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51.
C. Veigel R. D. von Maydell K.R. Kress J. E. Molloy R. H. A. Fink 《Pflügers Archiv : European journal of physiology》1998,435(6):753-761
Recent atomic 3-D reconstructions of the acto-myosin interface suggest that electrostatic interactions are important in the
initial phase of cross-bridge formation. Earlier biochemical studies had also given strong evidence for the ionic strength
dependence of this step in the cross-bridge cycle. We have probed these interactions by altering the ionic strength (Γ/2)
of the medium mainly with K+, imidazole+ and EGTA2– to vary charge shielding. We examined the effect of ionic strength on the kinetics of rigor development at low Ca2+ (experimental temperature 18–22°C) in chemically skinned single fast-twitch fibres of mouse extensor digitorum longus (EDL)
muscle. On average the delay before rigor onset was 10 times longer, the maximum rate of rigor tension development was 10
times slower, the steady-state rigor tension was 3 times lower and the in-phase stiffness was 2 times lower at high (230 mM)
compared to low (60 mM) ionic strength. These results were modelled by calculating ATP depletion in the fibre due to diffusional
loss of ATP and acto-myosin Mg.ATPase activity. The difference in delay before rigor onset at low and high ionic strength
could be explained in our model by assuming a 15 times higher Mg.ATPase activity and a threefold increase in K
m in relaxing conditions at low ionic strength. Activation by Ca2+ induced at different time points before and during onset of rigor confirmed the calculated time course of ATP depletion.
We have also investigated ionic strength effects on rigor development with the activated troponin/tropomyosin complex. ATP
withdrawl at maximum activation by Ca2+ induced force transients which led into a ”high rigor” state. The peak forces of these force transients were very similar
at low and high ionic strength. The subsequent decrease in tension was only 10% slower and steady-state ”high rigor” tension
was reduced by only 27% at high compared to low ionic strength. Addition of 10 mM phosphate to lower cross-bridge attachment
strongly suppressed the transient increases in force at high ionic strength and reduced the steady-state rigor tension by
17%. A qualitatively similar but smaller effect of phosphate was observed at low ionic strength where steady-state rigor force
was reduced by 10%. The data presented in this study show a very strong effect of ionic strength on rigor development in relaxed
fibres whereas the ionic strength dependence of rigor development after thin filament activation was much less. The data confirm
the importance of electrostatic interactions in cross-bridge attachment and cross-bridge-attachment-induced activation of
thin filaments.
Received: 3 September 1997 / Received after revision and accepted: 12 December 1997 相似文献
52.
Harwant S. Gill Michael W. O'Boyle Julie Hathaway 《Cortex; a journal devoted to the study of the nervous system and behavior》1998,34(5):707-718
Alpha power (8–12 Hz) was monitored over the frontal, temporal, parietal and occipital lobes of the left and right cerebral hemispheres while participants mentally rotated three-dimensional shapes to match a specified target. By comparing the activational patterns generated during three experimental conditions, each designed to systematically isolate the involvement of the various subcomponents comprising this mental rotation task, it was suggested that the right frontal lobe mediates encoding and comparison/decision processes, while the left parietal and the left temporal region appear most involved in the generation of images and their mental rotation. A preliminary model describing the cooperative interaction of these cortical regions during mental rotation tasks is proposed. 相似文献
53.
Dr. Yoshihiko Hayakawa Ph.D. Allan G. Farman B.D.S Ph.D. D.Sc. William C. Scarfe B.D.S M.S. Kinya Kuroyanagi D.D.S Ph.D. 《Oral Radiology》1996,12(1):11-18
The RVG-4 permits automatic exposure compensation (AEC). The purpose of this investigation was to determine the effects of
AEC on image contrast. Images were made either with or without a dental QA jaw phantom using a fixed image projection geometry.
Exposures were 6.3 through 27.3 μC/kg using an X-ray generator operated at 70 kVp. Region of interest pixel value distributions
were measured at tissue thicknesses in this phantom, and the average pixel values and signal-to-noise ratios (SNR) were calculated.
The use of AEC without an object in place resulted in a disproportionate relationship between pixel value and exposure with
a marked reduction in SNR. The use of AEC on under- and over-exposed images of the phantom simultaneously enhanced image contrast
and reduced SNR. Thus, AEC provides a convenient and quick method for achieving high-contrast images with sub-optimal exposures,
however, this could lead to inappropriate patient dosages if the function is used for over-exposed images. AEC reduces the
SNR and produces disproportionate pixel values relative to exposure. 相似文献
54.
Abstract: Ultrafiltration of water from blood to dialysate decreases the rate of back–diffusion of solutes from dialysate to blood. Therefore, back–clearance ( bK ) of hemodialyzers may be expressed as bK = bK o – bTrQ u , where bK o is the diffusive back–clearance, bTr is the "back–"transmittance coefficient, and Qu is the net ultrafiltration rate. A formula for bK was derived from the one–dimensional theory of hemodialyzer, and bTr was described as a function of bK o and the Staverman reflection coefficient. The transport parameters, bK o and bTr , for creatinine and vitamin B12 were measured in two types of hemodialyzers with negligible back–filtration, using water solutions, and compared with the transport parameters, K o and Tr , for the case of both diffusion and ultrafiltration from blood to dialysate. bK o was in general equal to Ko . bTr was not different from Tr for creatinine whereas bTr was lower than Tr for vitamin B12 . Experimental values of bTr for vitamin B12 were in general agreement with theoretical predictions. However, experimental values of bTr for creatinine were lower than predicted values. We conclude that the impact of ultrafiltration on back–clearance for slowly diffusing solutes is weaker than on their clearance. 相似文献
55.
Atsuo Furuhashi M.D. Yasutaka Akasaki M.D. Masayasu Sato M.D. Koho Miyoshi M.D. 《Psychiatry and clinical neurosciences》1994,48(3):645-653
Abstract: The behavioral effects of ceroid-lipofuscin accumulation, induced by intraperitoneal administration of acetyl-ethyl-tetramethyl-tetralin (AETT) in Wistar rats for 3 months, were examined in the present studies. A significant increase in neuronal ceroid-lipofuscin was demonstrated neuropathologically as well as morphometrically. Although the AETT-intoxicated rats showed neither alteration of locomotor activity nor shock sensitivity, a significant impairment of learning ability, especially an acquisition trial in passive avoidance tests, was observed. Results of the present studies indicate the possibility that a diffuse lipofuscin accumulation causes a learning impairment in rats. The results also imply the possibility of a significant role of age-related lipofuscin accumulation in the dementing processes of human especially in the elderly. 相似文献
56.
57.
58.
Summary
Forty-one patients demonstrating clinical symptoms for cerebral infarction were investigated by magnetic resonance imaging
with diffusion-weighted echo-planar imaging (DWI) and T2-weighted imaging (T2WI). In 8 patients only DWI showed the cerebral
lesions clearly. One patient with positive DWI and T2WI suffered from HSV encephalitis. DWI is superior to T2WI in assessment
of small cortical infarcts and cerebral infarction in patients with preexisting vascular lesions. DWI is not specific, so
other causes like cerebral hematoma and encephalitis have to be considered.
相似文献
59.
60.
Purpose: Following the widespread adoption of energy-dependent rare earth intensifying screens this study investigates the relevance of two conventions (the 10 kVp and kVp-to-the-4th-power rules) commonly used to calculate the kVp/mAs relationship.Methods: A stepwedge was exposed at standard kVp and mAs setting. The kVp was then increased in increments of 5 kVp, and appropriate mAs values were determined using a densitometer. These values were then compared with approximations from the rules previously mentioned.Results: Wide discrepancies are reported between appropriate and approximated values with the latter overestimating exposure by a factor of two.Discussion: Traditional theorems used to estimate the kVp/mAs relationship should no longer be used in modern imaging departments. Each film screen combination should be evaluated individually to ensure appropriate exposure settings. 相似文献