Hypoglycemia-associated Autonomic Failure Research Articles

Cerebral glutamate metabolism during hypoglycaemia in healthy and type 1 diabetic humans

The mechanisms responsible for the progressive failure of hypoglycaemia counterregulation in long-standing type 1 diabetes are poorly understood. Increased brain glucose uptake during hypoglycaemia or alterations of brain energy metabolism could effect glucose sensing by the brain and thus contribute to hypoglycaemia-associated autonomic failure. Type 1 diabetic patients (T1DM) and healthy volunteers (CON) were studied before, during and after a hypoglycaemic (50 mg dL(-1)) hyperinsulinaemic (1.5 mU kg(-1) min(-1)) clamp test. The (1)H magnetic resonance spectroscopy of the occipital lobe of the brain was performed employing the STEAM localization technique. The water signal was suppressed by the modified SWAMP method. All spectra were acquired on a 3 Tesla scanner (80 cm MEDSPEC-DBX, Bruker Medical, Ettlingen, Germany) using a 10-cm diameter surface coil. During hypoglycaemia, T1DM showed blunted endocrine counterregulation. At baseline the brain tissue glucose : creatine ratio was lower in CON than in T1DM (CON 0.13 +/- 0.05 vs. T1DM 0.19 0.11; P < 0.01). During hypoglycaemia glucose : creatine ratios decreased in both groups (CON 0.07 +/- 0.08, P < 0.05; T1DM 0.03 +/- 0.03, P < 0.001). A significant drop in the glutamate : creatine ratio could only be found in CON during hypoglycaemia (CON 1.36 +/- 0.08 vs. 1.26 +/- 0.11; P < 0.01; T1DM 1.32 +/- 0.13 vs. 1.28 +/- 0.15; P = NS). The ratios of glutamine, N-acetylaspartate, choline and myo-inositol : creatine were not different between both groups and did not change throughout the experiment. Only in CON does moderate hypoglycaemia reduce intracerebral glutamate concentrations, possibly owing to a slower substrate flux through the tricarboxylic acid cycle in neurones. The maintenance of normal energy metabolism in T1DM during hypoglycaemia might effect glucose sensing in the brain and contribute to hypoglycaemia-associated autonomic failure.

Feeding and neuroendocrine responses after recurrent insulin-induced hypoglycemia

Prior exposure to hypoglycemia impairs neuroendocrine counterregulatory responses (CRR) during subsequent hypoglycemia. Defective CRR to hypoglycemia is a component of the clinical syndrome hypoglycemia-associated autonomic failure (HAAF). Hypoglycemia also potently stimulates food intake, an important behavioral CRR. Because the increased feeding response to hypoglycemia is behavioral and not hormonal, we hypothesized that it may be regulated differently with recurrent bouts of hypoglycemia. To test this hypothesis, we simultaneously evaluated neuroendocrine CRR and food intake in rats experiencing one or three episodes of insulin-induced hypoglycemia. As expected, recurrent hypoglycemia significantly reduced neuroendocrine hypoglycemic CRR. Epinephrine (E), norepinephrine (NE) and glucagon responses 120 min after insulin injection were significantly reduced in recurrent hypoglycemic rats, relative to rats experiencing hypoglycemia for the first time. Despite these neuroendocrine impairments, food intake was significantly elevated above baseline saline intake whether rats were experiencing a first (hypoglycemia: 3.4 ± 0.4 g vs. saline: 0.94 ± 0.3 g, P < 0.05) or third hypoglycemic episode (hypoglycemia: 3.8 ± 0.3 g vs. saline: 1.2 ± 0.3 g, P < 0.05). These findings demonstrate that food intake elicited in response to hypoglycemia is not impaired as a result of recurrent hypoglycemia. Thus, neuroendocrine and behavioral (stimulation of food intake) CRR are differentially regulated by recurrent hypoglycemia experience.

Mechanisms of Hypoglycemia-Associated Autonomic Failure and Its Component Syndromes in Diabetes

Iatrogenic hypoglycemia is a problem for people with diabetes. It causes recurrent morbidity, and sometimes death, as well as a vicious cycle of recurrent hypoglycemia, precluding maintenance of euglycemia over a lifetime of diabetes. Improved therapeutic approaches that will minimize both hypo- and hyperglycemia will be based on insight into the pathophysiology of glucoregulation, specifically glucose counterregulation, in insulin-deficient (type 1 and advanced type 2) diabetes. In such patients, hypoglycemia is the result of the interplay of relative or absolute therapeutic insulin excess and compromised physiological (the syndrome of defective glucose counterregulation) and behavioral (the syndrome of hypoglycemia unawareness) defenses against falling plasma glucose concentrations. The concept of hypoglycemia-associated autonomic failure (HAAF) in diabetes posits that recent antecedent iatrogenic hypoglycemia causes both defective glucose counterregulation (by reducing epinephrine responses to a given level of subsequent hypoglycemia in the setting of absent decrements in insulin and absent increments in glucagon) and hypoglycemia unawareness (by reducing sympathoadrenal and the resulting neurogenic symptom responses to a given level of subsequent hypoglycemia) and thus a vicious cycle of recurrent hypoglycemia. The clinical impact of HAAF is well established in type 1 diabetes; it also affects those with advanced type 2 diabetes. It is now known to be largely reversible, by as little as 2-3 weeks of scrupulous avoidance of hypoglycemia, in most affected patients. However, the mechanisms of HAAF and its component syndromes are largely unknown. Loss of the glucagon secretory response, a key feature of defective glucose counterregulation, is plausibly explained by insulin deficiency, specifically loss of the decrement in intraislet insulin that normally signals glucagon secretion as glucose levels fall. Reduced neurogenic symptoms, a key feature of hypoglycemia unawareness, are largely the result of reduced sympathetic neural responses to falling glucose levels. The mechanism by which hypoglycemia shifts the glycemic thresholds for sympathoadrenal activation to lower plasma glucose concentrations, the key feature of both components of HAAF, is not known. It does not appear to be the result of the release of a systemic mediator (e.g., cortisol, epinephrine) during antecedent hypoglycemia or of increased blood-to-brain glucose transport (although increased transport of alternative fuels is conceivable). It is likely the result of alterations of brain metabolism. Although there is an array of clues, the specific alteration remains to be identified. While the research focus has been largely on the hypothalamus, hypoglycemia is now known to activate widespread brain regions, including the medial prefrontal cortex. The possibility that HAAF could be the result of posthypoglycemic brain glycogen supercompensation has also been raised. Finally, there appear to be diverse causes of HAAF. In addition to recent antecedent hypoglycemia, these include exercise- and sleep-related HAAF. Clearly, a unifying mechanism of HAAF would need to incorporate these causes as well. Pending the prevention and cure of diabetes, critical fundamental, translational, and outcomes research is needed if we are to eliminate hypoglycemia from the lives of people affected by diabetes.

Cortical Fluoro-Jade Staining and Blunted Adrenomedullary Response to Hypoglycemia after Noncoma Hypoglycemia in Rats

Intensive insulin therapy in patients with type 1 diabetes mellitus reduces long-term complications; however, intensive therapy is also associated with a three-fold increase in hypoglycemic episodes. The present study in conscious rats characterizes the physiologic and neuropathologic consequences of a single episode of moderate hypoglycemia. In this model, intravenous insulin is used to reduce plasma glucose to 30 to 35 mg/dL for 75 mins. This single hypoglycemic insult acutely induces hypoglycemia-associated autonomic failure (HAAF), with epinephrine responses to hypoglycemia reduced more than 36% from control. Neuropathology after this insult includes the appearance of dying cells, assessed with the marker Fluoro-jade B (FJ). After hypoglycemic insult, FJ+ cells were consistently seen in subdivisions of the medial prefrontal cortex, the orbital cortex, and the piriform cortex. There was a significant correlation between depth of hypoglycemia and number of FJ+ cells, suggesting that there is a critical threshold below which vulnerable cells begin to die. These data suggest that there is a population of cells that are vulnerable to moderate levels of hypoglycemia commonly experienced by patients with insulin-treated diabetes. These cells, which may be neurons, are primarily found in cortical regions implicated in visceral perception and autonomic control, raising the possibility that their loss contributes to clinically reported deficits in autonomic and perceptual responses to hypoglycemia.

Recurrent insulin-induced hypoglycemia causes site-specific patterns of habituation or amplification of CNS neuronal genomic activation

Antecedent hypoglycemia is a primary factor in hypoglycemia-associated autonomic failure, a pathophysiological condition characterized by impaired glucose counterregulatory function. Conventional therapeutic strategies involving administration of intermediate dosage-release formulations of insulin in the management of insulin-dependent diabetes mellitus result in frequent iatrogenic hypoglycemia. This study investigated the neuroanatomical location, direction, and magnitude of CNS neuronal genomic activation by singular versus repeated induction of hypoglycemic bouts of greater than 6 h duration achieved by administration of the intermediate-acting insulin, humulin neutral protamine Hagedorn (NPH). Adult male rats injected subcutaneously with Humulin NPH exhibited robust immunolabeling for the nuclear transcription factor, Fos, in discrete telencephalic, diencephalic, midbrain, and caudal hindbrain loci in a pattern that was not identical to that described for regular insulin. Administration of four doses of insulin on as many days significantly diminished or extinguished Fos immunostaining within the parvocellular hypothalamic paraventricular nucleus, lateral hypothalamic area, dorsomedial hypothalamic nucleus, thalamic paraventricular nucleus, nucleus tractus solitarius, and area postrema, but did not modify labeling of other metabolic loci. However, numbers of Fos-immunoreactivity-positive magnocellular neurons in the hypothalamic paraventricular and supraoptic nuclei were significantly increased after the second and fourth insulin doses, relative to the single-dose group. Concurrent observations of exacerbated hypoglycemia and modified patterns of glucoregulatory hormone secretion after serial injections of intermediate-acting insulin suggest that central mechanisms governing compensatory endocrine responses, specifically glucagon, become habituated to repetitive hypoglycemia of extended duration. Resultant alterations in CNS-islet and -adrenomedullary output and hypothalamic–pituitary–adrenal activity may reflect diminished neuronal activation within one or more of the brain loci characterized here by nonuniform transcriptional activation. The current studies provide a neuroanatomical foundation for further investigation of the neurochemical phenotypes and interconnectivity of functionally adaptive neurons, underlying cellular and molecular mechanisms of diminished or enhanced activation, as well as the impact of these modified cellular responses on glucose counterregulation during administration of intermediate-acting insulin.

Morbidity of hypoglycemia in type 1 diabetes

Hypoglycemia is a common side effect of intensive insulin therapy in patients with type 1 diabetes. Mild hypoglycemia is any episode that can be self-treated, while a severe episode requires external help for recovery. Acute hypoglycemia produces autonomic and neuroglycopenic symptoms, including cognitive impairment and mood changes, while sympathoadrenal stimulation can provoke acute hemodynamic changes with alterations in regional vascular perfusion and a risk of cardiac dysrhythmias. Neurological manifestations include coma, convulsions and focal abnormalities. Long-term morbidities associated with hypoglycemia include impaired awareness of hypoglycemia, counterregulatory hormonal deficiencies, hypoglycemia-associated autonomic failure, and, in rare cases, permanent cognitive impairment. Hypoglycemia affects all aspects of life for the person with type 1 diabetes, including employment, social interactions, driving, sport and leisure activities, and sleep. Appreciation of the potential morbidities of hypoglycemia should encourage physicians to utilize therapeutic regimens that decrease the risk of severe hypoglycemia.

Diverse causes of hypoglycemia-associated autonomic failure in diabetes.

This review explains our current understanding of the syndromes of defective glucose counterregulation and hypoglycemia unawareness in persons with diabetes. The physiologic defenses against hypoglycemia — the glucagon and epinephrine responses — are compromised in most people with type 1 diabetes and in many with advanced type 2 diabetes. In addition to hypoglycemia, exercise and sleep are causes of autonomic failure that must be recognized and require changes in the management of diabetes.

A single episode of central glucoprivation reduces the adrenomedullary response to subsequent hypoglycemia in rats

Hypoglycemia-associated autonomic failure (HAAF) is a complication of recurrent hypoglycemia in patients with diabetes. The present experiments were performed to study the contribution of brain glucose-sensitive structures to HAAF. In conscious rats, brain glucoprivation was elicited by injection of 2 deoxy- d-glucose (2-DG) into the third cerebral ventricle (intracerebroventricular, ICV), while controls received saline ICV. Two days after ICV injection, hormone responses to systemic hypoglycemic challenge were measured. The epinephrine response 60 min after insulin injection was reduced by 46% in rats treated with prior ICV 2-DG, relative to saline-treated controls (SAL: 10,050±970 pg/ml; 2-DG: 5590±880 pg/ml). Thus, the adrenomedullary response to sustained hypoglycemia is attenuated after prior brain glucoprivation induced by intracerebroventricular injection of 2-DG. These data support the hypothesis that the brain contributes to the loss of responsiveness to repeated hypoglycemia that may ultimately result in HAAF.

Hypoglycemia in Type 1 Diabetes Mellitus

Spontaneous hypoglycemia is uncommon in the general (nondiabetic) population, but iatrogenic hypoglycemia is rife in patients with type 1 diabetes mellitus, among whom hypoglycemia constitutes a barrier to optimal glycemic control. The obligate dependence on exogenous insulin, together with the current imperfection in insulin therapies, generates degrees of blood glucose fluctuations that often exceed physiological boundaries in these patients. Downward swings in blood glucose levels, if sustained, result in hypoglycemia and significant morbidity and mortality. Hypoglycemia in type 1 diabetes indicates an imbalance between caloric supply and glucose use in response to insulin or exercise. Counterregulatory mechanisms that auto-correct iatrogenic hypoglycemia often become progressively impaired in these patients. This defective counterregulation, together with the imperfections in insulin delivery, set the stage for significant morbidity from iatrogenic hypoglycemia. Recurrent episodes of iatrogenic hypoglycemia induce a state of hypoglycemia unawareness and defective counterregulation, which defines the syndrome of hypoglycemia-associated autonomic failure (HAAF). The reduced awareness of, and counterregulatory responses to, hypoglycemia in patients with HAAF lead to worsening episodes of severe hypoglycemia. Approaches to the prevention of hypoglycemia include glucose monitoring, patient education, meal planning, and medication adjustment. In patients with HAAF, scrupulous avoidance of iatrogenic hypoglycemia may restore the symptomatic and counterregulatory responses to hypoglycemia. Behavioral training focusing on recognition of the more subtle symptoms and signs of evolving hypoglycemia may be beneficial to some patients with HAAF. A methodical search for the pattern and etiology of iatrogenic hypoglycemia is a prerequisite for the identification of the best preventive approach. With proper education, patients with type 1 diabetes and their physicians can learn to prevent or minimize the risk of hypoglycemia while pursuing excellence in glycemic control.

Glucosensing neurons in the ventromedial hypothalamic nucleus (VMN) and hypoglycemia-associated autonomic failure (HAAF).

Hypoglycemia is a profound threat to the brain since glucose is its preferred fuel. Thus, decreases in plasma glucose must be sensed and appropriate hormonal and neuroendocrine responses generated to restore glucose to safe levels (i.e. counterregulatory responses (CRR) to hypoglycemia). Recurrent hypoglycemia impairs these protective mechanisms, resulting in a potentially life-threatening condition known as hypoglycemia-associated autonomic failure (HAAF). During HAAF, the glycemic threshold is reset so that glucose levels must fall further before the CRR is initiated. The brain plays a critical role in sensing hypoglycemia and initiating the CRR. Additionally, many neurons may sense changes in plasma and extracellular glucose. However, the way in which central glucose sensing is integrated to lead to effective initiation of the CRR is unknown. Furthermore, the mechanisms by which this system becomes impaired during HAAF are also unknown. Glucosensing neurons in the ventromedial hypothalamic nucleus (VMN) are poised to serve an integrative function in glucose homeostasis. First, they sense glucose. Second, the VMN receives input from other glucose-sensing areas. Finally, the VMN projects to areas linked to the regulation of the sympathoadrenal system that mediates the CRR. This review discusses VMN glucosensing neurons relative to their capacity to play a role in the regulation of the CRR and the generation of HAAF. Glucosensing neurons in the hindbrain as well as peripheral glucosensors are also considered.

Hypoglycemia in diabetes.

Iatrogenic hypoglycemia causes recurrent morbidity in most people with type 1 diabetes and many with type 2 diabetes, and it is sometimes fatal. The barrier of hypoglycemia generally precludes maintenance of euglycemia over a lifetime of diabetes and thus precludes full realization of euglycemia's long-term benefits. While the clinical presentation is often characteristic, particularly for the experienced individual with diabetes, the neurogenic and neuroglycopenic symptoms of hypoglycemia are nonspecific and relatively insensitive; therefore, many episodes are not recognized. Hypoglycemia can result from exogenous or endogenous insulin excess alone. However, iatrogenic hypoglycemia is typically the result of the interplay of absolute or relative insulin excess and compromised glucose counterregulation in type 1 and advanced type 2 diabetes. Decrements in insulin, increments in glucagon, and, absent the latter, increments in epinephrine stand high in the hierarchy of redundant glucose counterregulatory factors that normally prevent or rapidly correct hypoglycemia. In insulin-deficient diabetes (exogenous) insulin levels do not decrease as glucose levels fall, and the combination of deficient glucagon and epinephrine responses causes defective glucose counterregulation. Reduced sympathoadrenal responses cause hypoglycemia unawareness. The concept of hypoglycemia-associated autonomic failure in diabetes posits that recent antecedent hypoglycemia causes both defective glucose counterregulation and hypoglycemia unawareness. By shifting glycemic thresholds for the sympathoadrenal (including epinephrine) and the resulting neurogenic responses to lower plasma glucose concentrations, antecedent hypoglycemia leads to a vicious cycle of recurrent hypoglycemia and further impairment of glucose counterregulation. Thus, short-term avoidance of hypoglycemia reverses hypoglycemia unawareness in most affected patients. The clinical approach to minimizing hypoglycemia while improving glycemic control includes 1) addressing the issue, 2) applying the principles of aggressive glycemic therapy, including flexible and individualized drug regimens, and 3) considering the risk factors for iatrogenic hypoglycemia. The latter include factors that result in absolute or relative insulin excess: drug dose, timing, and type; patterns of food ingestion and exercise; interactions with alcohol and other drugs; and altered sensitivity to or clearance of insulin. They also include factors that are clinical surrogates of compromised glucose counterregulation: endogenous insulin deficiency; history of severe hypoglycemia, hypoglycemia unawareness, or both; and aggressive glycemic therapy per se, as evidenced by lower HbA(1c) levels, lower glycemic goals, or both. In a patient with hypoglycemia unawareness (which implies recurrent hypoglycemia) a 2- to 3-week period of scrupulous avoidance of hypoglycemia is advisable. Pending the prevention and cure of diabetes or the development of methods that provide glucose-regulated insulin replacement or secretion, we need to learn to replace insulin in a much more physiological fashion, to prevent, correct, or compensate for compromised glucose counterregulation, or both if we are to achieve near-euglycemia safely in most people with diabetes.

Sleep-related hypoglycemia-associated autonomic failure in type 1 diabetes: reduced awakening from sleep during hypoglycemia.

Given that iatrogenic hypoglycemia often occurs during the night in people with type 1 diabetes, we tested the hypothesis that physiological, and the resulting behavioral, defenses against developing hypoglycemia-already compromised by absent glucagon and attenuated epinephrine and neurogenic symptom responses-are further compromised during sleep in type 1 diabetes. To do so, we studied eight adult patients with uncomplicated type 1 diabetes and eight matched nondiabetic control subjects with hyperinsulinemic stepped hypoglycemic clamps (glucose steps of approximately 85, 75, 65, 55, and 45 mg/dl) in the morning (0730-1230) while awake and at night (2100-0200) while awake throughout and while asleep from 0000 to 0200 in random sequence. Plasma epinephrine (P = 0.0010), perhaps norepinephrine (P = 0.0838), and pancreatic polypeptide (P = 0.0034) responses to hypoglycemia were reduced during sleep in diabetic subjects (the final awake versus asleep values were 240 +/- 86 and 85 +/- 47, 205 +/- 24 and 148 +/- 17, and 197 +/- 45 and 118 +/- 31 pg/ml, respectively), but not in the control subjects. The diabetic subjects exhibited markedly reduced awakening from sleep during hypoglycemia. Sleep efficiency (percent time asleep) was 77 +/- 18% in the diabetic subjects, but only 26 +/- 8% (P = 0.0109) in the control subjects late in the 45-mg/dl hypoglycemic steps. We conclude that autonomic responses to hypoglycemia are reduced during sleep in type 1 diabetes, and that, probably because of their reduced sympathoadrenal responses, patients with type 1 diabetes are substantially less likely to be awakened by hypoglycemia. Thus both physiological and behavioral defenses are further compromised during sleep. This sleep-related hypoglycemia-associated autonomic failure, in the context of imperfect insulin replacement, likely explains the high frequency of nocturnal hypoglycemia in type 1 diabetes.

Role of corticotrophin-releasing hormone in the impairment of counterregulatory responses to hypoglycemia.

We have explored the role of individual elements of the hypothalamic pituitary adrenal axis on the pathogenesis of hypoglycemia-associated autonomic failure. Five groups of male Sprague-Dawley rats were used. Control animals had 3 days of sham treatment followed by a hyperinsulinemic/hypoglycemic glucose clamp on day 4. A second group underwent 3 days of antecedent insulin-induced hypoglycemia then a subsequent clamp. Three more groups underwent pretreatment with corticosterone, adrenocorticotrophic hormone (ACTH), or corticotrophin-releasing hormone (CRH) mirroring the glucocorticoid response of the hypoglycemic group. Subsequent counterregulatory responses showed marked differences. CRH- (and insulin-treated) animals showed markedly reduced epinephrine responses (CRH 1,276 +/- 404 pg/ml, controls 3,559 +/- 563 pg/ml; P < 0.05). In contrast, ACTH pretreatment augmented epinephrine responses (6,681 +/- 814 pg/ml; P = 0.007 versus controls); corticosterone pretreatment caused a similar but nonsignificant enhancement. The same pattern was seen for norepinephrine. CRH pretreatment also suppressed glucagon responses to hypoglycemia (control 157 +/- 21, CRH 68 +/- 10 pg/ml; P = 0.004). The addition of a CRH receptor 1 (CRHr1) antagonist to the antecedent CRH reversed the subsequent suppression of epinephrine. These findings suggest that CRH acting via CRHr1 plays an important role in the sympathoadrenal downregulation seen in this rodent model of antecedent hypoglycemia; this action is not mediated via activation of the hypothalamic-pituitary-adrenal axis.

Inactivation of the PVN during hypoglycemia partially simulates hypoglycemia-associated autonomic failure.

The anatomic connections of the paraventricular nucleus of the hypothalamus (PVN) are such that it is ideally situated to modulate and/or control autonomic responses to a variety of stressors, including hypoglycemia. In our experimental model of hypoglycemia-associated autonomic failure (HAAF), a syndrome in which the counterregulatory response to hypoglycemia is partially compromised via unknown mechanisms, activation of the PVN is blunted (15). We hypothesized that this blunted PVN activation during HAAF may be sufficient to cause the impaired counterregulatory response. To test this hypothesis, we anesthetized the PVN with lidocaine during insulin-induced hypoglycemia in rats and measured counterregulatory hormone levels. PVN inactivation decreased indexes of the sympathoadrenal response (plasma epinephrine and norepinephrine) and the hypothalamic-pituitary axis response (ACTH). Inactivation decreased the peak epinephrine response to hypoglycemia by almost half (-42 +/- 6% from control; P = 0.04) and the peak norepinephrine response by 34 +/- 5% (P = 0.01). The peak plasma ACTH levels attained were suppressed by 35 +/- 6% (P = 0.02). Adrenal corticosterone and pancreatic glucagon responses were not impaired. This pattern of neuroendocrine response is unlike that previously seen with our HAAF model. Control infusions of lidocaine >or=1 mm anterior or posterior to the PVN did not simulate this neuroendocrine pattern. Thus it appears that decreased PVN activation, as occurs with HAAF, may be involved in specific components of HAAF (i.e., blunting the sympathoadrenal and hypothalamic-pituitary-adrenocortical axis response), but not in others (i.e., blunting the glucagon response).

Hypoglycaemia: the limiting factor in the glycaemic management of Type I and Type II diabetes.

Hypoglycaemia is the limiting factor in the glycaemic management of diabetes. Iatrogenic hypoglycaemia is typically the result of the interplay of insulin excess and compromised glucose counterregulation in Type I (insulin-dependent) diabetes mellitus. Insulin concentrations do not decrease and glucagon and epinephrine concentrations do not increase normally as glucose concentrations decrease. The concept of hypoglycaemia-associated autonomic failure (HAAF) in Type I diabetes posits that recent antecedent iatrogenic hypoglycaemia causes both defective glucose counterregulation (by reducing the epinephrine response in the setting of an absent glucagon response) and hypoglycaemia unawareness (by reducing the autonomic and the resulting neurogenic symptom responses). Perhaps the most compelling support for HAAF is the finding that as little as 2 to 3 weeks of scrupulous avoidance of hypoglycaemia reverses hypoglycaemia unawareness and improves the reduced epinephrine component of defective glucose counterregulation in most affected patients. The mediator and mechanism of HAAF are not known but are under active investigation. The glucagon response to hypoglycaemia is also reduced in patients approaching the insulin deficient end of the spectrum of Type II (non-insulin-dependent) diabetes mellitus, and glycaemic thresholds for autonomic (including epinephrine) and symptomatic responses to hypoglycaemia are shifted to lower plasma glucose concentrations after hypoglycaemia in Type II diabetes. Thus, patients with advanced Type II diabetes are also at risk for HAAF. While it is possible to minimise the risk of hypoglycaemia by reducing risks -- including a 2 to 3 week period of scrupulous avoidance of hypoglycaemia in patients with hypoglycaemia unawareness -- methods that provide glucose-regulated insulin replacement or secretion are needed to eliminate hypoglycaemia and maintain euglycaemia over a lifetime of diabetes.

Elevated endogenous cortisol reduces autonomic neuroendocrine and symptom responses to subsequent hypoglycemia.

We tested the hypothesis that increased endogenous cortisol secretion reduces autonomic neuroendocrine and neurogenic symptom responses to subsequent hypoglycemia. Twelve healthy young adults were studied on two separate occasions, once after infusions of a pharmacological dose of alpha-(1-24)-ACTH (100 microg/h) from 0930 to 1200 and 1330 to 1600, which raised plasma cortisol levels to approximately 45 microg/dl on day 1, and once after saline infusions on day 1. Hyperinsulinemic (2.0 mU x kg(-1) x min(-1)) stepped hypoglycemic clamps (90, 75, 65, 55, and 45 mg/dl glucose steps) were performed on the morning of day 2 on both occasions. These markedly elevated antecedent endogenous cortisol levels reduced the adrenomedullary (P = 0.004, final plasma epinephrine levels of 489 +/-64 vs. 816 +/-113 pg/ml), sympathetic neural (P = 0.0022, final plasma norepinephrine levels of 244 +/-15 vs. 342 +/-22 pg/ml), parasympathetic neural (P = 0.0434, final plasma pancreatic polypeptide levels of 312 +/- 37 vs. 424 +/- 56 pg/ml), and neurogenic (autonomic) symptom (P = 0.0097, final symptom score of 7.1 +/-1.5 vs. 10.6 +/- 1.6) responses to subsequent hypoglycemia. Growth hormone, but not glucagon or cortisol, responses were also reduced. The findings that increased endogenous cortisol secretion reduces autonomic neuroendocrine and neurogenic symptom responses to subsequent hypoglycemia are potentially relevant to cortisol mediation of hypoglycemia-associated autonomic failure, and thus a vicious cycle of recurrent iatrogenic hypoglycemia, in people with diabetes mellitus.

Hypoglycemia-Associated Autonomic Failure in Advanced Type 2 Diabetes

We tested the hypotheses that the glucagon response to hypoglycemia is reduced in patients who are approaching the insulin-deficient end of the spectrum of type 2 diabetes and that recent antecedent hypoglycemia shifts the glycemic thresholds for autonomic (including adrenomedullary epinephrine) and symptomatic responses to hypoglycemia to lower plasma glucose concentrations in type 2 diabetes. Hyperinsulinemic stepped hypoglycemic clamps (85, 75, 65, 55, and 45 mg/dl steps) were performed on two consecutive days, with an additional 2 h of hypoglycemia (50 mg/dl) in the afternoon of the first day, in 13 patients with type 2 diabetes---7 treated with oral hypoglycemic agents (OHA R(X); mean [+/- SD] HbA(1c) 8.6 +/- 1.1%) and 6 requiring therapy with insulin for an average of 5 years and with reduced C-peptide levels (insulin R(X), HbA(1c) 7.5 +/- 0.7%)---and 15 nondiabetic control subjects. The glucagon response to hypoglycemia was virtually absent (P = 0.0252) in the insulin-deficient type 2 diabetic patients (insulin R(X) mean [+/- SE] final values of 52 plus minus 16 vs. 93 plus minus 15 pg/ml in control subjects and 98 +/- 16 pg/ml in type 2 diabetic patients, OHA R(X) on day 1). Glucagon (P = 0.0015), epinephrine (P = 0.0002), and norepinephrine (P = 0.0138) responses and neurogenic (P = 0.0149) and neuroglycopenic (P = 0.0015) symptom responses to hypoglycemia were reduced on day 2 after hypoglycemia on day 1 in type 2 diabetic patients; these responses were not eliminated, but their glycemic thresholds were shifted to lower plasma glucose concentrations. In addition, the glycemic thresholds for these responses were at higher-than-normal plasma glucose concentrations (P = 0.0082, 0.0028, 0.0023, and 0.0182, respectively) at baseline (day 1) in OHA R(X) type 2 diabetic patients, with relatively poorly controlled diabetes. Because the glucagon response to falling plasma glucose levels is virtually absent and the glycemic thresholds for autonomic and symptomatic responses to hypoglycemia are shifted to lower glucose concentrations by recent antecedent hypoglycemia, patients with advanced type 2 diabetes, like those with type 1 diabetes, are at risk for hypoglycemia-associated autonomic failure and the resultant vicious cycle of recurrent iatrogenic hypoglycemia.

Effects of Repetitive Hypoglycemia on Neuroendocrine Response and Brain Tyrosine Hydroxylase Activity in the Rat

Hypoglycemia-associated autonomic failure (HAAF) is a syndrome of acute adaptation to a metabolic stressor, in which neuroendocrine responses to repetitive hypoglycemic bouts are blunted. The CNS mechanisms that contribute to HAAF are unknown. In the present study, we modeled HAAF in the rat and measured the activity of tyrosine hydroxylase (TH) as an index of acute noradrenergic activation, to test the hypothesis that noradrenergic activation of the hypothalamus might be impaired. In association with a significant counter-regulatory response to a single bout of hypoglycemia (elevated corticosterone, catecholamines, and glucagon), TH activity was elevated overall in brainstem NE cell body areas and hypothalamus. With multiple hypoglycemic episodes in a 24 h period, the counter-regulatory response was blunted, and hypothalamic TH activity was comparable to that of saline-infused controls. In a similar paradigm, multiple bouts of CNS neuroglucopenia did not blunt the hyperglycemic or corticosterone responses, and were required for elevation of TH activity. This alternate response pattern suggests that insulin-induced hypoglycemia and cerebral neuroglucopenia represent somewhat different metabolic stressors at the CNS.

Hypoglycemia-associated autonomic failure in diabetes.

Hypoglycemia is the limiting factor in the glycemic management of diabetes. The concept of hypoglycemia-associated autonomic failure (HAAF) in diabetes posits that recent antecedent iatrogenic hypoglycemia causes both defective glucose counterregulation (by reducing the epinephrine response to falling glucose levels in the setting of an absent glucagon response) and hypoglycemia unawareness (by reducing the autonomic and the resulting neurogenic symptom responses) and thus a vicious cycle of recurrent hypoglycemia. Perhaps the most compelling support for HAAF is the finding that as little as 2-3 wk of scrupulous avoidance of hypoglycemia reverses hypoglycemia unawareness and improves the reduced epinephrine component of defective glucose counterregulation in most affected individuals. Insight into this pathophysiology has led to a broader view of the clinical risk factors for hypoglycemia to include indexes of compromised glucose counterregulation and provided a framework for the study of the mechanisms of iatrogenic hypoglycemia and, ultimately, its elimination from the lives of people with diabetes.

Acute 2DG-induced glucoprivation or dexamethasone abolishes 2DG-induced glucoregulatory responses to subsequent glucoprivation.

Behavioral, neuroendocrine, and autonomic responses to glucoprivation are impaired after a glucoprivic episode. A life-threatening manifestation of this effect, known as hypoglycemia-associated autonomic failure (HAAF), occurs in diabetic patients as a result of prior inadvertent hypoglycemia resulting from insulin therapy. Glucocorticoids, which are elevated by glucoprivation, have been implicated in the pathogenesis of HAAF. The goal of the present study was to examine the effect of glucocorticoids on glucoregulatory responses in a rat model of HAAF. 2-deoxy-D-glucose (2DG; 200 mg/kg) was used to induce glucoprivation. Rats were injected with saline, 2DG, or the synthetic glucocorticoid, dexamethasone (DEX; 250 microg/rat) in the morning. Then 6 h later, rats were injected with 2DG, and their feeding and hyperglycemic responses were measured. Both 2DG and DEX in the morning eliminated glucoprivic feeding and hyperglycemic responses in the afternoon test. Epinephrine (0.3 mg/kg) administration in the afternoon elicited marked hyperglycemia in animals given 2DG that morning, demonstrating that glycogen depletion from morning glucoprivation was not responsible for the absence of the hyperglycemic response in the afternoon test. The effects of prior saline or 2DG treatment on subsequent glucoprivic feeding were also examined in adrenalectomized rats in which the source of endogenous glucocorticoids was removed. In these animals, prior glucoprivation did not attenuate 2DG-induced feeding in the afternoon test. These findings demonstrate that a single glucoprivic episode is sufficient to cause impairment in glucoregulatory responses to a second glucoprivic episode in the same day. In addition, these results strongly implicate glucocorticoids in the pathogenesis of HAAF.