The Endocrine Pancreas

1. The Endocrine Pancreas Regulation of Carbohydrate Metabolism

2. Nutritional Requirements • Living tissue is maintained by constant expenditure of energy (ATP). • Indirectly from glucose, fatty acids, ketones, amino acids, and other organic molecules. • Energy of food is commonly measured in kilocalories. • One kilocalorie is = 1000 calories. • One calorie = amount of heat required to raise the temperature of 1 cm3 of H20 from 14.5o to 15.5o C. • The amount of energy released as heat when food is combusted in vitro = amount of energy released within cells through aerobic respiration.

3. Metabolic Rate and Caloric Requirements • Metabolic rate is the total rate of body metabolism. • Metabolic rate measured by the amount of oxygen consumed by the body/min. • BMR: • Oxygen consumption of an awake relaxed person 12–14 hours after eating and at a comfortable temperature. • BMR determined by: • Age. • Gender. • Body surface area. • Thyroid secretion.

4. Anabolic Requirements • Anabolism: • Food supplies raw materials for synthesis reactions. • Synthesize: • DNA and RNA. • Proteins. • Triglycerides. • Glycogen. • Must occur constantly to replace molecules that are hydrolyzed.

5. Aerobic Requirements (continued) • Catabolism: • Hydrolysis (break down monomers down to CO2 and H2O.): • Hydrolysis reactions and cellular respiration. • Gluconeogenesis. • Glycogenolysis. • Lipolysis.

6. How do we use food components in catabolic and anabolic pathways? Involves specific chemical reactions: - Each reaction is catalyzed by a specific enzyme. - Other compounds, besides those being directly metabolized, are required as intermediates or catalysts in metabolic reactions - adenosine triphosphate (ATP) - nicotinamide adenine dinucleotide (NAD+) - flavin adenine dinucleotide (FAD+) - Coenzyme A

7. ATP • ATP is the energy currency of the cell • The structure of ATP is similar to that of nucleic acids • The energy in ATP is “carried” in the phosphate groups - to convert ADP into ATP requires energy - the energy is stored as potential energy in the phosphate group bond - removal of the third phosphate releases that energy

8. NADH, FADH2 • NAD+ can accept a hydrogen ion and become reduced to NADH: NAD+ + 2[H+] + 2e-  NADH + H+ • The added hydrogen ion (and electrons) can be carried to and used in other reactions in the body. • FAD+ is similarly reduced to FADH2. • NADH and FADH carry hydrogen ions and electrons to the enzymes in the electron transport chain of the mitochondria, allowing ATP production there.

9. Coenzyme A • The enzyme coenzyme A converts acetyl groups (2-carbon structures) into acetyl CoA, which can then be used in metabolic reactions • During the course of acetyl CoA production, energy is released and is used to convert NAD+ to NADH

10. Cellular Respiration • Generating ATP from food requires glycolysis, the Krebs Cycle, and the electron transport chain. • Overall reaction: C6H12O6 + 6 O2----> 6 CO2 + 6 H2O + 38 ATP+ heat • The Main point: the break down of glucose releases LOTS of energy: - about 40% in usable form (ATP) - about 60% as heat

11. Glycolysis • Glycolysis is the breakdown of glucose into pyruvic acid • Two main steps are involved, occurring in the cytoplasm of cells (no organelles involved).

12. Step one: glucose glucose 6-phosphate fructose 1,6- diphosphate ATP ATP Step two: fructose 1,6- diphosphate 2 pyruvic acid 2 NADH 2 ATP 2 ATP The two main steps of glycolysis:

13. What happens to pyruvic acid? • In aerobic respiration (oxygen present): - pyruvic acid moves from cytoplasm to mitochondria - pyruvic acid (3 carbons) is converted to acetyl group (2 carbons), producing CO2 in the process - acetyl group is converted to acetyl CoA by coenzyme A - acetyl CoA is used in the Krebs cycle.

14. Krebs Cycle • Acetyl CoA combines with oxaloacetic acid, forming citric acid • A series of reactions then occurs resulting in: - one ATP produced - three NADH and one FADH2 produced (go to electron transport chain) - two CO2 molecules produced

15. Electron-transport Chain • The main point: NADH and FADH2 carry H+ ions to the electron-transport chain, resulting in production of ATP • To do this, the H+ ions are moved along the transport chain, eventually accumulating in the outer mitochondrial compartment • The H+ ions move back into the inner mitochondrial compartment via hydrogen channels, which are coupled to ATP production. • At the end of the transport chain, four hydrogen ions join with two oxygen molecules to form water: 4 H+ + O2 ----> 2 H2O • In the absence of oxygen, the transport chain stalls (no ATP production)

16. Net Result of Glycolysis, Citric Acid Cycle, and Electron Transport Chain: • Production of ATP (stored, potential energy for chemical reactions in the body; 40% of energy released). • Production of heat (maintains body temperature; 60% of energy released). • Also, production of CO2 and H2O.

17. Excess glucose can be stored as glycogen. Stored glycogen can be utilized, by glycogenolysis. Glycogenolysis: -glycogen is broken down into glucose 6- phosphate - liver transforms glucose 6-phosphate to glucose, maintaining blood glucose levels glucose glucose glucose glycogen 6-phosphate 1-phosphate Storage and Utilization of Glycogen

18. Lipid Metabolism • Over 95% of stored energy in the body is in the form of triacylglycerol • During lipid catabolism (lipolysis), triacylglycerol is broken down into free fatty acids and glycerol • Free fatty acids are metabolized by beta-oxidation: 1) fatty acid (18 C) + coenzyme A 2) fatty acid (18 C)-coA 3) fatty acid (16 C) and acetyl-coA • Acetyl-CoA used in citric acid cycle • This reaction also yields NADH => electron transport chain • Excess acetyl-CoA forms ketone bodies

19. Lipid Metabolism (cont.) • The glycerol is convertedinto glyceraldehyde 3-phosphate, which is converted to pyruvic acid • Pyruvic acid is metabolized under aerobic conditions into acetyl-coA • While lipids are major storage form of energy, accessing lipids for metabolism takes time - water insoluble - less efficient energy source - potential for keto-acidosis

20. Protein Metabolism • Amino acids are NOT stored for energy • However, protein can be broken down, and amino acids can be modified and utilized to create glucose or for metabolism • Modification of amino acids to produce substrate for energy involves oxidative deamination

21. Oxidative Deamination • Oxidative deamination removes the amino group from the amino acid, forming ammonia, NADH, and a keto acid: • NADH => electron transport chain • ammonia => liver, converted to urea • keto acid => acetyl-coA => citric acid cycle

22. Proteins and Energy • Utilization of proteins for quick energy is not very efficient: - more difficult to break apart (multiple proteases) - toxic byproduct (ammonia) - can get accumulation of keto acids - proteins are important structural and functional components of cells

23. glucose glucose 6-phosphate glyceraldehyde 3-phosphate glycerol triglycerides amino acidsacetyl fatty CoA acids Interconversion of Nutrients • Lipogenesis: once glycogen stores are filled, glucose and amino acids are converted to lipids • Rate limiting enzyme: acetyl CoA carboxylase acetyl CoA carboxylase

24. Glycerol glyceraldehyde glucose 3- phosphate 6-phosphate phosphoenol pyruvate oxaloacetate Amino pyruvic acid glucose acids Interconversion of Nutrients (cont.) • Gluconeogenesis: amino acids and glycerol can be used to produce glucose (liver) • More glucose is produced via gluconeogenesis than glycogenolysis • Rate-limiting enzyme: phosphoenolpyruvate carboxykinase PEPCK

25. Importance of Blood Glucose Homeostasis • Blood glucose levels must be maintained as a nutrient source for nervous tissue (no glucose stores) • What mechanisms regulate blood nutrient levels in tissues and blood glucose levels?

26. The Endocrine Pancreas: Regulation of Nutrient Metabolism • Located on the posterior abdominal wall, retroperitoneal. • Exocrine portion: secretes digestive enzymes via pancreatic duct, to small intestine. • Endocrine portion: pancreatic islets (of Langerhans), involved in regulation of blood glucose levels.

27. Production of Pancreatic Hormones by Three Cell Types • Alpha cells produce glucagon. • Beta cells produce insulin. • Delta cellsproduce somatostatin.

28. Structure of Insulin • Insulin is a polypeptide hormone, composed of two chains (A and B) • BOTH chains are derived from proinsulin, a prohormone. • The two chains are joined by disulfide bonds.

29. Roles of Insulin • Acts on tissues (especially liver, skeletal muscle, adipose) to increase uptake of glucose and amino acids. - without insulin, most tissues do not take in glucose and amino acids well (except brain). • Increases glycogen production (glucose storage) in the liver and muscle. • Stimulates lipid synthesis from free fatty acids and triglycerides in adipose tissue. • Also stimulates potassium uptake by cells (role in potassium homeostasis).

30. phosphorylation of insulin responsive substrates (IRS) RAS RAF-1 MAP-K MAP-KK Final actions The Insulin Receptor • As we previously saw, the insulin receptor is composed of two subunits, and has intrinsic tyrosine kinase activity. • Activation of the receptor results in a cascade of phosphorylation events:

31. Specific Targets of Insulin Action: Carbohydrates • Increased activity of glucose transporters. Moves glucose into cells. • Activation of glycogen synthetase. Converts glucose to glycogen. • Inhibition of phosphoenolpyruvate carboxykinase. Inhibits gluconeogenesis.

32. lipoprotein lipase Specific Targets of Insulin Action: Lipids • Activation of acetyl CoA carboxylase. Stimulates production of free fatty acids from acetyl CoA. • Activation of lipoprotein lipase (increases breakdown of triacylglycerol in the circulation). Fatty acids are then taken up by adipocytes, and triacylglycerol is made and stored in the cell.

33. Regulation of Insulin Release • Major stimulus: increased blood glucose levels - after a meal, blood glucose increases - in response to increased glucose, insulin is released - insulin causes uptake of glucose into tissues, so blood glucose levels decrease. - insulin levels decline as blood glucose declines

34. Storage In Fat Depots Inhibition of Lipolysis Restrain of HGO Uptake of glucose FOOD I G Glucose I Insulin Secretion I G Insulin G G I G I Pancreas G I G I G G I I G G Insulin Effects

35. Effect of Glucose on Insulin Release • Glucose enters beta cell through a glucose transporter. • Glucose is utilized to generate ATP. • ATP closes a potassium channel, depolarizing the beta cell membrane (normally, K+ leaks out of cell). • Depolarization activates a voltage-dependent calcium channel, increasing intracellular calcium levels. • Increased calcium triggers insulin release.

36. The synthesis and release of insulin is modulated by: Glucose (most important), AAs, FAs and ketone bodies stimulate release. Glucagon and somatostation inhibit relases α-Adrenergic stimulation inhibits release (most important). β-Adrenergic stimulation promotes release. Elevated intracellular Ca2+ promotes release. Insulin secretion - Insulin secretion in beta cells is triggered by rising blood glucose levels. Starting with the uptake of glucose by the GLUT2 transporter, the glycolytic phosphorylation of glucose causes a rise in the ATP:ADP ratio. This rise inactivates the potassium channel that depolarizes the membrane, causing the calcium channel to open up allowing calcium ions to flow inward. The ensuing rise in levels of calcium leads to the exocytotic release of insulin from their storage granule.

37. Mechanism of Insulin Action • Insulin binds to specific high affinity membrane receptors with tyrosine kinase activity • Phosphorylation cascade results in translocation of Glut-4 (and some Glut-1) transport proteins into the plasma membrane. • It induces the transcription of several genes resulting in increased glucose catabolism and inhibits the transcription of genes involved in gluconeogenesis. • Insulin promotes the uptake of K+ into cells.

38. Other Factors Regulating Insulin Release • Amino acids stimulate insulin release (increased uptake into cells, increased protein synthesis). • Keto acids stimulate insulin release (increased glucose uptake to prevent lipid and protein utilization). • Insulin release is inhibited by stress-induced increase in adrenal epinephrine - epinephrine binds to alpha adrenergic receptors on beta cells - maintains blood glucose levels • Glucagon stimulates insulin secretion (glucagon has opposite actions).

39. Structure and Actions of Glucagon • Peptide hormone, 29 amino acids • Acts on the liver to cause breakdown of glycogen (glycogenolysis), releasing glucose into the bloodstream. • Inhibits glycolysis • Increases production of glucose from amino acids (gluconeogenesis). • Also increases lipolysis, to free fatty acids for metabolism. • Result: maintenance of blood glucose levels during fasting.

40. Mechanism of Action of Glucagon • Main target tissues: liver, muscle, and adipose tissue • Binds to a Gs-coupled receptor, resulting in increased cyclic AMP and increased PKA activity. • Also activates IP3 pathway (increasing Ca++)

41. Targets of Glucagon Action • Activates a phosphorylase, which cleaves off a glucose 1-phosphate molecule off of glycogen. • Inactivates glycogen synthase by phosphorylation (less glycogen synthesis). • Increases phosphoenolpyruvate carboxykinase, stimulating gluconeogenesis • Activates lipases, breaking down triglycerides. • Inhibits acetyl CoA carboxylase, decreasing free fatty acid formation from acetyl CoA • Result: more production of glucose and substrates for metabolism

42. Regulation of Glucagon Release • Increased blood glucose levels inhibit glucagon release. • Amino acids stimulate glucagon release (high protein, low carbohydrate meal). • Stress: epinephrine acts on beta-adrenergic receptors on alpha cells, increasing glucagon release (increases availability of glucose for energy). • Insulin inhibits glucagon secretion.

43. Other Factors Regulating Glucose Homeostasis • Glucocorticoids (cortisol): stimulate gluconeogenesis and lipolysis, and increase breakdown of proteins. • Epinephrine/norepinephrine: stimulates glycogenolysis and lipolysis. • Growth hormone: stimulates glycogenolysis and lipolysis. • Note that these factors would complement the effects of glucagon, increasing blood glucose levels.

44. Hormonal Regulation of Nutrients • Right after a meal (resting): • - blood glucose elevated • - glucagon, cortisol, GH, epinephrine low • - insulin increases (due to increased glucose) • - Cells uptake glucose, amino acids. • - Glucose converted to glycogen, amino acids into protein, lipids stored as triacylglycerol. • - Blood glucose maintained at moderate levels.

45. Hormonal Regulation of Nutrients • A few hours after a meal (active): • - blood glucose levels decrease • - insulin secretion decreases • - increased secretion of glucagon, cortisol, GH, epinephrine • - glucose is released from glycogen stores (glycogenolysis) • - increased lipolysis (beta oxidation) • - glucose production from amino acids increases (oxidative deamination; gluconeogenesis) • - decreased uptake of glucose by tissues • - blood glucose levels maintained

46. Turnover Rate • Rate at which a molecule is broken down and resynthesized. • Average daily turnover for carbohydrates is 250 g/day. • Some glucose is reused to form glycogen. • Only need about 150 g/day. • Average daily turnover for protein is 150 g/day. • Some protein may be reused for protein synthesis. • Only need 35 g/day. • 9 essential amino acids. • Average daily turnover for fats is 100 g/day. • Little is actually required in the diet. • Fat can be produced from excess carbohydrates. • Essential fatty acids: • Linoleic and linolenic acids.

47. Regulation of Energy Metabolism • Energy reserves: • Molecules that can be oxidized for energy are derived from storage molecules (glycogen, protein, and fat). • Circulating substrates: • Molecules absorbed through small intestine and carried to the cell for use in cell respiration. Insert fig. 19.2

48. Eating • Eating behaviors partially controlled by hypothalamus. • Lesions in vetromedial area produce hyperphagia (obesity). • Lesions in lateral hypothalamus produces hypophagia (weight loss). • Endorphins, NE, serotonin, and CCK affect hunger and satiety.

49. Regulatory Functions of Adipose Tissue • Adipostat regulatory system (negative feedback loops) to defend amount of adipose tissue. • Differentiation of adipocytes require nuclear receptor protein (PPARg) which is activated when bound to 15-D PGJ2: • Stimulates adipogenesis by promoting development of preadipocytes into mature adipocytes. • Number of adipocytes increase after birth. • Differentiation promoted by high [fatty acids]. • Adipocytes store fat within large vacuoles. • May secrete hormones involved in regulation of metabolism.

50. Regulatory Functions of Adipose Tissue (continued) • Leptin: • Hormone that signals the hypothalamus to indicate the level of fat storage. • Involved in long-term regulation of eating. • Satiety factor in obese have decreased sensitivity to leptin in the brain. • Neuropeptide Y: • Potent stimulator of appetite. • Functions as a NT within the hypothalamus. • These neurons are inhibited by leptin. • TNFa: • Acts to reduce the sensitivity of cells to insulin. • Increased in obesity. • May contribute to insulin resistance.