W3006    Fall 2002       Physiology      Mini-Exam #5        NAME __________________________________

1.  (15 pts)  You feed your pet rabbit some lettuce and grass and soon enough, he starts to defecate.  Suddenly, he bends down, pulls out a soft, juicy fecal pellet… and eats it.  Yum!  (Or Yuk!, if you prefer).    If you dissect a rabbit (not yours, of course), you’d see an enlarged pouch, called the caecum, in the area where the end of the small intestine joins the large intestine.  There’s lots of cellulase in this area.  Assume that in all other ways the rabbit’s digestive tract resembles that of the human.  Explain why the rabbit needs to eat its feces.

Cellulase is produced in the caecum, where it is able to digest cellulose, a polysaccharide, into its constituent monosaccharides (glucose).  However, since this occurs at the end of the small intestine, the glucose moves into the large intestine, which lacks villi and glucose transporters, so it cannot be absorbed.  When the rabbit eats the pellet, the glucose (and any remaining cellulose and cellulase) goes back through the stomach to the small intestine.  In the small intestine, glucose is absorbed in co-transport with Na+, and water follows by osmosis.  So the second time around, the feces are dry and hard, and nowhere near as tasty as the first time, so, of course, even a rabbit doesn't eat those pellets.

 2.  (15 pts)  A patient’s blood is tested and found to be deficient in Vitamin B12, and is treated with injections of the vitamin. Considering the fate of Vitamin B12 in the digestive tract, describe two different problems that the patient might have had, which would have led the doctor to inject her with the vitamin, rather than giving her the vitamin in a pill to swallow.  

Vitamin B12  is found in foods that come from other animals  - meat, poultry, milk, eggs, fish. Why can those animals make it and we can't?  Actually, they can't either.  They have bacteria living in their intestines, which make Vitamin B12.  In the rabbit, these bacteria are found in the caecum....  This is another reason why they eat their feces, to ingest the bacteria and absorb B12.  You might think that the patient has insufficient B12 in the diet, but if this were the only problem, the doctor could have given the vitamin in a pill.  So the injection implies that her problem was not with intake of the vitamin, but rather a problem with its absorption.  arietal cells in the stomach secrete Intrinsic Factor, which binds to Vitamin B12 in the intestine.  The IF-B12 complex is then taken up by receptor-mediated endocytosis in the ileum, and from there goes to the blood.    When Vitamin B12 is ingested, it is bound to proteins in the food.  For B12 to be released, it's necessary for pepsin to digest the protein, and the Vitamin B12 binds to a different protein, called R protein.  In the alkaline intestine, proteases break down R protein, liberating B12, which now binds IF.  So deficiencies in pepsin or in bicarbonate secretion could also lead to deficiencies in B12 absorption.

3.  (30 pts)  We discussed how fats are digested and absorbed, but sometimes fat remains in the digestive tract, and eventually exits the body in the feces.   Steatorrhea refers to the symptom of fatty stools (feces), which can occur in several different disorders.  Explain why it would occur in the following conditions: 
A.  A disease that significantly decreases the activity of the exocrine pancreas.

The exocrine pancreas secretes digestive enzymes, such as lipase, which digests triglycerides to produce monoglycerides and free fatty acids.  If lipase is not secreted, then the fats are not digested, so can't be absorbed, and leave in the feces.

B.  Because of cancer of the ileum, this section of the small intestine was removed.  

In the ileum, bile salts are reabsorbed.  These are recycled by the liver, and secreted again in the bile.  Bile is needed to emulsify fat, and if the bile is not reabsorbed, then the liver's supply is depleted.  With less bile, less fat is emulsified, so lipase is able to digest less fat, so less is digested and more of the fat leaves in the feces. 

  4.  (10 pts)  The sympathetic nervous system innervates adipose tissue and liver.  When these neurons stimulate these targets, they are most likely going to mimic the way those tissues respond to         a. insulin    b. glucagon       One sentence:  explain why you chose this answer.   The parasympathetic nervous system stimulates digestion, and you'd expect insulin to predominate during the absorptive state.  OR The sympathetic nervous system is activated in times of stress, to ensure adequate glucose delivery to the brain, so you'd expect glucagon to predominate, since it raises blood glucose.

5.  (10 pts)  A. How does insulin affect the glucose transport protein in the liver? It doesn't.  This GLUT-2 protein is not insulin-dependent.

B. How does insulin affect the glucose transport protein in the muscle? This protein is insulin-dependent, and under the influence of insulin, more of them (GLUT-4) are inserted in the plasma membrane.