Chapters
In this article, we will discuss the principles of cell signalling using the example of the control of blood glucose concentration by glucagon, limited to binding of the hormone to cell surface receptor which causes a conformational change, activation of G-protein resulting in the stimulation of adenylyl cyclase, creation of the second messenger, cyclic AMP (cAMP), activation of protein kinase A by cAMP resulting in the initiation of an enzyme cascade, amplification of the signal through the enzyme cascade as a result of activation of more and more enzymes by phosphorylation, cellular response in which the final enzyme in the pathway is activated and catalyzing the breakdown of glycogen. Moreover, we will also explain how negative feedback control mechanisms regulate blood glucose concentration, by throwing light on how insulin affects muscle cells and liver cells and how glucagon affects liver cells. So, let us get started.
Controlling Blood Glucose
- When the concentration of glucose in the blood declines below a specific level, then cells may not have sufficient glucose for respiration. As a result, they may not be able to work properly
- When the concentration of glucose in the blood rises above a specific level, the normal function of the cells may get disrupted, which can potentially cause major issues
- Controlling the concentration of blood glucose is the fundamental part of the hemostasis
- Two hormones that are secreted by the endocrine tissue in the pancreas are responsible for controlling the concentration of blood glucose
- This tissue is composed of a group of cells, referred to as islets of Langerhans
- There are two types of cells in the islets of Langerhans:
- α cells that secrete the hormone known as glucagon
- β cells that secrete the hormone known as insulin
- The α and β cells play the role of the receptors and start the response to control the concentration of glucose in the blood
- Controlling blood glucose concentration through glucagon can be employed to describe the principles of cell signalling
In the next section of the article, we will discuss what happens when the glucose concentration in the blood decreases.
What Happens When Blood Glucose Concentration Decreases?
- α and β cells in the pancreas detect the decrease in blood glucose concentration
- To respond to a decrease in the blood glucose concentration, the α cells secrete glucagon
- To respond to a decrease in blood glucose concentration, the β cells respond by stopping the secretion of insulin
- The decline in the concentration of blood insulin minimizes the use of glucose by liver and muscle cells
- The binding of glucagon to receptors in the cell surface membranes of liver cells causes a conformational shift in the receptor protein that activates a G protein
- The activated G protein stimulates the enzyme known as adenylyl cyclase which catalyzes, i.e. speeds up the conversion of ATP to the second messenger, cyclic AMP (cAMP)
- Cyclic AMP cAMP activates the protein kinase A enzymes by binding to them
- Active protein kinase A enzyme adds phosphate groups to phosphorylase kinase enzymes and activates them
- Glycogen phosphorylase enzymes get activated by active phosphorylase kinase enzymes
- The breakdown of glycogen to glucose is catalyzed by the active glycogen phosphorylase enzymes in a process called glycogenolysis
- The original signal from glucagon is magnified by the enzyme cascade mentioned above. It leads to the release of additional glucose by the liver to enhance the blood glucose concentration back to the normal level
In the next section of the article, we will discuss what happens when the glucose concentration in the blood increases.
What Happens When Blood Glucose Concentration Increases?
- The β cells in the pancreas detect the increase in the blood glucose concentration if it rises above a normal range
- The molecules of glucose enter the β cells through a process known as facilitated diffusion when the concentration of glucose is high
- The cells produce ATP by respiring this glucose
- If the concentration of ATP is high, the potassium channels in the β cells are close to producing a shift in the membrane potential
- This variation in the membrane potential opens the voltage-gated calcium channels
- Due to the influx of calcium ions, the β cells secrete a hormone referred to as insulin
- The vesicles that contain insulin travel towards the cell surface membrane where they release the insulin into the capillaries
- The insulin circulates throughout the body once it enters the bloodstream
- It triggers the uptake of glucose by fat cells, muscle cells, and the liver
In the next section of the article, we will discuss how insulin acts
How Insulin Act?
- Fat storage cells, muscle cells, adipose tissue, and liver cells contain glucose transport proteins in their cell surface membranes. The insulin targets these cells.
- These membrane proteins enable the uptake of glucose molecules through a process known as facilitated diffusion
- The rate at which these cells uptake the glucose is restricted by the number of glucose transporter proteins available
- The glucose transporter proteins on the target cells mentioned above are sensitive to insulin
- The insulin binds to certain receptors on the target cells' membranes. It in turn triggers them to include more glucose transporter proteins in their cell surface membrane which enhances the permeability of the cells to glucose.
- This, in turn, enhances the facilitated diffusion
In the next section of the article, we will discuss the negative feedback control of blood glucose.
How Blood Glucose Controlled by Negative Feedback?
- Negative feedback control mechanisms regulate the concentration of glucose in the blood
- In negative feedback systems, receptors identify whether a certain level is low to high enough. They communicate this crucial information through the hormonal or nervous system to effectors which in turn counteract the change by bringing the glucose level back to the normal
- To control the concentration of glucose in the blood α and β cells in the pancreas play the role of the receptors
- These α and β cells in the pancreas release hormones glucagon and insulin respectively
- Liver cells play the role of the effectors by responding to glucagon and the liver. On the other hand, muscle and fat cells play the role of the effectors by responding to insulin.
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