In the fasting state, as the concentration of glucose in the poal blood coming from the small intestine falls, insulin secretion decreases, and skeletal muscle and adipose tissue take up less glucose. The increase in secretion of glucagon by a cells of the pancreas inhibits glycogen synthetase and activates glycogen phosphorylase in the liver leading to glycogenolysis. The resulting glucose-6-phosphate is hydrolyzed by glucose 6-phosphatase, and glucose is released into the bloodstream for use by the brain and erythrocytes. Reference: Harper; 30th edition; Chapter 14; Page no: 148