LECTURE 20 - hormonal control of blood sugar Flashcards
How is energy stored in our body?
- stored as fat
- stored as glycogen and triglycerides
How are carbohydrates metabolised?
Glycogenesis
- conversion of GLUCOSE –> GLYCOGEN
- occurs in liver and muscles
- glycogen soured within cytoplasmic granules
Glycogenolysis
- conversion of glycogen –> glucose
Gluconeogenesis
- synthesising glucose from non-carbohydrate sources
- lactic acid, amino acids and glycerol can be converted into glucose
What are the key players in fuel metabolism?
LIVER
- glucose uptake
- glycolysis
- gluconeogenesis
- de novo lipogenesis
PANCREAS
- insulin and glucagon secretion
ADIPOSE TISSUE
- glucose uptake
- triglyceride storage
- fatty acid release
MUSCLE
- glucose uptake
- glycogen storage
- triglyceride storage
What is the importance of blood sugar control?
- ensures energy can be given on demand
- carbohydrate is the body’s primary energy source in cell metabolism
What are the consequences of too high blood sugar?
Hyperglycaemia Symptoms: - none - polyuria - thirst - weight loss - fatigue
Consequences:
- neuropathy
- nepropathy
- heart disease
- cataracts and blinders
- diabetic coma
- death
What are the consequences of too low blood sugar?
Hypoglycaemia Symptoms: - irritability, fatigue - food cravings - headaches - dizziness - shaking - confusion
Consequences:
- loss of consciousness
- accidents and injury
- weight gain
- reduced IQ
- brain abnormalities
What are the Islets of Langerhans?
- insulin levels regulate the secretion of glucagon
- alpha cells secrete glucagon
- beta cells secrete insulin
- delta cells secrete somatostatin
- alpha cells see high levels of insulin thus don’t produce glucagon
Describe the structure of insulin and how it is processed
- insulin is a peptide hormone
- gene synthesis, gene is transcribed and mRNA passed onto RER
- initially produce preproinsulin
- preproinuslin has a signal peptide an A, B and C chain
- within lumen of RER, endopeptidases will cleave off signal peptide, leaving proinsulin
- proinsulin gets packaged into vesicles (disulphide bridges between A and B chain occur in these vesicles)
- vesicles are secreted towards Golgi apparatus, once here, contents released onto Golgi where there are more endopeptidases where the C chain gets cleaved off
- mature insulin and C chain packaged into secretory vesicles (in certain cases there is proinsulin)
- insulin needs to be held with zinc
How is insulin secreted?
- insulin stored in vesicles
- high glucose levels = we want insulin secretion
- glucose cannot cross plasma membrane => requires transporters which are the glucosensors (GLTU2 transporters (bidirectional) on pancreas)
- if blood supply has high levels of glucose => diffusion of glucose into the cell through transporters
- a mechanism is required to keep glucose in cell to avoid equilibrium, this is phosphorylation
- glucokinase in pancreas (hexokinase in liver) adds phosphate to form glucose-6-phosphate - can be metabolised
- metabolism = ATP production
- on beta cell, K channels are open when bound to ADP - usually net movement = K out leaving inside relatively -ve ~-70mV
- glucose entering = channels close due to more ATP = K cannot move out, build up in cell = -70 gets more +ve
- at -50mV, voltage gated Ca channel open allowing Ca to enter cell => exocytosis
How does insulin signalling occur?
- insulin binds to receptor
- in liver, more GLUT transporters can be brought to surface, these are GLUT4
What are the metabolic effects of insulin?
- more glucose and amino acids imported
- glucose stored as glycogen
- amino acids stored as proteins
- fats stored as fatty acids such as adipose tissue
What is glucagon?
- hormone that has antagonistic action to insulin
- released by alpha cells of the Islands of Langherhans
- secreted in response to LOW blood sugar
How is glucagon synthesised?
- synthesised by proteolytic processing in the alpha cells from proglucagon
- made from a gene that also exists in the intestines
What are the actions of glucagon?
- promotes gluconeogeneis
- promotes glycogenolysis
- increases ketogenesis and lipolysis (the formation of ketone bodies from fatty acid oxidation)
- ketogenesis occurs in the liver, lipolysis occurs in adipose tissue
What other hormones are involved in glucose homeostasis?
- growth hormone
- epinephrine
- glucocorticoids
- somatostatin: inhibitor of other peptide, produced by delta cells
- GLP-1
