organ systems: glucose regulation

this lecture focused on glucose metabolism: specifically, the interplay of insulin and glucagon secretion and their effect of glucose metabolism and storage in different organs and tissues. the pancreatic islets of langerhans secrete glucagon from alpha cells, insulin from beta cells, and somatostatin from delta cells. glucagon is secreted in response to low blood sugar and has a variety of effects which ultimately serve to raise blood glucose levels. in the liver, glycogenolysis is initiated, freeing glucose units from storage (see the biochem chapter on glycogen for more detail), gluconeogenesis creates glucose from non-carbon precursors such as amino acids. additionally, triacylglyceride stores are converted into fatty acids, which can be cleaved into ketone bodies, which are used as an alternative fuel source. glucagon is released in response to hypoglycemic (low blood sugar) conditions. other mechanisms are in place to raise blood sugar: sympathetic stimulation, cortisol, growth hormone. the symptoms from severe hypoglycemia are caused by these mechanisms; hunger by the hypothalamus and anxiety/tremors/sweating by sympathetic stimulation.

on the other hand, insulin is released in hyperglycemic conditions (high blood sugar) and facilitates uptake of glucose into cells. it accomplishes this by binding to receptors that translocate glucose transport proteins to surface of cell membranes, thereby allowing glucose to enter. there are 5 categories of glucose transport proteins, with different affinities for glucose and found in different locations of the body (see the carb digestion biochem chapter for some more physiology of the GLUT transporters): GLUT1 are found everywhere in the body and also present in placenta. GLUT2 transporters are in the pancreas, liver, kidney, and intestine. GLUT3 transporters are everywhere in the body. GLUT4 transporters are in muscle and adipose tissue and are the only insulin dependent glucose transporters. GLUT5 transporters are in the jejunum.

insulin has a variety of actions on organs and tissues; in the liver it initiates glycogen storage, fatty acid synthesis and subsequent triacylglyceride synthesis. in adipose tissue it stimulates uptake of glucose, triacylglyceride synthesis, and triacylglyceride release (through VLDL's). in muscle it stimulates uptake of glucose and amino acids and promotes glycogen storage from the excess glucose. insulin release is stimulated by a variety of factors- primarily high blood glucose levels, but also via parasympathetic stimulation, amino acids, growth hormone, and various GI hormones. insulin release is inhibited by catecholamines, somatostatin, and glucagon. note: glucagon and insulin reciprocally regulate each other- the release of one inhibits the release of the other. in addition, the release of somatostatin inhibits the release of both insulin and glucagon- preventing "rapid nutrient exhaustion".

diabetes type I is caused by an autoimmune destruction of pancreatic beta cells, resulting in low insulin levels and therefore low glucose metabolism and therefore a shift to ketone body metabolism. this also results in increased glucose levels in the urine, which can cause polyuria (excess urine volume), polydipsia (excess thirst), and polyphagia (excess hunger). type II diabetes is a resistance to insulin that is associated with high visceral fat deposits with high lipolytic activity (releasing fatty acids into the bloodstream) that are resistant to the anti-lipolytic properties of insulin. this can be induced by high free fatty acid, cortisol, or testesterone levels.

questions
glucagon...
1. what are the three types of cells in the pancreatic islets of langerhans and what do they secrete?
2. describe glucagon's effect on the liver.
3. what are the specific processes that occur that release glucose and ketones from the liver?
4. what is glucagon release from alpha cells stimulated by?
5. what is glucagon release from alpha cells inhibited by?
6. how does somatostatin "prevent rapid nutrient exhaustion"?
7. glucagon corrects...
8. what are the other ways that the body corrects for hypoglycemia?
9. what are the symptoms of severe hypoglycemia and what are they caused by?
10. what is reactive hypoglycemia and what is it caused by?

insulin...
11. describe the general function of insulin.
12. how does insulin facilitate the uptake of glucose into cells?
13. where are GLUT1-GLUT5 found?
14. which glucose transporter protein requires insulin?
15. describe insulin's action on muscle.
16. describe insulin's action on the liver.
17. describe insulin's action on adipose tissue.
18. what are some factors that facilitate release of insulin from the pancreas?
19. what are inhibitors of insulin release?

hormonal regulation...
20. describe the "reciprocal regulation" of insulin and glucagon.
21. describe the concept of a "basin of attraction" in regards to glucose regulation.

diabetes...
22. what is IDDM? what is it caused by?
23. what do high glucose levels in urine cause?
24. what is type II diabetes? what is it caused by and what does it result in?

answers
1. alpha cells secrete glucagon, beta cells secrete insulin, delta cells secrete glucagon.
2. increases glucose and ketone production and secretion.
3. glycogenolysis, gluconeogenesis, lipolysis, ketogenesis.
4. amino acids, decreased bloods sugar, CCK, VIP, catecholamines.
5. insulin/glucose, somatostatin.
6. by inhibiting both alpha and beta cell secretion of glucagon and insulin secretion.

7. hypoglycemia.
8. sympathetic stimulation, cortisol, growth hormone.
9. anxiety, tremors, sweating are caused by sympathetic action and hunger is caused by hypothalamus.
10. low blood sugar levels after a meal that results from excess release of insulin triggered by high content of high glycemic index carbohydrates (or insufficient protein).

11. to store metabolic fuels.
12. by binding to receptors which translocate glucose transporter proteins into the cell membrane.
13. GLUT1- ubiquitous, placenta, GLUT2- beta cell, liver, kidney, intestine, GLUT3- ubiquitous, GLUT4- muscle,adipose, GLUT5-jejunum.
14. GLUT4.
15. causes uptake of amino acids and sugar (and therefore promotes glycogen storage).
16. glycogen synthesis, fatty acid synthesis.
17. uptake of glucose and converion into fatty acids and glycerols, triglyceride synthesis, and uptake of fatty acids from blood lipoproteins.
18. high glucose levels, amino acids, parasympathetic stimulation (cephalic phase of pancreatic secretion), growth hormone, cortisol, GI hormones such as gastrin, secretin, CCK, GIP.
19. somatostatin, catecholamines.

20. insulin and glucagon inhibit each other's release from islet cells via paracrine actions.
21. the basin of attraction is the set of homeostatic variables which the body settles into over time; long term changes in hormone levels or autonomic activity can shift this basin of attraction to a new equilibrium point.

22. autoimmune destruction of pancreatic beta cells which results in low levels of insulin, and thus a shift from glucose metabolism to ketone body metabolism.
23. polyuria (excess urine volume), polydipsia (excess thirst), polyphagia (excess hunger)
24. type II diabetes is an insulin resistance that is caused by excess fatty acids, cortisol, or testosterone, which blocks insulin's anti-lipolytic effect on adipose tissue. can not be compensated by excess insulin secretion.

biochem: mark's medical biochemistry chapter 31- gluconeogenesis

gluconeogenesis is the process that occurs mainly in the liver in which glucose is produced from non-carbohydrate sources during times of fasting, exercise, or stress. it is essentially the opposite of glycolysis, in that instead of producing pyruvate from glucose, it synthesizes glucose from pyruvate. pyruvate itself is supplied by several different sources: lactate can be oxidized into pyruvate and is available from anaerobic glycolysis or by adipocytes in red blood cells. alanine can be transaminated into pyruvate and is produced from other amino acids released from the muscle. glycerol also serves as the precursor to an intermediate of gluconeogenesis, DHAP. the reactions of gluconeogenesis can be divided into three major sections:

conversion of pyruvate to phosphoenolpyruvate (PEP)
1. pyruvate is formed from alanine or lactate in the cytosol.
2. pyruvate diffuses into the mitochondria and is carboxylated to oxaloacetate via pyruvate carboxylase (recall that this is an anaplerotic reaction of the TCA cycle)
3. oxaloacetate is either transaminated to aspartate, or reduced to malate (using NADH as an electron source) and transported back out of the mitochondria.
4. oxaloacetate is reformed in the cytosol either by transamination of aspartate or oxidation of malate.
5. oxaloacetate is converted to phosphoenolpyruvate via phosphoenolpyruvate carboxykinase, using one GTP.

conversion of PEP to fructose 1,6 bisphosphate (reverse of glycolysis)
6. PEP is converted to 2-phosphoglycerate
7. 2-phosphoglycerate is converted to 3-phosphoglycerate
8. 3-phosphoglycerate is converted to 1,3-bisphosphoglycerate
9. 1,3-bisphosphoglycerate is converted to G3P.
10. for every two molecules of G3P that are formed, one isomerizes to DHAP
11. G3P condenses with DHAP to form fructose 1,6 bisphosphate.

conversion of fructose 1,6 bisphosphate to glucose (reverse of glycolysis)
12. fructose 1,6 bisphosphate is converted to fructose 6-phosphate via fructose 1,6 bisphosphatase.
13. fructose 6-phosphate is isomerized to glucose 6-phosphate via phosphoglucoisomerase.
14. glucose 6-phosphate is converted to glucose via glucose 6-phosphatase.

the reactions in bold are irreversible, endergonic reactions that use enzymes that are not used in the reverse glycolytic pathway. this is significant because the relative activity of these competing enzymes determines whether the reaction will proceed in the glycolytic or gluconeogenic pathway. the regulation of the first of these reactions, the conversion of oxaloacetate to PEP, is the most complex and is regulated by several enzymes in upstream reactions beginning with pyruvate production. the first, pyruvate dehydrogenase, is normally responsible for oxidizing pyruvate to acetyl CoA but is deactivated during gluconeogenesis, allowing pyruvate to instead be carboxylated into oxaloacetate. pyruvate carboxylase, the enzyme that catalyzes this reaction, is in turn activated by acetyl CoA, which is produced during the fatty acid oxidation which occurs during fasting or stress. these two reactions work in tandem during fasting conditions to ensure production of oxaloacetate from pyruvate rather than acetyl CoA.

the third enzyme which takes place in the regulation of the production of PEP is PEP carboxykinase, which converts oxaloacetate to PEP. in fasting conditions, glucagon and epinephrine stimulate cAMP to increase transcription of PEPCK enzymes, increasing the quantity of enzyme in the cell (called inducing). finally, the last enzyme involved is pyruvate kinase, which normally converts PEP back into pyruvate (recall the last step of glycolysis). high glucagon levels causes phosphorylation of the enzyme (using a mechanism involving cAMP and protein kinase A) and inactivates it-- thus allowing PEP to be used for gluconeogenesis instead of being uselessly cycled back to pyruvate. these four enzymes basically act as "switches" which first turn on the gluconeogenic pathway by allowing pyruvate to be converted to PEP.

the next places for enzymatic regulation of the gluconeogenic pathway are: the conversion of fructose 1,6-bisphosphate to fructose 6-phosphate, and the conversion of glucose 6-phosphate into glucose. both reactions are similar in that (as mentioned earlier) they use enzymes that are not the same as the reverse glycolytic reaction. in fasting conditions, the enzymes that catalyze the glycolytic reaction are deactivated, allowing the reaction to proceed in the gluconeogenic direction.

the book then talks about what happens in the liver and body tissues during, after, and long after a meal. during a high carbohydrate meal, blood glucose levels can rise from the normal 80-100 mg/dL to a high of 140 mg/dL. during this time insulin is secreted from the beta cells in the pancreas, and glucagon levels decrease. the net result is a storage of glucose in the liver as glycogen. within a few hours after eating, blood glucose and insulin levels fall back down, and glucagon levels start to rise- this initiates the process of glycogenolysis, which is the conversion of the stored glycogen in the liver back into glucose to maintain blood glucose levels. glucagon stimulates glycogenolysis and inhibits glycogen storage concurrently via production of cAMP, which stimulates protein kinase A to inactivate the enzyme related to glycogen synthesis as well as activate the glycogenolytic pathway. within 4 hours after a meal, as the liver's glycogen supply is decreasing (it takes about 30 hours to deplete the liver's supply of glycogen), gluconeogenesis is also stimulated by glucagon and falling blood sugar levels.

questions
1. what happens in the liver during fasting?
2. what is gluconeogenesis?
3. what are the three carbon sources for gluconeogenesis in humans?

4. describe the role of lactate as a gluconeogenic precursor.
5. describe the role of alanine as a gluconeogenic precursor.
6. describe the role of glycerol in gluconeogenesis.

7. describe the conversion of pyruvate to PEP.
8. what determines the path in which oxaloacetate will be converted and transported across the mitochondrial membrane?
9. describe the conversion of PEP to fructose 1,6 bisphosphate.
10. describe the conversion of fructose 1,6 bisphosphate to glucose.
11. describe the conversion of glycerol to DHAP.

12. what are other factors that can stimulate gluconeogenesis?
13. what are the three main reactions that are regulated in gluconeogenesis?
14. how does the fasting state deactivate pyruvate dehydrogenase?
15. how does the fasting state activate pyruvate carboxylase?
16. how is PEP carboxykinase regulated?
17. what is pyruvate kinase and how is it regulated?
18. describe the regulation of the reaction from fructose 1,6 bisphosphate to fructose 6-phosphate.
19. describe the regulation of the reaction from glucose 6-phosphate to glucose.

20. what is the energy consumption during gluconeogenesis and where does it happen?

21. what are normal blood glucose levels for fasting, right after a meal, 2 hours after a meal, and starvation?
22. describe the pancreas's actions after ingestion of a high glucose meal.
23. glycerol, glucagon, glycogen.
24. describe the stimulation of glycogenolysis in the liver.
25. describe what happens roughly 4 hours after a meal.
26. describe what happens during prolonged starvation.
27. how long does it take to deplete liver glycogen stores? (and therefore halt glycogenolysis)


answers

1. liver releases glucose into the blood via glycogenolysis and gluconeogenesis.
2. the process by which glucose is created in the liver from non carbohydrate sources.
3. lactate, glycerol, and amino acids- particularly alanine.

4. lactate is produced by anaerobic glycolysis through reduction of pyruvate or by adipocytes in the fed state or by red blood cells. lactate is oxidized into pyruvate, which is a precursor for gluconeogenesis.
5. alanine is produced in the muscle from other amino acids (whenever insulin is low or stress hormones are high) and from glucose. it is converted to pyruvate via alanine aminotransferase.
6. glycerol is released from adipose tissue whenever insulin levels are low or stress hormones are high. it is converted to DHAP, which is a gluconeogenetic intermediate (as well as a glycolytic one)

7. pyruvate is created from alanine or lactate in the cytosol, and then travels into the mitochondria, where it is carboxylated to oxaloacetate via pyruvate carboxylate (an anaplerotic reaction of the TCA cycle). oxaloacetate is then transaminated to aspartate or reduced to malate and transported back out into the cytosol, and reformed back into oxaloacetate (via oxidation or transamination). in the cytosol, oxaloacetate is decarboxylated by phosphoenolpyruvate carboxylkinase to form PEP.
8. the reduction of oxaloacetate into malate requires reducing equivalents; if the mitochondria has need for reducing equivalents for other reactions, it will use the other venue, the conversion to aspartate.
9. PEP is converted to fructose 1,6 bisphophate through a reversal of the glycolytic reactions. PEP is converted into 2-phosphoglycerate, to 3-phosphoglycerate, to 1,3 bisphosphoglycerate, and reduced to G3P. for every two molecules of G3P produced, one isomerizes to DHAP. G3P and DHAP condense to form fructose 1,6 bisphosphate.
10. fructose 1,6 bisphosphate has a phosphate removed by fructose 1,6bisphosphatase to form fructose 6 phosphate. fructose 6 phosphate is isomerized to glucose 6 phosphate by phosphoglucose isomerase. glucose 6 phosphate has a phosphate removed by glucose 6-phosphatase, producing glucose.
11. glycerol is converted to glycerol 3-phosphate via glycerol kinase, and then oxidized to DHAP.

12. prolonged exercise, stress, and a high protein diet.
13. OAA to PEP, fructose 1,6 bisphosphate to fructose 6 phosphate, glucose 6 phosphate to glucose. all three reactions use regulatory enzymes which are not involved in the reverse glycolytic pathway.

14. during the fasting state, fatty acids are released from adipose tissue and undergo beta oxidation, producing NADH, acetyl CoA, and ATP. the higher ATP / ADP ratio phosphorylates pyruvate dehydrogenase into the inactive form.
15. fatty acid oxidation produces acetyl CoA, which activates pyruvate carboxylase.
16. glucagon is released during fasting and EP is released during exercise/stress, both of which stimulate production of cAMP, which increases transcription of PEPCK genes.
17. pyruvate kinase is the enzyme that catalyzes the conversion of PEP back into pyruvate. when glucagon levels are high, pyruvate kinase is phosphorylated and inactive through a mechanism involving cAMP and protein kinase A.

18. this reaction occurs via the fructose 1,6 bisphosphotase enzyme, and normally would compete with the reverse reaction from glycolysis, fructose 6-phosphate to fructose 1,6 biphosphate via PFK-1. however, under conditions favoring gluconeogenesis, the enzymes that stimulate PFK-1 are inactive, allowing the reaction to head towards the production of glucose.
19. low insulin and glucose levels deactivate the enzyme for the glycolytic forward reaction and allow the glucose synthesis to occur.


20. for every mole of glucose that is produced, 6 moles of ATP and 2 moles of NADH are used. 2 moles of ATP at the conversion of pyruvate to oxaloacetate, 2 moles of ATP at the conversion of oxaloacetate to PEP, 2 moles of ATP at the conversion from 3-phosphoglycerate to 1,3 bisphosphoglycerate, and 2 moles of NADH at the reduction of 1,3 bisphospholycerate to G3P. (2 moles at each reaction because 2 molecules of pyruvate combine into one molecule of glucose)

21. fasting: 80-100mg/dL. right after a meal: up to 140mg/dL. 2 hours after a meal: back to 80-100mg/dL. starvation: not lower than 65mg/dL.
22. during a meal, the high glucose concentration in the blood stimulates the beta cells of the pancreas to increase insulin production. glucagon levels decrease in response to a high carbohydrate meal but increase in response to a high protein meal.
23. glycerol is released from adipose whenever levels of insulin are low and levels of glucagon is high-- and is converted into DHAP. glucagon is a hormone released by the alpha cells of the pancreas in response to decreasing blood glucose levels-- stimulating gluconeogenesis. glucagon also activates production of cAMP in liver cells, which activates protein kiase A, which inactivates glycogen synthase-- thus high glucagon levels inhibit glycogen production. glycogen is synthesized from glucose and stored in the liver.
24. high glucagon levels stimulate adenylate cyclate, which synthesizes cAMP. cAMP activates protein kinase A, which inactivates glycogen synthase, and activates phosphorylase kinase. phosphorylase activates phosphorylase b, which converts glycogen to glucose 1-P, which is then converted to glucose 6-P and then free glucose in the liver, which can then enter the blood.
25. in addition to supplementing blood glucose levels with glycogenolysis, gluconeogenesis is stimulated by the release of precursor material such as glycerol, alanine, and lactate from peripheral body tissues.
26. the body switches to fatty acid and ketone body oxidation and requires much less glucose.
27. ~30 hours