Adrenergic mechanisms contribute to the late phase of hypoglycemic glucose counterregulation in humans by stimulating lipolysis

Effects of insulin on the counterregulatory response to equivalent hypoglycemia in normal females

The aim of this study was to determine if insulin could augment the counterregulatory response to equivalent hypoglycemia in normal females similarly to males. Experiments were carried out in nine normal lean overnight-fasted female subjects. Insulin was infused in two separate randomized protocols so that steady-state levels of 794 +/- 62 (low) and 3,620 +/- 476 pM (high) were obtained. Despite an identical plasma glucose level (2.8 +/- 0.1 mM), epinephrine (5.7 +/- 0.9 vs. 3.9 +/- 0.6 nM), norepinephrine (2.7 +/- 0.4 vs. 1.8 +/- 0.3 nM), cortisol (918 +/- 55 vs. 826 nM), and growth hormone (35.8 +/- 3.7 vs. 28.4 +/- 2.7 micrograms/l) were increased (P < 0.05) during high compared with low insulin infusion, respectively. Glucagon and pancreatic polypeptide levels increased significantly but were not different during the two insulin infusions. Hepatic glucose production was increased during the high-compared with low-dose infusions (9.5 +/- 1.1 vs. 5.1 +/- 2.2 mumol.kg-1 x min-1; P < 0.05). Lipolysis, as indicated by the blood glycerol level, increased significantly during high- compared with low-dose insulin infusions (121 +/- 29 vs. 65 +/- 13 microM; P < 0.05). The hormonal and metabolic responses to hypoglycemia were significantly different in females compared with previous results in males.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration of a critical role for free fatty acids in mediating counterregulatory stimulation of gluconeogenesis and suppression of glucose utilization in humans

In vitro studies indicate that FFA compete with glucose as an oxidative fuel in muscle and, in addition, stimulate gluconeogenesis in liver. During counterregulation of hypoglycemia, plasma FFA increase and this is associated with an increase in glucose production and a suppression of glucose utilization. To test the hypothesis that FFA mediate changes in glucose metabolism that occur during counterregulation, we examined the effects of acipimox, an inhibitor of lipolysis, on glucose production and utilization ([3-3H]glucose), and incorporation of [U-14C]-alanine into glucose during insulin-induced hypoglycemia. Eight normal volunteers were infused with insulin for 8 h to produce modest hypoglycemia (approximately 3 mM) on two occasions, first without acipimox (control) and then with acipimox administration (250 mg per os at 60 and 240 min). Despite identical plasma insulin concentrations, glucose had to be infused in the acipimox experiments (glucose-clamp technique) to maintain plasma glucose concentrations identical to those in control experiments. Acipimox completely prevented counterregulatory increases in lipolysis so that during the last 4 h plasma FFA were below baseline values and averaged 67 +/- 13 vs. 725 +/- 65 microM in control experiments, P < 0.001. Concomitantly, overall glucose production was reduced by 40% (5.5 +/- 11 vs. 9.3 +/- 0.7 mumol/kg per min, P < 0.001), and gluconeogenesis from alanine was reduced by nearly 70% (0.32 +/- 0.09 vs. 1.00 +/- 0.18 mumol/kg per min, P < 0.001), while glucose utilization increased by 15% (10.8 +/- 1.4 vs. 9.3 +/- 0.7 mumol/kg per min). We conclude that FFA play a critical role in mediating changes in glucose metabolism during counterregulation, and that under these conditions, FFA exert a much more profound effect on hepatic glucose production than on glucose utilization.

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.

The contribution of naturally labelled 13C fructose to glucose appearance in humans

Among monosaccharides, fructose has a small hyperglycaemic effect. In order to better explain the mechanisms which cause this metabolic property, we used tracers labelled with stable isotopes (deuterated glucose and naturally 13C labelled fructose) to quantify the overall glucose appearance, the rate of appearance in plasma of the 13C glucose synthesized from fructose, and the fructose oxidation in vivo in man during a 6-h period following ingestion of 0.5 and 1 g.kg-1 fructose. Fructose had a very small effect on overall glucose appearance (NS). During the 6 h of the study, it was found that the overall glucose appearance was 0.87 +/- 0.06 and 0.89 +/- 0.06 g.kg-1 (NS). The amount of glucose synthesized from fructose was 0.27 +/- 0.04 and 0.51 +/- 0.03 g.kg-1 (p < 0.01) representing 31% and 57% of overall glucose appearance (p < 0.01); the non-fructose glucose production was 0.60 +/- 0.02 and 0.38 +/- 0.03 g.kg-1 (p < 0.05) after the 0.5 and 1 g.kg-1 load, respectively. Fructose oxidation was 0.28 +/- 0.03 and 0.59 +/- 0.07 g.kg-1 after the 0.5 and 1 g.kg-1 load respectively (p < 0.01) representing 56% and 59% of the fructose loads (NS). These data show that the low hyperglycaemic effect of fructose is explained by its very small effect on overall glucose appearance and that fructose has a sparing effect on glucose metabolism.

Glucose counterregulation: prevention and correction of hypoglycemia in humans

The prevention or correction of hypoglycemia in humans is the result of both dissipation of insulin and activation of glucose counterregulatory (glucose-raising) systems. Whereas insulin is the dominant glucose-lowering factor, there are redundant glucose counterregulatory factors. Furthermore, there is a hierarchy among the glucoregulatory factors. The first defense against a decrement in plasma glucose is decreased insulin secretion; this occurs with glucose decrements within the physiological range at a glycemic threshold of 4.6 +/- 0.2 mmol/l. However, biological glucose recovery from hypoglycemia can occur despite mild (approximately 2-fold) peripheral hyperinsulinemia and can occur in the absence of portal hypoinsulinemia. Thus additional (glucose counterregulatory) factors must be involved. Critical glucose counterregulatory systems are activated at glycemic thresholds of approximately 3.8 mmol/l (the level at which brain glucose uptake is first measurably reduced), well above the thresholds for symptoms of hypoglycemia (approximately 3.0 mmol/l) and those for cognitive dysfunction resulting from neuroglycopenia (approximately 2.7 mmol/l). Among the glucose counterregulatory factors, glucagon plays a primary role. Indeed, it may be that hypoglycemia does not occur if the secretion and actions of both glucagon and insulin, among the glucoregulatory hormones, are normal. Epinephrine is not normally critical, but it becomes critical to glucose counterregulation when glucagon is deficient. Because hypoglycemia develops or progresses when both glucagon and epinephrine are deficient and insulin is present, these three hormones stand high in the hierarchy of redundant glucoregulatory factors.(ABSTRACT TRUNCATED AT 250 WORDS)