Hypoglycemia-induced Increase Research Articles

OR12-05 Blunting Of Sympathetic Activation Of Pancreas As A Cause For Hypoglycemic Unawareness In Hypoglycemia Associated Autonomic Failure

Abstract Disclosure: K. Thakkar: None. R. Hampton: None. M. Jimenez Gonzalez: None. D. Espinoza: None. S. Stanley: None. Introduction: Hypoglycemia activates the autonomic nervous system to suppress pancreatic insulin and increase glucagon release, restoring blood glucose levels to normal. These effects occur partly via sympathetic innervation of pancreatic islets. Recurrent hypoglycemia leads to a blunted autonomic response known as Hypoglycemia Associated Autonomic Failure (HAAF), which further exacerbates hypoglycemia. The mechanisms underlying HAAF are not well-understood, but changes in pancreatic innervation may contribute. Hypothesis: Decreased activity and remodeling of sympathetic innervation to the pancreas contribute to autonomic dysfunction with repeated hypoglycemia. Methods: To examine pancreatic sympathetic innervation with repeated hypoglycemia, C57Bl6 mice were treated with 0, 1, or 5 episodes (0, 1x, or 5x) of insulin-induced hypoglycemia. Pancreata, sympathetic celiac ganglia, and brains were harvested. Pancreatic tissue was immunostained for insulin, glucagon, and tyrosine hydroxylase (TH), a marker for sympathetic innervation. Samples were then cleared using iDISCO+, imaged, and analyzed using Imaris to examine the 3D distribution of innervation and islets. To determine if pancreatic sympathetic nerve function is altered in HAAF, we used viral delivery to express activating chemogenetic constructs specifically in pancreas-projecting sympathetic nerves. We assessed the effects of chemogenetic activation of pancreatic sympathetic innervation during insulin tolerance testing (ITT) in C57Bl6 mice with or without previous recurrent hypoglycemia. Results: 3D image analyses suggest acute hypoglycemia (1x) upregulates islet glucagon staining intensity. By comparison, recurrent hypoglycemia (5x) ​decreases pancreatic TH+ innervation and may blunt hypoglycemia-induced increases in islet glucagon staining intensity. Chemogenetic activation of pancreas sympathetic nerves in mice with repeated hypoglycemia increased blood glucose during an ITT while activation in mice without previous hypoglycemia did not significantly alter blood glucose. These data suggest reduced activity in pancreatic sympathetic innervation with recurrent hypoglycemia that can be restored by chemogenetic activation. Conclusions: Our data suggest repeated episodes of hypoglycemia may remodel the structure and function of peripheral pancreatic sympathetic innervation as compared to one episode of hypoglycemia. Restoring activity in pancreatic sympathetic innervation may partially reverse the severity of hypoglycemia in HAAF. Staining and analysis of celiac ganglia and sympathetic outflow circuits in the CNS will allow determination of the level at which blunting of sympathetic activation and/or remodeling of innervation occurs. Presentation: Friday, June 16, 2023

Astrocyte regulation of cerebral blood flow during hypoglycemia

Hypoglycemia triggers increases in cerebral blood flow (CBF), augmenting glucose supply to the brain. We have tested whether astrocytes, which can regulate vessel tone, contribute to this CBF increase. We hypothesized that hypoglycemia-induced adenosine signaling acts to increase astrocyte Ca2+ activity, which then causes the release of prostaglandins (PGs) and epoxyeicosatrienoic acids (EETs), leading to the dilation of brain arterioles and blood flow increases. We used an awake mouse model to investigate the effects of insulin-induced hypoglycemia on arterioles and astrocytes in the somatosensory cortex. During insulin-induced hypoglycemia, penetrating arterioles dilated and astrocyte Ca2+ signaling increased when blood glucose dropped below a threshold of ∼50 mg/dL. Application of the A2A adenosine receptor antagonist ZM-241385 eliminated hypoglycemia-evoked astrocyte Ca2+ increases and reduced arteriole dilations by 44% (p < 0.05). SC-560 and miconazole, which block the production of the astrocyte vasodilators PGs and EETs respectively, reduced arteriole dilations in response to hypoglycemia by 89% (p < 0.001) and 76% (p < 0.001). Hypoglycemia-induced arteriole dilations were decreased by 65% (p < 0.001) in IP3R2 knockout mice, which have reduced astrocyte Ca2+ signaling compared to wild-type. These results support the hypothesis that astrocytes contribute to hypoglycemia-induced increases in CBF by releasing vasodilators in a Ca2+-dependent manner.

ACTH Infusion Impairs Baroreflex Sensitivity-Implications for Cardiovascular Hypoglycemia-Associated Autonomic Failure.

Hypoglycemia attenuates cardiovascular homeostatic autonomic control. This attenuation, known as the cardiovascular component of hypoglycemia-associated autonomic failure (HAAF), is characterized most notably by decreased baroreflex sensitivity (BRS) that begins during hypoglycemia and persists until at least the next day, despite return to euglycemia. Understanding the mechanisms underlying this reduction in BRS is important because BRS attenuation is associated with increased morbidity and mortality. The objective of this work is to investigate the role of the adrenocorticotropin (ACTH)-adrenal axis in decreasing BRS. We tested the hypothesis that infusion of ACTH 1-24 (cosyntropin), as compared to placebo, would acutely suppress BRS, and that this decrease in BRS would be present the next day. A double-blind, placebo-controlled, random-order, cross-over study was conducted. This study took place in a clinical research center. Participants included healthy men and women. Interventions included an intravenous infusion of cosyntropin (70 μg/hour for 2.5 hours in the morning and again in the early afternoon) vs normal saline placebo. Outcome measures included BRS during and 16 hours after cosyntropin vs placebo infusions. Cosyntropin infusion attenuated BRS (mm Hg/ms) as compared to placebo (baseline 17.8 ± 1.38 vs 17.0 ± 2.07; during 14.4 ± 1.43 vs 17.3 ± 1.65; and next day 14.8 ± 1.42 vs 18.9 ± 2.04; P < .05, time by treatment, analysis of variance). BRS was decreased during the final 30 minutes of the morning cosyntropin infusion as compared to baseline (P < .01) and remained suppressed the next day (16 hours after afternoon infusion) (P < .025). Placebo infusion did not significantly change BRS. Corrected QT interval was not affected. ACTH attenuates BRS, raising the possibility that hypoglycemia-induced increases in ACTH may contribute to the cardiovascular component of HAAF.

Glucose transporter 2 mediates the hypoglycemia-induced increase in cerebral blood flow

Glucose transporter 2 (Glut2)-positive cells are sparsely distributed in brain and play an important role in the stimulation of glucagon secretion in response to hypoglycemia. We aimed to determine if Glut2-positive cells can influence another response to hypoglycemia, i.e. increased cerebral blood flow (CBF). CBF of adult male mice devoid of Glut2, either globally (ripglut1:glut2−/−) or in the nervous system only (NG2KO), and their respective controls were studied under basal glycemia and insulin-induced hypoglycemia using quantitative perfusion magnetic resonance imaging at 9.4 T. The effect on CBF of optogenetic activation of hypoglycemia responsive Glut2-positive neurons of the paraventricular thalamic area was measured in mice expressing channelrhodopsin2 under the control of the Glut2 promoter. We found that in both ripglut1:glut2−/− mice and NG2KO mice, CBF in basal conditions was higher than in their respective controls and not further activated by hypoglycemia, as measured in the hippocampus, hypothalamus and whole brain. Conversely, optogenetic activation of Glut2-positive cells in the paraventricular thalamic nucleus induced a local increase in CBF similar to that induced by hypoglycemia. Thus, Glut2 expression in the nervous system is required for the control of CBF in response to changes in blood glucose concentrations.

Acidosis mediates recurrent hypoglycemia-induced increase in ischemic brain injury in treated diabetic rats

ObjectivesCerebral ischemia is a serious possible manifestation of diabetic vascular disease. Recurrent hypoglycemia (RH) enhances ischemic brain injury in insulin-treated diabetic (ITD) rats. In the present study, we determined the role of ischemic acidosis in enhanced ischemic brain damage in RH-exposed ITD rats. MethodsDiabetic rats were treated with insulin and mild/moderate RH was induced for 5 days. Three sets of experiments were performed. The first set evaluated the effects of RH exposure on global cerebral ischemia-induced acidosis in ITD rats. The second set evaluated the effect of an alkalizing agent (Tris-(hydroxymethyl)-aminomethane: THAM) on ischemic acidosis-induced brain injury in RH-exposed ITD rats. The third experiment evaluated the effect of the glucose transporter (GLUT) inhibitor on ischemic acidosis-induced brain injury in RH-exposed ITD rats. Hippocampal pH and lactate were measured during ischemia and early reperfusion for all three experiments. Neuronal survival in Cornu Ammonis 1 (CA1) hippocampus served as a measure of ischemic brain injury. FindingsPrior RH exposure increases lactate concentration and decreases pH during ischemia and early reperfusion when compared to controls. THAM and GLUT inhibitor treatments attenuated RH-induced increase in ischemic acidosis. GLUT inhibitor treatment reduced the RH-induced increase in lactate levels. Both THAM and GLUT inhibitor treatments significantly decreased ischemic damage in RH-exposed ITD rats. ConclusionsIschemia causes increased acidosis in RH-exposed ITD rats via a GLUT-sensitive mechanism. Exploring downstream pathways may help understand mechanisms by which prior exposure to RH increases cerebral ischemic damage.

Hypoglycemia-Induced Increases in Thalamic Cerebral Blood Flow are Blunted in Subjects with Type 1 Diabetes and Hypoglycemia Unawareness

The thalamus has been found to be activated during the early phase of moderate hypoglycemia. Here, we tested the hypothesis that this region is less activated during hypoglycemia in subjects with type 1 diabetes (T1DM) and hypoglycemia unawareness relative to controls. Twelve controls (5 F/7 M, age 40 ± 14 years, body mass index 24.2 ± 2.7 kg/m(2)) and eleven patients (7 F/4 M, age 39 ± 13 years, body mass index 26.5 ± 4.4 kg/m(2)) with well-controlled T1DM (A1c 6.8 ± 0.4%) underwent a two-step hyperinsulinemic (2.0 mU/kg per minute) clamp. Cerebral blood flow (CBF) weighted images were acquired using arterial spin labeling to monitor cerebral activation in the midbrain regions. Blood glucose was first held at 95 mg/dL and then allowed to decrease to 50 mg/dL. The CBF image acquisition during euglycemia and hypoglycemia began within a few minutes of when the target blood glucose values were reached. Hypoglycemia unaware T1DM subjects displayed blunting of the physiologic CBF increase that occurs in the thalamus of healthy individuals during the early phase of moderate hypoglycemia. A positive correlation was observed between thalamic response and epinephrine response to hypoglycemia, suggesting that this region may be involved in the coordination of the counter regulatory response to hypoglycemia.

Crosstalk Between Calcium and cAMP in Pancreatic Alpha Cells

Glucagon secretion from pancreatic alpha cells can be controlled by a variety of factors including hypoglycemia-induced increase in calcium or by hormones that stimulate cAMP production. The mechanisms by which calcium and cAMP signals influence each other and glucagon secretion have yet to be resolved. Using fura2 imaging, we found that acute hypoglycemia-triggered calcium-induced calcium release (CICR) in clonal mouse alpha cells was sensitive to the calcium channel blockers, nifedipine and w-conotoxin-GVIA. CICR with similar channel pharmacology was triggered by forskolin-induced elevation of [cAMP] revealing cAMP-stimulation of [Ca2+]i. This CICR likely involved multiple cAMP targets as both PKA-selective and EPAC-selective cAMP analogs each triggered CICR. In addition, a role for calcium regulation of [cAMP] was revealed by hypoglycemia-triggered increase in [cAMP] as reported by imaging of an EPAC-based cAMP FRET indicator. This was presumably due to calcium regulation of adenylyl cyclase or cAMP phosphodiesterase (PDE). Multiple isoforms of adenylyl cyclase and PDEs including Ca-sensitive AC3 and PDE1 were detected by RT-PCR in alpha cells. PDE3- and PDE4-selective inhibitors had distinct effects on calcium and cAMP dynamics and whole cell calcium currents. These data suggest multiple points of cAMP and calcium crosstalk in regulating glucagon secretion.

Serious Cardiovascular Outcomes in Diabetes

Hypoglycemia is a well-recognized side effect of glucose-lowering therapies in patients with diabetes mellitus. The incidence of mild self-reported hypoglycemic episodes in patients with type 1 diabetes mellitus is approximately 30 episodes per patient per year, whereas the incidence of severe hypoglycemic episodes (ie, those that require third-party assistance) may be as high as 3.2 episodes per patient per year.1,–,3 Hypoglycemic episodes occur much less frequently in patients with type 2 diabetes mellitus, in whom the incidence of mild and severe hypoglycemic episodes is 2 to 10 per patient per year and 0.1 to 0.7 per patient per year, respectively.2 When patients are alerted to the occurrence of a hypoglycemic episode by symptoms such as tremor, diaphoresis, tachycardia, malaise, hunger, and anxiety, they can abort these episodes by consuming carbohydrates. However, if hypoglycemic episodes occur rapidly or are unrecognized and untreated, the resulting neuroglycopenia may cause confusion, seizures, accidents, angina, and, rarely, death or permanent cognitive impairment.3,4 Patients who experience frequent episodes of hypoglycemia are especially at risk of having unrecognized hypoglycemic episodes (and their sequelae), because their counterregulatory response to hypoglycemia becomes blunted. Indeed, the rare occurrence of sudden death during sleep in young patients with type 1 diabetes mellitus (the so-called dead-in-bed syndrome) has been attributed to hypoglycemia, although this cause is seldom proven.5,–,7 It is well known that hypoglycemic episodes are associated with a surge of sympathetic activity and a release of catecholamines.8,–,10 These observations have supported the suggestion that the tachycardia and the rise in blood pressure observed during a hypoglycemic episode might destabilize an atherosclerotic plaque.11 These hemodynamic changes, the increased myocardial work, and hypoglycemia-induced increases in platelet aggregation, platelet activity,12,–, …

Attenuated Neuroendocrine Response to Hypoglycemic Stress in Patients with Panic Disorder

Background/Aims: There is a lack of information on the effects of metabolic stress exposure on hormone release in patients with panic disorder. The aim of this study was to test the hypothesis that neuroendocrine activation during hypoglycemic stress is altered in panic disorder patients compared to healthy subjects. Methods: Hormone responses to an intravenous bolus of insulin (0.1 IU/kg) were evaluated in both fully remitted, medication-free panic disorder patients and healthy controls (n = 9/group). Blood samples for determination of cortisol, growth hormone, prolactin, adrenaline and noradrenaline concentrations were obtained at rest and over a 90-min period after insulin injection. Results: In patients with panic disorder, basal prestress hormone levels were comparable to those in healthy subjects with the exception of plasma adrenaline, which was higher in panic disorder patients compared to controls. The degree of hypoglycemia induced by insulin administration was similar in patients and healthy subjects. Hypoglycemia-induced increases in growth hormone, prolactin and cortisol concentrations were significantly attenuated in panic disorder patients compared to healthy individuals. No differences between patients and controls were observed in adrenalin release induced by hypoglycemia. Conclusions: The present data demonstrate that somatotropic, lactotropic and corticotropic activation during hypoglycemic stress is blunted in patients with panic disorder. It is suggested that in contrast to the effects of relatively mild stress conditions used in other studies published in the literature, intensive stressors inducing a broad spectrum of hormonal changes fail to provoke an adequate neuroendocrine response in patients with panic disorder.

Antioxidant capacity contributes to protection of ketone bodies against oxidative damage induced during hypoglycemic conditions

Ketone bodies play a key role in mammalian energy metabolism during the suckling period. Normally ketone bodies' blood concentration during adulthood is very low, although it can rise during starvation, an exogenous infusion or a ketogenic diet. Whenever ketone bodies' levels increase, their oxidation in the brain rises. For this reason they have been used as protective molecules against refractory epilepsy and in experimental models of ischemia and excitotoxicity. The mechanisms underlying the protective effect of these compounds are not completely understood. Here, we studied a possible antioxidant capacity of ketone bodies and whether it contributes to the protection against oxidative damage induced during hypoglycemia. We report for the first time the scavenging capacity of the ketone bodies, acetoacetate (AcAc) and both the physiological and non-physiological isomers of β-hydroxybutyrate ( d- and l-BHB, respectively), for diverse reactive oxygen species (ROS). Hydroxyl radicals ( OH) were effectively scavenged by d- and l-BHB. In addition, the three ketone bodies were able to reduce cell death and ROS production induced by the glycolysis inhibitor, iodoacetate (IOA), while only d-BHB and AcAc prevented neuronal ATP decline. Finally, in an in vivo model of insulin-induced hypoglycemia, the administration of d- or l-BHB, but not of AcAc, was able to prevent the hypoglycemia-induced increase in lipid peroxidation in the rat hippocampus. Our data suggest that the antioxidant capacity contributes to protection of ketone bodies against oxidative damage in in vitro and in vivo models associated with free radical production and energy impairment.

Modulation of hypoglycemia‐induced increases in plasma epinephrine by estrogen in the female rat

Clinical studies have demonstrated that estrogen replacement therapy suppresses stress-induced increases in plasma catecholamines. The present study determined whether normal circulating levels of estrogen can modulate hypoglycemia-induced increases in plasma epinephrine (EPI). In anesthetized female rats, insulin-induced hypoglycemia (0.25 U/kg) increased plasma EPI concentration to a significantly greater extent in 14-day ovariectomized (OVEX) rats compared to that in sham-operated controls. In 17beta-estradiol (E2)-replaced OVEX rats, the hypoglycemia-induced rise in plasma EPI was reduced significantly when compared to that in vehicle-replaced OVEX rats. OVEX and E2 replacement had no effect on tyrosine hydroxylase or phenylethanolamine N-methyltransferase mRNA levels in the adrenal medulla. In isolated adrenal medullary chromaffin cells, agonist-induced increases in intracellular Ca2+ were unaffected by 48-hr exposure to 10 nM E2. In contrast, acute (3-min) exposure to micromolar concentrations of E2 dose-dependently and reversibly inhibited agonist-induced Ca2+ transients. In addition, in OVEX rats, a constant infusion of E2 significantly reduced the insulin-induced increase in plasma EPI concentration compared to that in vehicle-infused controls. These data demonstrate that physiologic levels of circulating E2 can modulate hypoglycemia-induced increases in plasma EPI. This effect seems independent of steroid influence on adrenal medullary secretion or biosynthesis. In contrast, acute exposure to high levels of E2 can also suppress hypoglycemia-induced increases in plasma epinephrine, due at least in part to inhibition of stimulus-secretion coupling.

Pancreatic islet blood flow in conscious rats during hyperglycemia and hypoglycemia

Anesthesia affects general hemodynamics and regulation of organ perfusion. We used colored microspheres to measure pancreatic islet blood flow in conscious rats at two time points, during either hyperglycemia or hypoglycemia. This method, using black and green microspheres, was validated by comparison with previous microsphere experiments and by lack of effect of a nonmetabolizable glucose analog, 3-O-methylglucose, on islet perfusion. Basal and glucose-stimulated islet blood flow levels were similar in pentobarbital sodium-anesthetized and conscious rats. However, the basal distribution of pancreatic blood flow was altered by anesthesia (fractional islet blood flow 5.8 +/- 0.4% in conscious rats, 7.9 +/- 0.8% in pentobarbital-anesthetized rats, P < 0.05). Insulin-induced hypoglycemia significantly increased whole pancreatic blood flow in conscious rats, whereas islet blood flow remained unchanged and fractional islet blood flow was decreased (5.8 +/- 0.5% in the basal state, 4.2 +/- 0.4% during hypoglycemia, P < 0.001). Methylatropine pretreatment significantly increased islet blood flow during hypoglycemia by 181%. This result suggests that prevention of hypoglycemia-induced increase in islet perfusion may be mediated, at least in part, by a cholinergic, vagal muscarinic mechanism.

Effect of vagal cooling on the counterregulatory response to hypoglycemia induced by a low dose of insulin in the conscious dog.

We previously demonstrated, using a nerve-cooling technique, that the vagus nerves are not essential for the counterregulatory response to hypoglycemia caused by high levels of insulin. Because high insulin levels per se augment the central nervous system response to hypoglycemia, the question arises whether afferent nerve fibers traveling along the vagus nerves would play a role in the defense of hypoglycemia in the presence of a more moderate insulin level. To address this issue, we studied two groups of conscious 18-h-fasted dogs with cooling coils previously placed on both vagus nerves. Each study consisted of a 100-min equilibration period, a 40-min basal period, and a 150-min hypoglycemic period. Glucose was lowered using a glycogen phosphorylase inhibitor and a low dose of insulin infused into the portal vein (0.7 mU.kg(-1) min(-1)). The arterial plasma insulin level increased to 15 +/- 2 microU/ml and the plasma glucose level fell to a plateau of 57 +/- 3 mg/dl in both groups. The vagal cooling coils were perfused with a 37 degrees C (SHAM COOL; n = 7) or a -20 degrees C (COOL; n = 7) ethanol solution for the last 90 min of the study to block parasympathetic afferent fibers. Vagal cooling caused a marked increase in the heart rate and blocked the hypoglycemia-induced increase in the arterial pancreatic polypeptide level. The average increments in glucagon (pg/ml), epinephrine (pg/ml), norepinephrine (pg/ml), cortisol (microg/dl), glucose production (mg.kg(-1). min(-1)), and glycerol (micromol/l) in the SHAM COOL group were 53 +/- 9, 625 +/- 186, 131 +/- 48, 4.63 +/- 1.05, -0.79 +/- 0.24, and 101 +/- 18, respectively, and in the COOL group, the increments were 39 +/- 7, 837 +/- 235, 93 +/- 39, 6.28 +/- 1.03 (P < 0.05), -0.80 +/- 0.20, and 73 +/- 29, respectively. Based on these data, we conclude that, even in the absence of high insulin concentrations, afferent signaling via the vagus nerves is not required for a normal counterregulatory response to hypoglycemia.

Prevention of an increase in plasma cortisol during hypoglycemia preserves subsequent counterregulatory responses.

The aim of this study was to determine whether preventing increases in plasma cortisol during antecedent hypoglycemia preserves autonomic nervous system counterregulatory responses during subsequent hypoglycemia. Experiments were carried out on 15 (8 male/7 female) healthy, overnight-fasted subjects and 8 (4 male/4 female) age- and weight-matched patients with primary adrenocortical failure. 5 d before a study, patients had their usual glucocorticoid therapy replaced with a continuous subcutaneous infusion of cortisol programmed to produce normal daily circadian levels. Both groups underwent identical 2-d experiments. On day 1, insulin was infused at a rate of 1.5 mU/kg per min, and 2-h clamped hypoglycemia (53+/-2 mg/dl) was obtained during the morning and afternoon. The next morning, subjects underwent an additional 2-h hypoglycemic (53+/-2 mg/ dl) hyperinsulinemic clamp. In controls, day 2 steady state epinephrine, norepinephrine, pancreatic polypeptide, glucagon, growth hormone, and muscle sympathetic nerve activity were significantly blunted (P < 0.01) compared with day 1 hypoglycemia. In marked contrast, when increases of plasma cortisol were prevented in the patient group, day 2 neuroendocrine, muscle sympathetic nerve activity, hypoglycemic symptoms, and metabolic counterregulatory responses were equivalent with day 1 results. We conclude that (a) prevention of increases of cortisol during antecedent hypoglycemia preserves many critical autonomic nervous system counterregulatory responses to subsequent hypoglycemia; (b) hypoglycemia-induced increases in plasma cortisol levels are a major mechanism responsible for causing subsequent hypoglycemic counterregulatory failure; and (c) our results suggest that other mechanisms, apart from cortisol, do not play a major role in causing hypoglycemia-associated autonomic failure.

Adenosine and cerebrovascular hyperemia during insulin-induced hypoglycemia in newborn piglet.

The present study tested the hypothesis that adenosine is involved in mediating the hyperemic response of the newborn brain to hypoglycemia. By use of the cranial window and microdialysis-H2 clearance methodologies, changes in the diameter of pial arterioles (25-50 microns), extracellular adenosine concentrations ([ADO]), and local cerebral blood flow (CBF) were examined in isoflurane-anesthetized piglets subjected to insulin-induced hypoglycemia. Blood glucose concentrations ranged from 10 to 18 mg/dl after insulin administration (25 IU/kg iv). Local CBF in the frontal cortex increased 36 +/- 12% (P = 0.014) at 30 min of hypoglycemia (group 1, n = 12; control = 43 +/- 3 ml.min-1.100 g-1). The mean increase in dialysate [ADO] sampled concurrently from the same cortical area was 59 +/- 29% (P = 0.011; control = 0.11 +/- 0.02 microM). At 30 min of hypoglycemia, pial diameters increased 55 +/- 10% (P = 0.001; group 2, n = 9). The [ADO] in cranial window cerebrospinal fluid (CSF) increased 217 +/- 71% (P = 0.04) in response to hypoglycemia (group 3, n = 8; control = 0.016 +/- 0.006 microM). Local administration of an adenosine antagonist, 10 microM 8-sulfophenyltheophylline, to the cerebral cortex before hypoglycemia caused a 38% reduction (P = 0.011) in the pial arteriolar response at 30 min of hypoglycemia (group 4, n = 9). Similarly, local superfusion of CSF with 3.7 mM glucose attenuated the hypoglycemia-induced pial dilation 33% (P = 0.039; group 5, n = 9). Perfusion of microdialysis probes with 3.7 mM glucose in the CSF abolished the hypoglycemia-induced increase in dialysate [ADO] (group 1).(ABSTRACT TRUNCATED AT 250 WORDS)

Hypoglycemia and cerebral autoregulation in anesthetized dogs

We examined the effects of moderate hypoglycemia with near-normal cerebral glucose consumption on cerebral autoregulation during graded hemorrhagic hypotension in anesthetized dogs. Four groups of animals (n = 8 each) were studied: normoglycemia, insulin-induced hypoglycemia (1.1 mM), normoglycemia plus alpha- and beta-adrenergic blockade [propranolol (1 mg/kg loading dose + 1 mg.kg-1.h-1 infusion) plus phentolamine (2 mg/kg loading dose + 2 mg/min infusion)], and hypoglycemia plus alpha- and beta-adrenergic blockade. As cerebral perfusion pressure was reduced to 40-50 mmHg, cerebral blood flow and O2 consumption increased in the hypoglycemic group. These increases were not observed after adrenergic blockade or in the normoglycemic groups. The perfusion pressure at which cerebrovascular resistance was minimal was higher during hypoglycemia (40 mmHg) and hypoglycemia plus blockade (50 mmHg) than in either of the normoglycemic groups (30 mmHg). This study demonstrates that hypoglycemia increases the lower limit of cerebral autoregulation during hypotension. Furthermore, adrenergic mechanisms acting during combined hypoglycemia and hemorrhagic hypotension increase cerebral blood flow and O2 consumption and attenuate the hypoglycemia-induced increase in the lower limit of autoregulation.

Aspects of neural plasticity in the central nervous system—VI. Studies of the effects of gangliosides on brain metabolic lesions

The effects of gangliosides have been studied in two models of metabolic insult (insulin-induced hypoglycemia and transient forebrain ischemia) of the central nervous system. In the severe hypoglycemia experiments lactate extracellular fluid levels were evaluated by means of a microdialysis probe implanted in the frontoparietal cortex. Ganglioside GM1, given either peripherally (10 mg/kg, i.p.) or intracerebrally (2 × 10 −4 M, via the microdialysis probe) 2 h before insulin injection, was able to reduce the decay of the perfusate levels of lactate induced by the insulin injection. In the same animal model peripheral, but not central, administration of GM1 reduced the hypoglycemia-induced increase of cerebral blood flow and increased the survival time observed after the insulin injection. In the experiments on transient forebrain ischemia, a GM1 derivative, AGF2 (5 mg/kg/day, i.p.), was administered chronically, starting 5 days before or the day after the ischemic insult. With both treatment schedules a similar protective effect was observed in a neurological test battery and in the step-through latency in a passive avoidance test.

Insulin-induced hypoglycemia activates the release of adrenocorticotropin predominantly via central and propranolol insensitive mechanisms.

The dynamic patterns of pituitary-adrenocortical and sympatho-adrenal hormone responses to insulin hypoglycemia as well as the relative importance of central vs. peripheral control of hypoglycemia-induced ACTH secretion were evaluated. In conscious rats bearing indwelling cannulae, the changes in hormone concentrations after insulin injection were dependent on the changes in blood glucose levels with respect to both time course and magnitude. ACTH, corticosterone, epinephrine, and norepinephrine levels were found to be maximal at 60 min after 2.5 IU kg-1 insulin injected ip, whereas earlier (20 min) but smaller increases were obtained in response to 0.5 IU kg-1 insulin injected iv. In rats 6-7 days after lesions of the medial basal hypothalamus (MBH), the rise of ACTH during insulin hypoglycemia was markedly inhibited and corticosterone levels were significantly reduced. Simultaneously, the hypoglycemia-induced increase in plasma epinephrine was unchanged and that in plasma norepinephrine was significantly enhanced in rats with the MBH destroyed. The beta-adrenoreceptor blocker propranolol did not inhibit ACTH and corticosterone responses to hypoglycemia in either sham-operated or MBH-lesioned animals. We conclude that the main factors triggering ACTH release during insulin-induced hypoglycemia are of central rather than peripheral origin. The high concentrations of circulating catecholamines occurring during insulin hypoglycemia are not responsible for pituitary-adrenocortical activation by direct, beta-adrenoreceptor mediated action at the pituitary level.

Studies on mechanisms involved in hypoglycemia-induced platelet activation.

The aim of our study was to investigate the mechanisms involved in hypoglycemia-induced platelet activation. Sixteen healthy male subjects received a 60-min intravenous infusion of human regular insulin at the rate of 64 mU . m-2 . min-1: throughout 150 min, we serially measured plasma concentrations of glucose, insulin, and counterregulatory hormones; platelet sensitivity to ADP, thrombin and platelet-activating factor; plasma concentrations of platelet markers for specific proteins of in vivo release reaction (beta-thromboglobulin and platelet factor 4). Our study showed that insulin-induced hypoglycemia causes a significant increase in platelet sensitivity to aggregating agents in vitro and a platelet release reaction in vivo. Hypoglycemia-induced platelet activation was not correlated with plasma glucose concentrations at nadir and occurred before the increase of plasma growth hormone and cortisol. To further elucidate the mechanisms of hypoglycemia-induced platelet activation, we incubated in vitro platelet-rich plasma (PRP) of seven fasting healthy subjects with the same concentrations of insulin, epinephrine, glucagon, growth hormone, and cortisol measured in vivo during insulin-induced hypoglycemia. Only epinephrine was able to increase platelet sensitivity to aggregating agents. To investigate the role of alpha-adrenergic receptors in this phenomenon, we also studied four healthy subjects on another occasion, repeating the above-described insulin infusion together with intravenous infusion of phentolamine (-15 to +150 min), 5 mg over 2 min followed by 500 micrograms/min. alpha-Blockade was able to suppress hypoglycemia-induced increase of platelet sensitivity to aggregating agents.(ABSTRACT TRUNCATED AT 250 WORDS)

Increases in plasma cyclic AMP dependent on endogenous catecholamines

Administration of tyramine (with or without phentolamine) as well as induction of ether anesthesia or insulin hypoglycemia caused a sharp increase in plasma cyclic AMP in rats. Based onthe findings that the treatment of rats with reserpine, 6-hydroxydopamine, cocaine or propranolol totally abolished tyramine-induced increases in plasma cyclic AMP, it was concluded that catecholamines released from sympathetic neuronal terminals by tyramine could activate adenylate cyclase via the stimulation of postsynaptic β-adrenoceptors. In contrast, catecholamines secreted from adrenal medulla were largely responsible for the increase in plasma cyclic AMP induced by ether anesthesia; whereas glucagon, in addition to adrenal catecholamines, played a significant role in hypoglycemia-induced increases in plasma cyclic AMP. Assay of plasma cyclic AMP following these stimuli is very promising as a test for adrenergic activities in experimental and clinical studies.