Control of Blood Glucose

Abstract

Some molecular control mechanisms of blood glucose are described schematically in Fig. 4.1. Glucose comes from food and liver, and is utilized by brain and nerve cells (insulin-independent) via the glucose transporter 3 (GLUT3) or by tissue cells such as muscle, kidney, and fat cells (insulin-dependent) via the glucose transporter 4 (GLUT4). Glucose is transported into and out of liver cells by the concentrationdriven glucose transporter 2 (GLUT2), which is insulin-independent. In response to a low blood glucose level (<80 mg/dl or 4.4 mmol/L), α cells of the pancreas produce the hormone glucagon. The glucagon initiates a series of activations of kinases, and finally leads to the activation of the glycogen phosphorylase, which catalyzes the breakdown of glycogen into glucose. In addition, the series of activations of kinases also result in the inhibition of glycogen synthase and then stop the conversion of glucose to glycogen. In response to a high blood glucose level (> 120 mg/dl or 6.7 mmol/L), β cells of the pancreas secrete insulin. Insulin triggers a series of reactions to activate the glycogen synthase, which catalyzes the conversion of glucose into glycogen. Insulin also initiates a series of activations of kinases in tissue cells to lead to the redistribution of GLUT4 from intracellular storage sites to the plasma membrane. Once at the cell surface, GLUT4 transports glucose into the muscle or fat cells.