Intraperitoneal Glucose Tolerance Test, Measurement of Lung Function, and Fixation of the Lung to Study the Impact of Obesity and Impaired Metabolism on Pulmonary Outcomes
March 15th, 2018
The incidence of obesity is rising and increases the risk of chronic lung diseases. To establish the underlying mechanisms and preventive strategies, well-defined animal models are needed. Here, we provide three methods (glucose-tolerance-test, body plethysmography, and lung fixation) to study the effect of obesity on pulmonary outcomes in mice.
The overall goal of the following three methods is to study the impact of obesity and metabolic disorders on lung structure and function. These methods can help to answer key questions in the respiratory research field about the impact of metabolic disorders on lung structure and lung function. The advantage of these techniques is that they can be combined to assess the functional role of metabolism on the pathogenesis of chronic lung diseases and to test different therapeutical approaches.
The implications of these techniques extend beyond phenotyping to what's elucidating the specific functional role of metabolic targets in the pathogenesis of lung diseases. These methods can also be applied to the study of structure and function in both healthy animals with genetic modifications but also in animal models of COPD and asthma. Generally, individuals new to these methods will struggle as insertion of the tracheal tube, the insufflation of the lung and the administration of the deep anesthesia, they all require experience and training.
We first had the idea for these methods when we discovered that early onset obesity in mice but also in humans were intimately linked to impact metabolism and to chronic lung diseases beyond infancy. To measure intraperitoneal glucose tolerance, gently restrain a 12-hour fasted adult mouse by the tail and use sterile scissors to incise the tail tip. Immediately apply a free-flowing drop of blood to the test strip of the blood glucose meter to measure the fasted blood glucose level.
The test result will appear on the glucometer screen after four seconds. After measuring the glucose for each animal, label them individually with colored markers and weigh the animals. Then use a 27-gauge needle attached to a one-milliliter syringe to administer 100 microliters of glucose per ton grams of body weight to each animal via intraperitoneal injection and measure the blood glucose levels 15, 30, 60 and 120 minutes later with new test strips as just demonstrated.
To assess the lung function, place an adult mouse in the supine position on a pad and apply ointment to the animal's eyes. After confirming a lack of response to toe pinch, wet the fur in the thyroid region with 70%ethanol and use forceps to lift the skin at the midline between the jugular notch at the sternum and the tuber symphysis of the mentum. Using blunt scissors, carefully make a one-centimeter skin incision in the tented skin to visualize the underlying subcutaneous adipose tissue and thyroid gland and expose the trachea with careful blunt separation of both thyroid lobes at the isthmus and dissection of the sternothyroid and sternothyroid muscles.
Using sharp forceps, pass a 4-0 braided surgical suture between the trachea and the esophagus and carefully incise the trachea close the the larynx between the tracheal cartilages with microscissors. Intubate with a tracheal tube securing the tube with a surgical suture and move the animal to the heated bed of a body chamber. Connect the tracheal tube to the face plate and turn on the ventilator.
Confirm movement of the thorax contemporaneously with the ventilation rate to confirm proper placement of the tracheal tube and watch the pressure signal on the computer screen. To control changes in the transpulmonary pressure during ventilation, insert an esophageal tube to the level of the lungs watching the screen to facilitate placement of the tube to where maximal pressure deflection and minimal heart artifacts can be observed. Begin the data acquisition according to the manufacturer's instructions and apply 10 microliters of PBS on the nebulizer.
Start nebulization after five minutes of acclamation followed by a response phase of three minutes during which the airway resistance and respiratory system compliance are measured. Allow the animal to recover for three minutes before starting the next nebulization and follow the software instructions for stepwise application of increasing 10-microliter concentrations of methacholine on the ventilator. For quantitative histomorphometric analysis of the lung tissue, use blunt scissors to make a small incision in the diaphragm of an adult mouse and use curved blunt scissors to make a peristernal incision along the entire length of the rib cage to gently open the thorax.
Lift the rib cage to expose the pleural cavity and remove the thymus. After removing the heart, use sharp forceps to prepare and expose the trachea in order to pass a 4-0 braided surgical suture between the trachea and the esophagus followed by careful incision of the trachea close to the larynx between the tracheal cartilages. Intubate with a 26-gauge intravenous cannula and use a fixative agent to inflate the lungs by pressure fixation at a constant pressure of 20 centimeters of water.
After 30 minutes at room temperature, ligate the trachea and remove the cannula at the same time. Then carefully excise the lungs without damaging the tissue and store the lungs in fresh fixative agent at four degrees Celsius overnight. Obese mice demonstrate an increase in serum glucose levels 15 and 30 minutes after intraperitoneal glucose injection indicating an impaired cellular glucose uptake compared to standard diet fed mice.
Invasive lung function analysis shows an up to 1.5-fold increase of airway resistance in obese, high-fat diet fed mice after stimulation with methacholine compared to control animals. Hematoxylin and eosin staining of lung parenchyma sections after intratracheal installation reveals multiple uninflated areas, thick alveolar septa and polygonal-shaped alveoli in lungs instilled with too little pressure. In contrast, too much instillation pressure leads to overinflated areas with destructed alveolar septa.
Application of the appropriate amount of pressure during lung fixation however leads to a completely inflated lung with round-shaped alveoli. Once mastered, the intraperitoneal glucose tolerance tests can be completed in two and a half hour. The lung function tests can be completed in one hour, and the lung insufflation can be completed in under an hour.
While attempting this procedure, it's important to remember that adapting the mice, training the investigator and working in a quiet and stress-reduced environment all are recommended for optimal data acquisition. After its development, this technique paved the way for researchers in the fields of metabolism and respiratory diseases to explore the mechanisms involved in and potention of treatment strategies of chronic lung disease using small animal models. After watching this video, you should have a good understanding how to investigate the impact of obesity and metabolic disorders on lung structure and function.
Don't forget that working with animals or surgical instruments can cause injuries or skin reactions and that precautions such as wearing the appropriate personal protection equipment should always be taken while performing these procedures.
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Overview
This article discusses methods to study the impact of obesity on lung structure and function using animal models. It highlights the importance of these techniques in understanding chronic lung diseases.
Key Study Components
Area of Science
- Neuroscience
- Respiratory Research
- Metabolic Disorders
Background
- Obesity is linked to chronic lung diseases.
- Understanding the mechanisms requires well-defined animal models.
- Research aims to explore the functional role of metabolism in lung pathogenesis.
- Techniques can be applied to both healthy and diseased models.
Purpose of Study
- To assess the impact of obesity on pulmonary outcomes.
- To establish preventive strategies for chronic lung diseases.
- To elucidate the functional role of metabolic targets.
Methods Used
- Glucose-tolerance test
- Body plethysmography
- Lung fixation
- Combination of techniques for comprehensive assessment
Main Results
- Methods provide insights into lung structure and function.
- Techniques can help test therapeutic approaches.
- Results extend to understanding COPD and asthma.
- Experience is required for proper technique execution.
Conclusions
- These methods are crucial for studying obesity's effects on lung health.
- They facilitate the exploration of metabolic disorders in respiratory diseases.
- Training is essential for effective application of these techniques.
Frequently Asked Questions
Executive Industry Relevance
Obesity-related metabolic dysfunction represents a growing risk factor for chronic lung diseases, creating a need for preclinical models that can mechanistically link systemic metabolism to pulmonary outcomes. The integrated assessment of glucose tolerance, lung function, and histological structure enables de-risking of therapeutic hypotheses by providing quantitative, reproducible readouts across metabolic and respiratory domains. This approach supports target validation and predictive confidence in early discovery by elucidating whether metabolic interventions modify lung pathology in disease-relevant systems.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Interrogates the functional role of metabolic targets in lung pathogenesis by measuring glucose intolerance as a biomarker of impaired cellular uptake.
- Operational Value: Enables pathway clarification through standardized ipGTT procedures that detect diet-induced metabolic dysregulation in obese models.
- Predictive Value: Supports portfolio triage by linking metabolic impairment to functional lung outcomes via concurrent airway resistance and compliance measurements.
Screening & Assay Development
- Scientific Value: Prepares validated biological systems for downstream workflows by fixing lungs at standardized pressure to ensure reproducible alveolar morphology for histomorphometric analysis.
- Operational Value: Addresses assay standardization through stepwise methacholine challenge protocols that quantify dose-dependent bronchoconstriction in obese versus control animals.
- Scalability: Highlights platform reuse across COPD and asthma models, enabling consistent lung structure-function assessment in genetically modified or diseased cohorts.
Translational & Preclinical Research
- Scientific Value: Discusses disease relevance by connecting systemic inflammatory and metabolic dysregulation to lung structural changes such as thickened septa and polygonal alveoli.
- Operational Value: Describes continuity from discovery through preclinical validation by demonstrating how lung fixation at 20 cm H2O pressure yields round-shaped alveoli, enabling reliable quantitative endpoints.
- Risk-Adjusted Advancement: Addresses mechanistic de-risking by showing that appropriate insufflation pressure prevents overinflation or collapse, reducing artifact-driven variability in histological assessments.
Pipeline & Workflow Integration
The method positions itself within the discovery continuum from hypothesis testing to lead identification, supporting iterative evaluation of metabolic interventions on lung structure and function in preclinical models.
- Discovery Biology: Explains how the method supports hypothesis testing by measuring serum glucose levels at 15 and 30 minutes post-injection to evaluate cellular glucose uptake in obesity models.
- Screening: Describes assay readiness through reproducible airway resistance and compliance measurements following nebulization and methacholine challenge, enabling compound effect comparison.
- Analytics: Highlights quantitative dependent variable measurements such as increased airway resistance (up to 1.5-fold) and altered alveolar morphology that help teams compare metabolic and pulmonary phenotypes.
- Translational Research: Connects the method to preclinical continuity by demonstrating lung preparation and fixation protocols that preserve tissue integrity for histological analysis across study groups.
- Enterprise Reuse: Frames the method as a reusable capability rather than a single-use technique, applicable to both healthy animals with genetic modifications and disease models of COPD and asthma.
Operational & Enterprise Impact
- Scientific Value: Predictive confidence, target validation, reduction of mechanistic ambiguity in obesity-lung disease linkages.
- Operational Value: Standardization, reproducibility, and scalability of metabolic and functional pulmonary assessments.
- Strategic Value: Better go/no-go decisions, capital efficiency, and reduced late-stage biological risk through early metabolic phenotype stratification.
- Portfolio Impact: Risk-adjusted prioritization and advancement decisions based on concordant glucose tolerance, lung function, and histological outcomes.
Implementation Considerations
- Required scientific expertise in surgical procedures including tracheal intubation, lung insufflation, and glucose administration.
- Instrumentation and analytical infrastructure needs such as glucometers, ventilators, pressure monitoring systems, and nebulizers for methacholine delivery.
- Cross-team standardization requirements for consistent animal fasting, injection dosing, and timing of metabolic and functional readouts.
- Adaptation considerations across model systems including healthy, genetically modified, and disease-induced obese mice.
- Practical limitations supported by source material: investigator training and stress-reduced environments are recommended for optimal data acquisition due to technical complexity of surgical steps.
