Temperature effects on the inflation reflex during expiratory time in the cat

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.

Hypothermia and hypoxia inhibit the Hering-Breüer reflex in the marsupial newborn

The effects of lowering body temperature (Tb) on metabolic rate, ventilation, and the strength of the Hering-Breüer expiratory promoting reflex (HB reflex; determined from an inhibitory ratio calculated from volumetric measurements of the respiratory rhythm) were examined in 18-day-old ectothermic pouch young of the tammar wallaby during normoxia or hypoxia (10% O2). Hypoxia and hypothermia, either singularly or combined, depressed metabolic rate. At all Tb, the hypoxic hyperventilation was associated with a significant hyperpnea. At pouch Tb (36.5°C) during normoxia, inflation of the lungs with -5 or -10 cmH2O extrathoracic pressure induced a significant HB reflex. Exposure to cold reduced the strength of the reflex, almost abolishing it at 28°C. For Tb above 28°C, the reflex in hypoxia was always less than the corresponding normoxic value. Taken in context with the changes in metabolic state that occurred, these data in the ectothermic marsupial newborn suggest that the decline in the HB reflex during moderate hypothermia is the result of a direct effect of Tb on vagal mechanisms rather than a temperature-driven decline in metabolic rate that should have acted to strengthen the HB reflex. Therefore, it seems that inputs inhibitory to breathing are more negatively affected during cold than those inputs that are excitatory.

Effects of Hyperthermia on Ventilation and Metabolism during Hypoxia in Conscious Mice

Hyperthermia and hypoxia influence ventilation and metabolism; however, their synergistic effects remain unanswered. We hypothesized that an enhancement of ventilation induced by hyperthermia is competitive with hypoxic hypometabolism. We then examined the relationship of body temperature, hypoxia, and respiration in conscious mice, measuring minute ventilation (VE), aerobic metabolism, and arterial blood gases. All parameters were measured at two different body temperatures (BTs), approximately 37 degrees C (normothermia) and 39 degrees C (hyperthermia), under both normoxia (room air inhalation) and hypoxia (7% O2 inhalation). Under normoxia, VE and O2 consumption (VO2) were lower at hyperthermia than at normothermia, and the VE-VO2 ratio remained constant. PaCO2 values were normal at both BTs under normoxia. Hypoxic gas inhalation increased VE, which reached a peak in 2 min, then decreased at both BTs. VE remained at a higher level during hyperthermia than during normothermia throughout the 10 min experiment. VO2 decreased during hypoxia at both BTs. Hypoxia increased the VE-VO2 ratio because of relatively high VE with respect to the decreased VO2, which means hyperventilation. At hypoxia under hyperthermia, serious hyperventilation occurred with a further increase in VE. The augmented ventilation may be due to the thermal stimulus and a lowered thermoregulatory set point for hypoxia. Thus hyperthermia reduces ventilation and metabolism to maintain normocapnia; as a result, thermogenesis is reduced under normoxia. Hyperthermia augments hyperventilation induced by hypoxia, leading to severe hypoxic hypocapnia. Thermal stimuli may impair the adjustment of ventilation and metabolism when O2 is limited.

Selective vagal afferent dysfunction in dogs with congenital idiopathic megaoesophagus

Congenital idiopathic megaoesophagus (CIM) is a rare, naturally occurring disorder of the dog that is characterised by deficient motility and dilatation of the oesophagus. Recent studies indicate that the vagal sensory system mediating reflexes induced by oesophageal distension is defective in, and may underlie the pathomechanism of this disorder. We sought to establish whether other distension sensitive vagal afferent systems were impaired in CIM, or whether the vagal afferent dysfunction was selective. Thus, we examined the Hering-Breuer lung inflation reflex (HBR), which is subserved by a contiguous and physiologically similar vagal afferent system, in five dogs with CIM in which oesophageal vagal afferent dysfunction had been demonstrated. At varying levels of lung inflation, we found the HBR to be normally graded and of normal strength in affected dogs and that this result was unlikely to be influenced by other factors known to alter the strength of the reflex. These observations provide evidence for an organ specific, selective vagal afferent dysfunction in dogs with CIM. It is possible that similar processes may be active in disorders of visceral organ systems subserved by vagal afferents in other species, including man.

Vagal afferent activity and body temperature in 3 to 10-day-old and adult rats

This study examined the possible contribution of vagal stretch receptor activity to the increased power of the Hering-Breuer reflex in hyperthermia in rats during the early postnatal period. Experiments were performed on 10 anesthetized (pentobarbital 40 mg/kg, ip) 3 to 10-day-old (body weight of 16 ± 1 g; SE) and, for comparison, 18 adult Sprague-Dawley rats (body weight of 336 ± 35 g). Animals were tracheostomized and artificially ventilated with oxygen. The left vagus nerve was cut. In adult animals, single receptor fibers or a bundle of a few fibers were recorded using a bipolar stainless-steel electrode under mineral oil. In the young rats, a suction electrode filled with normal saline was used. Positive pressure of either 5 or 10 cmH2O was applied to the trachea when the respirator was turned off. The vagal activity was amplified and monitored on a storage oscilloscope for calculation of the frequency of vagal afferent activity during a given pressure application at different rectal temperatures (TR; range 28 to 42°C). In total, 30 and 31 sets of vagal activity in the young and adult rats, respectively, were analyzed. In all cases, an increase in tracheal pressure (PTR) from 5 to 10 cmH2O increased the frequency of vagal firing. The increase was greater in the adult versus the young animals; at 36°C the increase was 49 ± 11% and 16 ± 3% in the adult and young rats, respectively (P < 0.01). In all animals, vagal receptors showed temperature-sensitivity, but less so in the young than in the adult rats (P < 0.0004 and P < 0.003; for PTR of 5 and 10 cmH2O, respectively). In addition, the relationship between temperature-sensitivity and TR had significant slopes (P < 0.001 for both inflation pressures) in the adults but not in the young rats, indicating that in the latter the temperature-sensitivity of vagal receptors is independent of TR. These results imply that temperature-sensitivity of vagal receptors could have contributed to the increased power of the Hering-Breuer reflex in rats during the early postnatal period in the warmer environment.Key words: vagal receptors, temperature, Hering-Breuer reflex, young, adults, rats.

Ventilatory responses to changes in temperature in mammals and other vertebrates

This article reviews the relationship between pulmonary ventilation (VE) and metabolic rate (oxygen consumption) during changes in ambient temperature. The main focus is on mammals, although for comparative purposes the VE responses of ectothermic vertebrates are also discussed. First, the effects of temperature on pulmonary mechanics, chemoreceptors, and airway receptors are summarized. Then we review the main VE responses to cold and warm stimuli and their interaction with exercise, hypoxia, or hypercapnia. In these cases, mammals attempt to maintain both oxygenation and body temperature, although conflicts can arise because of the respiratory heat loss associated with the increase in ventilation. Finally, we consider the VE responses of mammals when body temperature changes, as during torpor, fever, sleep, and hypothermia. In ectotherms, during changes in temperature, VE control becomes part of a general strategy to maintain constant relative alkalinity and ensure a constancy of pH-dependent protein functions (alphastat regulation). In mammals on the other hand, VE control is aimed to balance metabolic needs with homeothermy. Therefore, alphastat regulation in mammals seems to have a low priority, and it may be adopted only in exceptional cases.

Central histamine contributed to temperature-induced polypnea in mice

Breathing pattern is influenced by body temperature. However, the central mechanism for changing breathing patterns is unknown. Central histamine is involved in heat loss mechanisms in behavioral studies, but little is known about its effect on breathing patterns. We examined first the effect of body temperature on breathing patterns with increasing hypercapnia in conscious mice and then that of the depletion of central histamine by S(+)-alpha-fluoromethylhistidine hydrochloride (alpha-FMH) (100 mg/kg ip), a specific inhibitor of histidine decarboxylase, at normal and raised body temperatures. A raised body temperature increased respiratory frequency with reductions in both inspiratory and expiratory time and decreased tidal volume. On the other hand, alpha-FMH lowered respiratory frequency with a prolongation of expiratory time at the raised temperature; however, this was not observed at a normal temperature. These results indicate that central histamine contributes to an increase in respiratory frequency as a result of a reduction in expiratory time when body temperature is raised.

Hering-Breuer reflex in conscious newborn rats: effects of changes in ambient temperature during hypoxia

In a previous study in conscious normoxic newborn rats, we found that the strength of the Hering-Breuer reflex (HB reflex) was greater (188%) at high (36 degrees C) than at low (24 degrees C) ambient temperature (T(a); D. Merazzi and J. P. Mortola. Pediatr. Res. 45: 370-376, 1999). We now asked what the effect would be of changes in T(a) during hypoxia. Rat pups at 3-4 days of age were studied in a double-chamber airflow plethysmograph. The HB reflex was induced by negative body surface pressures of 5 or 10 cmH(2)O and quantified from the inhibition of breathing during maintained lung inflation. Rats were first studied at T(a) = 32 degrees C in normoxia, followed by hypoxia (10% O(2) breathing). During hypoxia, oxygen consumption (VO(2)) averaged 47%, and HB reflex 115%, of the corresponding normoxic values, confirming that in the newborn, differently from the adult, hypoxia does not decrease the strength of the HB reflex. As hypoxia was maintained, lowering T(a) to 24 degrees C or increasing it to 36 degrees C, on average, had no significant effects on VO(2) and the HB reflex. However, with 5-cmH(2)O inflations, the HB reflex during the combined hypoxia and hyperthermia was significantly stronger than in normoxia. We conclude that in conscious newborn rats during normoxia the T(a) sensitivity of the HB reflex is largely mediated by the effects of T(a) on thermogenesis and VO(2); in hypoxia, because thermogenesis is depressed and VO(2) varies little with T(a), the HB reflex is T(a) independent. The observation that the reflex response to lung inflations during hypoxic hyperthermia can be greater than in normoxia may be of importance in the pathophysiology of apneas during the neonatal period.

Interaction between somatic and vagal afferent inputs in control of ventilation in 2-week-old rabbits

Effects of saphenous nerve stimulation (SNS) on the Hering-Breuer expiratory-promoting reflex evoked by a positive tracheal pressure (PTR; 5 cmH2O) and on the diaphragmatic EMG (EMG(DI)), inspiratory (TI) and expiratory (TE) time, were studied in 16 urethane-anesthetized (1.2-1.6 g/kg, i.p.) spontaneously breathing 2-week-old rabbits. Positive P(TR) applied at the end of T(I) increased the subsequent TE to 255+/-29% (+/-S.E.; P < 0.0001) of control. SNS (1 sec train, 2 msec pulse, 6 Hz) applied at the onset of TE, shortened this TE by 42+/-3% (P < 0.0001). When SNS preceded positive PTR or positive PTR preceded SNS, the TE increased to 163+/-20 and 184+/-21% of control, respectively. These responses were not different, and smaller than that provoked by the PTR test alone (P < 0.003 and 0.05, respectively). The results show that in newborns somatic afferent stimulation attenuates the vagally mediated respiratory inhibition, whether immediately before or during the vagal stimulation.

Effects of changes in ambient temperature on the Hering-Breüer reflex of the conscious newborn rat

We questioned to what extent changes in temperature could affect the newborn's ventilatory inhibition provoked by lung inflation, or Hering-Breüer (HB) inflation reflex. Conscious newborn rats (3-4 d old) were studied in a double chamber airflow plethysmograph at ambient temperatures of 32 degrees C (slightly below their thermoneutrality), 24 degrees C (cold), and 36 degrees C (warm). At these ambient temperatures, the corresponding body temperatures averaged 35.4, 31.0, and 37 degrees C. The HB reflex was triggered by negative body surface pressures of 5 or 10 cm H2O, and quantified as the duration of the expiratory time during the maintained inflation, either in absolute values or in relation to the control expiratory time. In cold the HB reflex decreased to 80%, and in warm it increased to 150%, of the values measured at 32 degrees C. Oxygen consumption, measured by an open flow system, averaged 59, 47, and 29 mL x kg(-1) x min(-1) at, respectively, 24, 32, and 36 degrees C. In cold, the ventilatory response to hypoxia (10% O2) was almost absent, that to hypercapnia (5% CO2) was greater, and that to hypoxia and hypercapnia combined was less than in warm. We conclude that in newborn rats the strength of the vagal inhibition on breathing, evaluated in the form of the HB reflex, is sensitive to temperature, becoming stronger as temperature increases. One contributing factor is the temperature-induced change in metabolic rate, whereas the role of temperature-induced changes in ventilatory chemosensitivity is variable. The strong temperature-dependency of the neonatal HB reflex implies that in newborns exposed to a warm environment breathing is more susceptible to inhibitory inputs.

Effects of baclofen on the Hering-Breuer inspiratory-inhibitory and deflation reflexes in rats

The objective of this study was to evaluate effects of baclofen, a gamma-aminobutyric acid type B (GABAB) receptor agonist, injected into the nucleus of the solitary tract, on the Hering-Breuer inspiratory-inhibitory (TI-inhibitory) and deflation reflexes in urethan-anesthetized adult Wistar rats (n = 7). The TI-inhibitory reflex was estimated from changes in peak amplitude of the integrated diaphragmatic electromyogram and inspiratory time (TI) provoked by airway occlusion at end expiration. The deflation reflex was evaluated from changes in TI and expiration (TE) of the first two breaths (TI-1, TE-1 and TI-2, TE-2) immediately after a decrease in tracheal pressure (Ptr). Under control conditions, airway occlusion at end-TE prolonged TI (66 +/- 5%; mean +/- SE) and the following TE (54 +/- 11%). Decreases in Ptr, from -2 to -5 cmH2O, evoked an increase in TI and shortening of TE of both breaths. Both effects were Ptr dependent, and TI-1 and TE-1 differed from TI-2 and TE-2, suggesting a rapid adaptation to the stimulus. At Ptr of -5 cmH2O, TI-1 and TI-2 increased by 30 +/- 2 and 43 +/- 6%, respectively, and TE-1 and TE-2 decreased by 53 +/- 4 and 33 +/- 7%, respectively. During unloaded breathing, 60 pmol baclofen prolonged TI by 120 +/- 11% and left TE unaffected. Baclofen abolished vagally mediated changes in TE. On the other hand, the TI increases caused by either airway occlusion (24 +/- 8%) or Ptr of -5 cmH2O (TI-1; 16 +/- 5%) were still significant, but TI-1 and TI-2 were not different. A GABAB receptor antagonist, CGP-35348 (2.8 nmol), reversed these effects of baclofen. These results imply that stimulation of GABAB receptors attenuates but does abolish vagally mediated control of TI. The difference in effects of baclofen on the central and vagal control of TI and TE suggests different distribution of GABAB receptors in neuronal networks controlling each of these respiratory phases.

Effects of graded focal cold block in the solitary and para-ambigual regions of the medulla in the cat

Unilateral focal cold blocks in the region of the nucleus tractus solitarius and the dorsal respiratory group of neurons, DRG, of anaesthetized cats consistently caused apneustic-type breathing. There was no concomitant change in the initial rate of rise of inspiratory activity. The apneustic prolongation of inspiratory duration, TI, was most pronounced in, but was not confined to, the DRG. The apneustic effects were more marked after vagotomy. In cats with intact vagus nerves being given artificial ventilation, focal cooling at certain sites of the DRG region could produce 'unlocking' of the respiratory rhythm from that of the respiratory pump. At other sites in this region, focal cooling could selectively block the effects of the inspiration-facilitating reflex induced by deflation without blocking the inspiration-inhibiting Hering-Breuer reflex. Unilateral focal cold blocks in the region of the intermediate part of the ventral respiratory group of neurons, VRG, generally caused depression of the rate of rise of inspiratory activity, but almost never apneustic effects. All effects of unilateral focal cooling both in the DRG and VRG were bilaterally symmetrical. No systematic differences between the effects on phrenic and external intercostal inspiratory activity were found in response to focal cooling either of the DRG or VRG suggesting that differential control of phrenic and external intercostal motoneurons is not exerted mainly at the level of these medullary structures. The results suggest that the DRG and VRG areas exert somewhat different effects on the respiratory pattern: DRG appears to be more concerned with integration of vagal and other inputs contributing to the inspiratory off-switch mechanisms which, however, are not confined only to the DRG. The VRG inspiratory mechanisms, on the other hand, appear to be more involved in the gain control of the inspiratory output intensity.

The effect of vagal blockade on the variability of ventilation in the awake dog

The present study examined the variability of breathing in five (5) awake tracheostomized dogs with the vagi intact and during complete vagal blockade produced by cooling exteriorized cervical vagal loops (VC). Breath by breath variations in both respiratory timing (assessed from the airflow signal) and the drive to the respiratory muscles (as assessed from the rate of inspiratory airflow (VI/TI) and occlusive pressure (P100) were examined. The degree of variability in the parameters characterizing breathing was evaluated from frequency distribution histograms and by calculation of the standard deviation. VC increased the mean values of VT, TI, TE, TI/TTOT, and decreased VT/TI and occlusion pressure, but had no consistent effect on the mean value of VE. The variability of VE, PACO2, VT, TI, TE, TI/TTOT was greater during VC in 4 of the 5 dogs. The increased variability of VE and PACO2 during VC appeared to be due to a poorer correlation between TI and TE. The present study suggests that vagal mechanoreceptors, presumably pulmonary stretch receptors, minimize breath by breath fluctuations in both the level and pattern of ventilation by controlling respiratory timing. An explanation, based on the model of inspiratory off-switching proposed by Beadley et al. (1975) is invoked.

Excitability changes of the inspiratory "off-switch" mechanism tested by electrical stimulation in nucleus parabrachialis in the cat

The time course of the excitability of the inspiratory "off-switch" mechanism with and without phasic vagal stretch receptor feedback has been studied in cats under light pentobarbitone anesthesia by electrical stimulation in the rostral pons using brief tetanic stimulation (300 Hz for 0.2 s). The threshold strength required just to elicit inspiratory "off-switch" was high early in inspiration and fell steeply with time. The threshold curves were steeper with than without phasic vagal feedback, and the difference reflects the phasic vagal contribution to the excitability of the inspiratory "off-switch" the absence of phasic vagal vagal feedback the time course of this threshold curve usually corresponded closely to that of the "integrated" phrenic activity at all PCO2 levels and body temperatures tested indicating that the "integrated" phrenic activity can be used as an index of the centrally generated inspiratory activity. In response to a rise in PCO2 both the rate of change of excitability of the inspiratory "off-switch" mechanism and its initial threshold level was increased. Changes in body temperature caused no change in the initial threshold but produced marked changes in the rate of rise of the "off-switch" excitability; Following an "augmented breath" the inspiratory "off-switch" threshold was markedly reduced

Effects of lesions in the parabrachial nucleus on the mechanisms for central and reflex termination of inspiration in the cat

The effects of PCO2 and body temperature on the time course and peak amplitude of the central inspiratory activity (CIA) and the inspiratory "off-switch" threshold was studied in apneustic and non-apneustic cats. Apneusis resulted from lesions of the inspiratory inhibiting structures of the medial parabrachial nucleus (NPBM) and by interrupting vagal volume feedback. The cats were paralyzed and ventilated either proportionally to their phrenic output or at predetermined rate and volume. The dependence of the rate of rise and maximal amplitude of phrenic activity on PCO2 and body temperature were comparable in apneustic and non-lesioned animals. The Hering-Breuer volume threshold for inspiratory termination was increased following the rostral pontine lesions. Both hyperthermia and hypercapnia caused augmentation of the absolute rate of rise of inspiratory activity but hypercapnia, in contrast to hyperthermia, caused virtually no change in the fractional increment per unit time. With hypercapnia the inspiratory "off-switch" threshold was raised in the apneustic animals in intact ones, whereas hyperthermia did not seem to influence this threshold. In apneustic conditions expiratory duration remained constant, independent of the large variations in the inspiratory durations. Our results suggest that the NPBM merely provides an excitatory, threshold-lowering input to the inspiratory "off-switch" mechanism.