Control of glycaemia.
Maintenance of plasma glucose concentrations within a narrow range despite wide fluctuations in the demand (e.g. vigorous exercise) and supply (e.g. large carbohydrate meals) of glucose results from coordination of factors that regulate glucose release into and removal from the circulation. On a moment-to-moment basis these processes are controlled mainly by insulin and glucagon, whose secretion is reciprocally influenced by the plasma glucose concentration. In the resting postabsorptive state, release of glucose from the liver (equally via glycogenolysis and gluconeogenesis) is the key regulated process. Glycogenolysis depends on the relative activities of glycogen synthase and phosphorylase, the latter being the more important. The activities of fructose-1,6-diphosphatase, phosphoenolpyruvate carboxylkinase and pyruvate dehydrogenase regulate gluconeogenesis, whose main precursors are lactate, glutamine and alanine. In the postprandial state, suppression of liver glucose output and stimulation of skeletal muscle glucose uptake are the most important factors. Glucose disposal by insulin-sensitive tissues is regulated initially at the transport step and the mainly by glycogen synthase, phosphofructokinase and pyruvate dehydrogenase. Hormonally induced changes in intracellular fructose 2,6-bisphosphate concentrations play a key role in muscle glycolytic flux and both glycolytic and gluconeogenic flux in the liver. Under stressful conditions (e.g. hypoglycaemia, trauma, vigorous exercise), increased secretion of other hormones such as adrenaline, cortisol and growth hormone, and increased activity of the sympathetic nervous system, come into play; their actions to increase hepatic glucose output and to suppress tissue glucose uptake are partly mediated by increases in tissue fatty acid oxidation. In diabetes, the most common disorder of glucose homeostasis, fasting hyperglycaemia, results primarily from excessive release of glucose by the liver due to increased gluconeogenesis; postprandial hyperglycaemia results from both impaired suppression of hepatic glucose release and impaired skeletal muscle glucose uptake. These abnormalities are usually due to the combination of impaired insulin secretion and tissue resistance to insulin, the causes of which remain to be determined.