Ventilatory phenotypes among four strains of adult rats

Altered control of breathing in a rat model of allergic lower airway inflammation

Obstructive sleep apnea (OSA) is highly prevalent in patients with asthma. Asthma, dose-dependently to its duration, promotes incident OSA, suggesting that asthma plays a role in OSA pathogenesis. We hypothesized that asthma-related inflammation alters breathing control mechanisms, specifically the carotid chemoreflex. Accordingly, we measured hypoxic ventilatory responses (HVR) in awake, unrestrained, ovalbumin (OVA)-sensitized Brown Norway rats and compared them with responses in sham-sensitized (SALINE) controls. To differentiate the role of allergic inflammation from bronchoconstriction, we repeated hypoxic ventilatory response (HVR) after administration of formoterol, a long-acting bronchodilator. Blood and bronchoalveolar lavage (BAL) fluid were collected for quantification of inflammatory cytokines. The rise in ventilatory equivalent for O2 evoked by acute exposure to hypoxia was augmented following sensitization by OVA, whereas it remained stable after SALINE. This augmentation was driven by increased breathing frequency with no change in tidal volume. Tachypneic hyperventilation in normoxia was also observed with OVA. Neither the increased HVR nor excessive normoxic ventilation was affected by formoterol, suggesting that they were not secondary to lung mechanical constraints. Higher levels of inflammatory cytokines were observed in BAL fluid and serum of OVA versus SALINE. In OVA, serum interleukin-5 levels significantly correlated with change from baseline in ventilatory responses to severe hypoxia ([Formula: see text], 0.09). These observations are consistent with inflammation-induced enhancement of carotid chemoreflex function, i.e., increased controller gain, and they suggest a possible role for asthma-related allergic inflammation in the ventilatory instability known to promote upper airway collapse and sleep apnea in humans.NEW & NOTEWORTHY Asthma is a risk factor for obstructive sleep apnea (OSA); however, the mechanisms are incompletely understood. In a rat model of allergic inflammation associated with asthma, we found that ventilation in normoxia and ventilatory responses to hypoxia were markedly enhanced and related with systemic inflammation. These alterations indicating carotid chemoreflex sensitization, known to promote ventilatory instability during sleep in humans, may contribute to the increased OSA risk in asthma.

Repeated seizure-induced brainstem neuroinflammation contributes to post-ictal ventilatory control dysfunction

Patients with epilepsy face heightened risk of post-ictal cardiorespiratory suppression and sudden unexpected death in epilepsy (SUDEP). Studies have shown that neuroinflammation, mediated by the activation of microglia and astrocytes, may be a cause or consequence of seizure disorders. Kcnj16 (Kir5.1) knockout rats (SS kcnj16-/- ) are susceptible to repeated audiogenic seizures and recapitulate features of human SUDEP, including post-ictal ventilatory suppression, which worsens with repeated seizures and seizure-induced mortality. In this study, we tested the hypothesis that repeated seizures cause neuroinflammation within key brainstem regions that contribute to the control of breathing. Audiogenic seizures were elicited once/day for up to 10 days in groups of adult male SS kcnj16-/- rats, from which frozen brainstem biopsies of the pre-Bötzinger complex/nucleus ambiguus (preBötC/NA), Bötzinger complex (BötC), and raphe magnus (RMg) regions were subjected to a cytokine array. Several cytokines/chemokines, including IL-1α and IL-1ß, were increased selectively in preBötC/NA after 3 or 5 days of seizures with fewer changes in other regions tested. In additional groups of male SS kcnj16-/- rats that underwent repeated seizures, we quantified microglial (IBA-1+) cell counts and morphology, specifically within the preBötC/NA region, and showed increased microglial cell counts, area, and volume consistent with microglial activation. To further test the role of inflammation in physiological responses to seizures and seizure-related mortality, additional groups of SS kcnj16-/- rats were treated with anakinra (IL-1R antagonist), ketoprofen (non-selective COX inhibitor), or saline for 3 days before and up to 10 days of seizures (1/day), and breathing was measured before, during, and after each seizure. Remarkably, IL-1R antagonism mitigated changes in post-ictal ventilatory suppression on days 7-10 but failed to prevent seizure-related mortality, whereas ketoprofen treatment exacerbated post-ictal ventilatory suppression compared to other treatment groups but prevented seizure-related mortality. These data demonstrate neuroinflammation and microglial activation within the key brainstem region of respiratory control following repeated seizures, which may functionally but differentially contribute to the pathophysiological consequences of repeated seizures.

Influence of chronic hypoxia on the hypoxic ventilatory response of juvenile and adult rats

Chronic hypoxia (CH) from birth attenuates the acute hypoxic ventilatory response (HVR) in rats and other mammals, but CH is often reported to augment the HVR in adult mammals. To test the hypothesis that this transition - from blunting to augmenting the HVR - occurs in the third or fourth postnatal week in rats, juvenile and adult rats were exposed to normobaric CH (12% O2) for 7 days and the HVR was assessed by whole-body plethysmography. No transition was observed, however, and the acute HVR was reduced by 61 - 85% across all ages studied. The failure to observe an augmented HVR in adult rats could not be explained by the substrain of Sprague Dawley rats used, the duration of the CH exposure, the order in which test gases were presented, the level of hypoxia used for CH and to assess the HVR, or the effects of CH on the metabolic response to hypoxia and the hypercapnic ventilatory response. A literature survey revealed several distinct patterns of ventilatory acclimatization to hypoxia (VAH) in adult rats, with most studies (77%) revealing a decrease or no change in the acute HVR after CH. In conclusion, the effects of CH on respiratory control are qualitatively similar across age groups, at least within the populations of Sprague Dawley rats used in the present study, and there does not appear to be one "typical" pattern for VAH in adult rats.

Control of Breathing

Substantial advances have been made recently into the discovery of fundamental mechanisms underlying the neural control of breathing and even some inroads into translating these findings to treating breathing disorders. Here, we review several of these advances, starting with an appreciation of the importance of V̇A:V̇CO2:PaCO2 relationships, then summarizing our current understanding of the mechanisms and neural pathways for central rhythm generation, chemoreception, exercise hyperpnea, plasticity, and sleep-state effects on ventilatory control. We apply these fundamental principles to consider the pathophysiology of ventilatory control attending hypersensitized chemoreception in select cardiorespiratory diseases, the pathogenesis of sleep-disordered breathing, and the exertional hyperventilation and dyspnea associated with aging and chronic diseases. These examples underscore the critical importance that many ventilatory control issues play in disease pathogenesis, diagnosis, and treatment.

Correlation between convection requirement and carotid body responses to hypoxia and hemoglobin affinity: comparison between two rat strains

We tested the hypothesis that differences in ventilatory ([Formula: see text]) or convection requirement ([Formula: see text]/[Formula: see text]O₂₎ response to hypoxia would be correlated with differences in hemoglobin (Hb) oxygen affinity between two strains of rats, as they have been shown to be among several species of mammals, birds and reptiles. Brown Norway (BN) rats reduce metabolism more than they increase ventilation in response to hypoxia and both the ventilatory and convection requirement responses to hypoxia are lower in the BN than the Sprague-Dawley (SD) rat. The lower threshold of the ventilation/convection requirement responses of the BN to hypoxia are associated with a higher affinity Hb than the SD rats, (P₅₀ values of 32.4 (± 0.6) versus 34.4 (± 0.5), respectively (P < 0.05), and P₇₅ values of 46.1 (± 0.5) for BN versus 50.7 (± 0.8) for SD (P < 0.001). This significant difference, particularly near the inflection point of the dissociation curve, supported our hypothesis. A reduced sensitivity of BN compared to SD carotid bodies was found. BN carotid bodies (from 36 20-60-day-olds) had a mean estimated volume of 26.64 ± 1.47 × 10⁶ μm³, significantly (P < 0.0001) smaller than SD carotid bodies (from 46 16-40-day-olds) at 50.66 ± 3.41 × 10⁶ μm³. Both genetic and epigenetic/developmental mechanisms may account for the observed inter-strain differences.

Carotid sinus nerve transection abolishes the facilitation of breathing that occurs upon cessation of a hypercapnic gas challenge in male mice

Arterial pCO₂ elevations increase minute ventilation via activation of chemosensors within the carotid body (CB) and brainstem. Although the roles of CB chemoafferents in the hypercapnic (HC) ventilatory response have been investigated, there are no studies reporting the role of these chemoafferents in the ventilatory responses to a HC challenge or the responses that occur upon return to room air, in freely moving mice. This study found that an HC challenge (5% CO₂, 21% O₂, 74% N₂ for 15 min) elicited an array of responses, including increases in frequency of breathing (accompanied by decreases in inspiratory and expiratory times), and increases in tidal volume, minute ventilation, peak inspiratory and expiratory flows, and inspiratory and expiratory drives in sham-operated (SHAM) adult male C57BL6 mice, and that return to room air elicited a brief excitatory phase followed by gradual recovery of all parameters toward baseline values over a 15-min period. The array of ventilatory responses to the HC challenge in mice with bilateral carotid sinus nerve transection (CSNX) performed 7 days previously occurred more slowly but reached similar maxima as SHAM mice. A major finding was responses upon return to room air were dramatically lower in CSNX mice than SHAM mice, and the parameters returned to baseline values within 1-2 min in CSNX mice, whereas it took much longer in SHAM mice. These findings are the first evidence that CB chemoafferents play a key role in initiating the ventilatory responses to HC challenge in C57BL6 mice and are essential for the expression of post-HC ventilatory responses.NEW & NOTEWORTHY This study presents the first evidence that carotid body chemoafferents play a key role in initiating the ventilatory responses, such as increases in frequency of breathing, tidal volume, and minute ventilation that occur in response to a hypercapnic gas challenge in freely moving C57BL6 mice. Our study also demonstrates for the first time that these chemoafferents are essential for the expression of the ventilatory responses that occur upon return to room air in these mice.

Mortality and ventilatory effects of central serotonin deficiency during postnatal development depend on age but not sex

Serotonin (5-HT) influences brain development and has predominantly excitatory neuromodulatory effects on the neural respiratory control circuitry. Infants that succumb to sudden infant death syndrome (SIDS) have reduced brainstem 5-HT levels and Tryptophan hydroxylase 2 (Tph2). Furthermore, there are age- and sex-dependent risk factors associated with SIDS. Here we utilized our established Dark Agouti transgenic rat lacking central serotonin KO to test the hypotheses that CNS 5-HT deficiency leads to: (1) high mortality in a sex-independent manner, (2) age-dependent alterations in other CNS aminergic systems, and (3) age-dependent impairment of chemoreflexes during post-natal development. KO rat pups showed high neonatal mortality but not in a sex-dependent manner and did not show altered hypoxic or hypercapnic ventilatory chemoreflexes. However, KO rat pups had increased apnea-related metrics during a specific developmental age (P12-16), which were preceded by transient increases in dopaminergic system activity (P7-8). These results support and extend the concept that 5-HT per se is a critical factor in supporting respiratory control during post-natal development.

Carotid Body and Metabolic Syndrome: Mechanisms and Potential Therapeutic Targets

The carotid body (CB) is responsible for the peripheral chemoreflex by sensing blood gases and pH. The CB also appears to act as a peripheral sensor of metabolites and hormones, regulating the metabolism. CB malfunction induces aberrant chemosensory responses that culminate in the tonic overactivation of the sympathetic nervous system. The sympatho-excitation evoked by CB may contribute to the pathogenesis of metabolic syndrome, inducing systemic hypertension, insulin resistance and sleep-disordered breathing. Several molecular pathways are involved in the modulation of CB activity, and their pharmacological manipulation may lead to overall benefits for cardiometabolic diseases. In this review, we will discuss the role of the CB in the regulation of metabolism and in the pathogenesis of the metabolic dysfunction induced by CB overactivity. We will also explore the potential pharmacological targets in the CB for the treatment of metabolic syndrome.

Neurokinin-1 receptor activation is sufficient to restore the hypercapnic ventilatory response in the Substance P-deficient naked mole-rat

Naked mole-rats (NMRs) live in large colonies within densely populated underground burrows. Their collective respiration generates significant metabolic carbon dioxide (CO₂) that diffuses slowly out of the burrow network, creating a hypercapnic environment. Currently, the physiological mechanisms that underlie the ability of NMRs to tolerate environmental hypercapnia are largely unknown. To address this, we used whole-body plethysmography and respirometry to elucidate the hypercapnic ventilatory and metabolic responses of awake, freely behaving NMRs to 0%-10% CO₂. We found that NMRs have a blunted hypercapnic ventilatory response (HCVR): ventilation increased only in 10% CO₂. Conversely, metabolism was unaffected by hypercapnia. NMRs are insensitive to cutaneous acid-based pain caused by modified substance P (SP)-mediated peripheral neurotransmission, and SP is also an important neuromodulator of ventilation. Therefore, we re-evaluated physiological responses to hypercapnia in NMRs after an intraperitoneal injection of exogenous substance P (2 mg/kg) or a long-lived isoform of substance P {[pGlu5-MePhe8-MeGly9]SP(5-11), DiMe-C7; 40-400 μg/kg}. We found that both drugs restored hypercapnia sensitivity and unmasked an HCVR in animals breathing 2%-10% CO₂. Taken together, our findings indicate that NMRs are remarkably tolerant of hypercapnic environments and have a blunted HCVR; however, the signaling network architecture required for a "normal" HCVR is retained but endogenously inactive. This muting of chemosensitivity likely suits the ecophysiology of this species, which presumably experiences hypercapnia regularly in their underground niche.

Genetic mutation of Kcnj16 identifies Kir5.1-containing channels as key regulators of acute and chronic pH homeostasis

Acute and chronic homeostatic pH regulation is critical for the maintenance of optimal cellular function. Renal mechanisms dominate global pH regulation over longer time frames, and rapid adjustments in ventilation compensate for acute pH and CO2 changes. Ventilatory CO2 and pH chemoreflexes are primarily determined by brain chemoreceptors with intrinsic pH sensitivity likely driven by K+ channels. Here, we studied acute and chronic pH regulation in Kcnj16 mutant Dahl salt-sensitive (SS Kcnj16-/-) rats; Kcnj16 encodes the pH-sensitive inwardly rectifying K+ 5.1 (Kir5.1) channel. SS Kcnj16-/- rats hyperventilated at rest, likely compensating for a chronic metabolic acidosis. Despite their resting hyperventilation, SS Kcnj16-/- rats showed up to 45% reduction in the ventilatory response to graded hypercapnic acidosis vs. controls. SS Kcnj16-/- rats chronically treated with bicarbonate or the carbonic anhydrase inhibitor hydrochlorothiazide had partial restoration of arterial pH, but there was a further reduction in the ventilatory response to hypercapnic acidosis. SS Kcnj16-/- rats also had a nearly absent hypoxic ventilatory response, suggesting major contributions of Kir5.1 to O2- and CO2-dependent chemoreflexes. Although previous studies demonstrated beneficial effects of a high-K+ diet (HKD) on cardiorenal phenotypes in SS Kcnj16-/- rats, HKD failed to restore the observed ventilatory phenotypes. We conclude that Kir5.1 is a key regulator of renal H+ handling and essential for acute and chronic regulation of arterial pH as determinants of the ventilatory CO2 chemoreflex.-Puissant, M. M., Muere, C., Levchenko, V., Manis, A. D., Martino, P., Forster, H. V., Palygin, O., Staruschenko, A., Hodges, M. R. Genetic mutation of Kcnj16 identifies Kir5.1-containing channels as key regulators of acute and chronic pH homeostasis.

Acute and chronic changes in the control of breathing in a rat model of bronchopulmonary dysplasia

Infants born very prematurely (<28 wk gestation) have immature lungs and often require supplemental oxygen. However, long-term hyperoxia exposure can arrest lung development, leading to bronchopulmonary dysplasia (BPD), which increases acute and long-term respiratory morbidity and mortality. The neural mechanisms controlling breathing are highly plastic during development. Whether the ventilatory control system adapts to pulmonary disease associated with hyperoxia exposure in infancy remains unclear. Here, we assessed potential age-dependent adaptations in the control of breathing in an established rat model of BPD associated with hyperoxia. Hyperoxia exposure ( FI O 2 ; 0.9 from 0 to 10 days of life) led to a BPD-like lung phenotype, including sustained reductions in alveolar surface area and counts, and modest increases in airway resistance. Hyperoxia exposure also led to chronic increases in room air and acute hypoxic minute ventilation (V̇e) and age-dependent changes in breath-to-breath variability. Hyperoxia-exposed rats had normal oxygen saturation ( S p O 2 ) in room air but greater reductions in S p O 2 during acute hypoxia (12% O2) that were likely due to lung injury. Moreover, acute ventilatory sensitivity was reduced at P12 to P14. Perinatal hyperoxia led to greater glial fibrillary acidic protein expression and an increase in neuron counts within six of eight or one of eight key brainstem regions, respectively, controlling breathing, suggesting astrocytic expansion. In conclusion, perinatal hyperoxia in rats induced a BPD-like phenotype and age-dependent adaptations in V̇e that may be mediated through changes to the neural architecture of the ventilatory control system. Our results suggest chronically altered ventilatory control in BPD.

Recent advances in understanding the physiology of hypoxic sensing by the carotid body

Hypoxia resulting from reduced oxygen (O ₂) levels in the arterial blood is sensed by the carotid body (CB) and triggers reflex stimulation of breathing and blood pressure to maintain homeostasis. Studies in the past five years provided novel insights into the roles of heme oxygenase-2 (HO-2), a carbon monoxide (CO)-producing enzyme, and NADH dehydrogenase Fe-S protein 2, a subunit of the mitochondrial complex I, in hypoxic sensing by the CB. HO-2 is expressed in type I cells, the primary O2-sensing cells of the CB, and binds to O ₂ with low affinity. O ₂-dependent CO production from HO-2 mediates hypoxic response of the CB by regulating H ₂S generation. Mice lacking NDUFS2 show that complex I-generated reactive oxygen species acting on K ⁺ channels confer type I cell response to hypoxia. Whether these signaling pathways operate synergistically or independently remains to be studied.

Systemic Oxygen Transport with Rest, Exercise, and Hypoxia: A Comparison of Humans, Rats, and Mice

The objective of this article is to compare and contrast the known characteristics of the systemic O₂ transport of humans, rats, and mice at rest and during exercise in normoxia and hypoxia. This analysis should help understand when rodent O₂ transport findings can-and cannot-be applied to human responses to similar conditions. The O₂ -transport system was analyzed as composed of four linked conductances: ventilation, alveolo-capillary diffusion, circulatory convection, and tissue capillary-cell diffusion. While the mechanisms of O₂ transport are similar in the three species, the quantitative differences are naturally large. There are abundant data on total O₂ consumption and on ventilatory and pulmonary diffusive conductances under resting conditions in the three species; however, there is much less available information on pulmonary gas exchange, circulatory O₂ convection, and tissue O₂ diffusion in mice. The scarcity of data largely derives from the difficulty of obtaining blood samples in these small animals and highlights the need for additional research in this area. In spite of the large quantitative differences in absolute and mass-specific O₂ flux, available evidence indicates that resting alveolar and arterial and venous blood PO₂ values under normoxia are similar in the three species. Additionally, at least in rats, alveolar and arterial blood PO₂ under hypoxia and exercise remain closer to the resting values than those observed in humans. This is achieved by a greater ventilatory response, coupled with a closer value of arterial to alveolar PO₂ , suggesting a greater efficacy of gas exchange in the rats. © 2018 American Physiological Society. Compr Physiol 8:1537-1573, 2018.

How important is the CO2 chemoreflex for the control of breathing? Environmental and evolutionary considerations

Haldane and Priestley (1905) discovered that the ventilatory control system is highly sensitive to CO₂. This "CO₂ chemoreflex" has been interpreted to dominate control of resting arterial PCO₂/pH (P_aCO₂/pH_a) by monitoring P_aCO₂/pH_a and altering ventilation through negative feedback. However, P_aCO₂/pH_a varies little in mammals as ventilation tightly couples to metabolic demands, which may minimize chemoreflex control of P_aCO₂. The purpose of this synthesis is to (1) interpret data from experimental models with meager CO₂ chemoreflexes to infer their role in ventilatory control of steady-state P_aCO₂, and (2) identify physiological causes of respiratory acidosis occurring normally across vertebrate classes. Interestingly, multiple rodent and amphibian models with minimal/absent CO₂ chemoreflexes exhibit normal ventilation, gas exchange, and P_aCO₂/pH_a. The chemoreflex, therefore, plays at most a minor role in ventilatory control at rest; however, the chemoreflex may be critical for recovering P_aCO₂ following acute respiratory acidosis induced by breath-holding and activity in many ectothermic vertebrates. An apparently small role for CO₂ feedback in the genesis of normal breathing contradicts the prevailing view that central CO₂/pH chemoreceptors increased in importance throughout vertebrate evolution. Since the CO₂ chemoreflex contributes minimally to resting ventilation, these CO₂ chemoreceptors may have instead decreased importance throughout tetrapod evolution, particularly with the onset and refinement of neural innovations that improved the matching of ventilation to tissue metabolic demands. This distinct and elusive "metabolic ventilatory drive" likely underlies steady-state P_aCO₂ in air-breathers. Uncovering the mechanisms and evolution of the metabolic ventilatory drive presents a challenge to clinically-oriented and comparative respiratory physiologists alike.

Influence of estrous cycle hormonal fluctuations and gonadal hormones on the ventilatory response to hypoxia in female rats

Sex hormones may influence many physiological processes. Recently, we demonstrated that hormonal fluctuations of cycling female rats do not affect respiratory parameters during hypercapnia. However, it is still unclear whether sex hormones and hormonal fluctuations that occur during the estrous cycle can affect breathing during a hypoxic challenge. Our study aimed to evaluate respiratory, metabolic, and thermal responses to hypoxia in female rats on different days of the estrous cycle (proestrus, estrus, metestrus, and diestrus) and in ovariectomized rats that received replacement with oil (OVX), estradiol (OVX + E₂), or a combination of estradiol and progesterone (OVX + E₂P). Ventilation (V _E), tidal volume (V _T), respiratory frequency (fR), oxygen consumption (VO₂), and V _E/VO₂ were not different during the estrous cycle in normoxia or hypoxia. Body temperature (Tb) was higher during estrus, but decreased similarly in all groups during hypoxia. Compared with intact females in estrus, gonadectomized rats also had lower Tb in normoxia, but not in hypoxia. OVX rats experienced a significant drop in the ventilatory response to hypoxia, but hormonal replacement did not restore values to the levels of an intact animal. Our data demonstrate that the different phases of the estrous cycle do not alter ventilation during normoxia and hypoxia, but OVX animals display lower ventilatory responses to hypoxia compared with ovary-intact rats. Because estradiol and progesterone replacement did not cause significant differences in ventilation, our findings suggest that a yet-to-be-defined non-steroidal ovarian hormone is likely to stimulate the ventilatory responses to hypoxia in females.

Analysis of Hypoxic and Hypercapnic Ventilatory Response in Healthy Volunteers

Introduction

A previous study has suggested that the Human Leukocyte Antigen (HLA) allele DQB1*06:02 affects hypoxic ventilatory response (HVR) but not hypercapnic ventilatory response (HCVR) in an Asian population. The current study evaluated the relationship in Caucasians and Asians. In addition we assessed whether gender or polymorphisms in genes participating in the control of breathing affect HVR and HCVR.

Methods

A re-breathing system was used to measure HVR and HCVR in 551 young adults (56.8% Caucasians, 30% Asians). HLA-DQB1*06:02 and tagged polymorphisms and coding variants in genes participating in breathing (PHOX2B, GPR4 and TASK2/KCNK5) were analyzed. The associations between HVR/HCVR and HLA-DQB1*06:02, genetic polymorphisms, and gender were evaluated using ANOVA or frequentist association testing with SNPTEST.

Results

HVR and gender are strongly correlated. HCVR and gender are not. Mean HVR in women was 0.276±0.168 (liter/minute/%SpO2) compared to 0.429±0.266 (liter/minute/%SpO2) in men, p<0.001 (55.4% higher HVR in men). Women had lower baseline minute ventilation (8.08±2.36 l/m vs. 10.00±3.43l/m, p<0.001), higher SpO2 (98.0±1.3% vs. 96.6±1.7%, p<0.001), and lower EtCO2 (4.65±0.68% vs. 4.82±1.02%, p = 0.025). One hundred and two (18.5%) of the participants had HLA-DQB1*06:02. No association was seen between HLA-DQB1*06:02 and HVR or HCVR. Genetic analysis revealed point wise, uncorrected significant associations between two TASK2/KCNK5 variants (rs2815118 and rs150380866) and HCVR.

Conclusions

This is the largest study to date reporting the relationship between gender and HVR/ HCVR and the first study assessing the association between genetic polymorphisms in humans and HVR/HCVR. The data suggest that gender has a large effect on hypoxic breathing response.

Oxygen Sensing and Homeostasis

The discovery of carotid bodies as sensory receptors for detecting arterial blood oxygen levels, and the identification and elucidation of the roles of hypoxia-inducible factors (HIFs) in oxygen homeostasis have propelled the field of oxygen biology. This review highlights the gas-messenger signaling mechanisms associated with oxygen sensing, as well as transcriptional and non-transcriptional mechanisms underlying the maintenance of oxygen homeostasis by HIFs and their relevance to physiology and pathology.

Adrenaline release evokes hyperpnoea and an increase in ventilatory CO2 sensitivity during hypoglycaemia: a role for the carotid body

KEY POINTS

Hypoglycaemia is counteracted by release of hormones and an increase in ventilation and CO2 sensitivity to restore blood glucose levels and prevent a fall in blood pH. The full counter-regulatory response and an appropriate increase in ventilation is dependent on carotid body stimulation. We show that the hypoglycaemia-induced increase in ventilation and CO2 sensitivity is abolished by preventing adrenaline release or blocking its receptors. Physiological levels of adrenaline mimicked the effect of hypoglycaemia on ventilation and CO2 sensitivity. These results suggest that adrenaline, rather than low glucose, is an adequate stimulus for the carotid body-mediated changes in ventilation and CO2 sensitivity during hypoglycaemia to prevent a serious acidosis in poorly controlled diabetes.

ABSTRACT

Hypoglycaemia in vivo induces a counter-regulatory response that involves the release of hormones to restore blood glucose levels. Concomitantly, hypoglycaemia evokes a carotid body-mediated hyperpnoea that maintains arterial CO2 levels and prevents respiratory acidosis in the face of increased metabolism. It is unclear whether the carotid body is directly stimulated by low glucose or by a counter-regulatory hormone such as adrenaline. Minute ventilation was recorded during infusion of insulin-induced hypoglycaemia (8-17 mIU kg(-1)  min(-1) ) in Alfaxan-anaesthetised male Wistar rats. Hypoglycaemia significantly augmented minute ventilation (123 ± 4 to 143 ± 7 ml min(-1) ) and CO2 sensitivity (3.3 ± 0.3 to 4.4 ± 0.4 ml min(-1)  mmHg(-1) ). These effects were abolished by either β-adrenoreceptor blockade with propranolol or adrenalectomy. In this hypermetabolic, hypoglycaemic state, propranolol stimulated a rise in P aC O2, suggestive of a ventilation-metabolism mismatch. Infusion of adrenaline (1 μg kg(-1)  min(-1) ) increased minute ventilation (145 ± 4 to 173 ± 5 ml min(-1) ) without altering P aC O2 or pH and enhanced ventilatory CO2 sensitivity (3.4 ± 0.4 to 5.1 ± 0.8 ml min(-1)  mmHg(-1) ). These effects were attenuated by either resection of the carotid sinus nerve or propranolol. Physiological concentrations of adrenaline increased the CO2 sensitivity of freshly dissociated carotid body type I cells in vitro. These findings suggest that adrenaline release can account for the ventilatory hyperpnoea observed during hypoglycaemia by an augmented carotid body and whole body ventilatory CO2 sensitivity.

Raphe gene expression changes implicate immune-related functions in ventilatory plasticity following carotid body denervation in rats

The regulation of blood gases in mammals requires precise feedback mechanisms including chemoreceptor feedback from the carotid bodies. Carotid body denervation (CBD) leads to immediate hypoventilation (increased PaCO₂) in adult rats, but over a period of days and weeks ventilation normalizes due in part to central (brain) mechanisms. Here, we tested the hypothesis that functional ventilatory recovery following CBD correlated with significant shifts in medullary raphe gene expression of molecules/pathways associated with known or novel forms of neuroplasticity. Tissue punches were obtained from snap frozen brainstems collected from rats 1-2days or 14-15days post-sham or post-bilateral CBD surgery (verified by physiologic measurements), and subjected to mRNA sequencing to identify, quantify, and statistically compare gene expression level differences among these groups of rats. We found the greatest number of gene expression changes acutely after CBD (154 genes), with fewer changes in the weeks after CBD (69-80 genes) and the fewest changes in expression among the time control groups (39 genes). Little or no changes were observed for multiple genes associated with serotonin- or glutamate receptor-dependent forms of neuroplasticity. However, an unbiased assessment of gene expression changes using a bioinformatics pathway analysis highlighted multiple changes in gene expression in signaling pathways associated with immune function. These included several growth factors and cytokines associated with peripheral and innate immune systems. Thus, these medullary raphe gene expression data support a role for immune-related signaling pathways in the functional restoration of blood gas control after CBD, but little or no role for serotonin- or glutamate receptor-mediated plasticity.

Is plasticity within the retrotrapezoid nucleus responsible for the recovery of the PCO2 set-point after carotid body denervation in rats?

KEY POINTS

Arterial PCO2 is kept constant via breathing adjustments elicited, at least partly, by central chemoreceptors (CCRs) and the carotid bodies (CBs). The CBs may be active in a normal oxygen environment because their removal reduces breathing. Thereafter, breathing slowly returns to normal. In the present study, we investigated whether an increase in the activity of CCRs accounts for this return. One week after CB excision, the hypoxic ventilatory reflex was greatly reduced as expected, whereas ventilation and blood gases at rest under normoxia were normal. Optogenetic inhibition of Phox2b-expressing neurons including the retrotrapezoid nucleus, a cluster of CCRs, reduced breathing proportionally to arterial pH. The hypopnoea was greater after CB excision but only in a normal or hypoxic environment. The difference could be simply explained by the loss of fast feedback from the CBs. We conclude that, in rats, CB denervation may not produce CCR plasticity. We also question whether the transient hypoventilation elicited by CB denervation means that these afferents are active under normoxia.

ABSTRACT

Carotid body denervation (CBD) causes hypoventilation and increases the arterial PCO2 set-point; these effects eventually subside. The hypoventilation is attributed to reduced CB afferent activity and the PCO2 set-point recovery to CNS plasticity. In the present study, we investigated whether the retrotrapezoid nucleus (RTN), a group of non-catecholaminergic Phox2b-expressing central respiratory chemoreceptors (CCRs), is the site of such plasticity. We evaluated the contribution of the RTN to breathing frequency (FR ), tidal volume (VT ) and minute volume (VE ) by inhibiting this nucleus optogenetically for 10 s (archaerhodopsinT3.0) in unanaesthetized rats breathing various levels of O2 and/or CO2 . The measurements were made in seven rats before and 6-7 days after CBD and were repeated in seven sham-operated rats. Seven days post-CBD, blood gases and ventilation in 21% O2 were normal, whereas the hypoxic ventilatory reflex was still depressed (95.3%) and hypoxia no longer evoked sighs. Sham surgery had no effect. In normoxia or hypoxia, RTN inhibition produced a more sustained hypopnoea post-CBD than before; in hyperoxia, the responses were identical. Post-CBD, RTN inhibition reduced FR and VE in proportion to arterial pH or PCO2 (ΔVE : 3.3 ± 1.5% resting VE /0.01 pHa). In these rats, 20.7 ± 8.9% of RTN neurons expressed archaerhodopsinT3.0. Hypercapnia (3-6% FiCO2 ) increased FR and VT in CBD rats (n = 4). In conclusion, RTN regulates FR and VE in a pH-dependent manner after CBD, consistent with its postulated CCR function. RTN inhibition produces a more sustained hypopnoea after CBD than before, although this change may simply result from the loss of the fast feedback action of the CBs.

Chronic intermittent hypoxia alters ventilatory and metabolic responses to acute hypoxia in rats

We determined the effects of chronic exposure to intermittent hypoxia (CIH) on chemoreflex control of ventilation in conscious animals. Adult male Sprague-Dawley rats were exposed to CIH [nadir oxygen saturation (SpO2), 75%; 15 events/h; 10 h/day] or normoxia (NORM) for 21 days. We assessed the following responses to acute, graded hypoxia before and after exposures: ventilation (V̇e, via barometric plethysmography), V̇o2 and V̇co2 (analysis of expired air), heart rate (HR), and SpO2 (pulse oximetry via neck collar). We quantified hypoxia-induced chemoreceptor sensitivity by calculating the stimulus-response relationship between SpO2 and the ventilatory equivalent for V̇co2 (linear regression). An additional aim was to determine whether CIH causes proliferation of carotid body glomus cells (using bromodeoxyuridine). CIH exposure increased the slope of the V̇e/V̇co2/SpO2 relationship and caused hyperventilation in normoxia. Bromodeoxyuridine staining was comparable in CIH and NORM. Thus our CIH paradigm augmented hypoxic chemosensitivity without causing glomus cell proliferation.

Chronic central serotonin depletion attenuates ventilation and body temperature in young but not adult Tph2 knockout rats

Genetic deletion of brain serotonin (5-HT) neurons in mice leads to ventilatory deficits and increased neonatal mortality during development. However, it is unclear if the loss of the 5-HT neurons or the loss of the neurochemical 5-HT led to the observed physiologic deficits. Herein, we generated a mutant rat model with constitutive central nervous system (CNS) 5-HT depletion by mutation of the tryptophan hydroxylase 2 (Tph2) gene in dark agouti (DA(Tph2-/-)) rats. DA(Tph2-/-) rats lacked TPH immunoreactivity and brain 5-HT but retain dopa decarboxylase-expressing raphe neurons. Mutant rats were also smaller, had relatively high mortality (∼50%), and compared with controls had reduced room air ventilation and body temperatures at specific postnatal ages. In adult rats, breathing at rest and hypoxic and hypercapnic chemoreflexes were unaltered in adult male and female DA(Tph2-/-) rats. Body temperature was also maintained in adult DA(Tph2-/-) rats exposed to 4°C, indicating unaltered ventilatory and/or thermoregulatory control mechanisms. Finally, DA(Tph2-/-) rats treated with the 5-HT precursor 5-hydroxytryptophan (5-HTP) partially restored CNS 5-HT and showed increased ventilation (P < 0.05) at a developmental age when it was otherwise attenuated in the mutants. We conclude that constitutive CNS production of 5-HT is critically important to fundamental homeostatic control systems for breathing and temperature during postnatal development in the rat.

Fluoxetine prevents respiratory arrest without enhancing ventilation in DBA/1 mice

Sudden unexpected death in epilepsy (SUDEP) is a fatal epileptic event. DBA/1 mice are a relevant animal model for the study of SUDEP, as these mice exhibit seizure-induced respiratory arrest (S-IRA) leading to death, which has been observed in patients with witnessed SUDEP. Fluoxetine, a selective serotonin (5-hydroxytryptamine or 5-HT) reuptake inhibitor (SSRI), reduces S-IRA in DBA/1 mice. Given that DBA/1 mice with S-IRA can be resuscitated using a ventilator, we hypothesized that breathing stimulants can prevent S-IRA and that fluoxetine prevents S-IRA by enhancing ventilation in these mice. Spontaneous respiratory function in anesthetized or awake DBA/1 mice was examined using noninvasive plethysmography before and after administering fluoxetine or breathing stimulants, doxapram, and 5,6,7,8-tetrahydropyrido[4,3-d]pyrimidine (PK-THPP). The effects of these drugs on S-IRA in DBA/1 mice were tested. As reported previously, systemic administration of fluoxetine reduced S-IRA in awake DBA/1 mice, but fluoxetine in anesthetized and awake DBA/1 mice did not increase basal ventilation or the ventilatory response to 7% CO2. Both doxapram and PK-THPP increased ventilation in room air and in air+7% CO2 in anesthetized DBA/1 mice. However, neither of the breathing stimulants reduced the incidence of S-IRA. Our studies confirm that fluoxetine reduces S-IRA in DBA/1 mice without enhancing basal ventilation in the absence of seizures. Although breathing stimulants increased ventilation in the absence of seizures, they were ineffective in reducing S-IRA, indicating that drug-induced increases in ventilation are insufficient to compensate for S-IRA in DBA/1 mice.

Tasting arterial blood: what do the carotid chemoreceptors sense?

The carotid bodies are sensory organs that detect the chemical composition of the arterial blood. The carotid body sensory activity increases in response to arterial hypoxemia and the ensuing chemoreflex regulates vital homeostatic functions. Recent studies suggest that the carotid bodies might also sense arterial blood glucose and circulating insulin levels. This review focuses on how the carotid bodies sense O₂, glucose, and insulin and some potential implications of these sensory functions on physiological regulation and in pathophysiological conditions. Emerging evidence suggests that carbon monoxide (CO)-regulated hydrogen sulfide (H₂S), stemming from hypoxia, depolarizes type I cells by inhibiting certain K⁺ channels, facilitates voltage-gated Ca²⁺ influx leading to sensory excitation of the carotid body. Elevated CO and decreased H₂S renders the carotid bodies insensitive to hypoxia resulting in attenuated ventilatory adaptations to high altitude hypoxia, whereas reduced CO and high H₂S result in hypersensitivity of the carotid bodies to hypoxia and hypertension. Acute hypoglycemia augments the carotid body responses to hypoxia but that a prolonged lack of glucose in the carotid bodies can lead to a failure to sense hypoxia. Emerging evidence also indicates that carotid bodies might sense insulin directly independent of its effect on glucose, linking the carotid bodies to the pathophysiological consequences of the metabolic syndrome. How glucose and insulin interact with the CO-H₂S signaling is an area of ongoing study.

Functional and developmental identification of a molecular subtype of brain serotonergic neuron specialized to regulate breathing dynamics

Serotonergic neurons modulate behavioral and physiological responses from aggression and anxiety to breathing and thermoregulation. Disorders involving serotonin (5HT) dysregulation are commensurately heterogeneous and numerous. We hypothesized that this breadth in functionality derives in part from a developmentally determined substructure of distinct subtypes of 5HT neurons each specialized to modulate specific behaviors. By manipulating developmentally defined subgroups one by one chemogenetically, we find that the Egr2-Pet1 subgroup is specialized to drive increased ventilation in response to carbon dioxide elevation and acidosis. Furthermore, this subtype exhibits intrinsic chemosensitivity and modality-specific projections-increasing firing during hypercapnic acidosis and selectively projecting to respiratory chemosensory but not motor centers, respectively. These findings show that serotonergic regulation of the respiratory chemoreflex is mediated by a specialized molecular subtype of 5HT neuron harboring unique physiological, biophysical, and hodological properties specified developmentally and demonstrate that the serotonergic system contains specialized modules contributing to its collective functional breadth.

RNASeq-derived transcriptome comparisons reveal neuromodulatory deficiency in the CO₂ insensitive brown Norway rat

Raphé-derived serotonin (5-HT) and thyrotropin-releasing hormone (TRH) play important roles in fundamental, homeostatic control systems such as breathing and specifically the ventilatory CO2 chemoreflex. Brown Norway (BN) rats exhibit an inherent and severe ventilatory insensitivity to hypercapnia but also exhibit relatively normal ventilation at rest and during other conditions, similar to multiple genetic models of 5-HT system dysfunction in mice. Herein, we tested the hypothesis that the ventilatory insensitivity to hypercapnia in BN rats is due to altered raphé gene expression and the consequent deficiencies in raphé-derived neuromodulators such as TRH. Medullary raphé transcriptome comparisons revealed lower expression of multiple 5-HT neuron-specific genes in BN compared to control Dahl salt-sensitive rats, predictive of reduced central nervous system monoamines by bioinformatics analyses and confirmed by high-performance liquid chromatography measurements. In particular, raphé Trh mRNA and peptide levels were significantly reduced in BN rats, and injections of the stable TRH analogue Taltirelin (TAL) stimulated breathing dose-dependently, with greater effects in BN versus control Sprague-Dawley rats. Importantly, TAL also effectively normalized the ventilatory CO2 chemoreflex in BN rats, but TAL did not affect CO2 sensitivity in control Sprague-Dawley rats. These data establish a molecular basis of the neuromodulatory deficiency in BN rats, and further suggest an important functional role for TRH signalling in the mammalian CO2 chemoreflex.

Strain differences in pH-sensitive K+ channel-expressing cells in chemosensory and nonchemosensory brain stem nuclei

The ventilatory CO2 chemoreflex is inherently low in inbred Brown Norway (BN) rats compared with other strains, including inbred Dahl salt-sensitive (SS) rats. Since the brain stem expression of various pH-sensitive ion channels may be determinants of the CO2 chemoreflex, we tested the hypothesis that there would be fewer pH-sensitive K(+) channel-expressing cells in BN relative to SS rats within brain stem sites associated with respiratory chemoreception, such as the nucleus tractus solitarius (NTS), but not within the pre-Bötzinger complex region, nucleus ambiguus or the hypoglossal motor nucleus. Medullary sections (25 μm) from adult male and female BN and SS rats were stained with primary antibodies targeting TASK-1, Kv1.4, or Kir2.3 K(+) channels, and the total (Nissl-stained) and K(+) channel immunoreactive (-ir) cells counted. For both male and female rats, the numbers of K(+) channel-ir cells within the NTS were reduced in the BN compared with SS rats (P < 0.05), despite equal numbers of total NTS cells. In contrast, we found few differences in the numbers of K(+) channel-ir cells among the strains within the nucleus ambiguus, hypoglossal motor nucleus, or pre-Bötzinger complex regions in both male and female rats. However, there were no predicted functional mutations in each of the K(+) channels studied comparing genomic sequences among these strains. Thus we conclude that the relatively selective reductions in pH-sensitive K(+) channel-expressing cells in the NTS of male and female BN rats may contribute to their severely blunted ventilatory CO2 chemoreflex.

Inherent variations in CO-H2S-mediated carotid body O2 sensing mediate hypertension and pulmonary edema

Oxygen (O2) sensing by the carotid body and its chemosensory reflex is critical for homeostatic regulation of breathing and blood pressure. Humans and animals exhibit substantial interindividual variation in this chemosensory reflex response, with profound effects on cardiorespiratory functions. However, the underlying mechanisms are not known. Here, we report that inherent variations in carotid body O2 sensing by carbon monoxide (CO)-sensitive hydrogen sulfide (H2S) signaling contribute to reflex variation in three genetically distinct rat strains. Compared with Sprague-Dawley (SD) rats, Brown-Norway (BN) rats exhibit impaired carotid body O2 sensing and develop pulmonary edema as a consequence of poor ventilatory adaptation to hypobaric hypoxia. Spontaneous Hypertensive (SH) rat carotid bodies display inherent hypersensitivity to hypoxia and develop hypertension. BN rat carotid bodies have naturally higher CO and lower H2S levels than SD rat, whereas SH carotid bodies have reduced CO and greater H2S generation. Higher CO levels in BN rats were associated with higher substrate affinity of the enzyme heme oxygenase 2, whereas SH rats present lower substrate affinity and, thus, reduced CO generation. Reducing CO levels in BN rat carotid bodies increased H2S generation, restoring O2 sensing and preventing hypoxia-induced pulmonary edema. Increasing CO levels in SH carotid bodies reduced H2S generation, preventing hypersensitivity to hypoxia and controlling hypertension in SH rats.

Physiological and pathological angiogenesis in the adult pulmonary circulation

Angiogenesis occurs during growth and physiological adaptation in many systemic organs, for example, exercise-induced skeletal and cardiac muscle hypertrophy, ovulation, and tissue repair. Disordered angiogenesis contributes to chronic inflammatory disease processes and to tumor growth and metastasis. Although it was previously thought that the adult pulmonary circulation was incapable of supporting new vessel growth, over that past 10 years new data have shown that angiogenesis within this circulation occurs both during physiological adaptive processes and as part of the pathogenic mechanisms of lung diseases. Here we review the expression of vascular growth factors in the adult lung, their essential role in pulmonary vascular homeostasis and the changes in their expression that occur in response to physiological challenges and in disease. We consider the evidence for adaptive neovascularization in the pulmonary circulation in response to alveolar hypoxia and during lung growth following pneumonectomy in the adult lung. In addition, we review the role of disordered angiogenesis in specific lung diseases including idiopathic pulmonary fibrosis, acute adult distress syndrome and both primary and metastatic tumors of the lung. Finally, we examine recent experimental data showing that therapeutic enhancement of pulmonary angiogenesis has the potential to treat lung diseases characterized by vessel loss.

Fluoxetine augments ventilatory CO2 sensitivity in Brown Norway but not Sprague Dawley rats

The Brown Norway (BN; BN/NHsdMcwi) rat exhibits a deficit in ventilatory CO2 sensitivity and a modest serotonin (5-HT) deficiency. Here, we tested the hypothesis that the selective serotonin reuptake inhibitor fluoxetine would augment CO2 sensitivity in BN but not Sprague Dawley (SD) rats. Ventilation during room air or 7% CO2 exposure was measured before, during and after 3 weeks of daily injections of saline or fluoxetine (10mg/(kgday)) in adult male BN and SD rats. Fluoxetine had minimal effects on room air breathing in BN and SD rats (p>0.05), although tidal volume (VT) was reduced in BN rats (p<0.05). There were also minimal effects of fluoxetine on CO2 sensitivity in SD rats, but fluoxetine increased minute ventilation, breathing frequency and VT during hypercapnia in BN rats (p<0.05). The augmented CO2 response was reversible upon withdrawal of fluoxetine. Brain levels of biogenic amines were largely unaffected, but 5-HIAA and the ratio of 5-HIAA/5-HT were reduced (p<0.05) consistent with selective and effective 5-HT reuptake inhibition. Thus, fluoxetine increases ventilatory CO2 sensitivity in BN but not SD rats, further suggesting altered 5-HT system function may contribute to the inherently low CO2 sensitivity in the BN rat.

Erythropoietin and its antagonist regulate hypoxic fictive breathing in newborn mice

Clinical use of erythropoietin in adult and newborn patients has revealed its involvement in neuroprotection, neurogenesis, and angiogenesis. More recently, we showed in adult mouse, that brain erythropoietin interacts with the major brainstem centers associated with respiration to enhance the ventilatory response to acute and chronic conditions of physiological hypoxia (e.g., as occurring at high altitude). However, whether brain erythropoietin is involved in breathing regulation in newborns remains unknown. In this study, en bloc brainstem-spinal cord preparations were obtained from mice at postnatal day 4. After various periods (30, 60, or 90 min) of incubation with 0, 25, or 250 U of erythropoietin, preparations were superfused with artificial cerebrospinal fluid bubbled with normoxic or hypoxic gas mixtures. The electrophysiological fictive breathing produced by axons at the C4 ventral root was next recorded. Our results show that erythropoietin attenuates the hypoxia-mediated decrease of the central respiratory activity and improves post-hypoxic recovery. Additional analysis revealed that the soluble erythropoietin receptor (the endogenous erythropoietin antagonist) dramatically decreases neural hypoxic respiratory activity, confirming the specific erythropoietin effect on respiratory drive. These results imply that erythropoietin exerts main modulation and maintenance of respiratory motor output during hypoxic and post-hypoxic challenges in 4-days old mice.

Acute and chronic effects of carotid body denervation on ventilation and chemoreflexes in three rat strains

Brown Norway (BN) rats have a relatively specific deficit in CO2 sensitivity. This deficit could be due to an abnormally weak carotid body contribution to CO2 sensitivity. Accordingly, we tested the hypothesis that CBD would have less of an effect on eupnoeic breathing and CO2 sensitivity in the BN rats compared to other rat strains.We measured ventilation and blood gases at rest (eupnoea) and during hypoxia (FIO2 =0.12) or hypercapnia (FICO2 =0.07) before and up to 23 days after bilateral or Sham CBD in BN, Sprague–Dawley (SD) and Dahl Salt-Sensitive (SS) rats. In all three rat strains, CBD elicited eupnoeic hypoventilation (PaCO2 +8.7–11.0 mmHg) 1–2 days post-CBD (P <0.05), and attenuated ventilatory responses to hypoxia (P <0.05) and venous sodium cyanide (NaCN; P<0.05), while sham CBD had no effect on resting breathing, blood gases or chemoreflexes (P >0.05). In contrast, CBD had no effect on CO2 sensitivity (˙VE/PaCO2) in all strains (P>0.05). Eupnoeic PaCO2 returned to pre-CBD values within 15–23 days post-CBD. Thus, the effects of CBD in rats (1) further support an important role for the carotid bodies in eupnoeic blood gas regulation, (2) suggest that the carotid bodies are not a major determinant of CO2 sensitivity in rats, and (3) may not support the concept of an interaction among the peripheral and central chemoreceptors in rats.

Ventilatory behavior and carotid body morphology of Brown Norway and Sprague Dawley rats

Differences in acute ventilatory behavior are associated with carotid body (CB) structural and immunohistologic profiles in some, but not all, reports. Brown Norway (BN) rats exhibit lower acute ventilatory responses to hypoxia and hypercapnia compared to Sprague Dawley (SD) rats. We hypothesized that BN rats possess CB with fewer glomus cells. Ventilation was recorded in 6-month-old BN and SD rats exposed to hypoxia-reoxygenation and hypercapnia. Extracted CBs were examined using H&E staining, and immunohistochemistry with antibodies specific for tyrosine hydroxylase (TH), neural nitric oxide synthase (nNOS), and pyruvate dehydrogenase (PD). Sections were analyzed for cell and immunostaining density. SD displayed greater hypoxic and hypercapnic responses, and post-hypoxic short term potentiation, whereas BN exhibited post-hypoxic frequency decline. Contrary to our hypothesis, BN demonstrated a denser arrangement of glomus cells with a larger TH stained area (31.7% BN, 22.6% SD; p<0.0001), and nNOS stained area (37.3% BN, 32.1%; SD; p=0.01). Hence, respiratory phenotype does not correlate intuitively with these anatomic features.

Differences between three inbred rat strains in number of K+ channel-immunoreactive neurons in the medullary raphé nucleus

Ventilatory sensitivity to hypercapnia is greater in Dahl salt-sensitive (SS) rats than in Fawn Hooded hypertensive (FHH) and Brown Norway (BN) inbred rats. Since pH-sensitive potassium ion (K(+)) channels are postulated to contribute to the sensing and signaling of changes in CO(2)-H(+) in chemosensitive neurons, we tested the hypothesis that there are more pH-sensitive K(+) channel-immunoreactive (ir) neurons within the medullary raphé nuclei of the highly chemosensitive SS rats than in the other two strains. Medullary tissues from male and female BN, FHH, and SS rats were stained with cresyl violet or with antibodies targeting TASK-1, K(v)1.4, and Kir2.3 channels. K(+) channel-ir neurons were quantified and compared with the total neurons in the region. The total number of neurons in the medullary raphé 1) was greater in male FHH than the other male rats, 2) did not differ among the female rats, and 3) did not differ between sexes. The average number of K(+) channel-ir neurons per section was 30-60 neurons higher in the male SS than in the other rat strains. In contrast, for the females, the number of K(+) channel-ir neurons was greatest in the BN. We also found significant differences in the number of K(+) channel-ir neurons between sexes in SS (males > females) and BN (females > males) rats, but not the FHH strain. Our findings support the hypothesis for males but not for females, suggesting that both genetic background and sex are determinants of K(+) channel immunoreactivity of medullary raphé neurons, and that the expression of pH-sensitive K(+) channels in the medullary raphé does not correlate with the ventilatory sensitivity to hypercapnia.

A perinatal nitric oxide donor increases renal vascular resistance and ameliorates hypertension and glomerular injury in adult fawn-hooded hypertensive rats

Enhancing perinatal nitric oxide (NO) availability persistently reduces blood pressure in spontaneously hypertensive rats. We hypothesize that this approach can be generalized to other models of genetic hypertension, for instance those associated with renal injury. Perinatal exposure to the NO donor molsidomine was studied in fawn-hooded hypertensive (FHH) rats, a model of mild hypertension, impaired preglomerular resistance, and progressive renal injury. Perinatal molsidomine increased urinary NO metabolite excretion at 8 wk of age, i.e., 4 wk after treatment was stopped (P < 0.05). Systolic blood pressure was persistently reduced after molsidomine (42-wk females: 118 +/- 3 vs. 141 +/- 5 and 36-wk males: 139 +/- 4 vs. 158 +/- 4 mmHg; both P < 0.001). Perinatal treatment decreased glomerular filtration rate (P < 0.05) and renal blood flow (P < 0.01) and increased renal vascular resistance (P < 0.05), without affecting filtration fraction, suggesting persistently increased preglomerular resistance. At 4 wk of age natriuresis was transiently increased by molsidomine (P < 0.05). Molsidomine decreased glomerulosclerosis (P < 0.05). Renal blood flow correlated positively with glomerulosclerosis in control (P < 0.001) but not in perinatally treated FHH rats. NO dependency of renal vascular resistance was increased by perinatal molsidomine. Perinatal enhancement of NO availability can ameliorate development of hypertension and renal injury in FHH rats. Paradoxically, glomerular protection by perinatal exposure to the NO donor molsidomine may be due to persistently increased preglomerular resistance. The mechanisms by which increased perinatal NO availability can persistently reprogram kidney function and ameliorate hypertension deserve further study.

Disinhibition of the dorsomedial hypothalamus increases the frequency of augmented breaths in the anesthetized rat

The present study was undertaken to identify if activation of the dorsomedial hypothalamus (DMH) elicits augmented breaths (ABs). DMH disinhibition in urethane anesthetized rats produced both an increase in baseline respiratory rate (RR) and an increase in the number of ABs. The increase in RR was associated with a decrease in both the time of inspiration (T(i)) and expiration (T(e)) and the peak change in RR was observed 5 min post DMH activation. In contrast, the increase in ABs was greatest during the first 1.25 min, and both T(i)s of the ABs did not change significantly from pre-injection values. The T(e) of the ABs did decrease but remained significantly greater than the T(e) of the normal breath during DMH disinhibition. Our results support the hypothesis that the central neural pathway involved in the maintenance of normal respiratory pattern may be distinct from pathways involved in the generation of ABs.

Cerebral angiogenic factors, angiogenesis, and physiological response to chronic hypoxia differ among four commonly used mouse strains

Angiogenesis is a critical element for adaptation to low levels of oxygen and occurs following long-term exposure to mild hypoxia in rats. To test whether a similar response in mice occurs, CD1, 129/Sv, C57Bl/6, and Balb/c mice were exposed to 10% oxygen for up to 3 wk. All mice showed significant increases in the percentage of packed red blood cells, and CD1 and 129/Sv mice showed increased respiration frequency and minute volume, common physiological measures of hypoxia. Significant angiogenesis was observed in all strains except Balb/c following 3-wk exposure to chronic hypoxia. CD1 hypoxic mice had the largest increase (88%), followed by C57Bl/6 (48%), 129/Sv (41%), and Balb/c (12%), suggesting that some mice undergo more remodeling than others in response to hypoxia. Protein expression analysis of vascular endothelial growth factor (VEGF), angiopoietin (Ang)-1 and Ang2, and Tie2 were examined to determine whether regulation of different angiogenic proteins could account for the differences observed in hypoxia-induced angiogenesis. CD1 mice showed the strongest upregulation of VEGF, Ang2, Ang1, and Tie2, whereas Balb/c had only subtle increases in VEGF and no change in the other proteins. C57Bl/6 mice showed a regulatory response that fell between the CD1 and Balb/c mice, consistent with the intermediate increase in angiogenesis. Our results suggest that genetic heterogeneity plays a role in angiogenesis and regulation of angiogenic proteins and needs to be accounted for when designing and interpreting experiments using transgenic mice and when studying in vivo models of angiogenesis.

MK-801 alters diaphragmatic activities in unanesthetized rats differently between normoxia and hypoxia

This study was designed to examine how systemic administration of an N-methyl-d-aspartate (NMDA) receptor antagonist, MK-801, altered respiratory timing in unanesthetized rats under normoxia and hypoxia. To detect fine changes in inspiratory time (TI) and expiratory time (TE), and cycle duration (TTOT), we prepared a diaphragmatic electromyogram (EMGdia). Diaphragm electrodes and arterial and venous catheters were inserted into Wistar rats (n = 8) under pentobarbital anesthesia. The next day, EMGdia was recorded before and after intravenous administration of MK-801 (3 mg/kg) under normoxia and hypoxia (12% O2) without anesthesia, and the respiratory timing (TI, TE, TTOT), respiratory frequency (fR), and amplitude of the integrated EMGdia were measured. Arterial blood gases (ABGs), mean arterial pressure (MAP), and heart rate (fH) were also measured with the EMGdia. Under normoxia, MK-801 increased fR owing to a significant decrease in TE, and elevated both MAP and fH. Under hypoxia, MK-801 suppressed an increase in fR owing to a significant increase in TI, and did not accelerate fH. In both gaseous conditions, on ABGs, MK-801 did not alter partial pressure of O2 (PaO2) or CO2 (PaCO2), and slightly decreased pH (but not less than 7.4). MK-801 significantly decreased hypoxic response (%change from normoxia) in fR, and increased that in EMGdia amplitude, and did not alter a total ventilatory index (fRxEMGdia amplitude). The results suggest that an NMDA receptor-mediated mechanism partially determines fR through significant alterations in respiratory timing, particularly in which the hypoxic ventilatory response was obtained in unanesthetized rats.

Domperidone and ventilatory behavior: Sprague–Dawley versus Brown Norway rats

Domperidone, a dopamine D(2) receptor antagonist, is a tool for uncovering the tonic and dynamic effects of the peripheral dopaminergic system in unanesthestized animals. The hypothesis was that domperidone effects would vary between strains of the same species. Ventilatory behavior -- frequency and minute ventilation -- was measured by the plethysmographic method in unrestrained adult male Sprague-Dawley (SD: n=8) and Brown Norway (BN: n=8) rats before, during and after rapid transition to 100% O(2) after 5 min of 13% O(2)/3% CO(2). Tests were done 60 min after intraperitoneal injection of either vehicle (0.1% lactic acid in saline) or a dose of domperidone (0.1, 0.5, 1.0, or 5.0mg/kg) dissolved in vehicle, each on a separate day. Resting frequency and minute ventilation (mean+/-standard deviation) decreased after domperidone in the BN strain (e.g. 94.63/min+/-4.99 versus 87.37/min+/-9.59, p=0.42; 77.3 ml/min+/-9.25 versus 62.13 ml/min+/-11.5, p=0.019, respectively), but did not change in the SD. With increasing doses of domperidone the ventilatory response to hypoxia and reoxygenation became similar owing to a decrease in frequency and minute ventilation in the SD. At a dose altering SD hypoxic responses, the hypercapnic ventilatory response was not significantly affected. In conclusion, breathing frequency and minute ventilation over a challenge with hypoxia and reoxygenation differ with domperidone depending upon genetic background. We speculate that hypoxic ventilatory responses may be differently configured even among strains of the same species.

Postnatal developmental changes in CO2 sensitivity in rats

Ventilatory sensitivity to CO2 in awake adult Brown Norway (BN) rats is 50–75% lower than in adult Sprague-Dawley (SD) and salt-sensitive Dahl S (SS) rats. The purpose of the present study was to test the hypothesis that this difference would be apparent during the development of CO2 sensitivity. Four litters of each strain were divided into four groups such that rats were exposed to 7% inspired CO2 for 5 min in a plethysmograph every third day from postnatal day (P) 0 to P21 and again on P29 and P30. From P0 to P14, CO2 exposure increased pulmonary ventilation (V̇e) by 25–50% in the BN and SD strains and between 25 to over 200% in the SS strain. In all strains beginning around P15, the response to CO2 increased progressively reaching a peak at P19–21 when V̇e during hypercapnia was 175–225% above eucapnia. There were minimal changes in CO2 sensitivity between P21 and P30, and at both ages there were minimal between-strain differences. At P30, the response to CO2 in the SS and SD strains was near the adult response, but the response in the BN rats was 100% greater at P30 than in adults. We conclude that 1) CO2-sensing mechanisms, and/or mechanisms downstream from the chemoreceptors, change dramatically at the age in rats when other physiological systems are also maturing (∼P15), and 2) there is a high degree of age-dependent plasticity in CO2 sensitivity in rats, which differs between strains.

BN phenome: detailed characterization of the cardiovascular, renal, and pulmonary systems of the sequenced rat

The postgenome era has provided resources to link disease phenotypes to the genomic sequence, i.e., creating a disease "phenome." Our detailed characterization of the sequenced BN rat strain (BN/NHsdMcwi) provides the first concerted effort in creating a direct link between a sequenced genome and its resulting biology. For the BN sequence to be of broad value to investigators, these measures need to be put into the context of the spectrum of the laboratory rats, so that their physiology can be benchmarked against the sequenced BN. As a major step in generating a comprehensive cardiovascular and pulmonary disease phenome, we measured 281 traits related to diseases of the heart, lung, and blood (http://pga.mcw.edu) in the sequenced BN. We compared these data with those of the same traits measured across multiple genetic backgrounds, both genders, and differing environments. We show that no single strain, inbred or outbred, can be considered a physiological control strain; what is normal depends on what trait is being measured and the strains' genome backgrounds. We find vast differences between the genders, also dependent on genome background. By combining the values across all strains studied, we generated a "population" mean and normal range of values for each of these traits, which are more genetically representative than the measured values in any single inbred or outbred strain. These data provide a baseline for physiological comparison of traits related to cardiovascular, lung, blood, and renal function in the sequenced BN rats relative to the major strains of rats studied in biomedical research.

Differences in time-dependent hypoxic phrenic responses among inbred rat strains

Hypoxic ventilatory responses differ between rodent strains, suggesting a genetic contribution to interindividual variability. However, hypoxic ventilatory responses consist of multiple time-dependent mechanisms that can be observed in different respiratory motor outputs. We hypothesized that strain differences would exist in discrete time-dependent mechanisms of the hypoxic response and, furthermore, that there may be differences between hypoglossal and phrenic nerve responses to hypoxia. Hypoglossal and phrenic nerve responses were assessed during and after a 5-min hypoxic episode in anesthetized, vagotomized, and ventilated rats from four inbred strains: Brown Norway (BN), Fischer 344 (FS), Lewis (LW), and Piebald-viral-Glaxo (PVG). During baseline, burst frequency was higher in PVG than LW rats ( P < 0.05), phrenic burst amplitude was higher in PVG vs. other strains ( P < 0.05), and hypoglossal burst amplitude was higher in PVG and BN vs. FS and LW ( P < 0.05). During hypoxia, burst frequency did not change in BN or LW rats, but it increased in PVG and FS rats. The phrenic amplitude response was smallest in PVG vs. other strains ( P < 0.05), and the hypoglossal response was similar among strains. Short-term potentiation posthypoxia was slowest in FS and fastest in LW rats ( P < 0.05). Posthypoxia frequency decline was absent in PVG, but it was observed in all other strains. Augmented breaths were observed during hypoxia in FS rats only. Thus genetic differences exist in the time domains of the hypoxic response, and these are differentially expressed in hypoglossal and phrenic nerves. Furthermore, genetic diversity observed in hypoxic ventilatory responses in unanesthetized rats may arise from multiple neural mechanisms.

Genetic determinants on rat chromosome 6 modulate variation in the hypercapnic ventilatory response using consomic strains

To understand the genetic basis of pathways involved in the control of breathing, a large scale, high-throughput study using chromosomal substitution strains of rats is underway. Eight new consomic rat stains (SS-2(BN), SS-4(BN), SS-6(BN), SS-7(BN), SS-8(BN), SS-11(BN), SS-12(BN), SS-14(BN), SS-Y(BN)), containing one homozygous BN/NHsdMcwi (BN) chromosome on a background of SS/JrHsdMcwi (SS), were created by PhysGen (http://pga.mcw.edu) Program for Genomic Applications. Male and female rats were studied using standard plethysmography under control conditions and during acute hypoxia (inspired oxygen fraction = 0.12) and hypercapnia (inspired CO(2) fraction = 0.07). The rats were also studied during treadmill exercise. Both male and female BN rats had a significantly lower ventilatory response during 7% CO(2) compared with SS rats of the same gender. SS-6(BN) female rats had a significantly reduced ventilatory response, similar to BN rats due primarily to a reduced tidal volume. Male SS-6(BN) rats had a significantly reduced tidal volume response to hypercapnia but a slightly increased frequency response during hypercapnia. Gene(s) on the Y chromosome may play a role in this increased frequency response in the male rats because the SS-Y(BN) hypercapnic ventilatory response involves a significantly increased frequency response. Several chromosomal substitutions slightly altered the ventilatory responses to hypoxia and exercise. However, genes on chromosomes 6 and Y of those studied are of primary importance in aspects of ventilatory control currently studied.

Ageing and gonadectomy have similar effects on hypoglossal long-term facilitation in male Fischer rats

Long-term facilitation (LTF), a form of serotonin-dependent respiratory plasticity induced by intermittent hypoxia, decreases with increasing age or following gonadectomy in male Sprague-Dawley (SD) rats. Ageing is accompanied by decreasing levels of testosterone, which in turn influences serotonergic function. In addition, LTF in young male rats differs among strains. Thus, we tested the hypothesis that LTF is similar in middle-aged and gonadectomized young male rats of an inbred rat strain commonly used in studies on ageing (F344) by comparison with SD rats. We further tested whether the magnitude of LTF correlates with circulating serum levels of testosterone and/or progesterone. Young and middle-aged intact and young gonadectomized (GDX) male Fischer 344 rats were anaesthetized, neuromuscularly blocked and ventilated. Integrated phrenic and hypoglossal (XII) nerve activities were measured before, during and 60 min following three 5-min episodes of isocapnic hypoxia. LTF was observed in phrenic motor output in young and middle-aged intact and young GDX rats. In contrast, XII LTF was observed only in young intact rats. In middle-aged and young GDX rats, XII LTF was significantly lower than in young intact rats (P < 0.05). Furthermore, XII LTF was positively correlated with the testosterone/progesterone ratio. These data show that serotonin-dependent plasticity in upper airway respiratory output is similar in F344 and SD rat strains. Furthermore, LTF is similarly impaired in middle-aged and gonadectomized male rats, suggesting that gonadal hormones play an important role in modulating the capacity for neuroplasticity in upper airway motor control.

Hypercapnia induces long-term changes in postganglionic renal nerve activity in the piglet

In developing swine, time and frequency domain analyses were used to compare changes in discharge features of efferent phrenic and postganglionic renal nerve activities evoked by prolonged (1 h) exposure to severe hypercapnia (10% CO2, balance O2), before and after combined carotid sinus and aortic depressor nerve (CSN-AOD) sectioning. With intact CSN-AOD innervation, respiration-related activity in renal nerve discharge was rare (3 of 11 animals) during baseline periods with intact innervation, but was observed in most cases (10 of 11 animals) during baseline following denervation. Renal nerve respiration-related activity was recruited by hypercapnic stimulation in animals with intact CSN-AOD innervation, and was augmented in denervated animals with ongoing respiratory activity. Phrenic nerve discharge was markedly augmented during hypercapnia, whether CSN-AOD innervation was intact or not, and it did not exhibit a post-hypercapnic depression. Autopower spectra of renal nerve activity revealed the presence of two coexisting rhythms, 2-6 and 7-13 Hz, which were present whether CSN-AOD innervation was intact or not. The hypercapnic-induced increases of activity in the 2-6 and 7-13 Hz bands were not comparable, with the latter region exhibiting a much more robust response to hypercapnia, especially following CSN-AOD denervation. Thus, prolonged exposure to hypercapnia evoked changes in renal nerve discharge that involved increased coupling to neuronal ensembles shaping central inspiratory activity and those generating central sympathetic outflows, especially to networks generating 7-13 Hz rhythm. Such changes may permit more efficient modulation of innervated structures during exposure to stressors.

The fa leptin receptor mutation and the heritability of respiratory frequency in a Brown Norway and Zucker intercross

We sought to determine whether the fa (leptin receptor) mutation was a major determinant of the putative obesity effects on respiratory frequency in an intercross between the Brown Norway (low breathing frequency, nonobese strain) and the Zucker (moderately high breathing frequency, with the fa mutation) strains. The hypothesis was that rats bearing one (heterozygote) or two (homozygote) alleles of the Glu296Pro point mutation (fa) would have a uniformly high respiratory frequency in the second filial (F2) generation, compared with wild-type animals. In addition to breathing frequency, tidal volume and minute ventilation were assessed during baseline, acute hypoxic (10% O2-0% CO2-balance nitrogen), hypercapnic (93% O2-7% CO2), hyperoxic (100% O2-0% CO2), and combined (10% O2-3% CO2-balance nitrogen) challenges in fa homozygote (fa/fa; n = 24), fa heterozygote (fa/wt; n = 33), and wild-type (wt/wt; n = 19) animals. Phenotypes were adjusted with stepwise regression analyses for the effects of age, sex, length, and litter size. Broad-sense heritability was estimated by examining the variance of the traits in first filial and F2 generations. ANOVAs were used to determine the mode of inheritance of the fa allele in the F2 generation. As anticipated, weight demonstrated the greatest overall broad-sense heritability (77%) and was the result of the recessive mutation. Breathing parameters during the hypoxic, hypercapnic, and combined challenges demonstrated a wide range of heritability from 5 to 96%, with a very nonuniform proportion of heritability explained by the leptin receptor. At best, for frequency 4.5 min into the hypercapnic hypoxic challenge, approximately 20% of the total heritability (approximately 67%) could be attributed to an effect of the leptin receptor mutation. We conclude that, unlike its major effect on weight, the effect of the fa allele is not a major gene involved in the regulation of breathing frequency.

Contribution of airway closure to chronic postbronchiolitis airway dysfunction in rats

Genetically susceptible Brown Norway rats develop a chronic asthmalike syndrome after recovering from viral bronchiolitis at an early age. We hypothesized that airway closure is an important mechanism of airflow obstruction in postbronchiolitis rats. Rats were studied 8–12 wk after inoculation with Sendai virus or sterile vehicle at 3–4 wk of age. Under light pentobarbital anesthesia, rats were instrumented with an orotracheal catheter and an esophageal pressure monitor and placed in a total body plethysmograph. Lung volumes and forced-expiratory maneuvers were measured using the Boyle's law method and software-controlled valving of positive and negative pressures to elicit lung inflations and rapid deflations; pulmonary resistance was measured during spontaneous tidal breathing; and quasi-static pressure-volume curves were obtained with passive inflations and deflations in fully anesthetized, paralyzed rats. Compared with controls, the postbronchiolitis rats had elevated pulmonary resistance and reduced forced-expiratory volume in 0.2 s. Most of the reduced forced-expiratory volume in 0.2 s was associated with reduced forced vital capacity, indicating premature airway closure as a prominent mechanism. The reduced airflow in postbronchiolitis rats was highly dependent on lung volume, being nearly normal at 70% lung capacity, but sevenfold less than normal at 30% lung capacity. Increased respiratory system hysteresis between functional reserve capacity and total lung capacity was evidence for increased airway closure at normal end-expiratory lung volumes in postbronchiolitis rats. We conclude that airway instability and closure is a prominent mechanism of the chronic airway dysfunction in rats that have recovered from viral bronchiolitis at an early age.