1. Insulin, Glucagon, and Diabetes Mellitus
Prof. dr. Zoran Valić
Department of Physiology
University of Split School of Medicine

2. digestive functions
two important hormones: insulin and glucagon (crucial for normal regulation of glucose, lipid, and protein metabolism)
other hormones: amylin, somatostatin, and pancreatic polypeptide

3. acini (secrete digestive juices)
islets of Langerhans (secrete insulin and glucagon directly into the blood)
1-2 million islets of Langerhans (0.3 mm in diameter)
three major types of cells: alpha, beta, and delta cells

5. beta cells – 60%, lie mainly in the middle of each islet, secrete insulin and amylin
alpha cells – 25%, secrete glucagon
delta cells – 10%, secrete somatostatin
PP cells – present in small numbers, secrete pancreatic polypeptide?
insulin inhibits glucagon secretion, amylin inhibits insulin secretion, and somatostatin inhibits secretion of insulin and glucagon

7. Insulin and Its Metabolic Effects
Banting and Best (1922) – isolated inzulin
insulin has always been associated with "blood sugar“, but it has profound effect on metabolism of fat (acidosis, arteriosclerosis) and proteins
insulin secretion is associated with energy abundance

8. excess amounts of carbohydrates cause increase in insulin secretion
insulin stores the excess carbohydrates as glycogen (liver and muscles)
inzulin stores adipose tissue
inzulin promotes amino acid uptake by cells and conversion of amino acids into protein
inzulin inhibits the breakdown of proteins

9. Insulin Chemistry and Synthesis
small protein (5808)
it is composed of two amino acid chains connected by disulfide linkages
spliting of the chains – loss of activity
translation of the insulin RNA by ribosomes  preproinsulin (11500)  in the ER proinsulin (9000)  in Golgi apparatus insulin and the C chain peptide

12. circulates almost entirely in an unbound form
plasma half-life averages only about 6 min
cleared from circulation within 10 to 15 min
degraded by the enzyme insulinase mainly in the liver, to a lesser extent in the kidneys and muscles

13. Activation of Target Cell Receptors and Resulting Effects
activation of membrane receptor protein
autophosphorylation of beta subunits  activation of tyrosine kinase  phosphorylation of multiple other intracellular enzymes including a group called insulin-receptor substrates (IRS)

15. after few seconds 80% of cells increase their uptake of glucose (muscle and adipose cells)
cells become more permeable for many amino acids, potassium and phosphate ions
within min change in activity of many intracellular metabolic enzymes
within few hours or days – rates of translation and transcription

16. Inzulin  Muscle Glucose Uptake and Metabolism
muscle tissue depends on fatty acids, resting muscle membrane is only slightly permeable to glucose
moderate or heavy exercise – fibers more permeable in absence of insulin
few hours after a meal (BGC is high and pancreas is secreting large quantities of insulin)

17. if the muscles are not exercising after a meal, glucose is stored in the form of muscle glycogen (2-3%)
spurts of anaerobic energy
insulin facilitates glucose transport through the muscle cell membrane

20. Insulin  Liver Uptake, Storage, and Use of Glucose
storing most of glucose in the form of glycogen after a meal and its release between two meals
inactivates liver phosphorylase (glikogen)
increasing the activity of glucokinase (initial phosphorylation of glucose)
increase activity of glycogen synthase
about 5-6% of the liver mass (100g)

21. Glucose Release Between Meals
pancreas decrease its insulin secretion
stopping synthesis of glycogen and preventing uptake of glucose in the liver
lack of insulin & increase of glucagon activates phosphorylase (splitting of glycogen into glucose phosphate)
activation of glucose phosphatase (spliting of phosphate radical, diffusion of glucose)

22. insulin promotes conversion of excess glucose into fatty acids (lipoproteins – fat)
inzulin inhibits gluconeogenesis (decreasing the quantities and activities of the liver enzymes and availability of precursors required for gluconeogenesis)

23. Lack of effect of insulin on glucose uptake and usage by brain
most of the brain cells are permeable to glucose and can use glucose without the intermediation of insulin
only glucose for energy
when the BGC falls too low (1-3 mmol/L) – hypoglycemic shock (nervous irritability, fainting, seizures, and even coma)

24. Effect of Insulin on Fat Metabolism
not quite as visible, but equally important
long-term effect of insulin lack – extreme atherosclerosis (MI, cerebral strokes)
insulin promotes fat synthesis and storage
increases the utilization of glucose (fat sparer)
promotes fatty acid synthesis (in liver):

25. insulin increases transport of glucose into liver cells (glucose  pyruvate  acetyl-CoA  fatty acids)
excess of citrate and isocitrate ions formed by citric acid cycle when excess amounts of glucose are being used for energy  activation of acetyl-CoA carboxylase
formation of triglycerides and release from liver cells in form of lipoproteins; insulin activates lipoprotein lipase in adipose tissue

26. Storage of Fat in Adipose Cells
insulin inhibits the action of hormone-sensitive lipase (hydrolysis of the triglycerides in fat cells)
insulin promotes glucose transport through the cell membrane into the fat cells; forms large quantities of α-glycerol phosphate (base for glycerol)

27. Insulin Deficiency Increases Use of Fat for Energy
hormone-sensitive lipase in the fat cells becomes strongly activated (release of fatty acids and glycerol into circulating blood)
increases plasma cholesterol and phospholipid concentrations (3x increase in plasma lipoproteins)
ketosis and acidosis (acetoacetic acid – acidosis, β-hydroxybutyric acid & acetone)

30. Effect of Insulin on Protein Metabolism and on Growth
insulin stimulates transport of many amino acids into the cells (valine, leucine, isoleucine, tyrosine, and phenylalanine)
increases translation of mRNA ("turns on" the ribosomal machinery)
increases transcription of DNA (enzymes)
inhibits catabolism of proteins (lysosomes)
depresses gluconeogenesis (enzymes)

31. insulin and growth hormone interact synergistically to promote growth
insulin is essential for growth
two hormones function synergistically to promote growth
each promotes cellular uptake of a different selection of amino acids, all of which are required for growth

33. Mechanisms of Insulin Secretion
response to increased BGC
glucose transporters (GLUT 2) in beta cells
glucose is phosphorylated to glucose-6-phosphate by glucokinase
“rate limiting step”
ATP is formed – inhibits KATP channels (sulfonylurea) – opening Ca channels – exocytosis of inzulina

35. Control of Insulin Secretion
formerly, it was believed that insulin secretion was controlled almost entirely by BCG

37. increased BGC stimulates insulin secretion
at BGC 4,5-5,0 mmol/L (80-90 mg/100 ml) – lučenje 25 ng/min/kg
increase in BGC insulin secretion increases markedly in two stages

40. feedback relation between BGC and insulin secretion rate
BGC of mmol/L ( mg/100 ml) insulin secretion is reaching a peak (10-25x basal level)

43. Other Factors That Stimulate Insulin Secretion
amino acids (arginine & lysine; little alone, but combined with BGC x2; important)
gastrointestinal hormones (gastrin, secretin, cholecystokinin, GIP; amplify the action of glucose; anticipatory effect)
other hormones and autonomic nervous system (glucagon, hGH, cortisol, progesterone and estrogen; prolonged secretion – exhaustion of beta cells)

44. Switching Between Carbohydrate and Lipid Metabolism
insulin promotes use of carbohydrates for energy, depresses the utilization of fats
control – BGC
hGH, cortisol – are secreted in response to hypoglycemia, inhibit cellular utilization of glucose while promoting fat utilization
epinephrine – increasing both (BGC and fatty acids)

45. Glucagon and Its Functions
alpha cells of the islets of Langerhans
increase BGC
large polypeptide (3485); composed of a chain of 29 amino acids
hyperglycemic effect/hyperglycemic hormone

46. Effects on Glucose Metabolism
breakdown of liver glycogen (glycogenolysis; adenylyl cyclase  cAMP  protein kinase  phosphorylase b  phosphorylase a  degradation of glycogen into glucose-1-phosphate  dephosphorylation  BGC; amplifying mechanism)

47. Effects on Glucose Metabolism
increased gluconeogenesis in the liver (increase in the rate of amino acid uptake by the liver cells and then the conversion of many of the amino acids to glucose by gluconeogenesis (pyruvate  phosphoenolpyruvate))

48. Other Effects of Glucagon
activation of adipose cell lipase
inhibition of storage of triglycerides in liver
in high concentrations :
enhances the strength of the heart, increases blood flow in some tissues, especially the kidneys, enhances bile secretion, inhibits gastric acid secretion

49. Regulation of Glucagon Secretion
increased BGC inhibits glucagon secretion (exactly opposite from effect of insulin)
increased blood amino acids stimulate glucagon secretion (same as insulin; alanine and arginine)
exercise stimulates glucagon secretion (4-5x, prevents a decrease in BGC)

52. Somatostatin Inhibits Glucagon and Insulin Secretion
delta cells, 14 amino acid polypeptide, extremely short half-life, extend the period of absorption, GHIH in hypothalamus
locally, depress secretion of both insulin and glucagon
decreases the motility of the stomach, duodenum, and gallbladder
decreases both secretion and absorption in the gastrointestinal tract

53. Importance of BGC Regulation
glucose is the only nutrient used by brain, retina, and germinal epithelium of gonads
most of the glucose between meals is used in the brain
glucose can exert high osmotic pressure (cellular dehydration), loss of glucose in urine, osmotic diuresis by the kidneys (loss of fluids and electrolytes), damage to many tissues, especially to blood vessels

54. Diabetes Mellitus
syndrome of impaired carbohydrate, fat, and protein metabolism, caused by either lack of insulin secretion or decreased sensitivity of the tissues to insulin
two general types :
type I – insulin-dependent diabetes mellitus (IDDM)
type II – non-insulin-dependent diabetes mellitus (NIDDM, insulin resistance)

55. metabolism of all main foodstuffs is altered
prevent the efficient uptake and utilization of glucose by most cells of the body
increase in BGC
cell utilization of glucose falls increasingly lower, and utilization of fats and proteins increases

56. Type I Diabetes
injury to beta cells or diseases that impair insulin production (viral infections or autoimmune disorders, heredity)
usual onset of type I diabetes occurs at about 14 years of age – juvenile diabetes mellitus
may develop abruptly (few days or weeks)

57. increase in BGC to 17-66 mmol/L (300-1200 mg/100 ml :
loss of glucose in urine (BCG > 10 mmol/L)
cell dehydration (osmotic transfer of water out of the cells; osmotic diuresis  extracellular dehydration)
chronic increase – tissue injury(MI, stroke, kidney failure, retinopathy ,blindness, and ischemia and gangrene of the limbs, peripheral neuropathy, autonomic nervous system dysfunction)

58. severe metabolic acidosis (excess keto acids) + dehydration = severe acidosis  diabetic coma  death
physiological compensations: rapid and deep breathing, decrease in bicarbonate
depletion of body's proteins (asthenia (lack of energy) despite eating large amounts of food (polyphagia))

61. Type II Diabetes
diminished sensitivity of target tissues to the metabolic effects of insulin – insulin resistance
far more common (90-95%)
occurs after age of 30 – adult-onset diabetes
in recent years – younger individuals (increasing prevalence of obesity)

62. increased plasma insulin concentration (hyperinsulinemia)
compensatory response by the pancreatic beta cells
in the early stages of the disease – moderate hyperglycemia; in the later stages – beta cells become "exhausted" or damaged – more severe hyperglycemia

64. gradual process, beginning with excess weight gain and obesity (fewer insulin receptors or abnormalities of the signaling pathways)
in some individuals pancreas is “used up”
in the beginning – limitation of food intake, decrease in body weight

65. “metabolic syndrome” obesity, especially accumulation of abdominal fat
insulin resistance
fasting hyperglycemia
increased blood triglycerides and decreased blood high-density lipoprotein-cholesterol
hypertension

67. drugs that increase insulin sensitivity – thiazolidinediones
drugs that suppress liver glucose production – metformin
drugs that cause additional release of insulin by the pancreas – sulfonylureas
in the later stages of type II diabetes, insulin administration is usually required

70. Physiology of Diagnosis of DM
urinary glucose (qualitative & quantitative tests)
BGC (4,5-5,0 mmol/L – normala, 6,0 mmol/L gornja granica) & insulin levels
oral glucose tolerance test (OGTT) – 1 g glucose / kg body mass
acetone breath

73. Treatment of Diabetes
insulin is available in several forms (regular, longer-acting insulins)
individualized pattern of treatment
dieting and exercise
animal / recombinant DNA
lipid-lowering drugs to control high level of blood cholesterol

74. Insulinoma – Hyperinsulinism
excessive insulin production from an adenoma of an islet of Langerhans
much rarer than diabetes
10-15% of these adenomas are malignant – tremendous production of insulin
more than 1 kg of glucose have had to be administered every 24 hours

75. Insulin Shock and Hypoglycemia
insulin-secreting tumors or administration of too much insulin
BGC: 3-4 mmol/L – CNS becomes excitable, hallucinations, extreme nervousness, trembling, sweating
BGC: 1-3 mmol/L – clonic seizures and loss of consciousness
state of coma, permanent damage