Strain variation in the adaptation of C57Bl6 and BALBc mice to chronic hypobaric hypoxia

The interplay of environmental and genetic factors may lead to a spectrum of physiological and behavioral outcomes. How environmental stress factors interact with the diverse mouse genomes is still poorly understood and elucidating the underlying interactions requires specific stress models that can target integrated physiological systems. Here, we employ behavioral tests and whole-body plethysmography to examine the effects of 12 weeks of simulated high altitude (HA) exposure on two inbred mouse strains, BALBc and C57Bl6. We find that HA induced- weight loss recovers at significantly different rates in these two strains. Even at 12 weeks, however, both strains fail to reach body weight levels of controls. Performance on two motor tasks, rotarod and treadmill, improve with HA exposure but more prominently in BALBc mice. Whole-body plethysmography outcomes indicate that compensation to chronic HA includes increased respiratory frequencies and tidal volumes in both strains. However, the effects on tidal volume are significantly greater in BALBc mice and showed a biphasic course. Whole- body metabolic rates are also increased in both strains with prolonged HA exposure, but were more pronounced in BALBc mice suggestive of less successful adaptation in this strain. These adaptations occur in the absence of gross pathological changes in all major organs. Together these results indicate that chronic HA exposure results in environmental stressors that impact the specific physiological responses of BALBc more than C57Bl6 mice. Thus, these strains provide a promising platform for investigating how genetic backgrounds can differentially reinforce the effects of long-lasting environmental stressors and their potential to interact with psychological stressors.

Breathing and temperature control disrupted by morphine and stabilized by clonidine in neonatal rats

BACKGROUND

Sedative-analgesics are often given to newborn infants and are known to affect many components of the autonomic nervous system. While morphine is most frequently used, α-2 adrenergic receptor agonists are being increasingly used in this population. Alpha-2 adrenergic receptors agonists also have anti-shivering properties which may make it a desirable drug to give to infants undergoing therapeutic hypothermia. The aim of this study was to systematically compare two different classes of sedative-analgesics, morphine, a μ-opioid receptor agonist, and clonidine an α-2 adrenergic receptor agonist on breathing, metabolism and core body temperature (CBT) in neonatal rodents.

METHODS

Breathing parameters, oxygen consumption (VO2) and carbon dioxide production (VCO2), were measured prior to, 10 and 90 min after intraperitoneal (IP) administration of morphine (2, 10 or 20 mg/kg), clonidine (40, 200 or 400 μg/kg), or saline in Sprague-Dawley rat pups at postnatal day 7 (p7) while continuously monitoring CBT.

RESULTS

Morphine reduced the respiratory rate, VO2 and VCO2 greater than clonidine at all dosages used (p<0.05, morphine vs. clonidine, for all metabolic and respiratory parameters). Furthermore, morphine induced prolonged respiratory pauses, which were not observed in animals treated with clonidine or saline. Morphine caused hypothermia which was dose dependent, while clonidine stabilized CBT in comparison to saline treated animals (p<0.0001).

CONCLUSION

In the newborn rat, morphine causes profound respiratory depression and hypothermia while clonidine causes minimal respiratory depression and stabilizes CBT. All together, we suggest that clonidine promotes autonomic stability and may be a desirable agent to use in infants being treated with therapeutic hypothermia.

Perinatal hyperoxic exposure reconfigures the central respiratory network contributing to intolerance to anoxia in newborn rat pups

Perinatal exposure to hyperoxia (30-60% O2) alters the respiratory control system via modulation of peripheral arterial chemoreceptor development and function. Furthermore, hyperoxic exposure during the first two postnatal weeks of life can alternatively modulate the different phases of the hypoxic ventilatory response. Given the effects of perinatal hyperoxia, the aims of our study were 1) to determine the effect on survival time in response to lethal anoxic stimuli in rat pups and 2) to characterize the output of the isolated central respiratory network in response to acute hypoxic stimuli. We hypothesized that perinatal hyperoxic exposure would modify the neonatal rat ventilatory response to anoxia by affecting a central component of the respiratory network in addition to the maturation of the carotid body chemoreceptors. We found that animals continuously exposed to 60% oxygen up to age 5 days after parturition (P5) have reduced breathing frequency at baseline and within the first 10 min of a fatal anoxic challenge. Hyperoxic rat pups also have a shortened time to last gasp in response to anoxia that is not associated with lung injury or inflammation. This study is the first to demonstrate that these in vivo findings correlate with reduced phrenic burst frequency from the isolated brainstem ex vivo. Thus hyperoxic exposure reduced the phrenic burst frequency at baseline and in response to ex vivo anoxia. Importantly, our data suggest that perinatal hyperoxia alters ventilation and the response to anoxia at P5 in part by altering the frequency of phrenic bursts generated by the central respiratory network.

Altered Th1/Th2 commitment contributes to lung senescence in CXCR3-deficient mice

Aging is an inevitable process associated with immune imbalance, which is characterized by a progressive functional decline in major organs, including lung. However, effects of altered Th1/Th2 commitment on lung senescence are largely unknown. To examine effects of altered Th1/Th2 balance on lung aging, we measured proportions of Th1 and Th2 cells and expression of cytokines, chemokines, collagen deposition and other relevant physiological and pathological parameters in 2- and 20-months-old (mo) CXCR3-deficient (CXCR3(-/-)) C57BL/6J mice compared with wild-type (WT) mice. There was a significant weight-loss observed in 20-mo CXCR3(-/-) mice compared with the same aged WT group. Although lung function and structure changed with age in both groups, central airway resistance (Rn), tissue elastance (H) and damping (G) were significantly lower in 20-mo CXCR3(-/-) mice than those of WT mice. In contrast, the whole lung volume (V(L)), the mean linear intercept length of alveolar (L(m)), and the total lung collagen content were significantly elevated in 20-mo CXCR3(-/-) mice. With aging, the lungs of WT mice had typical Th1-type status (increased population of Th1 cells and concentrations of cytokine IFN-γ and CXCR3 ligands) while CXCR3(-/-) mice showed Th2-type polarization (decreased proportion of Th1 cells and concentrations of CXCR3 ligands but increased level of IL-4). Our data suggest that Immunosenescence is associated with lung aging, and that altered Th1/Th2 imbalance favors Th2 predominance in CXCR3(-/-) mice, which contributes to the process of accelerated lung aging in this model.

The effects of age and carbon black on airway resistance in mice

CONTEXT

Ambient particulate matter (PM) is associated with acute exacerbations of airflow obstruction. Additionally, elderly individuals are more susceptible to increased functional morbidity following acute PM exposure.

OBJECTIVE

The purpose of this study is to determine the aging effects of PM exposure on the responsiveness of airway smooth muscle in mice. We hypothesized that airway reactivity induced by methacholine (Mch) will increase with age in PM exposed mice.

MATERIALS AND METHODS

Male C57BL/6 (B6) mice at 11, 39, 67, and 96 weeks of age were exposed to carbon black (CB) or room air (RA) for 3 h on 3 consecutive days. One day after the last exposure, mice were anesthetized and airways resistance (R(aw)) was measured by forced oscillation following half-log dose increases of aerosolized Mch.

RESULTS

Baseline R(aw) was significantly lower in 67 and 96 week mice compared to 11-week mice (p < 0.05). In RA exposed mice, an age-dependent decline in Mch-induced airway reactivity occurred in association with the highest Mch doses at ages 67 and 96 weeks (p < 0.05). A significantly (p < 0.05) greater Mch-induced R(aw) response occurred in 67-week mice exposed to CB compared with age-matched RA-exposed mice.

DISCUSSION AND CONCLUSION

Our results show a progressive decrease in the Mch-induced R(aw) response with age in mice. The effect of CB exposure resulted in greater airway reactivity in middle-aged mice, which highlights the effects of PM exposure on the lung as it relates to increased morbidity and mortality with older age.

Strain variation in response to lung ischemia: role of MMP-12

Background

Systemic neovascularization of the lung during chronic ischemia has been observed in all mammals studied. However, the proteins that orchestrate the complex interaction of new vessel growth and tunneling through lung tissue matrix have not been described. Although previous work has demonstrated the CXC chemokines are essential growth factors in the process of angiogenesis in mice and rats, key matrix proteins have not been identified.

Methods

Since the degradation of chemokines has been shown to be dependent on metalloproteinases (MMP), we first surveyed gene expression patterns (real time RT-PCR) of several lung matrix proteins in DBA/J (D2) mice and C57Bl/6 (B6) mice, strains known to have divergent parenchymal responses in other lung disease models. We studied changes in the time course of MMP-12 activity in D2 and B6 mice. Functional angiogenesis was determined 14 days after the onset of complete left lung ischemia induced by left pulmonary artery ligation (LPAL), using fluorescent microspheres.

Results

Our results confirmed higher levels of MMP-12 gene expression in D2 mice relative to B6, which corresponded to a phenotype of minimal systemic angiogenesis in D2 mice and more robust angiogenesis in B6 mice (p < 0.01). MMP-12 activity decreased over the course of 14 days in B6 mice whereas it increased in D2 mice (p < 0.05). MMP-12 was associated largely with cells expressing the macrophage marker F4/80. Genetic deficiency of MMP-12 resulted in significantly enhanced neovascularization (p < 0.01 from B6).

Conclusion

Taken together, our results suggest macrophage-derived MMP-12 contributes to angiostasis in the ischemic lung.

Effects of ozone and particulate matter on cardiac mechanics: role of the atrial natriuretic peptide gene

A positive association between air pollution exposure and increased human risk of chronic heart disease progression is well established. In the current study, we test two hypotheses: (1) the cardiac compensatory changes in response to air pollution are dependent on its composition and (2) specific cardiac adaptations are regulated by atrial natriuretic peptide (ANP). We address these hypotheses by initially examining the exposure effects of ozone (O(3)) and/or particulate matter (PM) on cardiac function in C57Bl/6J (B6) mice. Subsequently, the results are compared with cardiac functional changes to the same exposures in Nppa (the precursor gene for ANP) knockout (KO) mice. Separate groups of mice underwent 3 consecutive days of the same exposure sequence for 3h each consisting of the following: (1) 6h of filtered air (FAFA), (2) O(3) then FA (O(3)FA), (3) FA then carbon black (FACB), or (4) O(3) then CB. Cardiac function was assessed using a conductance catheter to generate cardiac pressure-volume loops 8-10h following each exposure sequence. As compared with FAFA, each sequence led to a substantial drop (as much as 33%) in stroke volume and cardiac output. However, these losses of cardiac function occurred by different compensatory mechanisms dependent on the pollutant composition. For example, O(3)FA exposure led to reductions in both end-systolic and end-diastolic left ventricular (LV) volumes, whereas FACB exposure led an increase in end-diastolic LV volume. These same cardiac compensatory changes were largely abolished in Nppa KO mice following O(3)FA or FACB exposure. These results suggest that cardiac functional changes in response to air pollution exposure are strongly dependent on the pollutant constituents, especially related to O(3) and/or PM. Furthermore, ANP regulation appears to be crucial to these cardiac compensatory mechanisms induced by air pollution.

A critical role for muscle ring finger-1 in acute lung injury-associated skeletal muscle wasting

RATIONALE

Acute lung injury (ALI) is a debilitating condition associated with severe skeletal muscle weakness that persists in humans long after lung injury has resolved. The molecular mechanisms underlying this condition are unknown.

OBJECTIVES

To identify the muscle-specific molecular mechanisms responsible for muscle wasting in a mouse model of ALI.

METHODS

Changes in skeletal muscle weight, fiber size, in vivo contractile performance, and expression of mRNAs and proteins encoding muscle atrophy-associated genes for muscle ring finger-1 (MuRF1) and atrogin1 were measured. Genetic inactivation of MuRF1 or electroporation-mediated transduction of miRNA-based short hairpin RNAs targeting either MuRF1 or atrogin1 were used to identify their role in ALI-associated skeletal muscle wasting.

MEASUREMENTS AND MAIN RESULTS

Mice with ALI developed profound muscle atrophy and preferential loss of muscle contractile proteins associated with reduced muscle function in vivo. Although mRNA expression of the muscle-specific ubiquitin ligases, MuRF1 and atrogin1, was increased in ALI mice, only MuRF1 protein levels were up-regulated. Consistent with these changes, suppression of MuRF1 by genetic or biochemical approaches prevented muscle fiber atrophy, whereas suppression of atrogin1 expression was without effect. Despite resolution of lung injury and down-regulation of MuRF1 and atrogin1, force generation in ALI mice remained suppressed.

CONCLUSIONS

These data show that MuRF1 is responsible for mediating muscle atrophy that occurs during the period of active lung injury in ALI mice and that, as in humans, skeletal muscle dysfunction persists despite resolution of lung injury.

Age-induced augmentation of p38 MAPK phosphorylation in mouse lung

The p38 mitogen-activated protein kinase (p38 MAPK) pathway is a key regulator of pro-inflammatory cytokine biosynthesis, which may contribute to the chronic low-grade inflammation observed with aging. We hypothesize that aging up-regulates the activation of p38 MAPK as well as the pro-inflammatory cytokines tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6) in mouse lung, and is accompanied by disturbances in oxidant-antioxidant status. In addition, the elevated protein levels of phosphorylated active form of p38 MAPK (phospho-p38 MAPK) with age are tissue-specific. To test this hypothesis, protein levels of phospho-p38 MAPK were determined using Western blot analysis in isolated lung, brain, heart, spleen, kidney and muscle of young (2-month-old) and aged (20-month-old) male C57BL/6J mice. Results show that phospho-p38 MAPK protein levels, not total-p38 MAPK, increased significantly (p<0.01, n=8) in lung and brain of 20-month-old mice. The activation of p38 MAPK in other tissues was not altered with age. Immunostaining showed that epithelial cells and alveolar macrophages in lung parenchyma were the major cellular sources of phospho-p38 MAPK immunity. As measured by enzyme-linked immunosorbent assay (ELISA), TNF-α, IL-1β and IL-6 in lung homogenates were elevated significantly with age, but there were no differences with age in serum levels except for IL-6. In addition, IL-1β and IL-6 were increased notably while TNF-α was not different with age in bronchoalveolar lavage fluid (BALF). Furthermore, the oxidant-antioxidant status was evaluated by measuring pro-oxidant malondialdehyde (MDA) levels and the activity of reactive oxygen species scavenging enzymes (i.e. superoxide dismutase (SOD) and glutathione (GSH)) in lung homogenates. The results showed that SOD and GSH decreased with age, while MDA did not change. In conclusion, our data demonstrate that p38 MAPK is activated during lung aging with a corresponding increase in pro-inflammatory cytokines and decrease in antioxidant capacity.

Variation in echocardiographic and cardiac hemodynamic effects of PM and ozone inhalation exposure in strains related to Nppa and Npr1 gene knock-out mice

Elevated levels of ambient co-pollutants are associated with adverse cardiovascular outcomes shown by epidemiology studies. The role of particulate matter (PM) and ozone (O3) as co-pollutants in this association is unclear. We hypothesize that cardiac function following PM and O3 exposure is variably affected by genetic determinants (Nppa and Npr1 genes) and age. Heart function was measured before and after 2 days each of the following exposure sequence; (1) 2-h filtered air (FA) and 3-h carbon black (CB; 0.5 microg/m(3)); (2) 2-h O3 (0.6 ppm) and 3-h FA; (3) 5-h FA; and, (4) 2-h O3 and 3-h CB. Two age groups (5 and 18 months old (mo)) were tested in C57Bl/6J (B6) and 129S1/SvImJ (129) mice using echocardiographic (echo) and in vivo hemodynamic (IVH) measurements. With echo, posterior wall thickness was significantly (P < 0.01) greater in 129 relative to B6 mice at baseline. With CB exposure, young B6 and older 129 mice show significant (P < 0.01) reductions in fractional shortening (FS) compared to FA. With O3 exposure, FS was significantly (P < 0.01) diminished in young 129, which was attributable to significant increases in end-systolic left ventricular diameter. With O3 and CB combined, notable (P < 0.01) declines in heart rate and end-systolic posterior wall thickness occurred in young 129 mice. The IVH measurements showed striking (P < 0.05) compromises in cardiac function after CB and O3 exposure; however, strain differences were undetectable. These results suggest that PM and O3 exposures, alone and combined, lead to different cardiac functional changes, and these unique changes are age-specific and dependent on Nppa and Npr1 genes.

Age-related changes in cardiac and respiratory adaptation to acute ozone and carbon black exposures: interstrain variation in mice

CONTEXT

Epidemiological studies show positive associations between increased ambient air pollutant levels and adverse cardiopulmonary effects. These studies suggest that the elderly and those with certain genetic polymorphisms are susceptible to adverse air pollution-associated health events.

HYPOTHESIS/OBJECTIVE

We hypothesize that physiological responses to air pollutants vary with age and are genetically influenced.

MATERIALS AND METHODS

To test this hypothesis, we exposed mice from three inbred strains (C57BL/6J, B6; C3H/HeJ, HeJ; C3H/HeOuJ, OuJ) to ozone (O(3)) and carbon black (CB) at two ages, (5 months, 12 months), for 3 consecutive days, to either filtered air (FA), CB particles, or O(3) and CB sequentially (O(3)CB) (CB, 550 µg/m(3); O(3), 600 ppb). Heart rate (HR), HR variability (HRV), breathing, and core temperature (Tco) responses were analyzed.

RESULTS

We observed time-dependent physiological changes in response to O(3)CB exposure in each strain, relative to FA exposure for both age groups. Each mouse strain showed distinct adaptation profiles to repeated acute exposures to O(3). In younger mice, several time-dependent effects (decreased HR and increased HRV) were prominent in HeJ and OuJ mice but not B6 mice. We also observed variability in adaptation in older mice. However, responses in older mice were generally attenuated when compared to the younger mice. In addition, cardiac-respiratory interactions were affected with CB and O(3)CB exposures albeit with patterns differing by age or exposure.

DISCUSSION/CONCLUSION

Our results suggest that age considerably attenuates physiological responses to O(3) and O(3)CB exposures. Age-related physiological changes such as increased oxidative stress in mouse tissue may be involved in this attenuation.

Cigarette smoke-induced emphysema in A/J mice is associated with pulmonary oxidative stress, apoptosis of lung cells, and global alterations in gene expression

Cigarette smoking is the major risk factor for developing chronic obstructive pulmonary disease, the fourth leading cause of deaths in the United States. Despite recent advances, the molecular mechanisms involved in the initiation and progression of this disease remain elusive. We used Affymetrix Gene Chip arrays to determine the temporal alterations in global gene expression during the progression of pulmonary emphysema in A/J mice. Chronic cigarette smoke (CS) exposure caused pulmonary emphysema in A/J mice, which was associated with pronounced bronchoalveolar inflammation, enhanced oxidative stress, and increased apoptosis of alveolar septal cells. Microarray analysis revealed the upregulation of 1,190, 715, 260, and 246 genes and the downregulation of 1,840, 730, 442, and 236 genes in the lungs of mice exposed to CS for 5 h, 8 days, and 1.5 and 6 mo, respectively. Most of the genes belong to the functional categories of phase I genes, Nrf2-regulated antioxidant and phase II genes, phase III detoxification genes, and others including immune/inflammatory response genes. Induction of the genes encoding multiple phase I enzymes was markedly higher in the emphysematous lungs, whereas reduced expression of various cytoprotective genes constituting ubiquitin-proteasome complex, cell survival pathways, solute carriers and transporters, transcription factors, and Nrf2-regulated antioxidant and phase II-responsive genes was noted. Our data indicate that the progression of CS-induced emphysema is associated with a steady decline in the expression of various genes involved in multiple pathways in the lungs of A/J mice. Many of the genes discovered in this study could rationally play an important role in the susceptibility to CS-induced emphysema.

Interstrain variation in cardiac and respiratory adaptation to repeated ozone and particulate matter exposures

Increased ambient particulate matter (PM) is associated with adverse cardiovascular and respiratory outcomes, as demonstrated by epidemiology studies. Several studies have investigated the role of copollutants, such as ozone (O(3)), in this association. It is accepted that physiological adaptation involving the respiratory system occurs with repeated exposures to O(3). We hypothesize that adaptation to PM and O(3) varies among different inbred mouse strains, and cardiopulmonary adaptation to O(3) is a synchronized response between the cardiac and respiratory systems. Heart rate (HR), HR variability (HRV), and the magnitude and pattern of breathing were simultaneously measured by implanted telemeters and by plethysmography in three inbred mouse strains: C57Bl/6J (B6), C3H/HeJ (HeJ), and C3H/HeOuJ (OuJ). Physiological responses were assessed during dual exposures to filtered air (FA), O(3) (576 +/- 32 parts/billion), and/or carbon black (CB; 556 +/- 34 mug/m(3)). Exposures were repeated for 3 consecutive days. While each strain showed significant reductions in HR during CB with O(3) preexposure (O(3)CB) on day 1, prominent HRV responses were observed in only HeJ and OuJ mice. Each strain also differed in their adaptation profile in response to repeated O(3)CB exposures. Whereas B6 mice showed rapid adaptation in HR after day 1, HeJ mice generally showed more moderate HR and HRV adaptation after day 2 of exposure. Unlike either B6 or HeJ strains, OuJ mice showed little evidence of HR or HRV adaptation to repeated O(3)CB exposure. Adaptation profiles between HR regulation and breathing characteristics were strongly correlated, but these associations also varied significantly among strains. These findings suggest that genetic factors determine the responsivity and adaptation of the cardiac and respiratory systems to repeated copollutant exposures. During O(3)CB exposure, adaptation of cardiac and respiratory systems is markedly synchronized, which may explain a potential mechanism for adverse effects of PM on heart function.

Particle Effects on Heart-Rate Regulation in Senescent Mice

Because epidemiology studies consistently identify the elderly at risk for air pollution-related morbidity and mortality, we developed a model of senescent-dependent susceptibility based on indices of physiological aging. In the current study, we hypothesized that heart-rate regulation during particulate matter (PM) exposure differs with senescence-dependent susceptibility owing to variation in autonomic nervous control. Heart rate (HR) and heart-rate variability (HRV) parameters were measured from 162 samples of 2-min electrocardiograph (ECG) recordings in age-matched healthy (n = 5) and terminally senescent (n = 3) AKR mice during 3-h exposures to filtered-air (FA, day 1) and carbon black (CB, day 4; <200 microg/m(3)). On day 1, HR was significantly (p <.01) depressed during FA in terminally senescent mice. By day 4, HR was further slowed significantly (p <.01) due to the effects of CB exposure for 3 days. The combined effects of terminal senescence and CB exposure acted to depress HR to an average (+/-SEM) 445 +/- 40 bpm, or approximately 80 bpm lower compared to healthy HR responses. The change in rMSSD, an HRV parameter corresponding to relative influences of parasympathetic tone on HR, was significantly (p <.01) greater on day 1 and day 4 in terminally senescent mice compared to healthy mice. In contrast, the LF/HF ratio, an HRV parameter derived from spectral analysis indicating relative changes in cardiac sympathetic tone, was significantly (p <.01) depressed in terminally senescent mice on day 1. By day 4, significant increases in LF/HF were evident in healthy mice during CB exposure, suggesting that HR regulation was associated with an increase in sympathetic tone. Alternatively, terminally senescent mice appeared to modulate a lower HR without change in LF/HF ratio during CB exposure, suggesting an absence of sympathetic tone. In conclusion, older healthy mice increase cardiac sympathetic tone during PM exposure while terminally senescent mice show a greater PM-induced parasympathetic tone in regulating HR. The significance of the current results suggest that PM-induced HR regulatory changes may ultimately depend on the degree of physiological aging.

Adverse cardiovascular effects with acute particulate matter and ozone exposures: interstrain variation in mice

OBJECTIVES

Increased ambient particulate matter (PM) levels are associated with cardiovascular morbidity and mortality, as shown by numerous epidemiology studies. Few studies have investigated the role of copollutants, such as ozone, in this association. Furthermore, the mechanisms by which PM affects cardiac function remain uncertain. We hypothesized that PM and O(3) induce adverse cardiovascular effects in mice and that these effects are strain dependent.

STUDY DESIGN

After implanting radiotelemeters to measure heart rate (HR) and HR variability (HRV) parameters, we exposed C57Bl/6J (B6), C3H/HeJ (HeJ), and C3H/HeOuJ (OuJ) inbred mouse strains to three different daily exposures of filtered air (FA), carbon black particles (CB), or O(3) and CB sequentially [O(3)CB; for CB, 536 +/- 24 microg/m(3); for O(3), 584 +/- 35 ppb (mean +/- SE)].

RESULTS

We observed significant changes in HR and HRV in all strains due to O(3)CB exposure, but not due to sequential FA and CB exposure (FACB). The data suggest that primarily acute HR and HRV effects occur during O(3)CB exposure, especially in HeJ and OuJ mice. For example, HeJ and OuJ mice demonstrated dramatic increases in HRV parameters associated with marked brady-cardia during O(3)CB exposure. In contrast, depressed HR responses occurred in B6 mice without detectable changes in HRV parameters.

CONCLUSIONS

These findings demonstrate that important interstrain differences exist with respect to PM- and O(3)-induced cardiac effects. This interstrain variation suggests that genetic factors may modulate HR regulation in response to and recuperation from acute copollutant exposures.

Exposure to inhaled particulate matter impairs cardiac function in senescent mice

Daily exposure to particulate matter (PM) is known to adversely affect cardiac function and is also known to be exaggerated with senescence. This study tests the hypothesis that cardiac function is uniquely altered by PM exposure in senescent mice. A mechanism for PM-induced cardiac effects is also postulated by examining the activity of nitric oxide synthase (NOS) and the generation of reactive oxygen species (ROS) in heart tissue. Echocardiography is performed in awake 18- and 28-mo-old mice at baseline and immediately following 3-h exposures to either filtered air or carbon black (CB; approximately 400 microg/m3) on 4 days. At 28 mo, left ventricular diameter at end-systole and end-diastole is significantly (P < 0.05) elevated, and fractional shortening is significantly reduced (49 +/- 3% vs. 56 +/- 3%) with CB exposure. In vivo hemodynamic measurements at 28 mo also demonstrate significant (P < 0.05) reductions in ejection fraction and increases in right ventricular and pulmonary vascular pressures following CB exposure. Functional changes at 28 mo are associated with increased ROS production as suggested by enhanced luminol activity. This elevated ROS production with aging and CB exposure is attributable to NOS uncoupling. Measurements of natriuretic peptide (atrial and brain) transcription and matrix metalloproteinase (MMP2 and MMP9) activity in heart tissue are significantly (P < 0.05) amplified with senescence and exposure to CB, pointing to increased cardiac stress and remodeling. These results demonstrate that acute PM exposure reduces cardiac contractility in senescent mice, and this decline in function is associated with increased ROS production linked to NOS uncoupling.

Effects of leptin deficiency on postnatal lung development in mice

Leptin modulates energy metabolism and lung development. We hypothesize that the effects of leptin on postnatal lung development are volume dependent from 2 to 10 wk of age and are independent of hypometabolism associated with leptin deficiency. To test the hypotheses, effects of leptin deficiency on lung maturation were characterized in age groups of C57BL/6J mice with varying Lep(ob) genotypes. Quasi-static pressure-volume curves and respiratory impedance measurements were performed to profile differences in respiratory system mechanics. Morphometric analysis was conducted to estimate alveolar size and number. Oxygen consumption was measured to assess metabolic rate. Lung volume at 40-cmH(2)O airway pressure (V(40)) increased with age in each genotypic group, and V(40) was significantly (P < 0.05) lower in leptin-deficient (ob/ob) mice beginning at 2 wk. Differences were amplified through 7 wk of age relative to wild-type (+/+) mice. Morphometric analysis showed that alveolar surface area was lower in ob/ob compared with +/+ and heterozygote (ob/+) mice beginning at 2 wk. Unlike the other genotypic groups, alveolar size did not increase with age in ob/ob mice. In another experiment, ob/ob at 4 wk received leptin replacement (5 microg.g(-1) x day(-1)) for 8 days, and expression levels of the Col1a1, Col3a1, Col6a3, Mmp2, Tieg1, and Stat1 genes were significantly increased concomitantly with elevated V(40). Leptin-induced increases in V(40) corresponded with enlarged alveolar size and surface area. Gene expression suggested a remodeling event of lung parenchyma after exogenous leptin replacement. These data support the hypothesis that leptin is critical to postnatal lung remodeling, particularly related to increased V(40) and enlarged alveolar surface area.

Global expression profiles from C57BL/6J and DBA/2J mouse lungs to determine aging-related genes

This study identified gene expression profiles that provided evidence for genomic mechanisms underlying the pathophysiology of aging lung. Aging lungs from C57BL/6 (B6) and DBA/2 (D2) mouse strains differ in physiology and morphometry. Lungs were harvested from B6 mice at 2, 18, and 26 mo and from D2 mice at 2 and 18 mo of age. Purified RNA was subjected to oligonucleotide microarray analyses, and differential expression analyses were performed for comparison of various data sets. A significant majority of differentially expressed genes were upregulated with aging in both strains. Aging D2 lungs uniquely exhibited upregulation in stress-response genes including xenobiotic detoxification cascades. In contrast, aging B6 lungs showed downregulation of heat shock-response genes. Age-dependent downregulation of genes common to both B6 and D2 strains included several collagen genes (e.g., Col1a1 and Col3a1). There was a greater elastin gene (Eln) expression in D2 mice at 2 mo, and Eln was uniquely downregulated with age in this strain. The matrix metalloproteinase 14 gene (Mmp14), critical to alveolar structural integrity, was also downregulated with aging in D2 mice only. Several polymorphisms in the regulatory and untranslated regions of Mmp14 were identified between strains, suggesting that variation in Mmp14 gene regulation contributes to accelerated aging of lungs in D2 mice. In summary, lungs of B6 and D2 mice age with variable rates at the gene expression level, and these quantifiable genomic differences provide a template for understanding the variability in age-dependent changes in lung structure and function.

Variation in heart rate regulation and the effects of particle exposure in inbred mice

Altered autonomic control of heart rate (HR) rhythm during exposure to particulate matter (PM) has been suggested in human and animal studies. Our lab has shown strain variation in HR regulation between quiescent C3H/HeJ (C3) and C57BL/6J (B6) mice: that is, C3 mice show a consistently higher HR by approximately 80 bpm compared with B6 mice during a normal 24-h circadian cycle. In the current study, we hypothesize that the balance between sympathetic and parasympathetic control of HR during PM exposure varies between C3 and B6 mice. Radiotelemeters were implanted in C3 and B6 mice to measure HR responses and HR variability (HRV) parameters during successive 3-h exposures to filtered air (FA) or carbon black (CB, < 300 mug/m3). Exposures were repeated following administration of saline or parasympathetic (PS; atropine, 0.5 mg/kg i.p.) and sympathetic (S; propranolol, 1 mg/kg i.p.) blockade to study the autonomic regulation of HR during CB exposure. During FA exposure with saline, a significantly (p < .05) greater 3-h average HR response (bpm +/- SEM) occurred in C3 compared with B6 mice (496 +/- 22 vs. 427 +/- 3). With PS blockade, the strain difference between C3 and B6 mice was not evident (485 +/- 23 vs. 503 +/- 61). With S blockade, the 3-h average HR responses for C3 mice were significantly (p < .05) reduced compared with saline (413 +/- 18 vs. 392 +/- 15 for B6). During CB exposure with saline, HR responses were again significantly (p < 0.05) elevated in C3 compared with B6 mice, but these HR responses were not different relative to FA exposure. With S blockade, HR was significantly (p < .05) elevated in B6 mice during CB relative to FA, but was unchanged in C3 mice. Collectively, these results suggest that strain variation in HR regulation is due to a robust PS tone evident in B6 mice and a predominant S tone in C3 mice. Furthermore, CB exposure alters HR regulation in B6 mice by modulating a withdrawal of PS tone. Finally, strain variation in HR between B6 and C3 mice in responding to acute PM exposure implies that robust genetic determinants modulate altered autonomic regulation in susceptible individuals.

Variation in senescent-dependent lung changes in inbred mouse strains

Previous studies from our laboratories showed lung development differences between inbred strains of mice. In the present study, the C57BL/6J (B6) and DBA/2J (D2) strains were examined for senescent-dependent differences with respect to the lung structure and function. Specifically, we hypothesize that senescent changes in lung vary between strains due to identifiable gene expression differences. Quasi-static pressure-volume curves and respiratory impedance measurements were performed on 2- and 20-mo-old B6 and D2 mice. Lung volume at 30 cm H(2)O (V(30)) pressure was significantly (P < 0.01) increased with age in both strains, but the increase was proportionally greater in D2 (68%) than in B6 (40%) mice. In addition, decreased elastic recoil pressure at 50% of V(30) and a reduction in airway resistance as a function of positive end-expiratory pressure were observed in 20-mo-old D2 mice but not in B6 mice. Morphometric analysis of lung parenchyma showed significant decreases in elastic fiber content with age in both strains, but the collagen content was significantly (P < 0.01) increased with age in D2 but not B6 mice at 20 mo. Furthermore, using quantitative RT-PCR methods, gene expression differences between strains suggested that D2 mice significantly (P < 0.05) downregulated the expressions of elastin (Eln) and procollagen I, III, and VI (Col1a1, Col3a1, and Col6a3) in lung tissue at 20 mo of age. These age-dependent changes were accompanied by an increased gene expression in matrix metalloproteinase 9 (Mmp9) in D2 and an increase in tissue inhibitor of matrix metalloproteinase (Timp1 and Timp4) in B6 mice. In conclusion, the results from the present study demonstrate that lung mechanics of both strains show significant age-dependent changes. However, changes in D2 mice are accelerated relative to B6 mice. Moreover, gene expression differences appear to be involved in the strain-specific changes of lung mechanic properties.

Age-dependent changes of airway and lung parenchyma in C57BL/6J mice

In the current study, we hypothesize that senescent-dependent changes between airway and lung parenchymal tissues of C57BL/6J (B6) mice are not synchronized with respect to altered lung mechanics. Furthermore, aging modifications in elastin fiber and collagen content of the airways and lung parenchyma are remodeling events that differ with time. To test these hypotheses, we performed quasi-static pressure-volume (PV) curves and impedance measurements of the respiratory system in 2-, 20-, and 26-mo-old B6 mice. From the PV curves, the lung volume at 30 cmH2O pressure (V30) and respiratory system compliance (Crs) were significantly ( P < 0.01) increased between 2 and 20 mo of age, representing about 80–84% of the total increase that occurred between 2 and 26 mo of age. Senescent-dependent changes in tissue damping and tissue elastance were analogous to changes in V30and Crs; that is, a majority of the parenchymal alterations in the lung mechanics occurred between 2 and 20 mo of age. In contrast, significant decreases in airway resistance (R) occurred between 20 and 26 mo of age; that is, the decrease in R between 2 and 20 mo of age represented only 29% ( P > 0.05) of total decrease occurring through 26 mo. Morphometric analysis of the elastic fiber content in lung parenchyma was significantly ( P < 0.01) decreased between 2 and 20 mo of age. To the contrary, increased collagen content was significantly delayed until 26 mo of age ( P < 0.01, 2 vs. 26 mo). In conclusion, our data demonstrate that senescent-dependent changes in airway and lung tissue mechanics are not synchronized in B6 mice. Moreover, the reduction in elastic fiber content with age is an early lung remodeling event, and the increased collagen content in the lung parenchyma occurs later in senescence.

Respiratory impairment in a mouse model of amyotrophic lateral sclerosis

Amyothrophic lateral sclerosis (ALS) is a progressive, lethal neuromuscular disease that is associated with the degeneration of cortical and spinal motoneurons, leading to atrophy of limb, axial, and respiratory muscles. Patients with ALS invariably develop respiratory muscle weakness and most die from pulmonary complications. Overexpression of superoxide dismutase 1 (SOD1) gene mutations in mice recapitulates several of the clinical and pathological characteristics of ALS and is therefore a valuable tool to study this disease. The present study is intended to evaluate an age-dependent progression of respiratory complications in SOD1(G93A) mutant mice. In each animal, baseline measurements of breathing pattern [i.e., breathing frequency and tidal volume (VT)], minute ventilation (VE), and metabolism (i.e., oxygen consumption and carbon dioxide production) were repeatedly sampled at variable time points between 10 and 20 wk of age with the use of whole-body plethysmographic chambers. To further characterize the neurodegeneration of breathing, VE was also measured during 5-min challenges of hypercapnia (5% CO(2)) and hypoxia (10% O(2)). At baseline, breathing characteristics and metabolism remained relatively unchanged from 10 to 14 wk of age. From 14 to 18 wk of age, there were significant (P < 0.05) increases in baseline VT, VE, and the ventilatory equivalent (VE/oxygen consumption). After 18 wk of age, there was a rapid decline in VE due to significant (P < 0.05) reductions in both breathing frequency and VT. Whereas little change in hypoxic VE responses occurred between 10 and 18 wk, hypercapnic VE responses were significantly (P < 0.05) elevated at 18 wk due to an augmented VT response. Like baseline breathing characteristics, hypercapnic VE responses also declined rapidly after 18 wk of age. The phenotypic profile of SOD1(G93A) mutant mice was apparently unique because similar changes in respiration and metabolism were not observed in SOD1 controls. The present results outline the magnitude and time course of respiratory complications in SOD1(G93A) mutant mice as the progression of disease occurs in this mouse model of ALS.

PEEP-induced changes in epithelial permeability in inbred mouse strains

Inbred mouse strains have demonstrated a range of susceptibilities to inhaled environmental irritants. C57Bl/6J mice are highly susceptible while C3H/HeJ mice are resistant to ozone exposures, as assessed by lavaged protein. However, lavaged protein reflects a loss of both the endothelial and epithelial barrier. To determine whether basal differences exist in the epithelial barrier, we measured soluble tracer ((99m)technetium-diethylenetriamine pentaacetic acid, (99m)Tc-DTPA) clearance from the lung in spontaneously breathing, anesthetized mice and mice ventilated with increased lung volume with applied positive end-expiratory pressure (PEEP; 1, 6, or 10cmH(2)O). Both strains showed more rapid clearance during ventilation with 10cmH(2)O PEEP compared with other ventilation pressures (p<0.001). There was a substantial difference in clearance between the two strains during ventilation with 10cmH(2)O PEEP (mean half time for C57Bl/6J mice=19+/-4min versus 34+/-3min for C3H/HeJ mice; p<0.001). Thus, when lung volume is increased, the susceptible C57Bl/6J strain shows a greater change in epithelial barrier than the resistant C3H/HeJ strain. These results may reflect fundamental differences in lung architecture.

Heritable and pharmacological influences on pauses and apneas in inbred mice during anesthesia and emergence

Inherited differences in response to hypercapnia are augmented by volatile anesthetics. Therefore, the authors tested the hypotheses that (1) the incidence of pauses and apneas also increase under anesthesia; (2) there is a difference in the incidence between mouse strains; and (3) there is a difference in the incidence of pauses and apneas depending on the volatile agent. The authors assessed respiratory pauses and apneas at rest; during anesthesia with isoflurane, sevoflurane, and desflurane; and at recovery in C3, B6, and F1 mice. The results are compared using analysis of variance (ANOVA) (P<.05). Awake, there was no difference between mouse strains (Bb, 0.3+/-0.7 P min(-1), C3, 0.4+/-0.7 P min(-1) and F1, 0.4+/-0.5 P min(-1)). In contrast, during anesthesia C3 mice showed a significantly higher incidence of pauses and apneas. There was no increase for B6 and F1 mice. There was no difference depending on the volatile agent. These results indicate an inheritance of a susceptibility to pauses and apneas under the influence of volatile anesthetics, albeit independent of the agent used. This response seems to be independent from the inherited response to hypercapnia.

Strain-specific differences in sensitivity to ischemia-reperfusion lung injury in mice

Ischemia-reperfusion (I/R) lung injury is characterized by increased pulmonary endothelial permeability and edema, but the genetic basis for this injury is unknown. We utilized an in vivo mouse preparation of unilateral lung I/R to evaluate the genetic determinants of I/R lung injury. An index of pulmonary vascular protein permeability was measured by the ratio of left-to-right lung Evans blue dye of eight inbred mouse strains after 30 min of left lung ischemia and 150 min of reperfusion. The order of strain-specific sensitivity to I/R lung injury was BALB/c < SJL/J < CBA/J < C57BL/6J < 129/J < A/J < C3H/H3J < SWR/J. The reciprocal F1 offspring of the BALB/c and SWR/J progenitor strains had intermediate phenotypes but a differing variance. A similar pattern of right lung Evans blue dye content suggested the presence of contralateral injury because baseline vascular permeability was not different. Lung I/R injury was attenuated by NADPH oxidase inhibition, indicating a role for NADPH oxidase-derived reactive oxygen species (ROS). There was no strain-dependent difference in lung NADPH oxidase expression. Strain-related differences in zymosan-stimulated neutrophil ROS production did not correlate with I/R lung injury in that neutrophil ROS production in SWR/J mice was greater than C57BL/6J but not different from BALB/c mice. These data indicate the presence of a genetic sensitivity to lung I/R injury that involves multiple genes including a maternal-related factor. Although neutrophil-derived ROS production is also modulated by genetic factors, the pattern did not explain the genetic sensitivity to lung I/R injury.

The effect of leptin on the ventilatory responseto hyperoxia

Leptin-deficient mice show a blunted response to hypercapnia explained by central nervous system effects. The impact of leptin on peripheral chemoreceptor function is unclear. Therefore, 9 mutant (ob/ob) and 9 wild-type (+/+) mice were exposed to room air or 100% oxygen and respiratory rate (RR) and tidal volume (Vt) were measured. Subsequently, ob/ob mice received either leptin or vehicle and measurements were repeated. Compared to baseline, for +/+ mice, RR decreased significantly by 9.4% +/- 3.0% (means +/- SD), whereas Vt remained unchanged. Transition from normoxia to hyperoxia did not change RR and Vt in untreated ob/ob mice, whereas after leptin treatment, RR and Vt decreased significantly. Leptin deficiency abolishes the response to hyperoxia, which is restored by leptin replacement. Thus, leptin seems to be influential for a competent peripheral chemoreceptor function.

Phenotypic variation in cardiovascular responses to acute hypoxic and hypercapnic exposure in mice

The impact of genetic variation on cardiovascular responses to hypoxia and hypercapnia is not well understood. Therefore, we determined the acute changes in systemic arterial blood pressure (PSA) and heart rate (HR) in seven strains of commonly used inbred mice exposed to acute periods of hypoxia (10% O2), hypercapnia (5% CO2), and hypoxia/hypercapnia (10% O2 + 5% CO2) during wakefulness. Hypercapnia induced an essentially homogeneous response across strains, with PSA maintained at or slightly above baseline and with HR exhibiting a typical baroreceptor-mediated bradycardia. In contrast, exposure to hypoxia elicited a marked heterogeneity in cardiovascular responses between strains. The change in PSA during hypoxia ranged from maintenance of normotension in the FVB/J strain to profound hypotension of ∼30 mmHg in the DBA/2J strain. HR responses were highly variable between strains during hypoxia, and with the exception of the DBA/2J strain that exhibited significant bradyarrhythmias and consequent hypotension, the HR responses were unrelated to changes in PSA. The PSA response to combined hypoxia/hypercapnia represented a balance of the hypertension of hypercapnia and the hypotension of hypoxia in six of the seven strains. In the FVB/J strain, combined hypoxia/hypercapnia produced a hypertensive response that was greater than that of hypercapnia alone. These results suggest that genetic background affects the cardiovascular response to hypoxia, but not hypercapnia.

Differences in alveolar size in inbred mouse strains

In this paper we examined structural differences in alveolar size among inbred mouse strains which are known to have significant differences in lung pressure-volume relations. Accordingly, we assessed whether the relative size or number of alveoli in the C3H/HeJ, C57BL/6J, and A/J strains are related to these lung volume differences. Lungs from each of these strains were fixed in situ and then excised for quantitative morphometric analysis of airspace chord lengths. Mean chord lengths (in microm) were significantly different (P < 0.0001) among the three strains, with the largest alveoli found in the C3H/HeJ mice (45 +/- 5), the smallest in the C57BL/6J mice (35 +/- 3), and intermediate in the A/J strain (38 +/- 2). These findings provide clear evidence that there are significant genetic differences in the lung structure among different mouse strains. However, since the A/J and C57BL/6J mice had similar lung volumes, there does not yet seem to be a clear link between the macroscopic manifestations of the microscopic structure. We speculate that these structural differences might have significant influence on several mouse models of lung disease, especially those involving the development of emphysema.

Interactions in hypoxic and hypercapnic breathing are genetically linked to mouse chromosomes 1 and 5

The genetic basis for differences in the regulation of breathing is certainly multigenic. The present paper builds on a well-established genetic model of differences in breathing using inbred mouse strains. We tested the interactive effects of hypoxia and hypercapnia in two strains of mice known for variation in hypercapnic ventilatory sensitivity (HCVS); i.e., high gain in C57BL/6J (B6) and low gain in C3H/HeJ (C3) mice. Strain differences in the magnitude and pattern of breathing were measured during normoxia [inspired O2fraction (FiO2) = 0.21] and hypoxia (FiO2= 0.10) with mild or severe hypercapnia (inspired CO2fraction = 0.03 or 0.08) using whole body plethysmography. At each level of FiO2, the change in minute ventilation (V̇e) from 3 to 8% CO2was computed, and the strain differences between B6 and C3 mice in HCVS were maintained. Inheritance patterns showed potentiation effects of hypoxia on HCVS (i.e., CO2potentiation) unique to the B6C3F1/J offspring of B6 and C3 progenitors; i.e., the change in V̇e from 3 to 8% CO2was significantly greater ( P < 0.01) with hypoxia relative to normoxia in F1 mice. Linkage analysis using intercross progeny (F2; n = 52) of B6 and C3 progenitors revealed two significant quantitative trait loci associated with variable HCVS phenotypes. After normalization for body weight, variation in V̇e responses during 8% CO2in hypoxia was linked to mouse chromosome 1 (logarithm of the odds ratio = 4.4) in an interval between 68 and 89 cM (i.e., between D1Mit14 and D1Mit291). The second quantitative trait loci linked differences in CO2potentiation to mouse chromosome 5 (logarithm of the odds ratio = 3.7) in a region between 7 and 29 cM (i.e., centered at D5Mit66). In conclusion, these results support the hypothesis that a minimum of two significant genes modulate the interactive effects of hypoxia and hypercapnia in this genetic model.

Influence of volatile anaesthetics on hypercapnoeic ventilatory responses in mice with blunted respiratory drive

BACKGROUND

Subanaesthetic concentrations of volatile anaesthetics significantly affect the respiratory response to hypoxia and hypercapnoeia. Individuals with an inherited blunted respiratory drive are more affected than normal individuals. To test the hypothesis that subjects with blunted hypercapnoeic respiratory drive are diversely affected by different anaesthetics, we studied the effects of three volatile anaesthetics on the control of breathing in C3H/HeJ (C3) mice, characterized by a blunted hypercapnoeic respiratory response.

METHODS

Using whole body plethysmography, we assessed respiratory rate (RR) and pressure amplitude in 11 male C3 mice at rest, during anaesthesia with isoflurane, sevoflurane or desflurane, and during recovery. To test respiratory drive, mice were exposed to 8% carbon dioxide. Data were analysed by two-way-analysis of variance with post hoc tests and Bonferroni correction.

RESULTS

RR was unaffected during sevoflurane anaesthesia up to 1.0 MAC. Likewise, sevoflurane at 1.5 MAC affected RR less than either isoflurane (P=0.0014) or desflurane (P=0.0048). The increased RR to a carbon dioxide challenge was blocked by all three anaesthetics even at the lowest concentration, and remained depressed during recovery (P<0.0001). Tidal volume was unaffected by all three anaesthetics.

CONCLUSIONS

In C3 mice, spontaneous ventilation was less affected during sevoflurane compared with either isoflurane or desflurane anaesthesia. However, the RR response to hypercapnoeia was abolished at 0.5 MAC for all the anaesthetic agents and remained depressed even at the end of recovery. Our data suggest that different volatile anaesthetics have varying effects on the control of breathing frequency but all block the respiratory response to carbon dioxide. Therefore, a genetic predisposition to a blunted carbon dioxide response represents a susceptibility factor that interacts with hypercapnoeic hypoventilation during maintenance of anaesthesia and in the emergence from anaesthesia, regardless of the agent used.

Differences in Sleep-induced Hypoxia between A/J and DBA/2J Mouse Strains

In obstructive sleep apnea, hypoxic ventilatory sensitivity may affect the degree of hypoxic stress and sleep disruption that occurs in response to upper airway obstruction. We induced (1) sleep-induced hypoxia (SIH) or (2) sleep fragmentation (SF) without hypoxia for 5 days (12-hour light/dark cycle) in two inbred mouse strains with low (A/J) and high (DBA/2J) hypoxic ventilatory sensitivities. During SIH, the time to arousal (26.4 +/- 1.1 vs. 21.3 +/- 1.5 seconds, p<0.025) and the severity of hypoxic exposure (nadir FIO2: 11.5 +/- 0.4 vs. 13.6 +/- 0.1%, p<0.002) was greater in A/J than DBA/2J mice. Furthermore, A/J mice had a greater frequency of hypoxic events (640 +/- 29 vs. 368 +/- 33 events per 24 hours, p<0.001) and total sleep time (47.5 +/- 2.8% vs. 26.5 +/- 2.4% per 24 hours, p<0.0001) during SIH than DBA/2J mice. In contrast, the event characteristics and total sleep time during SF were the same in both strains. Furthermore, in the light phase, both strains showed a longer (p<0.01) time to arousal during SIH and SF compared with the dark phase. We conclude that genetic background can influence respiratory events and sleep architecture during SIH and that the arousal threshold is subject to circadian variation. Our data imply that individuals with low hypoxic sensitivity may be at a greater risk for hypoxia-related complications of obstructive sleep apnea.

Impact of interrupted leptin pathways on ventilatory control

Leptin deficiency in ob/ob mice produces marked depression of the hypercapnic ventilatory response, particularly during sleep. We now extend our previous findings to determine whether 1) leptin deficiency affects the hypoxic ventilatory response and 2) blockade of the downstream excitatory actions of leptin on melanocortin 4 receptors or inhibitory actions on neuropeptide Y (NPY) pathways has an impact on hypercapnic and hypoxic sensitivity. We have found that leptin-deficient ob/ob mice have the same hypoxic ventilatory response as weight-matched wild-type obese mice. There were no differences in the hypoxic sensitivity between agouti yellow mice and weight-matched controls, or NPY-deficient mice and wild-type littermates. Agouti yellow mice, with blocked melanocortin pathways, exhibited a significant depression of the hypercapnic sensitivity compared with weight-matched wild-type controls during non-rapid eye movement sleep (5.8 +/- 0.7 vs. 8.9 +/- 0.7 ml x min(-1) x %CO(2)(-1), P < 0.01), but not during wakefulness. NPY-deficient transgenic mice exhibited a small increase in the hypercapnic ventilatory response compared with wild-type littermates, but this was only present during wakefulness. We conclude that interruption of leptin pathways does not affect hypoxic sensitivity during sleep and wakefulness but that melanocortin 4 blockade is associated with depressed hypercapnic sensitivity in non-rapid eye movement sleep.

Heritable differences in respiratory drive and breathing pattern in mice during anaesthesia and emergence

BACKGROUND

Postanaesthetic hypoxia and ischaemia can lead to postoperative morbidity and mortality. We studied the effect of isoflurane anaesthesia in two inbred mouse strains known for phenotypic differences in breathing pattern and respiratory drive during carbon dioxide challenge and their first-generation offspring (F(1)).

METHODS

Using whole body plethysmography, we assessed respiratory rate (RR) and pressure amplitude (Amp) in male B6 (high responder to hypercapnia), C3 (low responder), and F(1) mice at rest, during anaesthesia with isoflurane, and during recovery from anaesthesia. At each time point, the magnitude and pattern of breathing were determined during hypercapnic challenge (FI(CO(2)) = 0.08). Data (mean (SD)) were analysed by generalized ANOVA with post hoc Bonferroni's correction (P<0.05).

RESULTS

During isoflurane anaesthesia, strain differences between B6 and C3 mice in RR were obscured while differences in Amp persisted. In contrast to baseline RR responses to carbon dioxide were significantly reduced at 0.5 MAC (increase in RR: 175 (33) bpm, 147 (44) bpm, 127 (33) bpm, for B6, C3, and F(1) strains respectively) and completely blocked at 1.5 MAC (change in RR: -3 (10) bpm, -2 (1) bpm, -4 (5) bpm, for B6, C3, and F(1) strains, respectively). During recovery, B6 mice showed a significant increase in RR (77 (33) bpm; P<0.0001) as well as in Amp. This was not observed in either C3 (-22 (31) bpm) or F(1) mice (23 (51) bpm).

CONCLUSION

Isoflurane anaesthesia abolished the strain differences in respiratory drive between B6, C3, and F(1) mice. However, during recovery from anaesthesia, significant strain variation in respiratory drive reappeared and was more pronounced compared with pre-anaesthetic levels. These results suggested, that genetic differences may have minimal contribution to decreased respiratory drive during anaesthesia, but may be a major risk factor for post-operative hypoventilation and the associated morbidity and mortality.

Changes in lung permeability and lung mechanics accompany homeostatic instability in senescent mice

Aging and lung disease are recognized factors that increase mortality risk in subjects exposed to ambient particulate matter (PM). In an effort to understand the mechanisms of enhanced susceptibility, the present study examined an inbred mouse model of senescence to 1) determine changes in lung permeability as animals approach the end-of-life and 2) characterize age-dependent changes in lung mechanics in presenescent and terminally senescent mice. The clearance of technetium-99m (99mTc)-diethylenetriamine pentaacetic acid (DTPA) was used to test the hypothesis that lung permeability increases with age and enhances uptake of soluble components of PM principally during the period several weeks before death in AKR/J mice. Quasistatic pressure-volume curves were conducted on robust and on terminally senescent AKR/J mice several weeks before death to assess the relative importance of lung mechanics. Abrupt body weight loss was used to signal imminent death because it accompanies indexes of physiological aging and terminal senescence. 99mTc-DTPA clearance from the lung 30 min after tracheal instillation was significantly (P < 0.05) enhanced in senescent mice. Age-dependent changes in lung mechanics were indicative of significant (P < 0.05) decrements in lung volume and compliance several weeks before death. Thus, during a period of homeostatic instability leading toward natural death, AKR/J mice showed enhanced permeability of soluble particles despite a decrease in lung volume and concomitant alveolar surface area. These results suggest that pulmonary epithelial-endothelial barrier dysfunction occurs in terminally senescent mice just before death. Furthermore, this senescent-dependent increase in lung permeability may be a contributing factor for increased PM susceptibility in the elderly and patients with lung disease.

Hypercapnic duty cycle is an intermediate physiological phenotype linked to mouse chromosome 5

We hypothesized that upper airway obstruction (UAO) leads to a compensatory increase in the duty cycle [ratio of inspiratory time to respiratory cycle length (Ti/Tt)], which is determined by genetic factors. We examined the compensatory Ti/Tt responses to 1). UAO and hypercapnia among normal individuals and 2). hypercapnia in different inbred strains, C3H/HeJ (C3) and C57BL/6J (B6), and their first- and second-generation (F2) offspring. 3). We then used the compensatory Ti/Tt response in the F2 to determine genetic linkage to the mouse genome. First, normal individuals exhibited a similar increase in the Ti/Tt during periods of hypercapnia (0.11 +/- 0.07) and UAO (0.09 +/- 0.06) compared with unobstructed breathing (P < 0.01). Second, the F2 offspring of C3 and B6 progenitors showed an average Ti/Tt response to 3% CO2 (0.42 +/- 0.005%) that was significantly (P < 0.01) greater than that of the two progenitors. Third, with a peak log of the odds ratio score of 4.4, Ti/Tt responses of F2 offspring are genetically linked to an interval between 58 and 64 centimorgans (cM) on mouse chromosome 5. One gene in the interval, Dagk4 at 57 cM, is polymorphic for C3 and B6 mice. Two other genes, Adrbk2 at 60 cM and Nos1 at 65 cM, have biological plausibility in mechanisms of upper airway patency and chemosensitivity, respectively. In summary, Ti/Tt may serve as an intermediate physiological phenotype for compensatory neuromuscular response mechanisms for maintaining ventilation in the face of UAO and hypoventilation and to help target specific candidate genes that may play a role in the expression of sleep-disordered breathing.

Genetic aspects of breathing: on interactions between hypercapnia and hypoxia

Indeed, specific genes in humans and mice regulate breathing pattern at baseline and breathing control during chemical stimulation. The current review addresses the question of coupling plausible candidate genes to physiological variation in control of breathing. That is, can genes discovered in mice be candidates assigned to similar physiological mechanisms as genetic control of breathing in humans? As an illustration, this review examines the interaction of hypoxia in affecting the hypercapnic ventilatory sensitivity (HCVS) curve in humans and mice. Strain distribution patterns (SDPs) incorporating ten inbred mouse strains demonstrate that hypoxic stimulation likely influences HVCS via an additive mechanism rather than synergy between hypercapnia and hypoxia (i.e. CO(2) potentiation). As a mechanism associated with the chemical control of breathing in humans, the absence of CO(2) potentiation in mice suggests that specific genes interact to establish variation in complex breathing traits among mouse strains and between species. If future studies support the current evidence, the absence of CO(2) potentiation in mice compared with humans suggest a clearly defined species difference, which may depend on alternative hypoxic interactions such as hypometabolic and central neuronal depressive mechanisms in mice. Because the complexity of breathing mechanisms varies with modest adjustments in the environment, gene-targeting strategies that achieve 'one-gene, one-phenotype' results must be complimented with alternative strategies that consider integrating complex respiratory mechanisms with gene-to-gene interactions.

Structural and functional differences of the carotid body between DBA/2J and A/J strains of mice

In a previous study, DBA/2J and A/J inbred mice showed extremely different hypoxic ventilatory responses, suggesting variations in their carotid bodies. We have assessed the morphological and functional differences of the carotid bodies in these mice. Histological examination revealed a clearly delineated carotid body only in the DBA/2J mice. Many typical glomus cells and glomeruli appeared in the DBA/2J but not in the A/J mice. The size of the carotid body in the DBA/2J and A/J mice was 6.3 +/- 0.5 x 10(6) and 1.5 +/- 0.3 x 10(6) micro m(3), respectively. The area immunostained for tyrosine hydroxylase, an estimation of the glomus cell quantity, was four times larger in the DBA/2J mice than in the A/J mice. The individual data points in the DBA/2J mice segregated from those in the A/J mice. ACh increased intracellular Ca(2+) in most clusters (81%) of cultured carotid body cells from the DBA/2J mice, but only in 18% of clusters in the A/J mice. These data suggest that genetic determinants account for the strain differences in the structure and function of the carotid body.

Unstable heart rate and temperature regulation predict mortality in AKR/J mice

Elderly populations face greater risks of mortality when exposed to changes in environmental stress. The purpose of the following study was to develop an age-dependent susceptibility model that achieved the following three goals: 1) to operationally define homeostasis by assessing the stability and periodicity in physical activity, heart rate (HR), and deep body temperature (T(db)), 2) to specify alterations in activity, HR, and T(db) regulation that signal imminent death, and 3) to test the hypothesis that the decay in homeostasis associated with imminent death incorporates the coincident disintegration of multiple physiological systems. To achieve these goals, the circadian regulation of activity, HR, and T(db) was assessed using radiotelemeters implanted in AKR/J (n = 17) inbred mice at approximately 190 days of age. During a 12:12-h light-dark cycle, weekly measurements were obtained at 30-min intervals for 48-h periods until each animal's natural death. The average (+/-SE) life span of surgically treated animals did not differ from untreated controls (319 +/- 12 vs. 319 +/- 14 days). Cardiac and thermal stability were characterized by a circadian periodicity, which oscillated around stable daily averages of 640 +/- 14 beats/min in HR and 36.6 +/- 0.1 degrees C in T(db). Stable HR and T(db) responses were compared with extreme conditions 3 days before death, during which a disintegration of circadian periodicity was coincident with a fall in the daily average HR and T(db) of approximately 29 and approximately 13% lower (i.e., 456 +/- 22 beats/min and 31.7 +/- 0.6 degrees C), respectively. The results further suggested that multiple predictors of cardiac and thermal instability in AK mice, including significant bradycardia, hypothermia, and a loss of circadian periodicity, forecast life span 5-6 wk before expiration.

Differential CO(2)-induced c-fos gene expression in the nucleus tractus solitarii of inbred mouse strains

Genetic determinants confer variation between inbred mouse strains with respect to the magnitude and pattern of ventilation during hypercapnic challenge. Specifically, inheritance patterns derived from low-responsive C3H/HeJ (C3) and high-responsive C57BL/6J (B6) mouse strains suggest that differential hypercapnic ventilatory sensitivity (HCVS) is controlled by two independent genes. The present study also tests whether differential neuronal activity in respiratory control regions of the brain is positively associated with strain variation in HCVS. With the use of whole body plethysmography, ventilation was assessed in C3 and B6 strains at baseline and during 30 min of hypercapnia (inspired CO(2) fraction = 0.15, inspired O(2) fraction = 0.21 in N(2)). Subsequently, in situ hybridization histochemistry was performed to determine changes in c-fos gene expression in the commissural subnucleus of the nucleus tractus solitarius (NTS). During hypercapnia, breathing frequency and tidal volume were significantly (P < 0.01) different between strains: C3 mice showed a slow, deep-breathing pattern relative to a rapid, shallow phenotype of B6 mice. CO(2)-induced increase in c-fos gene expression was significantly (P < 0.01) greater in NTS regions of B6 compared with C3 mice. In this genetic model of differential HCVS, the results suggest that a genomic basis for varied hypercapnic chemoreception or transduction confers greater afferent neuronal activity in the caudal NTS for high-responsive B6 mice compared with low-responsive C3 mice.