Oxygen consumption in new-born rats

Hypothalamic thermo-responsive neurones in the new-born rat

1. Single unit activities were recorded from the neurones in the preoptic area and anterior hypothalamus of developing new-born rats (aged 1-24 days old) during thermal stimulation of the brain. During the first 2 weeks of life, about 80% of these neurones had low spontaneous firing rates between 0.1 and 5 impulses/sec at 38 degrees C hypothalamic temperature (Thyp). 2. Out of 640 units studied, 118 units increased the firing rate upon elevation of Thyp (warm-units) and fourteen showed the opposite type of response to temperature changes (cold-units). Warm-units were found in the rats of all the age span studied and cold-units were recorded in the rats more than 8 days old. 3. Thermal coefficients of warm-units and cold-units varied between +0.11 and +2.47 and between -0.10 and -0.49 impulses/sec, degrees C, respectively. Number of warm-units with higher rates of firing and greater thermal coefficients, comparable to those of warm-units in the adult, gradually increased with growth. The thermal responsiveness of warm-units, when expressed by Q10, are already high even in the immediate neonatal period. Their Q10 values were in the range between 2 and 38.5 (mean 6.4). 4. Units responding to extrahypothalamic temperatures were only found in the rats more than 14 days old. 5. All the six warm-units tested increased the firing rates following subcutaneous injections of capsaicin, while the majority of thermo-unresponsive units were not affected by this drug. 6. It is suggested that thermo-responsive neurones in the preoptic area and anterior hypothalamus in the new-born rat have attained some degree of electrophysiological maturity, despite their slowly firing characteristics.

Post-natal development of brown adipose tissue in the rat bred at 23 degrees C or 28 degrees C

In view to study the effects of thermal environment on the development and the thermogenic activity of interscapular brown adipose tissue (BAT), young rats born at 23 degrees C or 28 degrees C were sacrificed at 1, 3, 7, 11, 14 or 21 days after birth. The rate of increase in animal weight was quite the same at both temperatures up to the 14th day. The development of BAT and its contents in lipids, in water and in noradrenaline indicate that the energetic activity of the tissue is greatly stimulated in rats kept at 23 degrees C up to the 11th day. It is concluded that in rats bred in the habitual thermal conditions (23 degrees C), the occurrence of non shivering thermogenesis (NST) is important during the period of ten days after birth; in the following period NST could be progressively replaced by other thermoregulatory processes.

Quantitative lung morphology in newborn mammals

We have examined the lungs of eleven species of newborn mammals by quantitative morphometric techniques and related the findings to body weight (BW) and O(2) consumption (Vo2 is found to be proportional to BW(0.88); this exponent is significantly greater than the value of about 0.73, repeatedly found for adult mammals. Lung volume (LV) is essentially proportional to BW, and neonatal alveolar size - as indicated by mean chord length - is nearly constant among species and independent of BW. Thus the respiratory surface area (SA) is not proportional to Vo2, as in adult mammals, but to Vo2(1.23). Neonates of small species have lower SA/Vo2 ratios than adult mammals, possibly owing to constraints of surface forces on alveolar size. Newborn members of larger species have SA/Vo2 values in excess of the adult range. Published data on human newborn lung dimensions suggest that the human infant may have an unexpectedly low SA/Vo2 value in relation to BW. Whether this is a valid finding, or whether it derives from methodological differences between studies cannot be ascertained from the available data.

The development of the EEG in the rat

The development of the electroencephalogram (EEG) and, especially, behavioral state-specific EEG patterns was studied in white and black hooded rats of the Lister strain, aged 9-30 days. Movements of the rat were recorded and the behavioral state was monitored by means of a push-button device. The EEG was collected when the rat was in State 1 (regular respiration, absence of movements, and, after the 14th day when the rat eye opens, eyes closed), State 2 (irregular respiration, continual occurrence of twitches, and, after the 14th day, eyes closed), and in State 4 (irregular respiration, presence of gross body movements, and, after the 14th day, eyes opened). The EEG of rats on the 9th and 10th day did not reveal behavioral state-specific patterns. The amplitudes of the EEG were low and only low frequencies occurred. Between the 10th and 13th day an EEG pattern specific for State 1 containing high amplitudes developed. From the 14th day onwards spindles (frequencies from 14-18 Hz) occurred in the EEG during State 1. From the 14th day onwards, the EEG from the visual cortex during State 2 showed a regular and continually occurring 5-Hz rhythm. Bursts with 5-Hz waves were recorded from the visual cortex only intermittently during State 4. The EEG frequencies during the bursts varied between 5 and 7 Hz from the 17th day onwards. Computer analysis of the amplitude distributions showed a considerable increase in the power after the 10th day. The analysis of the frequency spectra indicated that the power increase occurs expecially in the higher frequencies of the EEG signal. Visual analysis as well as computer analysis of the EEG did not reveal systematic changes in the EEG after the 18th day when the EEG was similar to that recorded at older ages.

EEG development in normal and undernourished rats

The development of EEG was studied in normal and undernourished white and black hooded rats, aged 9-30 days. Undernutrition of the young was accomplished by underfeeding the mothers with restricted amounts of a standard laboratory food from the 7th day after conception until the 21st day after birth. Bipolar EEG recordings were made from the sensorimotor cortex and from the visual cortex in rats with chronically implanted electrodes. The EEG was recorded, stored on magnetic tape and analyzed for amplitude distributions and frequency spectra. During the recording 3 behavioural states (defined on the basis of behavioural criteria) were distinguished.

The response of the newborn rat to injury. I. Vascular and humoral aspects

The development of oedema in response to irritants injected into the skin, or pleural cavity of rats, has been shown to differ according to the age of rat studied. It was found that newborn rats were less responsive, with respect to oedema formation, at certain times after the injection of a particular irritant. Attempts to suppress the reactions of these animals using anti-histamines, anti-serotonin and kinin-depleting agents were less successful than in adults. Analysis of serum, plasma and exudate proteins revealed quantitative and qualitative differences between newborn and adult rats. The ability of various agents to cause increased vascular permeability in the skin of newborn rats was found to vary with the age of animal studied. Thus histamine, 5-hydroxytryptomine, cellulose sulphate and prostaglandin E2, all failed to cause increased vascular permeability in the skin of rats younger than 3 wk of age. However, lymph-node permeability factor induced increased vascular permeability in newborn (i.e., 6-hr-old) rats. An attempt was made to correlate these results with those of oedema formation following the infections of certain irritants.

The post-natal changes in minimal metabolic rate in the pig

1. Minimal metabolic rate was measured as oxygen consumption in new-born pigs when environmental temperature was rising and passing through thermal neutrality.2. Three different periods were observed in 20 days after birth. The first period included the time from birth to 82 hr and was characterized by a continuous rise in minimal metabolic rate from 9.4 to 16.0 ml. O(2)/kg.min (70% increase). The second period extended from 82 hr to 9.5 days with values near to 16 ml. O(2)/kg.min. The third period beginning on the eleventh day lasted until observations ceased on the twentieth day and was characterized by a 25-30% fall in minimal metabolic rate when compared with the second period.3. Minimal metabolic rates from the second and third period were proportional to body weight and not to the surface area. Exponential factors for body weight were 0.93 and 0.96 for the second and third periods respectively.4. Rectal temperature rose from 38.6 degrees C on the first day to 39.3 degrees C on the second and 39.5 degrees C on the fourth day after birth.5. Environmental temperature associated with minimal metabolic rate showed a general tendency to fall from 36.5 in the first 4 days to between 33 and 34 degrees C in the following days.6. Age rather than body weight is the decisive factor influencing changes in minimal metabolic rate in the new-born pig.

Summit metabolism and cardiovascular function in young lambs during hyperoxia and hypoxia

1. Exposure of lambs at sea level to air containing more than 33% oxygen resulted in a small increase in summit metabolism (maximum oxygen consumption on severe cold exposure) in some lambs, and in an improvement in the ability to maintain summit metabolism in others; there was a concomitant improvement in the maintenance of cardiac output, but no change in the degree of right to left shunting.2. Hypoxia (12-13% oxygen in inspired air) reduced summit metabolism of new-born lambs by 25%; but there was no effect in lambs 6-11 days old. Hypoxia neither reduced cardiac output, nor affected the degree of right to left shunting.3. The results suggest that in very young lambs there is some limitation of summit metabolism imposed by the supply of either oxygen or substrate to thermogenic tissues.

The effect of ambient temperature upon post-natal changes in oxygen consumption of the guinea-pig

1. The effect of thermal environment and administration of progesterone upon minimal oxygen consumption and thermal stability of the new-born guinea-pig has been investigated.2. It was found that the ambient temperature at which the new-born is reared does not influence minimal oxygen consumption although it modifies the response to cold.3. Thermal stability was decreased in animals raised in a warm environment.4. Minimal oxygen consumption per unit body weight during the neonatal period increased 25%, most of the increase taking place within 16 hr after birth.5. Decrease in the relative mass of brown adipose tissue was least in new-borns maintained at a high ambient temperature.6. Administration of progesterone did not affect oxygen consumption, body temperature and thermal stability of the new-born.

Shivering and non-shivering therogenesis during summit metabolism in young lambs

1. Summit metabolism of lambs declined steadily from about 3.5 l. O(2)/kg.hr during the first day of life, to about 2.0 l. O(2)/kg.hr at 2 months of age.2. The contributions of shivering and non-shivering thermogenesis to these changes were estimated by three independent methods; non-shivering thermogenesis was stimulated by catecholamines in a thermoneutral environment, shivering was suppressed by curariform drugs during summit metabolism, and an attempt was made to suppress non-shivering thermogenesis during summit metabolism by use of the sympatholytic drugs phentolamine and propranolol. Drugs were given by intravenous infusion during measurement of oxygen consumption in a closed circuit respiration chamber.3. ;Resting' metabolic rate of lambs during the first day of life was increased two to three-fold, from 1 l. O(2)/kg.hr, by either adrenaline or noradrenaline infused at 1-10 mug/kg.min. The increase declined with increasing age of lamb and was virtually absent by 3 weeks. The response to catecholamines appeared maximal at the dose levels used.4. Muscular paralysis induced by suxamethonium or gallamine reduced summit metabolism by about 2 l. O(2)/kg.hr in all lambs examined within the first 2 months of life. The residual metabolic rate, and the metabolic response to catecholamines under thermoneutral conditions, declined with age in the same manner, and their magnitudes were similar.5. Summit metabolism in lambs aged up to 2 months was depressed to varying degrees by the sympathetic inhibitors phentolamine, propranolol and hexamethonium. The depression with propranolol was greater, and the decline with age clearer, than with phentolamine. Hexamethonium and phentolamine depressed blood pressure, propranolol decreased heart rate and phentolamine and propranolol each suppressed shivering in some experiments.6. In 1 day-old lambs estimates of non-shivering thermogenesis, by the various methods, ranged from 0.8 to 1.4 l. O(2)/kg.hr (mean 1.1 l. or 31% of summit metabolism), and the estimates of shivering ranged from 1.3 to 1.9 l. O(2)/kg.hr (mean 1.6 l. or 46% of summit metabolism). However, in lambs 1-month old, estimates of non-shivering thermogenesis from sympathetic inhibition (0.6 and 0.8 l. O(2)/kg.hr) were considerably higher than estimates from muscular paralysis or stimulation by catecholamines (0.2 and 0.1 l. O(2)/kg.hr). It is suggested that the depression of summit metabolism by the sympathetic inhibitors is not solely due to specific inhibition of non-shivering thermogenesis, at least in the older lambs.7. The possession of a non-shivering thermogenic mechanism in addition to shivering is of clear survival value to new-born lambs.

Adipose tissue and heat production in the new-born ox (Bos taurus)

1. A histological examination of adipose tissue from 1- and 6-day-old calves showed a structure typical of white adipose tissue and no evidence of brown adipose tissue.2. Infusion of noradrenaline (1.0 mug/kg.min I.V.) into 1-to 6-day-old calves affected heart rate and respiratory rate but did not increase heat production, rectal temperature, skin temperature or skin evaporative loss.3. Cold exposure led to shivering and an increased oxygen consumption in the 6-day-old calf.4. Blood samples taken from 6-day-old calves in the cold (-1 degrees C) appeared to have a higher proportion of unesterified fatty acids in the total blood lipid than samples taken in an environment of 20 degrees C, but no change in fatty acid composition was found.5. It has been concluded that there is no non-shivering thermogenesis in the young calf.

Oxygen consumption during pouch development of the macropod marsupial Setonix brachyurus

1. Measurements of O(2) consumption at 9 or 10 temperatures in the 20-40 degrees C ambient temperature range were made on joeys with ages selected to cover the 180-day period of pouch occupancy.2. The rate of O(2) consumption of joeys younger than 100 days increased directly with ambient temperature.3. After 100 days of age the O(2) consumption rate at low temperatures rose and at about 140 days of age a constant rate was maintained over the full ambient temperature range.4. Heat transfer from joey to mother commenced after 100 days of age.5. At 150-180 days of age the rate of O(2) consumption at 20 degrees C was approximately 12 times greater than at ages less than 100 days. A thermal neutral zone was established in the range 32-36 degrees C by joeys older than 150 days.6. At the usual pouch temperature of 36.5 degrees C, O(2) consumption per unit wet body weight rose from 12 ml./kg.min at birth to 17 ml./kg.min at the end of pouch life. On a unit dry body weight basis it fell from 120 to 56 ml./kg.min. This decline parallels the decrease in growth rate.

Chemoreceptor reflexes in the new-born infant: effects of varying degrees of hypoxia on heart rate and ventilation in a warm environment

1. We studied the effects of varying degrees of hypoxia for 3 min periods on the heart rate and respiration of thirty-three healthy full-term infants in a warm environment.2. During the first 5 days of life a decrease in alveolar oxygen tension (P(A, CO2)) below 80 mm Hg induced hyperventilation, a decreased alveolar carbon dioxide tension (P(A, CO2)), and tachycardia during the first minute of hypoxia. During the second and third minute, while the decreased P(A, CO2) and tachycardia persisted, ventilation fell. There was a further fall in ventilation when the baby breathed 21% O(2) again. This response was also observed when the inspired gas was heated to 35 degrees C.3. During the first 5 days of life a decrease in P(A, CO2) between 81 and 100 mm Hg did not affect ventilation or P(A, CO2) during the first minute of hypoxia, but still induced a tachycardia and a fall in minute volume during the second and third minute.4. When the P(A, CO2) was elevated and maintained constant during hypoxia, ventilation increased during the first minute and fell during the second and third minutes, suggesting that hypocapnia did not explain the transient ventilatory response to hypoxia.5. After the first week of life a greater and maintained increase in ventilation was seen during hypoxia. This response was potentiated by the addition of CO(2).6. The possibility that changes in the pulmonary circulation, associated with a functionally patent ductus arteriosus, may explain these differences, is discussed.