Myocardial oxygenation and transmural lactate metabolism during experimental acute coronary stenosis in pigs

Human cardiac fibroblasts adaptive responses to controlled combined mechanical strain and oxygen changes in vitro

Upon cardiac pathological conditions such as ischemia, microenvironmental changes instruct a series of cellular responses that trigger cardiac fibroblasts-mediated tissue adaptation and inflammation. A comprehensive model of how early environmental changes may induce cardiac fibroblasts (CF) pathological responses is far from being elucidated, partly due to the lack of approaches involving complex and simultaneous environmental stimulation. Here, we provide a first analysis of human primary CF behavior by means of a multi-stimulus microdevice for combined application of cyclic mechanical strain and controlled oxygen tension. Our findings elucidate differential human CFs responses to different combinations of the above stimuli. Individual stimuli cause proliferative effects (PHH3⁺ mitotic cells, YAP translocation, PDGF secretion) or increase collagen presence. Interestingly, only the combination of hypoxia and a simulated loss of contractility (2% strain) is able to additionally induce increased CF release of inflammatory and pro-fibrotic cytokines and matrix metalloproteinases.

DOI:http://dx.doi.org/10.7554/eLife.22847.001

When the supply of oxygen to the heart is reduced, its cells start to die within hours, the heart muscle becomes less able to contract, and the area becomes inflamed. This inflammation is accompanied by an influx of immune cells. It also activates other cells known as cardiac fibroblasts that help to break down the framework of molecules that supported the damaged heart tissue and replace it with a scar. This response is part of the normal repair process, but it can lead to the formation of scar tissue in non-damaged areas of the heart. Excess scar tissue makes the heart muscle less able to contract and increases the affected individual’s chance of dying.

Understanding how this repair process works is an important step in developing strategies to minimise the damage caused by coronary artery disease or heart attacks. However, existing laboratory models are only partly able to recreate the conditions seen in real heart tissue. To properly understand the response at the level of living cells, a more complete model is needed.

Ugolini et al. now report improvements to a small device, referred to as a lab-on-chip, that can subject cells to mechanical strain. The improvements mean the device could also recreate other conditions seen early on in damaged heart tissue, specifically the reduced supply of oxygen. Replicating combinations of mechanical changes and oxygen supplies meant that the impact of these conditions on human cardiac fibroblasts could be directly observed in the laboratory for the first time. Ugolini et al. found that a lack of contraction and low oxygen levels triggered the cardiac fibroblasts to produce inflammatory molecules and molecules associated with the formation of scar tissue. This resembles the response seen in living hearts.

The next step is to improve the lab-on-chip device further by adding other cell types, including heart muscle cells and immune cells. A more complete model may aid future research into how our hearts operate in both health and disease.

DOI:http://dx.doi.org/10.7554/eLife.22847.002

Detection of moderate regional ischemia in pig hearts in vivo by near-infrared and thermal imaging: effects of dipyridamole

Effects of coronary vasodilator, dipyridamole, on epicardial oxygenation and flow were investigated under conditions of moderate coronary occlusion using near-infrared spectroscopic (NIRS) and thermal imaging. In anesthetized open chest pigs an inflatable occluder and flow probe were placed around the left anterior descending artery (LAD). In the ischemic group (n = 11) LAD occlusion (50% flow, 80 min) was followed by complete occlusion (10 min, n = 4), and reflow. Dipyridamole was infused (0.14 mg/min/kg/4 min) intravenously during 50% occlusion. In the control group (n = 6) LAD flow was temporarily increased (hyperemic response) by two 2-min periods of complete LAD occlusion applied 120 min apart, with a 4-min period of dipyridamole infusion between the two occlusions. NIRS and thermal images were acquired throughout the protocol. Maps of subepicardial oxygen saturation parameter (OSP), and epicardial temperature (T) were obtained. Partial occlusion reduced OSP and the temperature by 0.23 +/- 0.08 and 0.88 +/- 0.39 degrees C versus remote region, respectively. Dipyridamole decreased systolic blood pressure by 36%, which caused further decline in the LAD flow to 18% and OSP and T by 0.37 +/- 0.01 and 2.46 +/- 0.32 degrees C, respectively. Reflow restored OSP and T to their baseline levels. In control group dipyridamole and hyperemia increased LAD flow 2-4-fold associated with moderate increase in OSP and T. OSP and T showed linear dependence on the flow below 100%, which is leveled-off at flows above normal. Dipyridamole increases differences in the epicardial oxygenation and T between normal and moderately ischemic areas due to enhancement of disparity in perfusion of these areas.

Vectorcardiographic ST deviations related to increased heart rate in the absence of ischemia in an experimental pig model

The electrocardiographic ST segment may change when heart rate (HR) increases. We aimed to analyze vectorcardiographic ST relation and myocardial conditions during controlled HR increases in anesthetized pigs. The relative parameters ST change vector magnitude and ST change vector angle were calculated at paced HRs ranging from 85 to 175 beats per minute. ST change vector magnitude increased from baseline 6.3 +/- 1.3 to 26.0 +/- 3.1 microV (P < .01; range, 4-50 microV) at HR 175 beats per minute with similar changes in ST change vector angle, whereas the absolute parameter ST vector magnitude demonstrated a heterogeneous pattern without any systematic relation to HR changes. Microdialysis results from left ventricular wall, with analysis of glucose, lactate, and pyruvate, showed no sign of ischemia during pacing. Potassium concentrations did not change during pacing. We conclude that significant HR-related ST vector changes can occur in the absence of myocardial ischemia.

Fast spectroscopic imaging using online optimal sparsek-space acquisition and projections onto convex sets reconstruction

Long acquisition times, low resolution, and voxel contamination are major difficulties in the application of magnetic resonance spectroscopic imaging (MRSI). To overcome these difficulties, an online-optimized acquisition of k-space, termed sequential forward array selection (SFAS), was developed to reduce acquisition time without sacrificing spatial resolution. A 2D proton MRSI region of interest (ROI) was defined from a scout image and used to create a region of support (ROS) image. The ROS was then used to optimize and obtain a subset of k-space (i.e., a subset of nonuniform phase encodings) and hence reduce the acquisition time for MRSI. Reconstruction and processing software was developed in-house to process and reconstruct MRSI using the projections onto convex sets method. Phantom and in vivo studies showed that good-quality MRS images are obtainable with an approximately 80% reduction of data acquisition time. The reduction of the acquisition time depends on the area ratio of ROS to FOV (i.e., the smaller the ratio, the greater the time reduction). It is also possible to obtain higher-resolution MRS images within a reasonable time using this approach. MRSI with a resolution of 64 x 64 is possible with the acquisition time of the same as 24 x 24 using the traditional full k-space method.

Characterization of perceived hyperoxia in isolated primary cardiac fibroblasts and in the reoxygenated heart

Under normoxic conditions, pO2 ranges from 90 to <3 torr in mammalian organs with the heart at approximately 35 torr (5%) and arterial blood at approximately 100 torr. Thus, "normoxia" for cells is an adjustable variable. In response to chronic moderate hypoxia, cells adjust their normoxia set point such that reoxygenation-dependent relative elevation of pO2 results in perceived hyperoxia. We hypothesized that O2, even in marginal relative excess of the pO2 to which cells are adjusted, results in the activation of specific O2-sensitive signal transduction pathways that alter cellular phenotype and function. Thus, reperfusion causes damage to the tissue at the focus of ischemia while triggering remodeling in the peri-infarct region by means of perceived hyperoxia. We reported first evidence demonstrating that perceived hyperoxia triggers the differentiation of cardiac fibroblasts (CF) to myofibroblasts by a p21-dependent mechanism (Roy, S., Khanna, S., Bickerstaff, A. A., Subramanian, S. V., Atalay, M., Bierl, M., Pendyala, S., Levy, D., Sharma, N., Venojarvi, M., Strauch, A., Orosz, C. G., and Sen, C. K. (2003) Circ. Res. 92, 264-271). Here, we sought to characterize the genomic response to perceived hyperoxia in CF using GeneChips trade mark. Candidate genes were identified, confirmed and clustered. Cell cycle- and differentiation-associated genes represented a key target of perceived hyperoxia. Bioinformatics-assisted pathway reconstruction revealed the specific signaling processes that were sensitive to perceived hyperoxia. To test the significance of our in vitro findings, a survival model of rat heart focal ischemia-reperfusion (I-R) was investigated. A significant induction in p21 mRNA expression was observed in I-R tissue. The current results provide a comprehensive molecular definition of perceived hyperoxia in cultured CF. Furthermore, the first evidence demonstrating activation of perceived hyperoxia sensitive genes in the cardiac I-R tissue is presented.

Functional effects of acute coronary occlusion and catecholinergic stimuli on the isolated normothermic hemoperfused porcine heart

The aim of this study was to characterize functional parameters in the isolated and normothermic hemoperfused porcine beating heart model after pathophysiological stimuli for extended perfusion periods. Hearts were prepared and connected to a specially developed perfusion equipment, which simultaneously allowed perfusion with warm autologous blood as well as blood dialysis. Two groups were established: group A (12 hearts: no intervention) and group B (6 hearts: occlusion of the ramus circumflexus of the left coronary artery for 2 hours). Blood gas analyses and oxymetry were performed at baseline and every 30 min during a 6 hours perfusion period. Coronary perfusion pressure (CPP) and blood flow (CBF), right and left ventricular pressure, blood and dialyzate pH-values, and temperature were monitored online by a microcontroller. A steady state regarding the CPP and the CBF was achieved after 1 hour of perfusion for both groups. In group B, CPP increased during occlusion. Comparison of both groups showed no significant differences in the bicarbonate and sodium levels in blood and dialyzate. The potassium concentration in blood and dialyzate increased in both groups constantly during the experiments. No clear alteration of the oxygen consumption was observed. Lactate levels in blood and dialyzate increased during occlusion as did the aspartataminotransferase (AST) venous levels (both determined only for group B). Four concentrations of norepinephrine were injected into the stem of the coronary arteries (10, 20, 40, 80 microg). A clear inotropic effect of this hormone on right and left ventricular pressure was observed. It was concluded that longer perfusion periods and simulation of myocardial infarction for a clinically relevant period can be performed using this model. In addition, right and left ventricular function appear to be well preserved in this model, since the isolated porcine heart responded to norepinephrine stimuli.

Oxygen sensing by primary cardiac fibroblasts: a key role of p21(Waf1/Cip1/Sdi1)

In mammalian organs under normoxic conditions, O2 concentration ranges from 12% to <0.5%, with O2 approximately 14% in arterial blood and <10% in the myocardium. During mild hypoxia, myocardial O2 drops to approximately 1% to 3% or lower. In response to chronic moderate hypoxia, cells adjust their normoxia set point such that reoxygenation-dependent relative elevation of PO2 results in perceived hyperoxia. We hypothesized that O2, even in marginal relative excess of the PO2 to which cardiac cells are adjusted, results in activation of specific signal transduction pathways that alter the phenotype and function of these cells. To test this hypothesis, cardiac fibroblasts (CFs) isolated from adult murine ventricle were cultured in 10% or 21% O2 (hyperoxia relative to the PO2 to which cells are adjusted in vivo) and were compared with those cultured in 3% O2 (mild hypoxia). Compared with cells cultured in 3% O2, cells that were cultured in 10% or 21% O2 demonstrated remarkable reversible G2/M arrest and a phenotype indicative of differentiation to myofibroblasts. These effects were independent of NADPH oxidase function. CFs exposed to high O2 exhibited higher levels of reactive oxygen species production. The molecular signature response to perceived hyperoxia included (1) induction of p21, cyclin D1, cyclin D2, cyclin G1, Fos-related antigen-2, and transforming growth factor-beta1, (2) lowered telomerase activity, and (3) activation of transforming growth factor-beta1 and p38 mitogen-activated protein kinase. CFs deficient in p21 were resistant to such O2 sensitivity. This study raises the vital broad-based issue of controlling ambient O2 during the culture of primary cells isolated from organs.

Fast kappa-space sample selection in MRSI with a limited region of support

One of the primary drawbacks in the application of magnetic resonance spectroscopic imaging is the long acquisition times required to obtained the desired resolution. When region of support information is available, the number of phase-encoding steps and thus time can be reduced without loss of information if the kappa-space locations are chosen well. We propose to select locations using a rectangular sampling array that is shifted to various positions in kappa-space to obtain the necessary sampling density. This method allows multiple samples to be selected simultaneously and reduces the computation required to evaluate the selection criterion. We present an efficient forward selection algorithm for optimizing the shift pattern so that the image can be reconstructed as reliably as possible from a periodic nonuniform set of samples. The proposed algorithm has important practical potential in that it can finish the selection in less than half a minute for typical image sizes and can reconstruct the image with fewer samples than regular sampling. With appropriate imaging hardware, this new algorithm makes selective sampling possible in a real-time image acquisition setting.