A method for measuring the rate of oxygen release from single microvessels

Oxygen carrier in core-shell fibers synthesized by coaxial electrospinning enhances Schwann cell survival and nerve regeneration

Rationale: Local hypoxia is a challenge for fabrication of cellular grafts and treatment of peripheral nerve injury. In our previous studies, we demonstrated that perfluorotributylamine (PFTBA) could provide short term oxygen supply to Schwann cells (SCs) and counteract the detrimental effects of hypoxia on SCs during the early stages of nerve injury. However, the quick release of oxygen in PFTBA compromised its ability to counteract hypoxia over an extended time, limiting its performance in peripheral nerve injury.

Methods: In this study, PFTBA-based oxygen carrier systems were prepared through coaxial electrospinning to prolong the time course of oxygen release. Core-shell structures were fabricated, optimized, and the oxygen kinetics of PFTBA-enriched core-shell fibers evaluated. The effect of core-shells on the survival and function of SCs was examined in both 2D and 3D systems as well as in vivo. The system was used to bridge large sciatic nerve defects in rats.

Results: PFTBA core-shell fibers provided high levels of oxygen to SCs in vitro, enhancing their survival, and increasing NGF, BDNF, and VEGF expression in 2D and 3D culture systems under hypoxic condition. In vivo analysis showed that the majority of GFP-expressing SCs in the PFTBA conduit remained viable 14 days post-implantation. We found that axons in PFTBA oxygen carrier scaffold improved axonal regeneration, remyelination, and recovery.

Conclusion: A synthetic oxygen carrier in core-shell fibers was fabricated by the coaxial electrospinning technique and was capable of enhancing SC survival and nerve regeneration by prolonged oxygen supply. These findings provide a new strategy for fabricating cellular scaffolds to achieve regeneration in peripheral nerve injury treatment and other aerobic tissue injuries.

Periarteriolar Glioblastoma Stem Cell Niches Express Bone Marrow Hematopoietic Stem Cell Niche Proteins

In glioblastoma, a fraction of malignant cells consists of therapy-resistant glioblastoma stem cells (GSCs) residing in protective niches that recapitulate hematopoietic stem cell (HSC) niches in bone marrow. We have previously shown that HSC niche proteins stromal cell-derived factor-1α (SDF-1α), C-X-C chemokine receptor type 4 (CXCR4), osteopontin (OPN), and cathepsin K (CatK) are expressed in hypoxic GSC niches around arterioles in five human glioblastoma samples. In HSC niches, HSCs are retained by binding of SDF-1α and OPN to their receptors CXCR4 and CD44, respectively. Protease CatK cleaves SDF-1α to release HSCs out of niches. The aim of the present study was to reproduce the immunohistochemical localization of these GSC markers in 16 human glioblastoma samples with the addition of three novel markers. Furthermore, we assessed the type of blood vessels associated with GSC niches. In total, we found seven GSC niches containing CD133-positive and nestin-positive GSCs as a single-cell layer exclusively around the tunica adventitia of 2% of the CD31-positive and SMA-positive arterioles and not around capillaries and venules. Niches expressed SDF-1α, CXCR4, CatK, OPN, CD44, hypoxia-inducible factor-1α, and vascular endothelial growth factor. In conclusion, we show that GSC niches are present around arterioles and express bone marrow HSC niche proteins.

Enhanced in vivo survival of Schwann cells by a synthetic oxygen carrier promotes sciatic nerve regeneration and functional recovery

Local hypoxia in the early stages of peripheral nerve injury is a challenge for axonal regeneration. To address this issue, perfluorotributylamine (PFTBA)-based oxygen carrying fibrin hydrogel was prepared and injected into Schwann cell (SC)-seeded collagen-chitosan conduits to increase oxygen supply to SCs within the conduits. The conduit containing PFTBA-SC gel was then applied to bridge a 15-mm sciatic nerve defect in rats. It was observed that most of the GFP-labeled SCs initially seeded in the PFTBA hydrogel remained alive for approximately 28 days after their in vivo implantation. The number of SCs was significantly higher in the PFTBA-SC scaffold than that in the SC scaffold without PFTBA. In addition, nerve regeneration and functional recovery were examined after nerve injury repair. We found that the PFTBA-SC scaffold was capable of promoting axonal regeneration and remyelination of the regenerated axons. Further studies showed the PFTBA-SC scaffold was able to accelerate the recovery of motor and sensory function of the regenerating nerves. Electrophysiological analysis showed area under the curve of compound muscle action potential and nerve conduction velocity were also improved, and gastrocnemius muscle atrophy was partially reversed by PFTBA-SC scaffold. Furthermore, microvessel density analysis showed PFTBA-SC composites were beneficial for microvascular growth, which provided sustained oxygen for regenerating nerve in the later stages of nerve regeneration. In conclusion, enhanced survival of SCs by PFTBA is capable of promoting sciatic nerve regeneration and functional recovery, which provides a new avenue for achieving better functional recovery in the treatment of peripheral nerve injuries. Copyright © 2016 John Wiley & Sons, Ltd.

A synthetic oxygen carrier-olfactory ensheathing cell composition system for the promotion of sciatic nerve regeneration

The treatment of lengthy peripheral nerve defects is challenging in the field of the regenerative medicine. Thus far, many nerve scaffolds with seeded cells have been developed, which hold great potential to replace nerve autograft in bridging lengthy nerve defects by providing guiding and bioactive cues. However, low oxygen status has been found within nerve scaffolds after their implantation in vivo, which has been shown to result in death or loss of function of supportive cells, and significantly limit nerve regeneration and functional recovery after nerve injury. In the present study, perfluorotributylamine (PFTBA) was introduced into a collagen-chitosan conduit within which olfactory ensheathing cells (OECs) were seeded to increase oxygen supply to OECs, as well as regenerating axons. The "PFTBA-OECs" enriched scaffolds were then used to bridge a 15-mm-long sciatic nerve defect in rats. Both nerve regeneration and functional recovery were examined at pre-defined time points after surgery. We found that the number of GFP-labeled OECs was significantly higher in the "PFTBA-OECs" scaffold than that in the single OECs scaffold. In addition, PFTBA was found to enhance the beneficial effect of OECs-enriched scaffold on axonal regeneration and functional recovery. All these findings indicate that the "PFTBA-OECs" enriched scaffolds are capable of promoting nerve regeneration and functional recovery, which might be attributable, at least in part, to their beneficial effect on the survival of OECs after their implantation in vivo.

The rate of O₂ loss from mesenteric arterioles is not unusually high

The O(2) disappearance curve (ODC) recorded in an arteriole after the rapid arrest of blood flow reflects the complex interaction among the dissociation of O(2) from hemoglobin, O(2) diffusivity, and rate of respiration in the vascular wall and surrounding tissue. In this study, the analysis of experimental ODCs allowed the estimation of parameters of O(2) transport and O(2) consumption in the microcirculation of the mesentery. We collected ODCs from rapidly arrested blood inside rat mesenteric arterioles using scanning phosphorescence quenching microscopy (PQM). The technique was used to prevent the artifact of accumulated O(2) photoconsumption in stationary media. The observed ODC signatures were close to linear, in contrast to the reported exponential decline of intra-arteriolar Po(2). The rate of Po(2) decrease was 0.43 mmHg/s in 20-μm-diameter arterioles. The duration of the ODC was 290 s, much longer than the 12.8 s reported by other investigators. The arterioles associated with lymphatic microvessels had a higher O(2) disappearance rate of 0.73 mmHg/s. The O(2) flux from arterioles, calculated from the average O(2) disappearance rate, was 0.21 nl O(2)·cm(-2)·s(-1), two orders of magnitude lower than reported in the literature. The physical upper limit of the O(2) consumption rate by the arteriolar wall, calculated from the condition that all O(2) is consumed by the wall, was 452 nl O(2)·cm(-3)·s(-1). From consideration of the microvascular tissue volume fraction in the rat mesentery of 6%, the estimated respiration rate of the vessel wall was ∼30 nl O(2)·cm(-3)·s(-1). This result was three orders of magnitude lower than the respiration rate in rat mesenteric arterioles reported by other investigators. Our results demonstrate that O(2) loss from mesenteric arterioles is small and that the O(2) consumption by the arteriolar wall is not unusually large.

Microvascular oxygen tension in the rat mesentery

Longitudinal Po2profiles in the microvasculature of the rat mesentery were studied using a novel phosphorescence quenching microscopy technique that minimizes the accumulated photoconsumption of oxygen by the method. Intravascular oxygen tension (Po2, in mmHg) and vessel diameter ( d, in μm) were measured in mesenteric microvessels ( n = 204) of seven anesthetized rats (275 g). The excitation parameters were as follows: 7 × 7-μm spot size; 410 nm laser; and 100 curves at 11 pulses/s, with pulse parameters of 2-μs duration and 80-pJ/μm2energy density. The mean Po2(± SE) was 65.0 ± 1.4 mmHg ( n = 78) for arterioles ( d = 18.8 ± 0.7 μm), 62.1 ± 2.0 mmHg ( n = 38) at the arteriolar end of capillaries ( d = 7.8 ± 0.3 μm), and 52.0 ± 1.0 mmHg ( n = 88) for venules ( d = 22.5 ± 1.0 μm). There was no apparent dependence of Po2on d in arterioles and venules. There were also no significant deviations in Po2based on d (bin width, 5 μm) from the general mean for both of these types of vessels. Results indicate that the primary site of oxygen delivery to tissue is located between the smallest arterioles and venules (change of 16.3 mmHg, P = 0.001). In conclusion, oxygen losses from mesenteric arterioles and venules are negligible, indicating low metabolic rates for both the vascular wall and the mesenteric tissue. Capillaries appear to be the primary site of oxygen delivery to the tissue in the mesenteric microcirculation. In light of the present results, previously reported data concerning oxygen consumption in the mesenteric microcirculation can be explained as artifacts of accumulated oxygen consumption due to the application of instrumentation having a large excitation area for Po2measurements in slow moving and stationary media.

Apparent diffusion time of oxygen from blood to tissue in rat cerebral cortex: implication for tissue oxygen dynamics during brain functions

To investigate the dynamics of tissue oxygen demand and supply during brain functions, we simultaneously recorded Po(2) and local cerebral blood flow (LCBF) with an oxygen microelectrode and laser Doppler flowmetry, respectively, in rat somatosensory cortex. Electrical hindlimb stimuli were applied for 1, 2, and 5 s to vary the duration of evoked cerebral metabolic rate of oxygen (CMR(O(2))). The electrical stimulation induced a robust increase in Po(2) (4-9 Torr at peak) after an increase in LCBF (14-26% at peak). A consistent lag of approximately 1.2 s (0.6-2.3 s for individual animals) in the Po(2) relative to LCBF was found, irrespective of stimulus length. It is argued that the lag in Po(2) was predominantly caused by the time required for oxygen to diffuse through tissue. During brain functions, the supply of fresh oxygen further lagged because of the latency of LCBF onset ( approximately 0.4 s). The results indicate that the tissue oxygen supports excess demand until the arrival of fresh oxygen. However, a large drop in Po(2) was not observed, indicating that the evoked neural activity demands little extra oxygen or that the time course of excess demand is as slow as the increase in supply. Thus the dynamics of Po(2) during brain functions predominantly depend on the time course of LCBF. Possible factors influencing the lag between demand and supply are discussed, including vascular spacing, reactivity of the vessels, and diffusivity of oxygen.

Rate of decrease of PO2 from an arteriole with arrested flow

When flow to a region is arrested, the amount of oxygen contained within the stationary blood decreases at a rate dependent on the oxygen utilization of the surrounding tissue. We used phosphorescence quenching microscopy to measure arteriolar PO2 in the mesentery of male Sprague-Dawley rats. Flow was quickly stopped (< 1 s) by occluding the microvessels using an inflatable Saran bag attached to the microscope objective. The rate of decline in PO2 following occlusion yielded a calculated initial flux of oxygen out of the vessel lumen of 8.0 x 10(-7) ml O2 cm(-2) sec(-1). An upper limit on the oxygen consumption of the arteriolar wall was calculated by assuming that all of the oxygen in the lumen was consumed by the wall at the initial rate. This value was 2.5 x 10(-3) ml O2 cm(-3) sec(-1) and is an overestimate since the oxygen consumption of the nearby parenchymal cells was neglected. The calculated maximum oxygen consumption of the wall is more than an order of magnitude smaller than that reported previously for arterioles in the rat mesentery (6.5 x 10(-2) ml O2 cm(-3) sec(-1)). We conclude that oxygen consumption of the arteriolar wall is similar to previous values for other vascular tissues.

The influence of environmentalPO2 on hemoglobin oxygen saturation in developing zebrafishDanio rerio

Several studies suggest that during early larval development of lower vertebrates convective blood flow is not essential to supply oxygen to the tissues, but information about the oxygenation status of larvae during the time of cutaneous respiration is still missing. If convective oxygen transport contributes to the oxygen supply to tissues, venous blood in the central circulatory system should be partly deoxygenated, and hyperoxia should increase the oxygen saturation of the hemoglobin. To analyze the changes in hemoglobin oxygen saturation induced by hyperoxic incubation, zebrafish larvae were incubated in a tiny chamber between polytetrafluoroethylene membranes(Teflon), so that the oxygen supply could be rapidly modified. Hemoglobin oxygen saturation was measured in vivo by combining video imaging techniques with a spectrophotometrical analysis of hemoglobin light absorption at specific wavelengths for maximal absorption of oxygenated and deoxygenated blood (413 nm and 431 nm, respectively) under normoxic conditions and after a 10 min period of hyperoxia (PO2=100 kPa),assuming that at a PO2 of 100 kPa the hemoglobin is fully saturated. The results demonstrated that red blood cell oxygenation of zebrafish larvae at 4 days post fertilization (d.p.f.), 5 d.p.f. and 12 d.p.f. could be increased by hyperoxia. The data suggest that at the time of yolk sac degradation (i.e. 4 d.p.f. and 5 d.p.f.), when the total surface area of the animal is reduced, bulk diffusion of oxygen may not be sufficient to prevent a partial deoxygenation of the hemoglobin. The decrease in hemoglobin oxygenation observed at 12 d.p.f. confirms earlier studies indicating that at 12–14 d.p.f., convective oxygen transport becomes necessary to ensure oxygen supply to the growing tissues.

O2 release from Hb vesicles evaluated using an artificial, narrow O2-permeable tube: comparison with RBCs and acellular Hbs

A phospholipid vesicle that encapsulates a concentrated hemoglobin (Hb) solution and pyridoxal 5'-phosphate as an allosteric effector [Hb vesicle (HbV) diameter, 250 nm] has been developed to provide an O2 carrying ability to plasma expanders. The O2 release from flowing HbVs was examined using an O2-permeable, fluorinated ethylenepropylene copolymer tube (inner diameter, 28 microm) exposed to a deoxygenated environment. Measurement of O2 release was performed using an apparatus that consisted of an inverted microscope and a scanning-grating spectrophotometer with a photon-count detector, and the rate of O2 release was determined based on the visible absorption spectrum in the Q band of Hb. HbVs and fresh human red blood cells (RBCs) were mixed in various volume ratios at a Hb concentration of 10 g/dl in isotonic saline that contained 5 g/dl albumin, and the suspension was perfused at the centerline flow velocity of 1 mm/s through the narrow tube. The mixtures of acellular Hb solution and RBCs were also tested. Because HbVs were homogeneously dispersed in the albumin solution, increasing the volume of the HbV suspension resulted in a thicker marginal RBC-free layer. Irrespective of the mixing ratio, the rate of O2 release from the HbV/RBC mixtures was similar to that of RBCs alone. On the other hand, the addition of 50 vol% of acellular Hb solution to RBCs significantly enhanced the rate of deoxygenation. This outstanding difference in the rate of O2 release between the HbV suspension and the acellular Hb solution should mainly be due to the difference in the particle size (250 vs. 7 nm) that affects their diffusion for the facilitated O2 transport.

Oxygen gradients in the microcirculation

As arterialized blood transits from the central circulation to the periphery, oxygen exits through the vessel walls driven by radial oxygen gradients that extend from the red blood cell column, through the plasma, the vessel wall, and the parenchymal tissue. This exit determines a longitudinal gradient of blood oxygen saturation whose extent is inversely related to the level of metabolic activity of the tissue, being small for the brain and considerable for skeletal muscle at rest where hemoglobin is only half-saturated with oxygen when blood arrives to the capillaries. Data obtained by a variety of methods show that the oxygen loss is too great to be explained by diffusion alone, and oxygen gradients measured in the arteriolar wall provide evidence that this structure in vivo is a very large oxygen sink, and suggests a rate of oxygen consumption two orders of magnitude greater than seen in in vitro studies. Longitudinal gradients in the capillary network and radial gradients in surrounding tissue also show a dependence on the metabolic rate of the tissue, being more pronounced in brain than in resting skeletal muscle and mesentery. Mean PO2 values increase from the postcapillary venules to the distal vessels of this network while radial gradients indicate additional oxygen loss. This circumstance may be due to pathways with higher flow having higher oxygen content than low flow pathways as well as possible oxygen uptake from adjacent arterioles. Taken together, these newer findings on oxygen gradients in the microcirculation require a reexamination of existing concepts of oxygen delivery to tissue and the role of the capillaries in this process.

Reduced oxygen release from erythrocytes by the acceleration-induced flow shift, observed in an oxygen-permeable narrow tube

The oxygen release from flowing erythrocytes under accelerational force (0-4 g) was examined using an oxygen-permeable, fluorinated ethylenepropylene copolymer tube (25 microm in inner diameter). The narrow tube was fixed vertically on the rotating disk of a new centrifuge apparatus, and erythrocyte suspension was perfused in the direction of Earth gravity. The accelerational force was applied perpendicularly to the flow direction of cells by centrifugation. The microscopic images of the flowing cells obtained at five different wavelengths were analyzed, and marginal cell-free layer and oxygen saturation of the cells were measured. By lowering oxygen tension around the narrow tube, erythrocytes were deoxygenated in proportion to their traveling distance, and the deoxygenation was enhanced with decreasing flow velocity and hematocrit. With increase of the g-value, the shift of flowing erythrocyte column to the centrifugal side was increased, the column was compressed, and the oxygen release from the cells was suppressed. Qualitatively, similar results were obtained by inducing erythrocyte aggregation with Dextran T-70 (MW = 70,400), without accelerational force. These results conclude that both the accumulation of erythrocytes under accelerational force and the enhancement of erythrocyte aggregation by macromolecules lead to the reduction of oxygen release from the flowing cells.

O(2) release from erythrocytes flowing in a narrow O(2)-permeable tube: effects of erythrocyte aggregation

The effects of erythrocyte aggregation on O(2) release were examined using O(2)-permeable fluorinated ethylenepropylene copolymer tubes (inner diameter, 25 microm; outer diameter, 100 microm). Measurements were performed using an apparatus built on an inverted microscope that contained a scanning-grating spectrophotometer with a photon count detector connected to two photomultipliers and an image processor through a video camera. The rate of O(2) release from the cells flowing in the narrow tube was determined based on the visible absorption spectrum and the flow velocity of the cells as well as the tube size. When the tube was exposed to nitrogen-saturated deoxygenated saline containing 10 mM sodium dithionite, the flowing erythrocytes were deoxygenated in proportion to the traveling distance, and the deoxygenation at a given distance increased with decreasing flow velocity and cell concentration (hematocrit). Adding Dextran T-70 to the cell suspension increased erythrocyte aggregation in the tube, which resulted in suppressed cell deoxygenation and increased marginal cell-free-layer thickness. The deoxygenation was inversely proportional to the cell-free-layer thickness. The relation was not essentially altered even when the medium viscosity was adjusted with Dextran T-40 to remain constant. The rate of O(2) release from erythrocytes in the tube was discussed in relation to the O(2) diffusion process. We conclude that the diffusion of O(2) from erythrocytes flowing in narrow tubes is inhibited primarily by erythrocyte aggregation itself and partly by thickening of the cell-free layer.

Increase in O(2) delivery with hyperoxia does not increase O(2) uptake in tetanically contracting dog muscle

We investigated the influence of hyperoxia on O(2) uptake in tetanically contracting canine gastrocnemius. Hyperoxia showed neither increase in O(2) uptake nor decrease in lactate release, irrespective of increased O(2) supply, venous Po(2) and vascular resistance, as compared to normoxia, suggesting that hyperoxia decreases O(2) diffusion conductance and/or effective O(2) supply probably due to arteriovenous O(2) diffusion shunt.

CCD imaging in cryospectrophotometric determination of microvascular oxyhemoglobin saturations

A microspectrophotometric imaging method has been developed for localized measurements of intravascular oxyhemoglobin (HbO2) saturations in microvessels from sections of quick-frozen tissue. HbO2 saturation was calculated from the absorption spectrum of red blood cells measured at five selected wavelengths in the 520- to 570-nm range. We combined the use of narrow-bandwidth interference filters and a CCD camera mounted on a microscope to obtain one gray image of the sample at each wavelength. Each pixel is a quantitative measure of transmitted light intensity from the tissue sample at that location. A linear calibration curve for blood frozen in vitro (humans and mice) and in vivo (mice) was obtained using a multicomponent analysis. Oxy- and deoxyhemoglobin were assumed to be the only hemoglobin components present. A constant term compensates for light loss due to scattering on red blood cells and ice crystals. The standard error in single measurements of HbO2 saturation was 5%. The present method allows off-line analysis of the HbO2 saturation distribution within a microvessel network and offers new possibilities for comparative morphological studies.

Development of an Optical Triplicator for intravital video microscopy of oxygen saturation

The Optical Triplicator produces three copies of a portion of a microscopic image and places them side-by-side on the face of a video image tube, so that all three images can be viewed simultaneously in each video frame. The Optical Triplicator was used in an intravital microscopy assembly to obtain simultaneous images of a microvessel at three visible wavelengths selected to enable the accurate determination of oxygen saturation in microvessels of the hamster retractor muscle. An image processing system was used to obtain light intensity and optical density from video recordings made using the triplicator. Lumenal oxygen saturation profiles were determined using the measured intensity values and a published three wavelength photometric method.

Simulation of intraluminal gas transport processes in the microcirculation

Intraluminal resistance to gas transport between the microcirculation and tissue was neglected for a half-century following the early work of Krogh. In recent years it has come to be understood that this neglect is seriously in error. This paper reviews the background for the long period of misdirection, and progress in placing the simulation of gas transport processes on a more accurate, quantitative basis.

Flow dynamics of erythrocytes in microvessels of isolated rabbit mesentery: cell-free layer and flow resistance

A part of microvascular bed isolated from rabbit mesentery (composed of a few branches of superior mesenteric artery) was used for the study of the flow dynamics of erythrocytes in microvessels. The flow resistance in the microvascular bed was analyzed with respect to the thickness of a cell-free layer formed along the inner wall of vessels and the suspension viscosity of erythrocytes (in terms of hematocrit and erythrocyte deformability). The thickness of the cell-free layer increased with the increase in the inner diameter of microvessels. By lowering the hematocrit, the thickness increased and the flow resistance decreased. Meanwhile, by decreasing the erythrocyte deformability with diamide, the thickness decreased and the flow resistance increased. However, the thickness was not altered in microvessels less than 10 microns by the diamide treatment. The maximum inner diameter of microvessel required to induce parachute- and/or slipper-like deformation (at flow velocity of erythrocytes less than 2 mm s-1) was 13 microns for control cells and 6 microns for diamide-treated cells. The thickness of the cell-free layer could not be closely related to the flow resistance, while a good relationship applicable to both control and diamide-treated cells was obtained between the flow resistance and the suspension viscosity. The present results conclude that the flow resistance in the isolated microvascular bed is mainly provided by both hematocrit and erythrocyte deformability.

ESR spectral transition by arteriovenous cycle in nitric oxide hemoglobin of cytokine-treated rats

Nitric oxide (NO) generation was induced in rats by Escherichia coli lipopolysaccharide (LPS) as detected by electron spin resonance (ESR) signals of NO hemoglobin (HbNO). However, there were inconsistencies in ESR spectral shape among them. We have therefore carried out a systematic study to clarify the in vivo spectral changes. First, the spectra of the alpha-NO heme species had the distinct three-line hyperfine structure in venous blood but not in arterial blood in all rats treated with tumor necrosis factor, interleukin-1, and/or LPS, and methemoglobin was not detected at the g = 6 (high-spin methemoglobin) region. Second, when the treated rats died, the three-line hyperfine structure was very distinct even in arterial blood. Third, even if HbNO was formed by injection of nitrite to rats, the three-line hyperfine structure of HbNO in venous blood was more marked than that in arterial blood, independent of the appearance of the methemoglobin signal. Fourth, an ex vivo study using whole blood demonstrated that the three-line hyperfine structure intensified lineally when O2 saturation of hemoglobin decreased but disappeared on reoxygenation of hemoglobin. These results directly demonstrate in vivo quaternary structural transition of the hemoglobin tetramer from the high-affinity state in the arterial cycle to the low-affinity state in the venous cycle. The transition makes the diverse ESR spectra of HbNO in vivo.

Oxygen exchange in the isolated, arrested guinea pig heart: theoretical and experimental observations

A model of oxygen transport in perfused myocardial tissue is presented. Steady-state conditions are assumed in order to mimic the metabolic rate of the arrested heart. The model incorporates Michaelis-Menten dependence of mitochondrial oxygen consumption, oxymyoglobin saturation and oxyhemoglobin saturation on oxygen partial pressure (PO2). The transport equations model both the advective supply of oxygen via the coronary circulation and the diffusive exchange of oxygen between tissues and environment across the epicardial and endocardial surfaces. The left ventricle is approximated by an axisymmetric prolate spheroid and the transport equations solved numerically using finite element techniques. Solution yields the PO2 profile across the heart wall. Integration of this profile yields the simulated rate of metabolic oxygen uptake determined according to the Fick principle. Correction for the diffusive flux of oxygen across the surfaces yields the simulated true metabolic rate of oxygen consumption. Simulated values of oxygen uptake are compared with those measured experimentally according to the Fick principle, using saline-perfused, Langendorff-circulated, K(+)-arrested, guinea pig hearts. Four perfusion variables were manipulated: arterial PO2, environmental PO2, coronary flow and perfusion pressure. In each case agreement between simulated and experimentally determined rates of oxygen consumption gives confidence that the model adequately describes the advective and diffusive transport of oxygen in the isolated, arrested, saline-perfused heart.

Intravital microreflectometry of individual pial vessels and capillary region of rat

A microscopic reflectance spectrophotometer was constructed to obtain the spectra of single pial vessels and of a region containing only capillaries (capillary region). The difference in the oxygen saturation (SO2) of hemoglobin between the regional arteriole and venule [R(A - V)] and that between the regional arteriole or capillaries [R(A - C)] were calculated. The reduction of cytochrome aa3 was also estimated in the capillary region. This method was applied to the brain surface of spontaneously breathing rats subjected to hypoxic and anemic hypoxia. On decreasing the inhaled O2 from 100 to 15%, elevation of R(A - V) and R(A - C) with slight arteriolar dilatation (though statistically not significant) was observed. Below 10% O2 (especially at 4 and 3% O2), the R(A - V) and R(A - C) decreased in spite of significant arteriolar dilatation with progressive reduction of cytochrome aa3, indicating suppression of oxygen transport to mitochondria. In the case of hemodilution down to 37% hematocrit (Ht), elevation of R(A - V) and R(A - C) occurred with a slight tendency toward arteriolar dilatation. Below 32% Ht, the R(A - V) decreased but the R(A - C) remained steady, while reduction of cytochrome aa3 progressed. Altogether, the SO2 in the capillary region decreased and the reduction of cytochrome aa3 progressed with the decline of arteriolar O2 supply in both hypoxic and anemic hypoxia.