Heart rate variability in rodents: uses and caveats in toxicological studies

Heart rate variability (HRV) is a measure of cardiac pacing dynamics that has recently garnered a great deal of interest in environmental health studies. While the use of these measures has become popular, much uncertainty remains in the interpretation of results, both in terms of human and animal research. In humans, HRV endpoints, specifically chronic alterations in baseline HRV patterns, have been reasonably well characterized as prognostic indicators of adverse outcomes for a variety of diseases. However, such information is lacking for reversible HRV changes that may be induced by short-term exposures to environmental toxicants. Furthermore, there are minimal substantive data, either acute or chronic, regarding the pathological interpretation or prognostic value of toxicant-induced changes in HRV in rodents. The present report summarizes the physiological and clinical aspects of HRV, the methodological processes for obtaining these endpoints, and previous human and animal studies in the field of environmental health. Furthermore, we include a discussion of important caveats and recommendations for the interpretation of HRV data in animal research.

Fresh gasoline emissions, not paved road dust, alter cardiac repolarization in ApoE-/- mice

Fresh vehicular emissions potentially represent a ubiquitous environmental concern for cardiovascular health. We compared electrocardiographic effects of fresh gasoline engine emissions with resuspended paved road dust in a mouse model of coronary insufficiency. Apolipoprotein E (ApoE)-/- mice on a high fat diet were exposed by whole-body inhalation to either gasoline emissions at 60 microg/m3 particulate matter (PM), an equivalent atmosphere with particles filtered out of the whole exhaust, or paved road dust at 0.5 and 3.5 mg /m3 for 6 h/d for 3 d. Radiotelemetry recordings of electrocardiogram (ECG) were analyzed for changes in T-wave morphology (QT interval, T-wave amplitude, and T-wave Area). Following exposures, lung lavage and blood samples were obtained to assay for markers of pulmonary and systemic inflammation. No exposure induced significant changes in heart rate and only the high concentration of road dust induced signs of pulmonary inflammation. T-wave area exhibited significant deviation from baseline values during exposure to gasoline exhaust particulates, but not to either concentration of road dust or gasoline emissions sans particulates. Gasoline-exposed mice demonstrated elevated plasma endothelin-1, but did not cause systemic inflammation. These data support the hypothesis that freshly-generated engine emissions, as opposed to resuspended paved road dust, may drive cardiac effects that have been observed at road-sides in the environment. The absence of ECG effects for both very high concentrations of road dust PM and equivalent concentrations of the vapor/gas phase of gasoline engine exhaust further indicate the specific risk conferred by fresh vehicular PM.

Development of a head-out plethysmograph system for non-human primates in an Animal Biosafety Level 3 facility

INTRODUCTION

The term 'select agent' (SA) refers to a list of microorganisms and toxins and are defined as those that have the potential to pose a severe threat to public health and safety (42 C.F.R. Part 73). In order to carry out a research with SAs, an Animal Biosafety Level 3 (ABSL3) containment facility is required. Our newly completed ABSL3 facility is developing protocols for implementing safety and efficacy studies of therapeutics for SAs.

METHODS

The primary purpose of this study was to develop methods for exposing non-human primates (NHP) to aerosolized SAs in the ABSL3 and systematically measure specific ventilatory endpoints (frequency, tidal volume, minute volume, and accumulated volume) using a head-out plethysmograph to more precisely control dosimetry. This report details the equipment and protocols used to conduct such studies within a containment facility.

RESULTS

After validating the performance of the plethysmography system, we successfully exposed NHPs to an agent using the integrated plethysmography system. The system enabled an acquisition and analysis of ventilatory characteristics, facilitating accurate estimations of the inhaled dose.

DISCUSSION

This system will have clear uses in the development of novel therapeutics and vaccines for the treatment of SAs in NHPs.

Gasoline exhaust emissions induce vascular remodeling pathways involved in atherosclerosis

Epidemiological evidence indicates that environmental air pollutants are positively associated with the development of chronic vascular disease; however, the mechanisms involved have not been fully elucidated. In the present study we examined molecular pathways associated with chronic vascular disease in atherosclerosis-prone apolipoprotein E-deficient (ApoE(-/-)) mice, including markers of vascular remodeling and oxidative stress, in response to exposure to the ubiquitous environmental pollutant, gasoline engine emissions. ApoE(-/-) mice, on a high-fat diet, were exposed by inhalation to either filtered air; 8, 40, or 60 mug/m(3) particulate matter whole exhaust; or filtered exhaust with gases matching the 60-mug/m(3) concentration, for 7 weeks. Aortas and plasma were collected and assayed for changes in histochemical markers, real-time reverse transcriptase-polymerase chain reaction, and indicators of oxidative damage. Inhalational exposure to gasoline engine emissions resulted in increased aortic mRNA expression of matrix metalloproteinase-3 (MMP-3), MMP-7, and MMP-9, tissue inhibitor of metalloproteinases-2, endothelin-1 and heme oxygenase-1 in ApoE(-/-) mice; increased aortic MMP-9 protein levels were confirmed through immunohistochemistry. Elevated reactive oxygen species were also observed in arteries from exposed animals, despite absence of plasma markers. Similar findings were also observed in the aortas of ApoE(-/-) mice exposed to particle-filtered atmosphere, implicating the gaseous components of the whole exhaust in mediating the expression of markers associated with the vasculopathy. These findings demonstrate that exposure to gasoline engine emissions results in the transcriptional upregulation of factors associated with vascular remodeling, as well as increased markers of vascular oxidative stress, which may contribute to the progression of atherosclerosis and reduced stability of vulnerable plaques.

Characterization of shock in a hamster model of hantavirus infection

Human hantavirus cardiopulmonary syndrome (HCPS) due to Andes, sin nombre and other hantaviruses is characterized by severe pulmonary capillary leak and cardiogenic shock. Hamsters, the only animal manifesting HCPS-like disease, were instrumented with radiotelemeters that enabled ambulatory intracarotid blood pressure recording within an animal biosafety level-4 facility. Following infection with Andes virus, blood pressure and heart rate decreased slowly in a biphasic manner during the first 7 days of infection, followed by a rapid fall in pressure and rapid increase in heart rate during the 10-20 h preceding death on day 9 or 10. The preterminal narrowing of pulse pressure was consistent with a cardiogenic impairment. Heart rate variability analysis implicated increased sympathetic nervous system activity as seen in human HCPS. The hamster model of HCPS mimics not only the pulmonary capillary leak but also the hypotension characteristic of human HCPS.

Modulators of cigarette smoke-induced pulmonary emphysema in A/J mice

Mice develop pulmonary emphysema after chronic exposure to cigarette smoke (CS). In this study, the influence of gender, exposure duration, and concentration of CS on emphysema, pulmonary function, inflammation, markers of toxicity, and matrix metalloproteinase (MMP) activity was examined in A/J mice. Mice were exposed to CS at either 100 or 250 mg total particulate material/m(3) (CS-100 or CS-250, respectively) for 10, 16, or 22 weeks. Evidence of emphysema was first seen in female mice after 10 weeks of exposure to CS-250, while male mice did not develop emphysema until 16 weeks. Female mice exposed to CS-100 did not have emphysema until 16 weeks, suggesting that disease development depends on the concentration and duration of exposure. Airflow obstruction and increased pulmonary compliance were observed in mice exposed to CS-250 for 22 weeks. Decreased elasticity was likely the major contributor to airflow obstruction because substantial remodeling of the conducting airways, beyond mild mucous cell hyperplasia, was lacking. Exposure to CS increased the number of macrophages, neutrophils, lymphocytes (B cells and activated CD4- and CD8-positive T cells), and activity of MMP-2 and -9 in the bronchoalveolar lavage fluid (BALF). Treatment with antioxidants N-acetylcysteine or epigallocatechin gallate (EGCG) did not decrease emphysema severity, but EGCG slightly decreased BALF inflammatory cell numbers and lactate dehydrogenase activity. Inflammation and emphysema persisted after a 17-week recovery period following exposure to CS-250 for 22 weeks. The similarities of this model to the human disease make it promising for studying disease pathogenesis and assessing new therapeutic interventions.

Low-level subchronic exposure to wood smoke exacerbates inflammatory responses in allergic rats

Epidemiological studies have implicated wood smoke as a risk factor for exacerbating asthma. However, comparisons of findings in animal models with those in humans are currently not possible, because detailed clinically relevant measurements of pulmonary function are not available in animal studies. Brown Norway rats were immunized with ovalbumin and exposed to either filtered air or wood smoke at 1 mg particulate matter/m(3) for 70 days and challenged with allergen during the last 4 days of exposure. Baseline values for dynamic lung compliance were lower while functional residual capacity was increased in rats exposed to wood smoke compared to rats exposed to filtered air. IFN-gamma levels were reduced and IL-4 levels increased in the bronchoalveolar lavage fluid and blood plasma, inflammatory lesions in the lungs were 21% greater, and airway mucous cells/mm basal lamina were non-significantly increased in rats exposed to wood smoke compared to controls. Collectively, these studies suggest that the pulmonary function was affected in rats by exposure to wood smoke and this decline was associated with only minor increases in inflammation of the lung. Therefore, this animal model may be useful to elucidate the mechanisms of the decline in pulmonary function caused by environmental pollutants when asthmatics are exposed to allergen.

Nonparticulate components of diesel exhaust promote constriction in coronary arteries from ApoE-/- mice

Air pollution is positively associated with increased daily incidence of myocardial infarction and cardiovascular mortality. We hypothesize that air pollutants, primarily vapor phase organic compounds, cause an enhancement of coronary vascular constriction. Such events may predispose susceptible individuals to anginal symptoms and/or exacerbation of infarction. To develop this hypothesis, we studied the effects of nonparticulate diesel exhaust constituents on (1) electrocardiographic traces from ApoE-/- mice exposed whole-body and (2) isolated, pressurized septal coronary arteries from ApoE-/- mice. ApoE-/- mice were implanted with radiotelemetry devices to assess electrocardiogram (ECG) waveforms continuously throughout exposures (6 h/day x 3 days) to diesel exhaust (0.5 and 3.6 mg/m3) in whole-body inhalation chambers with or without particulates filtered. Significant bradycardia and T-wave depression were observed, regardless of the presence of particulates. Pulmonary inflammation was present only in the whole exhaust-exposed animals at the highest concentration. Fresh diesel exhaust or air was bubbled through the physiologic saline tissue bath prior to experiments to enable the isolated tissue exposure; exposed saline contained elevated levels of several volatile carbonyls and alkanes, but low to absent levels of polycyclic aromatic hydrocarbons. Vessels were then assayed for constrictive and dilatory function. Diesel components enhanced the vasoconstrictive effects of endothelin-1 and reduced the dilatory response to sodium nitroprusside. These data demonstrate that nonparticulate compounds in whole diesel exhaust elicit ECG changes consistent with myocardial ischemia. Furthermore, the volatile organic compounds in the vapor phase caused enhanced constriction and reduced dilatation in isolated coronary arteries caused by nonparticulate components of diesel exhaust.

Acute and chronic cardiovascular effects of intermittent hypoxia in C57BL/6J mice

We investigated the effects of 1) acute hypoxia and 2) 5 wk of chronic intermittent hypoxia (IH) on the systemic and pulmonary circulations of C57BL/6J mice. Mice were chronically instrumented with either femoral artery or right ventricular catheters. In response to acute hypoxia (4 min of 10% O2; n = 6), systemic arterial blood pressure fell (P < 0.005) from 107.7 +/- 2.5 to 84.7 +/- 6.5 mmHg, whereas right ventricular pressure increased (P < 0.005) from 11.7 +/- 0.8 to 14.9 +/- 1.3 mmHg. Another cohort of mice was then exposed to IH for 5 wk (O2 nadir = 5%, 60-s cycles, 12 h/day) and then implanted with catheters. In response to 5 wk of chronic IH, mice (n = 8) increased systemic blood pressure by 7.5 mmHg, left ventricle + septum weight by 32.2 +/- 7.5 x 10(-2) g/100 g body wt (P < 0.015), and right ventricle weight by 19.3 +/- 3.2 x 10(-2) g/100 g body wt (P < 0.001), resulting in a 14% increase in the right ventricle/left ventricle + septum weight (P < 0.005). We conclude that in C57BL/6J mice 1) acute hypoxia causes opposite effects on the pulmonary and systemic circulations, leading to preferential loading of the right heart; and 2) chronic IH in mice results in mild to moderate systemic and pulmonary hypertension, with resultant left- and right-sided ventricular hypertrophy.

Heart rate variability responses to hypoxic and hypercapnic exposures in different mouse strains

Heart rate variability (HRV) is a well-characterized, noninvasive means of assessing cardiac autonomic nervous system activity. This study examines the basic cardiac responses to hypoxic and hypercapnic challenges in seven strains of commonly used inbred mice (A/J, BALB/cJ, C3H/HeJ, C57BL/6J, CBA/J, DBA/2J, and FVB/J). Adult male mice, 8-12 wk of age, were chronically instrumented to a femoral artery catheter for the continuous measurement of systemic arterial blood pressure and heart rate. Mice were exposed to multiple 4-min periods of hypoxia (10% O2), hypercapnia (5% CO2), and combined hypoxia/hypercapnia (10% O2 + 5% CO2). HRV was derived from pulse intervals of the blood pressure tracings. Hypoxia induced increases in high-frequency HRV power and decreased low-frequency (LF) HRV power in most strains. Hypercapnia led to decreased high-frequency HRV power and increased LF HRV power in most strains. Strain differences were most notable in regard to the concomitant exposures of hypoxia and hypercapnia, with FVB/J mice mirroring their own response to hypercapnia alone, whereas CBA/J mice mirrored their own responses to hypoxia. As blood pressure is most likely the driving factor for heart rate changes via the baroreflex pathway, it is interesting that LF, considered to reflect cardiac sympathetic activity, was negatively correlated with heart rate, suggesting that LF changes are driven by baroreflex oscillation and not necessarily by absolute sympathetic or parasympathetic activity to the heart. These findings suggest that genetic background can influence the centrally mediated cardiovascular responses to basic hypoxic and hypercapnic challenges.

Persistence of mitochondrial toxicity in hearts of female B6C3F1 mice exposed in utero to 3'-azido-3'-deoxythymidine

Cardiac toxicity has been associated with HIV infection and exposure to nucleoside reverse transcriptase inhibitors (NRTIs), but the role of the latter in the development of cardiac disease of HIV-infected patients is uncertain. To investigate the cardiotoxicity of transplacentally administered zidovudine (AZT) or AZT plus lamivudine (3TC) in the absence of HIV infection, we evaluated several biomarkers of cardiac mitochondrial structure and cardiac structure and function in a B6C3F1 mouse model. In utero exposure to AZT-3TC resulted in ultrastructural pathology, loss of mitochondria, and altered echocardiographic measurements in newborn mice. Cardiac pathology and dysfunction persisted into the adult life of female mice exposed in utero to AZT, as evidenced by significant dose-dependent heart enlargement, clusters of atypical mitochondria and myofibril alterations, significantly increased cytochrome c oxidase activity, and significantly higher numbers of mutations in mitochondrial tRNA genes compared with unexposed controls at 18 to 24 mo of age. These data led to the hypothesis that the long-term pathology of peri-natal exposure to these NRTIs is related to persistent mitochondrial DNA mutations in cardiac tissue; that is, the primary damage during drug treatment is mutational (as opposed to affecting polymerase gamma and/or other mitochondrial elements) and leads over time to delayed, progressive cardiotoxicity.

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.

Cardiac and thermoregulatory responses to inhaled pollutants in healthy and compromised rodents: modulation via interaction with environmental factors

Rodents often demonstrate a profound depression in physiological function following acute exposure to toxic xenobiotic agents. This effect, termed the hypothermic response, is primarily characterized by significant decreases in core temperature and heart rate and is generally accompanied by similar deficits in other important functional parameters. This response appears to be remarkably consistent across a wide variety of toxic agents and exposure regimens; however, the magnitude and duration of the induced effects may be modulated by changes in dose, animal mass, and environmental conditions. While the initiating stimulus and underlying mechanism(s) remains elusive, this response may represent an inherent reflexive pattern that is unique to the rodent and serves to attenuate the induced toxicity. Given that rodents are the primary animal species used in toxicological studies, it is important to consider this hypothermic response and its modulatory factors when interpreting the results of such studies and extrapolating those results to man.

Cardiovascular Effects of Inhaled Diesel Exhaust in Spontaneously Hypertensive Rats

Particulate matter air pollution is associated with increased cardiovascular mortality. The present study examined the cardiac effects of diesel exhaust exposure in spontaneously hypertensive rats. These rats (4 mo old, n = 6 males and 4-6 females/concentration) were exposed to one of five diesel exhaust levels (0, 30, 100, 300, and 1000 micrograms particles/m3) for 6 h per day for 7 d. Electrocardiographic measurements were obtained by radiotelemetry beginning 3 d prior to exposure and ending 4 d after exposure cessation. Control rats displayed a reduced daytime heart rate from the beginning of the protocol, whereas exposed rats maintained a significantly elevated heart rate throughout the exposure. Daytime heart rate values for male control rats averaged 265 +/- 5 beats/min (mean +/- standard error [SE]), whereas values for exposed rats averaged 290 +/- 7 beats/min. This difference persisted during the evenings of the exposure period but was not observed at any time during the preexposure or postexposure periods. The PQ interval, an index of atrioventricular node sensitivity, was significantly prolonged among exposed animals in a concentration-dependent manner. Increased heart rate with prolongation of the PQ interval may represent a substrate for ventricular arrhythmias. These results concur with previous reports suggesting that realistic exposure concentrations of air pollution affect the pacemaking system of rats.

Phenotypic differences in the hemodynamic response during REM sleep in six strains of inbred mice

The pattern of cardiovascular changes that occur at nighttime can have an impact on morbidity and mortality. Rapid-eye-movement (REM) sleep, in particular, represents a period of increased risk due to marked cardiovascular instability. We hypothesized that genetic differences between inbred strains of mice would affect the phenotypic expression of cardiovascular responses that occur in REM sleep. We monitored polysomnography and arterial blood pressure (P(SA)) simultaneously in six inbred strains of mice as they naturally cycled through sleep/wake states. Two strains elevated their P(SA) above non-REM (NREM) levels for 57.9 +/- 6.6% (BALB/cJ) and 51.8 +/- 8.4% (DBA/2J) of the REM period and exhibited a significant (P < 0.05) number of P(SA) surges greater than 10 mmHg (0.78 +/- 0.36 surges/min for BALB/cJ; 0.63 +/- 0.13 surges/min for DBA/2J). Despite similar P(SA) responses, the DBA/2J strain exhibited a decreased heart rate and the BALB/cJ strain exhibited an increased heart rate during REM sleep. The four other strains (A/J, C57BL/6J, C3H/HeJ, and CBA/J) exhibited a significant hypotensive response associated with no change in heart rate in three of the strains and a significant decrease in heart rate in the A/J strain. The overall variability in P(SA) during REM sleep was significantly greater in the C3H/HeJ strain (26.8 +/- 2.0 mmHg; P < 0.0125) compared with the other five strains. We conclude that genetic background contributes to the magnitude, variability, and arterial baroreceptor buffering capacity of cardiovascular responses during REM sleep.

Cardiac and thermoregulatory effects of instilled particulate matter-associated transition metals in healthy and cardiopulmonary-compromised rats

Particulate matter air pollution has been associated with cardiopulmonary morbidity and mortality in many recent epidemiological studies. Previous toxicological research has demonstrated profound cardiac and thermoregulatory changes in rats following exposure to residual oil fly ash (ROFA), a combustion-derived particulate. The response to ROFA appeared biphasic, consisting of both immediate (0-6 h) and delayed (24-96 h) bradycardia and hypothermia. Other studies have demonstrated that much of the pulmonary toxicity of ROFA was caused by its constitutive transition metals, namely, Fe, Ni, and V. This study examined the contributions of these metals to the observed cardiac and thermoregulatory changes caused by ROFA in conscious, unrestrained rats. Prior to exposure, each animal was surgically implanted with a radiotelemetry device capable of continuously monitoring heart rate, electrocardiographic, and core temperature data. Individual metals were intratracheally instilled in healthy rats (n = 4 per metal species) and in rats with monocrotaline (MCT; 60 mg/kg)-induced pulmonary hypertension (n = 10 per metal species); combinations of metals were instilled in MCT-treated rats only (n = 6 per combination of metal species). Metals were administered in doses equivalent to those found in the highest dose of ROFA used in previous studies, that is, 105 microg Fe(2)(SO(4))(3), 263 microg NiSO(4), and 245 microg VSO(4). Healthy and MCT-treated rats demonstrated similar responses to metals. Fe caused little response, whereas V caused marked bradycardia, arrhythmogenesis, and hypothermia immediately following instillation and lasting approximately 6 h. Ni caused no immediate response, but induced a delayed bradycardia, arrhythmogenesis, and hypothermia that began approximately 24 h after instillation and lasted for several days. When instilled in combination, Ni appeared to exacerbate the immediate effects of V, whereas Fe attenuated them. These data suggest that the biphasic response to instilled ROFA may result from a summation of the temporally different effects of V and Ni.

A model of sleep-disordered breathing in the C57BL/6J mouse

To investigate the pathophysiological sequelae of sleep-disordered breathing (SDB), we have developed a mouse model in which hypoxia was induced during periods of sleep and was removed in response to arousal or wakefulness. An on-line sleep-wake detection system, based on the frequency and amplitude of electroencephalograph and electromyograph recordings, served to trigger intermittent hypoxia during periods of sleep. In adult male C57BL/6J mice (n = 5), the sleep-wake detection system accurately assessed wakefulness (97.2 +/- 1.1%), non-rapid eye movement (NREM) sleep (96.0 +/- 0.9%) and rapid eye movement (REM) sleep (85.6 +/- 5.0%). After 5 consecutive days of SDB, 554 +/- 29 (SE) hypoxic events were recorded over a 24-h period at a rate of 63.6 +/- 2.6 events/h of sleep and with a duration of 28.2 +/- 0.7 s. The mean nadir of fraction of inspired O(2) (FI(O(2))) on day 5 was 13.2 +/- 0.1%, and 137.1 +/- 13.2 of the events had a nadir FI(O(2)) <10% O(2). Arterial blood gases confirmed that hypoxia of this magnitude lead to a significant degree of hypoxemia. Furthermore, 5 days of SDB were associated with decreases in both NREM and REM sleep during the light phase compared with the 24-h postintervention period. We conclude that our murine model of SDB mimics the rate and magnitude of sleep-induced hypoxia, sleep fragmentation, and reduction in total sleep time found in patients with moderate to severe SDB in the clinical setting.

Cardiovascular and thermoregulatory effects of inhaled PM-associated transition metals: a potential interaction between nickel and vanadium sulfate

Recent epidemiological studies have shown an association between daily morbidity and mortality and ambient particulate matter (PM) air pollution. It has been proposed that bioavailable metal constituents of PM are responsible for many of the reported adverse health effects. Studies of instilled residual oil fly ash (ROFA) demonstrated immediate and delayed responses, consisting of bradycardia, hypothermia, and arrhythmogenesis in conscious, unrestrained rats. Further investigation of instilled ROFA-associated transition metals showed that vanadium (V) induced the immediate responses, while nickel (Ni) was responsible for the delayed effects. Furthermore, Ni potentiated the immediate effects caused by V when administered concomitantly. The present study examined the responses to these metals in a whole-body inhalation exposure. To ensure valid dosimetric comparisons with instillation studies, 4 target exposure concentrations ranging from 0.3-2.4 mg/m(3) were used to incorporate estimates of total inhalation dose derived using different ventilatory parameters. Rats were implanted with radiotelemetry transmitters to continuously acquire heart rate (HR), core temperature (T(CO)), and electrocardiographic data throughout the exposure. Animals were exposed to aerosolized Ni, V, or Ni + V for 6 h per day x 4 days, after which serum and bronchoalveolar lavage samples were taken. Even at the highest concentration, V failed to induce any significant change in HR or T(CO). Ni caused delayed bradycardia, hypothermia, and arrhythmogenesis at concentrations > 1.2 mg/m(3). When combined, Ni and V produced observable delayed effects at 0.5 mg/m(3) and potentiated responses at 1.3 mg/m(3), greater than were produced by the highest concentration of Ni (2.1 mg/m(3)) alone. These results indicate a possible synergistic relationship between inhaled Ni and V, and provide insight into potential interactions regarding the toxicity of PM-associated metals.

Cardiovascular and systemic responses to inhaled pollutants in rodents: effects of ozone and particulate matter

Striking similarities have been observed in a number of extrapulmonary responses of rodents to seemingly disparate ambient pollutants. These responses are often characterized by primary decreases in important indices of cardiac and thermoregulatory function, along with secondary decreases in associated parameters. For example, when rats are exposed to typical experimental concentrations of ozone (O(3), they demonstrate robust and consistent decreases in heart rate (HR) ranging from 50 to 100 beats per minute, whereas core temperature (T(co) often falls 1.5-2.5 degrees C. Other related indices, such as metabolism, minute ventilation, blood pressure, and cardiac output, appear to exhibit similar deficits. The magnitudes of the observed decreases may be modulated by changes in experimental conditions and appear to vary inversely with both ambient temperature and body mass. More recent studies in which both healthy and compromised rats were exposed to either particulate matter or its specific components yielded similar results. The agents studied included representative examples of ambient, combustion, and natural source particles, along with individual or combined exposures to their primary metallic constituents. In addition to the substantial decreases in HR and T(co), similar to those seen with the O(3)-exposed rats, these animals also displayed numerous adverse changes in electrocardiographic waveforms and cardiac rhythm, frequently resulting in fatal outcomes. Although there is only limited experimental evidence that addresses the underlying mechanisms of these responses, there is some indication that they may be related to stimulation of pulmonary irritant receptors and that they may be at least partially mediated via the parasympathetic nervous system.

The spontaneously hypertensive rat as a model of human cardiovascular disease: evidence of exacerbated cardiopulmonary injury and oxidative stress from inhaled emission particulate matter

Cardiovascular disease is considered a probable risk factor of particulate matter (PM)-related mortality and morbidity. It was hypothesized that rats with hereditary systemic hypertension and underlying cardiac disease would be more susceptible than healthy normotensive rats to pulmonary injury from inhaled residual oil fly ash (ROFA) PM. Eight spontaneously hypertensive (SH) and eight normotensive Wistar-Kyoto (WKY) rats (12-13 weeks old) were implanted with radiotelemetry transmitters on Day -10 for measurement of electrocardiographic (ECG) waveforms. These and other nonimplanted rats were exposed to filtered air or ROFA (containing leachable toxic levels of metals) on Day 0 by nose-only inhalation (ROFA, 15 mg/m(3) x 6 h/day x 3 days). ECGs were monitored during both exposure and nonexposure periods. At 0 or 18 h post-ROFA exposure, rats were assessed for airway hyperreactivity, pulmonary and cardiac histological lesions, bronchoalveolar lavage fluid (BALF) markers of lung injury, oxidative stress, and cytokine gene expression. Comparisons were made in two areas: (1) underlying cardiopulmonary complications of control SH rats in comparison to control WKY rats; and (2) ROFA-induced cardiopulmonary injury/inflammation and oxidative burden. With respect to the first area, control air-exposed SH rats had higher lung and left ventricular weights when compared to age-matched WKY rats. SH rats had hyporeactive airways to acetylcholine challenge. Lung histology revealed the presence of activated macrophages, neutrophils, and hemorrhage in control SHrats. Consistently, levels of BALF protein, macrophages, neutrophils, and red blood cells were also higher in SH rats. Thiobarbituric acid-reactive material in the BALF of air-exposed SH rats was significantly higher than that of WKY rats. Lung inflammation and lesions were mirrored in the higher basal levels of pulmonary cytokine mRNA expression. Cardiomyopathy and monocytic cell infiltration were apparent in the left ventricle of SH rats, along with increased cytokine expression. ECG demonstrated a depressed ST segment area in SH rats. With regard to the second area of comparison (ROFA-exposed rats), pulmonary histology indicated a slightly exacerbated pulmonary lesions including inflammatory response to ROFA in SH rats compared to WKY rats and ROFA-induced increases in BALF protein and albumin were significantly higher in SH rats than in WKY rats. In addition, ROFA caused an increase in BALF red blood cells in SH rats, indicating increased hemorrhage in the alveolar parenchyma. The number of alveolar macrophages increased more dramatically in SH rats following ROFA exposure, whereas neutrophils increased similarly in both strains. Despite greater pulmonary injury in SH rats, ROFA-induced increases in BALF GSH, ascorbate, and uric acid were attenuated when compared to WKY rats. ROFA inhalation exposure was associated with similar increases in pulmonary mRNA expression of IL-6, cellular fibronectin, and glucose-6-phosphate dehydrogenase (relative to that of beta-actin) in both rat strains. The expression of MIP-2 was increased in WKY but attenuated in SH rats. Thus, SH rats have underlying cardiac and pulmonary complications. When exposed to ROFA, SH rats exhibited exacerbated pulmonary injury, an attenuated antioxidant response, and acute depression in ST segment area of ECG, which is consistent with a greater susceptibility to adverse health effects of fugitive combustion PM. This study shows that the SH rat is a potentially useful model of genetically determined susceptibility with pulmonary and cardiovascular complications.

Lung injury from intratracheal and inhalation exposures to residual oil fly ash in a rat model of monocrotaline-induced pulmonary hypertension

A rat model of monocrotaline (MCT)-induced pulmonary injury/hypertension has been recently used in particulate matter (PM) health effects studies, however, results have been equivocal. Neither the mechanism by which mortality occurs in this model nor the variation in response due to differences in PM exposure protocols (i.e., a bolus dose delivered intratracheally versus a similar cumulative dose inhaled over three days) have been fully investigated. Sprague Dawley rats (SD, 60 d old; 250-300 g) were injected with either saline (healthy) or MCT, 60 mg/kg, i.p. (to induce pulmonary injury/hypertension). Ten days later they were exposed to residual oil fly ash (ROFA), either intratracheally (IT; saline, 0.83 or 3.33 mg/kg) or by nose-only inhalation (15 mg/m3 x 6 h/d x 3 d). Lung histology, pulmonary cytokine gene expression (0 and 18 h postinhalation), and bronchoalveolar lavage fluid (BALF) markers of injury were analyzed (24 and 96 h post-IT; or 18 h post-inhalation). Data comparisons examined three primary aspects, 1) ROFA IT versus inhalation effects in healthy rats; 2) pulmonary injury caused by MCT; and 3) exacerbation of ROFA effects in MCT rats. In the first aspect, pulmonary histological lesions following ROFA inhalation in healthy rats were characterized by edema, inflammatory cell infiltration, and thickening of alveolar walls. Increases in BALF markers of lung injury and inflammation were apparent in ROFA-IT or nose-only exposed healthy rats. Increased IL-6, and MIP-2 expression were also apparent in healthy rats following ROFA inhalation. In regards to the second aspect, MCT rats exposed to saline or air showed perivascular inflammatory cell infiltrates, increased presence of large macrophages, and alveolar thickening. Consistently, BALF protein, and inflammatory markers (macrophage and neutrophil counts) were elevated indicating pulmonary injury. In regards to the third aspect, 58% of MCT rats exposed to ROFA IT died within 96 h regardless of the dose. No mortality was observed using the inhalation protocol. ROFA inhalation in MCT rats caused exacerbation of lung lesions such as increased edema, alveolar wall thickening, and inflammatory cell infiltration. This exacerbation was also evident in terms of additive or more than additive increases in BALF neutrophils, macrophages and eosinophils. IL-6 but not MIP-2 expression was more than additive in MCT rats, and persisted over 18 h following ROFA. IL-10 and cellular fibronectin expression was only increased in MCT rats exposed to ROFA. In summary, only the bolus IT ROFA caused mortality in the rat model of lung injury/hypertension. Exacerbation of histological lesions and cytokine mRNA expression were most reflective of increased ROFA susceptibility in this model.

Cardiac arrhythmia induction after exposure to residual oil fly ash particles in a rodent model of pulmonary hypertension

Recent epidemiological studies have reported a positive association between exposure to ambient concentrations of particulate matter (PM) and the incidence of cardiopulmonary-related morbidity and mortality. The present study examined the effects of fugitive residual oil fly ash (ROFA) PM on cardiac arrhythmia induction in healthy and cardiopulmonary-compromised rodents. Male Sprague-Dawley rats were implanted with radiotelemetry transmitters capable of monitoring the electrocardiogram and were subjected to one of two treatment regimens. Rats in the first treatment regimen (n = 16) served as normal control animals whereas rats in the second treatment regimen (n = 16) were injected with monocrotaline (MCT, 60 mg/kg, ip) to induce pulmonary vascular inflammation and hypertension and served as a model of cardiopulmonary disease. Rats within each treatment regimen were equally divided into four dose groups (0.0, 0.25, 1.0, 2.5 mg ROFA), instilled intratracheally, and monitored for 96 h. In the animals in the first treatment regimen, ROFA instillation caused dose-related increases in the incidence and duration of serious arrhythmic events that appeared to be associated with impaired atrioventricular conduction and myocardial hypoxia. There were no lethalities in the normal animals following ROFA instillation. The frequency and severity of arrhythmias were greatly exacerbated in the MCT-treated animals in the second treatment regimen and were accompanied by one, three, and two deaths in the low-, medium-, and high-dose groups, respectively. The results of the present study demonstrate substantial cardiac effects in normal and compromised rats after exposure to ROFA PM and implicate both conductive and hypoxemic arrhythmogenic mechanisms in the observed cardiac-related lethalities. These results support previous epidemiological studies that suggest a link between preexisting cardiopulmonary disease and potentiation of adverse health effects following exposure to anthropogenic particulates.

Impact of the hypothermic response in inhalation toxicology studies

Previous studies from this laboratory showed that the decreases in Tco and associated functional parameters often observed in rodents following exposure to xenobiotic agents are capable of modulating the subsequent toxic response and that the magnitude of this induced hypothermic response may itself be modified by a number of experimental conditions. A moderate hypothermic response, characterized by a temperature drop of approximately 2 degrees C, appears to afford the optimal protection. Studies in which exposures occur through inhalation of harmful gases or particles present a special set of problems. In such studies, the dose of the toxic agent to which the animal is exposed is a function of the concentration of the agent in the atmosphere and the minute ventilation of the animal. Although ambient concentrations is generally held constant in laboratory studies, minute ventilation varies directly with metabolism, and both of these parameters may change significantly across experimental conditions. Thus, at low Tas, metabolism and minute ventilation are relatively high and uptake of inhalable toxic agents is increased. However, the development of the hypothermic response during the exposure entails a directly correlated reduction in these parameters and, presumably, in dose. For the most part, inhalation toxicological studies are conducted using resting animals or exercising humans. Animals are sometimes concurrently exposed to CO2 to simulate the increased ventilation of exercise and more closely mimic human studies. The experimental protocols employed in the above inhalation studies permitted examination of (1) the impact of species, size, handling stress, and changes in Ta on both the induced hypothermic response and the concomitant pulmonary toxicity; (2) the additive impact of exercise stress on O3 toxicity; and (3) the toxicity of ambient-derived particulate matter in normal rats and in rats with preexisting pulmonary inflammation. The results of these studies demonstrate that the magnitude of the induced hypothermic response is directly proportional to the uptake of the toxic agent by the lung and inversely proportional to the mass of the animal and the ambient temperature at which the exposure is conducted. The hypothermic response is sensitive to a number of experimental stresses including handling and changes in cage conditions. Exercise attenuates the hypothermic response, whereas CO2-stimulated increases in ventilation employed as an exercise surrogate may potentiate the response. Toxic exposures conducted in animals with lung disease or compromised pulmonary function may induce a severe hypothermic response while comparable exposures in normal animals produce only mild or moderate responses. In general, the development of the hypothermic response in the presence of ambient pollutants serves to decrease the minute ventilation of the animal and therefore limits the uptake and dose of the airborne toxicant. The results of these inhalation studies support our previous conclusions concerning the impact of the hypothermic response on toxicity and emphasize the need to monitor and incorporate these changes in functional parameters into analyses of toxicological data. Furthermore, because humans do not demonstrate a robust hypothermic response following exposure to toxic agents, extrapolation of the results obtained from animal studies and comparisons with data from human studies are considerably more complicated.