Functional roles and regulation of gastrointestinal transporter proteins in urea-nitrogen salvaging in ruminants

点击翻译按钮获取中文摘要

In domesticated ruminants like beef and dairy cattle, the protein that is provided in the diet contains amino acids that are required for the production of edible products like meat and milk. However, the efficiency of converting dietary protein into milk or meat protein is poor at only 15 to 30%; in other words, most of the dietary protein that is consumed by beef or dairy cattle is excreted in the manure in one form or another. This excessive excretion of diet protein in the manure is undesirable for a number of reasons, including: 1) protein supplements are expensive, so it is important for economic reasons that animals use them efficiently; and 2) protein excretion in manure poses a threat to the environment. The ruminant digestive tract is different from that of non-ruminants (e.g., pigs) in that ruminants possess a four-compartment stomach, with the first three compartments being the rumen, reticulum, and omasum (collectively referred to as the fore-stomachs). In the fore-stomachs, there is a very diverse population of microorganisms that ferments the feed. In part, the inefficiency of dietary protein utilization arises because microorganisms in the fore-stomachs ferment a lot of the dietary protein to ammonia, which is then absorbed into the bloodstream. Ammonia is toxic to the animal, so essentially all of the ammonia is removed by the liver when blood from the fore-stomachs passes through the liver. The liver converts ammonia to urea, which is a non-toxic end-product. Urea that is produced in the liver is released into the bloodstream and most of it ends up being excreted in the urine, thus representing an irreversible loss of nitrogen (originally contained in dietary protein) to the animal. However, ruminants have evolved a mechanism that allows urea in the bloodstream to pass into the fore-stomachs where it can be used by the microorganisms for their own growth. This benefits the animal because these microorganisms eventually pass out of the fore-stomachs to the small intestine where they are digested by the animal’s enzymes to yield amino acids that are then absorbed and can be used for milk and meat production. This research program’s broad objective is to enhance our understanding of what controls the passage of urea from the bloodstream into the fore-stomachs. What we know at this time is that there are two types of protein molecules that are referred to as urea transporters and aquaporins that are found in the wall of the fore-stomachs and appear to be responsible for the passage of urea from the bloodstream into the fore-stomachs; however, we do not know what controls the activity of these protein molecules. In this research program, I will study how the activity of these protein molecules is affected by the composition of the diet, the products of fermentation in the fore-stomachs, and hormones. If we can understand what nutritional or physiological factors regulate the activity of these protein molecules, then we can develop strategies to increase the passage of urea from the bloodstream into the fore-stomachs. This would improve the utilization of diet protein and environmental stewardship in intensive ruminant production systems.