Divergent evolution of the alcohol-forming pathway of wax biosynthesis among bryophytes

The plant cuticle is a hydrophobic barrier, which seals the epidermal surface of most aboveground organs. While the cuticle biosynthesis of angiosperms has been intensively studied, knowledge about its existence and composition in nonvascular plants is scarce. Here, we identified and characterized homologs of Arabidopsis thaliana fatty acyl-CoA reductase (FAR) ECERIFERUM 4 (AtCER4) and bifunctional wax ester synthase/acyl-CoA:diacylglycerol acyltransferase 1 (AtWSD1) in the liverwort Marchantia polymorpha (MpFAR2 and MpWSD1) and the moss Physcomitrium patens (PpFAR2A, PpFAR2B, and PpWSD1). Although bryophyte harbor similar compound classes as described for angiosperm cuticles, their biosynthesis may not be fully conserved between the bryophytes M. polymorpha and P. patens or between these bryophytes and angiosperms. While PpFAR2A and PpFAR2B contribute to the production of primary alcohols in P. patens, loss of MpFAR2 function does not affect the wax profile of M. polymorpha. By contrast, MpWSD1 acts as the major wax ester-producing enzyme in M. polymorpha, whereas mutations of PpWSD1 do not affect the wax ester levels of P. patens. Our results suggest that the biosynthetic enzymes involved in primary alcohol and wax ester formation in land plants have either evolved multiple times independently or undergone pronounced radiation followed by the formation of lineage-specific toolkits.

Tetracosanoic acids produced by 3-ketoacyl-CoA synthase 17 are required for synthesizing seed coat suberin in Arabidopsis

Very long-chain fatty acids (VLCFAs) are precursors for the synthesis of membrane lipids, cuticular waxes, suberins, and storage oils in plants. 3-Ketoacyl CoA synthase (KCS) catalyzes the condensation of C2 units from malonyl-CoA to acyl-CoA, the first rate-limiting step in VLCFA synthesis. In this study, we revealed that Arabidopsis KCS17 catalyzes the elongation of C22-C24 VLCFAs required for synthesizing seed coat suberin. Histochemical analysis of Arabidopsis plants expressing GUS (β-glucuronidase) under the control of the KCS17 promoter revealed predominant GUS expression in seed coats, petals, stigma, and developing pollen. The expression of KCS17:eYFP (enhanced yellow fluorescent protein) driven by the KCS17 promoter was observed in the outer integument1 of Arabidopsis seed coats. The KCS17:eYFP signal was detected in the endoplasmic reticulum of tobacco epidermal cells. The levels of C22 VLCFAs and their derivatives, primary alcohols, α,ω-alkane diols, ω-hydroxy fatty acids, and α,ω-dicarboxylic acids increased by ~2-fold, but those of C24 VLCFAs, ω-hydroxy fatty acids, and α,ω-dicarboxylic acids were reduced by half in kcs17-1 and kcs17-2 seed coats relative to the wild type (WT). The seed coat of kcs17 displayed decreased autofluorescence under UV and increased permeability to tetrazolium salt compared with the WT. Seed germination and seedling establishment of kcs17 were more delayed by salt and osmotic stress treatments than the WT. KCS17 formed homo- and hetero-interactions with KCR1, PAS2, and ECR, but not with PAS1. Therefore, KCS17-mediated VLCFA synthesis is required for suberin layer formation in Arabidopsis seed coats.

Functional conservation of an AP2/ERF transcription factor in cuticle formation suggests an important role in the terrestrialization of early land plants

The formation of a hydrophobic cuticle layer on aerial plant parts was a critical innovation for protection from the terrestrial environment during the evolution of land plants. However, little is known about the molecular mechanisms underlying cuticle biogenesis in early terrestrial plants. Here, we report an APETALA2/Ethylene Response Factor (AP2/ERF) transcriptional activator, PpWIN1, involved in cutin and cuticular wax biosynthesis in Physcomitrium patens and Arabidopsis. The transcript levels of PpWIN1 were 2.5-fold higher in gametophores than in the protonema, and increased by approximately 3- to 4.7-fold in the protonema and gametophores under salt and osmotic stresses. PpWIN1 harbouring transcriptional activation activity is localized in the nucleus of tobacco leaf epidermal cells. Δppwin1 knockout mutants displayed a permeable cuticle, increased water loss, and cutin- and wax-deficient phenotypes. In contrast, increased total cutin and wax loads, and decreased water loss rates were observed in PpWIN1-overexpressing Arabidopsis plants. The transcript levels of genes involved in cutin or wax biosynthesis were significantly up-regulated in PpWIN1-overexpressing Arabidopsis lines, indicating that PpWIN1 acts as a transcriptional activator in cuticle biosynthesis. This study suggests that Arabidopsis WIN1/SHN1 orthologs may be functionally conserved from early to vascular land plants.

Protein-protein interactions in fatty acid elongase complexes are important for very-long-chain fatty acid synthesis

Fatty acid elongase (FAE), which catalyzes the synthesis of very-long-chain fatty acids (VLCFAs), is a multiprotein complex; however, little is known about its quaternary structure. In this study, bimolecular fluorescence complementation and/or yeast two-hybrid assays showed that homo-interactions were observed in β-ketoacyl-CoA synthases (KCS2, KCS9, and KCS6), Eceriferum2-like proteins [CER2 and CER2-Like2 (C2L2)], and FAE complex proteins (KCR1, PAS2, ECR, and PAS1), except for CER2-Like1 (C2L1). Hetero-interactions were observed between KCSs (KCS2, KCS9, and KCS6), between CER2-LIKEs (CER2, C2L2, and C2L1), and between FAE complex proteins (KCR1, PAS2, ECR, and PAS1). PAS1 interacts with FAE complex proteins (KCR1, PAS2, and ECR), but not with KCSs (KCS2, KCS9, and KCS6) and CER2-LIKEs (CER2, C2L2, and C2L1). Asp308 and Arg309-Arg311 of KCS9 were essential for the homo-interactions of KCS9 and hetero-interactions between KCS9 and PAS2 or ECR. Asp339 of KCS9 is involved in its homo- and hetero-interactions with ECR. Complementation analysis of the Arabidopsis kcs9 mutant by the expression of amino acid-substituted KCS9 mutant genes showed that Asp308 and Asp339 of KCS9 are involved in the synthesis of C24 VLCFAs from C22. This study suggests that protein-protein interaction in FAE complexes is important for VLCFA synthesis and provides insight into the quaternary structure of FAE complexes for efficient synthesis of VLCFAs.

Antioxidant effects on hypoxia-induced oxidative stress and apoptosis in rat rotator cuff fibroblasts

Most cells, highly sensitive to oxygen levels, undergo apoptosis under hypoxia. Therefore, the involvement of hypoxia in rotator cuff tendon degeneration has been proposed. While previous studies have reported that hypoxia induces apoptosis in rotator cuff fibroblasts (RCFs), little research has investigated whether antioxidants have cytoprotective effects against RCF apoptosis. The present study aimed at determining whether the antioxidant N-acetylcysteine (NAC) exerted cytoprotective effects against hypoxia-induced RCF apoptosis. Third-passage rat RCFs were divided into normoxia, NAC, hypoxia and NAC-hypoxia groups. The hypoxia inducer was 1,000 µmol/L cobalt chloride (CoCl2); the antioxidant was 20 mmol/L NAC. Expressions of hypoxia-inducible factor-1α (HIF-1α) and heme oxygenase-1 (HO-1), cell viability, intracellular reactive oxygen species (ROS) production, apoptosis rates as well as expressions of cleaved caspase-3, cleaved poly ADP-ribose polymerase-1 (PARP-1), vascular endothelial growth factors-β (VEGF-β) and matrix metalloproteinase-2 (MMP-2) were evaluated. Expression of HIF-1α and HO-1 was significantly higher in the hypoxia group than in the normoxia group (p < 0.001). Cell viability was significantly lower in the hypoxia group than in the normoxia group (p < 0.001). Intracellular ROS production, apoptosis rate and expressions of cleaved caspase-3, cleaved PARP-1, VEGF-β and MMP-2 were significantly higher in the hypoxia group than in the normoxia group (p < 0.001). All these responses were significantly attenuated by pre-treatment with NAC (p ≤ 0.001). ROS were involved in hypoxic RCF apoptosis induced by CoCl2; NAC, an ROS scavenger, inhibited hypoxia-induced RCF apoptosis by inhibiting ROS production.

AP2/DREB Transcription Factor RAP2.4 Activates Cuticular Wax Biosynthesis in Arabidopsis Leaves Under Drought

Drought is a critical environmental stress that limits growth and development of plants and reduces crop productivity. The aerial part of land plants is covered with cuticular waxes to minimize water loss. To understand the regulatory mechanisms underlying cuticular wax biosynthesis in Arabidopsis under drought stress conditions, we characterized the role of an AP2/DREB type transcription factor, RAP2.4. RAP2.4 expression was detected in one-week-old seedlings and rosette leaves, stems, stem epidermis, cauline leaves, buds, flowers, and siliques of 6-week-old Arabidopsis. The levels of RAP2.4 transcripts increased with treatments of abscisic acid (ABA), mannitol, NaCl, and drought stress. Under drought, total wax loads decreased by approximately 11% and 10%, and in particular, the levels of alkanes, which are a major wax component, decreased by approximately 11% and 12% in rap2.4-1 and rap2.4-2 leaves, respectively, compared with wild type (WT) leaves. Moreover, the transcript levels of cuticular wax biosynthetic genes, KCS2 and CER1, decreased by approximately 15-23% and 32-40% in rap2.4-1 and rap2.4-2 leaves, respectively, relative to WT 4 h after drought treatment, but increased by 2- to 12-fold and 3- to 70-fold, respectively, in three independent RAP2.4 OX leaves relative to WT. Epicuticular wax crystals were observed on the leaves of RAP2.4 OX plants, but not on the leaves of WT. Total wax loads increased by 1.5- to 3.3-fold in leaves of RAP2.4 OX plants relative to WT. Cuticular transpiration and chlorophyll leaching occurred slowly in the leaves of RAP2.4 OX plants relative to WT. Transcriptional activation assay in tobacco protoplasts showed that RAP2.4 activates the expression of KCS2 and CER1 through the involvement of the consensus CCGAC or GCC motifs present in the KCS2 and CER1 promoter regions. Overall, our results revealed that RAP2.4 is a transcription factor that activates cuticular wax biosynthesis in Arabidopsis leaves under drought stress conditions.

P1021Comparative efficacy of microfidelity technology vs standard ablation for atrioventricular nodal ablation

Background

Microfidelity Cateter Technology has proven efficacy in ablating atrial arrhythmias in multiple pilot studies. Closely spaced radial microelectrodes render a focused near-field electrogram. Case series suggest that this catheter design facilitates accurate ablations with fewer radiofrequency (RF) lesions. Atrioventricular junction (AVJ) ablation is regarded as a straightforward procedure, but case records show wide variance in procedure times and number of RF lesions required.

Methods

Twenty-four patients scheduled for AVJ ablation were randomized to treatment with either the Microfidelity technology or standard 8mm/8 French ablation catheter. Both groups located the AVJ by fluoroscopic landmarks and His electrograms, and the MiFi group used electroanatomical mapping to create the location of his electrograms. The primary endpoints were development of Junctional Rhythm (JR) or Complete Heart Block (CHB), and time from first RF lesion until rhythm change. Secondary endpoints included number of RF applications.

Results

Patients were randomized one-to-one to the MiFi arm or standard ablation arm. JR or CHB was achieved in all patients. Time from first RF lesion until JR/CHB was: (Median/IQR) 325 sec/250–1270 sec. vs 287 sec/101–406 sec. Number of RF applications was 5/3–15 applications vs 4.5/1–5 applications. Total procedure time in the lab was 134 min/73.5–172.5 min vs 58 min/52–146 min.

Microfidelity Technology vs Standard

Conclusion

Analysis suggests that the MiFi catheter is efficacious in ablating the AVJ, but requires greater RF duration and number of lesions, with wider case-by-case variability to achieve JR or CHB. Microfidelity technology and electroanatomical mapping did not result in faster time to completion than using fluoroscopic landmarks and His electrograms alone. Preoperative choice of sheath for catheter stability and contact may also play a role in a more efficient timely successful ablation of the AV node.

Acknowledgement/Funding

Boston Scientific

87Effects of the chymase inhibitor fulacimstat on adverse cardiac remodelling after acute myocardial infarction - Results of the CHIARA MIA 2 trial

Introduction

Adverse cardiac remodelling represents the most important risk factor for the development of heart failure (HF) after myocardial infarction (MI). Chymase is a protease that generates locally pro-fibrotic factors such as angiotensin II, TGFβ, and matrixmetallproteases that contribute to tissue remodelling.

Purpose

This phase IIa study examined the effects of the chymase inhibitor fulacimstat on functional parameters of adverse cardiac remodelling after acute MI.

Methods

A double-blind, multinational, randomized, placebo-controlled study was performed in patients after first STEMI who were treated with primary percutaneous coronary intervention within 24h of symptom onset. To enrich for patients at risk of adverse remodelling, main inclusion criteria were a left-ventricular ejection fraction (LVEF)≤45% and an infarct size>10% on day 5 to 9 post MI as measured by cardiac MRI. On day 6 to 12 post MI, patients were randomized to treatment with either 25 mg fulacimstat (n=54) or placebo (n=53) twice daily on top of standard of care. The changes in LVEF, LVEDVI, and LVESVI from baseline to 6 months of treatment were analyzed by a central blinded cardiac MRI core laboratory.

Results

Fulacimstat was safe and well tolerated, 64.8% of patients treated with fulacimstat and 75.5% of patients treated with placebo reported treatment emergent adverse events. Fulacimstat achieved exposures that were approximately 10-fold higher than those predicted to be required for minimal therapeutic activity. After six months of treatment, there were no effects of fulacimstat compared to placebo on the changes in LVEF, LVEDVI, and LVESVI (see Table).

Analysis of primary efficacy parameters Parameter Placebo Fulacimstat p-value LVEF (%) baseline 37.2±6.1 39.1±5.5 0.15 6 months 41.2±8.4 42.6±8.4 0.45 delta 4.0±5.0 3.5±5.4 0.69 LVEDVI (mL/m2) baseline 80.0±17.1 77.4±18.2 0.51 6 months 85.1±19.1 84.7±23.4 0.94 delta 5.1±18.9 7.3±13.3 0.54 LVESVI (mL/m2) baseline 50.5±13.0 47.3±12.3 0.26 6 months 51.1±16.9 49.6±18.1 0.71 delta 0.6±14.8 2.3±11.2 0.56 Data are given as mean ± standard deviation.

Conclusion

Fulacimstat was safe and well tolerated in patients with left-ventricular dysfunction (LVD) after first STEMI but had no effect on adverse cardiac remodelling in the experimental setting of this study.

Acknowledgement/Funding

The study was funded by its sponsor BAYER AG

Plastidial and Mitochondrial Malonyl CoA-ACP Malonyltransferase is Essential for Cell Division and Its Overexpression Increases Storage Oil Content

Malonyl-acyl carrier protein (ACP) is a key building block for the synthesis of fatty acids, which are important components of cell membranes, storage oils and lipid-signaling molecules. Malonyl CoA-ACP malonyltransferase (MCAMT) catalyzes the production of malonyl-ACP and CoA from malonyl-CoA and ACP. Here, we report that MCAMT plays a critical role in cell division and has the potential to increase the storage oil content in Arabidopsis. The quantitative real-time PCR and MCAMT promoter:GUS analyses showed that MCAMT is predominantly expressed in shoot and root apical meristems, leaf hydathodes and developing embryos. The fluorescent signals of MCAMT:eYFP were observed in both chloroplasts and mitochondria of tobacco leaf protoplasts. In particular, the N-terminal region (amino acid residues 1-30) of MCAMT was required for mitochondrial targeting. The Arabidopsis mcamt-1 and -2 mutants exhibited an embryo-lethal phenotype because of the arrest of embryo development at the globular stage. The transgenic Arabidopsis expressing antisense MCAMT RNA showed growth retardation caused by the defects in cell division. The overexpression of MCAMT driven by the promoter of the senescence-associated 1 (SEN1) gene, which is predominantly expressed in developing seeds, increased the seed yield and storage oil content of Arabidopsis. Taken together, the plastidial and mitochondrial MCAMT is essential for Arabidopsis cell division and is a novel genetic resource useful for enhancing storage oil content in oilseed crops.

Molecular and biochemical characterizations of the monoacylglycerol lipase gene family of Arabidopsis thaliana

Monoacylglycerol lipase (MAGL) catalyzes the last step of triacylglycerol breakdown, which is the hydrolysis of monoacylglycerol (MAG) to fatty acid and glycerol. Arabidopsis harbors over 270 genes annotated as 'lipase', the largest class of acyl lipid metabolism genes that have not been characterized experimentally. In this study, computational modeling suggested that 16 Arabidopsis putative MAGLs (AtMAGLs) have a three-dimensional structure that is similar to a human MAGL. Heterologous expression and enzyme assays indicated that 11 of the 16 encoded proteins indeed possess MAG lipase activity. Additionally, AtMAGL4 displayed hydrolase activity with lysophosphatidylcholine and lysophosphatidylethanolamine (LPE) substrates and AtMAGL1 and 2 utilized LPE as a substrate. All recombinant AtMAGLs preferred MAG substrates with unsaturated fatty acids over saturated fatty acids and AtMAGL8 exhibited the highest hydrolase activities with MAG containing 20:1 fatty acids. Except for AtMAGL4, -14 and -16, all AtMAGLs showed similar activity with both sn-1 and sn-2 MAG isomers. Spatial, temporal and stress-induced expression of the 16 AtMAGL genes was analyzed by transcriptome analyses. AtMAGL:eYFP fusion proteins provided initial evidence that AtMAGL1, -3, -6, -7, -8, -11, -13, -14 and -16 are targeted to the endoplasmic reticulum and/or Golgi network, AtMAGL10, -12 and -15 to the cytosol and AtMAGL2, -4 and -5 to the chloroplasts. Furthermore, AtMAGL8 was associated with the surface of oil bodies in germinating seeds and leaves accumulating oil bodies. This study provides the broad characterization of one of the least well-understood groups of Arabidopsis lipid-related enzymes and will be useful for better understanding their roles in planta.

An annotated database of Arabidopsis mutants of acyl lipid metabolism

We have constructed and annotated a web-based database of over 280 Arabidopsis genes that have characterized mutants associated with Arabidopsis acyl lipid metabolism. Mutants have played a fundamental role in gene discovery and in understanding the function of genes involved in plant acyl lipid metabolism. The first mutant in Arabidopsis lipid metabolism (fad4) was described in 1985. Since that time, characterization of mutants in more than 280 genes associated with acyl lipid metabolism has been reported. This review provides a brief background and history on identification of mutants in acyl lipid metabolism, an analysis of the distribution of mutants in different areas of acyl lipid metabolism and presents an annotated database (ARALIPmutantDB) of these mutants. The database provides information on the phenotypes of mutants, pathways and enzymes/proteins associated with the mutants, and allows rapid access via hyperlinks to summaries of information about each mutant and to literature that provides information on the lipid composition of the mutants. In addition, the database of mutants is integrated within the ARALIP plant acyl lipid metabolism website ( http://aralip.plantbiology.msu.edu ) so that information on mutants is displayed on and can be accessed from metabolic pathway maps. Mutants for at least 30% of the genes in the database have multiple names, which have been compiled here to reduce ambiguities in searches for information. The database should also provide a tool for exploring the relationships between mutants in acyl lipid-related genes and their lipid phenotypes and point to opportunities for further research.

Overexpression of Arabidopsis MYB96 confers drought resistance in Camelina sativa via cuticular wax accumulation

Camelina has been highlighted as an emerging oilseed crop. Transgenic Camelina plants overexpressing Arabidopsis MYB96 exhibited drought resistance by activating expression of Camelina wax biosynthetic genes and accumulating wax load. Camelina (Camelina sativa L.) is an oilseed crop in the Brassicaeae family with potential to expand biofuel production to marginal land. The aerial portion of all land plants is covered with cuticular wax to protect them from desiccation. In this study, the Arabidopsis MYB96 gene was overexpressed in Camelina under the control of the CaMV35S promoter. Transgenic Camelina plants overexpressing Arabidopsis MYB96 exhibited normal growth and development and enhanced tolerance to drought. Deposition of epicuticular wax crystals and total wax loads increased significantly on the surfaces of transgenic leaves compared with that of non-transgenic plants. The levels of alkanes and primary alcohols prominently increased in transgenic Camelina plants relative to non-transgenic plants. Cuticular transpiration occurred more slowly in transgenic leaves than that in non-transgenic plants. Genome-wide identification of Camelina wax biosynthetic genes enabled us to determine that the expression levels of CsKCS2, CsKCS6, CsKCR1-1, CsKCR1-2, CsECR, and CsMAH1 were approximately two to sevenfold higher in transgenic Camelina leaves than those in non-transgenic leaves. These results indicate that MYB96-mediated transcriptional regulation of wax biosynthetic genes is an approach applicable to generating drought-resistant transgenic crops. Transgenic Camelina plants with enhanced drought tolerance could be cultivated on marginal land to produce renewable biofuels and biomaterials.

Comparative functional analysis of wheat (Triticum aestivum) zinc finger-containing glycine-rich RNA-binding proteins in response to abiotic stresses

Although the functional roles of zinc finger-containing glycine-rich RNA-binding proteins (RZs) have been characterized in several plant species, including Arabidopsis thaliana and rice (Oryza sativa), the physiological functions of RZs in wheat (Triticum aestivum) remain largely unknown. Here, the functional roles of the three wheat RZ family members, named TaRZ1, TaRZ2, and TaRZ3, were investigated using transgenic Arabidopsis plants under various abiotic stress conditions. Expression of TaRZs was markedly regulated by salt, dehydration, or cold stress. The TaRZ1 and TaRZ3 proteins were localized to the nucleus, whereas the TaRZ2 protein was localized to the nucleus, endoplasmic reticulum, and cytoplasm. Germination of all three TaRZ-expressing transgenic Arabidopsis seeds was retarded compared with that of wild-type seeds under salt stress conditions, whereas germination of TaRZ2- or TaRZ3-expressing transgenic Arabidopsis seeds was retarded under dehydration stress conditions. Seedling growth of TaRZ1-expressing transgenic plants was severely inhibited under cold or salt stress conditions, and seedling growth of TaRZ2-expressing plants was inhibited under salt stress conditions. By contrast, expression of TaRZ3 did not affect seedling growth of transgenic plants under any of the stress conditions. In addition, expression of TaRZ2 conferred freeze tolerance in Arabidopsis. Taken together, these results suggest that different TaRZ family members play various roles in seed germination, seedling growth, and freeze tolerance in plants under abiotic stress.

Multidisciplinary approach in the management of a complicated crown root fracture

BACKGROUND

This article describes the management of a complicated crown root fracture.

CASE REPORT

A young patient presented with a crown root fracture of the maxillary left central incisor with an oblique subgingival fracture line. A multidisciplinary treatment approach including endodontic treatment, orthodontic extrusion, surgical extraction and intra-alveolar repositioning was used to gain sufficient crown length of the fractured maxillary incisor. The coronally repositioned maxillary left central incisor was stabilised by sutures and a resin wire splint. A resin core was built up followed by fabrication of an all-ceramic crown. Clinical and radiographic follow-up of the maxillary left central incisor after 24 months showed no signs of bone resorption or pathology and good aesthetics and functions were maintained.

Safety of gadoversetamide in patients with acute and chronic myocardial infarction

PURPOSE

To assess the safety data from two large, multicenter, phase 2 trials on the use of gadoversetamide (OptiMARK, Tyco Healthcare/Mallinckrodt, St. Louis, MO) as a contrast agent in delayed hyperenhancement magnetic resonance imaging (DE-MRI) in patients with acute and chronic myocardial infarction (MI).

MATERIALS AND METHODS

The study population from both trials comprised 577 patients who were randomly assigned to one of four dose groups (0.05, 0.1, 0.2, or 0.3 mmol/kg) before undergoing DE-MRI. Safety evaluations included physical and electrocardiographic (ECG) examinations. Vital signs, laboratory values, adverse events (AE), and serious adverse events (SAE) were monitored before and after contrast administration.

RESULTS

Of the 577 patients who received gadoversetamide, 124 (21.5%) reported a total of 164 AEs; most were mild (139 AEs; 84.8%) or moderate (25 AEs; 15.2%). ECG-related changes were the most frequent AE. Site investigators judged only eight AEs as likely related to gadoversetamide and only two of the eight as clinically relevant. Further evaluation suggested neither AE was related to gadoversetamide. Two SAEs were reported, but none was judged related to gadoversetamide by the site investigators.

CONCLUSION

Gadoversetamide is safe for use in patients with acute or chronic MI up to a dose of 0.3 mmol/kg.

Limits of detection of regional differences in vasodilated flow in viable myocardium by first-pass magnetic resonance perfusion imaging

BACKGROUND

Perfusion imaging techniques intended to identify regional limitations in coronary flow reserve in viable myocardium need to identify 2-fold differences in regional flow during coronary vasodilation consistently. This study evaluated the suitability of current first-pass magnetic resonance approaches for evaluating such differences, which are 1 to 2 orders of magnitude less than in myocardial infarction.

METHODS AND RESULTS

Graded regional differences in vasodilated flow were produced in chronically instrumented dogs with either left circumflex (LCx) infusion of adenosine or partial LCx occlusion during global coronary vasodilation. First-pass myocardial signal intensity-time curves were obtained after right atrial injection of gadoteridol (0.025 mmol/kg) with an MRI inversion recovery true-FISP sequence. The area under the initial portion of the LCx curve was compared with that of a curve from a remote area of the ventricle. Relative LCx and remote flows were assessed simultaneously with microspheres. The ratio of LCx and remote MRI curve areas and the ratio of LCx and remote microsphere concentrations were highly correlated and linearly related over a 5-fold range of flow differences (y=0.96 x+/-0.07, P<0.0001, r(2)=0.87). The 95% confidence limits for individual MRI measurements were +/-35%. Regional differences of >/=2-fold were consistently apparent in unprocessed MR images.

CONCLUSIONS

Clinically relevant regional reductions in vasodilated flow in viable myocardium can be detected with 95% confidence over the range of 1 to 5 times resting flow. This suggests that MRI can identify and quantify limitations in perfusion reserve that are expected to be produced by stenoses of >/=70%.

Erythrocyte membrane ATP binding cassette (ABC) proteins: MRP1 and CFTR as well as CD39 (ecto-apyrase) involved in RBC ATP transport and elevated blood plasma ATP of cystic fibrosis

In addition to the better-known roles of the erythrocyte in the transport of oxygen and carbon dioxide, the concept that the red blood cell is involved in the transport and release of ATP has been evolving (J. Luthje, Blut 59, 367, 1989; G. R. Bergfeld and T. Forrester, Cardiovasc. Res. 26, 40, 1992; M. L. Ellsworth et al., Am. J. Physiol. 269, H2155, 1995; R. S. Sprague et al., Am. J. Physiol. 275, H1726, 1998). Membrane proteins involved in the release of ATP from erythrocytes now appear to include members of the ATP binding cassette (ABC) family (C. F. Higgins, Annu. Rev. Cell Biol. 8, 67, 1992; C. F. Higgins, Cell 82, 693, 1995). In addition to defining physiologically the presence of ABC proteins in RBCs, accumulating gel electrophoretic evidence suggests that the cystic fibrosis transmembrane conductance regulator (CFTR) and the multidrug resistance-associated protein (MRP1), respectively, constitute significant proteins in the red blood cell membrane. As such, this finding makes the mature erythrocyte compartment a major mammalian repository of these important ABC proteins. Because of its relative structural simplicity and ready accessibility, the erythrocyte offers an ideal system to explore details of the physiological functions of ABC proteins. Moreover, the presence of different ABC proteins in a single membrane implies that interaction among these proteins and with other membrane proteins may be the norm and not the exception in terms of modulation of their functions.

Transmural extent of acute myocardial infarction predicts long-term improvement in contractile function

BACKGROUND

Previous animal studies have demonstrated that the transmural extent of acute myocardial infarction defined by contrast-enhanced MRI (ceMRI) relates to early restoration of flow and future improvements in contractile function. We tested the hypothesis that ceMRI would have similar predictive value in humans.

METHODS AND RESULTS

Twenty-four patients who presented with their first myocardial infarction and were successfully revascularized underwent cine and ceMRI of their heart within 7 days (scan 1) of the peak MB band of creatine kinase. Cine MRI was repeated 8 to 12 weeks later (scan 2). The transmural extent of infarction on scan 1 and wall thickening on both scans were determined using a 72-segment model. A total of 524 of 1571 segments (33%) were dysfunctional on scan 1. Improvement in segmental contractile function on scan 2 was inversely related to the transmural extent of infarction on scan 1 (P=0.001). Improvement in global contractile function, as assessed by ejection fraction and mean wall thickening score, was not predicted by peak creatine kinase-MB (P=0.66) or by total infarct size, as defined by MRI (P=0.70). The best predictor of global improvement was the extent of dysfunctional myocardium that was not infarcted or had infarction comprising <25% of left ventricular wall thickness (P<0.005 for ejection fraction, P<0.001 for mean wall thickening score).

CONCLUSION

In patients with acute myocardial infarction, the transmural extent of infarction defined by ceMRI predicts improvement in contractile function.

Theory of high-speed MR imaging of the human heart with the selective line acquisition mode

Selective line acquisition mode (SLAM) reduces magnetic resonance imaging time by a factor n relative to conventional techniques. Seventeen patients with cardiac disease and three volunteers were examined with SLAM and two-frame interpolation (2FI). SLAM images were sharper than 2FI images and showed well-defined endocardial borders. SLAM is best suited for fast imaging of moving objects, such as the heart, confined to 1/n of the field of view.

Visualization of discrete microinfarction after percutaneous coronary intervention associated with mild creatine kinase-MB elevation

BACKGROUND

Mild elevations in creatine kinase-MB (CK-MB) are common after successful percutaneous coronary interventions and are associated with future adverse cardiac events. The mechanism for CK-MB release remains unclear. A new contrast-enhanced MRI technique allows direct visualization of myonecrosis.

METHODS AND RESULTS

Fourteen patients without prior infarction underwent cine and contrast-enhanced MRI after successful coronary stenting; 9 patients had procedure-related CK-MB elevation, and 5 did not (negative controls). The mean age of all patients was 61 years, 36% had diabetes, 43% had multivessel coronary artery disease, and all had a normal ejection fraction. Twelve patients (86%) received an intravenous glycoprotein IIb/IIIa inhibitor; none underwent atherectomy, and all had final TIMI 3 flow. Of the 9 patients with CK-MB elevation, 5 had a minor side branch occlusion during stenting, 2 had transient ECG changes, and none developed Q-waves. The median CK-MB was 21 ng/mL (range, 12 to 93 ng/mL), which is 2.3x the upper limit of normal. Contrast-enhanced MRI demonstrated discrete regions of hyperenhancement within the target vessel perfusion territory in all 9 patients. Only one developed a new wall motion abnormality. The median estimated mass of myonecrosis was 2.0 g (range, 0.7 to 12.2 g), or 1.5% of left ventricular mass (range, 0.4% to 6.0%). Hyperenhancement persisted in 5 of the 6 who underwent a repeat MRI at 3 to 12 months. No control patient had hyperenhancement.

CONCLUSIONS

Contrast-enhanced MRI provides an anatomical correlate to biochemical evidence of procedure-related myocardial injury, despite the lack of ECG changes or wall motion abnormalities. Mild elevation of CK-MB after percutaneous coronary intervention is the result of discrete microinfarction.

An improved MR imaging technique for the visualization of myocardial infarction

PURPOSE

To design a segmented inversion-recovery turbo fast low-angle shot (turboFLASH) magnetic resonance (MR) imaging pulse sequence for the visualization of myocardial infarction, compare this technique with other MR imaging approaches in a canine model of ischemic injury, and evaluate its utility in patients with coronary artery disease.

MATERIALS AND METHODS

Six dogs and 18 patients were examined. In dogs, infarction was produced and images were acquired by using 10 different pulse sequences. In patients, the segmented turboFLASH technique was used to acquire contrast material-enhanced images 19 days +/- 7 (SD) after myocardial infarction.

RESULTS

Myocardial regions of increased signal intensity were observed in all animals and patients at imaging. With the postcontrast segmented turboFLASH sequence, the signal intensity of the infarcted myocardium was 1,080% +/- 214 higher than that of the normal myocardium in dogs-nearly twice that of the next best sequence tested and approximately 10-fold greater than that in previous reports. All 18 patients with myocardial infarction demonstrated high signal intensity at imaging. On average, the signal intensity of the high-signal-intensity regions in patients was 485% +/- 43 higher than that of the normal myocardium.

CONCLUSION

The segmented inversion-recovery turboFLASH sequence produced the greatest differences in regional myocardial signal intensity in animals. Application of this technique in patients with infarction substantially improved differentiation between injured and normal regions.

Visualisation of presence, location, and transmural extent of healed Q-wave and non-Q-wave myocardial infarction

BACKGROUND

A technical advance in contrast-enhanced magnetic resonance imaging (MRI) has significantly improved image quality. We investigated whether healed myocardial infarction can be visualised as hyperenhanced regions with this new technique, and whether assessment of the transmural extent of infarction yields new physiological data.

METHODS

82 MRI examinations were carried out in three groups: patients with healed myocardial infarction; patients with non-ischaemic cardiomyopathy; and healthy volunteers. Patients with healed myocardial infarction were prospectively enrolled after enyzmatically proven necrosis and imaged 3 months (SD 1) or 14 months (7) later. The MRI procedure used a segmented inversion-recovery gradient-echo sequence after gadolinium administration. Findings were compared with those of coronary angiography, electrocardiography, cine MRI, and creatine kinase measurements.

FINDINGS

29 (91%) of 32 patients with infarcts imaged at 3 months (13 non-Q-wave) and all of 19 imaged at 14 months (eight non-Q-wave) showed hyperenhancement. In patients in whom the infarct-related-artery was identified by angiography, 24 of 25 imaged at 3 months and all of 14 imaged at 14 months had hyperenhancement in the appropriate territory. None of the 20 patients with non-ischaemic cardiomyopathy or the 11 healthy volunteers showed hyperenhancement. Irrespective of the presence or absence of Q waves, the majority of patients with hyperenhancement had only non-transmural involvement. Normal left-ventricular contraction was shown in seven patients examined at 3 months and three examined at 14 months, but in these cases hyperenhancement was limited to the subendocardium.

INTERPRETATION

The presence, location, and transmural extent of healed Q-wave and non-Q-wave myocardial infarction can be accurately determined by contrast-enhanced MRI.

An improved MR imaging technique for the visualization of myocardial infarction

PURPOSE

To design a segmented inversion-recovery turbo fast low-angle shot (turboFLASH) magnetic resonance (MR) imaging pulse sequence for the visualization of myocardial infarction, compare this technique with other MR imaging approaches in a canine model of ischemic injury, and evaluate its utility in patients with coronary artery disease.

MATERIALS AND METHODS

Six dogs and 18 patients were examined. In dogs, infarction was produced and images were acquired by using 10 different pulse sequences. In patients, the segmented turboFLASH technique was used to acquire contrast material-enhanced images 19 days +/- 7 (SD) after myocardial infarction.

RESULTS

Myocardial regions of increased signal intensity were observed in all animals and patients at imaging. With the postcontrast segmented turboFLASH sequence, the signal intensity of the infarcted myocardium was 1,080% +/- 214 higher than that of the normal myocardium in dogs-nearly twice that of the next best sequence tested and approximately 10-fold greater than that in previous reports. All 18 patients with myocardial infarction demonstrated high signal intensity at imaging. On average, the signal intensity of the high-signal-intensity regions in patients was 485% +/- 43 higher than that of the normal myocardium.

CONCLUSION

The segmented inversion-recovery turboFLASH sequence produced the greatest differences in regional myocardial signal intensity in animals. Application of this technique in patients with infarction substantially improved differentiation between injured and normal regions.

The use of contrast-enhanced magnetic resonance imaging to identify reversible myocardial dysfunction

BACKGROUND

Recent studies indicate that magnetic resonance imaging (MRI) after the administration of contrast material can be used to distinguish between reversible and irreversible myocardial ischemic injury regardless of the extent of wall motion or the age of the infarct. We hypothesized that the results of contrast-enhanced MRI can be used to predict whether regions of abnormal ventricular contraction will improve after revascularization in patients with coronary artery disease.

METHODS

Gadolinium-enhanced MRI was performed in 50 patients with ventricular dysfunction before they underwent surgical or percutaneous revascularization. The transmural extent of hyperenhanced regions was postulated to represent the transmural extent of nonviable myocardium. The extent of regional contractility at the same locations was determined by cine MRI before and after revascularization in 41 patients.

RESULTS

Contrast-enhanced MRI showed hyperenhancement of myocardial tissue in 40 of 50 patients before revascularization. In all patients with hyperenhancement the difference in image intensity between hyperenhanced regions and regions without hyperenhancement was more than 6 SD. Before revascularization, 804 of the 2093 myocardial segments analyzed (38 percent) had abnormal contractility, and 694 segments (33 percent) had some areas of hyperenhancement. In an analysis of all 804 dysfunctional segments, the likelihood of improvement in regional contractility after revascularization decreased progressively as the transmural extent of hyperenhancement before revascularization increased (P<0.001). For instance, contractility increased in 256 of 329 segments (78 percent) with no hyperenhancement before revascularization, but in only 1 of 58 segments with hyperenhancement of more than 75 percent of tissue. The percentage of the left ventricle that was both dysfunctional and not hyperenhanced before revascularization was strongly related to the degree of improvement in the global mean wall-motion score (P<0.001) and the ejection fraction (P<0.001) after revascularization.

CONCLUSIONS

Reversible myocardial dysfunction can be identified by contrast-enhanced MRI before coronary revascularization.

Contrast-enhanced magnetic resonance imaging of myocardium at risk: distinction between reversible and irreversible injury throughout infarct healing

OBJECTIVES

We sought to determine the relationship of delayed hyperenhancement by contrast magnetic resonance imaging (MRI) to viable and nonviable myocardium within the region at risk throughout infarct healing.

BACKGROUND

The relationship of delayed MRI contrast enhancement patterns to injured but viable myocardium within the ischemic bed at risk has not been established.

METHODS

We compared in vivo and ex vivo MRI contrast enhancement to histopathologic tissue sections encompassing the entire left ventricle in dogs (n = 24) subjected to infarction with (n = 12) and without (n = 12) reperfusion at 4 h, 1 day, 3 days, 10 days, 4 weeks and 8 weeks. In vivo MR imaging was performed 30 min after contrast injection.

RESULTS

The sizes and shapes of in vivo myocardial regions of elevated image intensity (828+/-132% of remote) were the same as those observed ex vivo (241 slices, r = 0.99, bias = 0.05+/-1.6% of left ventricle [LV]). Comparison of ex vivo MRI to triphenyltetrazolim chloride-stained sections demonstrated that the spatial extent of hyperenhancement was the same as the spatial extent ofinfarction at every stage of healing (510 slices, lowest r = 0.95, largest bias = 1.7+/-2.9% of LV). Conversely, hyperenhanced regions were smaller than the ischemic bed at risk defined by fluorescent microparticles at every stage of healing (239 slices, 35+/-24% of risk region, p<0.001). Image intensities of viable myocardium within the risk region were the same as those of remote, normal myocardium (102+/-9% of remote, p = NS).

CONCLUSIONS

Delayed contrast enhancement by MRI distinguishes between viable and nonviable regions within the myocardium at risk throughout infarct healing.

Microvascular integrity and the time course of myocardial sodium accumulation after acute infarction

Loss of membrane permeability caused by ischemia leads to cellular sodium accumulation and myocardial edema. This phenomenon has important implications to left ventricular structure and function in the first hours after myocardial infarction. We hypothesized that during this period of time, after prolonged coronary occlusion and complete reflow, the rate of myocardial sodium accumulation is governed by microvascular integrity. We used 3-dimensional (23)Na MRI to monitor myocardial sodium content changes over time in an in vivo closed-chest canine model (n=13) of myocardial infarction and reperfusion. Infarcts with microvascular obstruction (MO) defined by both radioactive microspheres and contrast-enhanced (1)H MRI showed a slower rate of sodium accumulation as well as lower blood flow at 20 minutes and 6 hours after reperfusion. Conversely, the absence of MO was associated with faster rates of sodium accumulation and greater blood flow restoration. In addition, infarct size by (23)Na MRI correlated best with infarct size by triphenyltetrazolium chloride and contrast-enhanced (1)H MRI at 9 hours after reperfusion. We conclude that in reperfused myocardial infarction, sodium accumulation is dependent on microvascular integrity and is slower in regions of MO compared with those with patent microvasculature. Finally, (23)Na MRI can be a useful tool for monitoring in vivo myocardial sodium content in acute myocardial infarction.

Early assessment of myocardial salvage by contrast-enhanced magnetic resonance imaging

BACKGROUND

Myocardial salvage after acute myocardial infarction is defined clinically by early restoration of flow and long-term improvement in contractile function. We hypothesized that contrast-enhanced magnetic resonance imaging (MRI), performed early after myocardial infarction, indexes myocardial salvage. We studied the relationship between the transmural extent of hyperenhancement by contrast-enhanced MRI, restoration of flow, and recovery of function.

METHODS AND RESULTS

The left anterior descending coronary artery was occluded in dogs (n=15) for either 45 minutes, 90 minutes, or permanently. Cine and contrast-enhanced MRI were performed 3 days after the procedure; cine MRI was also done 10 and 28 days after the procedure. The transmural extent of hyperenhancement and wall thickening were determined using a 60-segment model. The mean transmural extent of hyperenhancement for the 45-minute occlusion group was 22% of the 90-minute group and 18% of the permanent occlusion group (P:<0.05 for both). The transmural extent of hyperenhancement on day 3 was related to future improvement in both wall thickening score and absolute wall thickening at 10 and 28 days (P:<0.0001 for each). For example, of the 415 segments on day 3 that were dysfunctional and had <25% transmural hyperenhancement, 362 (87%) improved by day 28. Conversely, no segments (0 of 9) with 100% hyperenhancement improved. The transmural extent of hyperenhancement on day 3 was a better predictor of improvement in contractile function than occlusion time (P:<0.0001).

CONCLUSIONS

A reduction in the transmural extent of hyperenhancement by contrast-enhanced MRI early after myocardial infarction is associated with an early restoration of flow and future improvement in contractile function.

Evaluation of myocardial viability by MRI

Distinguishing between viable and non-viable myocardium is an important clinical issue. Several magnetic resonance (MR) techniques to address this issue have been proposed. Spectroscopy of phosphorus-31 and hydrogen-1 from creatine as well as imaging of sodium-23 and potassium-39 reflect information related to cellular metabolism. The spatial and temporal resolutions of these techniques are limited, however, by the small magnitude of the MR signal. Proton imaging techniques include examination of pathologic alterations in MR relaxation times (T1 and T2), wall thickness and thickening, cine MRI combined with low-dose dobutamine, first-pass contrast enhancement patterns, and delayed contrast enhancement patterns. Of the proton imaging approaches, cine MRI combined with low-dose dobutamine is supported by the largest body of clinical evidence supporting the hypothesis that the technique yields useful information regarding myocardial viability. Recent data suggest that delayed contrast enhancement examines the transmural extent of viable myocardium irrespective of contractile function and that this technique should also be considered in a clinical setting.

Relationship of MRI delayed contrast enhancement to irreversible injury, infarct age, and contractile function

BACKGROUND

Contrast MRI enhancement patterns in several pathophysiologies resulting from ischemic myocardial injury are controversial or have not been investigated. We compared contrast enhancement in acute infarction (AI), after severe but reversible ischemic injury (RII), and in chronic infarction.

METHODS AND RESULTS

In dogs, a large coronary artery was occluded to study AI and/or chronic infarction (n = 18), and a second coronary artery was chronically instrumented with a reversible hydraulic occluder and Doppler flowmeter to study RII (n = 8). At 3 days after surgery, cine MRI revealed reduced wall thickening in AI (5+/-6% versus 33+/-6% in normal, P<0.001). In RII, wall thickening before, during, and after inflation of the occluder for 15 minutes was 35+/-5%, 1+/-8%, and 21+/-10% and Doppler flow was 19.8+/-5.3, 0.2+/-0.5, and 56.3+/-17.7 (peak hyperemia) cm/s, respectively, confirming occlusion, transient ischemia, and reperfusion. Gd-DTPA-enhanced MR images acquired 30 minutes after contrast revealed hyperenhancement of AI (294+/-96% of normal, P<0.001) but not of RII (98+/-6% of normal, P = NS). Eight weeks later, the chronically infarcted region again hyperenhanced (253+/-54% of normal, n = 8, P<0.001). High-resolution (0.5 x 0.5 x 0.5 mm) ex vivo MRI demonstrated that the spatial extent of hyperenhancement was the same as the spatial extent of myocyte necrosis with and without reperfusion at 1 day (R = 0.99, P<0.001) and 3 days (R = 0.99, P<0.001) and collagenous scar at 8 weeks (R = 0.97, P<0.001).

CONCLUSIONS

In the pathophysiologies investigated, contrast MRI distinguishes between reversible and irreversible ischemic injury independent of wall motion and infarct age.

Relationship of elevated 23Na magnetic resonance image intensity to infarct size after acute reperfused myocardial infarction

BACKGROUND

Elevated 23Na MR image intensity after acute myocardial infarction has previously been shown to correspond to high tissue [Na+] and loss of myocardial viability. In this study, we explored the potential of in vivo 23Na MRI to assess infarct size and investigated possible mechanisms for elevated 23Na image intensity.

METHODS AND RESULTS

Thirteen dogs and 8 rabbits underwent in situ coronary artery occlusion and reperfusion and were imaged by 23Na MRI. For anatomically matched left ventricular short-axis cross sections (n=46), infarct size measured by in vivo 23Na MRI correlated well with triphenyltetrazolium chloride staining (r=0.87, y=0.92x+3.37, P<0.001). Elevated 23Na image intensity was observed in infarcted myocardium (206+/-37% of remote in dogs, P<0.001; 215+/-58% in rabbits, P<0.002) but was not observed after severe but reversible ischemic injury (101+/-11% of baseline, P=NS). High-resolution ex vivo imaging revealed that regions of elevated 23Na image intensity appeared to be identical to those of infarcted regions (r=0.97, y=0.92x+1.52, P<0.001). In infarcted regions, total tissue [Na+] was elevated (89+/-12 versus 37+/-9 mmol/L in control tissue, 156+/-60% increase, P<0.001) and was associated with increased intracellular sodium (254+/-68% of control, P<0.005) and an increased intracellular sodium/potassium ratio (868+/-512% of control, P<0.002). Morphometric analysis demonstrated only a minor increase in extracellular volume (17+/-8% versus 14+/-5%, P<0.05) in the infarcted territory.

CONCLUSIONS

Elevated 23Na MR image intensity in vivo measures infarct size after reperfused infarction in both a large and a small animal model. The mechanism of elevated 23Na image intensity is probably intracellular sodium accumulation secondary to loss of myocyte ionic homeostasis.

Physiological basis for potassium (39K) magnetic resonance imaging of the heart

The potassium cation (K+) is fundamentally involved in myocyte metabolism. To explore the potential utility of direct MRI of the most abundant natural isotope of potassium, 39K, we compared 39K magnetic resonance (MR) image intensity with regional myocardial K+ concentrations after irreversible injury. Rabbits were subjected either to 40 minutes of in situ coronary artery occlusion and 1 hour of reperfusion (n=26) or to 24 hours of permanent occlusion (n=4). The hearts were then isolated and imaged by 39K MRI (n=10), or tissue samples were analyzed for regional 39K content by MR spectroscopy (n=9), K+ and Na+ concentrations by atomic emission spectroscopy (inductively coupled plasma atomic emission spectroscopy; n=5), or intracellular K+ content by electron probe x-ray microanalysis (n=6). Three-dimensional 39K MR images of the isolated hearts were acquired in 44 minutes with 3 x 3 x 3-mm resolution. 39K MR image intensity was reduced in infarcted regions (51.7+/-4. 8% of remote; P<0.001). The circumferential extent and location of regions of reduced 39K image intensity were correlated with those of infarcted regions defined histologically (r=0.97 and r=0.98, respectively). Compared with remote regions, tissue analysis revealed that infarcted regions had reduced 39K concentration (by MR spectroscopy, 40.5+/-9.3% of remote; P<0.001), reduced potassium-to-sodium ratio (by inductively coupled plasma atomic emission spectroscopy, 20.7+/-2.1% of remote; P<0.01), and reduced intracellular potassium (by electron probe x-ray microanalysis, K+ peak-to-background ratio 0.95+/-0.32 versus 2.86+/-1.10, respectively; P<0.01). We acquired the first 39K MR images of hearts subjected to infarction. In the pathophysiologies examined, potassium (39K) MR image intensity primarily reflects regional intracellular K+ concentrations.

Quantification and time course of microvascular obstruction by contrast-enhanced echocardiography and magnetic resonance imaging following acute myocardial infarction and reperfusion

OBJECTIVES

We aimed to validate contrast-enhanced echocardiography (CE) in the quantification of microvascular obstruction (MO) against magnetic resonance imaging (MRI) and the histopathologic standards of radioactive microspheres and thioflavin-S staining. We also determined the time course of MO at days 2 and 9 after infarction and reperfusion.

BACKGROUND

Postinfarction MO occurs because prolonged ischemia produces microvessel occlusion at the infarct core, preventing adequate reperfusion. Microvascular obstruction expands up to 48 h after reperfusion; the time course beyond 2 days is unknown. Though used to study MO, CE has not been compared with MRI and thioflavin-S, which yield precise visual maps of MO.

METHODS

Ten closed-chest dogs underwent 90-min coronary artery occlusion and reperfusion. Both CE and MRI were performed at 2 and 9 days after reperfusion. The MO regions by both methods were quantified as percent left ventricular (% LV) mass. Radioactive microspheres were injected for blood flow determination. Postmortem, the myocardium was stained with thioflavin-S and 2,3,5-triphenyltetrazolium chloride.

RESULTS

Expressed as % total LV, MO by MRI matched in size MO by microspheres using a flow threshold of <40% remote (4.96+/-3.52% vs. 5.32+/-3.98%, p=NS). For matched LV cross sections, MO by CE matched in size MO by microspheres using a flow threshold of <60% remote (13.27+/-4.31% vs. 13.5+/-4.94%, p=NS). Both noninvasive techniques correlated well with microspheres (MRI vs. CE, r=0.87 vs. 0.74; p=NS). Microvascular obstruction by CE corresponded spatially to MRI-hypoenhanced regions and thioflavin-negative regions. For matched LV slices at 9 days after reperfusion, MO measured 12.94+/-4.51% by CE, 7.11+/-3.68% by MRI and 9.18+/-4.32% by thioflavin-S. Compared to thioflavin-S, both noninvasive techniques correlated well (CE vs. MRI, r=0.79 vs. 0.91; p=NS). Microvascular obstruction size was unchanged at 2 and 9 days (CE: 13.23+/-4.11% vs. 12.69+/-4.97%; MRI: 5.53+/-4.94% vs. 4.68+/-3.44%; p=NS for both).

CONCLUSIONS

Both CE and MRI can quantify MO. Both correlate well with the histopathologic standards. While MRI can detect regions of MO with blood flow <40% of remote, the threshold for MO by CE is <60% remote. The extent of MO is unchanged at 2 and 9 days after reperfusion.

DXplain on the Internet

DXplain, a computer-based medical education, reference and decision support system has been used by thousands of physicians and medical students on stand-alone systems and over communications networks. For the past two years, we have made DXplain available over the Internet in order to provide DXplain's knowledge and analytical capabilities as a resource to other applications within Massachusetts General Hospital (MGH) and at outside institutions. We describe and provide the user experience with two different protocols through which users can access DXplain through the World Wide Web (WWW). The first allows the user to have direct interaction with all the functionality of DXplain where the MGH server controls the interaction and the mode of presentation. In the second mode, the MGH server provides the DXplain functionality as a series of services, which can be called independently by the user application program.

Fast 23Na magnetic resonance imaging of acute reperfused myocardial infarction. Potential to assess myocardial viability

BACKGROUND

The ability of the myocyte to maintain an ionic concentration gradient is perhaps the best indication of myocardial viability. We studied the relationship of 23Na MRI intensity to viability and explored the potential of fast-imaging techniques to reduce 23Na imaging times in rabbits and dogs.

METHODS AND RESULTS

Eighteen rabbits underwent in situ coronary artery occlusion and reperfusion. The hearts were then either imaged following isolation and perfusion with cardioplegic solution (n = 6), imaged in vivo (n = 6), or analyzed for 23Na content and relaxation times (n = 12). Normal rabbits (n = 6) and dogs (n = 4) were imaged to examine the effect of animal size on 23Na image quality. 23Na imaging times were 7, 11, and 4 minutes for isolated rabbits, in vivo rabbits, and in vivo dogs, respectively. Infarcted, reperfused regions, identified by triphenyltetrazolium chloride staining, showed a significant elevation in 23Na image intensity compared with viable regions (isolated, 42 +/- 5%, P < .02; in vivo, 95 +/- 6%, P < .001), consistent with increased tissue sodium content. Similarly, 23Na MR spectroscopy showed that [Na+] was higher in nonviable than viable myocardium (isolated, 99 +/- 4 versus 61 +/- 2 mmol/L; in vivo, 91 +/- 2 versus 38 +/- 1 mmol/L; P < .001 for both). Image signal-to-noise ratios were higher in dogs than rabbits despite shorter imaging times, primarily due to larger voxels.

CONCLUSIONS

Following acute infarction with reperfusion, a regional increase in 23Na MR image intensity is associated with nonviable myocardium. Fast gradient-echo imaging techniques reduce 23Na imaging times to a few minutes, suggesting that 23Na MR imaging has the potential to become a useful experimental and clinical tool.

Myocardial Gd-DTPA kinetics determine MRI contrast enhancement and reflect the extent and severity of myocardial injury after acute reperfused infarction

BACKGROUND

Contrast medium-enhanced magnetic resonance images of acute, reperfused infarcts have shown hypoenhanced and hyperenhanced regions in areas of injured myocardium. The precise mechanisms that lead to these altered enhancement patterns are unknown. This study was designed to evaluate possible mechanisms and to relate altered enhancement patterns to myocardial perfusion and viability.

METHODS AND RESULTS

Thirteen rabbits underwent in situ coronary artery occlusion and reperfusion followed by isolated perfusion with cardioplegic solution. T1-weighted spin-echo images were acquired continuously during step changes in perfusate Gd-DTPA concentration. Regional blood flow was also measured by use of radioactive microspheres in all rabbits. There were marked differences in Gd-DTPA wash-in and washout time constants (wash-in, 0.8 +/- 0.1, 2.1 +/- 02, and 16.3 +/- 2.4 minutes, P < .001; washout, 1.6 +/- 0.1, 4.8 +/- 0.5, and 31.1 +/- 3.3 minutes, P < .001) in normal, infarct rim, and infarct core regions, respectively, resulting in differential enhancement of these regions. Microsphere flows in the infarct rim and core were 42.9 +/- 4.0% and 12.0 +/- 1.6% of normal myocardium and correlated well with washout time constants (r = .86, y = 0.77x - 0.002, P < .001), suggesting that these time constants index the severity of microvascular damage. In addition, spatial maps of washout time constants were produced. The extent of regions with abnormal time constants correlated well with triphenyltetrazolium chloride-determined infarct size (r = .94, y = 0.95x + 4.17, P < .001).

CONCLUSIONS

In contrast-enhanced magnetic resonance images of acute, reperfused rabbit infarcts, differential image intensity is primarily due to regional differences in contrast agent wash-in and washout time constants. These regional differences in time constants also indicate the extent and severity of myocardial injury.

Mouse astrocytes respond to the chemokines MCP-1 and KC, but reverse transcriptase-polymerase chain reaction does not detect mRNA for the KC or new MCP-1 receptor

Previous studies demonstrated the involvement of astrocytes in the development of astrogliosis, a condition in which these cells undergo proliferation and hypertrophy. To examine whether astrocytes could migrate into lesions, we tested the influence of the murine chemokines MCP-1, KC, TCA3, and MIP-1 beta on migration of cultured neonatal mouse astrocytes. Subnanomolar concentrations of MCP-1 and KC were active chemoattractants indicating that these molecules were effective at physiologic concentrations. Specificity of MCP-1 was demonstrated by antibody inhibition and by the finding that the chemokine MIP-1 beta failed to induce astrocyte migration. The migratory responses were sensitive to pertussis toxin; this finding is consistent with involvement of G protein-coupled receptors. To examine the receptors for these chemokines further, we cloned the mouse homolog of the human MCP-1 receptor from a mouse peritoneal exudate cell cDNA library. The gene had 78% nucleotide sequence homology with the human MCP-1 receptor (the nucleotide sequence of clone 1 encoding the mouse MCP-1 receptor can be obtained from the GenBank database, accession number U56819). However, reverse transcriptase-polymerase chain reaction (RT-PCR) failed to detect message for either the MCP-1 or KC receptors in astrocytes. The combined data suggest that mouse astrocytes use novel receptors to recognize these chemokines.