Mouse Graded Exercise Test to Obtain VO2 max

Hello, welcome to our exploration of graded exercise tests and the pivotal role they play in understanding cardiovascular fitness and exercise physiology. In addition to detailing different graded exercise tests, this video will describe how to perform a graded exercise test on research mice using a metabolic treadmill coupled to an indirect calorimeter, the goal of which is to determine the mouse’s VO2 max. We will show one protocol, but of course there are many different test protocols depending on the particulars of the mouse model and research aims.

There are numerous examples of mouse models of cardiovascular and metabolic diseases and responsiveness to therapeutics that translate to human physiology. For this reason, the mouse graded exercise test, executed by a metabolic treadmill, is an indispensable tool for understanding cardiovascular fitness and basic exercise physiology. A mouse graded maximal exercise test on a metabolic treadmill is the gold standard for studying cardiovascular fitness to measure CO2 produced and O2 consumed. You use this to calculate VO2 max, respiratory exchange ratio, RER, anaerobic threshold, and other cardiovascular measurements.

There are two main types of tests for understanding cardiovascular fitness. They are the progressive maximal exercise test and the graded maximal exercise test. Each test provides different but complementary information to understanding cardiovascular fitness.

Cardiovascular fitness is a key indicator of overall health and well-being. It is a measure of the heart and lungs’ ability to supply oxygen-rich blood to the body during physical activity. The graded exercise tests are vital to understanding mouse cardiovascular physiology with the goal of translating this understanding to humans. This is done by phenotyping rodent models of disease and testing new treatments.

The progressive maximal exercise test assesses how long an individual can sustain exercise at a constant level before reaching a state of exhaustion. This may be an indicator of impaired cardiovascular fitness. The test involves a steady increase in workload achieved by increasing treadmill speed while maintaining a set incline that typically lasts greater than 12 minutes. It is basically an endurance test. The PXT is a graded test that starts at a low intensity and incrementally increases in difficulty until a participant reaches their maximal exertion, typically measured by reaching their peak oxygen consumption.

On the other hand, graded maximal exercise tests involve a rapid increase in exercise intensity and workload until the mouse reaches its maximal effort level. This is done by the concurrent increase in both the treadmill speed and incline. The rapid increase in workload means that the maximal exertion is reached before the confounding effects of fatigue occur. This better enables the graded maximal exercise test to determine parameters such as VO2 max, which is important in assessing cardiovascular fitness.

Here we have in graphical form the GXT we use to determine VO2 max. Note this is one GXT protocol and the treadmill can be programmed to fit your individual research. In this example, the incline shown in green increases quickly to its maximum at 9 minutes. Speed in blue continues to increase past 9 minutes until 21 minutes. In practice, as we will see, the run may be stopped before 21 minutes because the mouse reached maximal exertion.

Here we show how the Promethion Metabolic Treadmill is used to obtain VO2 max, from training the mouse through setting up the graded maximal exercise test to evaluating the data. To increase the consistency of results, the mouse was acclimated to the treadmill. This was done by three short training sessions over the course of a week, where it was introduced to the treadmill and experienced speed and incline changes. Note the mouse was allowed 60 hours of recovery time between sessions and before the VO2 max run. For simplicity, the graded exercise test is created in a spreadsheet program such as Excel and transferred by an SD card to the treadmill control box. The profile may be reused to ensure consistency between runs. Concurrently, Promethion Core is set up to obtain VCO2 and VO2 measurements. Note Promethion enables baseline measurements before and after the duration of the test for a complete dataset.

Now that the mouse is acclimated to running on the treadmill, the graded maximal exercise test can be performed to obtain VO2 max. The treadmill automatically increases speed and actively gauges and adjusts the incline as programmed while Promethion Core collects the VO2 and VCO2 measurements. Here we can see what a GXT run to obtain VO2 max looks like while Promethion Live acquires and presents the respiratory data with real-time monitoring. To improve the consistency of each run, the treadmill incline is constantly monitored and adjusted for accuracy. Once started, the treadmill automatically changes speed and incline according to the profile.

During the GXT run, the mouse should be closely monitored, and the run is stopped when maximal exertion is achieved. The graded exercise test to determine VO2 max quickly takes the mouse to maximal exertion, which occurs when the mouse maintains continuous contact with the shock grid for 5 seconds and is unable to continue running. In this case, the maximal shock count occurs at 20 minutes and the run is stopped manually using the control box touchscreen. The data collected by Promethion Live of the shock counts will reflect the point of maximal exertion and can be indicated to correspond with the operator’s observations. Respirometry measurements continue and enable evaluation of recovery.

Lastly, the respirometry data is exported from Promethion Live into spreadsheet format for evaluation. The data is processed by the operator using Macro Interpreter and a custom VO2 max macro to determine VO2 max value, as well as VO2, VCO2, energy expenditure, and respiratory exchange ratio.

In summary, the Sable metabolic treadmill coupled to Promethion Live enables the routine and consistent execution of tests such as the graded exercise test. The treadmill is versatile so it can be programmed to execute a variety of tests, whether they be graded exercise tests to obtain VO2 max or more prolonged tests such as the progressive maximal tests. Thus, the Promethion Metabolic Treadmill makes routine graded exercise tests to consistently obtain important measures of cardiovascular fitness such as VO2 max. Thank you.