Hormonal regulation of glycaemia

1. Hormonal regulation of glycaemia Alice Skoumalová

2. The oral glucose tolerance test (oGTT): Used if: • elevated fasting levels of glucose - 5,3-6,7 mmol/l (for diagnosis of diabetes, screening of patients with impaired glucose tolerance) • screening of gestational diabetes Procedure: • administration of 75g glucose in an aqueous solution • after overnight fasting (10h) • „common“ diet and physical activity during previous three days • be seated and do not smoke during the test • determination of the glucose levels in the capillary blood before the glucose load and after 60 and 120 minutes Factors affecting oGTT: previous diet, infection, stress

4. Revision: • Glucose homeostasis (hypoglycemia and hyperglycemia prevention) • Insulin (which metabolic pathways are activated/inhibited) • Glucagon (which metabolic pathways are activated/inhibited)

5. Glucose homeostasis: • maintenance of blood glucose levels near 80 to 100 mg/dL (4,4-5,6 mmol/l) • insulin and glucagon (regulate fuel mobilization and storage) • Hypoglycemia prevention: • release of glucose from the large glycogen stores in the liver (glycogenolysis) • synthesis of glucose from lactate, glycerol, and amino acids in liver (gluconeogenesis) • release of fatty acids from adipose tissue (lipolysis) • Hyperglycemia prevention: • conversion of glucose to glycogen (glycogen synthesis) • conversion of glucose to triacylglycerols in liver and adipose tissue (lipogenesis)

6. Pathways regulated by the release of: • glucagon (in response to a lowering of blood glucose levels) • insulin (in response to an elevation of blood glucose levels)

7. Synthesis and secretion of insulin and glucagon: • the islets of Langerhans (β- and α-cells) • preprohormone (modification - in ER, GC, SV)

8. Cleavage of proinsulin to insulin: Proinsulin is converted to insulin by proteolytic cleavage, which removes the C-peptide

9. Major sites of insulin action on fuel metabolism: • The storage of nutriens • glucose transport into muscle and adipose tissue • glucose storage as glycogen (liver, muscle) • conversion of glucose to TG (liver) and their storage (adipose tissue) • protein synthesis (liver, muscle) • inhibition of fuel mobilization

10. Insulin receptor signaling: • the tyrosine kinase activity • a dimer (α and ß subunits) Signal transduction: 1.the ß-subunits autophosphorylate each other when insulin binds (activating the receptor) 2. the activated receptor binds and phosphorylates IRS (insulin receptor substrate) 3. multiple binding sites for different proteins

11. Major sites of glucagone action on fuel metabolism: • Mobilization of energy stores • release of glucose from liver glycogen • stimulating gluconeogenesis from lactate, glycerol, and amino acids (liver) • mobilizing fatty acids (adipose tissue)

12. Regulators of insulin and glucagon release:

15. Stimulation by insulin of glucose transport into muscle and adipose cells: Binding of insulin to its cell membrane receptor causes vesicles containing glucose transport proteins to move from inside the cell to the cell membrane

16. Diabetes mellitus • chronic disease characterized by derangements in carbohydrate, fat and protein metabolism • caused by either complete absence of insulin or relative insulin deficiency • 2 types: • Type 1 (insulin-dependent): • no insulin • defective ß-cells function (an autoimmune disease) • Type 2 (non-insulin-dependent): • „the insulin resistance“ (unknown cause, often obesity) • = impaired function of insulin receptors (TNF, resistin) • - the lower number of receptors • - signal cascade abnormalities

17. Pathways affected by insulin 1. Carbohydrate metabolism stimulation of glucose utilization: glycogen synthase ↑ glycolysis ↑ inhibition of gluconeogenesis the transport of glucose into tissues (muscle, adipose tissue) 2. Lipid metabolism stimulation of the glucose conversion into FA: acetyl CoA carboxylase ↑ NADPH (PPP ↑) storage of fat: lipoprotein lipase ↑ inhibition of the degradation of fat: hormone sensitive lipase ↓

18. Effects of insulin deficiency 1. Glucose uptake and utilization↓ 2. Proteolysis↑ 3. Gluconeogenesis↑ 3. Degradation of fat↑ Hypeglycemia (≥9mmol/l) Glucosuria Hyperlipidemia Metabolic acidosis Ketonuria

19. Types of diabetes:

20. The oral glucose tolerance test (oGTT): The blood glucose level returns to the basal level by 2 hours

21. The oral glucose tolerance test (oGTT): • diagnosis of diabetes; administration of glucose (75g) in an aqueous solution • glucose level determination before the glucose load and at 30, 60 and 120 minutes after

22. The chronic diabetes complications: • A. Microvascular (diabetic retinopathy, nefropathy, neuropathy) • nonenzymatic glycation of proteins in vascular tissue • B. Macrovascular (atherosclerosis) • nonenzymatic glycation of proteins in vascular tissue and lipoproteins • C. Diabetic cataract: • increased osmolarity of the lens (increased activity of the polyol pathway → ↑sorbitol) • nonenzymatic glycation of proteins of lens

23. Hyperglycemia - protein glycation: • hemoglobin • vascular tissue proteins → contribute to the diabetic complications (cataracta, atherosclerosis, retinopathy, nephropathy) Glycated proteins: - impaired structure and fucntion The importance of the maintance of low glucose levels in diabetic patients !

24. Lens metabolism: Diabetic cataract: ↑glucose concentration in the lens →↑aldose reductase activity → sorbitol accumulation → ↑osmolarity, structural changes of proteins

25. Questions: 1. Insulin (synthesis, receptor) 2. DM - major metabolic changes 3. Diabetes complications 4. oGTT