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FIGURE 2 Interaction of hormones to maintain the blood glucose concentration. Solid arrows (+) denote increase; dashed arrows (—) denote decrease.

The rate of glucose removal from the blood varies over a wide range depending on physical activity and environmental temperature. The rate of input largely reflects activity of the liver, since even immediately after eating, glucose absorbed from the intestines must pass through the liver before entering the circulation. Liver glycogen is the immediate source of blood glucose under most circumstances. Hepatic gluconeogenesis may contribute to blood glucose directly but is more important for replenishing glycogen stores. The kidneys are also capable of gluconeogenesis, but their role as providers of blood glucose has not been studied to any great extent, except in acidosis when glucose production from glutamate accompanies renal production and excretion of ammonia. Recent studies in patients undergoing liver transplantation, however, indicate that glucose production by the kidneys immediately after removal of the liver can be substantial, at least for a short time.

Minute-to-minute regulation of blood glucose depends on (1) insulin, which, in promoting fuel storage, drives glucose concentrations down; and (2) glucagon, and to a lesser extent catecholamines, which, in mobilizing fuel reserves, drive glucose concentrations up. Effects of these hormones are evident within seconds or minutes and dissipate as quickly. Insulin acts at the level of the liver to inhibit glucose output, and on muscle and fat to increase glucose uptake. Liver is more responsive to insulin than muscle and fat, and because of its anatomic location, is exposed to higher hormone concentrations. Smaller increments in insulin concentration are needed to inhibit glucose production than to promote glucose uptake. Glucagon and catecholamines act on hepatocytes to promote glycogenolysis and gluconeogenesis. They have no direct effects on glucose uptake by peripheral tissues, but epinephrine and norepinephrine may decrease the demand for blood glucose by mobilizing alternative fuels, glycogen and fat, within muscle and adipose tissue. Increased blood glucose is perceived directly by pancreatic beta cells, which respond by secreting insulin. Hypoglycemia is perceived not only by the glucagon-secreting alpha cells of pancreatic islets, but also by the central nervous system, which activates sympathetic outflow to the islets and the adrenal medullae. Sympathetic stimulation of pancreatic islets increases secretion of glucagon and inhibits secretion of insulin. In addition, hypoglycemia evokes secretion of the hypothalamic releasing hormones that stimulate ACTH and GH secretion from the pituitary gland (Fig. 3). Cortisol, secreted in response to ACTH, and GH act only after a substantial delay and hence are unlikely to contribute to rapid restoration of blood glucose. However, they are important for withstanding a sustained hypoglycemic challenge.

Long-term regulation, operative on a timescale of hours or perhaps days, depends on direct and indirect

FIGURE 3 Counter-regulatory hormonal responses to insulin-induced hypoglycemia. The infusion of insulin reduced plasma glucose concentration to 50-55 mg/dL. (From Sacca L, Sherwin R, Hendler R, Felig P. Influence of continuous physiologic hyperinsulinemia on glucose kinetics and counterregulatory hormones in normal and diabetic humans. J Clin Invest 1979;63:849-857.)

FIGURE 3 Counter-regulatory hormonal responses to insulin-induced hypoglycemia. The infusion of insulin reduced plasma glucose concentration to 50-55 mg/dL. (From Sacca L, Sherwin R, Hendler R, Felig P. Influence of continuous physiologic hyperinsulinemia on glucose kinetics and counterregulatory hormones in normal and diabetic humans. J Clin Invest 1979;63:849-857.)

actions of many hormones and ultimately ensures (1) that the peripheral drain on glucose reserves is minimized and (2) that liver contains an adequate reservoir of glycogen to satisfy the minute-to-minute needs of glucose-dependent cells. To achieve these ends, peripheral tissues, mainly muscle, must be provided with alternate substrate and limit their consumption of glucose. At the same time, gluconeogenesis must be stimulated and supplied with adequate precursors to provide the 150-200 g of glucose needed each day by the brain and other glucose-dependent tissues. Long-term regulation includes all of the responses that govern glucose utilization as well as all those reactions

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