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Depuis des siècles, la R.D. Congo est victime des agressions de la part de ses voisins de l’Est. En effet, en 2006, lors de ses cent neuvième congrès (S2125), les Etats-Unis d’Amérique, dans ses découvertes, ont remarqué que dans leurs stratégies nationales, en Afrique, et surtout en R.D. Congo, il y a trop de promesses et d’opportunités et pourtant, la maladie, la guerre et la misère de la pauvreté évoluent sans arrêt. De ce qui précède, cet article est une contribution à la recherche des voies et moyens de restaurer la paix et de rétablir la sécurité dans l’Est de la R.D. Congo. A cet effet, il envisage de démontrer que sans les efforts pertinents et concertés des Congolais, le développement sera toujours loin de se concrétiser et le conflit ou guerre continuera son bonhomme de chemin. C’est pourquoi, tout au long de notre cheminement, l’approche de politique publique, accompagnée des techniques d’observation, documentaire et d’interview nous serviront de guide. Cette recherche se structure au tour des deux chapitres : (i) le contexte de l’étude et la notion de sécurité et de recrutement dans l’armée nationale et, (ii) le modèle attrayant de recrutement des hommes dignes et crédibles dans les FARDC.

As injection drug users (IDUs) continue to be infected by HIV, an important component of prevention efforts is the identification of factors that are associated with utilization of HIV testing programs. The purpose of this study was to identify correlates of HIV testing within the past 12 months among IDUs. A sample of 558 IDUs, recruited from street-based and community venues, completed a cross-sectional survey. Multivariate logistic regression indicates factors independently associated with having been tested recently, including social structural factors such as being incarcerated, having interactions with an HIV prevention outreach worker, and having used a case manager. Furthermore, main sex partner's testing status was positively associated with recent testing. Support of programs that are designed to serve hard-to-reach populations and target couples may be effective in increasing HIV testing among active IDUs. [ABSTRACT FROM AUTHOR]

We examined the relationship between religious behavior and HIV antibody testing, serostatus, and receiving medical care among a sample of inner-city former and current injection-drug users. Recency of church attendance, guidance from religion, and religious denomination were used as indicators for religious behavior. More recent church attendance was significantly associated with HIV testing, HIV-positive serostatus, and receiving medical care for HIV in multivariate regression models, even after adjusting for possible confounders. The findings suggest that the church may be an important source of support and social regulation among HIV-infected inner-city African-American drug users. Further studies examining how the church serves as a mechanism for HIV testing and medical care are warranted. [ABSTRACT FROM AUTHOR]

Until recently, the Rwanda power sector increased rapidly to double the 2010 installed capacity. The energy consumption in Rwanda experienced a steady rise correspondingly with the population and modern socio-economic life. Consequently, Rwanda household access to electricity increased to 53% by September 2019. Not only does 47% of Rwanda's population lack electricity access, there are persistent power failures and the grid is also unstable. Using renewable energy hybrid technologies in off-grid areas might be a solution to this problem. However, the high cost of renewable energy hybrid systems has led to its slow adoption in many developing countries. Hence, it is important to find the most appropriate hybrid combinations that reduce energy cost and access electricity generation that maximizes the available renewable energy resources. This paper examines some new technology development needs related to the power sector in Rwanda. Secondly, four different 100% renewable energy hybrid systems were designed and simulated to support rural and remote areas considering an average load demand of 158.1 kWh/day with a peak load of 18 kW. The hybrid systems simulation and optimization were obtained using HOMER (hybrid optimization model for electric renewables) software. The input data were obtained from National Aeronautics and Space Administration (NASA) for solar and wind resources, and hydro resources were from real-time field data for selected study site. The simulation results indicate hydro/solar/battery hybrid is the most cost-effective and environmentally viable alternative for off-grid rural electrification because of low net present cost (NPC) and least greenhouse gas emissions. The proposed hybrid combination could apply to other rural areas in the region and elsewhere in the world especially where climate conditions are similar.

The original publication of the article contained the following errors.

The European CORE-MD consortium (Coordinating Research and Evidence for Medical Devices) proposes a score for medical devices incorporating artificial intelligence or machine learning algorithms. Its domains are summarised as valid clinical association, technical performance, and clinical performance. High scores indicate that extensive clinical investigations should be undertaken before regulatory approval, whereas lower scores indicate devices for which less pre-market clinical evaluation may be balanced by more post-market evidence., Competing Interests: Competing interests: F.R.: Occasional travel and accommodation costs to CORE–MD meetings were supported within the EU Horizon 2020 grant budget (2022-204); senior associate editor European Heart Journal, Cardiovascular Imaging. E.B.: external collaborative expert at the European Medicine Agency between 2023 and 2024 (unpaid collaboration); Occasional travel and accommodation costs to CORE–MD meetings were supported within the EU Horizon 2020 grant budget (2022-204). N.B.: Honorarium for presentation at Great Wall Cardiac Congress; ESC paid Article Processing Costs for one article in the European Heart Journal – Digital Health; Heart Rhythm Society paid travel costs for presentation at HRX; ESC Vice-chair Digital Health Committee; Editor-in-Chief of European Heart Journal – Digital Health. E.C.: Research contract with Luxottica S.p.A, Payment to my institution; Research contract with Croce Rossa Italiana, Payment to my institution; Project evaluator for the Autonomous Province of Trento, Italy; Honoraria for presentations from Dynamicom Education Srl, Summeet Srl, AIM Italy S.r.l.; Member of the Regulatory Affairs committee of the European Society of Cardiology. R.D.: minority shareholder in myocardium.AI; Consulting fees were paid directly to him. The company develops AI products for cardiac MR image analysis and is in the process of applying for regulatory approval. S.G.: German Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung, BMBF) through the European Union-financed NextGenerationEU programme under grant number 16KISA100K, project PATH—‘Personal Mastery of Health and Wellness Data’. The European Commission under the Horizon Europe Programme, as part of the projects CYMEDSEC (101094218) and ASSESS-DHT (101137347). S.G. has or has had consulting relationships with Una Health GmbH, Lindus Health Ltd.; Flo Ltd, Thymia Ltd., FORUM Institut für Management GmbH, High-Tech Gründerfonds Management GmbH, Ada Health GmbH, and he holds share options in Ada Health GmbH. Support and honoraria for TK Health Economics Forum at MEDICA. Advisory Group member of the EY-coordinated ‘Study on Regulatory Governance and Innovation in the Field of Medical Devices’ conducted on behalf of the DG SANTE of the European Commission. Share options in Ada Health GmbH. S.G. is a News and Views Editor for npj Digital Medicine. S.G. played no role in the internal review or decision to publish this article. G.M.: Occasional travel and accommodation costs to CORE–MD meetings were supported within the EU Horizon 2020 grant budget (2022-204). Health Products Regulatory Authority, Source of employment. G.O.: Health Products Regulatory Authority, Source of employment. J.B.R.: none. B.V.: Berrow Foundation Lord Florey scholarship. A.G.: Chair, Regulatory Affairs Committee, Biomedical Alliance in Europe., (© 2025. The Author(s).)

Mycobacterium tuberculosis ( Mtb ) is the world's most deadly infectious pathogen and new drugs are urgently required to combat the emergence of multi- (MDR) and extensively- (XDR) drug resistant strains. The bacterium specifically upregulates sterol uptake pathways in infected macrophages and the metabolism of host-derived cholesterol is essential for Mtb's long-term survival in vivo. Here, we report the development of antitubercular small molecules that inhibit the Mtb cholesterol oxidases CYP125 and CYP142, which catalyze the initial step of cholesterol metabolism. An efficient biophysical fragment screen was used to characterize the structure-activity relationships of CYP125 and CYP142, and identify a non-azole small molecule 1a that can bind to the heme cofactor of both enzymes. A structure-guided fragment-linking strategy was used to optimize the binding affinity of 1a , yielding a potent dual CYP125/142 inhibitor 5m (K D CYP125/CYP142 = 0.04/0.16 μM). Compound 5m potently inhibits the catalytic activity of CYP125 and CYP142 in vitro (K I values < 0.1 μM), and rapidly depletes Mtb intracellular ATP (IC 50 = 0.15 μM). The compound has antimicrobial activity against both drug susceptible and MDR Mtb ( MIC 99 values 0.4 - 1.5 μM ) in extracellular assays, and inhibits the growth of Mtb in human macrophages (MIC = 1.7 μM) with good selectivity over mammalian cytotoxicity (LD 50 ≥ 50 μM). The combination of small molecule inhibitors and structural data reported here provide useful tools to study the role of cholesterol metabolism in Mtb and are a promising step towards novel antibiotics targeting bioenergetic pathways, which could be used to help combat MDR-TB.

Introduction: Artificial intelligence (AI) encompasses a wide range of algorithms with risks when used to support decisions about diagnosis or treatment, so professional and regulatory bodies are recommending how they should be managed., Areas Covered: AI systems may qualify as standalone medical device software (MDSW) or be embedded within a medical device. Within the European Union (EU) AI software must undergo a conformity assessment procedure to be approved as a medical device. The draft EU Regulation on AI proposes rules that will apply across industry sectors, while for devices the Medical Device Regulation also applies. In the CORE-MD project (Coordinating Research and Evidence for Medical Devices), we have surveyed definitions and summarize initiatives made by professional consensus groups, regulators, and standardization bodies., Expert Opinion: The level of clinical evidence required should be determined according to each application and to legal and methodological factors that contribute to risk, including accountability, transparency, and interpretability. EU guidance for MDSW based on international recommendations does not yet describe the clinical evidence needed for medical AI software. Regulators, notified bodies, manufacturers, clinicians and patients would all benefit from common standards for the clinical evaluation of high-risk AI applications and transparency of their evidence and performance.

Introduction: Due to a global shortage of healthcare workers, there is a lack of basic healthcare for 4 billion people worldwide, particularly affecting low-income and middle-income countries. The utilisation of AI-based healthcare tools such as symptom assessment applications (SAAs) has the potential to reduce the burden on healthcare systems. The purpose of the AFYA Study (AI-based Assessment oF health sYmptoms in TAnzania) is to evaluate the accuracy of the condition suggestions and urgency advice provided by a user on a Swahili language Ada SAA., Methods and Analysis: This study is designed as an observational prospective clinical study. The setting is a waiting room of a Tanzanian district hospital. It will include patients entering the outpatient clinic with various conditions and age groups, including children and adolescents. Patients will be asked to use the SAA before proceeding to usual care. After usual care, they will have a consultation with a study-provided physician. Patients and healthcare practitioners will be blinded to the SAA's results. An expert panel will compare the Ada SAA's condition suggestions and urgency advice to usual care and study provided differential diagnoses and triage. The primary outcome measures are the accuracy and comprehensiveness of the Ada SAA evaluated against the gold standard differential diagnoses., Ethics and Dissemination: Ethical approval was received by the ethics committee (EC) of Muhimbili University of Health and Allied Sciences with an approval number MUHAS-REC-09-2019-044 and the National Institute for Medical Research, NIMR/HQ/R.8c/Vol. I/922. All amendments to the protocol are reported and adapted on the basis of the requirements of the EC. The results from this study will be submitted to peer-reviewed journals, local and international stakeholders, and will be communicated in editorials/articles by Ada Health., Trial Registration Number: NCT04958577., Competing Interests: Competing interests: EM, HA, LO, MS, PB, ET and SG are employees or company directors of Ada Health GmbH and some of the listed hold stock options in the company. RV is a former employee of Ada Health GmbH. HA is a director of the Ada Health Foundation gGmbH. The Ada Health GmbH research team has received research grant funding from Foundation Botnar and the Bill & Melinda Gates Foundation., (© Author(s) (or their employer(s)) 2022. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)

Objective: The aim of the study is to compare structure tensor imaging (STI) with diffusion tensor imaging (DTI) of the sheep heart (approximately the same size as the human heart)., Materials and Methods: MRI acquisition on three sheep ex vivo hearts was performed at 9.4 T/30 cm with a seven-element RF coil. 3D FLASH with an isotropic resolution of 150 µm and 3D spin-echo DTI at 600 µm were performed. Tensor analysis, angles extraction and segments divisions were performed on both volumes., Results: A 3D FLASH allows for visualization of the detailed structure of the left and right ventricles. The helix angle determined using DTI and STI exhibited a smooth transmural change from the endocardium to the epicardium. Both the helix and transverse angles were similar between techniques. Sheetlet organization exhibited the same pattern in both acquisitions, but local angle differences were seen and identified in 17 segments representation., Discussion: This study demonstrated the feasibility of high-resolution MRI for studying the myocyte and myolaminar architecture of sheep hearts. We presented the results of STI on three whole sheep ex vivo hearts and demonstrated a good correspondence between DTI and STI., (© 2021. The Author(s).)

High diastereoselectivity was observed in the Rh-catalysed reductive amination of 3-arylcyclohexanones to form tertiary amines. This was incorporated into a one-pot enantioselective conjugate addition and diastereoselective reductive amination, including an example of assisted tandem catalysis.

The U.S. Food and Drug Administration Center for Devices and Radiological Health (FDA/CDRH) has recently published several in vivo test guidance documents that mention refinements, reductions, or replacement animal testing strategies to facilitate the leveraging of data from large animal safety tests for conventional rodent testing. In response to the recently enacted Food and Drug Administration Safety and Innovation Act Section 907, which facilitates expedited access to novel therapies commonly described as Breakthrough Therapy Designation, FDA/CDRH has discussed efficient regulatory strategies for first-in-human investigation, including early feasibility study guidance. Large gains in humane care and translational research could also be attained by examples in FDA's Guidance for the Use of International Organization for Standardization 10993-1, which states that large animal safety studies may be considered as replacement rodent tests if the scientific principles, methods, and end points (SPME) are considered and applied. This article discusses SPME for the replacement of conventional rodent testing by the inclusion and integration of clinical, diagnostic, and pathologic data obtained from well-designed large animal studies. The recommendations include consideration for study designs that utilize methods for an overall more comprehensive interrogation of animal systems.

Background: Ventricular arrhythmias are frequent in patients with repaired tetralogy of Fallot (rTOF), but their origin and underlying mechanisms remain unclear. In this study, the involvement of left ventricular (LV) electrical and structural remodeling was assessed in an animal model mimicking rTOF sequelae., Methods: Piglets underwent a tetralogy of Fallot repair-like surgery (n=6) or were sham operated (Sham, n=5). Twenty-three weeks post-surgery, cardiac function was assessed in vivo by magnetic resonance imaging. Electrophysiological properties were characterized by optical mapping. LV fibrosis and connexin-43 localization were assessed on histological sections and protein expression assessed by Western Blot., Results: Right ventricular dysfunction was evident, whereas LV function remained unaltered in rTOF pigs. Optical mapping showed longer action potential duration on the rTOF LV epicardium and endocardium. Epicardial conduction velocity was significantly reduced in the longitudinal direction in rTOF LVs but not in the transverse direction compared with Sham. An elevated collagen content was found in LV basal and apical sections from rTOF pigs. Moreover, a trend for connexin-43 lateralization with no change in protein expression was found in the LV of rTOFs. Finally, rTOF LVs had a lower threshold for arrhythmia induction using incremental pacing protocols., Conclusions: We found an arrhythmogenic substrate with prolonged heterogeneous action potential duration and reduced conduction velocity in the LV of rTOF pigs. This remodeling precedes LV dysfunction and is likely to contribute to ventricular arrhythmias and sudden cardiac death in patients with rTOF.

Objective: The growing adult population with surgically corrected tetralogy of Fallot (TOF) is at risk of arrhythmias and sudden cardiac death. We sought to investigate the contribution of right ventricular (RV) structural and electrophysiological remodelling to arrhythmia generation in a preclinical animal model of repaired TOF (rTOF)., Methods and Results: Pigs mimicking rTOF underwent cardiac MRI functional characterisation and presented with pulmonary regurgitation, RV hypertrophy, dilatation and dysfunction compared with Sham-operated animals (Sham). Optical mapping of rTOF RV-perfused wedges revealed a significant prolongation of RV activation time with slower conduction velocities and regions of conduction slowing well beyond the surgical scar. A reduced protein expression and lateralisation of Connexin-43 were identified in rTOF RVs. A remodelling of extracellular matrix-related gene expression and an increase in collagen content that correlated with prolonged RV activation time were also found in these animals. RV action potential duration (APD) was prolonged in the epicardial anterior region at early and late repolarisation level, thus contributing to a greater APD heterogeneity and to altered transmural and anteroposterior APD gradients in rTOF RVs. APD remodelling involved changes in Kv4.3 and MiRP1 expression. Spontaneous arrhythmias were more frequent in rTOF wedges and more complex in the anterior than in the posterior RV., Conclusion: Significant remodelling of RV conduction and repolarisation properties was found in pigs with rTOF. This remodelling generates a proarrhythmic substrate likely to facilitate re-entries and to contribute to sudden cardiac death in patients with rTOF., (Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://www.bmj.com/company/products-services/rights-and-licensing/.)

The cyclo-dipeptide substrates of the essential M. tuberculosis (Mtb) enzyme CYP121 were deconstructed into their component fragments and screened against the enzyme. A number of hits were identified, one of which exhibited an unexpected inhibitor-like binding mode. The inhibitory pharmacophore was elucidated, and fragment binding affinity was rapidly improved by synthetic elaboration guided by the structures of CYP121 substrates. The resulting inhibitors have low micromolar affinity, good predicted physicochemical properties and selectivity for CYP121 over other Mtb P450s. Spectroscopic characterisation of the inhibitors' binding mode provides insight into the effect of weak nitrogen-donor ligands on the P450 heme, an improved understanding of factors governing CYP121-ligand recognition and speculation into the biological role of the enzyme for Mtb., (© 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Avoided resonance crossings (ARC) in plasmonic nanodisk structures due to near field or far field couplings were numerically demonstrated. Near field coupling in disk dimmer with both vertical or side-by-side arrangement leads to both energy and linewidth anti-crossing by varying one disk size across the other. Far field coupling in double layered disk arrays of extremely small gap size or gap size with Fabry Perot resonant condition close to the frequency selective surface (FSS) stopband center leads to non-reciprocal absorption spectrum as one disk size varying across the other. We observe linewidth anti-crossing but energy crossing of the absorption peak from different side illumination by varying either the size of one disk array or the gap in hetero disk arrays. The disappearing of Fabry-Perot resonant mode from one side illumination and the appearing of nonreciprocal nearly perfect absorption from the other side illumination are well explained by a FSS-Fabry-Perot model.

Mathematical modeling of excitation-contraction coupling (ECC) in ventricular cardiac myocytes is a multiscale problem, and it is therefore difficult to develop spatially detailed simulation tools. ECC involves gradients on the length scale of 100 nm in dyadic spaces and concentration profiles along the 100 μm of the whole cell, as well as the sub-millisecond time scale of local concentration changes and the change of lumenal Ca(2+) content within tens of seconds. Our concept for a multiscale mathematical model of Ca(2+) -induced Ca(2+) release (CICR) and whole cardiomyocyte electrophysiology incorporates stochastic simulation of individual LC- and RyR-channels, spatially detailed concentration dynamics in dyadic clefts, rabbit membrane potential dynamics, and a system of partial differential equations for myoplasmic and lumenal free Ca(2+) and Ca(2+)-binding molecules in the bulk of the cell. We developed a novel computational approach to resolve the concentration gradients from dyadic space to cell level by using a quasistatic approximation within the dyad and finite element methods for integrating the partial differential equations. We show whole cell Ca(2+)-concentration profiles using three previously published RyR-channel Markov schemes.

Surgical repair of Tetralogy of Fallot (TOF) is highly successful but may be complicated in adulthood by arrhythmias, sudden death, and right ventricular or biventricular dysfunction. To better understand the molecular and cellular mechanisms of these delayed cardiac events, a chronic animal model of postoperative TOF was studied using microarrays to perform cardiac transcriptomic studies. The experimental study included 12 piglets (7 rTOF and 5 controls) that underwent surgery at age 2 months and were further studied after 23 (+/- 1) weeks of postoperative recovery. Two distinct regions (endocardium and epicardium) from both ventricles were analyzed. Expression levels from each localization were compared in order to decipher mechanisms and signaling pathways leading to ventricular dysfunction and arrhythmias in surgically repaired TOF. Several genes were confirmed to participate in ventricular remodeling and cardiac failure and some new candidate genes were described. In particular, these data pointed out FRZB as a heart failure marker. Moreover, calcium handling and contractile function genes (SLN, ACTC1, PLCD4, PLCZ), potential arrhythmia-related genes (MYO5B, KCNA5), and cytoskeleton and cellular organization-related genes (XIRP2, COL8A1, KCNA6) were among the most deregulated genes in rTOF ventricles. To our knowledge, this is the first comprehensive report on global gene expression profiling in the heart of a long-term swine model of repaired TOF.

Background: Cardiovascular magnetic resonance (CMR) can through the two methods 3D FLASH and diffusion tensor imaging (DTI) give complementary information on the local orientations of cardiomyocytes and their laminar arrays., Methods: Eight explanted rat hearts were perfused with Gd-DTPA contrast agent and fixative and imaged in a 9.4T magnet by two types of acquisition: 3D fast low angle shot (FLASH) imaging, voxels 50 × 50 × 50 μm, and 3D spin echo DTI with monopolar diffusion gradients of 3.6 ms duration at 11.5 ms separation, voxels 200 × 200 × 200 μm. The sensitivity of each approach to imaging parameters was explored., Results: The FLASH data showed laminar alignments of voxels with high signal, in keeping with the presumed predominance of contrast in the interstices between sheetlets. It was analysed, using structure-tensor (ST) analysis, to determine the most (v1(ST)), intermediate (v2(ST)) and least (v3(ST)) extended orthogonal directions of signal continuity. The DTI data was analysed to determine the most (e1(DTI)), intermediate (e2(DTI)) and least (e3(DTI)) orthogonal eigenvectors of extent of diffusion. The correspondence between the FLASH and DTI methods was measured and appraised. The most extended direction of FLASH signal (v1(ST)) agreed well with that of diffusion (e1(DTI)) throughout the left ventricle (representative discrepancy in the septum of 13.3 ± 6.7°: median ± absolute deviation) and both were in keeping with the expected local orientations of the long-axis of cardiomyocytes. However, the orientation of the least directions of FLASH signal continuity (v3(ST)) and diffusion (e3(ST)) showed greater discrepancies of up to 27.9 ± 17.4°. Both FLASH (v3(ST)) and DTI (e3(DTI)) where compared to directly measured laminar arrays in the FLASH images. For FLASH the discrepancy between the structure-tensor calculated v3(ST) and the directly measured FLASH laminar array normal was of 9 ± 7° for the lateral wall and 7 ± 9° for the septum (median ± inter quartile range), and for DTI the discrepancy between the calculated v3(DTI) and the directly measured FLASH laminar array normal was 22 ± 14° and 61 ± 53.4°. DTI was relatively insensitive to the number of diffusion directions and to time up to 72 hours post fixation, but was moderately affected by b-value (which was scaled by modifying diffusion gradient pulse strength with fixed gradient pulse separation). Optimal DTI parameters were b = 1000 mm/s(2) and 12 diffusion directions. FLASH acquisitions were relatively insensitive to the image processing parameters explored., Conclusions: We show that ST analysis of FLASH is a useful and accurate tool in the measurement of cardiac microstructure. While both FLASH and the DTI approaches appear promising for mapping of the alignments of myocytes throughout myocardium, marked discrepancies between the cross myocyte anisotropies deduced from each method call for consideration of their respective limitations.

Objective: To develop a new method for the cardiac MR (CMR) quantification of peri-infarct ischaemia using fused perfusion and delayed-enhanced images and to evaluate this method using quantitative single photon emission CT (SPECT) imaging as a reference., Methods: 40 patients presenting with peri-infarct ischaemia on a routine stress (99m)Tc-SPECT imaging were recruited. Within 8 days of the SPECT study, myocardial perfusion was evaluated using stress adenosine CMR. Using fused perfusion and delayed-enhanced images, peri-infarct ischaemia was quantified as the percentage of myocardium with stress-induced perfusion defect that was adjacent to and larger than a scar. This parameter was compared with both the percent myocardium ischaemia (SD%) and the ischaemic total perfusion deficit (TPD). The diagnostic performance of CMR in detection of significant coronary artery stenosis (of ≥70%) was also determined., Results: On SPECT imaging, in addition to peri-infarct ischaemia, reversible perfusion abnormalities were detected in a remote zone in seven patients. In the 33 patients presenting with only peri-infarct ischaemia, the agreement between CMR peri-infarct ischaemia and both SD% and ischaemic TPD was excellent [intraclass coefficient of correlation (ICC) = 0.969 and ICC = 0.877, respectively]. CMR-defined peri-infarct ischaemia for the detection of a significant coronary artery stenosis showed an areas under receiver-operating characteristic curve of 0.856 (95% confidence interval, 0.680-0.939). The best cut-off value was 8.1% and allowed a 72% sensitivity, 96% specificity, 60% negative predictive value and 97% positive predictive value., Conclusion: This proof-of-concept study shows that CMR imaging has the potential as a test for quantification of peri-infarct ischaemia., Advances in Knowledge: This study demonstrates the proof of concept of a commonly known intuitive idea, that is, evaluating the peri-infarct ischaemic burden by subtracting delayed enhancement from first-pass perfusion imaging on CMR.

A series of synthetic analogues of 1-D-(2-amino-2-deoxy-α-D-glucopyranosyl)-myo-inositol 1-(1,2-di-O-hexadecanoyl-sn-glycerol 3-phosphate), consisting of 7 variants of either the D-myo-inositol, D-GlcpN or the phospholipid components, were prepared and tested as substrates and inhibitors of GlcNAc-PI de-N-acetylase, a genetically validated drug target enzyme responsible for the second step in the glycosylphosphatidylinositol (GPI) biosynthetic pathway of Trypanosoma brucei. The D-myo-inositol in the physiological substrate was successfully replaced by cyclohexanediol and is still a substrate for T. brucei GlcNAc-PI de-N-acetylase. However, this compound became sensitive to the stereochemistry of the glycoside linkage (the β-anomer was neither substrate or inhibitor) and the structure of the lipid moiety (the hexadecyl derivatives were inhibitors). Chemistry was successfully developed to replace the phosphate with a sulphonamide, but the compound was neither a substrate or an inhibitor, confirming the importance of the phosphate for molecular recognition. We also replaced the glucosamine by an acyclic analogue, but this also was inactive, both as a substrate and inhibitor. These findings add significantly to our understanding of substrate and inhibitor binding to the GlcNAc-PI de-N-acetylase enzyme and will have a bearing on the design of future inhibitors.

Pathological cardiac fibrosis is important in predisposing the heart to arrhythmia and mechanical failure. The regional distribution of fibrosis is often described qualitatively and quantitatively in histological studies of animal hearts after staining collagen with specific colored stains. Currently this description is often piecemeal, as it lacks rigorous spatial registration, matching and methodological standardization between animals and between study groups. We propose a strategy for the quantification of regional fibrosis using the American Heart Association (AHA) cardiac segmentation model. We quantify fibrosis after whole heart 3D histological reconstruction in one normal rat heart and in one rat heart in right heart failure induced by monocrotaline. We then assess the minimum spaced histological sampling which allows for accurate assessment of regional fibrosis. We show that using every section of a set of 5 μm serial sections quantifies regional right ventricular fibrosis, with highly significant (p <; 0.001) differences between heart failure and control hearts. We show that the absolute error of collagen quantification is low when sections are taken spaced by up to 100 μm (error 5.7±5.8%). Likewise, absolute error associated with sectioning starting position is low for sections spaced up to 100 μm (error 13.3±17.2%). Above 100 μm section spacing quantification error is large (tending to 50%) and error associated with sectioning starting position is large (tending to 60%).

Background: The cardiac conduction system consists of the sinus node, nodal extensions, atrioventricular (AV) node, penetrating bundle, bundle branches, and Purkinje fibers. Node-like AV ring tissue also exists at the AV junctions, and the right and left rings unite at the retroaortic node. The study aims were to (1) construct a 3-dimensional anatomical model of the AV rings and retroaortic node, (2) map electrical activation in the right ring and study its action potential characteristics, and (3) examine gene expression in the right ring and retroaortic node., Methods and Results: Three-dimensional reconstruction (based on magnetic resonance imaging, histology, and immunohistochemistry) showed the extent and organization of the specialized tissues (eg, how the AV rings form the right and left nodal extensions into the AV node). Multiextracellular electrode array and microelectrode mapping of isolated right ring preparations revealed robust spontaneous activity with characteristic diastolic depolarization. Using laser microdissection gene expression measured at the mRNA level (using quantitative PCR) and protein level (using immunohistochemistry and Western blotting) showed that the right ring and retroaortic node, like the sinus node and AV node but, unlike ventricular muscle, had statistically significant higher expression of key transcription factors (including Tbx3, Msx2, and Id2) and ion channels (including HCN4, Cav3.1, Cav3.2, Kv1.5, SK1, Kir3.1, and Kir3.4) and lower expression of other key ion channels (Nav1.5 and Kir2.1)., Conclusions: The AV rings and retroaortic node possess gene expression profiles similar to that of the AV node. Ion channel expression and electrophysiological recordings show the AV rings could act as ectopic pacemakers and a source of atrial tachycardia.

Photothermal cancer therapy using near-infrared (NIR) laser radiation is an emerging treatment. In the NIR region, two biological transparency windows are located in 650-950 nm (first NIR window) and 1000-1350 nm (second NIR window) with optimal tissue transmission obtained from low scattering and energy absorption, thus providing maximum radiation penetration through tissue and minimizing autofluorescence. To date, intensive effort has resulted in the generation of various methods that can be used to shift the absorbance of nanomaterials to the 650-950 nm NIR regions for studying photoinduced therapy. However, NIR light absorbers smaller than 100 nm in the second NIR region have been scant. We report that a Au nanorod (NR) can be designed with a rod-in-shell (rattle-like) structure smaller than 100 nm that is tailored to be responsive to the first and second NIR windows, in which we can perform hyperthermia-based therapy. In vitro performance clearly displays high efficacy in the NIR photothermal destruction of cancer cells, showing large cell-damaged area beyond the laser-irradiated area. This marked phenomenon has made the rod-in-shell structure a promising hyperthermia agent for the in vivo photothermal ablation of solid tumors when activated using a continuous-wave 808 m (first NIR window) or a 1064 nm (second NIR window) diode laser. We tailored the UV-vis-NIR spectrum of the rod-in-shell structure by changing the gap distance between the Au NR core and the AuAg nanoshell, to evaluate the therapeutic effect of using a 1064 nm diode laser. Regarding the first NIR window with the use of an 808 nm diode laser, rod-in-shell particles exhibit a more effective anticancer efficacy in the laser ablation of solid tumors compared to Au NRs.

DT-MRI has been widely used to quantify myocardial fiber and laminar orientations. These structural orientations influence both the spread of excitation and the reorganization of the myocardium during contraction and are altered in disease states. Studies have sought to validate DT-MRI but questions remain about the accuracy of the method and its sensitivity to the time post-fixation and imaging parameters, including b-value, number of diffusion directions and image voxel size. The advent of high-spatial resolution ex vivo MRI and structure tensor (ST) analysis provides a means of direct validation of DT-MRI and assessment of sensitivity to the b-value, the number of diffusion directions and the image voxel size. We find that, with the fixation method we used, structure does not change with time (up to 72 hours). We show that DT-MRI and ST/HR-MRI are markedly similar measures of fiber orientation but DT-MRI and ST are much less similar measures of laminar orientation. DT-MRI performance is not sensitive to the number of directions, with similar structural orientations measured with 6 or 12 directions. Likewise, DT-MRI performance is generally insensitive to b-value, but laminar measurement is moderately more accurate at b = 500 than for higher b-values.

Micro-computed tomography (micro-CT) has been widely used to generate high-resolution 3-D tissue images from small animals nondestructively, especially for mineralized skeletal tissues. However, its application to the analysis of soft cardiovascular tissues has been limited by poor inter-tissue contrast. Recent ex vivo studies have shown that contrast between muscular and connective tissue in micro-CT images can be enhanced by staining with iodine. In the present study, we apply this novel technique for imaging of cardiovascular structures in canine hearts. We optimize the method to obtain high-resolution X-ray micro-CT images of the canine atria and its distinctive regions-including the Bachmann's bundle, atrioventricular node, pulmonary arteries and veins-with clear inter-tissue contrast. The imaging results are used to reconstruct and segment the detailed 3-D geometry of the atria. Structure tensor analysis shows that the arrangement of atrial fibers can also be characterized using the enhanced micro-CT images, as iodine preferentially accumulates within the muscular fibers rather than in connective tissues. This novel technique can be particularly useful in nondestructive imaging of 3-D cardiac architectures from large animals and humans, due to the combination of relatively high speed ( ~ 1 h/per scan of the large canine heart) and high voxel resolution (36 μm) provided. In summary, contrast micro-CT facilitates fast and nondestructive imaging and segmenting of detailed 3-D cardiovascular geometries, as well as measuring fiber orientation, which are crucial in constructing biophysically detailed computational cardiac models.

Computational models of human atrial cells, tissues and atria have been developed. Cell models, for atrial wall, crista terminalis, appendage, Bachmann's bundle and pectinate myocytes are characterised by action potentials, ionic currents and action potential duration (APD) restitution. The principal effect of the ion channel remodelling of persistent atrial fibrillation (AF), and a mutation producing familial AF, was APD shortening at all rates. Electrical alternans was abolished by the modelled action of Dronedarone. AF induced gap junctional remodelling slows propagation velocity at all rates. Re-entrant spiral waves in 2-D models are characterised by their frequency, wavelength, meander and stability. For homogenous models of normal tissue, spiral waves self-terminate, due to meander to inexcitable boundaries, and by dissipation of excitation. AF electrical remodelling in these homogenous models led to persistence of spiral waves, and AF fibrotic remodelling to their breakdown into fibrillatory activity. An anatomical model of the atria was partially validated by the activation times of normal sinus rhythm. The use of tissue geometry from clinical MRI, and tissue anisotropy from ex vivo diffusion tensor magnetic resonance imaging is outlined. In the homogenous model of normal atria, a single scroll breaks down onto spatio-temporal irregularity (electrical fibrillation) that is self-terminating; while in the AF remodelled atria the fibrillatory activity is persistent. The persistence of electrical AF can be dissected in the model in terms of ion channel and intercellular coupling processes, that can be modified pharmacologically; the effects of anatomy, that can be modified by ablation; and the permanent effects of fibrosis, that need to be prevented., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Pulmonary hypertension provokes right heart failure and arrhythmias. Better understanding of the mechanisms underlying these arrhythmias is needed to facilitate new therapeutic approaches for the hypertensive, failing right ventricle (RV). The aim of our study was to identify the mechanisms generating arrhythmias in a model of RV failure induced by pulmonary hypertension. Rats were injected with monocrotaline to induce either RV hypertrophy or failure or with saline (control). ECGs were measured in conscious, unrestrained animals by telemetry. In isolated hearts, electrical activity was measured by optical mapping and myofiber orientation by diffusion tensor-MRI. Sarcoplasmic reticular Ca(2+) handling was studied in single myocytes. Compared with control animals, the T-wave of the ECG was prolonged and in three of seven heart failure animals, prominent T-wave alternans occurred. Discordant action potential (AP) alternans occurred in isolated failing hearts and Ca(2+) transient alternans in failing myocytes. In failing hearts, AP duration and dispersion were increased; conduction velocity and AP restitution were steeper. The latter was intrinsic to failing single myocytes. Failing hearts had greater fiber angle disarray; this correlated with AP duration. Failing myocytes had reduced sarco(endo)plasmic reticular Ca(2+)-ATPase activity, increased sarcoplasmic reticular Ca(2+)-release fraction, and increased Ca(2+) spark leak. In hypertrophied hearts and myocytes, dysfunctional adaptation had begun, but alternans did not develop. We conclude that increased electrical and structural heterogeneity and dysfunctional sarcoplasmic reticular Ca(2+) handling increased the probability of alternans, a proarrhythmic predictor of sudden cardiac death. These mechanisms are potential therapeutic targets for the correction of arrhythmias in hypertensive, failing RVs.

It has been shown by histology that cardiac myocytes are organized into laminae and this structure is important in function, both influencing the spread of electrical activation and enabling myocardial thickening in systole by laminar sliding. We have carried out high-spatial resolution three-dimensional MRI of the ventricular myolaminae of the entire volume of the isolated rat heart after contrast perfusion [dimeglumine gadopentate (Gd-DTPA)]. Four ex vivo rat hearts were perfused with Gd-DTPA and fixative and high-spatial resolution MRI was performed on a 9.4T MRI system. After MRI, cryosectioning followed by histology was performed. Images from MRI and histology were aligned, described, and quantitatively compared. In the three-dimensional MR images we directly show the presence of laminae and demonstrate that these are highly branching and are absent from much of the subepicardium. We visualized these MRI volumes to demonstrate laminar architecture and quantitatively demonstrated that the structural features observed are similar to those imaged in histology. We showed qualitatively and quantitatively that laminar architecture is similar in the four hearts. MRI can be used to image the laminar architecture of ex vivo hearts in three dimensions, and the images produced are qualitatively and quantitatively comparable with histology. We have demonstrated in the rat that: 1) laminar architecture is consistent between hearts; 2) myolaminae are absent from much of the subepicardium; and 3) although localized orthotropy is present throughout the myocardium, tracked myolaminae are branching structures and do not have a discrete identity.

Mechanisms underlying the genesis of re-entrant substrate for atrial fibrillation (AF) in the pulmonary veins (PVs) and left atrium (LA) are not well understood. We develop a biophysically detailed computational model for the PVs and surrounding LA tissue. The model integrates canine PV and LA single cell electrophysiology with the respective 3D tissue geometry and fiber orientation reconstructed from micro-CT data. The model simulations demonstrate that a combination of tissue anisotropy and electrical heterogeneity between the PVs and LA causes a break-down of normal electrical excitation wave-fronts. This leads to the generation of a high-frequency re-entrant source near the PV sleeves. Evidence of such sources have been seen clinically in AF patients. In summary, our modeling results provide new insights into the arrhythmogenic mechanisms of re-entrant excitation waves underlying AF.

The ventricular myocardium has a structure of branching laminae through which course regularly orientated myofibers, an architecture important in excitation and contraction. Quantifying this architecture is vital for understanding normal and disease states in the heart and for assessing their impact on electrical function. These data are also highly important in the construction of scientifically and clinically useful computer models of cardiac electrical behavior. Detailed structural information has previously been obtained from serial imaging. In this work we assess the potential for high-resolution (HR) MRI as a means to furnish useful myoarchitecture and compare and contrast this approach with the growing use of DT. Using rat hearts, we conclude that both approaches have strengths and weaknesses, however, HR-MRI may provide a consistently more robust picture of the myoarchitecture in small hearts.

We apply virtual tissue engineering to the full term human uterus with a view to reconstruction of the spatiotemporal patterns of electrical activity of the myometrium that control mechanical activity via intracellular calcium. The three-dimensional geometry of the gravid uterus has been reconstructed from segmented in vivo magnetic resonance imaging as well as ex vivo diffusion tensor magnetic resonance imaging to resolve fine scale tissue architecture. A late-pregnancy uterine smooth muscle cell model is constructed and bursting analysed using continuation algorithms. These cell models are incorporated into partial differential equation models for tissue synchronisation and propagation. The ultimate objective is to develop a quantitative and predictive understanding of the mechanisms that initiate and regulate labour., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Reaction-diffusion computational models of cardiac electrophysiology require both dynamic excitation models that reconstruct the action potentials of myocytes as well as datasets of cardiac geometry and architecture that provide the electrical diffusion tensor D, which determines how excitation spreads through the tissue. We illustrate an experimental pipeline we have developed in our laboratories for constructing and validating such datasets. The tensor D changes with location in the myocardium, and is determined by tissue architecture. Diffusion tensor magnetic resonance imaging (DT-MRI) provides three eigenvectors e(i) and eigenvalues λ(i) at each voxel throughout the tissue that can be used to reconstruct this architecture. The primary eigenvector e(1) is a histologically validated measure of myocyte orientation (responsible for anisotropic propagation). The secondary and tertiary eigenvectors (e(2) and e(3)) specify the directions of any orthotropic structure if λ(2) is significantly greater than λ(3)-this orthotropy has been identified with sheets or cleavage planes. For simulations, the components of D are scaled in the fibre and cross-fibre directions for anisotropic simulations (or fibre, sheet and sheet normal directions for orthotropic tissues) so that simulated conduction velocities match values from optical imaging or plunge electrode experiments. The simulated pattern of propagation of action potentials in the models is partially validated by optical recordings of spatio-temporal activity on the surfaces of hearts. We also describe several techniques that enhance components of the pipeline, or that allow the pipeline to be applied to different areas of research: Q ball imaging provides evidence for multi-modal orientation distributions within a fraction of voxels, infarcts can be identified by changes in the anisotropic structure-irregularity in myocyte orientation and a decrease in fractional anisotropy, clinical imaging provides human ventricular geometry and can identify ischaemic and infarcted regions, and simulations in human geometries examine the roles of anisotropic and orthotropic architecture in the initiation of arrhythmias.

Transmural myocardial activation is influenced by myocardial structure, including structural differences between the compacta (Cta) and the trabeculata (Tta), although this has not been fully explained. Hearts from rats were Langendorff perfused, stained with DI-4-ANEPPS, the apex was cut off and fluorescence acquired from the exposed short-axis surface. The hearts were stimulated at 160 ms cycle length at the anterior, lateral, posterior left ventricle (LV) and septal sub-epicardial sites. Conduction velocity perpendicular to the wave front orientation was measured in each pixel using a gradient-based approach. After optical mapping the cut surface was imaged using a light microscope and the extent of the Cta and Tta mapped and validated against 50 u, m isotropic MRI images. We used a 3D rat ventricle computational model, with architecture obtained from 200 u, m isotropic diffusion tensor MRI and kinetics from the modified Pandit model to determine the relative roles of fibers and sheets on propagation. We show in the experimental study that circumferential propagation around the LV cavity is fast in the Cta: 63.2±19.5 and is slower in the Tta: 32.7±11.0(∗) (mean ± s.d cms-1, ∗ p < 0.01 by two sample t test). In the simulation study the pattern and velocity are not replicated in an isotropic model (I), are partially replicated in a simulation study including fiber anisotropy (A) and is more fully replicated in orthotropic (O) ventricles (fiber and sheet anisotropy), where the circumferential propagation velocity is, I: Cta: 54.2±3.9; Tta:54.3±3.9; A: Cta:43.6±3.2; Tta: 40.6±6.6; O: Cta: 63.2±19.5; Tta: 32.7±11.9(∗). We show that sheet orientation is important in understanding activation differences between Cta and Tta.

Background: Epifluorescence imaging using voltage-sensitive dyes has provided unique insights into cardiac electrical activity and arrhythmias. However, conventional dyes use blue-green excitation light, which has limited depth penetration., Objective: The aim of this study was to demonstrate that combining a short and a long excitation wavelength using near-infrared (NIR) dyes allows for epifluorescence imaging of transmural electrophysiological properties in intact hearts., Methods: Epifluorescence imaging was performed in rat hearts (N = 11) using DI-4-ANEPPS and the NIR dye DI-4-ANBDQBS. Activation and action potential duration (APD) patterns were investigated at 2 excitation wavelengths (530 and 660 nm) after epicardial stimulation at various cycle lengths (160 to 70 ms)., Results: Optical action potential upstrokes acquired with 660-nm excitation of DI-4-ANBDQBS were significantly longer than upstrokes obtained with 530-nm excitation of DI-4-ANEPPS (P < .001). Comparison of activation maps showed counterclockwise rotation of isochrones consistent with a transmural rotation of myofibers. Pronounced APD modulation by the activation sequence was observed at both excitation wavelengths. Significantly prolonged APDs (P = .016) and steeper APD restitution curves were found with DI-4-ANBDQBS (660-nm excitation) when compared with DI-4-ANEPPS (530-nm excitation). Dual excitation wavelength experiments using solely DI-4-ANBDQBS yielded similar results. Monophasic action potential recordings showed prolonged APD and steeper APD restitution curves in the endocardium, indicating that 660-nm excitation provides a significant endocardial contribution to the signal. Three-dimensional computer simulations confirmed our findings., Conclusion: Dual excitation wavelength epifluorescence allows detecting transmural heterogeneity in intact hearts. It therefore has the potential to become an important tool in experimental cardiac electrophysiology., (Copyright © 2010 Heart Rhythm Society. Published by Elsevier Inc. All rights reserved.)

Over the last decade, magnetic resonance imaging (MRI) has become established as a useful referral diagnostic method in veterinary medicine that is widely used in small animal brain and spinal diseases, aural, nasal and orbital disorders, planning soft tissue surgery, oncology and small animal and equine orthopaedics. The use of MRI in these disciplines has grown due to its unparalleled capability to image soft tissue structures. This has been exploited in human cardiology where, despite the inherent difficulties in imaging a moving, contractile structure, cardiac MRI (CMRI) has become the optimal technique for the morphological assessment and quantification of ventricular function. Both CMRI hardware and software systems have developed rapidly in the last 10 years but although several preliminary veterinary CMRI studies have been reported, the technique's growth has been limited and is currently used primarily in clinical research. A review of published studies is presented with a description of CMRI technology and the potential of CMRI is discussed along with some of the reasons for its limited usage., (Copyright (c) 2008 Elsevier Ltd. All rights reserved.)

Mechanical stimulation has important consequences for myocardial function. However, this stimulation and the response to it, is not uniform. The right ventricle is thinner walled and operates at lower pressure than the left ventricle. Within the ventricles, differences in the orientation of myocardial fibres exist. These differences produce inhomogeneity in the stress and strain between and across the ventricles. Possibly as a result of these variations in mechanical stimulation, there are well characterised inhomogeneities in gene expression and protein function within the ventricular myocardium, for example in the transient outward K+ current and its associated Kv channels. Perhaps not surprisingly, it is becoming apparent that gradients of expression and function exist for proteins that are intimately involved in the response to mechanical stimulation in the heart, for example in the left ventricle of the rat there is a transmural gradient in mRNA and current density of the mechanosensitive two-pore domain K+ channel TREK-1 (ENDO>EPI). In healthy hearts it is assumed that these gradients are important for normal function and therefore that their disruption in diseased myocardium is involved in the dysfunction that occurs.

The architecture of the heart remains controversial despite extensive effort and recent advances in imaging techniques. Several opposing and non-mutually compatible models have been proposed to explain cardiac structure, and these models, although limited, have advanced the study and understanding of heart structure, function and development. We describe key areas of similarity and difference, highlight areas of contention and point to the important limitations of these models. Recent research in animal models on the nature, geometry and interaction of cardiac sheet structure allows unification of some seemingly conflicting features of the structural models. Intriguingly, evidence points to significant inter-individual structural variability (within constrained limits) in the canine, leading to the concept of a continuum (or distribution) of cardiac structures. This variability in heart structure partly explains the ongoing debate on myocardial architecture. These developments are used to construct an integrated description of cardiac structure unifying features of fibre, sheet and band architecture that provides a basis for (i) explaining cardiac electromechanics, (ii) computational simulations of cardiac physiology and (iii) designing interventions.

It has been reported that sensitization of animals to allergens increases both early shortening velocity and myosin light-chain kinase of their airway smooth muscle without increasing force generated by these muscles. Since early shortening sets muscle length for the duration of a contraction, these responses might be expected to produce greater airway obstruction. Here, it is explained how the more rapid early shortening without increased force production is predicted by the 2-stage process of activation followed by contraction posited by the crossbridge theory of contraction when the rate, but not the extent, of activation is increased. The experimental results are reproduced by a simple model in which activation rate is increased 1.6-fold without any other changes in contractile parameters. These results reinforce suggestions that sensitized animals are a model for reactive airway disease.

Excessive airway obstruction is the cause of symptoms and abnormal lung function in asthma. As airway smooth muscle (ASM) is the effecter controlling airway calibre, it is suspected that dysfunction of ASM contributes to the pathophysiology of asthma. However, the precise role of ASM in the series of events leading to asthmatic symptoms is not clear. It is not certain whether, in asthma, there is a change in the intrinsic properties of ASM, a change in the structure and mechanical properties of the noncontractile components of the airway wall, or a change in the interdependence of the airway wall with the surrounding lung parenchyma. All these potential changes could result from acute or chronic airway inflammation and associated tissue repair and remodelling. Anti-inflammatory therapy, however, does not "cure" asthma, and airway hyperresponsiveness can persist in asthmatics, even in the absence of airway inflammation. This is perhaps because the therapy does not directly address a fundamental abnormality of asthma, that of exaggerated airway narrowing due to excessive shortening of ASM. In the present study, a central role for airway smooth muscle in the pathogenesis of airway hyperresponsiveness in asthma is explored.

Muscle birefringence, caused mainly by parallel thick filaments, increases in smooth muscle during stimulation, signalling thick filament formation upon activation. The reverse occurs in skeletal muscle, where a decrease in birefringence has been correlated with crossbridge movement away from the thick filaments. When force generation by trachealis muscle was inhibited with wortmannin, which inhibits myosin light-chain phosphorylation and thick-filament formation, but not the calcium increase caused by stimulation, the birefringence response inverted, suggesting crossbridge movement similar to that of skeletal muscle. Resistance to quick stretches was much greater in stimulated muscle than in unstimulated muscle before wortmannin treatment and no different in stimulated and unstimulated muscle after force inhibition by wortmannin. Before wortmannin treatment, stimulation reduced thick-filament cross-sectional areas in electron micrographs by 44%. After force inhibition by wortmannin, filament areas were not significantly different in stimulated and unstimulated muscle and not significantly different from those of relaxed muscle without wortmannin treatment. These results suggest that myofibrillar-space calcium causes crossbridges to move away from the thick filaments without firmly attaching to thin filaments.

Birefringence and force produced by pig tracheal smooth muscles were recorded every 100 ms during electrically stimulated tetani at muscle lengths that varied 1.5-fold and at the peak of acetylcholine contractures at the same lengths. Isometric force was nearly the same at all lengths. Resting birefringence at the longest length was 30% greater than that at the shortest length. During tetani, birefringence increased with approximately the same time course as force, rising by 20% at the shortest length and 9% at the longest length, and continued to increase by an additional 0.5-1.5% of the resting value for 2-8 s after stimulation ended and force began to fall. This late increase was greatest and more sustained at longer lengths. During contractures, birefringence increased by 25 and 18% at the shortest and longest lengths, respectively. Comparison of these results with our published thick-filament densities suggests that thick-filament density increased by about 80, 72 and 50% during contractures at the short, intermediate and long lengths, and that approximately 35% of birefringence in the resting muscle at the longest length was not due to thick filaments. These findings support the hypotheses that tracheal smooth muscle adapts to longer lengths by increasing thick-filament mass and that myosin thick filaments are evanescent, dissociating partially during relaxation and reforming upon activation. The results further suggest that thick-filament formation is sufficiently rapid to account for the velocity slowing and some of the force increase observed during the rise of activation of tracheal smooth muscle.