• If the concentration of glucose in the blood?decreases?below a certain level, cells may not have enough glucose for?respiration?and may not be able to function normally
• The control of blood glucose concentration is a key part of?homeostasis
• Blood glucose concentration is controlled by?two hormones?secreted by?endocrine tissue?in the?pancreas
• This tissue is made up of groups of cells known as the?islets of Langerhans
• The islets of Langerhans contain?two cell types:
  • α cells?that secrete the hormone?glucagon
  • β cells?that secrete the hormone?insulin

   

• These α and β cells act as the?receptors?and initiate the response for controlling blood glucose concentration
• The control of blood glucose concentration by?glucagon?can be used to demonstrate the principles of?cell signalling
  • α and β cells?in the pancreas act as the?receptors
  • They release the hormone?glucagon
  • Liver cells?act as the?effectors?in response to?glucagon

   

The second messenger model

• If a?decrease?in blood glucose concentration occurs, it is?detected?by the?α and β cells?in the pancreas:
  • The?α cells?respond by?secreting?glucagon
  • The?β cells?respond by?stopping the secretion of insulin

   

• The decrease in blood?insulin?concentration?reduces?the use of?glucose?by liver and muscle cells
• Glucagon?(the first messenger) binds to?receptors?in the cell surface membranes of?liver cells
• This binding causes a?conformational change?in the receptor protein that activates a?G protein
• This activated G protein activates the enzyme?adenylyl cyclase
• Active adenylyl cyclase catalyses the conversion of?ATP?to the second messenger,?cyclic AMP?(cAMP)
• cAMP?binds to?protein kinase A enzymes, activating them
• Active protein kinase A enzymes activate?phosphorylase kinase enzymes?by adding?phosphate groups?to them
• Active phosphorylase kinase enzymes activate?glycogen phosphorylase enzymes
• Active glycogen phosphorylase enzymes?catalyse the breakdown of glycogen to glucose
  • This process is known as?glycogenolysis?and it occurs via the second messenger model

   

• The enzyme cascade described above?amplifies?the original signal from glucagon and results in the?releasing of extra glucose by the liver?to increase the blood glucose concentration back to a normal level

The second messenger model. Glucagon acts as the first messenger and cAMP acts as the second messenger

• Glucagon and insulin are both involved in the negative feedback loop that controls blood glucose concentration
• When blood glucose concentration is?low, more glucagon?is secreted and less insulin is secreted
• When blood glucose concentration is?high, more insulin?is secreted and less glucagon is secreted

The corrective mechanisms work together to regulate blood glucose concentration