Initial Increase In Ventilation Research Articles

Recovery of the biphasic hypoxic ventilatory response in neonatal rats after chronic hyperoxia

Newborn mammals exhibit biphasic hypoxic ventilatory responses (HVR) characterized by an initial increase in ventilation and a secondary ventilatory depression. The magnitude of the hypoxic ventilatory decline (HVD) in the late phase of the HVR normally decreases with age, but this occurs sooner in rats reared in 60% O2. We investigated whether a lower level of hyperoxia (30% O2) or a short period of recovery (1 or 3 d in 21% O2) would affect the expression of this plasticity. Similar to 60% O2, rat pups reared in 30% O2 until 3–4 days of age exhibited a less biphasic HVR to 12% O2. When pups reared in 60% O2 were returned to normoxia, the magnitude of HVD increased such that pups expressed a biphasic HVR appropriate for their chronological age. Blocking synaptic input from the carotid bodies revealed that CNS hypoxia depressed ventilation less in hyperoxia-reared rats immediately following hyperoxia and after 1 d in normoxia despite recovery of the biphasic HVR. This suggests that recovery of the biphasic HVR occurs in pathways regulating HVD that depend on carotid body activity. The early, carotid body-mediated phase of the HVR was also blunted immediately and 1 d after the hyperoxia exposure, but not after 3 d of recovery. These data confirm that short exposures to mild-to-moderate hyperoxia elicit developmental plasticity in the HVR. However, reemergence of the biphasic HVR after return to normoxia argues against a heterokairic process for the premature transition from biphasic HVR to sustained HVR in hyperoxia-reared rat pups.

P2Y 1 receptor modulation of Ih in the preBötzinger inspiratory rhythm generator may underlie the ATP‐mediated attenuation of the hypoxic respiratory depression

Premature infants often stop breathing briefly (apnea) because the brainstem respiratory network is still immature (apnea of prematurity, AOP). During these apneas, oxygen levels fall (hypoxia) triggering a biphasic hypoxic ventilatory response comprising an initial increase in ventilation followed by a centrally mediated secondary depression that can be life-threatening. Caffeine is used to stimulate breathing in AOP. However, ~20% of these infants do not respond to caffeine. Thus, alternate therapies are needed. Recent data suggest that during hypoxia, ATP is released in the preBötzinger Complex (preBötC, inspiratory rhythm generator) where it acts via P2Y1 receptors (Rs) to excite inspiratory neurons, increase ventilation and attenuate the secondary hypoxic depression. A mechanistic understanding of this P2Y1R excitation is lacking. Preliminary whole-cell analysis of inspiratory preBötC neurons in vitro revealed P2Y1R currents that reverse near -40 mV and increase in amplitude with neuron hyperpolarization. Based on these data, we tested the hypotheses that the increase in inspiratory frequency evoked by ATP in the preBötC in vitro and the ATP-mediated increase in ventilation during hypoxia in vivo reflects P2Y1R potentiation of the hyperpolarization-activated inward current, Ih (carried by cyclic nucleotide–gated, HCN, channels). Application of MRS2365 (P2Y1R agonist, 100 µM) to Ih-expressing inspiratory preBötC neurons in rhythmic brainstem slices from neonatal rat potentiated Ih by 32±6% (at -100 mV, n=18) and depolarized voltage-dependence of Ih activation by ~10 mV. The MRS2365 inward current and its potentiation of Ih were blocked by the Ih blocker, ZD7288 (100 μM). ZD7288 also attenuated the MRS2365-evoked frequency increase by 60-90%. Ih currents in preBötC neurons were sensitive to ZD7288 (100 µM) but not ivabradine (30 µM), suggesting HCN2 but not HCN1,4 subunit involvement. In paralyzed, anesthetized, vagotomized ventilated adult rats, bilateral microinfusion of ZD7288 (250 µM, but not 100 µM) into the preBötC increased the secondary hypoxic (10% O2) depression by 68±23% (n=6) compared to rats that received aCSF microinfusion (n=4). This effect was regionally specific. If the bilateral microinfusions of ZD7288 were not both in the preBötC (e.g., 1 in preBötC and the other in BötC or rVRG), ZD7288 had no effect on the hypoxic ventilatory response. Finally, in anesthetized, vagotomized, spontaneously-breathing adult rats, unilateral reverse microdialysis of ZD7288 (100 µM) in the preBötC area (n=13) increased the hypoxic frequency depression by 20±6% (compared to aCSF, n=6). These data suggest that the P2Y1R excitation of the preBötC network is produced, in part, via potentiation of Ih in a subgroup of inspiratory neurons, and that potentiation of Ih in the preBötC attenuates the hypoxic ventilatory depression in adult rats. Data also identify the HCN2 subunit as a potential target for new strategies to counteract respiratory depression in AOP.

Limited Capacity for Clearance of Adenosine from the preBötzinger Complex Inspiratory Network Contributes to the Profound Depression of Breathing by Hypoxia in the Very Young

Breathing in premature infants often stops briefly (apnea) because the brainstem network that controls breathing is immature (apnea of prematurity, AOP). Apneas cause hypoxia, triggering the hypoxic ventilatory response (HVR), which comprises an initial increase in ventilation followed by a centrally mediated 2° decrease. In adults, breathing remains above baseline during this 2° phase, but in the very young it falls below baseline leading to a life-threatening feedback loop where apnea causes hypoxia, respiratory depression, further hypoxia and so on. Understanding the cause of this powerful hypoxic inhibition of breathing in the very young, a major question in perinatal physiology, could inform new therapies for AOP. The transmitter ATP is released in the preBötzinger Complex (preBötC, site of inspiratory rhythm generation) during hypoxia where it increases breathing and attenuates the 2° depression. However, extracellular ATP (ATPe) is degraded to adenosine (ADO), which inhibits breathing and is implicated in AOP; e.g., caffeine blocks ADOe actions and is the first line treatment for AOP. We hypothesize that the greater 2° inhibition of breathing in prematurity is due to an immature system for clearing ADOe. ADOe-mediated inhibition stops with its removal from the extracellular space, which depends on i) equilibrative nucleoside transporters (ENTs) that move ADOe across cell membranes down its concentration gradient, and ii) ADO kinase (ADK), an enzyme that keeps the level of ADO inside cells lower than outside so ENTs clear ADOe. How ENT activity develops in the preBötC is unknown. However, the form of ADK that influences ADOe in the brain is minimally functional until 2 weeks of age in rodents. To assess the importance of ADOe clearance in the development of the HVR, we used plethysmography to measure the HVR (10% O2) in 0-56 day old (P0-14) wild-type (WT), ENT knockout (KO), and ADKtg mice engineered to have functional ADK throughout life. In WT and ENT KO neonates (P0-2), ventilation fell below baseline after ~7 min in hypoxia, but the hypoxia-evoked frequency increase was markedly reduced in the ENT KO. Strikingly, P0-2 ADKtg mice showed the mature breathing response; ventilation remained above baseline levels throughout hypoxia. At P12 in WT and ENT KO, the HVR had still not matured; ventilation fell below baseline during the 2° phase, yet it remained above baseline in P12 ADKtg mice. To assess the importance of ENT and ADK specifically in the preBötC, we isolated the preBötC network in rhythmic brain slices from P0-12 mice. The ADO inhibition of preBötC frequency lasted twice as long in P0-12 ENT KO mice compared to WT. Conversely, the decrease in basal frequency caused by ADK inhibition (ABT-702) increased with age (P0-2, 15%; P9-11, 23%). These data suggest that i) ENTs are important for ADO clearance in the preBötC after birth and ii) low ADK activity at birth is a major contributor to the greater hypoxic respiratory depression in early development.

Influence of chronic hyperoxia on the developmental time course of the hypoxic ventilatory response relative to other traits in rats

Newborn mammals exhibit a biphasic hypoxic ventilatory response (HVR) in which an initial increase in ventilation is followed by a decline back toward baseline levels. The magnitude of the secondary decline diminishes with postnatal age, but this transition occurs earlier in rat pups reared in moderate hyperoxia. This pattern is consistent with heterokairy, a form of developmental plasticity in which environmental factors alter the timing of developmental events. The present study investigated whether this plasticity is specific to the HVR or if hyperoxia instead accelerates overall development. Rat pups reared in 60 % O2 (Hyperoxia) exhibited a less biphasic ventilatory response to 12 % O2 than pups reared in 21 % O2 (Control) at 4 days of age (P4) and transitioned to a sustained HVR by P10-11; Control rats exhibited a biphasic HVR at both ages. However, the average ages at which pups attained other key developmental milestones (i.e., fur development at P5, incisor eruption at P9, and eye opening at P15) were similar between treatment groups. Moreover, growth rates and maturation of the metabolic response to cooling were not accelerated, and may have been delayed slightly, relative to Control rats. For example, the capacity for pups to increase their metabolic rate at low ambient temperatures increased with age, but this thermogenic capacity tended to be reduced in Hyperoxia pups at both P4 and P10-11 (i.e., lower CO2 production rates below the lower critical temperature). Collectively, these data support the conclusion that hyperoxia specifically advances the age at which rat pups exhibit a sustained HVR, altering the relative timing of developmental events rather than compressing the entire period of development.

Early Maturation of the Hypoxic Ventilatory Response in Hyperoxic Rat Pups: Evidence for Trait‐Specific Heterokairy

Newborn mammals exhibit a biphasic hypoxic ventilatory response (HVR) in which an initial increase in ventilation is followed by a decline back toward baseline levels. The magnitude of the secondary decline diminishes with age, but rat pups reared in moderate hyperoxia make this transition earlier (Bavis et al., J. Appl. Physiol. 109(3): 796–803, 2010). This is consistent with heterokairy, a form of developmental plasticity in which environmental factors alter the rate and/or timing of developmental events. The present study investigated whether this effect is specific to the HVR or if hyperoxia instead accelerates overall development. Rat pups reared in 60% O2 (Hyperoxia) exhibited a less biphasic HVR at both P4 and P10‐11 compared to pups reared in 21% O2 (Control). However, growth curves and the average ages at which pups attained key developmental milestones (i.e., fur development at P5, incisor eruption at P9, and eye opening at P15) were similar between treatment groups. We also studied the relationship between metabolic rate and ambient temperature as another indicator of physiological maturity. The lower critical temperature (LCT) decreased with age, but the LCT was the same for Control and Hyperoxia rats at both P4 (~32°C) and P10‐11 (~30°C). The ability to increase metabolic rate at low ambient temperatures also increased with age, but this thermogenic capacity tended to be reduced in Hyperoxia rats at both P4 and P10‐11 (i.e., lower rates of metabolic CO2 production at temperatures below the LCT). We conclude that hyperoxia accelerates the rate at which the HVR matures without affecting the maturation of other morphological and physiological traits.

Development of ventilatory chemoreflexes in Coturnix quail chicks

Compared to mammals, little is known about the development of the respiratory control system in birds. In the present study, ventilation and metabolism were measured in Coturnix quail chicks exposed to room air, hypoxia (11 % O2), and hypercapnia (4% CO2) at 0–1, 3–4, and 6–7 days posthatching (dph). Mass-specific ventilation and metabolic rate tended to increase between 0–1 and 3–4 dph and then decrease again between 3–4 and 6–7 dph. The magnitude of the hypoxic ventilatory response (HVR) increased with age. The HVR also exhibited a biphasic shape in younger quail: after the initial increase in ventilation, ventilation declined back to (0–1 dph), or toward (4 dph), baseline. Older chicks (6–7 dph) had a “sustained HVR” in which ventilation remained high throughout the hypoxic challenge. The biphasic HVR did not appear to be caused by a decline in metabolic rate; although hypoxic hypometabolism was observed in quail chicks in all three age groups, the metabolic response appeared to occur more slowly than the biphasic HVR. The biphasic ventilatory response was also specific to hypoxia since the hypercapnic ventilatory response (HCVR) was characterized by a sustained increase in ventilation in all three age groups. The magnitude of the HCVR decreased with age. These results point to several similarities in the development of ventilatory chemorflexes between Coturnix quail and newborn mammals, including age-dependent (1) increases in the HVR, (2) transitions from a biphasic to a sustained HVR, and (3) decreases in the HCVR. Whether homologous mechanisms underlie these developmental changes remains to be determined.

Impact of Equilibrative Nucleoside (Adenosine) Transporters on the preBötzinger Complex Inspiratory Network and the Hypoxic Ventilatory Response in Newborn Mice

Exposure to low oxygen (hypoxia) evokes a biphasic hypoxic ventilatory response (HVR) comprising of an initial increase in ventilation followed by a secondary depression. The secondary depression is much stronger and life‐threatening in premature infants and it is implicated in apneic events (Apnea of Prematurity). Despite the clinical significance, mechanisms underlying this secondary depression are poorly understood. Recent data suggest that during hypoxia, astrocytes in the preBötzinger complex (preBötC) release ATP, which increases breathing and attenuates the secondary depression. However, ATP is rapidly metabolized into extracellular adenosine (ADOe), which is implicated in the secondary depression. Thus, the overall effects of ATP are determined by a balance between its excitatory actions and the inhibitory actions of its by‐product, ADO. The importance of ADO is clear in that infants with apnea of prematurity are given caffeine (ADO receptor antagonist) to stimulate breathing. The ATP‐ADO balance is controlled by many factors, including equilibrative nucleoside transporters (ENTs) that move ADO across cell membranes along its concentration gradient. Our goal was to determine the role of ENTs in modulating preBötC network activity under baseline and hypoxic conditions. Bath application of the ENT‐1 inhibitor (NBMPR, 100 μM) to rhythmically‐active medullary slices from neonatal mice caused a significant 9.9±4% increase in frequency (n=5). However, when the preBötC was challenged with an ADO load (as might happen during hypoxia) via local injection of ADO (500 μM, 30 s) ENT‐1 inhibition enhanced the inhibitory effect. Similarly, when we compared the responses of wild type (WT) and ENT 1/2 knockout (ENT KO) mice to preBötC ADO injections, the duration of the frequency inhibition was almost doubled in the KO mice. We next compared the HVR of WT and ENT KO mice (1–3 days old) using whole‐body plethysmography (15 min control, 10 min 10% O2, 10 min recovery). Ventilation increased in both groups by 20% and gradually fell below baseline in WT mice after 4 min in hypoxia. In marked contrast, ventilation fell below baseline during the first 40 s of hypoxia in ENT KO mice by as much as 25% and never increased above baseline throughout the hypoxic challenge. These in vitro data suggest that ENTs contribute to setting ADOe tone in the preBötC under baseline conditions by transporting ADO into the extracellular space. However, under an extra load as might occur in hypoxia, ENTs become important in removing ADOe. In vivo data are consistent with this hypothesis, but it will be critical to determine whether the differences in the HVR of WT and ENT KO mice in vivo reflects loss of ENT activity in the carotid bodies or central nervous system.Support or Funding InformationCIHR, NSERC, WCHRI, AIHS, CFIThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Influence of GABA Receptor Antagonists on the Biphasic Hypoxic Ventilatory Response of Newborn Rats

Newborn mammals exhibit a characteristically biphasic hypoxic ventilatory response (HVR) in which an initial increase in ventilation is followed by a progressive decline toward (or below) baseline values. Studies in anesthetized, newborn piglets suggest that the secondary hypoxic depression is partially attributable to γ‐aminobutyric acid (GABA) signaling through GABAA and GABAB receptors (Huang et al. J Appl Physiol 77:1006–1010, 1994). To determine whether GABAA and GABAB receptors similarly contribute to the biphasic HVR of newborn rats, we measured normoxic (21% O2) and hypoxic (12% O2) ventilation of newborn (P3–5) rats by head‐body plethysmography 30 minutes after intraperitoneal injection with vehicle or a GABA receptor antagonist. Two different GABAB receptor antagonists, CGP‐35348 (200 mg kg−1) and CGP‐46381 (50 or 100 mg kg−1), were found to have no effect on normoxic ventilation or the HVR (all P>0.05). Similar results were obtained for the GABAA receptor antagonist (+)‐bicuculline (2 or 3 mg kg−1). Collectively, these data do not support a role for GABAergic signaling in the biphasic HVR of newborn rats.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Postnatal Development of Hypoxic and Hypercapnic Ventilatory Responses in Coturnix Quail

Newborn mammals generally exhibit a biphasic hypoxic ventilatory response (HVR) in which the initial increase in ventilation is followed by a progressive decline toward (or below) baseline; the magnitude of the secondary decline diminishes with age. Considerably less is known about postnatal development of the HVR in birds, although a recent study did not detect a biphasic HVR in nestling red‐winged blackbirds (Dzialowski et al., Am J Physiol 311:R1105–R1112, 2016). We used head‐body plethysmography to characterize the ventilatory responses of Coturnix quail chicks to 15‐min challenges of 11% O2 or 4% CO2 at 0–1, 3–4, and 6–7 days of age. The magnitude of the HVR increased with postnatal age (21±4, 35±3, and 42±4 % increase from baseline, respectively; mean±SEM, P<0.01). The HVR was biphasic at P0–1 and (to a lesser extent) at P3–4, but not at P6–7. In contrast, the magnitude of the hypercapnic ventilatory response decreased with age (76±8, 54±4, 31±3 % increase from baseline, respectively; P<0.001). The hypercapnic ventilatory response was characterized by a relatively sustained increase in ventilation in all age groups, suggesting that the biphasic ventilatory response is specific to hypoxia. Additional experiments assessed hypoxic hypometabolism, and decreased metabolic rate does not appear to explain the biphasic HVR in the youngest quail chicks. We conclude that newborn quail exhibit a biphasic HVR similar to that observed in newborn mammals.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Release of ATP by pre-Bötzinger complex astrocytes contributes to the hypoxic ventilatory response via a Ca2+ -dependent P2Y1 receptor mechanism.

The ventilatory response to reduced oxygen (hypoxia) is biphasic, comprising an initial increase in ventilation followed by a secondary depression. Our findings indicate that, during hypoxia, astrocytes in the pre-Bötzinger complex (preBötC), a critical site of inspiratory rhythm generation, release a gliotransmitter that acts via P2Y1 receptors to stimulate ventilation and reduce the secondary depression. In vitro analyses reveal that ATP excitation of the preBötC involves P2Y1 receptor-mediated release of Ca2+ from intracellular stores. By identifying a role for gliotransmission and the sites, P2 receptor subtype, and signalling mechanisms via which ATP modulates breathing during hypoxia, these data advance our understanding of the mechanisms underlying the hypoxic ventilatory response and highlight the significance of purinergic signalling and gliotransmission in homeostatic control. Clinically, these findings are relevant to conditions in which hypoxia and respiratory depression are implicated, including apnoea of prematurity, sleep disordered breathing and congestive heart failure. The hypoxic ventilatory response (HVR) is biphasic, consisting of a phase I increase in ventilation followed by a secondary depression (to a steady-state phase II) that can be life-threatening in premature infants who suffer from frequent apnoeas and respiratory depression. ATP released in the ventrolateral medulla oblongata during hypoxia attenuates the secondary depression. We explored a working hypothesis that vesicular release of ATP by astrocytes in the pre-Bötzinger Complex (preBötC) inspiratory rhythm-generating network acts via P2Y1 receptors to mediate this effect. Blockade of vesicular exocytosis in preBötC astrocytes bilaterally (using an adenoviral vector to specifically express tetanus toxin light chain in astrocytes) reduced the HVR in anaesthetized rats, indicating that exocytotic release of a gliotransmitter within the preBötC contributes to the hypoxia-induced increases in ventilation. Unilateral blockade of P2Y1 receptors in the preBötC via local antagonist injection enhanced the secondary respiratory depression, suggesting that a significant component of the phase II increase in ventilation is mediated by ATP acting at P2Y1 receptors. In vitro responses of the preBötC inspiratory network, preBötC inspiratory neurons and cultured preBötC glia to purinergic agents demonstrated that the P2Y1 receptor-mediated increase in fictive inspiratory frequency involves Ca2+ recruitment from intracellular stores leading to increases in intracellular Ca2+ ([Ca2+ ]i ) in inspiratory neurons and glia. These data suggest that ATP is released by preBötC astrocytes during hypoxia and acts via P2Y1 receptors on inspiratory neurons (and/or glia) to evoke Ca2+ release from intracellular stores and an increase in ventilation that counteracts the hypoxic respiratory depression.

Exocytotic Release of ATP from preBötzinger Complex Astrocytes Contributes to the Hypoxic Ventilatory Response via a P2Y1 Receptor, PLC, PKC‐Dependent Mechanism

The hypoxic ventilatory response (HVR) comprises an initial increase in ventilation followed by a secondary depression. We tested the hypothesis that the exocytotic release of ATP by astrocytes in the preBötC attenuates this secondary depression via a P2Y1 receptor (R)‐mediated increase in ventilation. The preBötC of adult rats was injected with an adenovirus expressing tetanus toxin light chain (TeLC) and GFP under the control of the GFAP promoter. 7 days later rats were anesthetized and the HVR (10% O2, 5 min) measured via the phrenic nerve. Ventilation increased initially in the control group to 1.7±0.21 of baseline and then fell to 1.4±0.14. In the TeLC group, the HVR was significantly reduced during the initial (1.5± 0.08) and the secondary (1.2±0.05) phases. Unilateral injection of P2Y1R antagonist MRS2279 (500 µM) into the preBötC revealed that endogenous activation of P2Y1Rs significantly attenuated the secondary depression. Multiple second messenger blockers attenuated the frequency increase evoked by injection of a P2Y1R agonist MRS2365 (10μM) into the preBötC of rhythmic medullary slices of neonatal rats. These data suggest that during hypoxia, preBötC astrocytes release ATP, which binds to P2Y1Rs that act through PLC‐IP3‐gated Ca2+ release and PKC to increase ventilation. Supported by CIHR, AIHS, CFI, WCHRI, Wellcome Trust.

ATP acts via P2Y1 receptors in the preBötzinger Complex in vivo to attenuate the secondary hypoxic respiratory depression

The ventilatory response to hypoxia comprises an initial increase in ventilation followed by a secondary depression that can be life‐threatening in premature infants. The mechanisms underlying the kinetics and magnitude of this secondary depression are not known, but ATP released within the ventrolateral medulla acts via purinergic (P2) receptors to attenuate this depression. We explored the hypothesis developed from analysis of the rhythmic medullary slice preparation that ATP acts via P2Y1 receptors in the preBötzinger Complex (preBötC, a critical site for inspiratory rhythm generation) to mediate this attenuation. Adult, urethane‐anesthetized, spontaneously‐breathing, vagotomized rats were exposed to 10% O2 (90% N2) for 5 min immediately following local unilateral injection of vehicle (0.2μl HEPES buffer) or 500 μM MRS2279 (P2Y1 receptor antagonist) into the preBötC. Respiratory frequency (fR) and minute ventilation (VE) increased rapidly by ~40% in both groups. fR fell to baseline levels in the control group but to 0.8 of baseline in the MRS2279‐injected animals. VE fell to levels that were 20% above baseline in the control group but to baseline in the MRS2279 group. Data suggest that during hypoxia endogenously‐released ATP acts via P2Y1 receptors in the preBötC to reduce the magnitude of the secondary hypoxic respiratory depression. Supported by CIHR, AIHS, CFI, WCHRI and ALA.

Ventilatory responses during and following exposure to a hypoxic challenge in conscious mice deficient or null in S-nitrosoglutathione reductase

Exposure to a hypoxic challenge increases ventilation in wild-type (WT) mice that diminish during the challenge (roll-off) whereas return to room air causes an increase in ventilation (short-term facilitation, STF). Since plasma and tissue levels of ventilatory excitant S-nitrosothiols such as S-nitrosoglutathione (GSNO) increase during hypoxia, this study examined whether (1) the initial increase in ventilation is due to generation of GSNO, (2) roll-off is due to increased activity of the GSNO degrading enzyme, GSNO reductase (GSNOR), and (3) STF is limited by GSNOR activity. Initial ventilatory responses to hypoxic challenge (10% O2, 90% N2) were similar in WT, GSNO+/− and GSNO−/− mice. These responses diminished markedly during hypoxic challenge in WT mice whereas there was minimal roll-off in GSNOR+/− and GSNOR−/− mice. Finally, STF was greater in GSNOR+/− and GSNOR−/− mice than in WT mice (especially females). This study suggests that GSNOR degradation of GSNO is a vital step in the expression of ventilatory roll-off and that GSNOR suppresses STF.

Purinergic modulation of preBötzinger Complex inspiratory rhythm generating networks

ATP actions on central inspiratory networks are determined by a 3-part signaling system comprising: i) the excitatory actions of ATP at P2 receptors (Rs) ii) ectonucleotidases that degrade ATP into adenosine (ADO), and iii) the inhibitory actions of ADO at P1Rs. During hypoxia, an initial increase in ventilation is followed by a secondary depression that is life threatening in premature infants. The release of ATP in respiratory networks, including the preBötzinger Complex (preBötC, important site of inspiratory rhythm generation) attenuates this secondary ventilatory depression.

Comparison of hyperthermic hyperpnea elicited during rest and submaximal, moderate-intensity exercise

We tested the hypothesis that, in humans, hyperthermic hyperpnea elicited in resting subjects differs from that elicited during submaximal, moderate-intensity exercise. In the rest trial, hot-water legs-only immersion and a water-perfused suit were used to increase esophageal temperature (T(es)) in 19 healthy male subjects; in the exercise trial, T(es) was increased by prolonged submaximal cycling [50% peak O(2) uptake (Vo(2))] in the heat (35 degrees C). Minute ventilation (Ve), ventilatory equivalent for Vo(2) (Ve/Vo(2)) and CO(2) output (Ve/Vco(2)), tidal volume (Vt), and respiratory frequency (f) were plotted as functions of T(es). In the exercise trial, Ve increased linearly with increases (from 37.0 to 38.7 degrees C) in T(es) in all subjects; in the rest trial, 14 of the 19 subjects showed a T(es) threshold for hyperpnea (37.8 +/- 0.5 degrees C). Above the threshold for hyperpnea, the slope of the regression line relating Ve and T(es) was significantly greater for the rest than the exercise trial. Moreover, the slopes of the regression lines relating Ve/Vo(2), Ve/Vco(2), and T(es) were significantly greater for the rest than the exercise trial. The increase in Ve reflected increases in Vt and f in the rest trial, but only f in the exercise trial, after an initial increase in ventilation due to Vt. Finally, the slope of the regression line relating T(es) and Vt or f was significantly greater for the rest than the exercise trial. These findings indicate that hyperthermic hyperpnea does indeed differ, depending on whether one is at rest or exercising at submaximal, moderate intensity.

Impaired ventilation and metabolism response to hypoxia in histamine H1 receptor-knockout mice

The role of central histamine in the hypoxic ventilatory response was examined in conscious wild-type (WT) and histamine type1 receptor-knockout (H1RKO) mice. Hypoxic gas (7% O(2) and 3% CO(2) in N(2)) exposure initially increased and then decreased ventilation, referred to as hypoxic ventilatory decline (HVD). The initial increase in ventilation did not differ between genotypes. However, H1RKO mice showed a blunted HVD, in which mean inspiratory flow was greater than that in WT mice. O(2) consumption (V(O2)) and CO(2) excretion were reduced 10min after hypoxic gas exposure in both genotypes, but (V(O2)) was greater in H1RKO mice than in WT mice. The ratio of minute ventilation to (V(O2)) during HVD did not differ between genotypes, indicating that ventilation is adequately controlled according to metabolic demand in both mice. Peripheral chemoreceptor sensitivity did not differ between genotypes. We conclude that central histamine contributes via the H1 receptor to changes in metabolic rate during hypoxia to increase HVD in conscious mice.

The role of gamma-aminobutyric acid and glutamate for hypoxic ventilatory response in anesthetized rabbits.

Acute hypoxia produces an increase in ventilation. When the hypoxia is sustained, the initial increase in ventilation is followed by a decrease in ventilation. Hypoxia causes changes in brain neurotransmitters depending on its severity and durations. The purpose of this study was to investigate the role of gamma-aminobutyric acid (GABA) and glutamate for hypoxic ventilatory response in rabbits. The experiments were performed in peripheral chemoreceptors intact and denervated rabbits anesthetized with Na-pentobarbitate. For intracerebroventricular (ICV) injections of reagents in each animal, cannula was placed in left lateral cerebral ventricle by stereotaxic method. After ICV injection of GABA (0.48 mg/kg), air breathing in both groups caused a depression of respiratory activity. On the other hand, after ICV injection of GABA, breathing of hypoxic gas mixture (8% O(2)-92% N(2)) in both groups produced the hypoxic hyperventilation. After ICV injection of GABA, blockade of GABA(A) receptors with bicuculline (0.2 mg/kg) did not prevent the hypoxic hyperventilation. In contrast, after ICV GABA injection, blockade of glutamate NMDA receptors with MK-801 (0.09 mg/kg) completely abolished the hypoxic hyperventilation observed while the animals were breathing hypoxic gas mixture. Our findings suggest that ICV injection of GABA causes respiratory depression in normoxic conditions, and that it increases ventilation in hypoxic conditions with or without peripheral chemoreceptor impulses by increasing glutamate.

Identification of neurons responding to hypoxia in sudden infant death syndrome.

The pathogenesis of sudden infant death syndrome (SIDS) is still not understood, although one of the most credited current hypotheses is the respiratory theory. Considerable evidence has been assembled suggesting that hypoxia in human infants produces an initial increase in ventilation, after which respiration is rapidly inhibited. We investigated the expression of the c-fos proto-oncogene, a marker of activated neurons, particularly by hypoxia, in the medulla oblongata nuclei involved in breathing after birth, with special reference to SIDS. We utilized c-fos protein immunohistochemistry on serial transverse sections of medulla oblongata from 22 SIDS victims. In 60% of the analyzed cases, we observed numerous positive c-fos neurons in the dorsal motor nucleus of the vagal nerve. In control cases, the immunohistochemical labeling was negative or very low. The c-fos protein was expressed in the rostral-intermediate portion of the dorsal motor vagal nucleus, where motoneurons with respiratory-related activity are located. The positive c-fos immunoreactivity observed in SIDS suggests that the neurons of the dorsal motor vagal nucleus involved in the regulation of breathing are able to yield an intense, immediate ventilatory response to hypoxia. Our results support the respiratory theory of SIDS.

Pathophysiologic Mechanisms Underlying Apnea of Prematurity

After completing this article, readers should be able to: 1. Characterize the unique features of neonatal respiratory control mechanisms. 2. Clarify how immature respiratory control contributes to neonatal apnea. 3. Identify the contribution of upper airway obstruction to episodes of mixed apnea. Idiopathic apnea of prematurity is a common disorder that requires therapeutic intervention to avoid potential morbidity in preterm infants who require neonatal intensive care. Severe, recurrent apneic episodes may lead to multiple investigations to rule out secondary disorders leading to apnea (see accompanying article in this issue). Although the incidence of neonatal apnea is inversely related to gestational age and is probably as high as 100% in the most immature preterm infants, the onset may be delayed by the presence of lung disease in the first days of life. Apnea of prematurity, especially if severe or persistent, has been associated with poor developmental outcome in school-age children, although a cause-and-effect relationship is difficult to establish. The definition of apnea varies among studies. Apnea of prematurity has been defined most widely as cessation of breathing in excess of 15 seconds’ duration, typically accompanied by desaturation and bradycardia. However, shorter episodes of apnea, and even periodic breathing, may be accompanied by bradycardia or hypoxemia. Prolonged desaturation episodes also have been reported in the absence of apnea or bradycardia, both in healthy preterm infants and in infants who have chronic lung disease. These episodes might represent obstructive apnea, hypoventilation, or possibly intrapulmonary right-to-left shunting, although episodes of desaturation and bradycardia almost always are preceded by apnea or hypoventilation. Apnea is classified traditionally into three categories based on the presence or absence of upper airway obstruction: central, obstructive, and mixed apneas. Central apnea is characterized by total cessation of inspiratory efforts with no evidence of obstruction. In obstructed apnea, the infant tries to breathe against …

The effect of intracerebroventricular dopamine administration on the respiratory response to hypoxia.

Acute hypoxia produces an increase in ventilation. When the hypoxia is sustained, the initial increase in ventilation is followed a decrease in ventilation. The precise mechanism of this decline in ventilation during sustained hypoxia is unknown. Recent studies hypothesized that the accumulation of dopamine in the central nervous system might have a major role in production of hypoxic respiratory depression. The purpose of this study was to examine whether dopamine has an effect on occurance of central ventilatory depression which is seen in acute hypoxia in peripheral chemoreceptors denervated animals. The experiment were conducted in rabbits anesthetized with Na-pentobarbital (25 mg x kg(-1) i.v.). For intracerebroventricular (i.c.v.) injections of dopamine (1 microg) in each animal, canula was placed in left lateral cerebral ventricle by stereotaxic method. Respiratory frequency (f x min(-1)), tidal volume (V(T)) ventilation minute volume (V(E)) and systemic arterial blood pressure (BP) were recorded during air and 3 minutes hypoxic gas mixture (8%O2-92%N2) breathing. I.c.v. administration of dopamine during normoxia decreased V(T), f, V(E) and BP, significantly. When rabbits were injected with an i.c.v. dopamine on hypoxic gas mixture breathing in control animals, there was depression of hypoxic ventilatory responses. After i.c.v. administration of dopamine antagonists haloperidol (0.1 mg) and domperidone (0.07 mg) in chemodenervated rabbits, the significant increases in V(T), V(E) and BP were observed. On the breathing of hypoxic gas mixture of chemodenervated and i.c.v. dopamine antagonists administrated rabbits, hypoxic depression was completely abolished. These results of this study show that accumulation of dopamine in the brain seems to reduce the response of the central control mechanisms to chemoreceptor impulses during normoxia and hypoxia. In conclusion present study suggests important role played by central dopaminergic pathways in the occurence of acute hypoxic ventilatory depression.