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* From the Department of Pathology, University of British Columbia, Vancouver, BC, Canada.
Correspondence to: Joanne L. Wright, MD, Department of Pathology, University of British Columbia, 2211 Westbrook Mall, Vancouver, BC, V6T 2B5 Canada; e-mail: jlwright{at}interchange.ubc.ca
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Abstract |
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 TOP Abstract Description of a Guinea...Comparison of Guinea Pig...Advantages of the Guinea...Problems and Prospects of...References |
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Key Words: animal models • cigarette smoke • COPD • emphysema • guinea pig • pulmonary hypertension • species differences
Although there are many potential causes of airflow obstruction, this article will be restricted to discussions of airflow obstruction that is related to cigarette smoke, hereafter termed COPD. Animal experimentation has been crucial to our understanding of the mechanisms of COPD, and both large and small animals have been utilized. Although the guinea pig has long been recognized as a very useful model in investigations related to asthma, it is only in the last few years that it has been used extensively in research related to COPD.
Investigations have centered on the lung parenchyma, most particularly examining for evidence of emphysematous destruction, although some studies have been directed specifically toward the effects of smoke on the airways. It is appropriate to note that the definition of emphysema in animal models is slightly different from that used in humans. In animals, emphysema is defined as follows: "... an abnormal state of the lungs in which there is enlargement of the airspaces distal to the terminal bronchiole. Airspace enlargement should be determined qualitatively in appropriate preparations and quantitatively by stereologic methods. Physiologic descriptions may or may not be used, depending on the purposes of the investigations, but in view of the great variability of the pathophysiology of human emphysema, it is not possible to establish specific physiologic requirements for animal models of emphysema."1 Many investigators have used both morphologic and physiologic modalities and have thus provided additional information in the form of pathophysiologic correlations.
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Description of a Guinea Pig Model |
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TOPAbstractDescription of a Guinea...Comparison of Guinea Pig...Advantages of the Guinea...Problems and Prospects of...References |
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In our laboratory, we selected the guinea pig as a model in which to investigate the effects of acute and chronic exposure to cigarette smoke on lung structure and function. After a short-term (10 min) exposure to smoke, there was airflow obstruction with air trapping, which was associated with neutrophilia in the peripheral blood and in the lung lavage fluid.14 Chronic exposure to cigarette smoke for periods of greater than 3 months was shown to produce emphysematous airspace enlargement.15 The increase appeared to be progressive with increasing lengths of exposure, with an increasing ratio to the measurements in the control animals. The mean chord length of alveoli and alveolar ducts were both increased, and this can be appreciated histologically in Fig 1 . We found that the increased airspace size was associated with gas trapping and increased total lung volumes, and with a shift of both the pressure-volume and flow-volume curves.15 A cessation of smoke exposure appeared to halt, but not reverse, emphysematous lung enlargement.16
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If the lung is destroyed, it may also be important to determine whether the destruction occurs randomly or occurs in a systematic fashion. Early workers suggested that the pores of Kohn, a normal anatomic feature responsible for collateral ventilation, were a site of destruction.20 To examine this hypothesis, we examined the lungs of guinea pigs that had been exposed to smoke for 12 months, and we found an increased number of alveolar holes, with a shift toward larger and more irregular hole configurations. These data imply that lung destruction results from the formation of multiple holes rather than from a simple enlargement of existing pores of Kohn.21
In humans, there is goblet cell metaplasia of the airway epithelium of the membranous bronchioles of subjects who smoke cigarettes.22
In animal models, the correlate is secretory cell metaplasia, which is detected by histochemical staining against neutral and acid mucins. Guinea pigs exposed to smoke for periods of 
3 months develop secretory cell metaplasia.23
We presume that this finding is indicative of a response to chronic irritation.
We also have used the guinea pig model to examine the effect of cigarette smoke on the pulmonary vasculature. In humans, a significant number of chronic smokers develop pulmonary hypertension,24 and this appears also to be true in the guinea pig.25 Interestingly, hypertension in the guinea pig does not appear to be due to capillary bed destruction.26 27 Long-term smoke exposure produces an increase in the percentage of muscularized small arteries and is associated with an increased level of cell proliferation in both muscular and partially muscularized vessels.28 However, there also appears to be a dynamic alteration of both airways and vessels, in that those animals with an increased pulmonary arterial pressure had greater airflow obstruction than those without.29 This would suggest that smoke alters the intrinsic tone of the muscle, in addition to altering the actual muscular structure.
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Comparison of Guinea Pig to Rat or Mouse Models of Smoking-Induced Lung Disease |
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Rubio and colleagues33 exposed rats to two cigarettes a day for 10 weeks and morphometrically demonstrated an increased thickness of the walls of bronchioles that were < 100 µm in diameter, and they showed that this was associated with an abnormal nitrogen washout curve. In our own study,34 using a dose of five cigarettes per day, we found that smoke increased aging effects on pulmonary function. We could not demonstrate significant airspace enlargement or airway abnormalities to explain this phenomenon (data not reported) or find any alterations in the response of the smoke-exposed rats to methacholine challenge.
The studies of Huber et al30 and Heckman and Dalbey31 demonstrated increased peribronchiolar chronic inflammatory cells, and Huber et al30 also found an increased number of metaplastic secretory cells in the tracheal epithelium. A morphometric study of the tracheal mucous glands35 after 30 consecutive days of smoke exposure for 30 min per day showed an increase in the volume proportion of glands, which was significant in female rats, but not in male rats, data that generally supported the conclusions of Jones et al.36
Ofalue et al32 used their rat model to investigate elastinolytic activity of the neutrophils and macrophages. Although lavage neutrophil numbers were increased after 1 month of smoke exposure, these numbers decreased to control levels thereafter. By contrast, the number of macrophages increased in both the BAL fluid and in the alveolar interstitium, with a concomitant increase in elastinolytic activity and an increase in levels of lavage elastin-derived peptides and desmosine.
Mice are relatively recent models of cigarette smoke-induced COPD, and their attractiveness as a model has been based on their ability to be transgenically modified. In addition, some isolated breeds normally develop emphysema as the animals age.37 Hautamaki and colleagues38 have exposed C57BL/6 and A/J mice to smoke for periods up to 6 months and have been able to document inflammation of the airways and within the airspaces, with associated airspace enlargement. The latter enlargement includes contributions from both the alveolar ducts and the alveolar spaces. Indeed, the mouse model has been instrumental in extending our knowledge of the mechanisms involved in emphysema to once again encompass the alveolar macrophage and metalloproteinases in addition to the serine proteinases produced by neutrophils.
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Advantages of the Guinea Pig Model |
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Anatomy
To calculate the surface area of the guinea pig,40
an equation has been established that is much like the one available for humans, although, unlike in humans, it depends on weight alone. Like humans, and compared to the mouse and even to the rat, a guinea pig has abundant lung tissue, allowing the research to partition the lungs of the same animal for investigations of, for example, molecular biology, histology, or lavage. The structure of the guinea pig lung is roughly analogous to the human lung, with three major lobes on the right and two major lobes on the left. Although the guinea pig, like the rat or mouse, does not have true respiratory bronchioles that would correspond to the three generations of respiratory bronchioles found in humans, they do have a well-defined terminal bronchiole with subtending alveolar ducts.
Responses to Smoke
The guinea pig develops morphologic and physiologic alterations after exposure to cigarette smoke at roughly the same concentrations as humans (as assessed by carboxyhemoglobin levels). As noted above, rats require a significantly greater exposure before developing disease, but mice will develop emphysema after exposure to two cigarettes per day.
A significant proportion of guinea pigs develop increases in pulmonary arterial pressure, although rat vessels clearly have a vascular proliferative response to smoke (at least after short-term exposures). We have no data that would indicate that they also have physiologic alterations. In our long-term exposure experiment, which was described above, we did not identify any alterations of vascular physiology, but it is also important to recognize that our exposures were at much lower levels than those used by others.
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Problems and Prospects of Model |
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Molecular Biology
The guinea pig has a very high level of endogenous ribonuclease. To obtain reliable results when measuring messenger RNA, which may be present in small quantities, it is necessary to take extensive precautions when handling the tissue and to make sure that the tissue is frozen or fixed appropriately as soon as possible after killing the animal.
The number of gene sequences for the guinea pig is much lower than that available for the mouse or rat. However, the number of sequences listed in the Gene Bank is increasing rapidly, and this should not be a major deterrent in planning future experiments with this model. Protein sequences also are limited. However, there is extensive crossover of guinea pig to human, and Western blot analyses, enzyme-linked immunosorbent assays, or immunohistochemistry techniques are often successful when using the antihuman antibodies that are commercially available.
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Footnotes |
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References |
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TOPAbstractDescription of a Guinea...Comparison of Guinea Pig...Advantages of the Guinea...Problems and Prospects of...References |
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B activation and IL-8 mRNA expression in guinea pigs in vivo. Am J Respir Cell Mol Biol 20,189-198This article has been cited by other articles:
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  S. Kubo, M. Kobayashi, Y. Masunaga, H. Ishii, Y. Hirano, K. Takahashi, and Y. Shimizu Cytokine and chemokine expression in cigarette smoke-induced lung injury in guinea pigs Eur. Respir. J., December 1, 2005; 26(6): 993 - 1001. [Abstract] [Full Text] [PDF]
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