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Biomedical subjects

A Engel

Publications and source records attributed to A Engel.

At least 199 records · Page 11Linked to original sources
PubMed1995

Increased production of erythropoietin after application of antidiuretic hormone. A consequence of renal vasoconstriction?

The renal glycoprotein hormone erythropoietin (Epo) is the key element in the feedback control of the production of red blood cells (RBC) in bone marrow. Excess of antidiuretic hormone (ADH) increases the RBC mass by increasing the synthesis of Epo. The mechanism of the Epo stimulating effect of ADH is not fully understood. Rats were treated with ADH with or without prior injection of a V1a-receptor antagonist. Additional experiments were carried out by stimulating the V2-receptor by desmopressin (DDAVP). Epo level in plasma was doubled following injections of ADH. Blockade of the V1a-receptor completely abolished the Epo stimulating effect of ADH. Neither ADH alone nor the combined giving of V1a-antagonist and ADH had an influence on the glomerular filtration rate or the renal plasma flow. Therefore, the increased Epo synthesis after application of ADH cannot be explained by a constriction of renal blood vessels with consecutive ischemic hypoxia. There is rather a direct stimulation of Epo synthesis by ADH via its receptors. Since a selective stimulation of the V2-receptor by DDAVP did not increase to Epo level in plasma, the observed increase of Epo is mediated by the V1a-receptor.

Animals↗
PubMed1995

Hybrid scanning transmission electron/scanning tunneling microscope system for the preparation and investigation of biomolecules.

A hybrid scanning transmission electron/scanning tunneling microscope vacuum system is introduced, which allows freeze drying and metal coating of biological samples and their simultaneous observation by scanning transmission electron microscopy and scanning tunnelling microscopy (STM). Different metal coatings and STM tips were analysed to obtain the highest possible resolution for such a system. Bovine liver catalase was used as a test sample and the STM results are compared to a molecular scale model.

Animals↗
PubMed1995

Muscle function after endoprosthetic replacement of the proximal tibia. Different techniques for extensor reconstruction in 17 tumor patients.

We analyzed in 17 patients the outcome of various surgical techniques for reconstruction of the extension apparatus after resection of the proximal tibia and tumor prosthesis implantation. The mean follow-up period was 5 (1.5-11) years. Knee extension and flexion strength were measured isokinetically by dynamometer and muscle activities of the vastus medialis, the vastus lateralis and the rectus femoris muscles determined by means of EMG. Muscle function of the operated leg was compared to that of the contralateral extremity by using various surgical techniques: fibula transposition, transposition of the gastrocnemius muscle, and combination technique. The results concerning the operated leg were compared within the 3 groups and the activity and strength of both legs were compared to those of a control group of healthy subjects matched for age and weight. The strength of extensor muscles of the healthy leg was greater than that of the control group in flexion position (60 degrees-90 degrees); the hamstring strength values were within the normal range. The strength of extensor muscles of the operated leg differed between groups II and III at 90 degrees in favor of group II (p < 0.01) and at 60 degrees to 20 degrees (p < 0.001) in favor of group III. Expressed in percentage ranges (nonoperated leg set at 100 percent) the flexor muscles averaged 30 percent, the extensor muscles represented on average 12 percent (9-17 percent) in group I, 9 percent (5-18 percent) in group II and 16 percent (6-26 percent) in group III, depending on the knee angle.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanical Phenomena↗
PubMed1995

[Effect of task difficulty on hemispheric differences in visual image processing].

Hemispheric specialization of visual imagery processes is a topic of recurrent debate. Recent theories have argued for a left postcentral center for the generation of visual images. Because of contradictory results the process of imaging has been divided into a left hemisphere component (visual imagery) and a right hemisphere component (supramodal spatial mental imagery). An alternative and more parsimonious account would be that the degree of difficulty of visual imagery tasks is responsible for hemispheric differences: easy tasks are solved faster within the left hemisphere because of a direct access to action control, whereas more difficult tasks rely on a right hemisphere specialization for spatial processing. In our experiment we varied the degree of difficulty by introducing concurrent tasks which were interspersed at random into the series of critical stimuli. Interspersing concurrent tasks prevents subjects from automatizing of critical processes and delays reaction times. For the critical visual imagery tasks we presented either upper or lower case letters to the different visual fields and asked for figural aspects of the corresponding lower or upper case letters, respectively. The interspersed stimuli concerned an estimation of the distance of the stimuli or the localization of the stimuli in the upper or lower part of the screen. We also presented upper and lower case letters in random order, thus forcing the subjects to shift between two different visual imagery tasks. In order to look for the effect of position-repetition priming upon visual field advantages we also analyzed the interaction of the position on the screen and its repetition with hemispheric advantages. In one of the visual imagery tasks, the experiments without concurrent tasks yielded a right visual field/left hemisphere advantage. On the other hand, interspersing other tasks resulted in a left visual field/right hemisphere advantage for visual imagery tasks, though none of the concurrent tasks showed a significant visual field advantage in itself. The same holds for combined and random presentation of the two different visual imagery tasks. The results therefore argue for a right hemisphere advantage for difficult visual imagery tasks, whereas easier tasks (monotonous presentation of only one visual imagery tasks) are processed faster in the left hemisphere. The random repetition of positions on the screen yielded a significant effect but had no influence on the visual field advantages found.

Adult↗
PubMed1994

Towards atomic interpretation of F-actin filament three-dimensional reconstructions.

We have recorded dark field images of negatively stained F-actin filaments polymerized with 2 mM MgCl2 and 50 mM KCl with a scanning transmission electron microscope and computed 3-D reconstructions using a helical parameter search to optimize simultaneously the helical repeat length, the radial position of the filament axis, and the helical selection rule. The resulting optimized averaged filament 3-D reconstruction at 2.5 nm resolution is remarkably similar to an atomic model of the F-actin filament. By comparison, several structural features of the reconstruction can be interpreted at the level of distinct secondary structure elements, and predictions made by the atomic model could be verified: for instance, the density connecting the two long-pitch helical strands in our reconstruction co-localizes with an extended beta-hairpin, the "hydrophobic loop" (i.e. residues 262 to 274), which according to the atomic model establishes the major intersubunit contact between the two long-pitch helical strands. The most pronounced structural variations among individual filament 3-D reconstructions were observed in (1) the details of the intersubunit contact pattern between the two long-pitch helical strands, and (2) the exact size and shape of subdomain 2 of the F-actin molecule, which appears rather flexible and easily deformed. In addition, we found that all phenotypes of F-actin filament 3-D reconstructions that arise from small deviations from the optimal helical parameters or from lowering the nominal resolution exhibited stronger intersubunit contacts between than along the two long-pitch helical strands, a structural feature that has been emphasized for a number of F-actin filament 3-D reconstructions in the past. Since this is clearly at variance with the relative strength of the intersubunit contacts as predicted by the atomic model, it may represent an artifactual structural feature arising from low-resolution data or suboptimal helical data processing, and should therefore be interpreted with caution in terms of indicating chemical, mechanical or conformational states of the F-actin filament.

Actins↗
PubMed1994

In vitro approaches to investigation of the early steps of colicin-OmpF interaction.

The OmpF porin plays a central role during the colicin uptake by sensitive Escherichia coli cells. Lipopolysaccharide-OmpF complexes (-1bLPS-OmpF), which contain one tightly bound and no loosely bound LPS molecules for each porin trimer, is able to recognize and bind to immobilized colicins. This association is specific to colicins A and N, which both use the OmpF porin as receptor, and depends on the presence of the porin-receptor domain on the bacteriocin molecule. The -1bLPS-OmpF complex protects sensitive cells against colicin A and N activity. The protection level depends on the native conformation, as demonstrated by heat denaturation of the trimeric porin which abolishes the protection. This indicates that the purified OmpF trimer presents an affinity site for the colicin which efficiently mimics the native cellular receptor site. These results are discussed with regard to the conformation of the receptor site and to the early steps of colicin uptake.

Antibodies↗
PubMed1994

The three-dimensional structure of human erythrocyte aquaporin CHIP.

Water-permeable membranes of several plant and mammalian tissues contain specific water channel proteins, the 'aquaporins'. The best characterized aquaporin is CHIP, a 28 kDa red blood cell channel-forming integral protein. Isolated CHIP and Escherichia coli lipids may be assembled into 2-D crystals for structural analyses. Here we present (i) a structural characterization of the solubilized CHIP oligomers, (ii) projections of CHIP arrays after negative staining or metal-shadowing, and (iii) the 3-D structure at 1.6 nm resolution. Negatively stained CHIP oligomers exhibited a side length of 6.9 nm with four-fold symmetry, and a mass of 202 +/- 3 kDa determined by scanning transmission electron microscopy. Reconstituted into lipid bilayers, CHIP formed 2-D square lattices with unit cell dimensions a = b = 9.6 nm and a p422(1) symmetry. The 3-D map revealed that CHIP tetramers contain central stain-filled depressions about the fourfold axis. These cavities extend from both sides into the transbilayer domain of the molecule leaving only a thin barrier to be penetrated by the water pores. Although CHIP monomers behave as independent pores, we propose that their particular structure requires tetramerization for stable integration into the bilayer.

Aquaporin 1↗
PubMed1994

Atomic force microscopy produces faithful high-resolution images of protein surfaces in an aqueous environment.

The atomic force microscope has the potential to monitor structural changes of a biological system in its native environment. To correlate them with the biological function at a molecular level, high lateral and vertical resolution are required. Here we demonstrate that the atomic force microscope is capable of imaging the surface of the hexagonally packed intermediate layer of Deinococcus radiodurans in buffer solution with a lateral resolution of 1 nm and a vertical resolution of 0.1 nm. On average, these topographs differ from those determined by electron microscopy by <0.5 nm.

Journal Article↗
PubMed1994

Biologically active two-dimensional crystals of aquaporin CHIP.

Plasma membranes of several mammalian tissues are highly permeable to water due to the presence of CHIP, the 28-kDa channel-forming integral protein which is the archetypal member of the aquaporin family of water channel proteins. To define its native structure, purified red cell CHIP protein was reconstituted into lipid bilayers at a high protein-to-lipid ratio, and the resulting 3-microns diameter membrane vesicles were examined by high resolution electron microscopy. The reconstituted membranes contained highly ordered two-dimensional crystalline lattices of p422(1) symmetry in which each CHIP tetramer contained a central depression extending from the outer and inner surfaces of the membrane into the transbilayer domain of the molecule. The reconstituted membranes also exhibited extremely high osmotic water permeability, Pf = 0.472 cm/s, corresponding to the sum of activities of all incorporated CHIP molecules. These studies report the first two-dimensional crystallization of a biologically active water channel and provide direct evidence of the structure responsible for its pore-like behavior.

Aquaporin 1↗
PubMed1994

Reproducible acquisition of Escherichia coli porin surface topographs by atomic force microscopy.

Crystalline membranes reconstituted from Escherichia coli OmpF porin and phospholipids were adsorbed to freshly cleaved mica and imaged in solution by the atomic force microscope. The extracellular as well as the periplasmic side of the porin trimers could be identified and the conditions to record topographs at 1-nm lateral and 0.1-nm vertical resolution were established.

Biophysical Phenomena↗
PubMed1994

Reconstitution of native-type noncrystalline lens fiber gap junctions from isolated hemichannels.

Gap junctions contain numerous channels that are clustered in apposed membrane patches of adjacent cells. These cell-to-cell channels are formed by pairing of two hemichannels or connexons, and are also referred to as connexon pairs. We have investigated various detergents for their ability to separately solubilize hemichannels or connexon pairs from isolated ovine lens fiber membranes. The solubilized preparations were reconstituted with lipids with the aim to reassemble native-type gap junctions and to provide a model system for the characterization of the molecular interactions involved in this process. While small gap junction structures were obtained under a variety of conditions, large native-type gap junctions were assembled using a novel two-step procedure: in the first step, hemichannels that had been solubilized with octylpolyoxyethylene formed connexon pairs by dialysis against n-decyl-beta-D-maltopyranoside. In the second step, connexon pairs were reconstituted with phosphatidylcholines by dialysis against buffer containing Mg2+. This way, double-layered gap junctions with diameter < or = 300 nm were obtained. Up to several hundred channels were packed in a noncrystalline arrangement, giving these reconstituted gap junctions an appearance that was indistinguishable from that of the gap junctions in the lens fiber membranes.

Animals↗
PubMed1994

Topology of the morphological domains of the chaperonin GroEL visualized by immuno-electron microscopy.

Electron microscopy of the tetradecameric double-ring complex of GroEL reveals a four-layered structure, indicating that the 58 kDa subunits are composed of two major morphological domains. We have used immuno-electron microscopy to assign these domains to the corresponding segments of the GroEL sequence. Upon chemical modification of GroEL with N-ethylmaleimide, protease treatment in the presence of ATP or ADP generates GroEL fragments of 15 kDa (N15; residues 1-141) and 40 kDa (C40; residues 153-531). As visualized by scanning transmission electron microscopy, affinity-purified antibodies directed against C40 recognize the outer layers, whereas antibodies against N15 interact with the equatorial portions of the GroEL double-ring. Thus, the two major domains of the subunits in the chaperonin complex are arranged in the order C40-N15:N15-C40. The single-ring chaperonin co-factor GroES interacts with the C40 domain while the ATP-binding site of GroEL is probably close to the junction between N15 and C40.

Antibodies↗
PubMed1994

[Osteonecrosis].

The causes of osteonecrosis are varied. The pathogenesis of osteonecrosis subsequent to trauma is well known. Furthermore, an association of this disorder with cortisone, haemoglobinopathies, rare metabolic diseases, and risk factors seems to be evident. However, a sizable proportion of patients have osteonecrosis in which none of these associations exists, referred to as idiopathic osteonecrosis. Generally, intraosseous vascularization represents the common pathway for the various causes of the disease. The prognosis is influenced by the age of the patient, the location of the necrosis and the stage of the disease at the time of diagnosis. In recent times diagnosis has been improved by the use of modern imaging modalities, and MR imaging is now widely accepted as the imaging method of choice. It not only allows early diagnosis but also yields exact staging information in advanced disease, which is a requirement for adequate therapy.

Adult↗
PubMed1994

[The non-traumatic femur head necrosis in the adult. I: pathophysiology, clinical picture and therapeutic options].

Intraosseous vascularization is the common pathway of the multifactorial causes of avascular necrosis of the hip (AVN). Recurring ischemic phases could lead to bone necrosis in the initial stage of the disease, when it is still reversible. A spontaneous repair mechanism can lead to complete healing at this stage. The necrotic area demarcates the superior-anterior aspect from the residual femoral head with a reactive interface in the irreversible early stage of the disease. In this stage damage to the femoral head can only be delayed with core decompression or femoral osteotomy, because a sufficient repair mechanism is no longer possible. When the articular surface collapses the early stage gives way to the late stage. Cartilage incongruence and microfracture lead to progressive destruction of the femoral head and to secondary osteoarthritis. In this late stage femoral osteotomy is only successful in carefully selected patients. Total hip replacement is the last resort for patients with painful destruction of the joint. As clinical symptoms are unspecific, MRI plays an important role for early diagnosis of AVN in the initial or early stages. Conservative treatment is not successful in any of the three stages of the disease.

Adult↗
PubMed1994

[Proton spin tomography in meniscus diagnosis].

Magnetic resonance imaging (MRI) is an excellent method of examining the meniscus with an accuracy of about 91-94% for the median meniscus and 88.4-100% for the lateral meniscus in T1-weighted images. The precision of the diagnosis can be increased to 100% by the use of paramagnetic contrast medium (gadolinium DTPA), as was shown by the evaluation of 170 knee joints. The reason it is difficult to evaluate the meniscus without contrast medium is a residual meniscus or regenerated meniscus. We are therefore dramatically able to reduce the number of diagnostic arthroscopies for evaluation of the meniscus.

Contrast Media↗
PubMed1993

[Transjugular liver biopsy in patients with bleeding tendency].

Transjugular liver biopsy (TJB) was successful in 14 of 23 patients with chronic liver disease and abnormal coagulation profiles. There were 7 men and 7 women between the ages of 20 and 65. No bleeding followed the procedure, nor were there any other significant complications. The size of the specimens and the number of portal spaces included were compared with those obtained by percutaneous liver biopsy from 12 patients with advanced liver cirrhosis. TJB samples were smaller than those obtained by percutaneous biopsy (0.56 +/- 1.6 cm vs 1.0 +/- 0.05) and contained fewer portal spaces (2.2 +/- 1.6 vs 3.4 +/- 2.3). Despite the smaller size, the contribution of TJB to diagnosis and prognosis was defined as good in 78% of the patients. We conclude that TJB is safe and is an important tool for liver tissue diagnosis in patients with bleeding tendency.

Adult↗