Glut1 is a specific transporter system that mediates glucose transfer across the blood-brain barrier (BBB). Although the main location of Glut1 is in the capillary endothelium of the brain, its local ...distribution in different brain regions is not as well defined. In the present investigation, the local pattern of Glut1 distribution was determined in 13 brain structures using an immunoautoradiographic method developed for this purpose. A polyclonal antibody directed against the C-terminal amino acid sequence of Glut1 was applied to cryosections of rat brains. A secondary antibody was added that had been coupled to 35S. Results show a heterogeneous distribution of Glut1 in the brain with activities of 35S ranging from 65% below to 15% above the mean. White matter activity was lower than gray matter activity. For comparison, capillary sections were counted in corresponding cryosections by indirect immunofluorescence using fibronectin antibodies. In addition, local cerebral glucose utilization (LCGU) was analyzed in identical brain structures of conscious rats by the quantitative autoradiographic 2-deoxyglucose method. Significant correlations were found between Glut1 density and either LCGU or capillary density. Results indicate a tight coupling of Glut1 transporter density and capillary density to the LCGU of different BBB structures in adult rats.
The effects of a blood exchange on cerebral blood flow and glucose utilization were studied. A near to total blood exchange (hematocrit < 3%) was achieved in conscious rats by isovolemic ...hemodilution. Ultrapurified, polymerized, bovine hemoglobin (UPBHB) served as a blood substitute. Local cerebral blood flow (LCBF) and local cerebral glucose utilization (LCGU) were measured in 34 brain structures of conscious rats by means of the ido14Cantipyrine and the 2-14C-deoxy-D-glucose methods. A group of rats without blood exchange served as control. After blood exchange LCBF increased from 36 to 126% in the different brain structures resulting in a nearly doubled mean cerebral blood flow (+82%). LCGU increased only moderately by 0-24%. Significant increases in LCGU were observed in 16 brain structures. Mean cerebral glucose utilization slightly increased (+14%). The relationship between LCGU and LCBF was found to be tight both in the control group (r = 0.95) as well as after blood replacement (r = 0.94), although it was reset to a higher overall LCBF-to-LCGU ratio. The profound increases in LCBF observed after blood exchange, which were not paralleled by comparable increases in LCGU, might be explained by a reduction of blood viscosity after blood exchange. Additional effects of blood exchange observed in the present study were an increase of mean arterial blood pressure and a decline of heart rate. The results indicate that replacement of blood with the hemoglobin-based oxygen carrier UPBHB appears to meet the cerebral circulatory and metabolic demands of the brain tissue.
The present study addresses the question whether a chronic decrease of plasma glucose concentration for 1 week induces a global or local increase in glucose transporter densities Glut1 and Glut3 in ...the brain. To induce chronic hypoglycemia insulin was infused into rats by osmotic minipumps for 1 week resulting in a mean plasma glucose concentration of 3.1±0.5 mmol/l (control group: 8.1±0.5 mmol/l). Global and local densities of Glut1 and Glut3 glucose transporters were measured by immunoautoradiographic methods. The mean density of glucose transporters Glut1 remained unchanged, whereas the mean density of Glut3 increased slightly, although significantly. To determine whether the increased density of Glut3 is related to a change in glucose metabolism, the local cerebral metabolic rate of glucose (lCMR
glc) was quantified by the 2-deoxyglucose method. Mean glucose utilization was decreased by 15%. Local analysis of transporter densities (Glut1 and Glut3) and glucose utilization showed a significant correlation between local glucose transporter densities (Glut1 and Glut3) and lCMR
glc during hypoglycemia as already previously observed during normoglycemia. It is concluded that 1 week of hypoglycemia is a stimulus for the induction of additional glucose transporters Glut3 in the brain. These additional neuronal glucose transporters may support the maintenance of glucose utilization which is not completely maintained under these conditions.
Regional cerebral blood flow and metabolism were investigated after addition of a small volume of perfluorocarbon (PFC) emulsion to the resuscitation fluid after hemorrhage. Severe volume-controlled ...hemorrhage (40 mL/kg body weight (bw) withdrawn over 30 min followed by hypovolemia of 30 min duration) was induced in conscious rats. While breathing 100% oxygen, the intravascular volume was repleted by the infusion of either 6% hydroxyethyl starch (mean mol wt 200,000/0.5; HES) or 6% hydroxyethyl starch plus perflubron (90% wt/vol emulsion of perfluoroctylbromide, 3 mL/kg bw; HES-PFOB). Two hours after fluid resuscitation either iodoCantipyrine or 2Cdeoxy-d-glucose were infused. Local cerebral blood flow (LCBF) or local cerebral glucose utilization (LCGU) were determined in 34 brain structures using quantitative autoradiography. Local cerebral metabolism was not disturbed in the HES and the HES-PFOB groups after fluid resuscitation, although slight reductions (mean −14%) were measured (HES-PFOB vs HES; P < 0.05). The HES-PFOB group showed LCBF values that were higher in the different brain structures than those of the HES group (mean +30%). A close correlation was found between LCGU and LCBF of the 34 brain structures in both groups (HESr = 0.96, P < 0.01; HES-PFOBr = 0.98, P < 0.01), whereas the LCBF-to-LCGU ratio was reset from 2.2 mL/μmol in the HES group to 3.4 mL/μmol in the HES-PFOB group (P < 0.05). The higher blood flows in the HES-PFOB group were sufficient to restore cerebral oxygen delivery to normal levels at a reduced arterial oxygen content. The results indicate that the addition of a small amount of perflubron to HES after fluid resuscitation from hemorrhage results in a restitution of cerebral oxygen delivery to normal levels due to an increased cerebral blood flow.
The rostroventrolateral medulla (RVLM) is the main integration center for the regulation of the sympathetic outflow. The present study had the aim of investigating the effects of stimulation of the ...RVLM on the glucose utilization of the brain. Local cerebral glucose utilization (LCGU) can be regarded as an indicator of the brain functional activity. In anesthetized (chloralose-urethan), paralyzed (pancuronium) and ventilated rats, the medulla was exposed by a ventral craniotomy. The RVLM was stimulated by microinjection of 100 nl of 0.5 M sodium glutamate (
n = 6). The effective stimulation was verified by the increase in arterial blood pressure. In a control group (
n = 7), an identical volume of saline was injected into the RVLM. Local cerebral glucose utilization was measured in both groups using the 2-
14Cdeoxyglucose method. The results showed a significant decrease in LCGU in the stimulated group in 33 of 39 brain structures examined. In order to investigate whether the decrease in brain glucose utilization is secondary to the stimulation of baroreceptor afferents by the increase in arterial blood pressure the carotid sinus nerves and both vagal nerves were cut. In this denervated group (
n = 5) the decrease in LCGU was abolished in all brain structures although blood pressure was increased to a degree comparable to the innervated group. It is concluded that cerebral glucose metabolism is decreased during stimulation of the RVLM and that this decrease is secondary to the activation of baroreceptor afferents by the increase in blood pressure.
The purpose of the present study was to investigate whether or not cerebral glucose utilization is changed locally after damage of the neuronal insulin receptor by means of intracerebroventricular ...(icv) streptozotocin (STZ) administered in a subdiabetogenic dosage (1.5 mg/kg bw.). STZ was administered at the start of the study, and 2 and 21 days later bilaterally into the cerebral ventricles in rats of a mean age of 18 months. The local distribution of cerebral glucose utilization was analyzed in conscious rats on the 42nd day after the first STZ injection using the quantitative (14C)-2-deoxyglucose method. Of the 35 brain structures investigated from autoradiograms of brain sections, 17 showed a reduction in glucose utilization. Decreases in glucose utilization were observed in the frontal, parietal, sensory motor, auditory and entorhinal cortex and in all hippocampal subfields. In contrast, glucose utilization was increased in two white matter structures. The decrease in cerebral glucose utilization observed in cortical and hippocampal areas in the present study may correspond to changes in morphobiological parameters which have been found in patients with Alzheimer's disease. The present data are in accordance with the hypothesis that an impairment in the control of neuronal glucose metabolism at the insulin receptor site may exist in sporadic dementia of Alzheimer type (DAT), and can be studied by the icv STZ animal model.
Since changes in the surface area of capillaries may be relevant to capillary exchange, the distensibility of brain capillaries was investigated. Brain capillary diameters were measured after ...perfusion fixation of brain tissue at a constant perfusion pressure during hypo- or hypercapnia. Sections were embedded, stained, and analyzed by light microscopy. The results showed significant differences in mean capillary diameter between the hypocapnic and the hypercapnic group. In the eight brain structures analyzed, capillary diameters were always larger in the hypercapnic group. Mean capillary diameter was 4.93 +/- 0.29 microns in the hypocapnic group and 5.91 +/- 0.10 microns in the hypercapnic group (means +/- SD). We conclude that brain capillaries exhibit a moderate degree of distensibility. Variations in the precapillary pressure of microvessels may therefore influence both capillary flow and capillary surface area.
Although the frequency for the use of moderate hypothermia in acute ischemic stroke is increasing, the optimal acid-base management during hypothermia remains unclear. This study investigates the ...effect of pH- and alpha-stat acid-base management on cerebral blood flow (CBF), infarct volume, and cerebral edema in a model of transient focal cerebral ischemia in rats.
Twenty Sprague-Dawley rats were subjected to transient middle cerebral artery occlusion (MCAO) for 2 h during normothermic conditions followed by 5 h of reperfusion during hypothermia (33 degrees C). Animals were artificially ventilated with either alpha- (n = 10) or pH-stat management (n = 10). CBF was analyzed 7 h after induction of MCAO by iodo(14)Cantipyrine autoradiography. Cerebral infarct volume and cerebral edema were measured by high-contrast silver infarct staining (SIS).
Compared with the alpha-stat regimen, pH-stat management reduced cerebral infarct volume (98.3 +/- 33.2 mm(3) vs. 53.6 +/- 21.6 mm(3); P > or = 0.05 mean +/- SD) and cerebral edema (10.6 +/- 4.0% vs. 3.1 +/- 2.4%; P > or = 0.05). Global CBF during pH-stat management exceeded that of alpha-stat animals (69.5 +/- 12.3 ml x 100 g(-1) x min(-1) vs. 54.7 +/- 13.3 ml x 100 g(-1) x min; P > or = 0.05). The regional CBF of the ischemic hemisphere was 62.1 +/- 11.2 ml x 100 g(-1) x min(-1) in the pH-stat group versus 48.2 +/- 7.2 ml x 100 g(-1) x min(-1) in the alpha-stat group ( P> or = 0.05).
In the very early reperfusion period (5 h), pH-stat management significantly decreases cerebral infarct volume and edema as compared with alpha-stat during moderate hypothermia, probably by increasing CBF.
The relationship between local cerebral glucose utilization (LCGU) and local cerebral blood flow (LCBF) was studied in two groups of normal conscious rats. LCGU was measured by the 14Cdeoxyglucose ...technique and LCBF by the 14Ciodoantipyrine technique. When the LCGU of 39 brain structures was correlated with their respective values of LCBF an excellent correlation (r = 0.96) was obtained between LCGU and LCBF, demonstrating a tight coupling at a local level. Chronic metabolic acidosis was induced in two other groups of rats by adding 0.35 M NH4Cl to the drinking water for 5-6 days. This resulted in a significant (P less than 0.01) reduction in overall cerebral glucose utilization (-29%) and a nonsignificant reduction in overall cerebral blood flow (-8%). This dissociation between the overall cerebral glucose utilization and blood flow during metabolic acidosis cannot be taken as an indicator of uncoupling because the values for LCGU were still well correlated (r = 0.95) with the values for LCBF, indicating a resetting of the coupling mechanism to a new level.
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