Changes in pulmonary blood flow distribution in monocrotaline compared with hypoxia-induced models of pulmonary hypertension: assessed using synchrotron radiation

Search for an animal model to investigate selective pulmonary vasodilation

Pulmonary arterial hypertension is a life-threatening disease with a poor prognosis. Oral treatment with vasodilators is often limited by systemic hypotension. Inhalation of vasodilators offers the opportunity for selective pulmonary vasodilation. Testing selective pulmonary vasodilation by inhaled nitric oxide or alternative substances in animal models requires an increased pulmonary vascular tone. The aim of this study was to identify animal models that are suitable for investigating selective pulmonary vasodilation. To do so, a haemodynamic stable pulmonary hypertension was initiated, with a 30 min duration deemed to be a sufficient time interval before and after a possible intervention. In anaesthetized and mechanically-ventilated Sprague-Dawley rats pulmonary hypertension was induced either by acute hypoxia due to reduction of the inspired oxygen fraction from 0.21 to 0.1 ( n = 6), a fixed infusion rate of the thromboxane analogue U46619 (240 ng/min; n = 6) or a monocrotaline injection (MCT; 60 mg/kg applied 23 days before the investigation; n = 7). The animals were instrumented to measure right ventricular and systemic arterial pressures. Acute hypoxia caused a short, and only transient, increase of pulmonary artery pressure as well as profound systemic hypotension which suggested haemodynamic instability. U46619 infusion induced variable changes in the pulmonary and systemic vascular tone without sufficient stabilization within 30 min. MCT provoked sustained pulmonary hypertension with normal systemic pressure values and inhalation of nitric oxide caused selective pulmonary vasodilation. In conclusion, out of the three examined rat animal models only MCT-induced pulmonary hypertension is a solid and reliable model for investigating selective pulmonary vasodilation.

β2-Adrenergic receptor-dependent attenuation of hypoxic pulmonary vasoconstriction prevents progression of pulmonary arterial hypertension in intermittent hypoxic rats

In sleep apnea syndrome (SAS), intermittent hypoxia (IH) induces repeated episodes of hypoxic pulmonary vasoconstriction (HPV) during sleep, which presumably contribute to pulmonary arterial hypertension (PAH). However, the prevalence of PAH was low and severity is mostly mild in SAS patients, and mild or no right ventricular hypertrophy (RVH) was reported in IH-exposed animals. The question then arises as to why PAH is not a universal finding in SAS if repeated hypoxia of sufficient duration causes cycling HPV. In the present study, rats underwent IH at a rate of 3 min cycles of 4-21% O2 for 8 h/d for 6 w. Assessment of diameter changes in small pulmonary arteries in response to acute hypoxia and drugs were performed using synchrotron radiation microangiography on anesthetized rats. In IH-rats, neither PAH nor RVH was observed and HPV was strongly reversed. Nadolol (a hydrophilic β(1, 2)-blocker) augmented the attenuated HPV to almost the same level as that in N-rats, but atenolol (a hydrophilic β1-blocker) had no effect on the HPV in IH. These β-blockers had almost no effect on the HPV in N-rats. Chronic administration of nadolol during 6 weeks of IH exposure induced PAH and RVH in IH-rats, but did not in N-rats. Meanwhile, atenolol had no effect on morphometric and hemodynamic changes in N and IH-rats. Protein expression of the β1-adrenergic receptor (AR) was down-regulated while that of β2AR was preserved in pulmonary arteries of IH-rats. Phosphorylation of p85 (chief component of phosphoinositide 3-kinase (PI3K)), protein kinase B (Akt), and endothelial nitric oxide synthase (eNOS) were abrogated by chronic administration of nadolol in the lung tissue of IH-rats. We conclude that IH-derived activation of β2AR in the pulmonary arteries attenuates the HPV, thereby preventing progression of IH-induced PAH. This protective effect may depend on the β2AR-Gi mediated PI3K/Akt/eNOS signaling pathway.

Assessment of the serotonin pathway as a therapeutic target for pulmonary hypertension

Blockade of the serotonin reuptake transporter (5-HTT), using fluoxetine, has been identified as a potential therapeutic target for preventing and, importantly, reversing pulmonary hypertension (PH). This study utilized synchrotron radiation microangiography to determine whether fluoxetine could prevent or reverse endothelial dysfunction and vessel rarefaction, which underpin PH. PH was induced by a single injection of monocrotaline (MCT; 60 mg kg(-1)). Following MCT administration, rats received daily injections of either saline or fluoxetine (MCT+Fluox; 10 mg kg(-1)) for three weeks. A third group of rats also received the fluoxetine regime, but only three weeks after MCT (MCT+FluoxDelay). Control rats received daily injections of saline. Pulmonary microangiography was performed to assess vessel branching density and visualize dynamic changes in vessel diameter following (i) acute fluoxetine or (ii) acetylcholine, sodium nitroprusside, BQ-123 (ET-1A receptor blocker) and L-NAME (NOS inhibitor). Monocrotaline induced PH that was inevitably terminal. `Delayed' treatment of fluoxetine (MCT+FluoxDelay) was unable to reverse the progression of PH. Early fluoxetine treatment pre-PH (i.e. MCT+Fluox) attenuated but did not completely prevent vascular remodeling, vessel rarefaction and an increase in pulmonary pressure, and it did not prevent pulmonary endothelial dysfunction. Interestingly, fluoxetine treatment did counter-intuitively prevent the onset of right ventricular hypertrophy. Using synchrotron radiation microangiography, selective blockade of the serotonin reuptake transporter alone is highlighted as not being sufficient to prevent pulmonary endothelial dysfunction, which is the primary instigator for the inevitable onset of vascular remodeling and vessel rarefaction. Accordingly, potential therapeutic strategies should aim to target multiple pathways to ensure an optimal outcome.

Impaired pulmonary blood flow distribution in congestive heart failure assessed using synchrotron radiation microangiography

Synchrotron radiation microangiography is a powerful tool for assessing adverse changes in pulmonary vessel density associated with primary pulmonary hypertension (PH). Congestive heart failure (CHF) leads to a `secondary' onset of PH, yet it is unknown whether secondary PH is also associated with reduced vessel density. This study utilized synchrotron radiation to assess both pulmonary vessel density and endothelial function in a Dahl rat model of CHF with secondary PH. High salt-fed Dahl salt-sensitive (Dahl-S) and salt-resistant (Dahl-R) rats were anesthetized and microangiography was performed to assess the pulmonary vessel density and vascular responses to (i) sodium nitroprusside (5.0 µg kg(-1) min(-1)), (ii) acetylcholine (3.0 µg kg(-1) min(-1)) and (iii) ET-1A receptor blockade, BQ-123 (1 mg kg(-1)). Dahl-S rats developed CHF and secondary PH as evident by endothelial dysfunction, impaired vasodilatory responses to acetylcholine, enhanced vasodilatory responses to BQ-123 and extensive pulmonary vascular remodeling. Consequently, the pulmonary vessel density was adversely reduced. Interestingly, the etiology of secondary PH manifests with structural and functional changes that are comparable with that previously reported for primary PH. One important discrepancy, however, is that ET-1 modulation of pulmonary vessels is most striking in vessels with a diameter range of 100-200 µm in secondary PH, in contrast to a range of 200-300 µm in primary PH. Such discrepancies should be considered in future studies investigating primary and secondary forms of PH.

X-ray intravital microscopy for functional imaging in rat hearts using synchrotron radiation coronary microangiography

An X-ray intravital microscopy technique was developed to enable in vivo visualization of the coronary, cerebral, and pulmonary arteries in rats without exposure of organs and with spatial resolution in the micrometer range and temporal resolution in the millisecond range. We have refined the system continually in terms of the spatial resolution and exposure time. X-rays transmitted through an object are detected by an X-ray direct-conversion type detector, which incorporates an X-ray SATICON pickup tube. The spatial resolution has been improved to 6 μm, yielding sharp images of small arteries. The exposure time has been shortened to around 2 ms using a new rotating-disk X-ray shutter, enabling imaging of beating rat hearts. Quantitative evaluations of the X-ray intravital microscopy technique were extracted from measurements of the smallest-detectable vessel size and detection of the vessel function. The smallest-diameter vessel viewed for measurements is determined primarily by the concentration of iodinated contrast material. The iodine concentration depends on the injection technique. We used ex vivo rat hearts under Langendorff perfusion for accurate evaluation. After the contrast agent is injected into the origin of the aorta in an isolated perfused rat heart, the contrast agent is delivered directly into the coronary arteries with minimum dilution. The vascular internal diameter response of coronary arterial circulation is analyzed to evaluate the vessel function. Small blood vessels of more than about 50 μm diameters were visualized clearly at heart rates of around 300 beats/min. Vasodilation compared to the control was observed quantitatively using drug manipulation. Furthermore, the apparent increase in the number of small vessels with diameters of less than about 50 μm was observed after the vasoactive agents increased the diameters of invisible small blood vessels to visible sizes. This technique is expected to offer the potential for direct investigation of mechanisms of vascular dysfunctions.

Synchrotron radiation imaging for advancing our understanding of cardiovascular function

Synchrotron radiation (SR) is increasingly being used for micro-level and nano-level functional imaging in in vivo animal experiments. This review focuses on the methodology that enables repeated and regional assessment of vessel internal diameter and flow in the resistance vessels of different organ systems. In particular, SR absorption microangiography approaches offer unique opportunities for real-time in vivo vascular imaging in small animals, even during dynamic motion of the heart and lungs. We also describe recent progress in the translation of multiple phase-contrast imaging techniques from ex vivo to in vivo small-animal studies. Furthermore, we also review the utility of SR for multiple pinpoint (dimensions 0.2×0.2 mm) assessments of myocardial function at the cross-bridge level in different regions of the heart using small-angle X-ray scattering, resulting from increases in SR flux at modern facilities. Finally, we present cases for the use of complementary SR approaches to study cardiovascular function, particularly the pathological changes associated with disease using small-animal models.

Exogenous ghrelin improves blood flow distribution in pulmonary hypertension-assessed using synchrotron radiation microangiography

Ghrelin has cardioprotective properties and, recently, has been shown to improve endothelial function and reduce endothelin-1 (ET-1)-mediated vasoconstriction in peripheral vascular disease. Recently, we reported that ghrelin attenuates pulmonary hypertension (PH) caused by chronic hypoxia (CH), which we hypothesized in this study may be via suppression of the ET-1 pathway. We also aimed to determine whether ghrelin's ability to prevent alterations of the ET-1 pathway also prevented adverse changes in pulmonary blood flow distribution associated with PH. Sprague-Dawley rats were exposed to CH (10% O(2) for 2 weeks) with daily subcutaneous injections of ghrelin (150 μg/kg) or saline. Utilizing synchrotron radiation microangiography, we assessed pulmonary vessel branching structure, which is indicative of blood flow distribution, and dynamic changes in vascular responsiveness to (1) ET-1 (1 nmol/kg), (2) the ET-1(A) receptor antagonist, BQ-123 (1 mg/kg), and (3) ACh (3.0 μg kg⁻¹ min⁻¹). CH impaired blood flow distribution throughout the lung. However, this vessel "rarefaction" was attenuated in ghrelin-treated CH-rats. Moreover, ghrelin (1) reduced the magnitude of endothelial dysfunction, (2) prevented an increase in ET-1-mediated vasoconstriction, and (3) reduced pulmonary vascular remodeling and right ventricular hypertrophy-all adverse consequences associated with CH. These results highlight the beneficial effects of ghrelin for maintaining optimal lung perfusion in the face of a hypoxic insult. Further research is now required to establish whether ghrelin is also an effective therapy for restoring normal pulmonary hemodynamics in patients that already have established PH.

Imaging of the closed-chest mouse pulmonary circulation using synchrotron radiation microangiography

Structural and functional changes of pulmonary circulation related to pathophysiology of pulmonary arterial hypertension (PAH) remain to be fully elucidated. Angiographic visualization in in vivo animals provided a powerful tool for assessing the major indexes associated with the pathogenesis of PAH. In this study, we have exploited the full potential of synchrotron radiation (SR) microangiography to show the ability to visualize pulmonary hemodynamics in a closed-chest mouse. Male adult mice were anesthetized and cannulated with a customized 24-gauge catheter into the right ventricle via the jugular vein for administering iodine contrast agent. The microangiography was performed on the left lung. We measured dynamic changes in vessel diameter in response to acetylcholine (ACh) and acute exposure to hypoxic gas (10% O2). Moreover, the pulmonary transit time was estimated by the time of contrast agent circulating. We were able to visualize the pulmonary arteries from the left pulmonary artery (LPA) to the third generation of branching (inner diameter <100 μm). ACh and acute hypoxia induced vascular responses chiefly in the second and third branching vessels rather than the LPA and the first branching vessels. The transit time was only 0.83 s. These results demonstrate the effectiveness of SR for visualizing the pulmonary circulation in a closed-chest mouse. Future studies using SR microangiography on specific gene-targeted knockout and transgenic mice will provide new insights into the pathophysiology of pulmonary dysfunction and functional adaptation to survive in hypoxic condition.

Role of Rho-kinase signaling and endothelial dysfunction in modulating blood flow distribution in pulmonary hypertension

Rho-kinase-mediated vasoconstriction and endothelial dysfunction are considered two primary instigators of pulmonary arterial hypertension (PAH). However, their contribution to the adverse changes in pulmonary blood flow distribution associated with PAH has not been addressed. This study utilizes synchrotron radiation microangiography to assess the specific role, and contribution of, Rho-kinase-mediated vasoconstriction and endothelial dysfunction in PAH. Male adult Sprague-Dawley rats were injected with saline (Cont-rats) or monocrotaline (MCT-rats) 3 wk before microangiography was performed on the left lung. We assessed dynamic changes in vessel internal diameter (ID) in response to 1) the Rho-kinase inhibitor fasudil (10 mg/kg iv); or 2) ACh (3 μg · kg−1 · min−1), sodium nitroprusside (SNP, 5 μg · kg−1 · min−1), and Nω-nitro-l-arginine methyl ester (l-NAME, 50 mg/kg iv). We observed that MCT-rats had fewer vessels of the microcirculation compared with Cont-rats. The fundamental result of this study is that fasudil improved pulmonary blood flow distribution and reduced pulmonary pressure in PAH rats, not only by dilating already-perfused vessels (ID > 100 μm), but also by restoring blood flow to vessels that had previously been constricted closed (ID < 100 μm). Endothelium-dependent vasodilation was impaired in MCT-rats primarily in vessels with an ID < 200 μm. Moreover the vasoconstrictor response to l-NAME was accentuated in MCT-rats, but only in the 200- to 300-μm vessels. These results highlight the importance of Rho-kinase-mediated control and endothelial control of pulmonary vascular tone in PAH. Indeed, an effective therapeutic strategy for treating PAH should target both the smooth muscle Rho-kinase and endothelial pathways.