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961.
Since the exact mechanism of manganese (Mn)-induced learning disability is not known, we investigated the role of elevated
cholesterol in rats exposed daily to 357 and 714 μg Mn/kg for 30 d. Significant Mn accumulation was accompanied by increased
cholesterol content in the hippocampal region of Mn-treated rats. The learning, which is based on the time needed to reach
food placed at the exit of a T-maze after a 1-d training period, was significantly slower in exposed rats than in unexposed
rats. The rats receiving 357 and 714 μg Mn/kg reached the food in 104.5±13.8 and 113.3±25.7 s, respectively, on d 30, whereas
their untreated counterparts reached the food in 28.7±11.4 s. This delay was completely corrected to 29.3±7.8 and 30.7±6.0
s in rats with coadministration of an inhibitor of cholesterol biosynthesis with 357 and 714 μg/kg of Mn. The correction of
impaired learning was associated with the normalization of hippocampal cholesterol, but the Mn level in this region of the
brain was not influenced in rats treated with a drug that inhibits cholesterol biosynthesis. These results suggested that
Mn-induced hypercholesterolemia is involved in Mn-dependent learning disability. 相似文献
962.
963.
964.
965.
966.
Markus Kurz Karin Schütz Michael Göbel 《Origins of life and evolution of the biosphere》1996,26(3-5):263-264
967.
Summary Late Arenigian biohermal reef mounds and biostromes within the shallow-marine platform facies of the upper San Juan Formation
of the Precordillera (Western Argentina) represent a new Early Ordovician reef type. The meter-sized reefs are dominated byZondarella communis n.g. n. sp. The new taxon is characterized by domical, bulbous and laminar morphotypes exhibiting growth layers and thin
horizontal and vertical as well as intermingled skeletal elements included within different sets. The fossil maybe compared
with stromatolites and stromatoporoids but an interpretation as primitive stromatoporoids is favoured. 相似文献
968.
Summary The microproblematicumPycnoporidium ? eomesozoicum
Flügel, 1972, from Upper Triassic reefs of the Alpine-Mediterranean region, Turkey Oman and Iran (originally interpreted as possible
alga) represents the type species of a new strophomenid brachiopod genus (Gosaukammerella n.g.). The genus is characterized by a very small, millimeter-sized plano-convex shell, whose ventral valve is attached to
the substratum (mainly sponges) by symmetrically arranged outgrowths developing from a pseudopunctate, lamellose foliated
shell wall and composed of densely spaced subparallel ‘tubes’ comparable with productide spines secreted by papillose extensions
of the mantle.Gosaukammerella seems to be the only reliable candidate for the existence of post-Paleozoic strophomenid (productid ?) brachiopods.
Gosaukammerella eomesozoica is restricted to possibly cryptic, shaded reef environments inhabited predominantly by sponges serving as substrates for
micromorphic brachiopods. 相似文献
969.
Klaus Zuberbühler Lorenz Gygax Nerida Harley Hans Kummer 《Primates; journal of primatology》1996,37(1):1-12
In a captive group of long-tailed macaques, tool-using behavior by a single competent individual had a significant effect on the synchronous manipulative behavior of naïve animals. Group members engaged in manipulations on the same object class more frequently during times when the model was working than when it was not. The form of their behavior, however, in no way resembled the technique used by the model. All three animals that later became successful tool users were among the few subjects that exhibited a significant increase in manipulations on the same object class while the model was working. Possible causal relationships between this stimulus enhancement and the transmission of the new behavior to other group members are analyzed and discussed. 相似文献
970.
Water and solute transport along developing maize roots 总被引:15,自引:0,他引:15
Hydraulic and osmotic properties were measured along developing maize (Zea mays L.) roots at distances between 15 and 465 mm from the root tip to quantify the effects of changes in root structure on the radial and longitudinal movement of water and solutes (ions). Root development generated regions of different hydraulic and osmotic properties. Close to the root tip, passive solute permeability (root permeability coefficient, Psr) was high and selectivity (root reflection coefficient, sr) low, indicative of an imperfect semipermeable root structure. Within the apical 100–150 mm, Psr decreased by an order of magnitude and sr increased significantly. Root hydraulic conductivity (Lpr) depended on the nature of the force (hydrostatic and osmotic). Osmotic Lpr was smaller by an order of magnitude than hydrostatic Lpr and decreased with increasing distance from the root tip. Throughout the root, responses in turgor of cortical cells and late metaxylem to step changes in xylem pressure applied to the base of excised roots were measured at high spatial resolution. The resulting profiles of radial and longitudinal propagation of pressure showed that the endodermis had become the major hydraulic barrier in older parts of the root, i.e. at distances from the apex ä 150 mm. Other than at the endodermis, no significant radial hydraulic resistance could be detected. The results permit a detailed analysis of the root's composite structure which is important for its function in collecting and translocating water and nutrients.Abbreviations and Symbols CPP
cell pressure probe
- IT
root segments with intact tips;
- Lpr
root hydraulic conductivity
- Lprh
hydrostatic hydraulic conductivity of root
- Lpro
osmotic hydraulic conductivity of root
- Papp
hydrostatic pressure applied to cut end of root
- Pc
cell turgor
- Pc, cor
turgor of cortical cell
- Pc,xyl
turgor of late metaxylem vessel
- Pro
stationary root pressure
- Pr0,seal
stationary root pressure of sealed root segment
- Psr
solute permeability coefficient of root
- RPP
root pressure probe
- TR
root segments with tip removed
- sr
reflection coefficient of root
Dedicated to Professor Andreas Sievers on the occasion of his retirement 相似文献