Insulin dependent tissues and insulin independent tissues
All those tissues that need insulin to absorb the glucose necessary for cellular metabolism from the bloodstream are defined as insulin dependent.
Examples of insulin dependent tissues are resting muscle, leukocytes, adipose tissue, and mammary glands.
All those tissues that do not depend directly on insulin to absorb the glucose necessary for cellular metabolism from the bloodstream are defined as independent insulin.
Examples of insulin independent tissues are nervous tissue, intestinal epithelium, erythrocytes, muscle during physical exertion and renal tubules.
The uptake of glucose by muscle and adipose tissue is regulated by the exocytosis of vesicles that contain GLUT4, special insulin-dependent glucose transporters. This biological event is stimulated by the interaction between insulin and the respective membrane receptor. With the removal of insulin, on the contrary, the process reverses from exocytosis to endocytosis, the GLUT-4 is again sequestered in the cytoplasmic vesicles and the glucose entry is drastically reduced.
In insulin-independent tissues, on the other hand, the entry of glucose into the cells is allowed by other isoforms of transporters, always present in the plasma membrane and independent of insulin levels.
The transport of glucose in the liver cells (hepatocytes) is not directly insulin-dependent, but is however influenced by the presence or absence of insulin. In fact, at the hepatic level we find special glucose transporters, called GLUT-2, which can operate in both directions: when insulin levels are high, glycolysis, glycogenosynthesis and lipogenesis predominate; consequently the glucose concentration in the hepatocytes remains lower than that of the blood plasma, so that the sugar continuously diffuses into the hepatocytes through the GLUT-2 carriers. On the other hand, when insulin levels are lowered, the glucose obtained from glycogenolysis and hepatic gluconeogenesis escapes from the hepatocytes using the same transport system, and enters the blood where it helps to maintain euglycemia.