Effect of hyperventilation on brain tissue oxygenation and cerebrovenous PO2 in rats

The Effects of Hypoxia on the Immune-Modulatory Properties of Bone Marrow-Derived Mesenchymal Stromal Cells

The therapeutic repertoire for life-threatening inflammatory conditions like sepsis, graft-versus-host reactions, or colitis is very limited in current clinical practice and, together with chronic ones, like the osteoarthritis, presents growing economic burden in developed countries. This urges the development of more efficient therapeutic modalities like the mesenchymal stem cell-based approaches. Despite the encouraging in vivo data, however, clinical trials delivered ambiguous results. Since one of the typical features of inflamed tissues is decreased oxygenation, the success of cellular therapy in inflammatory pathologies seems to be affected by the impact of oxygen depletion on transplanted cells. Here, we examine our current knowledge on the effect of hypoxia on the physiology of bone marrow-derived mesenchymal stromal cells, one of the most popular tools of practical cellular therapy, in the context of their immune-modulatory capacity.

Aspects on the Physiological and Biochemical Foundations of Neurocritical Care

Neurocritical care (NCC) is a branch of intensive care medicine characterized by specific physiological and biochemical monitoring techniques necessary for identifying cerebral adverse events and for evaluating specific therapies. Information is primarily obtained from physiological variables related to intracranial pressure (ICP) and cerebral blood flow (CBF) and from physiological and biochemical variables related to cerebral energy metabolism. Non-surgical therapies developed for treating increased ICP are based on knowledge regarding transport of water across the intact and injured blood-brain barrier (BBB) and the regulation of CBF. Brain volume is strictly controlled as the BBB permeability to crystalloids is very low restricting net transport of water across the capillary wall. Cerebral pressure autoregulation prevents changes in intracranial blood volume and intracapillary hydrostatic pressure at variations in arterial blood pressure. Information regarding cerebral oxidative metabolism is obtained from measurements of brain tissue oxygen tension (PbtO2) and biochemical data obtained from intracerebral microdialysis. As interstitial lactate/pyruvate (LP) ratio instantaneously reflects shifts in intracellular cytoplasmatic redox state, it is an important indicator of compromised cerebral oxidative metabolism. The combined information obtained from PbtO2, LP ratio, and the pattern of biochemical variables reveals whether impaired oxidative metabolism is due to insufficient perfusion (ischemia) or mitochondrial dysfunction. Intracerebral microdialysis and PbtO2 give information from a very small volume of tissue. Accordingly, clinical interpretation of the data must be based on information of the probe location in relation to focal brain damage. Attempts to evaluate global cerebral energy state from microdialysis of intraventricular fluid and from the LP ratio of the draining venous blood have recently been presented. To be of clinical relevance, the information from all monitoring techniques should be presented bedside online. Accordingly, in the future, the chemical variables obtained from microdialysis will probably be analyzed by biochemical sensors.

Hypoxia-inducible factors regulate human and rat cystathionine β-synthase gene expression

Increased catalytic activity of CBS (cystathionine β-synthase) was recently shown to mediate vasodilation of the cerebral microcirculation, which is initiated within minutes of the onset of acute hypoxia. To test whether chronic hypoxia was a stimulus for increased CBS expression, U87-MG human glioblastoma and PC12 rat phaeochromocytoma cells were exposed to 1% or 20% O2 for 24-72 h. CBS mRNA and protein expression were increased in hypoxic cells. Hypoxic induction of CBS expression was abrogated in cells transfected with vector encoding shRNA targeting HIF (hypoxia-inducible factor) 1α or 2α. Exposure of rats to hypobaric hypoxia (0.35 atm; 1 atm=101.325 kPa) for 3 days induced increased CBS mRNA, protein and catalytic activity in the cerebral cortex and cerebellum, which was blocked by administration of the HIF inhibitor digoxin. HIF-binding sites, located 0.8 and 1.2 kb 5' to the transcription start site of the human CBS and rat Cbs genes respectively, were identified by ChIP assays. A 49-bp human sequence, which encompassed an inverted repeat of the core HIF-binding site, functioned as a hypoxia-response element in luciferase reporter transcription assays. Thus HIFs mediate tissue-specific CBS expression, which may augment cerebral vasodilation as an adaptive response to chronic hypoxia.

Decreased spinal synaptic inputs to phrenic motor neurons elicit localized inactivity-induced phrenic motor facilitation

Phrenic motor neurons receive rhythmic synaptic inputs throughout life. Since even brief disruption in phrenic neural activity is detrimental to life, on-going neural activity may play a key role in shaping phrenic motor output. To test the hypothesis that spinal mechanisms sense and respond to reduced phrenic activity, anesthetized, ventilated rats received micro-injections of procaine in the C2 ventrolateral funiculus (VLF) to transiently (~30min) block axon conduction in bulbospinal axons from medullary respiratory neurons that innervate one phrenic motor pool; during procaine injections, contralateral phrenic neural activity was maintained. Once axon conduction resumed, a prolonged increase in phrenic burst amplitude was observed in the ipsilateral phrenic nerve, demonstrating inactivity-induced phrenic motor facilitation (iPMF). Inhibition of tumor necrosis factor alpha (TNFα) and atypical PKC (aPKC) activity in spinal segments containing the phrenic motor nucleus impaired ipsilateral iPMF, suggesting a key role for spinal TNFα and aPKC in iPMF following unilateral axon conduction block. A small phrenic burst amplitude facilitation was also observed contralateral to axon conduction block, indicating crossed spinal phrenic motor facilitation (csPMF). csPMF was independent of spinal TNFα and aPKC. Ipsilateral iPMF and csPMF following unilateral withdrawal of phrenic synaptic inputs were associated with proportional increases in phrenic responses to chemoreceptor stimulation (hypercapnia), suggesting iPMF and csPMF increase phrenic dynamic range. These data suggest that local, spinal mechanisms sense and respond to reduced synaptic inputs to phrenic motor neurons. We hypothesize that iPMF and csPMF may represent compensatory mechanisms that assure adequate motor output is maintained in a physiological system in which prolonged inactivity ends life.

Biochemistry and biology: heart-to-heart to investigate cardiac progenitor cells

BACKGROUND

Cardiac regenerative medicine is a rapidly evolving field, with promising future developments for effective personalized treatments. Several stem/progenitor cells are candidates for cardiac cell therapy, and emerging evidence suggests how multiple metabolic and biochemical pathways strictly regulate their fate and renewal.

SCOPE OF REVIEW

In this review, we will explore a selection of areas of common interest for biology and biochemistry concerning stem/progenitor cells, and in particular cardiac progenitor cells. Numerous regulatory mechanisms have been identified that link stem cell signaling and functions to the modulation of metabolic pathways, and vice versa. Pharmacological treatments and culture requirements may be exploited to modulate stem cell pluripotency and self-renewal, possibly boosting their regenerative potential for cell therapy.

MAJOR CONCLUSIONS

Mitochondria and their many related metabolites and messengers, such as oxygen, ROS, calcium and glucose, have a crucial role in regulating stem cell fate and the balance of their functions, together with many metabolic enzymes. Furthermore, protein biochemistry and proteomics can provide precious clues on the definition of different progenitor cell populations, their physiology and their autocrine/paracrine regulatory/signaling networks.

GENERAL SIGNIFICANCE

Interdisciplinary approaches between biology and biochemistry can provide productive insights on stem/progenitor cells, allowing the development of novel strategies and protocols for effective cardiac cell therapy clinical translation. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Brain tissue oxygen monitoring and hyperoxic treatment in patients with traumatic brain injury

Cerebral ischemia is a well-recognized contributor to high morbidity and mortality after traumatic brain injury (TBI). Standard of care treatment aims to maintain a sufficient oxygen supply to the brain by avoiding increased intracranial pressure (ICP) and ensuring a sufficient cerebral perfusion pressure (CPP). Devices allowing direct assessment of brain tissue oxygenation have showed promising results in clinical studies, and their use was implemented in the Brain Trauma Foundation Guidelines for the treatment of TBI patients in 2007. Results of several studies suggest that a brain tissue oxygen-directed therapy guided by these monitors may contribute to reduced mortality and improved outcome of TBI patients. Whether increasing the oxygen supply to supraphysiological levels has beneficial or detrimental effects on TBI patients has been a matter of debate for decades. The results of trials of hyperbaric oxygenation (HBO) have failed to show a benefit, but renewed interest in normobaric hyperoxia (NBO) in the treatment of TBI patients has emerged in recent years. With the increased availability of advanced neuromonitoring devices such as brain tissue oxygen monitors, it was shown that some patients might benefit from this therapeutic approach. In this article, we review the pathophysiological rationale and technical modalities of brain tissue oxygen monitors, as well as its use in studies of brain tissue oxygen-directed therapy. Furthermore, we analyze hyperoxia as a treatment option in TBI patients, summarize the results of clinical trials, and give insights into the recent findings of hyperoxic effects on cerebral metabolism after TBI.

Cerebral oxygenation in awake rats during acclimation and deacclimation to hypoxia: an in vivo electron paramagnetic resonance study

Exposure to high altitude or hypobaric hypoxia results in a series of metabolic, physiologic, and genetic changes that serve to acclimate the brain to hypoxia. Tissue Po(2) (Pto(2)) is a sensitive index of the balance between oxygen delivery and utilization and can be considered to represent the summation of such factors as cerebral blood flow, capillary density, hematocrit, arterial Po(2), and metabolic rate. As such, it can be used as a marker of the extent of acclimation. We developed a method using electron paramagnetic resonance (EPR) to measure Pto(2) in unanesthetized subjects with a chronically implanted sensor. EPR was used to measure rat cortical tissue Pto(2) in awake rats during acute hypoxia and over a time course of acclimation and deacclimation to hypobaric hypoxia. This was done to simulate the effects on brain Pto(2) of traveling to altitude for a limited period. Acute reduction of inspired O(2) to 10% caused a decline from 26.7 ± 2.2 to 13.0 ± 1.5 mmHg (mean ± SD). Addition of 10% CO(2) to animals breathing 10% O(2) returned Pto(2) to values measured while breathing 21% O(2,) indicating that hypercapnia can reverse the effects of acute hypoxia. Pto(2) in animals acclimated to 10% O(2) was similar to that measured preacclimation when breathing 21% O(2). Using a novel, individualized statistical model, it was shown that the T(1/2) of the Pto(2) response during exposure to chronic hypoxia was approximately 2 days. This indicates a capacity for rapid adaptation to hypoxia. When subjects were returned to normoxia, there was a transient hyperoxygenation, followed by a return to lower values with a T(1/2) of deacclimation of 1.5 to 3 days. These data indicate that exposure to hypoxia results in significant improvements in steady-state oxygenation for a given inspired O(2) and that both acclimation and deacclimation can occur within days.

HIF-1α in epidermis: oxygen sensing, cutaneous angiogenesis, cancer, and non-cancer disorders

Besides lung, postnatal human epidermis is the only epithelium in direct contact with atmospheric oxygen. Skin epidermal oxygenation occurs mostly through atmospheric oxygen rather than tissue vasculature, resulting in a mildly hypoxic microenvironment that favors increased expression of hypoxia-inducible factor-1α (HIF-1α). Considering the wide spectrum of biological processes, such as angiogenesis, inflammation, bioenergetics, proliferation, motility, and apoptosis, that are regulated by this transcription factor, its high expression level in the epidermis might be important to HIF-1α in skin physiology and pathophysiology. Here, we review the role of HIF-1α in cutaneous angiogenesis, skin tumorigenesis, and several skin disorders.

Reduced respiratory neural activity elicits phrenic motor facilitation

We hypothesized that reduced respiratory neural activity elicits compensatory mechanisms of plasticity that enhance respiratory motor output. In urethane-anesthetized and ventilated rats, we reversibly reduced respiratory neural activity for 25-30 min using: hypocapnia (end tidal CO(2)=30 mmHg), isoflurane (~1%) or high frequency ventilation (HFV; ~100 breaths/min). In all cases, increased phrenic burst amplitude was observed following restoration of respiratory neural activity (hypocapnia: 92±22%; isoflurane: 65±22%; HFV: 54±13% baseline), which was significantly greater than time controls receiving the same surgery, but no interruptions in respiratory neural activity (3±5% baseline, p<0.05). Hypocapnia also elicited transient increases in respiratory burst frequency (9±2 versus 1±1bursts/min, p<0.05). Our results suggest that reduced respiratory neural activity elicits a unique form of plasticity in respiratory motor control which we refer to as inactivity-induced phrenic motor facilitation (iPMF). iPMF may prevent catastrophic decreases in respiratory motor output during ventilatory control disorders associated with abnormal respiratory activity.

A method for measuring brain partial pressure of oxygen in unanesthetized unrestrained subjects: the effect of acute and chronic hypoxia on brain tissue PO(2)

The level of tissue oxygenation provides information related to the balance between oxygen delivery, oxygen utilization, tissue reactivity and morphology during physiological conditions. Tissue partial pressure of oxygen (PtO(2)) is influenced by the use of anesthesia or restraint. These factors may impact the absolute level of PtO(2). In this study we present a novel fiber optic method to measure brain PtO(2). This method can be used in unanesthetized, unrestrained animals, provides absolute values for PO(2), has a stable calibration, does not consume oxygen and is MRI compatible. Brain PtO(2) was studied during acute hypoxia, as well as before and after 28 days of high altitude acclimatization. A sensor was chronically implanted in the frontal cortex of eight Wistar rats. It is comprised of a fiber optic probe with a tip containing material that fluoresces with an oxygen dependent lifetime. Brain PtO(2) declines by 80% and 76% pre- and post-acclimatization, respectively, when the fraction of inspired oxygen declines from 0.21 to 0.08. In addition, a linear relationship between brain PtO(2) and inspired O(2) levels was demonstrated r(2)=0.98 and r(2)=0.99 (pre- and post-acclimatization). Hypoxia acclimatization resulted in an increase in the overall brain PtO(2) by approximately 35%. This paper demonstrates the use of a novel chronically implanted fiber optic based sensor for measuring absolute PtO(2). It shows a very strong linear relationship in awake animals between inspired O(2) and tissue O(2), and shows that there is a proportional increase in PtO(2) over a range of inspired values after exposure to chronic hypoxia.

Hypoxia increases the expression of stem-cell markers and promotes clonogenicity in glioblastoma neurospheres

Hypoxia promotes the expansion of non-neoplastic stem and precursor cell populations in the normal brain, and is common in malignant brain tumors. We examined the effects of hypoxia on stem-like cells in glioblastoma (GBM). When GBM-derived neurosphere cultures are grown in 1% oxygen, hypoxia-inducible factor 1alpha (HIF1alpha) protein levels increase dramatically, and mRNA encoding other hypoxic response genes, such as those encoding hypoxia-inducible gene-2, lysyl oxidase, and vascular endothelial growth factor, are induced over 10-fold. Hypoxia increases the stem-like side population over fivefold, and the percentage of cells expressing CD133 threefold or more. Notch pathway ligands and targets are also induced. The rise in the stem-like fraction in GBM following hypoxia is paralleled by a twofold increase in clonogenicity. We believe HIF1alpha plays a causal role in these changes, as when oxygen-stable HIF1alpha is expressed in normoxic glioma cells CD133 is induced. We used digoxin, which has been shown to lower HIF protein levels in vitro and in vivo, to inhibit the hypoxic response. Digoxin suppressed HIF1alpha protein expression, HIF1alpha downstream targets, and slowed tumor growth in vivo. In addition, pretreatment with digoxin reduced GBM flank xenograft engraftment of hypoxic GBM cells, and daily intraperitoneal injections of digoxin were able to significantly inhibit the growth of established subcutaneous glioblastoma xenografts, and suppressed expression of vascular endothelial growth factor.

High resolution ultra high field magnetic resonance imaging of glioma microvascularity and hypoxia using ultra-small particles of iron oxide

OBJECTIVES

This study assessed whether ultra-small particles of iron oxide (USPIO) intravascular contrast agent could enhance visualization of tumor microvascularity in F98 glioma bearing rats by means of ultra high field (UHF) high-resolution gradient echo (GRE) magnetic resonance imaging (MRI). In an effort to explain differences in visualization of microvascularity before and after USPIO administration, hypoxia and vessel diameters were assessed on corresponding histopathologic sections.

MATERIALS AND METHODS

F98 glioma cells were implanted stereotactically into the brains of syngeneic Fischer rats. Based on clinical criteria, rats were imaged 1 to 2 days before their death with and without USPIO contrast on an 8 Tesla MRI. To identify hypoxic regions of the brain tumor by immunohistochemical staining, a subset of animals also received a nitroimidazole-based hypoxia marker, EF5, before euthanasia. These sections then were compared with noncontrast enhanced MR images. The relative caliber of tumor microvasculature, compared with that of normal brain, was analyzed in a third group of animals.

RESULTS

After USPIO administration, UHF high-resolution GRE MRI consistently predicted increased microvascular density relative to normal gray matter when correlated with histopathology. The in-plane visibility of glioma microvascularity in 22 rats increased by an average of 115% and signal intensity within the tumor decreased by 13% relative to normal brain. Tumor microvascularity identified on noncontrast MR images matched hypoxic regions identified by immunohistochemical staining with a sensitivity of 83% and specificity of 89%. UHF GRE MRI was able to resolve microvessels less than 20 micro in diameter, although differences in tumor vessel size did not consistently account for differences in visualization of microvascularity.

CONCLUSIONS

USPIO administration significantly enhanced visualization of tumor microvascularity on gradient echo 8 T MRI and significantly improved visualization of tumor microvascularity. Microvascularity identified on precontrast images is suspected to be partly associated with hypoxia.

Hypoxia or in situ normoxia: The stem cell paradigm

Although O(2) concentrations are considerably lowered in vivo, depending on the tissue and cell population in question (some cells need almost anoxic environment for their maintenance) the cell and tissue cultures are usually performed at atmospheric O(2) concentration (20-21%). As an instructive example, the relationship between stem cells and micro-environmental/culture oxygenation has been recapitulated. The basic principle of stem cell biology, "the generation-age hypothesis," and hypoxic metabolic properties of stem cells are considered in the context of the oxygen-dependent evolution of life and its transposition to ontogenesis and development. A hypothesis relating the self-renewal with the anaerobic and hypoxic metabolic properties of stem cells and the actual O(2) availability is elaborated ("oxygen stem cell paradigm"). Many examples demonstrated that the cellular response is substantially different at atmospheric O(2) concentration when compared to lower O(2) concentrations which better approximate the physiologic situation. These lower O(2) concentrations, traditionally called "hypoxia" represent, in fact, an in situ normoxia, and should be used in experimentation to get an insight of the real cell/cytokine physiology. The revision of our knowledge on cell/cytokine physiology, which has been acquired ex vivo at non physiological atmospheric (20-21%) O(2) concentrations representing a hyperoxic state for most primate cells, has thus become imperious.

Respiratory rhythm of brainstem-spinal cord preparations: Effects of maturation, age, mass and oxygenation

We examined the effect of age, mass and the presence of the pons on the longevity (length of time spontaneous respiratory-related activity is produced) of brainstem-spinal cord preparations of neonatal rodents (rats and hamsters) and the level of oxygenation in the medulla respiratory network in these preparations. We found the longevity of the preparations from both species decreased with increasing postnatal age. Physical removal of the pons increased respiratory frequency and the longevity of the preparation. However, tissue oxygenation at the level of the medullary respiratory network was not affected by removal of the pons or increasing postnatal age (up to postnatal day 4). Taken together, these data suggest that the effect of removing the pons on respiratory frequency and the longevity of brainstem-spinal cord preparations with increasing postnatal age are primarily due to postnatal development and appear to be unrelated to mass or changes in levels of tissue oxygenation.

Severe hemodilutional anemia increases cerebral tissue injury following acute neurotrauma

Anemia may worsen neurological outcomes following traumatic brain injury (TBI) by undefined mechanisms. We hypothesized that hemodilutional anemia accentuates hypoxic cerebral injury following TBI. Anesthetized rats underwent unilateral TBI or sham injury (n > or = 7). Target hemoglobin concentrations between 50 and 70 g/l were achieved by exchanging 40-50% of the blood volume (1:1) with pentastarch. The effect of TBI, anemia, and TBI-anemia was assessed by measuring brain tissue oxygen tension (Pbr(O(2))), regional cerebral blood flow (rCBF), jugular venous oxygen saturation (Sjv(O(2))), cerebral contusion area, and nuclear staining for programmed cell death. Baseline postinjury Pbr(O(2)) values in the TBI and TBI-anemia groups (9.3 +/- 1.3 and 11.3 +/- 4.1 Torr, respectively) were lower than the uninjured controls (18.2 +/- 5.2 Torr, P < 0.05 for both). Hemodilution caused a further reduction in Pbr(O(2)) in the TBI-anemia group relative to the TBI group without anemia (7.8 +/- 2.7 vs. 14.8 +/- 3.9 Torr, P < 0.05). The rCBF remained stable after TBI and increased comparably after hemodilution in both anemia and TBI-anemia groups. The Sjv(O(2)) was elevated after TBI (87.4 +/- 8.9%, P < 0.05) and increased further following hemodilution (95.0 +/- 1.6%, P < 0.05). Cerebral contusion area and nuclear counts for programmed cell death were increased following TBI-anemia (4.1 +/- 3.0 mm(2) and 686 +/- 192, respectively) relative to TBI alone (1.3 +/- 0.3 mm(2) and 404 +/- 133, respectively, P < 0.05 for both). Hemodilutional anemia reduced cerebral Pbr(O(2)) and oxygen extraction and increased cell death following TBI. These results support our hypothesis that acute anemia accentuated hypoxic cerebral injury after neurotrauma.

Measuring hemoglobin oxygen saturation during graded hypoxic hypoxia in rat striatum

We evaluated in vivo reflectance spectroscopy of visible light as a method to assess brain tissue hemoglobin oxygen saturation in rat striatum (SstrO2). Seven anesthetized and mechanically ventilated rats were subjected to incremental reduction in the fraction of inspired oxygen (Fio2): 0.35, 0.25, 0.15, 0.12, and 0.10, followed by a reoxygenation period (Group 1). At each episode, local changes in SstrO2 and in cerebral blood flow (LCBF) were simultaneously determined in the two striatal regions, using reflectance spectroscopy and laser Doppler flowmetry, respectively. Another group of rats (Group 2, n = 6) was also studied to measure sagittal sinus blood hemoglobin saturation (SssO2) during graded hypoxic hypoxia. Corpus striatum exhibited a significant graded decrease in SstrO2, from 38% +/- 17% at Fio2 of 0.35 (control) to 16% +/- 10% at Fio2 of 0.12 and to 13% +/- 7% at Fio2 of 0.10 (P < 0.05), with no difference between the two hemispheres. These local changes in SstrO2 were associated with a significant graded increase in LCBF: 161% +/- 26% of control values and 197% +/- 34% during these 2 hypoxic episodes, respectively (P < 0.05). All local changes were fully reversed during the reoxygenation period. In Group 2, SssO2 decreased from 38% +/- 8% at Fio2 of 0.35 (control) to 10% +/- 3% at Fio2 of 0.10, closely related to SstrO2 decreasing in hypoxia. This study shows that reflectance spectroscopy of the visible light in rat striatum could be a possible measure of continuous changes in SstrO2. SssO2 and LCBF measurements during graded hypoxic hypoxia indicate that changes in SstrO2 reflect primarily those in brain venous oxygenation.

Regional temperature changes in the brain during somatosensory stimulation

Time-dependent variations in the brain temperature (Tt) are likely to be caused by fluctuations of cerebral blood flow (CBF) and cerebral metabolic rate of oxidative consumption (CMRO2) both of which are seemingly coupled to alterations in neuronal activity. We combined magnetic resonance, optical imaging, temperature sensing, and electrophysiologic methods in alpha-chloralose anesthetized rats to obtain multimodal measurements during forepaw stimulation. Localized changes in neuronal activity were colocalized with regional increases in Tt (by approximately 0.2%), CBF (by approximately 95%), and CMRO2 (by approximately 73%). The time-to-peak for Tt (42+/-11 secs) was significantly longer than those for CBF and CMRO2 (5+/-2 and 18+/-4 secs, respectively) with a 2-min stimulation. Net heat in the region of interest (ROI) was modeled as being dependent on the sum of heats attributed to changes in CMRO2 (Qm) and CBF (Qf) as well as conductive heat loss from the ROI to neighboring regions (Qc) and to the environment (Qe). Although tissue cooling because of Qf and Qc can occur and are enhanced during activation, the net increase in Tt corresponded to a large rise in Qm, whereas effects of Qe can be ignored. The results show that Tt increases slowly (by approximately 0.1 degrees C) during physiologic stimulation in alpha-chloralose anesthetized rats. Because the potential cooling effect of CBF depends on the temperature of blood entering the brain, Tt is mainly affected by CMRO2 during functional challenges. Implications of these findings for functional studies in awake humans and temperature regulation are discussed.

A multiparametric assessment of oxygen efflux from the brain

A quantitative understanding of unidirectional versus net extraction of oxygen in the brain is required because an important factor in calculating oxidative metabolism by calibrated functional magnetic resonance imaging (fMRI) as well as oxygen inhalation methods of positron emission tomography (15O2-PET) and nuclear magnetic resonance (17O2-NMR)) is the degree of oxygen efflux from the brain back into the blood. Because mechanisms of oxygen transport from blood to brain are dependent on cerebral metabolic rate of oxygen consumption (CMRO2), cerebral blood flow (CBF), and oxygen partial pressure (pO2) values in intravascular (Piv) and extravascular (Pev) compartments, we implemented multimodal measurements of these parameters into a compartmental model of oxygen transport and metabolism (i.e., hemoglobin-bound oxygen, oxygen dissolved in plasma and tissue spaces, oxygen metabolized in the mitochondria). In the alpha-chloralose anesthetized rat brain, we used magnetic resonance (7.0 T) and fluorescence quenching methods to measure CMRO2 (2.5+/-1.0 micromol/g min), CBF (0.7+/-0.2 mL/g min), Piv (74+/-10 mm Hg), and Pev (16+/-5 mm Hg) to estimate the degree of oxygen efflux from the brain. In the axially distributed compartmental model, oxygen molecules in blood had two possible fates: enter the tissue space or remain in the same compartment; while in tissue there were three possible fates: enter the blood or the mitochondrial space, or remain in the same compartment. The multiparametric results indicate that the probability of unmetabolized (i.e., dissolved) oxygen molecules reentering the blood from the tissue is negligible and thus its inclusion may unnecessarily complicate calculations of CMRO2 for 15O-PET, 17O-NMR, and calibrated fMRI methods.

Physiological and biochemical principles underlying volume-targeted therapy--the "Lund concept"

The optimal therapy of sustained increase in intracranial pressure (ICP) remains controversial. The volume-targeted therapy ("Lund concept") discussed in this article focuses on the physiological volume regulation of the intracranial compartments. The balance between effective transcapillary hydrostatic and osmotic pressures constitutes the driving force for transcapillary fluid exchange. The low permeability for sodium and chloride combined with the high crystalloid osmotic pressure (approximately 5700 mmHg) on both sides of the blood-brain barrier (BBB) counteracts fluid exchange across the intact BBB. Additionally, variations in systemic blood pressure generally are not transmitted to these capillaries because cerebral intracapillary hydrostatic pressure (and blood flow) is physio-logically tightly autoregulated. Under pathophysiological conditions, the BBB may be partially disrupted. Transcapillary water exchange is then determined by the differences in hydrostatic and colloid osmotic pressure between the intra- and extracapillary compartments. Pressure autoregulation of cerebral blood flow is likely to be impaired in these conditions. A high cerebral perfusion pressure accordingly increases intracapillary hydrostatic pressure and leads to increased intracerebral water content and an increase in ICP. The volume-targeted "Lund concept" has been evaluated in experimental and clinical studies to examine the physiological and biochemical (utilizing intracerebral microdialysis) effects, and the clinical experiences have been favorable.

The Difficulties in Comparing In Vivo Oxygen Measurements

There has been rapid development of effective new tools that provide information on oxygenation in vivo and an increased recognition of how valuable such information can be. Consequently, there also has been considerable interest in comparing and evaluating the accuracy and usefulness of the different types of measurements. The various types of measurements usually do not measure the same thing. They may measure PO2 or [O2] or something less directly related, such as hemoglobin saturation. They may make measurements in different compartments (e.g. intracellular, extracellular, vascular) in the volume that they sample, the time span over which they average, the local perturbation that they may cause, etc. They also differ in their sensitivity, accuracy, ability to measure repetitively. However, these potentially confounding and confusing differences can be made into an outstanding virtue, if their nature is considered carefully. Then a proper model can relate them to each other. The ability to relate the various measurements to each other can be a powerful tool to test the validity of models that attempt to explain fully the distribution of oxygen in real systems and the factors that affect this. We then could have a major advancement in our understanding of oxygen transport in tissues, with an ability to determine accurately the effects of physiological and pathophysiological perturbations on oxygenation at all levels of cells and tissues in vivo.

Modeling of the response of ptO2 in rat brain to changes in physiological parameters

It is known that oxygen tension in tissue (ptO2) will change in response to an alteration of physiological parameters including: pCO2 in arterial blood, blood flow, capillary density, oxygen carrying capacity, and p50 of hemoglobin. We have used modeling to compute the change of PtO2 in response to changes of each physiological parameter and related these changes to experimental data. The oxygen distribution in a Krogh cylinder was computed assuming a linear decrease of hemoglobin saturation from the arterial to the venous end of the capillary. Parameters of the model were used to compute the baseline cerebral PtO2 expressed as the mean value of the PtO2 over the whole cylinder. These parameters were adjusted to derive PtO2 values close to those measured at the relevant experimental conditions. Then each desired parameter was varied to calculate the change in PtO2 related to this parameter. Effects of different factors on cerebral PtO2 were modeled and compared with experimental values obtained with various experimental interventions including: changing CBF, modifying p50 with the allosteric modifier RSR13, modification of capillary density, and hemoglobin content. An acceptable agreement of the computed and the experimental changes of the cerebral PtO2 was obtained for these experimental conditions.

Hypercapnia increases cerebral tissue oxygen tension in anesthetized rats

PURPOSE

To test the hypotheses that deliberate elevation of PaCO(2) increases cerebral tissue oxygen tension (PBrO(2)) by augmenting PaO(2) and regional cerebral blood flow (rCBF).

METHODS

Anesthetized rats were exposed to increasing levels of inspired oxygen (O(2)) or carbon dioxide (CO(2); 5%, 10% and 15%, n = 6). Mean arterial blood pressure (MAP), PBrO(2) and rCBF were measured continuously. Blood gas analysis and hemoglobin concentrations were determined for each change in inspired gas concentration. Data are presented as mean +/- standard deviation with P < 0.05 taken to be significant.

RESULTS

The PBrO(2) increased in proportion to arterial oxygenation (PaO(2)) when the percentage of inspired O(2) was increased. Proportional increases in PaCO(2) (48.7 +/- 4.9, 72.3 +/- 6.0 and 95.3 +/- 15.4 mmHg), PaO(2) (172.2 +/- 33.1, 191.7 +/- 42.5 and 216.0 +/- 41.8 mmHg), and PBrO(2) (29.1 +/- 9.2, 49.4 +/- 19.5 and 60.5 +/- 23.0 mmHg) were observed when inspired CO(2) concentrations were increased from 0% to 5%, 10% and 15%, respectively, while arterial pH decreased (P < 0.05 for each). Exposure to CO(2) increased rCBF from 1.04 +/- 0.67 to a peak value of 1.49 +/- 0.45 (P < 0.05). Following removal of exogenous CO(2), arterial blood gas values returned to baseline while rCBF and PBrO(2) remained elevated for over 30 min. The hypercapnia induced increase in PBrO(2) was threefold higher than that resulting from a comparable increase in PaO(2) achieved by increasing the inspired O(2) concentration (34.9 +/- 14.5 vs 11.4 +/- 5.0 mmHg, P < 0.05).

CONCLUSION

These data support the hypothesis that the combined effect of increased CBF, PaO(2) and reduced pH collectively contribute to augmenting cerebral PBrO(2) during hypercapnia.

Baseline physiological state and the fMRI-BOLD signal response to apnea in anesthetized rats

To decipher the biophysical mechanism behind the fMRI-BOLD response to apnea and its dependence on the baseline cerebral blood flow and oxygenation, fMRI and laser Doppler flow (LDF) studies were carried out in anesthetized rats. Baseline cerebral blood flow (CBF) and PaO2 were modulated by ventilating with different gas mixtures namely, room air (21% O2), 100% O2, carbogen (95% O2+5% CO2), 2% CO2 in air or 5% CO2 in air, respectively. A decrease in BOLD signal intensity was observed after the onset of apnea with either room air, 2% CO2 or 5% CO2 ventilation. PaO2 and cerebral tissue PO2 decreased during apnea under these conditions. However, the apnea-induced BOLD signal intensity was unaffected with carbogen ventilation and increased with 100% O2 ventilation, during which PaO2 remained constant and cerebral tissue PO2 increased. When baseline CBF was high during hypercapnia, a faster decrease occurred in the apnea-induced BOLD signal. Apnea induced the largest increase in CBF of 85 +/- 25% when ventilated with 2% CO2 while a 44 +/- 8% increase was observed with room air. During the other ventilatory conditions, minimal or no significant change in CBF was observed during apnea. These results show a significant correlation between the BOLD signal change and tissue PO2 in response to apnea under different physiological conditions. Apnea-induced increase in CBF affects the magnitude of the BOLD signal response when PaO2 remains constant or changes minimally.

Hemodilutional anemia is associated with increased cerebral neuronal nitric oxide synthase gene expression

Severe hemodilutional anemia may reduce cerebral oxygen delivery, resulting in cerebral tissue hypoxia. Increased nitric oxide synthase (NOS) expression has been identified following cerebral hypoxia and may contribute to the compensatory increase in cerebral blood flow (CBF) observed after hypoxia and anemia. However, changes in cerebral NOS gene expression have not been reported after acute anemia. This study tests the hypothesis that acute hemodilutional anemia causes cerebral tissue hypoxia, triggering changes in cerebral NOS gene expression. Anesthetized rats underwent hemodilution when 30 ml/kg of blood were exchanged with pentastarch, resulting in a final hemoglobin concentration of 51.0 +/- 1.2 g/l (n = 7 rats). Caudate tissue oxygen tension (Pbr(O(2))) decreased transiently from 17.3 +/- 4.1 to 14.4 +/- 4.1 Torr (P < 0.05), before returning to baseline after approximately 20 min. An increase in CBF may have contributed to restoring Pbr(O(2)) by improving cerebral tissue oxygen delivery. An increase in neuronal NOS (nNOS) mRNA was detected by RT-PCR in the cerebral cortex of anemic rats after 3 h (P < 0.05, n = 5). A similar response was observed after exposure to hypoxia. By contrast, no increases in mRNA for endothelial NOS or interleukin-1beta were observed after anemia or hypoxia. Hemodilutional anemia caused an acute reduction in Pbr(O(2)) and an increase in cerebral cortical nNOS mRNA, supporting a role for nNOS in the physiological response to acute anemia.

Volume-targeted therapy of increased intracranial pressure: the Lund concept unifies surgical and non-surgical treatments

Opinions differ widely on the various treatment protocols for sustained increase in intracranial pressure (ICP). This review focuses on the physiological volume regulation of the intracranial compartments. Based on these mechanisms we describe a protocol called 'volume-targeted' ('Lund concept') for treatment of increased ICP. The driving force for transcapillary fluid exchange is determined by the balance between effective transcapillary hydrostatic and osmotic pressures. Fluid exchange across the intact blood-brain barrier (BBB) is counteracted by the low permeability to crystalloids (mainly Na+ and Cl-) combined with the high osmotic pressure (5500 mmHg) on both sides of the BBB. This contrasts to most other capillary regions where the osmotic pressure is mainly derived from the plasma proteins (approximately 25 mmHg). Accordingly, the level of the cerebral perfusion pressure (CPP) is of less importance under physiological conditions. In addition cerebral intracapillary hydrostatic pressure (and cerebral blood flow) is physiologically tightly autoregulated, and variations in systemic blood pressure are generally not transmitted to these capillaries. If the BBB is disrupted, transcapillary water transport will be determined by the differences in hydrostatic and colloid osmotic pressure between the intra- and extracapillary compartments. Under these pathological conditions, pressure autoregulation of cerebral blood flow is likely to be impaired and intracapillary hydrostatic pressure will depend on variations in systemic blood pressure. The volume-targeted 'Lund concept' can be summarized under four headings: (1) Reduction of stress response and cerebral energy metabolism; (2) reduction of capillary hydrostatic pressure; (3) maintenance of colloid osmotic pressure and control of fluid balance; and (4) reduction of cerebral blood volume. The efficacy of the protocol has been evaluated in experimental and clinical studies regarding the physiological and biochemical (utilizing intracerebral microdialysis) effects, and the clinical experiences have been favorable.

Differential fMRI-BOLD signal response to apnea in humans and anesthetized rats

Blood oxygenation level dependent (BOLD) signal intensity (SI) and regional cerebral blood flow (CBF) during a 20-s apnea stimulus in awake humans and pentobarbital-anesthetized rats were measured to assess the usefulness of apnea in estimating cerebral vasodilatory capacity for functional MRI (fMRI) experiments. Rats were ventilated with either room air or 100% O(2.) While breathing room air, apnea for 20 s increased the BOLD SI in humans but decreased it in rats. However, in rats ventilated with 100% O(2), BOLD SI increased upon apnea for 20 s. CBF measurements in rats using laser Doppler flowmetry (LDF) showed a 45% +/- 8% increase during apnea with room air ventilation, and a 10% +/- 3% increase with 100% O(2). Arterial blood oxygen saturation fell from 96% +/- 1% to 29% +/- 5%, and cerebral tissue PO(2) decreased from 15 +/- 3 mmHg to 6 +/- 2 mmHg by the end of 20-s apnea in rats breathing room air. However, with 100% O(2) respiration, apnea produced no change in the arterial blood oxygen saturation, which remained at 99%, but increased tissue PO(2) from 35 +/- 9 mmHg to 39 +/- 10 mmHg. From the results obtained in rats ventilated with room air, it is concluded that apnea induces hypoxia that results in a decrease in fMRI-BOLD signal. The signal decrease occurred despite an increase in P(a)CO(2) and CBF. This BOLD response is the opposite of that observed in humans, who presumably do not develop hypoxia within the applied apnea period. These studies highlight the importance of the choice of ventilating gas mixture on the outcome of BOLD experiments during systemic perturbations.