MINIRE VIEW Neural Control of Blood Glucose Level

It is well known that blood glucose is a major source of energy for cellular activities and the liver is an important organ for the storage of glucose in the form of glycogen. It is generally recognized that blood glucose level is maintained at a relatively constant level, and catecholamines (epinephrine and norepinephrine) released from the adrenal gland, and insulin and glucagon released from the pancreas play important roles in the regulation of the blood glucose level. Insulin is a key substance in controlling entry of glucose into the cell and utilization of glucose in the cell. Another insulin effect is to increase storage of glycogen in the liver and other tissues. These actions of insulin result in a fall in the blood glucose level. Glucagon, epinephrine, and norepinephrine cause release of glucose from the liver into the circulating blood by stimulating hepatic glycogenolysis. During starvation an increase in the rate of secretion of glucagon, epinephrine, and norepinephrine occurs. This facilitates release of glucose from the liver, thereby contributing to the maintenance of the blood glucose concentration above the lower limit of the physiological range. This is an important factor, guaranteeing reasonable utilization of glucose by the body and brain cells. It can be stated that insulin is an indispensable hormone for the utilization of glucose, and epinephrine, norepinephrine, and glucagon are the hormones which raise and maintain the glucose level in order to support utilization of the blood glucose. It has been reported that stimulation of the sympathetic nerve to the adrenal medulla causes release of epinephrine and norepinephrine into the circulating blood, resulting in release of glucose from the liver. Although it is generally accepted that the glucose concentration in the blood plasma has a direct effect on the islets of Langerhans in the pancreas in controlling the rate of insulin and glucagon secretion, the existence of an autonomic neural control mechanism for regulating the rate of secretion of these substances has been strongly suggested by WooDs and PoRTE [72]. This paper reviews the reports on the visceral afferents, glucose-responsive neurons in the central nervous system and autonomic outflows in relation to the regulation of blood glucose level.