Use of fluorescently labeled erythrocytes and digital cross-correlation for the measurement of flow velocity in the cerebral microcirculation

Timely Visualization of the Collaterals Formed during Acute Ischemic Stroke with Fe3 O4 Nanoparticle-based MR Imaging Probe

Ischemic stroke is one of the major leading causes for long-term disability and mortality. Collateral vessels provide an alternative pathway to protect the brain against ischemic injury after arterial occlusion. Aiming at visualizing the collaterals occurring during acute ischemic stroke, an integrin α_v β₃ -specific Fe₃ O₄ -Arg-Gly-Asp (RGD) nanoprobe is prepared for magnetic resonance imaging (MRI) of the collaterals. Rat models are constructed by occluding the middle cerebral artery for imaging studies of cerebral ischemia and ischemia-reperfusion on 7.0 Tesla MRI using susceptibility-weighted imaging sequence. To show the binding specificity to the collaterals, the imaging results acquired with the Fe₃ O₄ -RGD nanoprobe and the Fe₃ O₄ mother nanoparticles, respectively, are carefully compared. In addition, an RGD blocking experiment is also carried out to support the excellent binding specificity of the Fe₃ O₄ -RGD nanoprobe. Following the above experiments, cerebral ischemia-reperfusion studies show the collateral dynamics upon reperfusion, which is very important for the prognosis of various revascularization therapies in the clinic. The current study has, for the first time, enabled the direct observation of collaterals in a quasi-real time fashion and further disclosed that the antegrade flow upon reperfusion dominates the blood supply of primary ischemic tissue during the early stage of infarction, which is significantly meaningful for clinical treatment of stroke.

Blood vessel adaptation with fluctuations in capillary flow distribution

Throughout the life of animals and human beings, blood vessel systems are continuously adapting their structures – the diameter of vessel lumina, the thickness of vessel walls, and the number of micro-vessels – to meet the changing metabolic demand of the tissue. The competition between an ever decreasing tendency of luminal diameters and an increasing stimulus from the wall shear stress plays a key role in the adaptation of luminal diameters. However, it has been shown in previous studies that the adaptation dynamics based only on these two effects is unstable. In this work, we propose a minimal adaptation model of vessel luminal diameters, in which we take into account the effects of metabolic flow regulation in addition to wall shear stresses and the decreasing tendency of luminal diameters. In particular, we study the role, in the adaptation process, of fluctuations in capillary flow distribution which is an important means of metabolic flow regulation. The fluctuation in the flow of a capillary group is idealized as a switch between two states, i.e., an open-state and a close-state. Using this model, we show that the adaptation of blood vessel system driven by wall shear stress can be efficiently stabilized when the open time ratio responds sensitively to capillary flows. As micro-vessel rarefaction is observed in our simulations with a uniformly decreased open time ratio of capillary flows, our results point to a possible origin of micro-vessel rarefaction, which is believed to induce hypertension.

Long-term, repeated measurements of mouse cortical microflow at the same region of interest with high spatial resolution

A method for long-term, repeated, semi-quantitative measurements of cerebral microflow at the same region of interest (ROI) with high spatial resolution was developed and applied to mice subjected to focal arterial occlusion. A closed cranial window was chronically implanted over the left parieto-occipital cortex. The anesthetized mouse was placed several times, e.g., weekly, under a dynamic confocal microscope, and Rhodamine B-isothiocyanate-dextran was each time intravenously injected as a bolus, while microflow images were video recorded. Left and right tail veins were sequentially catheterized in a mouse three times at maximum over a 1.5 months' observation period. Smearing of the input function resulting from the use of intravenous injection was shown to be sufficiently small. The distal middle cerebral artery (MCA) was thermocoagulated through the cranial window in six mice, and five sham-operated mice were studied in parallel. Dye injection and video recording were conducted four times in this series, i.e., before and at 10 min, 7 and 30 days after sham operation or MCA occlusion. Pixelar microflow values (1/MTT) in a matrix of approximately 50×50 pixels were displayed on a two-dimensional (2-D) map, and the frequency distribution of the flow values was also calculated. No significant changes in microflow values over time were detected in sham-operated mice, while the time course of flow changes in the ischemic penumbral area in operated mice was similar to those reported in the literature. This method provides a powerful tool to investigate long-term changes in mouse cortical microflow under physiological and pathological conditions.

The scaling of blood flow resistance: from a single vessel to the entire distal tree

Although the flow resistance of a single vessel segment is easy to compute, the equivalent resistance of a network of vessel segments or the entire vasculature of an organ is difficult to determine in an analytic form. Here, we propose what we believe is a novel resistance scaling law for a vascular tree (i.e., the resistance of a vessel segment scales with the equivalent resistance of the corresponding distal tree). The formulation can be written as (R(s)/R(c)) proportional, variant(L(s)/L(c)) (where R(s) and L(s) are the resistance and length of a vessel segment, respectively, and R(c) and L(c) are the equivalent resistance and total length of the corresponding distal tree, respectively), which was validated for the coronary vascular systems of the heart. The scaling law was also shown to apply to the vascular systems of the lung, mesentery, muscle, eye, and so on. The novel resistance scaling law, coupled with the 3/4-power scaling law for metabolic rates, can predict several structure-function relations of vascular trees, albeit with a different exponent. In particular, the self-similar nature of the scaling law may serve as a diagnostic tool with the help of noninvasive imaging modalities.

Vascular endothelial growth factor-A is a survival factor for retinal neurons and a critical neuroprotectant during the adaptive response to ischemic injury

Vascular endothelial growth factor-A (VEGF-A) has recently been recognized as an important neuroprotectant in the central nervous system. Given its position as an anti-angiogenic target in the treatment of human diseases, understanding the extent of VEGF's role in neural cell survival is paramount. Here, we used a model of ischemia-reperfusion injury and found that VEGF-A exposure resulted in a dose-dependent reduction in retinal neuron apoptosis. Although mechanistic studies suggested that VEGF-A-induced volumetric blood flow to the retina may be partially responsible for the neuroprotection, ex vivo retinal culture demonstrated a direct neuroprotective effect for VEGF-A. VEGF receptor-2 (VEGFR2) expression was detected in several neuronal cell layers of the retina, and functional analyses showed that VEGFR2 was involved in retinal neuroprotection. VEGF-A was also shown to be involved in the adaptive response to retinal ischemia. Ischemic preconditioning 24 hours before ischemia-reperfusion injury increased VEGF-A levels and substantially decreased the number of apoptotic retinal cells. The protective effect of ischemic preconditioning was reversed after VEGF-A inhibition. Finally, chronic inhibition of VEGF-A function in normal adult animals led to a significant loss of retinal ganglion cells yet had no observable effect on several vascular parameters. These findings have implications for both neural pathologies and ocular vascular diseases, such as diabetic retinopathy and age-related macular degeneration.

Improved instrumentation for blood flow velocity measurements in the microcirculation of small animals

Microcirculation is the generic name of vessels with internal diameter less than 100 microm of the circulatory system, whose main functions are tissue nutrition and oxygen supply. In microcirculatory studies, it is important to know the amount of oxyhemoglobin present in the blood and how fast it is moving. The present work describes improvements introduced in a classical hardware-based instrument that has usually been used to monitor blood flow velocity in the microcirculation of small animals. It consists of a virtual instrument that can be easily incorporated into existing hardware-based systems, contributing to reduce operator related biases and allowing digital processing and storage. The design and calibration of the modified instrument are described as well as in vitro and in vivo results obtained with electrical models and small animals, respectively. Results obtained in in vivo studies showed that this new system is able to detect a small reduction in blood flow velocity comparing arteries and arterioles (p <0.002) and a further reduction in capillaries (p<0.0001). A significant increase in velocity comparing capillaries and venules (p <0.001) and venules and veins (p <0.001) was also observed. These results are in close agreement with biophysical principles. Moreover, the improvements introduced in the device allowed us to clearly observe changes in blood flow introduced by a pharmacological intervention, suggesting that the system has enough temporal resolution to track these microcirculatory events. These results were also in close conformity to physiology, confirming the high scientific potential of the modified system and indicating that this instrument can also be useful for pharmacological evaluations.

Long-term in vivo investigation of mouse cerebral microcirculation by fluorescence confocal microscopy in the area of focal ischemia

This study was designed to assess that mouse pial and cortical microcirculation can be monitored in the long term directly in the area of focal ischemia, using in vivo fluorescence microscopy. A closed cranial window was placed over the left parieto-occipital cortex of C57BL/6J mice. Local microcirculation was recorded in real time through the window using laser-scanning confocal fluorescence microscopy after intravenous injection of fluorescent erythrocytes and dextran. The basal velocity of erythrocytes through intraparenchymal capillaries was 0.53+/-0.30 mm/sec (n=121 capillaries in 10 mice). Two branches of the middle cerebral artery were topically cauterized through the window. Blood flow evaluated by laser-Doppler flowmetry in two distinct areas indicated the occurrence of an ischemic core (15.2%+/-5.9% of baseline for at least 2 h) and a penumbral zone. Magnetic resonance imaging and histology were used to characterize the ischemic area at 24 h after occlusion. The infarct volume was 7.3+/-3.2 mm(3) (n=6). Microcirculation was repeatedly videorecorded using fluorescence confocal microscopy over the next month. After the decrease following arterial occlusion, capillary erythrocyte velocity was significantly higher than baseline 1 week later, and attained 0.74+/-0.51 mm/sec (n=76 capillaries in six mice, P<0.005) after 1 month, while venous and capillary network remodeling was assessed, with a marked decrease in tortuosity. Immunohistochemistry revealed a zone of necrotic tissue into the infarct epicenter, with activated astrocytes at its border. Such long-term investigations in ischemic cortex brings new insight into the microcirculatory changes induced by focal ischemia and show the feasibility of long-term fluorescence studies in the mouse cortex.

Understanding cardiovascular physiology in zebrafish and Xenopus larvae: the use of microtechniques

Zebrafish and Xenopus, genetically accessible vertebrates with an externally developing, optically clear embryo, are ideally suited for in vivo functional dissection of the embryonic development of the circulatory system. Physiological characterizations of the cardiovascular system are still imperative for a more complete understanding of the connections between genetic/epigenetic factors and cardiovascular development. Here, we review experimental tools and methods that have been developed to measure numerous cardiovascular parameters in these millimetre-sized animals.

Cortical electrical stimulation alters erythrocyte perfusion pattern in the cerebral capillary network of the rat

The effect of direct cortical electrical stimulation on the pattern of erythrocyte perfusion in the capillary network of the rat cerebral cortex was studied by fluorescence intravital video-microscopy. The movement of fluorescently labeled red blood cells (FRBCs) in individual capillaries 50-70 microm subsurface in the dorsal somatosensory cortex was visualized using a closed cranial window. Cortical stimulation electrodes were placed on opposite sides of the window. FRBC velocity (mm/s) and supply rate (cells/s) were measured in 51 capillaries from six rats before and during electrical stimulation of increasing intensities (15-s trains of 3-Hz, 3-ms, 0.5-5.0-mA, square pulses). FRBC velocity, supply rate, and the instantaneous capillary erythrocyte content (lineal cell density, LCD, cells/mm) increased with the stimulation current and reached maxima of 110, 160 and 33% above control, respectively. Capillaries with low resting velocity showed a greater response than those with high resting velocity. The fraction of capillaries in which FRBC velocity increased was not constant, but increased with the stimulation current, as did the magnitude of the velocity change in these capillaries. A few capillaries showed a negative FRBC velocity response at stimulations <4 mA. These results suggest that a robust rise in the fraction of responding (engaged) capillaries and a smaller rise in the capillary LCD contribute to neuronal activation-induced cortical hyperemia. Thus, capillary engagement and erythrocyte recruitment appear to represent important components of the cortical functional hyperemic response. These results provide insight into some of the specific hemodynamic changes associated with functional hyperemia occurring at the capillary level.

Dynamic in vivo measurement of erythrocyte velocity and flow in capillaries and of microvessel diameter in the rat brain by confocal laser microscopy

A new method for studying brain microcirculation is described. Both fluorescently labeled erythrocytes and plasma were visualized on-line through a closed cranial window in anesthetized rats, using laser-scanning two-dimension confocal microscopy. Video images of capillaries, arterioles, and venules were digitized off-line to measure microvessel diameter and labeled erythrocyte flow and velocity in parenchymal capillaries up to 200 microm beneath the brain surface. The method was used to analyze the rapid adaptation of microcirculation to a brief decrease in perfusion pressure. Twenty-second periods of forebrain ischemia were induced using the tour-vessel occlusion model in eight rats. EEG, arterial blood pressure, and body temperature were continuously controlled. In all conditions, labeled erythrocyte flow and velocity were both very heterogeneous in capillaries. During ischemia, capillary perfusion was close to 0, but a low blood flow persisted in arterioles and venules, while EEG was flattening. The arteriole and venule diameter did not significantly change. At the unclamping of carotid arteries, there was an instantaneous increase (by about 150%) of arteriole diameter. Capillary erythrocyte flow and velocity increased within 5 seconds, up to, respectively, 346 +/- 229% and 233 +/- 156% of their basal value. No capillary recruitment of erythrocytes was detected. All variables returned to their basal levels within less than 100 seconds after declamping. The data are discussed in terms of a possible involvement of shear stress in the reperfusion period.

Regulation of oxygen supply in the cerebral circulation

The dynamics and regulation of red blood cell flow in the cerebral microcirculation was studied by intravital fluorescence video-microscopy in a closed cranial window preparation in the rat. The studies revealed that capillary perfusion in the brain is essentially continuous but a stationary difference from capillary to capillary within the same microvascular network exists. The main mechanism of an increase in flow in cerebral capillaries is an increase in linear velocity with no or minor role for classical capillary recruitment. While cyclic opening and closing of capillaries is not evident, low frequency oscillations in capillary flow velocity are present when perfusion or oxygen supply to tissue is challenged. In hypoxic hypoxia and moderate hypercapnia, RBC velocity increases in all capillaries while in severe hypercapnia, redistribution of RBC velocity in the capillary network occurs. Both systemic hypotension and severe hypercapnia are accompanied by an increase in the homogeneity of capillary flow; this change involves the redistribution of RBC flow between thoroughfare channels and exchange capillaries. Thoroughfare channels may thus provide a recruitable flow reserve in the cerebral microcirculation. The capillary flow response to hypoxic and anemic hypoxia depends on the activity neuronal nitric oxide synthase. These findings suggest the presence of a physiological regulatory mechanism of cerebral capillary red blood cell flow and oxygen supply which may involve neuronal nitric oxide as a mediator.

Effects of hypoxia and hypercapnia on capillary flow velocity in the rat cerebral cortex

The velocity of red blood cells (RBC) in individual capillaries of the rat cerebral cortex was assessed using direct, intravital video microscopy under normal conditions and during systemic hypoxia or hypercapnia. The movement of RBC in capillaries within 50-microm depth of the parietal cortex was visualized with the aid of fluorescent labeling of RBC in a closed cranial window preparation in pentobarbital-anesthetized, artificially ventilated adult rats. Hypoxia was produced by lowering the concentration of oxygen in the inspired gas from 30 to 15% for 5 min. Hypercapnia was achieved by increasing the inspired CO2 concentration (FiCO2) from 0 to 5% and then to 10% for 5 min at each level. The mean arterial pressure was maintained constant during both maneuvers. Under control conditions, fast and heterogeneous RBC flow in multioriented, tortuous capillaries was observed. During hypoxia, RBC velocity increased from 0.61 +/- 0.06 to 0.82 +/- 0.10 mm/sec (35% change). During hypercapnia, RBC velocity increased from 0.73 +/- 0.05 to 1.07 +/- 0. 11 mm/sec (46% change) at 5% CO2 and to 1.19 +/- 0.11 mm/sec (63% change) at 10% CO2. Corresponding changes in regional blood flow as assessed by laser-Doppler flowmetry during hypercapnia were 69 +/- 7 and 128 +/- 21%, respectively. The RBC velocity increased in almost all capillaries during hypoxia and during moderate hypercapnia. However, a substantial number of capillaries showed no change or a small decrease in RBC velocity during severe hypercapnia. A significant negative correlation between the velocity change at 10% CO2 and the normocapnic resting velocity was found in a group of capillaries isolated by cluster analysis. These results suggest that the dominant component of cerebral hyperemic response to hypoxia and to moderate hypercapnia is an increase in capillary RBC flow velocity. A more complex change in the velocity distribution occurs during severe hypercapnia and results in increased homogeneity of RBC perfusion in the cerebrocortical capillary network.

Mechanisms of Stroke in Sickle Cell Disease: Sickle Erythrocytes Decrease Cerebral Blood Flow in Rats After Nitric Oxide Synthase Inhibition

The etiology of stroke in sickle cell disease is unclear, but may involve abnormal red blood cell (RBC) adhesion to the vascular endothelium and altered vasomotor tone regulation. Therefore, we examined both the adhesion of sickle (SS)-RBCs to cerebral microvessels and the effect of SS-RBCs on cerebral blood flow when the nitric oxide (NO) pathway was inhibited. The effect of SS-RBCs was studied in the rat cerebral microcirculation using either a cranial window for direct visualization of infused RBCs or laser Doppler flowmetry (LDF ) to measure RBC flow. When fluorescently labeled human RBCs were infused into rats, SS-RBCs had increased adhesion to rat cerebral microvessels compared with control AA-RBCs (P = .01). Next, washed SS-RBCs or AA-RBCs were infused into rats prepared with LDF probes after pretreatment (40 mg/kg intravenously) with the NO synthase inhibitor, N-ω-nitro-L-arginine methyl ester (L-NAME), or the control isomer, D-NAME. In 9 rats treated with systemic L-NAME and SS-RBCs, 5 of 9 experienced a significant decrease in LDF and died within 30 minutes after the RBC infusion (P = .0012). In contrast, all control groups completed the experiment with stable LDF and hemodynamics. Four rats received a localized superfusion of L-NAME (1 mmol/L) through the cranial window followed by infusion of SS-RBCs. Total cessation of flow in all observed cerebral microvessels occurred in 3 of 4 rats within 15 minutes after infusion of SS-RBCs. We conclude that the NO pathway is critical in maintaining cerebral blood flow in the presence of SS-RBCs in this rat model. In addition, the enhanced adhesion of SS-RBCs to rat brain microvessels may contribute to cerebral vaso-occlusion either directly, by disrupting blood flow, or indirectly, by disturbing the vascular endothelium.

Postnatal remodeling of the leptomeningeal vascular network as assessed by intravital fluorescence video-microscopy in the rat

An intriguing characteristic of the ontogenic development of the cerebral vasculature is the rapid differentiation of the neonatal leptomeningeal vascular plexus into the mature, adult network form. The physiological and cellular mechanisms of this cerebrovascular remodeling process are unclear. The objective of this work was to determine and correlate changes in vascular density, network pattern and flow velocity in leptomeningeal microvessels of the rat during postnatal development in vivo. To this end, microvascular diameter, segment length, and vascular density of reconstructed leptomeningeal networks were measured from video-recordings of the microcirculation visualized through a cranial window in 0-15-day-old Sprague-Dawley rats. The velocity of erythrocytes in the microvessels was measured by frame to frame tracking of fluorescently labeled red blood cells. We found that surface vascular density (total vessel length per area), node density and segment density (object per area) decreased significantly by the second week after birth. Anastomosing vascular polygons, characteristic to newborn networks, became less numerous and larger in diameter during the postnatal 2-week period, indicating progressive rarefaction of the networks. Vessel diameter and red cell velocity showed transient increases at 1.5 weeks. The velocity/diameter ratio (V/D), an index of wall shear rate, increased by the age of 1.5 weeks and remained unchanged afterwards. There was a negative correlation between V/D and diameter at 1 week; this relationship was reversed to a positive correlation at 2 weeks. We conclude that postnatal remodeling of the leptomeningeal vascular network is associated with rarefaction and an adaptation of vessel caliber to wall shear rate. These changes may contribute to arterio-venous differentiation and redistribution of blood flow from the superficial to the intracortical vasculature in the developing brain.

Erythrocyte flow heterogeneity in the cerebrocortical capillary network

The heterogeneity of erythrocyte flow velocity and erythrocyte flux and their dependence on decreased cerebral perfusion pressure were studied in the rat cerebral cortex using intravital video microscopy. With decreased perfusion pressure, both mean and range of erythrocyte flow velocity of individual capillaries was reduced. Both cell velocity and cell flux decreased more in high flow capillaries than in low flow capillaries. The results are compatible with the hypothesis that redistribution of capillary flow during hypotension may help to maintain the perfusion of arterio-venous capillary flow pathways. Although the data are preliminary, they represent the first direct measurements of capillary flow path length and transit time in the capillary network of the cerebral cortex.