Bovine biology series
Part 3 - The Pancreas
The pancreas in mammals has a dual role, in that it is both an exocrine and endocrine secretory organ. Exocrine is defined as the exterior secretion of a gland whose secretion is to an epithelial surface either directly or through a duct. An endocrine secretion is one discharged directly into bloodstream or lymph and distributed to all parts of the body. Most hormones are secreted by endocrine glands like the pancreas, thyroid, parathyroid, adrenal cortex, anterior pituitary, and the male and female sex organs, the testes and ovaries.
The duodenum is part of the small intestine, and the pancreas, a rather small organ, is located next to this part of the digestive tract. The exocrine link is the pancreatic duct. The pancreas, in its role as an exocrine gland, secretes pancreatic juice from the cells of the acini. This secretion passes through the pancreatic ducts into the duodenum.
The pancreatic juice has a pH of 8, and this alkaline substance contains many enzymes that are responsible for the digestion of food. When the word digestion is used here, I am referring to the breaking apart of large molecules of food that has undergone some digestion either in the rumen by an acidic environment or bacterial action, or in the stomach, where also in an acidic bath food compounds are reduced in size.
The alkaline nature of the pancreatic juice presented to the small intestine is at this point, in the path of digestion, equalizing the acidic digesta, or food material. In other words, the acids have done their job, now the pancreas, through its exocrine function, secretes an alkaline solution that can digest the digesta in a different manner. The two main ingredients are the enzymes (most are really what we call proenzymes, that only become active once they have arrived in the duodenum), and numerous electrolytes (sodium, potassium, magnesium chloride, phosphates and sulfur). One important component of this fluid is sodium bicarbonate, a compound that we recognize as a buffer found in baking soda. It is this buffer which gives the pancreatic juice its alkalinity.
As you might expect, the control of this side of the pancreas, the exocrine side, is largely regulated by the acidity of digesta entering the small intestine. Actually, the presence of acid in the duodenum causes the release of a hormone called secretin from the intestinal wall, or mucosa. Secretin sends a signal to the pancreas saying, please secrete some alkaline juice, because the acids are building up. The pancreas does so, neutralizing the digesta, lowering the pH, and the secretin level drops away, and then so does the pancreatic juice until the acid levels once again result in secretin production. Then the cycle begins again.
The endocrine portion of the pancreas also serves a very important function related to the utilization of foodstuffs for energy and therefore work. Because the main function of the endocrine system is systemic, that is, spread throughout the entire body, the bloodstream becomes the media by which this is accomplished. Thus the secretory side of the endocrine pancreas is primarily the bloodstream.
As for the pancreas itself, the endocrine portion of the organ consists of cells called islets of Langerhorns, a rather unusual name but is aptly named by the biologist who first discovered and named them. These endocrine cells are really made up of two types, the alpha and beta cells, each with a slightly different role.
Both of these cell types regulate carbohydrate metabolism in the body, and as such are extremely important in all mammals because the majority of our food intake is carbohydrate (the others being protein and fat/lipid food).
Indeed, the relationship of the two secretions by these two cell types, insulin and glucagon, is antagonistic but yet required in balancing the need of cells to have energy, and therefore a fuel source in order to accomplish work. And yet at the same time, the body must guard against the rapid depletion of carbohydrate in times of heavy work demand, and thus a balance exists between carbohydrate usage and carbohydrate storage.
The alpha cells of the endocrine pancreas are responsible for the production and secretion of glucagon, a very large molecule. Simply put, glucagon secretion is elevated during times of glucose shortages in the bloodstream, known as a condition of hypoglycemia. We know, for instance, that the primary end product of carbohydrate metabolism (more correctly called catabolism) is glucose, a simple sugar that circulates as fuel in the blood. Glucagon, therefore, ensures the availability of glucose as an energy source.
How is this done? A series of receptors found throughout the body may find themselves too short of glucose in the blood. A signal is sent to the pancreas, specifically the alpha cells, saying please release some of that wonderful hormone, glucagon, into the bloodstream. This hormone travels throughout the body, and is received in the liver. The liver, then, stimulates the release of stored glucose, which is in the form of glycogen. But the liver can also do other things, which stimulate the formation of glucose, especially from other food sources such as protein and lipid material. Remember that one of the functions of the liver is to not only store glucose but be capable of making it too. So these organs work together very nicely.
A portion of the pancreas makes sure that we, and all of our mammal friends, have enough glucose, that high octane fuel required for energy production, and then work is accomplished (such as movement of muscle, brain functions, and the production of milk).
What do the beta cells, the other type of endocrine cells, of the pancreas do? If you guessed they produce the popular insulin then you are correct. What does insulin do? Well, this very important hormone facilitates the movement of glucose from the bloodstream, across the cell membrane, to the inner part of a cell. Thus when cells need glucose, they merely call upon the wonder of insulin and say, will you come over here and let some of this sugar inside so I may move this muscle? And the insulin does so, but at a price. The price is the lowering of blood glucose levels.
So in times of rapid muscle use, such as in the middle of the night when someone leaves a gate open and the top string of cows run absolutely crazy in the neighbors tidy lawn, blood glucose levels drop very fast (in cows and you as well). As I said, this is high octane fuel for cells, and is used very quickly because it can really make those muscles move fast (and those cows can cover a lot of ground!).
There is danger in all of this. The brain is a very large user of glucose, and if so much is used from the bloodstream before it can be replenished, then we can pass out. We have run out of sugar, so the body says, well, I better stop what I'm doing, and I can do this best by shutting down the brain, which is driving the muscles. A cow with milk fever, for instance, is rapidly using blood glucose as muscles are in shock. She cannot get up, but her cells are working overtime in response to a lack of calcium in the blood, and so she shivers. That is why the CMPK bottle uses sugar (dextrose) as a carrier. It is the high-octane fuel cells can use immediately, as long as insulin is present to open the door to these muscle cells.
Insulin serves another role as well. It suppresses fatty acid utilization. That can be helpful but yet harmful. Helpful in that by suppressing fatty acid utilization, it is causing these lipids to be stored in fat cells, those big balloon cells that serve as lipid banks for deposit. Harmful in that because lipid is stored, it is not utilized by the body. Perhaps from the standpoint of efficiency, we would like the body to utilize fatty acids as they are produced, rather than expanding these fat cells as these fatty acids are deposited. Such is biology, however, for this is a remarkable storage system when times get tough and food is not readily available. These fat cells then give up this fat deposit, and the liver converts these fatty acids into glucose. The process is much slower than carbohydrate metabolism, but these fatty acids are a very concentrated form of fuel.
It is the hormone glucagon that is largely responsible for the withdrawal of these fatty acids from fat cells, as you might expect. An example of this is the condition of ketosis, which is simply the overwhelming demand of the body for a tremendous volume of glucose. The liver cannot make it quick enough, in spite of the ration. Thus when milk production demands are very high, the glucose requirement is very high. Since it is glucagon that regulates the level of glucose in the body, it makes an adjustment when the liver is not able to secrete and make enough of this sugar. Thus fat is mobilized. As fat is mobilized, the liver is converting these fatty acids into glucose, albeit at a slower rate that normal carbohydrate metabolism.
Thus these fatty acids build up, like a traffic jam. Blood lipid levels are increased, and the smell of ketones is emitted via the rumen and belching.
What a remarkable organ, this pancreas! Not only does it enable the fuel of food to reach cells so that work can be accomplished, but it makes sure enough is held in reserve so that we do not pass out for lack of glucose used in the brain.
|