Evaluation of carbocyanine-labeled erythrocytes for microvascular measurements

Red blood cell-hitchhiking chitosan nanoparticles for prolonged blood circulation time of vitamin K1

Nanocarriers have been extensively applied for intravascular drug delivery. However, rapid clearance from circulation by mononuclear phagocyte system has limited their applications. Erythrocytes carriers are potential solutions to overcome the limitations of nanocarriers and considered to be ideal natural carriers for drug delivery because of their unique properties. The purpose of this work is to combine nanocarriers with erythrocytes carriers for sustained release and prolonged circulation time of vitamin K₁. Chitosan nanoparticles loading VK₁ (VK-CSNPs) were prepared using ionotropic gelation method, which was optimized using box-behnken design and response surface methodology. VK-CSNPs adsorbed onto red blood cells (RBC-VK-CSNPs) rapidly via electrostatic interactions. The exposure of phosphatidylserine, osmotic fragility and turbulence fragility of RBC loading nanoparticles were investigated to study the toxicity of nanoparticles to erythrocytes. In vivo pharmacokinetic study indicated that C_max, AUC and MRT of RBC-VK-CSNPs group were remarkably higher than that of VK-CSNPs group. Flow cytometry showed VK-CSNPs steadily retained on the surface of RBC for a long time without affecting the circulation profiles of RBC themselves. The nanoparticles carried on RBC released drug, desorbed and were eliminated in vivo. Therefore, the circulation time of RBC-hitchhiking chitosan nanoparticles was greatly prolonged compared with nanoparticles alone. RBC-hitchhiking could be a valuable hybrid strategy for prolonging the in vivo life of nanocarriers.

Labeling of Erythrocytes by Porphyrin-Phospholipid

A method is developed for membrane labeling of erythrocytes with porphyrin-phospholipid (PoP). To generate a concentrated PoP solution for labeling human red blood cells (RBCs), various surfactants and solvents are screened to identify conditions that avoid hemolysis, while minimizing non-specific PoP co-precipitation with RBCs in the pellet during centrifugation washes. Cholate, Tween 80 and Tween 40 are identified as useful surfactants for this purpose. When labeled RBCs are mixed with unlabeled ones, substantial non-specific PoP exchange is observed. Egg-yolk lecithin is included in a washing buffer to remove loosely bound PoP and reduce PoP exchange with unlabeled erythrocytes, based on flow cytometry and photodynamic hemolysis assays. Murine RBCs that are labeled with ⁶⁴Cu-chelated PoP displayed altered biodistribution with longer blood circulation relative to directly administered ⁶⁴Cu-chelated PoP.

Nitric oxide loading reduces sickle red cell adhesion and vaso-occlusion in vivo

Sickle red blood cells (SSRBCs) are adherent to the endothelium, activate leukocyte adhesion, and are deficient in bioactive nitric oxide (NO) adducts such as S-nitrosothiols (SNOs), with reduced ability to induce vasodilation in response to hypoxia. All these pathophysiologic characteristics promote vascular occlusion, the hallmark of sickle cell disease (SCD). Loading hypoxic SSRBCs in vitro with NO followed by reoxygenation significantly decreased epinephrine-activated SSRBC adhesion to the endothelium, the ability of activated SSRBCs to mediate leukocyte adhesion in vitro, and vessel obstruction in vivo. Because transfusion is frequently used in SCD, we also determined the effects of banked (SNO-depleted) red blood cells (RBCs) on vaso-occlusion in vivo. Fresh or 14-day-old normal RBCs (AARBCs) reduced epinephrine-activated SSRBC adhesion to the vascular endothelium and prevented vaso-occlusion. In contrast, AARBCs stored for 30 days failed to decrease activated SSRBC adhesivity or vaso-occlusion, unless these RBCs were loaded with NO. Furthermore, NO loading of SSRBCs increased S-nitrosohemoglobin and modulated epinephrine's effect by upregulating phosphorylation of membrane proteins, including pyruvate kinase, E3 ubiquitin ligase, and the cytoskeletal protein 4.1. Thus, abnormal SSRBC NO/SNO content both contributes to the vaso-occlusive pathophysiology of SCD, potentially by affecting at least protein phosphorylation, and is potentially amenable to correction by (S)NO repletion or by RBC transfusion.

Disrupting the vicious cycle created by NOX activation in sickle erythrocytes exposed to hypoxia/reoxygenation prevents adhesion and vasoocclusion

In sickle cell disease (SCD), recurrent painful vasoocclusive crisis are likely caused by repeated episodes of hypoxia and reoxygenation. The sickle erythrocyte (SSRBC) adhesion plays an active role in vasoocclusion. However, the effect of prolonged reoxygenation after hypoxic stress on the molecular mechanisms in SSRBCs involved in onset of episodic vasoocclusion remain unclear. Exposure of human SSRBCs to hypoxia followed by 2 h reoxygenation, increased reactive oxygen species (ROS) production. Using specific pharmacological inhibitors, we show that excess ROS production in both reticulocytes and mature SSRBCs is regulated by NADPH oxidases (NOXs), the mitogen-activated protein kinase (ERK1/2), and G-protein coupled-receptor kinase 2 (GRK2). Consequently, SSRBC ROS create an intracellular positive feedback loop with ERK1/2 and GRK2 to mediate SSRBC adhesion to endothelium in vitro, and vasoocclusion in a mouse model of vasoocclusion in vivo. Importantly, reducing ROS levels in SSRBCs with redox-active manganese (Mn) porphyrins, commonly known as mimics of superoxide dismutase (SOD), disrupted the cycle created by ROS by affecting NOX and GRK2 activities and ERK1/2 phosphorylation, thus abrogating RBC-endothelial interactions. Inhibition adhesion assays show that LW (ICAM-4, CD242) blood group glycoprotein and CD44 are the RBC adhesion molecules mediating endothelial binding. Conversely, hypoxia/reoxygenation of normal RBCs failed to activate this feedback loop, and adhesion. These findings provide novel insights into the pathophysiological significance of the deleterious cycle created by NOX-dependent ROS, GRK2 and ERK1/2 within SSRBCs activated by hypoxia/reoxygenation, and involved in SSRBC adhesion and vasoocclusion. Thus, this loop in SSRBCs, which can be disrupted by Mn porphyrins, likely drives the profound SCD vasculopathy, and may point to new therapeutic targets to prevent chronic vasoocclusive events.

Lipophilic indocarbocyanine conjugates for efficient incorporation of enzymes, antibodies and small molecules into biological membranes

Decoration of cell membranes with biomolecules, targeting ligands and imaging agents is an emerging strategy to improve functionality of cell-based therapies. Compared to covalent chemistry or genetic expression on the cell surface, lipid painting (i.e., incorporation of lipid-conjugated molecules into the cell bilayer) is a fast, non-damaging and less expensive approach. Previous studies demonstrated excellent incorporation and retention of distearyl indocarbocyanine dye DiI in membranes of cells in vitro and in vivo. In order to exploit the membrane stability of DiI, we synthesized an amino-DiI derivative, to which we subsequently conjugated an antibody (cetuximab), an enzyme (superoxide dismutase), and a small molecule (DyLight 800). Red blood cells have long been used as drug delivery vehicles so they were utilized as a model to study the incorporation of DiI conjugates in the plasma membrane. All the DiI constructs demonstrated fast and efficient ex vivo incorporation in the membrane of mouse RBCs, resulting in millions of exogenous molecules per RBC. Following an intravenous injection into mice, the molecules were detected on circulating RBCs for several days. DiI anchored molecules showed longer residence time in blood and significantly higher area under the curve (AUC) compared to free non-conjugated molecules. Thus, cetuximab, SOD and DyLight painted on RBC showed 5.5-fold, 6.5-fold and 78-fold increase in the AUC, respectively, compared to the non-modified molecules. Lipophilic indocarbocyanine anchors are a promising technology for incorporation of biomolecules and small molecules into biological membranes for in vivo applications.

Fluorescent Dye Labeling of Erythrocytes and Leukocytes for Studying the Flow Dynamics in Mouse Retinal Circulation

The retinal and choroidal blood flow dynamics may provide insight into the pathophysiology and sequelae of various ocular diseases, such as glaucoma, diabetic retinopathy, age-related macular degeneration (AMD) and other ocular inflammatory conditions. It may also help to monitor the therapeutic responses in the eye. The proper labeling of the blood cells, coupled with live-cell imaging of the labeled cells, allows for the investigation of the flow dynamics in the retinal and choroidal circulation. Here, we describe the standardized protocols of 1.5% indocyanine green (ICG) and 1% sodium fluorescein labeling of mice erythrocytes and leukocytes, respectively. Scanning laser ophthalmoscopy (SLO) was applied to visualize the labeled cells in the retinal circulation of C57BL/6J mice (wild type). Both methods demonstrated distinct fluorescently labeled cells in the mouse retinal circulation. These labeling methods can have wider applications in various ocular disease models.

Measurement of Retinal Blood Flow Using Fluorescently Labeled Red Blood Cells

Blood flow is a useful indicator of the metabolic state of the retina. However, accurate measurement of retinal blood flow is difficult to achieve in practice. Most existing optical techniques used for measuring blood flow require complex assumptions and calculations. We describe here a simple and direct method for calculating absolute blood flow in vessels of all sizes in the rat retina. The method relies on ultrafast confocal line scans to track the passage of fluorescently labeled red blood cells (fRBCs). The accuracy of the blood flow measurements was verified by (1) comparing blood flow calculated independently using either flux or velocity combined with diameter measurements, (2) measuring total retinal blood flow in arterioles and venules, (3) measuring blood flow at vessel branch points, and (4) measuring changes in blood flow in response to hyperoxic and hypercapnic challenge. Confocal line scans oriented parallel and diagonal to vessels were used to compute fRBC velocity and to examine velocity profiles across the width of vessels. We demonstrate that these methods provide accurate measures of absolute blood flow and velocity in retinal vessels of all sizes.

Regulation of blood flow in the retinal trilaminar vascular network

Light stimulation evokes neuronal activity in the retina, resulting in the dilation of retinal blood vessels and increased blood flow. This response, named functional hyperemia, brings oxygen and nutrients to active neurons. However, it remains unclear which vessels mediate functional hyperemia. We have characterized blood flow regulation in the rat retina in vivo by measuring changes in retinal vessel diameter and red blood cell (RBC) flux evoked by a flickering light stimulus. We found that, in first- and second-order arterioles, flicker evoked large (7.5 and 5.0%), rapid (0.73 and 0.70 s), and consistent dilations. Flicker-evoked dilations in capillaries were smaller (2.0%) and tended to have a slower onset (0.97 s), whereas dilations in venules were smaller (1.0%) and slower (1.06 s) still. The proximity of pericyte somata did not predict capillary dilation amplitude. Expression of the contractile protein α-smooth muscle actin was high in arterioles and low in capillaries. Unexpectedly, we found that blood flow in the three vascular layers was differentially regulated. Flicker stimulation evoked far larger dilations and RBC flux increases in the intermediate layer capillaries than in the superficial and deep layer capillaries (2.6 vs 0.9 and 0.7% dilation; 25.7 vs 0.8 and 11.3% RBC flux increase). These results indicate that functional hyperemia in the retina is driven primarily by active dilation of arterioles. The dilation of intermediate layer capillaries is likely mediated by active mechanisms as well. The physiological consequences of differential regulation in the three vascular layers are discussed.

MEK inhibitors, novel anti-adhesive molecules, reduce sickle red blood cell adhesion in vitro and in vivo, and vasoocclusion in vivo

In sickle cell disease, sickle erythrocyte (SSRBC) interacts with endothelial cells, leukocytes, and platelets, and activates coagulation and inflammation, promoting vessel obstruction, which leads to serious life-threatening complications, including acute painful crises and irreversible damage to multiple organs. The mitogen-activated protein kinase, ERK1/2, is abnormally activated in SSRBCs. However, the therapeutic potential of SSRBC ERK1/2 inactivation has never been investigated. I tested four different inhibitors of MEK1/2 (MEK), the kinase that activates ERK1/2, in a model of human SSRBC adhesion to TNFα-activated endothelial cells (ECs). SSRBC MEK inhibition abrogated adhesion to non-activated and TNFα-activated ECs to levels below baseline SSRBC adhesion to non-activated ECs in vitro. SSRBC MEK inhibition also prevented SSRBCs from activating naïve neutrophils to adhere to endothelium. To determine the effect of MEK inhibitors on SSRBC adherence in vivo, sham-treated or MEK inhibitor-treated SSRBCs were infused to nude mice previously treated with TNFα. Sham-treated SSRBCs displayed marked adhesion and occlusion of enflamed vessels, both small and large. However, SSRBC treatment with MEK inhibitors ex vivo showed poor SSRBC adhesion to enflamed vessels with no visible vasoocclusion in vivo. In addition, MEK inhibitor treatment of SSRBCs reduced SSRBC organ trapping and increased the number of SSRBCs circulating in bloodstream. Thus, these data suggest that SSRBC ERK1/2 plays potentially a critical role in sickle pathogenesis, and that MEK inhibitors may represent a valuable intervention for acute sickle cell crises.

Nanotopography as a trigger for the microscale, autogenous and passive lysis of erythrocytes

A mechano-responsive topology provides a highly active yet autogenous surface for erythrocyte lysis towards microfluidic haematology platforms.

Drug-loaded sickle cells programmed ex vivo for delayed hemolysis target hypoxic tumor microvessels and augment tumor drug delivery

Selective drug delivery to hypoxic tumor niches remains a significant therapeutic challenge that calls for new conceptual approaches. Sickle red blood cells (SSRBCs) have shown an ability to target such hypoxic niches and induce tumoricidal effects when used together with exogenous pro-oxidants. Here we determine whether the delivery of a model therapeutic encapsulated in murine SSRBCs can be enhanced by ex vivo photosensitization under conditions that delay autohemolysis to a time that coincides with maximal localization of SSRBCs in a hypoxic tumor. Hyperspectral imaging of 4T1 carcinomas shows oxygen saturation levels <10% in a large fraction (commonly 50% or more) of the tumor. Using video microscopy of dorsal skin window chambers implanted with 4T1 tumors, we demonstrate that allogeneic SSRBCs, but not normal RBCs (nRBCs), selectively accumulate in hypoxic 4T1 tumors between 12 and 24h after systemic administration. We further show that ex vivo photo-oxidation can program SSRBCs to postpone hemolysis/release of a model therapeutic to a point that coincides with their maximum sequestration in hypoxic tumor microvessels. Under these conditions, drug-loaded photosensitized SSRBCs show a 3-4 fold greater drug delivery to tumors compared to non-photosensitized SSRBCs, drug-loaded photosensitized nRBCs, and free drug. These results demonstrate that photo-oxidized SSRBCs, but not photo-oxidized nRBCs, sequester and hemolyze in hypoxic tumors and release substantially more drug than photo-oxidized nRBCs and non-photo-oxidized SSRBCs. Photo-oxidation of drug-loaded SSRBCs thus appears to exploit the unique tumor targeting and carrier properties of SSRBCs to optimize drug delivery to hypoxic tumors. Such programmed and drug-loaded SSRBCs therefore represent a novel and useful tool for augmenting drug delivery to hypoxic solid tumors.

Automated measurement of blood flow velocity and direction and hemoglobin oxygen saturation in the rat lung using intravital microscopy

Intravital microscopy of the pulmonary microcirculation in research animals is of great scientific interest for its utility in identifying regional changes in pulmonary microcirculatory blood flow. Although feasibility studies have been reported, the pulmonary window can be further refined into a practical tool for pharmaceutical research and drug development. We have established a method to visualize and quantify dynamic changes in three key features of lung function: microvascular red blood cell velocity, flow direction, and hemoglobin saturation. These physiological parameters were measured in an acute closed-chest pulmonary window, which allows real-time images to be captured by fluorescence and multispectral absorption microscopy; images were subsequently quantified using computerized analysis. We validated the model by quantifying changes in microcirculatory blood flow and hemoglobin saturation in two ways: 1) after changes in inspired oxygen content and 2) after pharmacological reduction of pulmonary blood flow via treatment with the β1 adrenergic receptor blocker metoprolol. This robust and relatively simple system facilitates pulmonary intravital microscopy in laboratory rats for pharmacological and physiological research.

Sickle erythrocytes target cytotoxics to hypoxic tumor microvessels and potentiate a tumoricidal response

Resistance of hypoxic solid tumor niches to chemotherapy and radiotherapy remains a major scientific challenge that calls for conceptually new approaches. Here we exploit a hitherto unrecognized ability of sickled erythrocytes (SSRBCs) but not normal RBCs (NLRBCs) to selectively target hypoxic tumor vascular microenviroment and induce diffuse vaso-occlusion. Within minutes after injection SSRBCs, but not NLRBCs, home and adhere to hypoxic 4T1 tumor vasculature with hemoglobin saturation levels at or below 10% that are distributed over 70% of the tumor space. The bound SSRBCs thereupon form microaggregates that obstruct/occlude up to 88% of tumor microvessels. Importantly, SSRBCs, but not normal RBCs, combined with exogenous prooxidant zinc protoporphyrin (ZnPP) induce a potent tumoricidal response via a mutual potentiating mechanism. In a clonogenic tumor cell survival assay, SSRBC surrogate hemin, along with H(2)O(2) and ZnPP demonstrate a similar mutual potentiation and tumoricidal effect. In contrast to existing treatments directed only to the hypoxic tumor cell, the present approach targets the hypoxic tumor vascular environment and induces injury to both tumor microvessels and tumor cells using intrinsic SSRBC-derived oxidants and locally generated ROS. Thus, the SSRBC appears to be a potent new tool for treatment of hypoxic solid tumors, which are notable for their resistance to existing cancer treatments.

Effect of propranolol as antiadhesive therapy in sickle cell disease

Sickle red blood cells (SSRBCs) adhere to both endothelial cells (ECs) and the extracellular matrix. Epinephrine elevates cyclic adenosine monophosphate in SSRBCs and increases adhesion of SSRBCs to ECs in a β-adrenergic receptor and protein kinase A-dependent manner. Studies in vitro as well as in vivo have suggested that adrenergic stimuli like epinephrine may contribute to vaso-occlusion associated with physiologic stress. We conducted both animal studies and a Phase I dose-escalation study in sickle cell disease (SCD) patients to investigate whether systemically administered propranolol inhibits SSRBC adhesion and to document the safety of propranolol in SCD. Systemically administered propranolol prevented SSRBC adhesion and associated vaso-occlusion in a mouse model. In patients receiving a single oral dose of 10, 20, or 40 mg propranolol, SSRBC adhesion to ECs was studied before and after propranolol, with and without stimulation with epinephrine. Propranolol administration significantly reduced epinephrine-stimulated SSRBC adhesion in a dose dependent manner (p = 0.03), with maximum inhibition achieved at 40 mg. Adverse events were not severe, did not show dose dependence, and were likely unrelated to drug. No significant heart rate changes occurred. These results imply that β-blockers may have a role as antiadhesive therapy for SCD.

Plasma expander and blood storage effects on capillary perfusion in transfusion after hemorrhage

BACKGROUND

Treating hemorrhage with blood transfusions in subjects previously hemodiluted with different colloidal plasma expanders, using fresh autologous blood or blood that has been stored for 2 weeks, allows identifying the interaction between type of plasma expander and differences in blood storage.

STUDY DESIGN AND METHODS

Studies used the hamster window chamber model. Fresh autologous plasma, 130-kDa starch-based plasma expander (hydroxyethyl starch [HES]), or 4% polyethylene glycol-conjugated albumin (PEG-Alb) was used for 20% of blood volume (BV) hemodilution. Hemodilution was followed by a 55% by BV 40-minute hemorrhagic shock period, treated with transfusion of fresh or blood that was stored for 2 weeks. Outcome was evaluated 1 hour after blood transfusion in terms of microvascular and systemic variables.

RESULTS

Results were principally dependent on the type of colloidal solution used during hemodilution, 4% PEG-Alb yielding the best microvascular recovery evaluated in terms of the functional capillary density. This result was consistent whether fresh blood or stored blood was used in treating the subsequent shock period. Fresh blood results were significantly better in systemic and microvascular terms relative to stored blood. HES and fresh plasma hemodilution yielded less favorable results, a difference that was enhanced when fresh versus stored blood was compared in their efficacy of correcting the subsequent hemorrhage.

CONCLUSION

The type of plasma expander used for hemodilution influences the short-term outcome of subsequent volume resuscitation using blood transfusion, 4% PEG-Alb providing the most favorable outcome by comparison to HES or fresh plasma.

RNA aptamer therapy for vaso-occlusion in sickle cell disease

Patients with sickle cell disease (SCD) often suffer painful vaso-occlusive episodes caused in part by the adhesion of sickle erythrocytes (SS-RBC) to the vascular endothelium. To investigate inhibition of SS-RBC adhesion as a possible treatment for vaso-occlusion, 2 adhesion molecules, α(v)β(3) and P-selectin, were targeted by high-affinity RNA aptamers. An in vitro flow chamber assay was used to test the antiadhesion activity of α(v)β(3) aptamer clone 17.16. Human SS-RBC were passed across a confluent monolayer of thrombin-stimulated human umbilical vein endothelial cells (HUVEC) at a constant rate. α(v)β(3) aptamer reduced SS-RBC adhesion to activated endothelial cells to the level seen with untreated HUVEC. An aptamer reactive with complement component 8 was used as a negative control and exerted no inhibition, confirming the specificity of α(v)β(3) aptamer (P=0.04). At 2 dyn/cm(2) shear stress, 30 nM α(v)β(3) aptamer showed maximal effect in decreasing SS-RBC adhesion to HUVEC. The antiadhesive activity of the P-selectin aptamer clone PF377 was also tested using HUVEC pretreated with IL-13 to upregulate expression of P-selectin as seen in activated endothelial cells. At 1 dyn/cm(2) shear stress, 60 nM of P-selectin aptamer had antiadhesion activity similar to heparin, a known inhibitor of SS-RBC adhesion to P-selectin. A negative control did not prevent adhesion (P=0.05). These data show the potential utility of aptamers to block endothelial adhesion molecules to prevent or treat vaso-occlusion in SCD.

Microflow of fluorescently labelled red blood cells in tumours expressing single isoforms of VEGF and their response to vascular targeting agents

In this work we studied the functional differences between the microcirculation of murine tumours that express only single isoforms of vascular endothelial growth factor-A (VEGF), namely VEGF120 and VEGF188, and the effect of VEGF receptor tyrosine kinase (VEGF-R TK) inhibition on their functional response to the vascular disrupting agent, combretastatin A-4 phosphate (CA-4-P), using measurement of red blood cell (RBC) velocity by a 'keyhole' tracking algorithm. RBC velocities in VEGF188 tumours were unaffected by chronic treatment with a VEGF-R tyrosine kinase inhibitor, SU5416, whereas RBC velocities in VEGF120 tumours were significantly increased compared to control VEGF120 tumours. This effect was accompanied by a reduced tumour vascularisation. Pre-treatment of VEGF120 tumours with SU5416 made them much more resistant to CA-4-P treatment, with a RBC velocity response that was very similar to that of the more mature vasculature of the VEGF188 tumours. This study shows that vascular normalisation following anti-angiogenic treatment with a VEGF-R tyrosine kinase inhibitor reduced the response of a previously sensitive tumour line to CA-4-P.

Spectral imaging reveals microvessel physiology and function from anastomoses to thromboses

Abnormal microvascular physiology and function is common in many diseases. Numerous pathologies include hypervascularity, aberrant angiogenesis, or abnormal vascular remodeling among the characteristic features of the disease, and quantitative imaging and measurement of microvessel function can be important to increase understanding of these diseases. Several optical techniques are useful for direct imaging of microvascular function. Spectral imaging is one such technique that can be used to assess microvascular oxygen transport function with high spatial and temporal resolution in microvessel networks through measurements of hemoglobin saturation. We highlight novel observation made with our intravital microscopy spectral imaging system employed with mouse dorsal skin-fold window chambers for imaging hemoglobin saturation in microvessel networks. Specifically, we image acute oxygenation fluctuations in a tumor microvessel network, the development of arteriovenous malformations in a mouse model of hereditary hemorrhagic telangiectasia, and the formation of spontaneous and induced microvascular thromboses and occlusions.

Real-time imaging of de novo arteriovenous malformation in a mouse model of hereditary hemorrhagic telangiectasia

Arteriovenous malformations (AVMs) are vascular anomalies where arteries and veins are directly connected through a complex, tangled web of abnormal arteries and veins instead of a normal capillary network. AVMs in the brain, lung, and visceral organs, including the liver and gastrointestinal tract, result in considerable morbidity and mortality. AVMs are the underlying cause of three major clinical symptoms of a genetic vascular dysplasia termed hereditary hemorrhagic telangiectasia (HHT), which is characterized by recurrent nosebleeds, mucocutaneous telangiectases, and visceral AVMs and caused by mutations in one of several genes, including activin receptor-like kinase 1 (ALK1). It remains unknown why and how selective blood vessels form AVMs, and there have been technical limitations to observing the initial stages of AVM formation. Here we present in vivo evidence that physiological or environmental factors such as wounds in addition to the genetic ablation are required for Alk1-deficient vessels to develop to AVMs in adult mice. Using the dorsal skinfold window chamber system, we have demonstrated for what we believe to be the first time the entire course of AVM formation in subdermal blood vessels by using intravital bright-field images, hyperspectral imaging, fluorescence recordings of direct arterial flow through the AV shunts, and vascular casting techniques. We believe our data provide novel insights into the pathogenetic mechanisms of HHT and potential therapeutic approaches.

Multimodal optical imaging of microvessel network convective oxygen transport dynamics

Convective oxygen transport by microvessels depends on several parameters, including red blood cell flux and oxygen saturation. We demonstrate the use of intravital microscopy techniques to measure hemoglobin saturations, red blood cell fluxes and velocities, and microvessel cross-sectional areas in regions of microvascular networks containing multiple vessels. With these methods, data can be obtained at high spatial and temporal resolution and correlations between oxygen transport and hemodynamic parameters can be assessed. In vivo data are presented for a mouse mammary adenocarcinoma grown in a dorsal skinfold window chamber model.

Erythrocyte flow in choriocapillaris of normal and diabetic rats

The choriocapillaris is a unique capillary bed that provides nutrients to the retinal photoreceptors. It changes anatomically in diabetes, but the impact of these changes on blood flow is unknown. In this study hemodynamic parameters in individual choriocapillaris vessels were compared in normal and diabetic rats. Three groups were studied: normal buffer-injected control rats, streptozotocin (STZ)-injected mildly hyperglycemic (STZ-MH) rats, and STZ-injected diabetic (STZ-D) rats. 7-8 weeks after STZ injection, the rats were anesthetized, and epifluorescent, intravital microscopy was used to record the flow of fluorescent red blood cells (RBC) in the choriocapillaris. Diameter, RBC flux, and RBC velocity were measured in 153 capillary pathways in five control rats, 98 pathways in four STZ-MH rats, and 153 pathways in seven STZ-D rats. There was no difference in capillary diameter among the groups. RBC flux and velocity were lower in the STZ-injected rats compared to the controls (p<or=0.023), which is similar to changes found in other capillary beds. RBC velocity and flux were significantly correlated in all three groups, but the correlations in the STZ-injected rats were much stronger than in the controls. This indicates a more heterogeneous distribution of RBCs at upstream arteriolar branch points in hyperglycemic rats, which could lead to a decrease in choriocapillaris hematocrit. These changes in the hyperglycemic choriocapillaris could contribute to impaired oxygen delivery to the photoreceptors in diabetic retina.

Attenuation of diabetes-induced retinal vasoconstriction by a thromboxane receptor antagonist

Retinal blood flow has been reported to decrease early in human diabetes as well as in diabetic animal models. The purpose of the present study is to investigate the role of thromboxane receptor binding in the decrease of flow. C57BL/6 mice were injected with streptozotocin (STZ) at 11-12 weeks of age and remained hyperglycemic for 4 weeks. The mice were treated with a selective thromboxane receptor antagonist, GR32191B (vapiprost), in drinking water for the final three weeks at a dose of 1mg/kg/day. In separate experiments, vapiprost was administered only once, as an acute injection 25min prior to the experimental measurements. The measurements included retinal arteriolar and venular diameters and red blood cell (RBC) velocities, from which retinal blood flow was calculated. STZ induced decreases in vascular diameters and RBC velocities, resulting in an approximate 30% decrease in overall retinal blood flow. However, these decreases were not seen in mice given the three-week administration of vapiprost. Acute administration to diabetic mice of 1mg/kg vapiprost, but not 0.1mg/kg, induced arteriolar vasodilation, with the dilation more substantial in smaller feed arterioles. In summary, STZ-induced decreases in retinal blood flow can be attenuated by the thromboxane receptor antagonist vapiprost.

Fast release of lipophilic agents from circulating PEG-PDLLA micelles revealed by in vivo forster resonance energy transfer imaging

Understanding the in vivo behavior of nanoparticles is critical for the translation of nanomedicine from laboratory research to clinical trials. In this work, in vivo Forster resonance energy transfer (FRET) imaging was employed to monitor the release of hydrophobic molecules from circulating poly(ethylene glycol)-poly( D, L-lactic acid) (PEG-PDLLA) micelles. A lipophilic FRET pair (DiIC(18) and DiOC(18)) was physically entrapped into micelle cores by mimicking the loading of hydrophobic drugs. The FRET efficiency was found significantly reduced within 15 min after intravenous injection, implying that DiIC(18) and DiOC(18) quickly escaped from the circulating micelles. FRET spectroscopy studies further demonstrated that alpha- and beta-globulins were major factors for the observed fast release, while gamma-globulins, albumin, and red blood cells played minor roles. These results provide useful information for developing blood-stable micelles to deliver hydrophobic drugs to the target site via prolonged circulation and extravasation from the vascular system.

Measuring the velocity of fluorescently labelled red blood cells with a keyhole tracking algorithm

In this paper we propose a tracking algorithm to measure the velocity of fluorescently labelled red blood cells travelling through microvessels of tumours, growing in dorsal skin flap window chambers, implanted on mice. Preprocessing removed noise and artefacts from the images and then segmented cells from background. The tracking algorithm is based on a 'keyhole' model that describes the probable movement of a segmented cell between contiguous frames of a video sequence. When a history of cell movement exists, past, present and a predicted landing position of the cells will define two regions of probability that resemble the shape of a keyhole. This keyhole model was used to determine if cells in contiguous frames should be linked to form tracks and also as a postprocessing tool to join split tracks and discard links that could have been formed due to noise or uncertainty. When there was no history, a circular region around the centroid of the parent cell was used as a region of probability. Outliers were removed based on the distribution of the average velocities of the tracks. Since the position and time of each cell is recorded, a wealth of statistical measures can be obtained from the tracks. The algorithm was tested on two sets of experiments. First, the vasculatures of eight tumours with different geometries were analyzed; average velocities ranged from 86 to 372 microm s(-1), with minimum and maximum track velocities 7 and 1212 microm s(-1), respectively. Second, a longitudinal study of velocities was performed after administering a vascular disrupting agent to two tumours and the time behaviour was analyzed over 24 h. In one of the tumours there is a complete shutdown of the vasculature whereas in the other there is a clear decrease of velocity at 30 min, with subsequent recovery by 6 h. The tracking algorithm enabled the simultaneous measurement of red blood cell velocity in multiple vessels within an intravital video sequence, enabling analysis of heterogeneity of flow and response to treatment in mouse models of cancer.

Ozagrel attenuates early streptozotocin-induced constriction of arterioles in the mouse retina

Retinal blood flow in human diabetics has been reported to follow a biphasic time course in which an initial period of reduced flow and ischemia is often followed by a hyperemic and angiogenic phase in which flow can exceed normal levels. The purpose of the present study is to investigate the mechanisms of the initial decrease in flow, since early interventions could provide the most effective treatment strategies. C57BL/6 mice were injected with streptozotocin (STZ) at 12 weeks of age and remained hyperglycemic until data were gathered 4 or 8 weeks later. Experimental measurements included retinal arteriolar red blood cell velocity and arteriolar diameters, with the diameters measured prior to and following an intravenous injection of the thromboxane synthase inhibitor ozagrel (100 mg/kg). Arterioles leading out of the optic disk constricted significantly at 4 weeks post-STZ (p<0.001) compared to age-matched controls, but not at 8 weeks post-STZ. Calculations of retinal blood flow indicated a 45% decrease at 4 weeks post-STZ, but only a 26% decrease by 8 weeks. Not all arterioles constricted equally in response to STZ; the most substantial constrictions were present in arterioles that were more closely arranged with countercurrent venules leading back into the optic disk. Injection of ozagrel provided significant dilation of constricted retinal arterioles. In addition, the pattern of dilation was consistent with the sites of the most severe constriction, i.e., ozagrel-induced dilation in the STZ mice occurred to the greatest extent in the arterioles more closely paired with the venules draining the microvascular bed. In summary, STZ induces a biphasic alteration in retinal blood flow in mice, in which thromboxane contributes to the initial reduction in blood flow at 4 weeks. Moreover, the thromboxane-induced arteriolar constriction is dependent on the proximity of the retinal arterioles to countercurrent venules.

Epinephrine-induced activation of LW-mediated sickle cell adhesion and vaso-occlusion in vivo

Sickle red cell (SS RBC) adhesion is believed to contribute to the process of vaso-occlusion in sickle cell disease (SCD). We previously found that the LW RBC adhesion receptor can be activated by epinephrine to mediate SS RBC adhesion to endothelial alphavbeta3 integrin. To determine the contribution of LW activation to vaso-occlusive events in vivo, we investigated whether in vitro treatment of SS RBCs by epinephrine resulted in vaso-occlusion in intact microvasculature after RBC infusion into nude mice. Epinephrine enhanced human SS but not normal RBC adhesion to murine endothelial cells in vitro and to endothelium in vivo, promoting vaso-occlusion and RBC organ sequestration. Murine sickle RBCs also responded to epinephrine with increased adhesion to postcapillary endothelium in nude mice. Epinephrine-induced SS RBC adhesion, vaso-occlusion, and RBC organ trapping could be prevented by the beta-adrenergic receptor (beta-AR) antagonist, propranolol. Infusion of soluble recombinant LW also significantly reduced adhesion and vaso-occlusion. In addition, epinephrine-treated SS RBCs induced activation of murine leukocyte adhesion to endothelium as well. We conclude that LW activation by epinephrine via beta-AR stimulation can promote both SS RBC and leukocyte adhesion as well as vaso-occlusion, suggesting that both epinephrine and LW play potentially pathophysiological roles in SCD.

Interaction between liposomes and RBC in microvessels in vivo

Liposomes are phospholipid vesicles that can serve as carriers of biologically active agents in vitro and in vivo. Here, we describe the movement of liposomes suspended with blood flowing in capillaries. Liposomes were coated with a polymer to extend their lifespan in rat mesenteric blood vessels and detected by fluorescent staining. Liposome activity was observed by intravital microscopy using a high-speed camera system at 5 and 60 min after liposome administration. Liposome velocity was determined using two-dimensional cross-correlation, and blood flow was measured by high-resolution PIV (particle image velocimetry). The results showed that the motion of polymer-coated liposome followed the phase averaged velocity distribution of heartbeats while flowing with red blood cells in microvessels. Liposome particles tend to move toward the near blood vessel wall in the low velocity of blood flow.

Direct Demonstration of Instabilities in Oxygen Concentrations within the Extravascular Compartment of an Experimental Tumor

To test the hypothesis that temporal variations in microvessel red cell flux cause unstable oxygen levels in tumor interstitium, extravascular oxygenation of R3230Ac mammary tumors grown in skin-fold window chambers was measured using recessed tip polarographic microelectrodes. Red cell fluxes in microvessels surrounding pO2 measurement locations were measured using fluorescently labeled red cells. Temporal pO2 instability was observed in all experiments. Median pO2 was inversely related to radial distance from microvessels. Transient fluctuations above and below 10 mm Hg were consistently seen, except in one experiment near the oxygen diffusion distance limit (140 microm) where pO2 fluctuations were <2 mm Hg and median pO2 was <5 mm Hg. Vascular stasis was not seen in these experiments. These results show that fluctuations in red cell flux, as opposed to vascular stasis, can cause temporal variations in pO2 that extend from perivascular regions to the maximum oxygen diffusion distance.

Tumor Vascular Permeabilization by Vascular-Targeting Photosensitization: Effects, Mechanism, and Therapeutic Implications

PURPOSE

Loss of vascular barrier function has been observed shortly following vascular-targeting photodynamic therapy. However, the mechanism involved in this event is still not clear, and the therapeutic implications associated with this pathophysiologic change have not been fully explored.

EXPERIMENTAL DESIGN

The effect of vascular-targeting photodynamic therapy on vascular barrier function was examined in both s.c. and orthotopic MatLyLu rat prostate tumor models and endothelial cells in vitro, using photosensitizer verteporfin. Vascular permeability to macromolecules (Evans blue-albumin and high molecular weight dextran) was assessed with dye extraction (ex vivo) and intravital microscopy (in vivo) methods. Intravital microscopy was also used to monitor tumor vascular functional changes after vascular-targeting photodynamic therapy. The effects of photosensitization on monolayer endothelial cell morphology and cytoskeleton structures were studied with immunofluorescence staining.

RESULTS

Vascular-targeting photodynamic therapy induced vascular barrier dysfunction in the MatLyLu tumors. Thus, tumor uptake of macromolecules was significantly increased following photodynamic therapy treatments. In addition to vascular permeability increase, blood cell adherence to vessel wall was observed shortly after treatment, further suggesting the loss of endothelial integrity. Blood cell adhesion led to the formation of thrombi that can occlude blood vessels, causing vascular shutdown. However, viable tumor cells were often detected at tumor periphery after vascular-targeting photodynamic therapy. Endothelial cell barrier dysfunction following photodynamic therapy treatment was also observed in vitro by culturing monolayer endothelial cells on Transwell inserts. Immunofluorescence study revealed microtubule depolymerization shortly after photosensitization treatment and stress actin fiber formation thereafter. Consequently, endothelial cells were found to retract, and this endothelial morphologic change led to the formation of intercellular gaps.

CONCLUSIONS

Vascular-targeting photodynamic therapy permeabilizes blood vessels through the formation of endothelial intercellular gaps, which are likely induced via endothelial cell microtubule depolymerization following vascular photosensitization. Loss of endothelial barrier function can ultimately lead to tumor vascular shutdown and has significant implications in drug transport and tumor cell metastasis.

Intravital microscopic observations of 15-microm microspheres lodging in the pulmonary microcirculation

Vascular infusions of 15-microm-diameter microspheres are used to study pulmonary blood flow distribution. The sites of microsphere lodging and their effects on microvascular perfusion are debated but unknown. Using intravital microscopy of the subpleural surface of rat lungs, we directly observed deposition of fluorescent microspheres. In a pump-perfused lung model, approximately 0.5 million microspheres were infused over 30 s into the pulmonary artery of seven rats. Microsphere lodging was analyzed for the location in the microvasculature and the effect on local flow after lodging. On average, we observed 3.2 microspheres per 160 alveolar facets. The microspheres always entered the arterioles as singlets and lodged at the inlets to capillaries, either in alveolar corner vessels or small arterioles. In all cases, blood flow continued either around the microspheres or into the capillaries via adjacent pathways. We conclude that 15-microm-diameter microspheres, in doses in excess of those used in typical studies, have no significant impact on pulmonary capillary blood flow distribution.

Prolonged circulation of large polymeric nanoparticles by non-covalent adsorption on erythrocytes

Polymeric nanoparticles have been extensively studied for use as intravascular drug delivery vehicles; however, their applications are limited by rapid clearance from circulation by the reticuloendothelial system (RES). Previous attempts to improve vascular circulation have focused on surface modification using polymers such as poloxamines, poloxamers, and polyethylene glycol, to prevent opsononization. We report on a novel method of prolonging intravascular particle circulation by anchoring the nanoparticles to the surface of red blood cells (RBCs). We hypothesize that particles adhered to RBCs can escape RES clearance due to the ability of RBCs to do so. This method is motivated by the strategy adopted by certain bacteria, for example, hemobartonella, that adhere to RBCs and remain in circulation for several weeks. Prolonged circulation of nanoparticles as large as 450 nm was observed after adsorption on RBCs. Although particles were eventually eliminated from circulation, RBCs were not cleared. RBC-anchored nanoparticles offer a novel approach for intravascular drug delivery and blood pool imaging.

Targeting tumor microvessels using doxorubicin encapsulated in a novel thermosensitive liposome

Liposomal drugs accumulate only in perivascular regions in tumors after i.v. injection. Thus, they cannot kill tumor cells in deeper tissue layers. To circumvent this problem, we investigated effects of doxorubicin (DOX) encapsulated in a lysolecithin-containing thermosensitive liposome (LTSL) on tumor microcirculation because damaging microvessels would stop nutrient supply to deeper tumor cells. We used LTSL-DOX in combination with hyperthermia to treat a human squamous carcinoma xenograft (FaDu) implanted in dorsal skinfold chambers in nude mice. Before the treatment, the RBC velocity in tumors was 0.428 ± 0.037 mm/s and the microvascular density was 3.93 ± 0.44 mm/mm2. At 24 hours after the treatment, they were reduced to 0.003 ± 0.003 mm/s and 0.86 ± 0.27 mm/mm2, respectively. The same treatment, however, caused only 32% decrease in the RBC velocity and no apparent change in microvascular networks in normal s.c. tissues over the same period. LTSL and LTSL-DOX alone had no effect on tumor microcirculation, and LTSL plus hyperthermia caused only a transient decrease in the RBC velocity in tumors. At 24 hours after treatments, tumor microcirculation in all these control experiments was insignificantly different from that before the treatments. We also examined apoptosis of cells in tumors at different time points after LTSL-DOX plus hyperthermia treatment and observed few apoptotic cells in tumor microvessels. In conclusion, the rapid release of DOX during hyperthermia could make the drug to shutdown tumor blood flow while have only minor effects on normal microcirculation in s.c. tissues.

Microcirculation Study of Rabbit Ear Arterial and Venous Flow-Through Flaps Using a Window Chamber Model

Venous flaps are used widely for finger reconstruction because they provide thin tissue, and the flap harvest is associated with less donor-site morbidity. The viability of the venous flap, however, is not as good as that of the ordinary perfused skin flap, and its microcirculation is questionable according to various indirect observations and hypotheses in the literature. Using a window chamber model in a rabbit ear, both arterial and venous flow-through flaps were studied. Factors evaluated were flap viability, flap weight, flap circulation as assessed by laser Doppler flowmetry, and direct observation of the microcirculation. Statistical analysis was performed using the two-sample t test. There was no statistically significant difference in viability between arterial and venous flow-through flaps (p = 0.661). The arterial flow-through flap had better perfusion than thevenous flow-through flap, as measured by laser Doppler perfusion studies (10.40 perfusion units [PU] vs 4.50 PU). However, no statistically significant difference was noted (p = 0.0717). Flap weight assesseed 1 week after surgery and oxygen saturation measured immediately after surgery showed significant differences between the arterial and venous flow-through flaps (p = 0.0001 and 0.0279). These datasuggest that the arterial flow-through flap is subjected to more congestion becauseof the abnormal flow pattern seen, andpossibly because of a superior inflow or nutritional status found in these flaps. Using vital microscopy, direct evaluation of the microcirculation was performed. A to-and-fro phenomenon was noted in both arterial and venous flow-through flaps,which was followed by a reversed direction of flow in part of the microvasculature. With both types of flaps, the blood was directed eventually from the postcapillary venules to the capillaries, through the terminal arterioles, and then to the arterioles. These findings may be explained partially by the normal physiologic pressure gradients present in the microvasculature of these flaps. In this study, direct observation of the microcirculation was used as well as other objective measures to determine the flow patterns and clinical behaviors found in these types of flaps. A model in a rabbit ear for the study of venous and arterial flow-through flaps is described, and clinical correlations are discussed.

Radial displacement of red blood cells during hemodilution and the effect on arteriolar oxygen profile

In this study, we assessed the magnitude of the erratic deviations in the radial position of red blood cells (RBCs) in the laminar flow regime of arterioles in a hamster window preparation and the intraluminal Po2 profile to determine whether this variability affects the intraluminal distribution of oxygen in conditions of normal hematocrit and hemodilution. A gated image intensifier was used to visualize fluorescently labeled RBCs in tracer quantities and obtain multiple measurements of RBC radial and longitudinal positions at time intervals on the order of 5 ms within single arterioles (diameter range 40–95 μm). RBCs in the velocity range of 0.3–14 mm/s exhibit a mean coefficient of variation of velocity of 16.9 ± 10.5% and a SD of the radial position of 1.98 ± 0.98 μm. Both quantities were inversely related to hematocrit, and the former was significantly lowered by hemodilution. Our experimental results presented very similar values and shape compared with the intraluminal oxygen profile derived theoretically for normal hematocrit, suggesting that shear-augmented diffusion due to the measured radial displacement of RBCs did not significantly affect oxygen diffusion from blood into the arteriolar vessel wall. Po2 profiles in the arterioles assumed an increasingly parabolic configuration with increasing levels of hemodilution.

Measurement of the cardiac output in small animals by thermodilution

Cardiac output (CO) measurements based on indicator dilution, microspheres, thermodilution and ultrasonic sensors are not suitable for small animals, because of limited blood volume, high heart rates and small caliber vessels that do not allow probe placement within the heart. We developed a modified thermodilution method to measure CO in awake animals weighing less than 100 g. Under anesthesia, the animal is instrumented with a jugular vein catheter placed proximal to the subclavian vein and a temperature probe in the carotid artery with the thermocouple positioned at the aortic arch. Two days after implantation, room temperature saline is injected (150 microl) into the jugular catheter and the temperature change recorded. This system uses the temperature probe as a digital feedback control: (1) to minimize recirculation error; (2) to adjust baseline temperature, thereby increasing sensitivity to small changes in temperature; and (3) to stabilize animal core temperature. The system was calibrated using a laboratory bench model with anatomically scaled components. CO was measured (n=29) in 16 hamsters (65-115 g), and was linearly related to body weight. Cardiac index (CI=CO/weight) was 197.0 +/- 18.8 (ml/min)/kg. Repeated measurements were made. This technique allows correlating systemic flow changes to be correlated to those measured in the microcirculation of window chamber preparations.

Effect of aggregation and shear rate on the dispersion of red blood cells flowing in venules

Previous in vitro studies of blood flow in small glass tubes have shown that red blood cells exhibit significant erratic deviations in the radial position in the laminar flow regime. The purpose of the present study was to assess the magnitude of this variability and that of velocity in vivo and the effect of red blood cell aggregation and shear rate upon them. With the use of a gated image intensifier and fluorescently labeled red blood cells in tracer quantities, we obtained multiple measurements of red blood cell radial and longitudinal positions at time intervals as short as 5 ms within single venous microvessels (diameter range 45-75 microm) of the rat spinotrapezius muscle. For nonaggregating red blood cells in the velocity range of 0.3-14 mm/s, the mean coefficient of variation of velocity was 16.9 +/- 10.5% and the SD of the radial position was 1.98 +/- 0.98 microm. Both quantities were inversely related to shear rate, and the former was significantly lowered on induction of red blood cell aggregation by the addition of Dextran 500 to the blood. The shear-induced random movements observed in this study may increase the radial transport of particles and solutes within the bloodstream by orders of magnitude.

Effects of erythrocyte aggregation and venous network geometry on red blood cell axial migration

Axial migration of red blood cells in small glass tubes can cause blood viscosity to be effectively independent of shear rate. However, this phase separation may not occur to the same degree in the venous network due to infusion of cells and aggregates at branch points. To investigate this hypothesis, we followed trajectories of fluorescently labeled red blood cells in the venular network of the rat spinotrapezius muscle at normal and reduced flow with and without red blood cell aggregation. Cells traveling near the wall of an unbranched venular segment migrated approximately 1% of the longitudinal path length without aggregation and migrated slightly more with aggregation. Venular segment length between branch points averaged three to five times the diameter. Cells in the main vessel were shifted centrally by up to 20% of diameter at branch points, reducing the migration rate of cells near the opposite wall to <1% even in the presence of aggregation. We conclude that formation of a cell-free marginal layer in the venular network is attenuated due to the time dependence of axial migration and the frequent branching of the network.

Effect of erythrocyte aggregation on velocity profiles in venules

A recent whole organ study in cat skeletal muscle showed that the increase in venous resistance seen at reduced arterial pressures is nearly abolished when the muscle is perfused with a nonaggregating red blood cell suspension. To explore a possible underlying mechanism, we tested the hypothesis that red blood cell aggregation alters flow patterns in vivo and leads to blunted red blood cell velocity profiles at reduced shear rates. With the use of fluorescently labeled red blood cells in tracer quantities and a video system equipped with a gated image intensifier, we obtained velocity profiles in venous microvessels (45-75 microm) of rat spinotrapezius muscle at centerline velocities between 0.3 and 14 mm/s (pseudoshear rates 3-120 s(-1)) under normal (nonaggregating) conditions and after induction of red blood cell aggregation with Dextran 500. Profiles are nearly parabolic (Poiseuille flow) over this flow rate range in the absence of aggregation. When aggregation is present, profiles are parabolic at high shear rates and become significantly blunted at pseudoshear rates of 40 s(-1) and below. These results indicate a possible mechanism for increased venous resistance at reduced flows.

Diameter changes in skeletal muscle venules during arterial pressure reduction

Previous studies in skeletal muscle have shown a substantial (>100%) increase in venous vascular resistance with arterial pressure reduction to 40 mmHg, but a microcirculatory study showed no significant venular diameter changes in the horizontal direction during this procedure. To examine the possibility of venular collapse in the vertical direction, a microscope was placed horizontally to view a vertically mounted rat spinotrapezius muscle preparation. We monitored the diameters of venules (mean diameter 73. 8 +/- 37.0 microm, range 13-185 microm) oriented horizontally and vertically with a video system during acute arterial pressure reduction by hemorrhage. Our analysis showed small but significant (P < 0.0001) diameter reductions of 1.0 +/- 2.5 microm and 1.8 +/- 3. 1 microm in horizontally and vertically oriented venules, respectively, upon reduction of arterial pressure from 115.0 +/- 26. 3 to 39.8 +/- 12.3 mmHg. The venular responses were not different after red blood cell aggregation was induced by Dextran 500 infusion. We conclude that diameter changes in venules over this range of arterial pressure reduction are isotropic and would likely increase venous resistance by <10%.

Fluorescent dyes for lymphocyte migration and proliferation studies

Fluorescent dyes are increasingly being exploited to track lymphocyte migration and proliferation. The present paper reviews the properties and performance of some 14 different fluorescent dyes that have been used during the last 20 years to monitor lymphocyte migration. Of the 14 dyes discussed, two stand out as being the most versatile in terms of long-term tracking of lymphocytes and their ability to quantify lymphocyte proliferation. They are the intracellular covalent coupling dye carboxyfluorescein diacetate succinimidyl ester (CFSE) and the membrane inserting dye PKH26. Both dyes have the advantage that they can be used to track cell division, both in vitro and in vivo, due to the progressive halving of the fluorescence intensity of the dyes in cells after each division. However, CFSE appears to have the edge over PKH26 based on homogeneity of lymphocyte staining and cost. Two other fluorescent dyes, although not suitable for lymphocyte proliferation studies, are valuable tracking dyes for short-term (up to 3 day) lymphocyte migration experiments, namely the DNA-binding dye Hoechst 33342 and the cytoplasmic dye calcein. In the future it is highly likely that additional fluorescent dyes, with different spectral properties to CFSE, will become available, as well as membrane inserting fluorescent dyes that more homogeneously label lymphocytes than PKH26.

Delivery of colloidal particles and red blood cells to tissue through microvessel ruptures created by targeted microbubble destruction with ultrasound

BACKGROUND

We have previously shown that the application of ultrasound to thin-shelled microbubbles flowing through small microvessels (<7 microm in diameter) produces vessel wall ruptures in vivo. Because many intravascular drug- and gene-delivery vehicles are limited by the endothelial barrier, we hypothesized that this phenomenon could be used to deliver drug-bearing vehicles to tissue.

METHODS AND RESULTS

An exteriorized rat spinotrapezius muscle preparation was used. Intravascular fluorescent red blood cells and polymer microspheres (PM) (205 and 503 nm in diameter) were delivered to the interstitium of rat skeletal muscle through microvessel ruptures created by insonifying microbubbles in vivo. On intravital microscopy, mean dispersion areas per rupture for red blood cells, 503-nm PM, and 205-nm PM were 14.5x10(3) microm2, 24. 2x10(3) microm2, and 27.2x10(3) microm2, respectively. PM dispersion areas were significantly larger than the mean dispersion area for red blood cells (P<0.05).

CONCLUSIONS

Microvessel ruptures caused by insonification of microbubbles in vivo may provide a minimally invasive means for delivering colloidal particles and engineered red blood cells across the endothelial lining of a targeted tissue region.

Anatomic distribution of pulmonary vascular compliance

Previously, the pressure changes after arterial and venous occlusion have been used to characterize the longitudinal distribution of pulmonary vascular resistance with respect to vascular compliance using compartmental models. However, the compartments have not been defined anatomically. Using video microscopy of the subpleural microcirculation, we have measured the flow changes in approximately 40-micron arterioles and venules after venous, arterial, and double occlusion maneuvers. The quasi-steady flows through these vessels after venous occlusion permitted an estimation of the compliance in three anatomic segments: arteries > 40 microns, veins > 40 microns, and vessels < 40 microns in diameter. We found that approximately 65% of the total pulmonary vascular compliance was in vessels < 40 microns, presumably mostly capillaries. The transient portions of the pressure and flow data after venous, arterial, and double occlusion were consistent with most of the arterial compliance being upstream from most of the arterial resistance and most of the venous compliance being downstream from most of the venous resistance.