Glucose and the ischemic brain: a sour grape or a sweet treat?

Delayed reperfusion deficits after experimental stroke account for increased pathophysiology

Cerebral blood flow and oxygenation in the first few hours after reperfusion following ischemic stroke are critical for therapeutic interventions but are not well understood. We investigate changes in oxyhemoglobin (HbO2) concentration in the cortex during and after ischemic stroke, using multispectral optical imaging in anesthetized mice, a remote filament to induce either 30 minute middle cerebral artery occlusion (MCAo), sham surgery or anesthesia alone. Immunohistochemistry establishes cortical injury and correlates the severity of damage with the change of oxygen perfusion. All groups were imaged for 6 hours after MCAo or sham surgery. Oxygenation maps were calculated using a pathlength scaling algorithm. The MCAo group shows a significant drop in HbO2 during occlusion and an initial increase after reperfusion. Over the subsequent 6 hours HbO2 concentrations decline to levels below those observed during stroke. Platelets, activated microglia, interleukin-1α, evidence of BBB breakdown and neuronal stress increase within the stroked hemisphere and correlate with the severity of the delayed reperfusion deficit but not with the ΔHbO2 during stroke. Despite initial restoration of HbO2 after 30 min MCAo there is a delayed compromise that coincides with inflammation and could be a target for improved stroke outcome after thrombolysis.

Hyperglycemia as a Risk Factor of Ischemic Stroke

Diabetes is considered a major risk factor for stroke and is associated with worsened stroke outcomes. Here, we discuss and summarize the mechanisms that have been associated with the increased risk of stroke due to the hyperglycemia in diabetes mellitus. In diabetic stroke models, hyperglycemia exaggerates the following damaging processes: acidosis, accumulation of reactive oxygen species/reactive nitrogen, inflammation and mitochondrial dysfunction. Understanding the mechanism of diabetes acting as a stroke risk factor will definitely assist to reveal issues related to drug metabolism and toxicity in diabetic stroke. In addition, it is suggested that future studies may focus on the mechanisms mediating blood-brain barrier and astrocytes dysfunction under hyperglycemic stroke.

Elevated lactate suppresses neuronal firing in vivo and inhibits glucose metabolism in hippocampal slice cultures

Glucose is well accepted as the major fuel for neuronal activity, while it remains controversial whether lactate also supports neural activity. In hippocampal slice cultures, synaptic transmission supported by glucose was reversibly suppressed by lactate. To test whether lactate had a similar inhibitory effect in vivo, lactate was perfused into the hippocampi of unanesthetized rats while recording the firing of nearby pyramidal cells. Lactate perfusion suppressed pyramidal cell firing by 87.5+/-8.3% (n=6). Firing suppression was slow in onset and fully reversible and was associated with increased lactate concentration at the site of the recording electrode. In vivo suppression of neural activity by lactate occurred in the presence of glucose; therefore we tested whether suppression of neural firing was due to lactate interference with glucose metabolism. Competition between glucose and lactate was measured in hippocampal slice cultures. Lactate had no effect on glucose uptake. Lactate suppressed glucose oxidation when applied at an elevated, pathological concentration (10 mM), but not at its physiological concentration (1 mM). Pyruvate (10 mM) also inhibited glucose oxidation but was significantly less effective than lactate. The greater suppressive effect of lactate as compared to pyruvate suggests that alteration of the NAD(+)/NADH ratio underlies the suppression of glucose oxidation by lactate. ATP in slice culture was unchanged in glucose (1 mM), but significantly reduced in lactate (1 mM). ATP in slice culture was significantly increased by combination of glucose (1 mM) and lactate (1 mM). These data suggest that alteration of redox ratio underlies the suppression of neural discharge and glucose metabolism by lactate.

Oxidation of 14C-labeled substrates by hippocampal slice cultures

In vitro studies of glucose and lactate utilization have been performed in acute hippocampal slices or dissociated neurons and glia. While some studies concluded that lactate and glucose are equivalent substrates to support evoked synaptic activity, others showed decreased synaptic activity in the presence of lactate as compared to glucose. We found diminished neural activity in the presence of lactate in hippocampal slice cultures. We developed a method to examine the oxidation rates of 14C-labeled substrates by hippocampal slice cultures. The rate of 14CO2 production from either 14C-glucose or 14C-lactate remained unchanged for 6 h suggesting that slice cultures are metabolically stable. While the glucose oxidation rate saturated between 2.8 and 10 mM, lactate oxidation rate had not saturated at 10 mM. These data suggest that organotypic slice cultures provide a method to examine elements of cerebral metabolism in vitro.

Prognostic value of relative adrenal insufficiency after out-of-hospital cardiac arrest

OBJECTIVE

To assess the prevalence of relative adrenal insufficiency in patients successfully resuscitated after cardiac arrest, and its prognostic role in post-resuscitation disease.

DESIGN AND SETTING

A prospective observational single-center study in a medical intensive care unit.

PATIENTS

64 patients hospitalised in the intensive care unit after successful resuscitation for out-of-hospital cardiac arrest.

MEASUREMENTS AND RESULTS

A corticotropin-stimulation test was performed between 12 and 24 h following admission: serum cortisol level was measured before and 60 min after administration of tetracosactide 250 microg. Patients with an incremental response less than 9 microg/dl were considered to have relative adrenal insufficiency (non-responders). Variables were expressed as medians and interquartile ranges. 33 patients (52%) had relative adrenal insufficiency. Baseline cortisol level was higher in non-responders than in responders (41 [27.2-55.5] vs. 22.8 [15.7-35.1] microg/dl respectively, P=0.001). A long interval before initiation of cardiopulmonary resuscitation was associated with relative adrenal insufficiency (5 [3-10] vs. 3 [3-5] min, P=0.03). Of the 38 patients with post-resuscitation shock, 13 died of irreversible multiorgan failure. The presence of relative adrenal insufficiency was identified as a poor prognostic factor of shock-related mortality (log-rank P=0.02). A trend towards higher mortality in non-responders was identified in a multivariate logistic regression analysis (odds ratio 6.77, CI 95% 0.94-48.99, P=0.058).

CONCLUSIONS

Relative adrenal insufficiency occurs frequently after successful resuscitation of out-of-hospital cardiac arrest, and appears to be associated with a poor prognosis in cases of post-resuscitation shock. The role of corticosteroid supplementation should be evaluated in this setting.

The impact of hyperglycemia on patients with severe brain injury

BACKGROUND

This study aimed to analyze the relation of hyperglycemia to outcome in cases of severe traumatic brain injury, and to examine factors that may be responsible for the hyperglycemic state.

METHODS

A retrospective analysis in an intensive care unit of a level 1 trauma center investigated 77 patients with severe traumatic brain injury. Patients with a Glasgow Coma Scale (GCS) of 8 or lower who survived more than 5 days were reviewed. Serum glucose, base deficit, GCS, use of steroids, and amounts of insulin and carbohydrates were recorded for 5 days, along with age. The Injury Severity Score (ISS) and the Abbreviated Injury Score (AIS) for the head, chest, and abdomen also were recorded. A hyperglycemia score (HS) was calculated as follows. A value of 1 was assigned each day the glucose exceeded 170 mg/dL (range, 0-5). A hyperglycemia score for days 3, 4, and 5 (HS day 3-5) also was calculated (range, 0-3). Outcomes included mortality, day 5 GCS, intensive care unit length of stay, and hospital length of stay.

RESULTS

Of the 77 patients, 24 (31.2%) died. Nonsurvivors had higher glucose levels each day. The HS was higher for those who died: 2.4 +/- 1.7 versus 1.5 +/- 1.4 (p = 0.02). Univariate analysis showed that only HS and ISS correlated with all four outcome variables studied. Cox's regression analysis showed that mortality was related to age and ISS. Head AIS and HS were independent predictors of lower day 5 GCS, whereas HS 3-5 and day 4 GCS were related to prolonged hospital length of stay. Older age, diabetes, and lower day 1 GCS were associated with higher HS, whereas carbohydrate infusion rate, ISS, head AIS, and steroid administration were not.

CONCLUSIONS

Early hyperglycemia is associated with poor outcomes for patients with severe traumatic brain injury. Tighter control of serum glucose without reduction of nutritional support may improve the prognosis for these critically ill patients.

Pathophysiology of stroke: lessons from animal models

The current pathophysiological understanding of stroke is substantially based on experimental studies. Brain injury after cerebral ischemia develops from a complex signaling cascade that evolves in an at least partially unraveled spatiotemporal pattern. Early excitotoxicity can lead to fast necrotic cell death, which produces the core of the infarction. The ischemic penumbra that surrounds the infarct core suffers milder insults. In this area, both mild excitotoxic and inflammatory mechanisms lead to delayed cell death, which shows biochemical characteristics of apoptosis. While brain cells are challenged by these deleterious mechanisms, they activate innate protective programs of the brain, which can be studied by means of experimentally inducing ischemic tolerance (i.e., ischemic preconditioning). Importantly, cerebral ischemia not only affects the brain parenchyma, but also impacts extracranial systems. For example, stroke induces a dramatic immunosuppression via an overactivation of the sympathetic nervous system. As a result, severe bacterial infections such as pneumonia occur. Complex signaling cascades not only decide about cell survival, but also about the neurological deficit and the mortality after stroke. These mechanisms of damage and endogenous protection present distinct molecular targets that are the rational basis for the development of neuroprotective drugs.

Effects of hyperglycemia and hypercapnia on lipid metabolism during complete brain ischemia

Ischemic damage is greatly enhanced by preischemic hyperglycemia or hypercapnia, which affects many intracellular responses including protein kinase C (PKC) translocation. We explored whether hyperglycemic or hypercapnic ischemia affects lipid metabolism, especially ischemia-induced release of free fatty acids (FFAs) and diacylglycerols (DAGs). A change in intraischemic level of acidosis was induced either by injecting glucose (hyperglycemic, HG) or by adding CO(2) (hypercapnic, HC). Complete cerebral ischemia was induced, and the brain was frozen in situ after 3, 5, and 10 min at 37 degrees C. Frontoparietal neocortex was dissected for FFA and DAG lipid analysis by thin-layer chromatography and gas-liquid chromatography. Significant differences were shown between normoglycemic and either hypercapnic or hyperglycemic values for individual and total FFAs. A significant delay in the release of FFA in ischemia with hyperglycemia or hypercapnia was observed. Significant differences were also shown in individual DAG-acyl groups and total DAGs. Hyperglycemic or hypercapnic ischemia resulted in a significant decrease of DAG at 10 min of ischemia. This was unexpected because a previous study showed that PKC translocation was significantly enhanced under similar condition at this time point. Upon cellular depolarization, massive influx of calcium and FFA accumulation may decrease the PKC dependence of DAG for translocation. In addition, PKC activation may lead to a negative feedback inhibition of phospholipase C.

Experience with ketamine and sodium pentobarbital as anesthetics in a rat model of cardiac arrest and resuscitation

We review 7 years experience with the chest compression model of cardiac arrest and resuscitation, comparing two different anesthetics. Ketamine stimulates cardiac function and only mildly depresses respiration; of the two it provides easier resuscitation. However, ketamine severely depresses brain protein synthesis; in studies using this measure ketamine is unsuitable and another agent must be used. Sodium pentobarbital mildly depresses brain protein synthesis, but depresses both cardiac and respiratory function, making resuscitation more difficult. Use of alternate chest/abdominal pumping (Babbs resuscitation technique), with judicious use of intra-cardiac epinephrine (adrenaline), made resuscitation reliable under sodium pentobarbital as well.

Correlation between extracellular glucose and seizure susceptibility in adult rats

In adult diabetic patients, periods of hyperglycemia may be associated with exacerbation of focal seizures. Our objective was to determine in the adult rats the correlation between seizure susceptibility and extracellular glucose concentration in two models of seizures. Male rats were injected with two doses of streptozocin (40 mg/kg IP) on 2 consecutive days to induce diabetic hyperglycemia. Controls either received vehicle or were not injected. After 2 weeks, blood glucose concentration was measured, and the rats were subjected to flurothyl seizure test. Another group of rats received glucose solution (20%, 5 ml IP) 30 minutes before testing to induce nondiabetic hyperglycemia. Thresholds for flurothyl-induced clonic and tonic-clonic seizures were determined. Finally, in vitro epileptiform activity was induced in the entorhinal cortex-hippocampal slices from naive rats by perfusing with magnesium-free medium with various glucose concentrations. In additional slices, paired-pulse paradigm was determined in the perforant path. Susceptibility to clonic and tonic-clonic flurothyl-induced seizures positively correlated with blood glucose concentrations as the increased glucose concentration was associated with proconvulsant effects. Similarly, in the in vitro experiments, epileptiform activity was promoted by increased and suppressed by decreased glucose concentrations. Data indicate that, in the adult rats, high glucose concentrations are associated with proconvulsant effects.

Energy metabolism, stress hormones and neural recovery from cerebral ischemia/hypoxia

All the advancements in the understanding of the molecular and cellular processes leading to the great investments in developing neuroprotection against cerebral ischemic/hypoxic damage cannot obscure the simple fact that exhaustion of energy supplies is still at the basis of this disorder. Much has been investigated and postulated over the years about the quick collapse of energy metabolism that follows oxygen and glucose deprivation in the brain. Anaerobic glycolysis, recognized as a pathway of paramount importance in keeping energy supplies, although, at bare minimum, has also presented a dilemma-a significant increase in lactate production during ischemia/hypoxia (IH). The dogma of lactate as a useless end product of anaerobic glycolysis and its postulated role as a detrimental player in the demise of the ischemic cell has persisted for the past quarter of a century. This persistence is due to, at least in part, the well-documented phenomenon termed "the glucose paradox of cerebral ischemia," the unexplained aggravation of postischemic neuronal damage by preischemic hyperglycemia. Recent studies have questioned the deleterious effect of lactic acid, while others even have offered the possibility that this monocarboxylate serves as an aerobic energy substrate during recovery from IH. Reviewed here are studies published over the past few years along with some key older papers on the topic of energy metabolism and recovery of neural tissue from IH. New insights gained from both in vitro and in vivo studies on energy metabolism of the ischemic/hypoxic brain should improve our understanding of this key metabolic process and the chances of protecting this organ from the consequences of energy deprivation.

The mRNA level of the potassium-chloride cotransporter KCC2 covaries with seizure susceptibility in inferior colliculus of the post-ischemic audiogenic seizure-prone rat

Cardiac arrest and resuscitation were used to induce brain damage and susceptibility to sound-triggered seizures in Sprague-Dawley rats. Glucose preloading was used to vary seizure susceptibility. Because loop diuretics can block these seizures, we investigated changes in KCC2, a potassium-chloride cotransporter, in the inferior colliculus - the origin of the seizures. Using polymerase chain reaction (PCR), we found that collicular KCC2 mRNA levels covaried with seizure susceptibility in these animals. Using quantitative PCR, we found that a fivefold increase in collicular KCC2 mRNA levels was associated with a doubling of seizure incidence. A hypothesis linking KCC2 activity to seizure susceptibility is presented.