Search PubMed⌕ Search

Biomedical subjects

J Brix

Publications and source records attributed to J Brix.

At least 19 recordsLinked to original sources
PubMed2002

[Effects of active and passive movement stimuli on cerebral hemodynamics and the cerebral metabolism].

INTRODUCTION: In contrast to the well-examined cardiovascular changes during movement stimuli, up to now changes of cerebral hemodynamics and cerebral metabolism have rarely been studied. We investigated the question if active and passive movement stimuli cause changes in the cerebral hemodynamics and the cerebral metabolism. METHOD: Active and passive repetitive movement stimuli on 14 volunteers (8 females, 6 males, age 35 +/- 8 years) were examined. As a parameter of cerebral hemodynamics the mean and the peak blood flow velocity (mCBFV(MCA), pCBFV(MCA)) in the middle cerebral artery (MCA) were recorded by transcranial Doppler sonography. At the same time the noninvasive blood pressure (Penaz method) and the CO(2) expiration concentration were investigated on 8 volunteers of the collective. As cerebral metabolic parameters we examined in 4 volunteers additionally the cerebral respiratory chain enzyme cytochrome aa3 (ccytaa3) and the cerebral oxygen saturation (cHbO(2)) by the transcranial near infrared spectroscopy. With each volunteer 4 measurement series were carried out with a special active and passive exercise program for the right upper as well as the right lower extremity. Each measurement series was formed according to the evoked flow test (R. Aaslid): Exercises were carried out for 20 s, followed by a break of 20 s; this was repeated 10 times for each series. RESULTS: During active exercises of the right lower extremity we found an increase of 13.6% (p < 0.001) of pCBFV(MCA) and an increase of 3.8% (p = 0.003) of mCBFV(MCA). During passive exercises of the lower extremity the increases ran up to 12.3% (p < 0.001) for pCBFV(MCA) and 3.4% (p = 0.004) for mCBFV(MCA). The increases of pCBFV(MCA) came up to 12.5% (p < 0.001) at active exercises of the right upper extremity, those of mCBFV(MCA) to 3.5% (p = 0.15). During passive exercises of the upper extremity the pCBFV(MCA) increased by 12.2% (p < 0.001) and the mCBFV(MCA) by 4.6% (p = 0.007). Significant increases of ccytaa3 were measured during active exercises of the upper extremity (1.6%; p = 0.04) and of the lower extremity (2.7%, p = 0.007). We also found an increase of ccytaa3 during passive exercises of the upper extremity (1.5%, p = 0.04). Significant changes of cHbO(2) were measured with 2.5% (p < 0.05) at active exercises of the lower extremity. CONCLUSION: These studies show that active as well as passive clinical exercises cause an increase of cerebral blood flow velocity. We attribute the increase of cerebral hemodynamics and cerebral metabolism to cerebral activation and autoregulative mechanisms.

Adult↗
PubMed2001

[Wrong again!].

Explore the source record for details and available documents.

Greek World↗
PubMed2001

Measurement of the individual exposure to 50 and 16 2/3 Hz magnetic fields within the Bavarian population.

This study investigates the individual magnetic field exposures at 16 2/3 and 50 Hz of 1952 people, selected from the Bavarian population. Personal flux density meters ("Field Watcher FW2A") were worn by the participants for 24 h. Every second, the flux density was recorded for both frequencies and for the three spatial axes (dynamic range per axis: several nT up to 100 microT at 50 Hz, 150 microT at 16 2/3 Hz). For 50 Hz fields, the mean of the 1,952 individual means was 0.101 microT and that of the individual medians was 0.047 microT. High level exposures occurred mainly during working hours. Only 2.4% of the subjects showed individual medians higher than 0.2 microT. About 53% of all volunteers were working on the day of recording. Levels for craftsmen (n = 148; mean individual mean: 0.166 microT) were generally higher than those for office workers (n = 624; mean individual mean: 0.107 microT). Flux densities exceeding 100 microT at 50 Hz were measured in 31 persons. The total time with such extreme exposures amounts to nearly 21 min, less than 0.001% of the total time for all measurements (5.3 years). To our knowledge, this is the first exposure study where 16 2/3 Hz magnetic fields (caused by electrified railways) have additionally been monitored over 24 h. For persons living next to railway lines, the mean individual mean (0.156 microT) and mean individual median (0.102 microT) were calculated. Over all, the mean exposures are only 0.1% of the magnetic flux density limit for 50 Hz (100 microT) and about 0.05% of the limit (300 microT) for 16 2/3 Hz recommended by the International Commission on Non-Ionizing Radiation Protection.

Adolescent↗
PubMed2000

The mitochondrial import receptor Tom70: identification of a 25 kDa core domain with a specific binding site for preproteins.

The mitochondrial import receptor of 70 kDa, Tom70, preferentially recognizes precursors of membrane proteins with internal targeting signals. We report the identification of a stably folded 25 kDa core domain located in the middle portion of Tom70 that contains two of the seven tetratricopeptide repeat motifs of the receptor. The core domain binds non-cleavable and cleavable preproteins carrying internal targeting signals with a specificity indistinguishable from the full-length receptor. Competition studies indicate that both types of preproteins interact with overlapping binding sites of the core domain and that at least one additional interaction site is present in the full-length receptor. We suggest a model of Tom70 function in import of membrane proteins whereby a hydrophobic preprotein concomitantly interacts with several binding sites of the receptor.

Binding, Competitive↗
PubMed1999

The preprotein translocase of the outer mitochondrial membrane: receptors and a general import pore.

Cytosol-synthesized preproteins destined for the mitochondria are transported across the outer membrane by the translocase of the mitochondrial outer membrane (TOM complex). This dynamic transport machinery can be divided into receptors that recognize preprotein targeting signals and components of the general import pore complex that mediate preprotein transport across the outer membrane. This review focuses on recent studies dealing with the central questions regarding the pore-forming subunits, and architecture and gating of the translocation channel of the outer membrane.

Adenosine Triphosphatases↗
PubMed1999

Tom22 is a multifunctional organizer of the mitochondrial preprotein translocase.

Mitochondrial preproteins are imported by a multisubunit translocase of the outer membrane (TOM), including receptor proteins and a general import pore. The central receptor Tom22 binds preproteins through both its cytosolic domain and its intermembrane space domain and is stably associated with the channel protein Tom40 (refs 11-13). Here we report the unexpected observation that a yeast strain can survive without Tom22, although it is strongly reduced in growth and the import of mitochondrial proteins. Tom22 is a multifunctional protein that is required for the higher-level organization of the TOM machinery. In the absence of Tom22, the translocase dissociates into core complexes, representing the basic import units, but lacks a tight control of channel gating. The single membrane anchor of Tom22 is required for a stable interaction between the core complexes, whereas its cytosolic domain serves as docking point for the peripheral receptors Tom20 and Tom70. Thus a preprotein translocase can combine receptor functions with distinct organizing roles in a multidomain protein.

Biological Transport↗
PubMed1999

Distribution of binding sequences for the mitochondrial import receptors Tom20, Tom22, and Tom70 in a presequence-carrying preprotein and a non-cleavable preprotein.

Preproteins destined for mitochondria either are synthesized with amino-terminal signal sequences, termed presequences, or possess internal targeting information within the protein. The preprotein translocase of the outer mitochondrial membrane (designated Tom) contains specific import receptors. The cytosolic domains of three import receptors, Tom20, Tom22, and Tom70, have been shown to interact with preproteins. Little is known about the internal targeting information in preproteins and the distribution of binding sequences for the three import receptors. We have studied the binding of the purified cytosolic domains of Tom20, Tom22, and Tom70 to cellulose-bound peptide scans derived from a presequence-carrying cleavable preprotein, cytochrome c oxidase subunit IV, and a non-cleavable preprotein with internal targeting information, the phosphate carrier. All three receptor domains are able to bind efficiently to linear 13-mer peptides, yet with different specificity. Tom20 preferentially binds to presequence segments of subunit IV. Tom22 binds to segments corresponding to the carboxyl-terminal part of the presequence and the amino-terminal part of the mature protein. Tom70 does not bind efficiently to any region of subunit IV. In contrast, Tom70 and Tom20 bind to multiple segments within the phosphate carrier, yet the amino-terminal region is excluded. Both charged and uncharged peptides derived from the phosphate carrier show specific binding properties for Tom70 and Tom20, indicating that charge is not a critical determinant of internal targeting sequences. This feature contrasts with the crucial role of positively charged amino acids in presequences. Our results demonstrate that linear peptide segments of preproteins can serve as binding sites for all three receptors with differential specificity and imply different mechanisms for translocation of cleavable and non-cleavable preproteins.

Amino Acid Sequence↗
PubMed1998

Preprotein translocase of the outer mitochondrial membrane: molecular dissection and assembly of the general import pore complex.

The preprotein translocase of the outer mitochondrial membrane (Tom) is a multisubunit machinery containing receptors and a general import pore (GIP). We have analyzed the molecular architecture of the Tom machinery. The receptor Tom22 stably associates with Tom40, the main component of the GIP, in a complex with a molecular weight of approximately 400,000 ( approximately 400K), while the other receptors, Tom20 and Tom70, are more loosely associated with this GIP complex and can be found in distinct subcomplexes. A yeast mutant lacking both Tom20 and Tom70 can still form the GIP complex when sufficient amounts of Tom22 are synthesized. Besides the essential proteins Tom22 and Tom40, the GIP complex contains three small subunits, Tom5, Tom6, and Tom7. In mutant mitochondria lacking Tom6, the interaction between Tom22 and Tom40 is destabilized, leading to the dissociation of Tom22 and the generation of a subcomplex of approximately 100K containing Tom40, Tom7, and Tom5. Tom6 is required to promote but not to maintain a stable association between Tom22 and Tom40. The following conclusions are suggested. (i) The GIP complex, containing Tom40, Tom22, and three small Tom proteins, forms the central unit of the outer membrane import machinery. (ii) Tom20 and Tom70 are not essential for the generation of the GIP complex. (iii) Tom6 functions as an assembly factor for Tom22, promoting its stable association with Tom40.

Adenosine Triphosphatases↗
PubMed1997

Differential recognition of preproteins by the purified cytosolic domains of the mitochondrial import receptors Tom20, Tom22, and Tom70.

The preprotein translocase of the outer mitochondrial membrane (Tom) is a multi-subunit complex required for specific recognition and membrane translocation of nuclear-encoded preproteins. We have expressed and purified the cytosolic domains of three postulated import receptors, Tom20, Tom22, and Tom70. Each receptor domain is able to bind mitochondrial preproteins but with different specificity. Tom20 binds both preproteins with N-terminal presequences and preproteins with internal targeting signals; the binding is enhanced by the addition of salt. Tom22 selectively recognizes presequence-carrying preproteins in a salt-sensitive manner. Tom70 preferentially binds preproteins with internal targeting information. A chemically synthesized presequence peptide competes with preproteins for binding to Tom20 and Tom22 but not to Tom70. We conclude that each of the three import receptors binds preproteins independently and by a different mechanism. Both Tom20 and Tom22 function as presequence receptors.

Animals↗
PubMed1994

Mechanical and electromechanical properties of the stereovillar bundles of isolated and cultured hair cells of the chicken.

Isolated single chicken hair cells and pieces of epithelium without the tectorial membrane, either freshly isolated or in tissue culture, were studied using water-jet stimulation of their stereovillar bundles and current injection. Responses were measured under enhanced video-microscopic observation or while using a differential photodiode technique sensitive down into the nanometer range. When stimulated with a water jet at low displacement amplitudes up to about 200 nm, the stereovillar bundle displacement was asymmetrical, indicating a lower stiffness in the excitatory direction, but the reverse was true at higher displacement amplitudes. Undamaged bundles showed no mechanical resonances below 1 kHz. In damaged bundles, however, such resonances were prominent and accompanied by splaying of the stereovilli. Hair cells in the epithelium showed small bundle movements (0.6 nm/mV) whose polarity depended on the polarity of the injected current. These movements probably resulted from activation of the bundle's adaptation motors.

Animals↗
PubMed1994

The cuticular plate of the hair cell in relation to morphological gradients of the chicken basilar papilla.

The aim of the present study was to provide details on the diversity and morphological gradients in the anatomy of the cuticular plate of hair cells in the chicken basilar papilla. The structure of the cuticular plate, which is mainly made up of a network of actin filaments, may be related to differences in the mechanical demands on the anchorage of the stereovillar bundle. We describe the morphological gradients in the cuticular plates as seen in transverse section for four positions along the basilar papilla. Three different shapes of cuticular plate could be distinguished. In general, cuticular plates in neurally-lying hair cells have their main mass on the neural side of the cells; for abneural cells, the converse is true. The shape of the plates changes gradually across the papilla; symmetrical forms exist. The hair-cell bundle orientation (and thus the preferred direction of stimulation of the bundle), as measured using scanning EM preparations, does not correlate with the shape of the plate in transverse section. The present data confirm the notion developed from other studies that (1) there are no distinct populations of hair cells, (2) there are no linear or monotonic morphological gradients, and (3) the gradients on the papilla are species- and position-specific.

Actins↗
PubMed1991

Contacts between hair cells in the avian cochlea.

In the avian papilla basilaris, contacts between hair cells are a common feature. With few exceptions, they only occur between tall hair cells (THC), and they are more frequent in the apical half of the papilla. In this quantitative study, four types of contacts are defined: Protrusion contacts, touch contacts, fusion contacts and multiple contacts. There are strong indications that these contacts are dynamic structures. The fact that avian hair cells probably communicate with each other means that their function can only be fully understood if cell groups are studied.

Animals↗
PubMed1991

Activity patterns of primary auditory-nerve fibres in chickens: development of fundamental properties.

We have examined the activity patterns of single auditory-nerve fibers in the chicken and tested for possible changes during post-hatching development. For this purpose, we recorded from fibres in the cochlear ganglion of chickens of two age groups (about P2 and P21) and investigated their spontaneous and sound-evoked activity patterns under nembutal-chloralhydrate anaesthesia. The spontaneous activity of primary auditory neurones was irregular, the average rates were between 20.5 (P2) and 23 (P21) spikes/s. Many low-frequency fibres from both age groups showed preferred intervals in their spontaneous activity. Tuning characteristics, including the range of characteristic frequencies, the presence of primary and two-tone suppression, the slopes of tuning-curve flanks and Q10dB values were similar to those previously reported for the starling and were statistically indistinguishable between the two age groups. However, there was a difference in fibre thresholds at the highest frequencies. Systematic differences were also present between the two age groups with regard to some characteristics of the rate-intensity functions. These data indicate that whereas the tuning properties of primary auditory fibres of the chicken cochlea are mature as early as post-hatching day 2, the intensity functions are not.

Acoustic Stimulation↗
PubMed1991

The development of links between stereocilia in hair cells of the chick basilar papilla.

Auditory papillae of chicks (embryonic age 6-21 days) were examined by scanning and transmission electron microscopy, in order to trace the development of the tip links between the stereocilia, and in order to trace the development of the spatial organisation of the tip links. In the most immature bundles, stereocilia were not graded in height, while strands of tenuous material interconnected adjacent stereocilia, this material being concentrated in a band near the tips of the stereocilia. The material joined the stereocilia in all directions, with no preferential direction for the interconnecting material being visible. Similarly, no columnar organisation of the stereocilia was visible. As soon as a gradation in height of the stereocilia began to appear, material could be seen running upwards from the shorter stereocilia to the adjacent lengthening stereocilia. There was a continuum in appearance between (i) the material running laterally between short immature stereocilia, (ii) the material running upwards between stereocilia which were developing a gradation in height, and (iii) the tip links seen in more mature bundles. It is suggested that tip links are a specialisation of the links which join immature stereocilia laterally near their tips. It is also suggested that the orientation of tip links, parallel to the hair cell axis of bilateral symmetry, is produced by the gradient in growth of the stereocilia.

Animals↗
PubMed1991

Innervation patterns and spontaneous activity of afferent fibres to the lagenar macula and apical basilar papilla of the chick's cochlea.

To investigate the origin of non-auditory fibres in the apical area of the avian cochlear ganglion, we recorded from nerve fibres in the young chick (87% of animals were aged between 5 and 10 days post-hatching). After characterization of their spontaneous activity patterns and, if present, their responses to sound, some fibres were stained with cobalt-ion injections and traced to their peripheral terminals. All stained fibres which were traced to the lagenar macula (N = 13) were non-auditory. They did not increase firing rate or phase-couple to sound stimuli. Their spontaneous activity was either regular (12 cases) or irregular (1 case). Regularly-firing cells all innervated several to very many hair cells, whereby there was no great difference in the pattern of spontaneous activity between those making calyx endings on relatively few hair cells in the striola region and those making small bouton endings on up to 80 hair cells outside the striola. All fibres that responded in any way to sound were irregularly spontaneously active. Three fibres, two of which only responded to sound with phase-coupling, innervated several hair cells in the apical, abneural region of the basilar papilla. Two other fibres traced to the basilar papilla are of previously undescribed types.

Acoustic Maculae↗
PubMed1990

Influence of collagenase on tip links in hair cells of the chick basilar papilla.

Chick basilar papillae were incubated in collagenase, using concentrations previously employed for the isolation of viable hair cells. When assessed by scanning electron microscopy, the hair bundles had a normal conformation, with no loss of tip links compared with control incubations. The results suggest that collagenase dose not destroy the integrity of structures on the apical surface of hair cells, and that tip links are not composed of collagen. The results are in agreement with the hypothesis that tip links are involved in mechanotransduction.

Animals↗