Regulation of Blood Glucose Concentration

Suppose the plasma glucose concentration rises because glucose is absorbed from food in the gut or released from the liver. Then:

• both a and b cells detect the rising concentration of glucose
• a cells cease to secrete glucagon, but b cells secrete insulin
• insulin binds to the receptors in the plasma (cell surface) membranes of muscle, fat and liver cells. 
• glucose uptake from the blood by these cells is increased
• use of glucose in respiration by the cells is increased
• liver cells convert glucose to glycogen and stop the reverse reaction. 

Conversely, when plasma glucose falls because it is used to rapidly by cells or is lacking in the food eaten: 

• both a and b cells detect the falling concentration of glucose 
• b cells cease to secrete insulin
• a cells to secrete glucagon
• target cells take up less glucose
• the rate of use of glucose decreases and fats or amino acids are used instead in respiration
• glycogen in the liver is converted to glucose and released into the blood. 

Insulin Secretion

A b cell has a resting potential across its membrane that is kept negative inside the cell by ATP-sensitive potassium ion (K+) channels. When plasma glucose concentration rises, glucose enters the b cell and is respired, allowing ATP production. The increased ATP concentration in the cell closes the K+ channels. The membrane depolarises and calcium ions rush into the cell. Insulin is released by exocytosis. In this way, K+ channels control the set point for plasma glucose homeostasis. 

Diabetes Mellitus

Diabetes Mellitus arises when no insulin, or not enough insulin, is produced, or…