Of Mice and Metabolism

One of the first sections we studied in my Physiology class was metabolism, and this is being followed up in the lab for the next several weeks. Most people know what metabolism is (or at least the popular culture definition), but many don't realize that metabolism involves more than the breaking down of food for energy in the body. That is only the half of metabolism called catabolism. There is also anabolism, which is the building up of bio-molecules in the body, proteins for example. Metabolism varies in people based on age, gender, activity level, percentage of lean muscle mass, and other biological factors. Part of the laboratory exercises we are doing involve measuring metabolism.

Measuring metabolism in humans is a very complex procedure, more complex than what our class can accomplish. It would involve a large, self-contained chamber to measure everything that is going on in the body. As a certain former president would say, "Wouldn't be prudent..." That is why we are using mice for this experiment. The mice we are using are a variety of black obese mice. They are obese because they have been genetically altered that way; this involves removing a gene from the DNA that codes for the feeling we get when we are full. So, generally speaking, these mice eat and eat and eat, and never get full, and therefore become obese. The purpose of using the obese mice is to test a dietary supplement called bitter orange, which has claimed to increase metabolism. By increasing metabolism, we expect to see weight loss in the mice.

Measuring metabolism in mice is also fairly complex, but is easier to achieve due to their smaller size. Whereas a chamber for humans would be bulky and quite expensive, a self-contained chamber for mice can be created using basic lab equipment. For our equipment, we are using large Erlenmeyer flasks, a rubber stopper with long rubber ports, a manometer (to measure changes in pressure in the chamber), a thermometer, a giant plastic syringe, and a balloon filled with 100% oxygen. The mice are coaxed into the flask, and the stopper is placed over the opening. We leave the ports open so that the mouse can breathe while everything else is set up. Our first objective is to flush the chamber with pure oxygen to get all the "room air" out of the chamber. By using 100% oxygen, we can get a better calculation of metabolism. After this is achieved, we fill a syringe with 60 cc of the oxygen and set that up on one of the ports. Then we close off the system and wait for the pressure to normalize.

Once the pressure is stable, we begin timing and watching the mouse. The mouse goes about its normal business moving around and cleaning itself, and is continuously breathing in the oxygen. However, there is the problem of carbon dioxide being produced by the mouse; one would assume that as the mouse is breathing in the oxygen, they are producing CO2 and ridding the air of the oxygen. Well, we accounted for this too. We hung a little bag of something called "soda lime" which apparently pulls the carbon dioxide from the chamber. This ensures that the only gas in the chamber is pure oxygen. Since we are drawing the CO2 out, this makes the pressure go up in the chamber. We can see the change in pressure on the manometer, since the liquid starts to move towards the higher pressure. When the liquid drops enough, we start to push more oxygen into the chamber with the syringe. Then the pressure stabilizes again, and the mouse just carries on as normal. We measure how much oxygen we have pushed every two minutes, until we push the full 60 cc. Once this has happened, we stop the time and open the system up to the atmosphere.

You might wonder how measuring the amount of oxygen can tell us metabolism. Well, there is a direct correlation between oxygen consumption and metabolism. We perform some calculations using the amount of oxygen consumed per minute, and convert that into kilocalories per hour (we call kilocalories simply Calories on our food labels). In this way, we are able to roughly determine the mouse's basal metabolic rate, or the rate at which the mouse body carries out all basic functions (breathing, heart beating, etc.). Over time, we will be measuring this again and again to determine if the dietary supplement is having an effect on the mouse metabolism. Interesting, huh?

Well, that's all I have to say about metabolism today. I hope I didn't bore you all with this, but I really thought it was interesting! If you have any questions about metabolism or the experiment in general, feel free to ask, and I'll do my best to answer!