An experimental model for simultaneous quantitative analysis of pulmonary micro- and macrocirculation during unilateral hypoxia in vivo

The Pulmonary Vasculature

The pulmonary circulation is a low-pressure, low-resistance circuit whose primary function is to deliver deoxygenated blood to, and oxygenated blood from, the pulmonary capillary bed enabling gas exchange. The distribution of pulmonary blood flow is regulated by several factors including effects of vascular branching structure, large-scale forces related to gravity, and finer scale factors related to local control. Hypoxic pulmonary vasoconstriction is one such important regulatory mechanism. In the face of local hypoxia, vascular smooth muscle constriction of precapillary arterioles increases local resistance by up to 250%. This has the effect of diverting blood toward better oxygenated regions of the lung and optimizing ventilation-perfusion matching. However, in the face of global hypoxia, the net effect is an increase in pulmonary arterial pressure and vascular resistance. Pulmonary vascular resistance describes the flow-resistive properties of the pulmonary circulation and arises from both precapillary and postcapillary resistances. The pulmonary circulation is also distensible in response to an increase in transmural pressure and this distention, in addition to recruitment, moderates pulmonary arterial pressure and vascular resistance. This article reviews the physiology of the pulmonary vasculature and briefly discusses how this physiology is altered by common circumstances.

Ventilation and Perfusion at the Alveolar Level: Insights From Lung Intravital Microscopy

Intravital microscopy (IVM) offers unique possibilities for the observation of biological processes and disease related mechanisms in vivo. Especially for anatomically complex and dynamic organs such as the lung and its main functional unit, the alveolus, IVM provides exclusive advantages in terms of spatial and temporal resolution. By the use of lung windows, which have advanced and improved over time, direct access to the lung surface is provided. In this review we will discuss two main topics, namely alveolar dynamics and perfusion from the perspective of IVM-based studies. Of special interest are unanswered questions regarding alveolar dynamics such as: What are physiologic alveolar dynamics? How do these dynamics change under pathologic conditions and how do those changes contribute to ventilator-induced lung injury? How can alveolar dynamics be targeted in a beneficial way? With respect to alveolar perfusion IVM has propelled our understanding of the pulmonary microcirculation and its perfusion, as well as pulmonary vasoreactivity, permeability and immunological aspects. Whereas the general mechanism behind these processes are understood, we still lack a proper understanding of the complex, multidimensional interplay between alveolar ventilation and microvascular perfusion, capillary recruitment, or vascular immune responses under physiologic and pathologic conditions. These are only part of the unanswered questions and problems, which we still have to overcome. IVM as the tool of choice might allow us to answer part of these questions within the next years or decades. As every method, IVM has advantages as well as limitations, which have to be taken into account for data analysis and interpretation, which will be addressed in this review.

Multicolor two-photon imaging of in vivo cellular pathophysiology upon influenza virus infection using the two-photon IMPRESS

In vivo two-photon imaging is a valuable technique for studies of viral pathogenesis and host responses to infection in vivo. In this protocol, we describe a methodology for analyzing influenza virus-infected lung in vivo by two-photon imaging microscopy. We describe the surgical procedure, how to stabilize the lung, and an approach to analyzing the data. Further, we provide a database of fluorescent dyes, antibodies, and reporter mouse lines that can be used in combination with a reporter influenza virus (Color-flu) for multicolor analysis. Setup of this model typically takes ~30 min and enables the observation of influenza virus-infected lungs for >4 h during the acute phase of the inflammation and at least 1 h in the lethal phase. This imaging system, which we termed two-photon IMPRESS (imaging pathophysiology research system), is broadly applicable to analyses of other respiratory pathogens and reveals disease progression at the cellular level in vivo.

Long-term High-Resolution Intravital Microscopy in the Lung with a Vacuum Stabilized Imaging Window

Metastasis to secondary sites such as the lung, liver and bone is a traumatic event with a mortality rate of approximately 90% ¹. Of these sites, the lung is the most difficult to assess using intravital optical imaging due to its enclosed position within the body, delicate nature and vital role in sustaining proper physiology. While clinical modalities (positron emission tomography (PET), magnetic resonance imaging (MRI) and computed tomography (CT)) are capable of providing noninvasive images of this tissue, they lack the resolution necessary to visualize the earliest seeding events, with a single pixel consisting of nearly a thousand cells. Current models of metastatic lung seeding postulate that events just after a tumor cell's arrival are deterministic for survival and subsequent growth. This means that real-time intravital imaging tools with single cell resolution ² are required in order to define the phenotypes of the seeding cells and test these models. While high resolution optical imaging of the lung has been performed using various ex vivo preparations, these experiments are typically single time-point assays and are susceptible to artifacts and possible erroneous conclusions due to the dramatically altered environment (temperature, profusion, cytokines, etc.) resulting from removal from the chest cavity and circulatory system ³. Recent work has shown that time-lapse intravital optical imaging of the intact lung is possible using a vacuum stabilized imaging window ^2,4,5 however, typical imaging times have been limited to approximately 6 hr. Here we describe a protocol for performing long-term intravital time-lapse imaging of the lung utilizing such a window over a period of 12 hr. The time-lapse image sequences obtained using this method enable visualization and quantitation of cell-cell interactions, membrane dynamics and vascular perfusion in the lung. We further describe an image processing technique that gives an unprecedentedly clear view of the lung microvasculature.

In vivo subcellular resolution optical imaging in the lung reveals early metastatic proliferation and motility

To better understand breast cancer metastatic cell seeding, we have employed multiphoton microscopy and a vacuum stabilized window which eliminates the need for complex registration software, video rate microscopy or specialized gating electronics to observe the initial steps of tumor cell seeding within the living, breathing lung. We observe that upon arrival to the lung, tumor cells are found exclusively in capillary vessels, completely fill their volume and display an initial high level of protrusive activity that dramatically reduces over time. Further, we observe a concomitant increase in positional stability during this same period. We employ several techniques accessible to most imaging labs for optimizing signal to noise and resolution which enable us to report the first direct observation, with subcellular resolution, of the arrival, proliferation, and motility of metastatic tumor cells within the lung.

An experimental rat model for studying pulmonary microcirculation by in vivo videomicroscopy

It is unclear what role pulmonary microcirculatory disorders play in the pathogenesis of adult respiratory distress syndrome. The aim of this study was to establish a rat model for the direct visualization of pulmonary microcirculation by in vivo fluorescence videomicroscopy. The pulmonary terminal vascular bed was visualized and the microcirculatory parameters of leukocyte sticking, erythrocyte velocity, capillary permeability, and interalveolar septal diameter were quantified. These parameters were examined simultaneously. The preparation was stable for 120 min. Under hyperthermia, there was increased permeability with a relative fluorescence of 0.39 +/- 0.19 compared to 0.16 +/- 0.13 in the control group, and interalveolar septal diameters were wider (30.7 +/- 2.9 microm) than in control animals (17.3 +/- 3 microm). Under hypothermia and hypovolemia, the erythrocyte velocity was lower (0.351 +/- 0.063 and 0.378 +/- 0.044 mm/s) than in control groups (0.527 +/- 0.07 mm/s). Under hypoventilation, we observed a higher amount of leukocyte sticking (3.1 +/- 1.1 vs 1.8 +/- 0.8 cells/alveolus) and increased permeability (relative fluorescence 1.03 +/- 0.37 vs 0.16 +/- 0.13 in the control group). The model of rat lung exposure for direct examination of microvascular structures in living animals was valuable because it remained stable for 2 h under baseline conditions and demonstrated distinct changes in microcirculatory parameters following specific pathophysiological interventions.