Regional cerebral and neural lobe blood flow during insulin-induced hypoglycemia in unanesthetized rats

Unraveling the brain's response to hypoglycemia: Neurovascular coupling

Functional magnetic resonance imaging has suggested the possibility that hypoglycemia could interfere with neurovascular coupling. Here we discuss the implications of a study by Nippert and colleagues showing that hypoglycemia does not impair neurovascular coupling.

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.

Energy Metabolism of the Brain, Including the Cooperation between Astrocytes and Neurons, Especially in the Context of Glycogen Metabolism

Glycogen metabolism has important implications for the functioning of the brain, especially the cooperation between astrocytes and neurons. According to various research data, in a glycogen deficiency (for example during hypoglycemia) glycogen supplies are used to generate lactate, which is then transported to neighboring neurons. Likewise, during periods of intense activity of the nervous system, when the energy demand exceeds supply, astrocyte glycogen is immediately converted to lactate, some of which is transported to the neurons. Thus, glycogen from astrocytes functions as a kind of protection against hypoglycemia, ensuring preservation of neuronal function. The neuroprotective effect of lactate during hypoglycemia or cerebral ischemia has been reported in literature. This review goes on to emphasize that while neurons and astrocytes differ in metabolic profile, they interact to form a common metabolic cooperation.

Real-time effects of insulin-induced hypoglycaemia on hippocampal glucose and oxygen

The hippocampus plays a vital role in learning and memory and is susceptible to damage following hypoglycaemic shock. The effect of an acute administration of insulin on hippocampal function has been described in terms of behavioural deficits but its effect on hippocampal oxygen and glucose is unclear. Glucose oxidase biosensors (detecting glucose) and carbon paste electrodes (detecting oxygen) were implanted into the hippocampus of Sprague Dawley rats. Animals were allowed to recover and real-time recordings were made in order to determine the effects of fasting, insulin administration (15 U/kg; i.p.) and reintroduction of food on hippocampal oxygen and glucose. Fasting caused a significant decrease in hippocampal glucose over the course of 24h. Insulin administration produced a significant decrease in hippocampal glucose along with a significant increase in hippocampal oxygen. Finally, the reintroduction of food resulted in glucose levels significantly increasing along with a transient but significant increase in oxygen levels. The findings presented here suggest that even a single acute period of hypoglycaemia may substantially disrupt hippocampal oxygen and glucose and therefore affect hippocampal function.

Ventromedial hypothalamic nitric oxide production is necessary for hypoglycemia detection and counterregulation

OBJECTIVE

The response of ventromedial hypothalamic (VMH) glucose-inhibited neurons to decreased glucose is impaired under conditions where the counterregulatory response (CRR) to hypoglycemia is impaired (e.g., recurrent hypoglycemia). This suggests a role for glucose-inhibited neurons in the CRR. We recently showed that decreased glucose increases nitric oxide (NO) production in cultured VMH glucose-inhibited neurons. These in vitro data led us to hypothesize that NO release from VMH glucose-inhibited neurons is critical for the CRR.

RESEARCH DESIGN AND METHODS

The CRR was evaluated in rats and mice in response to acute insulin-induced hypoglycemia and hypoglycemic clamps after modulation of brain NO signaling. The glucose sensitivity of ventromedial nucleus glucose-inhibited neurons was also assessed.

RESULTS

Hypoglycemia increased hypothalamic constitutive NO synthase (NOS) activity and neuronal NOS (nNOS) but not endothelial NOS (eNOS) phosphorylation in rats. Intracerebroventricular and VMH injection of the nonselective NOS inhibitor N(G)-monomethyl-l-arginine (l-NMMA) slowed the recovery to euglycemia after hypoglycemia. VMH l-NMMA injection also increased the glucose infusion rate (GIR) and decreased epinephrine secretion during hyperinsulinemic/hypoglycemic clamp in rats. The GIR required to maintain the hypoglycemic plateau was higher in nNOS knockout than wild-type or eNOS knockout mice. Finally, VMH glucose-inhibited neurons were virtually absent in nNOS knockout mice.

CONCLUSIONS

We conclude that VMH NO production is necessary for glucose sensing in glucose-inhibited neurons and full generation of the CRR to hypoglycemia. These data suggest that potentiating NO signaling may improve the defective CRR resulting from recurrent hypoglycemia in patients using intensive insulin therapy.

Cerebrovascular injury in premature infants: current understanding and challenges for future prevention

Cerebrovascular insults are a leading cause of brain injury in premature infants, contributing to the high prevalence of motor, cognitive, and behavioral deficits. Understanding the complex pathways linking circulatory immaturity to brain injury in premature infants remains incomplete. These mechanisms are significantly different from those causing injury in the mature brain. The gaps in knowledge of normal and disturbed cerebral vasoregulation need to be addressed. This article reviews current understanding of cerebral perfusion, in the sick premature infant in particular, and discusses challenges that lie ahead.

Global cerebral blood flow and metabolism during acute hyperketonemia in the awake and anesthetized rat

In the human setting, it has been shown that acute increase in the concentration of ketone bodies by infusion of beta-hydroxybutyrate increased the cerebral blood flow (CBF) without affecting the overall cerebral metabolic activity. The mechanism by which this effect of ketone bodies was mediated is not known. Alterations in several parameters may possibly explain the increase in CBF and the resetting of the relation between CBF and cerebral metabolism. To study this phenomenon further, we measured global CBF and global cerebral metabolism with the Kety-Schmidt technique in the wakeful rat before and during infusion of ketone bodies. During acute hyperketonemia (average concentration of beta-hydroxybutyrate: 6 mmol/L), global CBF increased 65% from 108 to 178 mL/100 g min and the cerebral metabolic rates for both oxygen and glucose remained constant. This resetting of the relation between CBF and cerebral metabolism could not be explained by alterations in blood pH or arterial CO2 tension. By measuring cerebral intracellular pH by 31P nuclear magnetic resonance spectroscopy, it could further be concluded that the brain pH was unchanged during acute hyperketonemia. These observations indicate that the mechanism responsible for the increase in CBF is rather a direct effect on the cerebral endothelium than via some metabolic interactions.

Is neuroprotective efficacy of nNOS inhibitor 7-NI dependent on ischemic intracellular pH?

The purpose of this study was to test the hypothesis that the efficacy of 7-nitroindazole (7-NI), a selective neuronal nitric oxide (NO) synthase (NOS) inhibitor, is pH dependent in vivo during focal cerebral ischemia. Wistar rats underwent 2 h of focal cerebral ischemia under 1% halothane anesthesia. 7-NI, 10 and 100 mg/kg in 0.1 ml/kg DMSO, was administered 30 min before occlusion. Ischemic brain acidosis was manipulated by altering serum glucose concentrations. Confirmation of the effects of these serum glucose manipulations on brain intracellular pH (pH(i)) was confirmed in a group of acute experiments utilizing umbelliferone fluorescence. The animals were euthanized at 72 h for histology. 7-NI significantly (P < 0.05) reduced infarction volume in both the normoglycemic by 93.3% and hyperglycemic animals by 27.5%. In the moderate hypoglycemic animals, the reduction in infarction volume did not reach significance because moderate hypoglycemia in itself dramatically reduced infarction volume. We hypothesize that a mechanism to explain the published discrepancies on the effects of neuronal NOS inhibitors in vivo may be due to the effects by differences in ischemic brain acidosis on the production of NO.

In vivo measurements of brain glucose transport using the reversible Michaelis-Menten model and simultaneous measurements of cerebral blood flow changes during hypoglycemia

Glucose is the major substrate that sustains normal brain function. When the brain glucose concentration approaches zero, glucose transport across the blood-brain barrier becomes rate limiting for metabolism during, for example, increased metabolic activity and hypoglycemia. Steady-state brain glucose concentrations in alpha-chloralose anesthetized rats were measured noninvasively as a function of plasma glucose. The relation between brain and plasma glucose was linear at 4.5 to 30 mmol/L plasma glucose, which is consistent with the reversible Michaelis-Menten model. When the model was fitted to the brain glucose measurements, the apparent Michaelis-Menten constant, Kt, was 3.3 +/- 1.0 mmol/L, and the ratio of the maximal transport rate relative to CMRglc, Tmax/CMRglc, was 2.7 +/- 0.1. This Kt is comparable to the authors' previous human data, suggesting that glucose transport kinetics in humans and rats are similar. Cerebral blood flow (CBF) was simultaneously assessed and constant above 2 mmol/L plasma glucose at 73 +/- 6 mL 100 g(-1) min(-1). Extrapolation of the reversible Michaelis-Menten model to hypoglycemia correctly predicted the plasma glucose concentration (2.1 +/- 0.6 mmol/L) at which brain glucose concentrations approached zero. At this point, CBF increased sharply by 57% +/- 22%, suggesting that brain glucose concentration is the signal that triggers defense mechanisms aimed at improving glucose delivery to the brain during hypoglycemia.

Regional differences in mechanisms of cerebral circulatory response to neuronal activation

Vibrissal stimulation raises cerebral blood flow (CBF) in the ipsilateral spinal and principal sensory trigeminal nuclei and contralateral ventroposteromedial (VPM) thalamic nucleus and barrel cortex. To investigate possible roles of adenosine and nitric oxide (NO) in these increases, local CBF was determined during unilateral vibrissal stimulation in unanesthetized rats after adenosine receptor blockade with caffeine or NO synthase inhibition with N(G)-nitro-L-arginine methyl ester (L-NAME) or 7-nitroindazole (7-NI). Caffeine lowered baseline CBF in all structures but reduced the percent increase during stimulation only in the two trigeminal nuclei. L-NAME and 7-NI lowered baseline CBF but reduced the percent increase during stimulation only in the higher stations of this sensory pathway, i.e., L-NAME in the VPM nucleus and 7-NI in both the VPM nucleus and barrel cortex. Combinations of caffeine with 7-NI or L-NAME did not have additive effects, and none alone or in combination completely eliminated functional activation of CBF. These results suggest that caffeine-sensitive and NO-dependent mechanisms are involved but with different regional distributions, and neither fully accounts for the functional activation of CBF.

Sustained elevation of cerebral blood flow after hypoglycaemia in normal man

During hypoglycaemia, counter-regulatory hormones are released, cognitive function is impaired and cerebral blood flow is increased. In the immediate period after normalisation of blood glucose only counter-regulatory hormones seem to be normalised. The aim of this study was to follow the changes in cerebral blood flow during a prolonged recovery period following moderate hypoglycaemia in normal man. In 15 healthy men, hypoglycaemia was induced by an intravenous infusion of insulin (2.5 mU/kg per min) to a blood glucose of 2.2 +/- 0.3 mmol/l (mean +/- S.D.) and was kept at this level for 66 +/- 11 min. The cerebral blood flow was measured by a single photon emission computed tomography camera (SPECT) recording the clearance of intravenously administered xenon-133. Measurements were performed before, at the beginning and at the end of the hypoglycaemic period, as well as 23 +/- 5, 51 +/- 7 and 97 +/- 7 min after normalisation of the blood glucose. The basal cerebral blood flow was 50.2 +/- 5.2 ml/100 g per min, increased to 55.6 +/- 4.9 ml/100 g per min (P < 0.001) during hypoglycaemia, and remained at this level at all measurements after normalisation of blood glucose. There was no relation between the rate of fall in blood glucose or level of hypoglycaemia and increment in cerebral blood flow or the actual blood flow during hypoglycaemia. The values of plasma adrenaline, serum ACTH, serum cortisol and symptom scores increased significantly during hypoglycaemia. The adrenaline level was back to the basal level at the first measurement after normalisation of blood glucose, while the ACTH level was normalised at the subsequent measurement and the cortisol level at the last measurement. In conclusion, the results show that despite normalisation of counter-regulatory hormones and hypoglycaemic symptoms, the cerebral blood flow remains elevated for at least 97 +/- 7 min following 66 +/- 11 min of moderate hypoglycaemia, indicating that additional factors which are not coupled to the cerebral metabolism influence this vasculatory response.

Effect of decreased glucose concentration on cerebrovascular tone in vitro

We studied the effect of decreased glucose concentration on cerebrovascular tone in vitro. Segments of rat middle cerebral arteries (MCA) were isolated, cannulated at both ends with glass micropipettes, and pressurized to 85 mm Hg. Decreasing the glucose in the extraluminal bath and luminal perfusate from 5.5 mmol/L to 1.0 or 0.5 mmol/L for 1.5 hours each had no significant effect on the diameter of the arteries. When all the glucose was removed from the extraluminal bath and luminal perfusate for 1.5 hours, the MCA dilated by 23% [252 +/- 24 (SD) microns to 311 +/- 7 microns (P < .5, n = 7)]. This dilation was 80% of the maximum dilation produced by removal of Ca+2 from the bathing solutions. Neither removal of the endothelium nor inhibition of the ATP-sensitive K channels with 10(-5) mol/L glibenclamide altered the response of the isolated MCA to the removal of glucose. We conclude that rat MCA are relatively more resistant to substrate limitation compared to the brain as a whole.

Blockade of cerebral blood flow response to insulin-induced hypoglycemia by caffeine and glibenclamide in conscious rats

The possibility that adenosine and ATP-sensitive potassium channels (KATP) might be involved in the mechanisms of the increases in cerebral blood flow (CBF) that occur in insulin-induced hypoglycemia was examined. Cerebral blood flow was measured by the [14C]iodoantipyrine method in conscious rats during insulin-induced, moderate hypoglycemia (2 to 3 mmol/L glucose in arterial plasma) after intravenous injections of 10 to 20 mg/kg of caffeine, an adenosine receptor antagonist, or intracisternal infusion of 1 to 2 mumol/L glibenclamide, a KATP channel inhibitor. Cerebral blood flow was also measured in corresponding normoglycemic and drug-free control groups. Cerebral blood flow was 51% higher in untreated hypoglycemic than in untreated normoglycemic rats (P < 0.01). Caffeine had a small, statistically insignificant effect on CBF in normoglycemic rats, but reduced the CBF response to hypoglycemia in a dose-dependent manner, i.e., 27% increase with 10 mg/kg and complete elimination with 20 mg/kg. Chemical determinations by HPLC in extracts of freeze-blown brains showed significant increases in the levels of adenosine and its degradation products, inosine and hypoxanthine, during hypoglycemia (P < 0.05). Intracisternal glibenclamide had little effect on CBF in normoglycemia, but, like caffeine, produced dose-dependent reductions in the magnitude of the increases in CBF during hypoglycemia, i.e., +66% with glibenclamide-free artificial CSF administration, +25% with 1 mumol/L glibenclamide, and almost complete blockade (+5%) with 2 mumol/L glibenclamide. These results suggest that adenosine and KATP channels may play a role in the increases in CBF during hypoglycemia.

Examination of potential mechanisms in the enhancement of cerebral blood flow by hypoglycemia and pharmacological doses of deoxyglucose

Cerebral blood flow (CBF) rises when the glucose supply to the brain is limited by hypoglycemia or glucose metabolism is inhibited by pharmacological doses of 2-deoxyglucose (DG). The present studies in unanesthetized rats with insulin-induced hypoglycemia show that the increases in CBF, measured with the [14C]iodoantipyrine method, are relatively small until arterial plasma glucose levels fall to 2.5 to 3.0 mM, at which point CBF rises sharply. A direct effect of insulin on CBF was excluded; insulin administered under euglycemic conditions maintained by glucose injections had no effects on CBF. Insulin administration raised plasma lactate levels and decreased plasma K+ and HCO3- concentrations and arterial pH. These could not, however, be related to the increased CBF because insulin under euglycemic conditions had similar effects without affecting CBF; furthermore, the inhibition of brain glucose metabolism with pharmacological doses (200 mg/kg intravenously) of DG increased CBF, just like insulin hypoglycemia, without altering plasma lactate and K+ levels and arterial blood gas tensions and pH. Nitric oxide also does not appear to mediate the increases in CBF. Chronic blockade of nitric oxide synthase activity by twice daily i.p. injections of NG-nitro-L-arginine methyl ester for 4 days or acutely by a single i.v. injection raised arterial blood pressure and lowered CBF in normoglycemic, hypoglycemic, and DG-treated rats but did not significantly reduce the increases in CBF due to insulin-induced hypoglycemia (arterial plasma glucose levels, 2.5-3 mM) or pharmacological doses of deoxyglucose.

Hypoglycemia prevents increase in lactic acidosis during reperfusion after temporary cerebral ischemia in rats

Sequential 31P and 1H MRS was used to measure cerebral phosphate metabolites, intracellular pH, and lactate in normoglycemic and hypoglycemic rats during 30 min of complete cerebral ischemia and 5.5 h of reperfusion. These results were correlated with brain levels of free fatty acids (FFAs), excitatory amino acids, cations, and water content at death. The lactate/N-acetyl aspartate ratio was not significantly different between groups before or during occlusion. During reperfusion, the ratio was higher in normoglycemic rats from 3 to 85 min (p < or = 0.05), and recovery time was faster in hypoglycemic rats (29 vs 45 min; p = 0.04), suggesting reduced lactate production and faster recovery of aerobic metabolism. During occlusion, significant but comparable decrease of intracellular pH occurred in each group. Intracellular pH was higher in hypoglycemic rats at 140 min and 260 min of reperfusion. Water content, Na and K+ concentrations, and FFA and excitatory amino acid levels were not significantly different between groups, but hypoglycemic rats had less depletion of levels of Mg2+ (p = 0.011). These results show that hypoglycemia has a limited but potentially beneficial effect on postischemic lactic acidosis.

The effects of pharmacologic doses of 2-deoxy-D-glucose on local cerebral blood flow in the awake, unrestrained rat

Previous studies on the effects of acute insulin-induced hypoglycemia on cerebral blood flow (CBF) have resulted in conflicting results. An alternate approach to the study of glucoprivation is the administration of pharmacologic doses of the glucose analogue, 2-deoxy-D-glucose (2-DG). 2-DG is transported across the blood-brain barrier into brain tissue where it is phosphorylated to 2-deoxy-D-glucose-6-phosphate (2-DG-6-P) but not metabolized further. The 2-DG-6-P accumulates and inhibits the conversion of glucose-6-phosphate to fructose-6-phosphate, thus blocking glycolysis and glucose metabolism. In the present study we have employed the [14C]iodoantipyrine method to examine the effects of a pharmacologic dose (500 mg/kg) of 2-DG on local cerebral blood flow (lCBF) in 29 regions of the brain in conscious, unrestrained, adult male rats. The 2-DG treatment raised arterial plasma glucose levels from 8 to 17 mM without affecting arterial blood pO2, pCO2, or pH but increased lCBF in most brain regions examined. The largest increases were in the cerebral cortex, basal ganglia, and thalamic nuclei (+65 to +157%). Smaller increases were found in most structures of the limbic system, brainstem, and white matter, and no changes in lCBF were seen in the cerebellar cortex and ventral medial hypothalamus. The results indicate that cerebral glucoprivation produced by pharmacological doses of 2-deoxyglucose is accompanied by substantial increase in blood flow in most regions of the brain.

The influence of hypoglycaemia on regional cerebral blood flow and cerebral volume in type 1 (insulin-dependent) diabetes mellitus

The effect of moderate hypoglycaemia (venous blood glucose 2.0 +/- 0.2 mmol/l; mean +/- SD) on regional cerebral blood flow and cerebral volume was studied in a group of ten right-handed patients with Type 1 (insulin-dependent) diabetes mellitus (age 26.0 +/- 2.4 years, duration 18.4 +/- 3.8 years) using an intravenous Xenon 133 single photon emission computed tomography technique. After 10 min of hypoglycaemia, global cerebral blood flow had increased to 55.8 +/- 4.5 ml.100 g-1.min-1 compared to the initial normoglycaemic flow of 49.5 +/- 3.7 ml.100 g-1.min-1 (p < 0.01). A further increase in global cerebral blood flow to 59.5 +/- 4.5 ml.100 g-1.min-1 (p < 0.05) occurred 15 min after normalization of the blood glucose level. The global cerebral blood flow change from before hypoglycaemia to after recovery was inversely related to the initial glucose level. No change in the relative distribution of the regional cerebral blood flow was found between the measurements. The cerebral blood flow was significantly higher in the right hemisphere compared with the left hemisphere (2.3, 1.6 and 2.2%, respectively; p < 0.05) in all measurements. Deeper hypoglycemia was associated with a more pronounced decrease in brain volume, while the length of the restitution time after hypoglycaemia correlated with a volume increase. Due to influences with opposite effects there was no mean change in the brain volume.

Regional cerebral blood flow in normal man during insulin-induced hypoglycemia and in the recovery period following glucose infusion

The effect of moderate hypoglycemia (p-glucose, 2.0 +/- 0.3 mmol/L; mean +/- SD) on regional cerebral blood flow (rCBF) was studied in a group of 10 healthy, right-handed men (aged 23 to 28 years) using an intravenous xenon 133 single photon emission computed tomography technique (SPECT). After 10 minutes of hypoglycemia, global CBF had increased to 46.3 +/- 9.6 mL/100 g/min compared with the initial normoglycemic flow of 38.6 +/- 6.8 mL/100 g/min (P less than .01). The relative distribution of the rCBF changed significantly (P less than .05, ANOVA) from before to during hypoglycemia. Of the 10 regions analyzed, the highest increments in rCBF during hypoglycemia were found in the frontal (21.5% +/- 15.2%) and parietal (20.6% +/- 14.2%) lobes, and the lowest (10.7% +/- 9.4%) were found in the pons/brainstem regions. The increase in rCBF persisted for 15 minutes after normalization of blood glucose. The persisting high flow after hypoglycemia affected all regions, but a further 10.1% +/- 7.2% increase was observed in the pons/brainstem area (P less than .05). The CBF was significantly higher in the right compared with the left hemisphere (2.8%, 1.2%, and 3.9%, respectively; P less than .05) in all measurements. A decrease in brain volume was found at the final examination, compared with the hypoglycemic state (2.6%; P less than .05). It is concluded that moderate hypoglycemia leads to a marked increase in CBF and in the relative distribution of rCBF, which persists in the immediate period after normalization of the blood glucose level.

Blood flow in portal systems with special reference to the rat pituitary gland

Regional pituitary blood flow has been studied in adult female Fischer 344 rats by [14C]iodoantipyrine autoradiography. A general mathematical solution has been derived to allow the calculation of blood flow in the second compartment of a portal system and the proportion of blood "shunted" through the first compartment without exposure to tissue uptake from a knowledge of (a) the volume ratios of the two compartments, (b) the tissue tracer uptakes of the two compartments, and (c) the arterial tracer concentration with respect to time of a freely diffusible tracer. Significant diffusion limitation and/or arteriovenous shunting has been demonstrated in the neurohypophysis, suggesting that the majority of incoming blood is "shunted" unchanged to the adenohypophysis. The mean value of the shunt is 89% (range of 84-93%) for the median eminence and lies between 72% (range of 52-82%) and 73% (range of 59-81%) for the posterior pituitary. Neurohypophysial flow rates of 1.20 (range of 0.99-1.55) ml g-1 min-1 for the median eminence and 1.68 (range of 0.83-3.53) ml g-1 min-1 for the posterior pituitary were measured. These values represent "tissue-available" (nonshunted) flow; estimated mean total (shunted plus nonshunted) neurohypophysial flow rates were 11.7 (range of 9.5-17.5) ml g-1 min-1 for the median eminence and 6.1 (range of 3.1-8.9) ml g-1 min-1 (minimum) for the posterior pituitary. Adenohypophysial blood flow is heterogeneous. In the long portal territory, the flow rate was 1.18 (range of 0.95-1.75) ml g-1 min-1 but short portal territory flow calculation is complicated by an unquantifiable nonportal venous drainage; using the natural limits of zero and 100% gives a minimum adenohypophysial flow rate of 1.42 (range of 0.76-2.07) ml g-1 min-1 and a maximum value of 1.97 (range of 1.03-2.82) ml g-1 min-1.

Control of cerebral circulation in the high-risk neonate

A knowledge of neonatal cerebrovascular physiology is essential to the understanding of diseases that frequently affect the subsequent development of the newborn brain. Recent observations indicate that the cerebral vessels of the healthy newborn infant, even the very preterm, respond to physiological stimuli in the same manner as in the mature organism. Thus, cerebral blood flow changes with changes in arterial carbon dioxide tension (PaCO2), oxygen concentration (CaO2), or glucose concentration, whereas cerebral blood flow remains constant at minor fluctuations in arterial blood pressure. In pathological states, pressure autoregulation may become impaired, and in severe cases the vessels do not react to chemical or metabolic stimuli. These infants are at high risk for developing cerebral lesion, and they may be candidates for new "brain-protecting regimens."

Chronic hyperglycemic diabetes in the rat is associated with a selective impairment of cerebral vasodilatory responses

Diabetes has been reported to impair vasodilatory responses in the peripheral vascular tissue. However, little is known about vasodilatory function in the diabetic brain. We therefore studied, in the N2O-sedated, paralyzed, and artificially ventilated rat, the effects of chronic hyperglycemic diabetes on the cerebral blood flow (CBF) responses to 3 acutely imposed vasodilatory stimuli: hypoglycemia (HG) (plasma glucose = 1.6-1.9 mumol ml-1), hypoxia (HX) (PaO2 = 35-38 mm Hg), or hypercarbia HC) (PaCO2 = 75-78 mm Hg). In addition, we evaluated the somatosensory evoked potential (SSEP) and plasma catecholamine changes in rats exposed to acute glycemic reductions. Diabetes was induced via streptozotocin (STZ, 60 mg kg-1 i.p.). All results in diabetic rats were compared to those obtained in age-matched nondiabetic controls. The animals were studied at 6-8 weeks (HG experiments) or 4-6 months (HG, HX, and HC experiments) post-STZ. Values for CBF were obtained for the cortex (CX), subcortex (SC), brainstem (BS), and cerebellum (CE) employing radiolabeled microspheres. Up to three CBF determinations were made in each animal. In 6-8 week diabetics vs. controls, CBF increased to a lesser value in the CX, SC, and BS (p less than 0.05). Thus, in the diabetics, going from chronic hyperglycemia to acute hypoglycemia, CBF values (in ml 100 g-1 min-1 +/- SD) increased (p less than 0.05) from 89 +/- 22 to 221 +/- 57 in the CX, from 82 +/- 21 to 160 +/- 52 in the SC, and from 79 +/- 34 to 237 +/- 125 in the BS. In controls, going from normoglycemia to acute hypoglycemia, the CBF changes (p less than 0.05) were 128 +/- 27 to 350 +/- 219 (CX), 117 +/- 11 to 358 +/- 206 (SC), and 130 +/- 29 to 452 +/- 254 (BS). CBF changes and absolute values in the CE were similar in the two groups. At 4-6 months post-STZ, a complete loss of the hypoglycemic CBF response was found in the CX, SC, and CE. In the BS, a CBF response to hypoglycemia was seen in the diabetic rats, with the CBF increasing from 114 +/- 28 (hyperglycemia) to 270 +/- 204 ml 100 g-1 min-1 (p less than 0.05), compared to a change from 147 +/- 36 (normoglycemia) to 455 +/- 299 ml 100 g-1 min-1 (p less than 0.05) in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain glucose utilization and transport and cortical function in chronic vs. acute hypoglycemia

We compared regional brain capillary permeability-surface area products for glucose transfer (PSin), cerebral glucose utilization (rCMRGlc) rates, and brain tissue glucose levels (GlCbr) in N2O-sedated, paralyzed, and artificially ventilated rats during normoglycemia (NG), insulin-induced acute hypoglycemia (AH), or chronic hypoglycemia (CH) [hypoglycemic plasma glucose (Glcp) = 2.2-2.3 mumol/ml]. In addition, a comparative assessment of brain function in AH vs. CH was performed employing somatosensory-evoked response (SSER) technology. A double-label (3H and 14C) 2-deoxy-D-glucose method was used for the simultaneous assessment of PSin and rCMRGlc. Compared with normoglycemic controls, AH resulted in significant 40-50% reductions in rCMRGlc in 10 of 11 regions analyzed (cerebellum unchanged). In CH vs. AH, significantly higher values for rCMRGlc, Glcbr/Glcp ratios, and PSin were seen in 8, 8, and 5 regions, respectively. No differences in rCMRGlc were observed when comparing CH vs. NG groups. Furthermore, CH rats were able to sustain normal SSER at levels of hypoglycemia (1.5 mumol/ml) that, when imposed acutely, resulted in attenuated SSER. Thus CH is associated with an enhanced blood-brain glucose transport capacity in many (but not all) brain regions. This in turn increases rCMRGlc and improves the general cerebral function compared with that seen during AH.

Cerebral blood flow during chronic hypoglycemia in the rat

Regional cerebral blood flow (rCBF) was measured in normoglycemic and chronically hypoglycemic rats. Chronic hypoglycemia was produced by continuously infusing insulin for 6-7 days. During chronic hypoglycemia (plasma glucose = 1.97 mumol/ml), rCBF increased in all regions except the cerebellum and hypothalamus. Blood flow increases present during chronic hypoglycemia were not as great as those previously measured during acute hypoglycemia. Therefore, adjustments in the regulation of rCBF occurred during chronic hypoglycemia compared to acute hypoglycemia.

Compensatory increase of CBF in preterm infants during hypoglycaemia

Cerebral blood flow (CBF00) was investigated in 24 preterm infants (mean 30.8 weeks of gestational age) by use of intravenous 133-Xe clearance technique while screening simultaneously for low blood glucose after birth (mean 3 hours). CBF was significantly increased in 10 infants with blood glucose lower than 1.7 mmol/l compared to normoglycaemic infants and tended to decrease rapidly after treatment. Nine of the 10 hypoglycaemic infants were monitored for cerebral function. Well defined visual evoked cortical potentials were elicitable in all and the aEEG was not less active during the hypoglycaemic episode. Therefore, it is suggested that compensatory increase of CBF may have supported the cerebral metabolism during uncomplicated hypoglycaemia.

Cerebral glucose utilization after aversive conditioning and during conditioned fear in the rat

Regional cerebral glucose utilization (rCMRglu) was studied in rats with and without previous aversive conditioning. Four groups of rats were studied. Two groups of rats were aversely conditioned by placing them in a shock chamber (conditioned stimulus) where they received random footshocks. The two remaining groups were placed in the shock chamber but not conditioned. Regional CMRglu and systemic parameters (heart rate, mean arterial blood pressure (MABP), blood gases and pH, plasma catecholamines, and plasma glucose) were measured in unconditioned and conditioned rats in the presence and in the absence of the conditioned stimulus. The changes in rCMRglu described below appeared to be global and not limited to specific regions. Results are as follows: (1) transferring unconditioned rats to the shock chamber had no significant effect on rCMRglu even though the systemic parameters indicated a stress response. It appears that stress capable of inducing changes in heart rate, MABP, and plasma catecholamines is not necessarily accompanied by increases in cerebral glucose utilization. (2) Conditioned rats not exposed to the shock chamber at the time rCMRglu was measured had decreased rates of rCMRglu compared to rats that were not conditioned. Except for plasma epinephrine, which increased after conditioning, systemic parameters were not affected. (3) Conditioned fear, elicited by transferring conditioned rats to the shock chamber, increased rCMRglu when compared to a control group that was conditioned to footshock using the same paradigm but not exposed to the shock chamber at the time rCMRglu was measured. The systemic parameters indicated a stress response in conditioned rats transferred to the shock chamber.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of angiotensin II and peptide YY on cerebral and circumventricular blood flow

Angiotensin II and peptide YY (PYY) are putative neuro/humoral agents acting at several circumventricular regions. These peptides also constrict cerebral vessels. We examined the effect of acute intravenous infusion of saline, angiotensin II and peptide YY on local cerebral blood flow (14C-iodoantipyrine autoradiography) in the circumventricular and non-circumventricular brain regions of 17 conscious rats. No reductions in brain blood flow (28 regions) were observed although angiotensin II and PYY infusion elevated arterial blood pressure 15-25% without influencing heart rate, suggesting an increase in peripheral resistance. However, local blood flow was dependent on the peptide infused. During PYY infusion, blood flow was rather constant in the 20 non-circumventricular regions examined whereas an increase in blood flow and a slight decrease in cerebrovascular resistance occurred in the circumventricular regions. The area postrema exhibited the most pronounced changes--an elevation in blood flow of 44 +/- 11% and a reduction in resistance of 20 +/- 5% in comparison to that in control animals. During angiotensin II infusion, local cerebral blood flow was similar to that in controls and local cerebrovascular resistance was elevated. Thus, the local cerebral circulatory response to peptide administration was dependent on the location of the region examined (circumventricular or non-circumventricular) and on the vasoactive peptide infused.

Electrolyte shifts between brain and plasma in hypoglycemic coma

Hypoglycemia of sufficient severity to cause cessation of EEG activity (coma) is accompanied by energy failure and by loss of ion homeostasis, the latter encompassing a marked rise in extracellular fluid (ECF) K+ concentration and a fall in ECF Ca2+ concentration. Presumably, ECF Na+ concentration decreases as well. In the present study, the extent that the altered ECF-plasma gradients give rise to net ion fluxes between plasma and tissue is explored. Accordingly, whole tissue contents of Ca2+, Mg2+, K+, and Na+ were measured. The experiments were carried out in anaesthetized and artificially ventilated rats given insulin i.p.; cerebral cortical tissue was sampled at the stage of slow-wave EEG activity, after 10, 30, and 60 min of coma (defined as isoelectric EEG), as well as after 1.5, 6, and 24 h of recovery. In the precomatose animals (with a slow-wave EEG pattern), no changes in electrolyte contents were observed. During coma, tissue Na+ content increased progressively and the K+ content fell (each by 20 mumol g-1 during 60 min). During recovery, these alterations were reversed within the first 6 h. The Mg2+ content remained unchanged. In spite of the appreciable plasma to ECF Ca2+ gradient, no significant calcium accumulation was observed. It is concluded significant calcium accumulation was observed. It is concluded that hypoglycemia leads to irreversible neuronal necrosis in the absence of gross accumulation of calcium in the tissue.