So what are the Intestinal secretions? One is mucus. The material entering the SI from the stomach is very acidic so the epithelium of the SI also needs protection and lubrication. The second major secretion from the crypts of Lieberkuhn is water and salts. It produces about two liters per day. This secretion is an active process, an energy dependent process of moving water and salts into the lumen of the SI. As mentioned earlier, the SI dumps a lot of stuff in, about two liters a day. Now sometimes certain bacteria such as ecoli or certain so called enterotoxins, such as toerotoxins by mechanisms which are not clear increase this rate of this secretion. And if you increase of the secretion you can overcome the ability of the SI to absorb all that material and that results in diarrhea. The major danger from tolera is the dehydration because you are pumping out all this water out into the lumen of the SI. One of the major secretory products is in fact water and salts ? important point. It is the movement of the salt that pulls the water by osmosis. Another major secretion into the SI, the duodenum, comes from the pancreas. There are two major secretions from the pancreas, 1) the Zymoges ? inactive enzymes packaged together and in fact pancreatic cells with the cells use. The zymogens are these packed enzymes consisting of amylases which digest starches, there are 3 lipases and many proteases, some DNAases and RNAases. These are all packaged in inactive form, they get secreted, they get dumped into the SI and they become activated. For example, one of the proteases is trypsinogen, which gets converted to trypsin once it reaches the interior, the lumen of the SI becuae there are enzymes known as enterokinases that line the surface of the SI that activate by breaking, cleaving a chunk off the trypsinogen and converting it to the active form of trypsin. It turns out that trypsin, aside form digestin proteins, it is a protease, will also activate many of the other enzymes that were secreted as zymogens. So the pancreas secretes zymogens, which is a package of inactive enzymes just for chewing up material. It gets into the lumen of SI, the trypsinogen gets converted to trypsin and that process, trypsin ? activates many of these enzymes. Now in addtion, the second major secretion of the pancreas is bicarbonate. The chyme that enters the SI, the duodenum, it is very acid because it is coming from the stomach. By the time you reach the end of the SI the chyme gets basic. In part it is due to bicarbonate ions secreted by duct cells of the pancreas. So how does the pancreatic duct cells secrete bicarbonate. They do so because they have a very active, as do all cells a sodium, potassium pump and they also have in the luminal membrane and ATP dependent pump that is secreting bicarbonate and bringing in chloride. So where does the bicarbonate come from. CO2? carbonic anydrase ? bicarbonate, so what happens ot the hydrogen. In the basolateral membrane there is a co-transporter that is driven by the inward movement of sodium which of course it wants to go into the cell because of the concentration gradient so it is like a water wheel. Sodium moves in and the hydrogen is secreted in the basolateral membrane. This is a co-transport system, which does not directly require ATP. It requires the gradient established by the sodium, potassium pump. But the transporter in the apical membrane use ATP, so it is a ATP dependent pump, so both these are ATP dependent. So again it is no surprise that pancreatic cells that are secreting bicarbonate have lots of mitochondria. These pancreatic secretions are controlled by hormones. Just like the gastric pits, the crypts of Lieberkuhn in addition to have cells that move water and salt into the lumen have endocrine cells. And they secrete three major hormones. 1) choliocystokinase (CCK) ? stimulates zymogen release, the cells of the pancrease synthesize zymogens and package them in granules and CCK released by cell in the crypts of Lieberkuhn stimulate the aster? Cells to release their zymogen. So what stimulates CCK? The presence of amino acids and fates in the SI stimulates CCK production which of course stimulates zymogen. As the level of amino acids and fats drop, the production of CCK drops and the amount of zymogen release will drop. 2) secretin? stimulates both bicarbonate and zymogen. It is sensitive to increase in H+ ion concentration in the chyme in the SI. So the more material that comes from the stomach with more H+ ions the more secretin will be produced and that will then stimulate bicarbonate and zymogen production. Secretin also has one other function and that is to inhibit gastrin. So as the food moves into the SI you get an inhibtion of HCl production from the stomach. 3) GIP, glucosedependent insulinotropic proteins? stimulated by the glucose and fats. It is secreted by the endocrine cells by the crypts of Lieberkuhn and it causes pancrease to secrete insulin. So these are the major hormones secreted by the crypts of Liberkuhn that controls pancreatic