Development of the chemical control of breathing in the Virginia opossum

Respiratory characteristics of the tammar wallaby pouch young and functional limitations in a newborn with skin gas exchange

A short gestation, low birth weight and presence of cutaneous exchange of O₂ and CO₂ comprise altricial features of newborn marsupials and that collectively implies a highly immature respiratory system. In the present study, we investigated various respiratory characteristics of the neonatal/postnatal tammar wallaby, a species of marsupial in which > 30% of the newborn's total O₂ demands are supported by cutaneous rather than pulmonary gas exchange. The ventilatory response (HVR) to acute hypoxia (10% inspired O₂) was absent in the newborn (1 day old) pouch young; a hypoxic hypometabolism contributed entirely to the hyperventilation (increased pulmonary convection requirement). A high (compared to older animals) resting metabolic cost to breathe and an inefficient respiratory system suggest the lack of a HVR might be due to an energetic constraint that impinges on their ability to sustain an increase in ventilation. The latter was supported by the inability of the newborn to tolerate metabolic-ventilatory stimulation following administration of the metabolic uncoupler, 2,4-dinitrophenol (2,4-DNP). At 1 week of age, the cost of breathing was reduced, which coincided with the expression of a significant ventilatory response to hypoxia, a more energetically efficient respiratory system, and tolerance to 2,4-DNP. These data suggest this species of marsupial is born with major respiratory insufficiency, and that their pronounced dependence on the skin for metabolic gas exchange is of critical importance for survival.

Respiratory development in burrowing rodents: Effect of perinatal hypercapnia

Burrowing rodents have a blunted hypercapnic ventilatory response compared to non-burrowing rodents, but semi-fossorial ground squirrels and hamsters are not born with this blunted response when raised in room conditions. This study examined the hypercapnic ventilatory response of rats, hamsters, and ground squirrels raised in burrow-like hypercapnia (∼3 % CO₂) through development (embryonic day 16-18 to postnatal day 30) to determine if chronic hypercapnia exerts any effect on the developing and adult semi-fossorial response. Chronic hypercapnia attenuated the ventilatory response to 5 % CO₂ by 60 % (rats), 150 % (hamsters), and 70 % (squirrels) in newborns when compared to newborns raised in normal conditions. When raised in burrow conditions, squirrels and hamsters reached the blunted adult response ∼8-12 days sooner in development than their room air counterparts, while burrow-reared rats maintained a consistently blunted response until removal from chronic hypercapnia. Our study revealed no lasting effect of chronic hypercarbia on the ventilatory responses to CO₂ in burrowing rodents, but rather a change in the developmental profile such that the blunted adult response was reached earlier in development.

The ventilatory response to hypoxia and hypercapnia is absent in the neonatal fat-tailed dunnart

At birth, the newborn fat-tailed dunnart relies on cutaneous gas exchange to meet metabolic demands, with continuous lung ventilation emerging several days later. We hypothesized that the delayed expression of lung ventilation (VE) in these animals is in part owing to a low responsiveness of the respiratory control system to blood gas perturbations. To address this hypothesis we assessed the ventilatory and metabolic response to hypoxia (10% O2) and hypercapnia (5% CO2) using closed-system respirometry from birth to 23 days postpartum (P). Neonatal fat-tailed dunnarts displayed no significant hypoxic or hypercapnic ventilatory responses at any age. Regardless, significant hyperventilation through a suppression of metabolic rate (Vo2) was observed at birth in response to hypercapnia and in response to hypoxia at all ages, except P12. Therefore, reliance on cutaneous gas exchange during early life may be partially attributed to reduced chemosensitivity or a lack of central integration of chemosensitive afferent information. This may be in part due to the relative immaturity of this species at birth, compared to other mammals.

Lung and metabolic development in mammals: contribution to the reconstruction of the marsupial and eutherian morphotype

Marsupials represent only 6% of all living mammals. Marsupialia and Placentalia are distinguished mainly by their modes of reproduction. In particular, the differences in the stage of development of the neonates may be one explanation for the divergent evolutionary success. In this respect one important question is whether the survivability of the neonate depends on the degree of maturation of the respiratory system relative to the metabolic capacity at the time of birth. Therefore, this review highlights the differences in lung morphology and metabolic development of extant Marsupialia and Placentalia. The Marsupial neonate is born with a low birth weight and is highly immature. The neonatal lung is characterized by large terminal sacs, a poorly developed bronchial system and late formation of alveoli. Marsupialia have a low metabolic rate at birth and attain adult metabolic rate and thermoregulatory capacity late in postnatal development. In contrast, the eutherian neonate is born with a relative high birth weight and is always more mature than marsupial neonates. The neonatal lung has small terminal sacs, the bronchial system is well developed and the formation of alveoli begins few days after birth. Placentalia have a high metabolic rate at birth and attain adult metabolic rate and thermoregulatory capacity early in postnatal development. The differences in the developmental degree of the newborn lung between Marsupialia and Placentalia have consequences for their metabolic and thermoregulatory capacity. These differences could be advantageous for Placentalia in the changing environments in which they evolved.

Postnatal lung and metabolic development in two marsupial and four eutherian species

Two marsupial species (Monodelphis domestica, Macropus eugenii) and four eutherian species (Mesocricetus auratus, Suncus murinus, Tupaia belangeri and Cavia aperea) were examined to compare and contrast the timing of lung and metabolic development during the postnatal maturation of the mammalian respiratory apparatus. Using light, scanning and transmission electron microscopy, the lung structural changes were correlated with indirect calorimetry to track the metabolic development. Marsupial and eutherian species followed the same pattern of mammalian lung development, but differed in the developmental pace. In the two newborn marsupial species, the lung parenchyma was at the early terminal sac stage, with large terminal air sacs, and the lung developed slowly. In contrast, the newborn eutherian species had more advanced lungs at the late terminal sac stage in altricial species (M. auratus, S. murinus) and at the alveolar stage in precocial species (T. belangeri, C. aperea). Postnatal lung development proceeded rapidly in eutherian species. The marsupial species had a low metabolic rate at birth and achieved adult metabolism late in postnatal development. In contrast, newborn eutherian species had high metabolic rates and reached adult metabolism during the first week of life. The time course of the metabolic development is thus tightly linked to the structural differentiation of the lungs and the timing of postnatal lung development. These differences in the neonatal lung structure and the timing of postnatal lung maturation between marsupial and eutherian species reflect their differing reproductive strategies.

Development of the respiratory system in marsupials

Marsupials at birth are small and relatively undeveloped. At birth, the lung in some species is at the canalicular stage of development and though lung diffusion and metabolic rate are strongly correlated, the allometric exponent suggests that smaller newborns have relatively smaller diffusing capacity with respect to their demand for oxygen. Without improvement in functional or structural parameters newborn marsupials are reliant to varying degrees on skin gas exchange to compensate for the immaturity of the lung. Indeed, in some species there is complete reliance on the skin for gas exchange at birth. Nevertheless, with an early dependence on ventilation, the CNS would appear already to contain neurons with properties and connections that permit rhythmic motor output at birth and pulmonary reflexes mature soon after. Despite appropriate neural control and the presence of surfactant, the highly compliant nature of the newborn chest wall results in substantial chest wall distortion during inspiratory effort which reduce the efficacy of the lung for ventilation. This review explores the morpho-functional development of the respiratory system, including oxygen transport and cardiac shunts, and the establishment of convective requirement in marsupials, a group that places emphasis on extended postnatal development.

Developmental changes in the hypoxic ventilatory response in C57BL/6 mice

C57BL/6 mice are the strain into which most null mutations for neurotransmitters or their receptors are backcrossed. A number of these transgenic mice have recently been shown to have an abnormal respiratory phenotype; however, the postnatal development of the ventilatory response to hypoxia has not been characterized in C57BL/6 mice. The effect of 8% oxygen for 5 min was examined in mice at five periods from P1 to P30 using a body plethysmograph. Neonatal and juvenile animals from P7 to P30 showed a biphasic pattern in hypoxia in which the increase in minute ventilation achieved in the first min declined towards baseline by the fifth minute and was decreased below baseline in the first minute of return to air breathing. In contrast P1-P3 C57BL/6 mice had a sustained increase in both respiratory frequency and tidal volume and their minute volume remained above baseline on return to air. The decline in oxygen consumption, measured in the fifth minute of hypoxia, was not different in P1-P3 mice compared to P8-P10. These results suggest that the earliest response to hypoxia of the respiratory system in this strain is not characterized by a time dependent depression as seen in older animals and in species whose motor systems are relatively more developed at birth.

Convection requirement is established by total metabolic rate in the newborn tammar wallaby

Ventilation (VE) and metabolic rate, determined from both pulmonary and cutaneous gas exchange, were measured in 39 newborn tammar wallabies, Macropus eugenii, aged between 0 and 3 days. In 1-day-old animals both total metabolic rate (skin+lung exchange) and ventilation were approximately 50% of the values predicted for eutherian newborns of equivalent body mass. Hence, the convection requirement (VE/total metabolic rate) of the newborn tammar is close to predicted values for newborns and adult mammals in general. Metabolic rate in the newborn tammar is supported to some extent by cutaneous gas exchange, approximately 30% of the total in the 1-day-old animal. This ratio diminishes with increasing age as the lung takes on an increasingly more important role for respiratory exchange. The early establishment of the convection requirement in the newborn tammar, despite significant cutaneous gas exchange, provides supporting evidence that metabolic rate per se is important in establishing the level of ventilation.

Control of respiration in the isolated central nervous system of the neonatal opossum, Monodelphis domestica

Respiration represents an unusual motor activity with respect to its development. As newly born mammals enter the world, their limb movements are not coordinated; time and experience are required for effective performance to be achieved. Yet the rhythm of respiration is of necessity functionally perfected and unfailing at birth. Inspiratory and expiratory motor neurons are already able to fire at appropriate rates, under the command of rhythmically active neurons in the medulla. In this review, we discuss refinements of control present in the newborn opossum, particularly with respect to mechanisms that allow adaptation of respiration to changes in the level of activity or in the outside environment. Our own studies have been aimed at analyzing respiration at the earliest stages, and at establishing the way in which important variables influence inspiration and expiration. To this end, we have used the central nervous system (CNS) of a neonatal opossum, isolated in its entirety and maintained in culture. Although the opossum is unable to walk and highly immature at birth, its respiration is regular and unfailing. The isolated CNS survives, undergoes development, and maintains its neural activity and fine structure in vitro. Moreover, fictive respiration persists for over a day or longer at rates similar to those of the intact pup. The effects of altered pH, of increased temperature, and of drugs known to alter respiratory rhythm in intact animals can be measured directly, by electrical recordings made from medullary neurons or ventral roots. As in a slice, fluids of different composition can be applied focally, through micropipettes to the surface of the ventral medulla, or diffusely to the brainstem, With highly localized application of procaine hydrochloride (2%) to selected areas of the ventral medulla, the respiratory rhythm is reduced or abolished. As in adult mammals, both the rate and the amplitude of respiration simultaneously increase in response to lowered pH (6.5-.7.1) or to topical application of 1.0 microM carbachol. Conversely, as expected, the rate and amplitude decrease in response to increased pH (pH 7.5-7.7), or 100 microM scopolamine. Two characteristic features of the control of respiration in the neonatal opossum are evident from such tests. First, changes in rate are achieved by changes in the duration of the expiratory phase of respiration. This result suggests that the timing of the respiratory cycle in the neonatal opossum is controlled by an expiratory instead of an inspiratory "off-switch". Second, the rate and the amplitude of the respiratory excursions can be controlled independently, depending on the stimulus. For example, an increase in temperature increases the rate of fictive respiration without changing its amplitude, whereas noradrenaline decreases the rate while increasing the amplitude. Thus, changes of timing and amplitude need not go hand in hand. The opossum CNS offers a favorable preparation for the analysis of neural mechanisms that generate and modulate a motor rhythm, as the animal develops from embryonic to adult stages.

Respiratory function in a newborn marsupial with skin gas exchange

The Julia Creek dunnart (Sminthopsis douglasi) is a marsupial born after approximately 12 days of gestation. At birth, the newborn is approximately 4 mm long and weighs approximately 15 mg. Gaseous metabolism (oxygen consumption rate, V(O2), rate of carbon dioxide production, V(CO2) was measured separately across the airways (lungs) and the rest of the body (skin). At pouch temperature (36 degrees C) total V(O2) (i.e. skin + lungs) averaged 15 +/- 2 S.E.M. ml x kg(-1) x min(-1). At birth the skin contributed almost the total gaseous metabolism, and at 3 weeks approximately 1/3 of the total. The compliance of the respiratory system, per unit of body weight, was similar to that of other newborn mammals. During the first postnatal days breathing was an occasional event determined by gross body movements. Artificial expansion of the lungs temporarily stopped breathing, presumably a manifestation of the Hering-Breuer reflex. By the 2nd-3rd week breathing was regular, pulmonary ventilation (V(E)) averaged 263 ml x kg(-1) x min(-1), tidal volume (V(T)) 3.4 ml x kg(-1), breathing frequency (f) 87 breaths x min(-1). Lowering ambient temperature in steps from 36 to 20 degrees C reduced both lung and skin gaseous metabolism. V(E) and f, at first, were little affected but eventually they dropped in approximate proportion to metabolism, whereas V(T) remained unchanged. In conclusion, for the newborn dunnart gas exchange through the skin is a requirement because of the inefficient V(E). To what extent the V(E) adjustments to changes in metabolic rate reflect mechanisms of regulation remains unresolved.

Development of ventilatory responsiveness to progressive hypoxia and hypercapnia in low-birth-weight lambs

Our aim was to determine the effects of low birth weight on ventilatory responses to progressive hypoxia and hypercapnia during early postnatal life. Seven low-birth-weight (2.7 +/- 0.3 kg) and five normal-birth-weight (4.8 +/- 0.2 kg) lambs, all born at term, underwent weekly rebreathing tests during wakefulness while arterial PO2, PCO2, and pH were measured. Hypoxic ventilatory responsiveness (HOVR; percent increase in ventilation when arterial PO2 fell to 605 of resting values) increased in normal lambs from 86.6 +/- 7.1% at week 1 to 227.4 +/- 24.9% at week 6. In low-birth-weight lambs, HOVR was not significantly different at week 1 (60.1 +/- 18.7%) from that of normal lambs but did not increase with postnatal age (56.6 +/- 19.3% at week 6). HOVR of all lambs at 6 wk was significantly correlated with birth weight (r2 = 0.8). Hypercapnic ventilatory responsiveness (gradient of ventilation vs. arterial PCO2) did not change with age and was not significantly different between groups [84.7 +/- 7.5 (low-birth-weight lambs) vs. 89.4 +/- 6.6 ml.min-1.kg-1.mmHg-1 (normal lambs)]. We conclude that intrauterine conditions that impair fetal growth lead to the failure of HOVR to increase with age.

Ventilatory responses to acute and chronic hypoxic hypercapnia in the ground squirrel

Golden-mantled ground squirrels exhibited a strong hypoxic ventilatory response but a blunted hypercapnic ventilatory response and showed no interactive effects when both stimuli were presented together. They exhibited a resting hypoxic ventilatory drive which was eliminated by carotid body denervation. Carotid denervation also shifted the threshold of the hypoxic ventilatory response but had no effect on the slope of either the hypoxic or hypercapnic ventilatory responses. Chronic exposure (2-12 months) to hypoxic-hypercapnic conditions (16% O2, 4% CO2) resulted in a sustained increase in ventilation. Initial increases in both tidal volume (VT) and breathing frequency (fR) were followed by a subsequent further increase in VT and concomitant decrease in fR (acclimation) which had little overall effect on ventilation (VE) but further increased calculated alveolar ventilation (VA). Respiratory sensitivity to hypoxia and hypercapnia were unaltered under these conditions. On acute return to breathing room air, VE remained elevated (approximately 35%) compared to control animals suggesting that deacclimation takes time. Carotid body denervation in these animals had similar effects to those seen in control animals suggesting that acclimation did not involve changes in carotid body input.

Hamsters versus rats: ventilatory responses in adults and newborns

Burrowing mammals often demonstrate structural and functional characteristics which could be interpreted as aspects of adaptation to the low PO2 and high PCO2 of their environment. Whether these characteristics are acquired by each individual as the result of life in the burrow or are genetic traits established through evolution is not clear. To examine the latter possibility aspects of ventilatory control were studied in newborns and adults of two rodents, the surface dwelling white rat (Rattus norvegicus, R) and the semifossorial syrian hamster (Mesocricetus auratus, H), both born and raised in normoxia. Adult H, but not R, manifested the burrowing preference whenever offered the opportunity. Adult H presented numerous differences from the adult R, including a higher normoxic oxygen consumption (VO2, +44%), higher hematocrit and heavier right heart. Ventilation (VE) was similar between the two species both in air and hyperoxia, although H had a deeper and slower pattern. Hence, resting VE/VO2 in H was low, probably contributing to the hypoxemia reported in this species. The acute ventilatory response to hypoxia (10 min 10% FIO2), was less in H, because of no increase in tidal volume. Most of the morphological and functional differences between adult hamsters and rats were not apparent between the newborns of the corresponding species. It is concluded that some of the physiological characteristics reported in burrowing species are also observed in adult hamsters which never lived in burrows, but not in the newborns; hence, they could represent genetic traits which are expressed postnatally with no needs of the special environmental conditions of the burrow.

Evidence that maturation of the peripheral chemoreceptors is not complete in childhood

We examined the hypothesis that the peripheral chemoreceptors contribute a different degree of tone to respiration during exercise in normal young children as compared to adults. To improve resolution of the peripheral chemoreceptor contribution, the studies were conducted during controlled levels of exercise. Peripheral chemoreceptor function was assessed by the hyperoxic (FIO2 = 0.80) switch technique during steady-state, sub-anaerobic threshold exercise during air (FIO2 = 0.21) and midly hypoxic gas (FIO2 = 0.15) breathing in 9 healthy children (mean +/- 1 SD age (years) = 8.2 +/- 1.4) and 10 healthy adults (28.2 +/- 6.5). Ventilation during exercise was significantly greater under hypoxic conditions in both children and adults. During air breathing exercise the mean ventilatory decrease in response to the hyperoxic switch was similar in the two groups (27.9 +/- 10.7% in children and 23.3 +/- 6.3% in adults). In contrast, during hypoxic gas breathing exercise the children demonstrated a much greater decrease in ventilation following the hyperoxic switch (57.9 +/- 3.6%) compared to adults (38.9 +/- 5.5%) (P less than 0.0001). Thus, the peripheral chemoreceptors have a greater role in the exercise hyperpnea during hypoxic exercise in young children as compared to adults, suggesting attenuation of peripheral chemoreceptor function during maturation.

Effects of hyperoxia on ventilatory and metabolic rates of newborn mice

Newborn mammals of medium or large sized species have ventilatory rates, expressed per kg body weight, larger than adults of corresponding size, while newborns of the smallest species do not. We hypothesized that the oxygen consumption of the smallest newborns is limited by the supply of oxygen and reasoned that if this were the case, an increase in Po2 of the inspired air should decrease their ventilation/oxygen consumption (VE/Vo2) ratio. We exposed 1-2 days old newborn mice for 5 min to 21% O2 in N2 or 100% O2, then measured their breathing pattern, by flow plethysmography, and Vo2 with an isovolume closed system. During hyperoxia the VE/Vo2 ratio dropped in average 36%, since VE decreased in 14 out of 18 animals and Vo2 increased in all the animals tested. The drop in VE was due to a prolongation of the expiratory time, with no changes in inspiratory time or tidal volume. During expiration, interruptions of the expiratory flow and tendency to maintain the lung inflated, a characteristic of neonatal respiration, were more pronounced with 100% O2 than 21% O2 breathing. We conclude that the resting metabolic rate of newborn mice is limited by the supply of oxygen; when Po2 is raised, metabolism increases and ventilatory rate decreases in favor of a breathing pattern aimed to preserve lung volume elevated.

Effect of perinatal hypercapnia on the adult ventilatory response to carbon dioxide

Burrowing mammals show a reduced ventilatory response to CO2 and CO2 retention. We examined whether this reduced responsiveness could be due to modification of chemoreceptors by persistent hypercapnia during development. Mice and rats were exposed to 6.0% CO2 throughout gestation and/or weaning and then removed to normocapnic air for a minimum of 6 weeks. Mouse gas pocket O2 and CO2 tensions and hematocrits were analyzed and compared with normocapnically raised controls. The ventilatory and blood gas and pH response to CO2 were compared in chronically cannulated test and control rats. Hematocrits and gas pocket CO2 and O2 tensions of mice and rat ventilatory and arterial blood CO2 and O2 tensions and pH responses were not different in test and control groups. There appears to be little or no developmental affect of CO2 suggesting that the reduced CO2 response seen in burrowers is genetically determined.

CO2-sensitivity of stretch receptors in the marsupial lung

Discharge activity in single fibres of the vagus of the brush tailed possum was examined for evidence of pulmonary CO2 receptors by artificially ventilating the lungs with gas mixtures which preserved, abolished or reversed the normal tidal oscillation in FCO2. No specific CO2 receptors were observed. A quantitative study of the CO2 sensitivity of thirteen pulmonary stretch receptors was carried out after stabilizing FACO2 at high (6.4-7.8%), low (1.4-2.5%) as well as intermediate values. In addition receptor responses to a series of sustained augmenting inflations were examined at different intrapulmonary CO2 concentrations. All thirteen receptors showed CO2 sensitivity, their frequency of discharge being reduced by hypercapnia and increased by hypocapnia. Five were low threshold receptors which discharged throughout the ventilatory cycle while the remaining eight were only phasically active. High threshold receptors were more sensitive to FACO2 changes than were low threshold units. The results from the series of augmenting inflations suggest that it is the receptor's threshold, but not its sensitivity, to tracheal pressure that is modulated by the co2 signal.

Laryngeal effects and respiration in the suckling opossum

Electromyogram (EMG) activity of the posterior cricoarytenoid (PCA muscles were active, but was otherwise closed. Larynx closure may contribute to the end-inspiratory pauses often observed in the intact animal. During inhalation of an asphyxiant test gas in place of room air, PCA and diaphragm activity generally increased; and increases in anesthesia decreased the EMG activity of both muscles. Water on the larynx transiently abolished both PCA and diaphragm discharge; but EMG activity returned to the PCA muscles before the diaphragm. Further, reactivation of the diaphragm was not always accompanied by a synchronized burst of PCA activity. Therefore, it is possible under some conditions to dissociate the motor outputs from the PCA muscles and diaphragm in this immature mammal.

An obstructive apnea in the suckling opossum

Pressure plethysmographic measurement of ventilation and electromyographic measurement of diaphragm activation in unanesthetized suckling opossums revealed spontaneous episodes of obstructive apnea. To better understand this phenomenon, intact animals were tested under different conditions of respiratory drive and during activation of pulmonary reflexes. Results showed that obstructed breaths were usually initiated with the lungs at or near the end-inspiratory level, and they occurred more frequently during air or oxygen breathing as compared with inhalation of hypercapnic or asphyxiant test gases. The latter two gas mixtures caused increases in ventilation; but depression of breathing resulting from pentobarbital anesthesia was not accompanied by obstructed breaths. It was initially considered that a reflex laryngeal response with changes in lung volume or lung irritation might trigger obstructive apnea; but pulmonary inflation and deflation as well as ammonia inhalation did not typically produce such an effect. The results indicate that mechanisms for obstructive apnea in the suckling opossum must account for the limited incidence of obstructed breaths during both ventilatory chemostimulation and anesthetic respiratory depression.

Ventilatory effects accompanying spontaneous movement in the suckling opossum

To determine whether spontaneous movement was associated with an organized respiratory response during early stages of mammalian development, we studied immature suckling opossums (Didelphis virginiana) between 30 and 60 days of age. Rate and depth of breathing were measured using pressure plethysmography and movement was assessed using electromyography of neck muscles. During air-breathing as well as during ventilatory chemostimulation with hypoxic, hypercapnic, and asphyxiant test gases, the onset of spontaneous movement was often marked by an increase in minute-ventilation, dominated by an increase in breathing frequency. In the absence of respiratory irregularities, ventilation and breathing frequency remained elevated throughout an episode of movement. After decerebration at the midcollicular level, the onset of spontaneous movement was accompanied by respiratory effects similar to those found in the intact preparation. The ventilation response with spontaneous movement contrasted to results during venilatory chemostimulation at rest, where increases in breathing resulted primarily from increases in tidal volume. It is concluded that a hyperpnea with spontaneous movement is present in the suckling opossum, and that the effect is primarily organized in the lower brainstem.