IBI variability was quantified by standard time-domain criteria. From beat-to-beat instantaneous heart rate (hR, beats/min), RSA was the difference between inspiratory peak (hR-peak) and expiratory trough (hR-trough), in percent of mean HR. RSA averaged 40.1 % ±4.5, or more than three times that of humans, with large inter-animal variability. On average, RSA contributed 38 % of the total IBI variability. RSA did not differ between sexes and did not correlate with body weight. It had modest negative correlations with HR (P < 0.05) and BR (P < 0.05), and a very strong negative correlation with hR-trough (P < 0.001). In two separate dogs, during panting, RSA was absent. In the transition from resting to panting, RSA continued like at rest for several breaths, despite the tachypnea, underlying the importance of central mechanisms in the origin of RSA. In conclusion, RSA in dogs is very large and explains less than half of their sinus arrhythmia. Rather than HR, BR or hR-peak, changes in the vago-sympathetic control, represented by hR-trough, are the most likely source of variability of RSA among subjects.

Growth of the chicken embryo: implications of egg size.

Among avian species with large differences in egg size, changes in eggshell conductance and incubation time permit the water loss necessary for embryonic development. To what extent this happens for different-size eggs within a species is much less known. Chicken eggs with fresh egg weight (Wegg) either large (L, approximately 66 g) or small (S, approximately 51 g) were incubated at 38 degrees C and 60% humidity; their yolk and albumen scaled almost in proportion to Wegg. Eggshell gas conductance scaled to 0.77 of Wegg, as it occurs inter-specifically, while external pipping and hatching occurred at similar times in S and L. Hence, L lost less water during incubation than S, and embryos of L were over-hydrated and those of S were dehydrated. The absolute values of embryo's weight, growth rate, oxygen consumption and the weight of the chorioallantoic membrane were similar between S and L during the first half of incubation, and greater in L in the second half. Incubation in hypoxia reduced growth rate in both sets and maintained the difference in growth trajectories between S and L. The energetic cost of growth and tissue maintenance did not differ significantly. It is concluded that, among chicken eggs of different sizes, 1) the growth rate of the embryo relates to the size of its egg, probably genetically and because of differences in water content, 2) eggshell conductance contributes, but incubation time does not, to the requirements for water loss. Therefore, the egg water balance during incubation may be the physiological constraint that limits the maximal variability in egg size compatible with embryonic survival.

Interactive effects of temperature and hypoxia on heart rate and oxygen consumption of the 3-day old chicken embryo.

In the chick embryo at day 3, gas exchange occurs by diffusion and oxygen consumption (V(O(2))) does not depend on the cardiovascular convection of O(2). Whether or not this is the case in hypoxia is not known and represents the aim of the study. The heart of chicken embryos at 72 h (stage HH18) was filmed through a window of the eggshell by a camera attached to a microscope. Stroke volume was estimated from the changes in heart silhouette between systole and diastole. V(O(2))was measured by a closed system methodology. In normoxia, a decrease in temperature (T) from 39 to 31 degrees C had parallel depressant effects on V(O(2))and HR. At 39 degrees C, a progressive decrease in O(2) lowered V(O(2)); HR was maintained until the O(2) threshold of approximately 15%. In severe hypoxia (4% O(2)) V(O(2))and HR were, respectively, approximately 12% and approximately 62% of normoxia. At 32 degrees C the hypoxic threshold for HR was significantly lower. During constant hypoxia (7% O(2)) V(O(2))did not respond to T, while the HR response was preserved. Stroke volume changed little with changes in T or O(2), except at 6 and 4% O(2), when it decreased by approximately 20 and 30%. In embryos growth-retarded because of incubation in chronic hypoxia, V(O(2))and HR responses to T and hypoxia were similar to those of normal embryos. We conclude that in the early embryo during hypoxia cardiovascular O(2) convection is not responsible for the drop in V(O(2)). The generalised hypometabolic response, in combination with the extremely small cardiac V(O(2)), probably explains the minor effects of hypoxia on cardiac activity.

Respiratory sinus arrhythmia during a mental attention task: the role of breathing-specific heart rate.

It is known that a mental attention task (MAT) can modify the magnitude of the increase in instantaneous heart rate (HR) with inspiration, or Respiratory Sinus Arrhythmia (RSA). Here, we asked whether the RSA changes were mediated by the changes in HR, breathing frequency (f) or HR/f ('breathing specific heart rate'). This latter reflects the degree of coupling between pulmonary blood and air flows, the optimization of which may be the function of RSA. RSA (computed as the difference between peak and trough instantaneous HR of each breath, in percent of mean HR) was measured breath-by-breath in 119 young men and women (19.6 ± 0.1 year old) during spontaneous breathing and during a MAT, which consisted in finger tapping at acoustic cues delivered in various patterns. During MAT, breathing became more rapid (+2.2 breaths/min, P < 0.001) and shallow (78% of rest, P < 0.001) and HR decreased slightly (-1 beats/min, P < 0.05). RSA dropped from 13.4 ± 0.7 to 11.6 ± 0.7% (P < 0.0001), because of the drop in the inspiratory peak of instantaneous HR, and so did HR/f, from 5.8 ± 0.2 to 4.9 ± 0.2 beats/breath (P < 0.0001).The results were very similar between genders. The magnitude of the changes in HR/f correlated linearly with those of RSA, so that those subjects who decreased HR/f the most also had the largest decrease in RSA and the few who increased HR/f during MAT also increased RSA. We conclude that this type of mental task changed RSA by a magnitude that depended on its effect on HR/f. The results support the concept that RSA is a central cardio-respiratory mechanism to ameliorate the matching between pulmonary blood and air flows, whether the ventilatory drive originates spontaneously or is under cortical influences.

Ventilatory response to hypoxia in chicken hatchlings: a developmental window of sensitivity to embryonic hypoxia.

We had reported previously [Szdzuy, K., Mortola, J.P., 2007b. Ventilatory chemosensitivity of the 1-day-old chicken hatchling after embryonic hypoxia. Am. J. Physiol. (Regul. Integr. Comp. Physiol.) 293, R1640-R1649] that hypoxia during incubation blunted ventilatory chemosensitivity in the hatchling. Because the carotid bodies become functional in the last portion of incubation, we asked whether these last days were the critical period for the effects of hypoxia on the development of ventilatory chemosensitivity. White Leghorn chicken eggs were incubated at 38 degrees C either in 21% O(2) (Controls) or in 15% O(2) for the whole 3-week incubation (HxTot) or for only the 1st (Hx1), 2nd (Hx2) or 3rd week of incubation (Hx3). Hatching time had a delay of half a day in HxTot, and was normal in the other groups. Body weight was similar in all hatchlings. Oxygen consumption ( [Formula: see text] ) and pulmonary ventilation (V e) were measured at about 20 h post-hatching. Ventilatory chemosensitivity was evaluated from the degree of hyperpnea (increase in V e) and hyperventilation (increase in [Formula: see text] ) during acute hypoxia (15 and 10% O(2), 20 min each) and acute hypercapnia (2 and 4% CO(2), 20 min each). The responses to hypoxia were similarly decreased in HxTot and in Hx3 compared to controls, and were normal in the other experimental groups; those to hypercapnia were blunted only in HxTot. The results are in agreement with the idea that prenatal hypoxia blunts V e chemosensitivity by interfering with the normal development of the carotid bodies.

Gas exchange in avian embryos and hatchlings.

The avian egg has been proven to be an excellent model for the study of the physical principles and the physiological characteristics of embryonic gas exchange. In recent years, it has become a model for the studies of the prenatal development of pulmonary ventilation, its chemical control and its interaction with extra-pulmonary gas exchange. Differently from mammals, in birds the initiation of pulmonary ventilation and the transition from diffusive to convective gas exchange are gradual and slow-occurring events amenable to detailed investigations. The absence of the placenta and of the mother permits the study of the mechanisms of embryonic adaptation to prenatal perturbations in a way that would be impossible with mammalian preparations. First, this review summarises the general aspects of the natural history of the avian egg that are pertinent to embryonic metabolism, growth and gas exchange and the characteristics of the structures participating in gas exchange. Then, the review focuses on the embryonic development of pulmonary ventilation, its regulation in relation to the embryo's environment and metabolic state, the effects that acute or sustained changes in embryonic temperature or oxygenation can have on growth, metabolism and ventilatory control.

End-tidal CO2 in some aquatic mammals of large size.

While resting on land or at the water surface, the breathing frequency (f) of aquatic mammals of medium and large size is lower than in terrestrial mammals of similar body weight (W), the difference widening with the increase in W. The allometric function for aquatic mammals is f proportional to W(-0.42) (f, breaths/min, W, kg) and that of terrestrial species is f proportional to W(-0.25). We asked whether or not resting breathing at such low f would entail high values of alveolar CO2. End-tidal alveolar CO2 pressure, taken as representative of alveolar CO2 pressure, PaCO2, was measured from the expired gas during resting breathing in captive specimens of aquatic species trained to rest in proximity of their keepers, either on land (walrus and sea lion) or at the water surface (dolphin, orca, beluga and hippopotamus). Their f during the recordings ranged from less than 1 (orca) to 6 (walrus) breaths/min. The average PaCO2 values ranged from 32 to 42 mm Hg, the peaks being a few mm Hg higher. These values were similar or slightly higher than literature data of many terrestrial species, with no relation to the animal f or W. The quasi-normality of PaCO2 in large aquatic species breathing at rest, despite their exceptionally low f and normal metabolism, can be explained mainly by two factors, their large tidal volume/W, about three times the average terrestrial value, and their peculiar breathing pattern with sustained high lung volume during the expiratory pause. This latter is key in avoiding a substantial rise in PaCO2 during the inter-breath pause.