Microcirculation and transport of oxygen to neurons of the brain

Measurement of Local Partial Pressure of Oxygen in the Brain Tissue under Normoxia and Epilepsy with Phosphorescence Lifetime Microscopy

In this work a method for measuring brain oxygen partial pressure with confocal phosphorescence lifetime microscopy system is reported. When used in conjunction with a dendritic phosphorescent probe, Oxyphor G4, this system enabled minimally invasive measurements of oxygen partial pressure (pO2) in cerebral tissue with high spatial and temporal resolution during 4-AP induced epileptic seizures. Investigating epileptic events, we characterized the spatio-temporal distribution of the "initial dip" in pO2 near the probe injection site and along nearby arterioles. Our results reveal a correlation between the percent change in the pO2 signal during the "initial dip" and the duration of seizure-like activity, which can help localize the epileptic focus and predict the length of seizure.

The metabolism of neurons and astrocytes through mathematical models

Mathematical modeling of the energy metabolism of brain cells plays a central role in understanding data collected with different imaging modalities, and in making predictions based on them. During the last decade, several sophisticated brain metabolism models have appeared. Unfortunately, the picture of the metabolic details that emerges from them is far from coherent: while each model has its justification and is in agreement with some experimental data, some of the predictions of different models can diverge from each other significantly. In this article, we review some of the recent published models, emphasizing similarities and differences between them to understand where the differences in predictions stem from. In that context we present a probabilistic approach, which rather than assigning fixed values to the model parameters, regard them as random variables whose distributions are inferred on in the light of stoichiometric information and different observations. The probabilistic approach reveals how much intrinsic variability a metabolic system may contain, which in turn may be a valid explanation of the different findings.

Dual responses of tissue partial pressure of oxygen after functional stimulation in rat somatosensory cortex

To compare the spatial heterogeneity of brain tissue partial pressure of oxygen (pO(2)) among local brain regions, we focused on functional and anatomical variations in rat somatosensory cortex. Tissue pO(2) was measured by using an oxygen microelectrode with high spatio-temporal resolution, and investigated in three somatosensory areas including hindlimb (HL), forelimb (FL), and trunk region (Tr). Their anatomical structures were determined with histological techniques (Nissl stain). In addition to the measurement of baseline tissue pO(2), we examined temporal shifts in tissue pO(2) distribution elicited by functional stimulation using the brushing stimulation to the hindlimb, forelimb, and trunk regions of the body. We observed that average tissue pO(2) in the Tr (14+/-10 Torr) was significantly lower than those in the HL (25+/-13 Torr) and FL (24+/-13 Torr). Such regional differences in tissue pO(2) were closely related to the cytoarchitectonic variations among these three areas. In addition, the functional stimulation enlarged the regional differences in the pO(2) depending on each somatosensory area; the pO(2) in the HL increased by 3.6+/-2.9% after the stimulation to hindlimb, whereas that in the Tr decreased by -2.9+/-2.5% after the stimulation to trunk region. Such dual responses of tissue pO(2) (i.e. increase or decrease) after the functional stimulation to the corresponding body regions may provide a criterion to clinically predict regions susceptible to tissue hypoxia, because considerable decrease in tissue pO(2) occurred in the Tr showing the lowest baseline pO(2).

Modeling pO(2) distributions in the bone marrow hematopoietic compartment. I. Krogh's model

Human bone marrow (BM) is a tissue of complex architectural organization, which includes granulopoietic loci, erythroblastic islets, and lymphocytic nodules. Oxygen tension (pO(2)) is an important determinant of hematopoietic stem and progenitor cell proliferation and differentiation. Thus, understanding the impact of the BM architectural organization on pO(2) levels in extravascular hematopoietic tissue is an important biophysical problem. However, currently it is impossible to measure pO(2) levels and their spatial variations in the BM. Homogeneous Kroghian models were used to estimate pO(2) distribution in the BM hematopoietic compartment (BMHC) and to conservatively simulate pO(2)-limited cellular architectures. Based on biophysical data of hematopoietic cells and characteristics of BM physiology, we constructed a tissue cylinder solely occupied by granulocytic progenitors (the most metabolically active stage of the most abundant cell type) to provide a physiologically relevant limiting case. Although the number of possible cellular architectures is large, all simulated pO(2) profiles fall between two extreme cases: those of homogeneous tissues with adipocytes and granulocytic progenitors, respectively. This was illustrated by results obtained from a parametric criterion derived for pO(2) depletion in the extravascular tissue. Modeling results suggest that stem and progenitor cells experience a low pO(2) environment in the BMHC.

Modeling pO(2) distributions in the bone marrow hematopoietic compartment. II. Modified Kroghian models

Hematopoietic cells of various lineages are organized in distinct cellular architectures in the bone marrow hematopoietic compartment (BMHC). The homogeneous Kroghian model, which deals only with a single cell type, may not be sufficient to accurately describe oxygen transfer in the BMHC. Thus, for cellular architectures of physiological significance, more complex biophysical-transport models were considered and compared against simulations using the homogeneous Kroghian model. The effects of the heterogeneity of model parameters on the oxygen tension (pO(2)) distribution were examined using the multilayer Kroghian model. We have also developed two-dimensional Kroghian models to simulate several cellular architectures in which a cell cluster (erythroid cluster) or an individual cell (megakaryocyte or adipocyte) is located in the BMHC predominantly occupied by mature granulocytes. pO(2) distributions in colony-type cellular arrangements (erythroblastic islets, granulopoietic loci, and lymphocytic nodules) in the BMHC were also evaluated by modifying the multilayer Kroghian model. The simulated results indicate that most hematopoietic progenitors experience low pO(2) values, which agrees with the finding that low pO(2) promotes the expansion of various hematopoietic progenitors. These results suggest that the most primitive stem cells, which are located even further away from BM sinuses, are likely located in a very low pO(2) environment.

Physiological blocking of the mechanisms of cold death: theoretical and experimental considerations

The cold inhibited functions of skin thermoreceptors, of the thermoregulation centre, and the respiration centre during deep hypothermia can be restored without rewarming the body. The methods used were developed to test the hypothesis that during deep hypothermia calcium ion concentration [Ca(2+)](i) in the cytoplasm increases. This causes a perturbation of cell metabolism, the impairment of cell membrane function that cause the inhibition of cell functioning, resulting in cell death. Such an increase in [Ca(2+)](i) most likely would result from an energy deficit in a deeply cooled cell, which would compromise the processes that maintain the [Ca(2+)](i) at about 10(-7) M. These processes require large amounts of energy since they occur against a large concentration gradient. With the use of EDTA the extracellular concentration of Ca(2+) has been lowered by 15-27%, so reducing the concentration gradient for Ca(2+) between the cell and the medium and in consequence facilitated the process the extrusion of cell Ca(2+).During a period of cooling, sufficient to impair normal functioning, the experimental lowering of blood Ca(2+) allowed the restoration of normal function without the need to rewarm. In such cases the animals survived after cooling the body to temperatures at which they would normally have succumbed. The data presented support the stated hypothesis that the impairment of cellular function in mammals by low temperatures is the result of an uncorrected rise in [Ca(2+)](i).

Microvessel surface area, density and dimensions in brain and muscle of the cephalopod Sepia officinalis

The microvasculature of brain and muscle in the cuttlefish Sepia was studied with stereological techniques to provide information about the surface area for exchange at the blood-tissue interface which was necessary for a parallel study of the permeability of the blood-brain barrier in Sepia . Microvessel density, length, dimensions and volume fraction, and the radius of the ‘Krogh cylinder’ of tissue supplied by each microvessel were also estimated. Vertical lobe (VL) and optic lobe (OL) of brain, outer collar valve muscle (VM) and tentacle muscle (TM) were analysed in 1 μm sections of aldehyde-fixed, Epon-embedded material. ‘Microvessels’ (diameter less than 20 μm) had a surface area density S v (in the order VL, OL, VM, TM) of 134, 176, 67.9 and 13.8 cm 2 cm -3 respectively. The numbers of microvessels per unit area tissue, Q A , were 211, 395, 157 and 43 mm -2 respectively. The length density of microvessels J V = 2 x Q A . The microvessel density was significantly greater in synaptic neuropil (NP) than neuron cell body (CB) zones. Total vessel volume density V V was 3.49, 4.73, 1.88 and 0.28%, in good agreement with previous estimates using intravascular tracers. Mean microvessel diameter d̄ was in the range 4.1-6.5 μm (mode 3.9-4.9 μm). The radius of the Krogh cylinder, R, was 28, 20, 32 and 61 μm. Calculations with the Krogh-Erlang equation show that brain and valve muscle are unlikely to be hypoxic under physiological conditions, while tentacle muscle may be. The vascular parameters correlate well with the known biochemistry of cephalopod tissues. This study represents a detailed analysis of the microvasculature in a complex invertebrate and permits useful comparisons with vertebrate tissues. Values for microvascular S V , Q A , J V and d̄ in Sepia brain are similar to those of the rat, while Sepia muscle vascularity is less than in the rat.

A compartmental model for oxygen transport in brain microcirculation

A compartmental model is formulated for oxygen transport in the cerebrovascular bed of the brain. The model considers the arteriolar, capillary and venular vessels. The vascular bed is represented as a series of compartments on the basis of blood vessel diameter. The formulation takes into account such parameters as hematocrit, vascular diameter, blood viscosity, blood flow, metabolic rate, the nonlinear oxygen dissociation curve, arterial PO2, P50 (oxygen tension at 50% hemoglobin saturation with O2) and carbon monoxide concentration. The countercurrent diffusional exchange between paired arterioles and venules is incorporated into the model. The model predicts significant longitudinal PO2 gradients in the precapillary vessels. However, gradients of hemoglobin saturation with oxygen remain fairly small. The longitudinal PO2 gradients in the postcapillary vessels are found to be very small. The effect of the following variables on tissue PO2 is studied: blood flow, PO2 in the arterial blood, hematocrit, P50, concentration of carbon monoxide, metabolic rate, arterial diameter, and the number of perfused capillaries. The qualitative features of PO2 distribution in the vascular network are not altered with moderate variation of these parameters. Finally, the various types of hypoxia, namely hypoxic, anemic and carbon monoxide hypoxia, are discussed in light of the above sensitivity analysis.

Microcirculation velocity changes under hypoxia in brain, muscles, liver, and their physiological significance

Linear blood flow velocity in brain, muscle capillaries and hepatic sinusoids was measured by means of microfilming in normal and hypoxemic rats (breathing with 7% O2 in N2). The mean flow velocity was found to increase by 66% in brain capillaries and by 12% in hepatic sinusoids. In skeletal muscle the blood flow ceased in about 40% of the capillaries under investigation and in the others the flow velocity slowed down twofold. Different response to hypoxemia was explained by the physiological function peculiarities of the organs in question as well as the type of the energetic exchange and that of the microvascular net structure.

Quantitative capillary topography and blood flow in the cerebral cortex of cats: an in vivo microscopic study

In 50 anesthetized cats the microcirculation in intermediate and deeper layers of the cerebral cortex was visualized in vivo by microtransillumination, and documented by high-speed microcinephotography. The viability of the preparation was verified in a series of experiments demonstrating spontaneous vasomotion and responsiveness to chemical stimulation of pial arterioles and small arteries. Stereological methods for quantitative analysis of projected images of capillaries in a comparatively large tissue volume were employed to determine morphometric and topographical parameters of the asymmetric, highly tortuous intracortical capillary network. Capillary diameters (5.1 +/- 0.84 micrometer), radii of curvature (median 57 micrometer), total capillary lengths per tissue volume 939 +/- 338.2 mm/cu.mm), capillary volume fractions (2.1 +/- 0.51%), total capillary surface areas per tissue volume (15.3 +/- 4.85 sq.mm/cu.mm), and intercapillary distances (median 24.2 micrometer) showed significant interregional differences. The frequency distribution of the lengths of capillary segments (median 108 micrometer) was best described by a Weibull distribution. On the average 90% of all capillaries were continuously perfused. Capillary red cell flow (median velocity 1500 micrometer/sec) was predominantly unidirectional and conspicuously irregular. The variance of capillary red cell velocities (CRCVs) was significantly correlated (tau = 0.48) with capillary tortuosity. An extreme value distribution best described the observed frequency distribution of CRCVs. Flow irregularities represented both white noise and a significant stochastic periodicity at frequencies between 40 and 90 Hz.