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Hormonal modulation of hepatic plasma membrane lactate transport was studied in primary cultures of isolated hepatocytes from fed rats to examine the mechanism for the known enhancement of lactate transport in starvation and diabetes. Total cellular lactate entry was increased by 14% in the presence of dexamethasone; this was accounted for by an approximately 40% increase in the carrier-mediated component of entry with no effect on diffusion. A trend of similar magnitude was evident with glucagon. The effects of dexamethasone and glucagon on lactate transport constitute an additional potential mechanism for enhancement of gluconeogenesis by these hormones. 相似文献
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The natural habitats of microbes are typically spatially structured with limited resources, so opportunities for unconstrained, balanced growth are rare. In these habitats, selection should favor microbes that are able to use resources most efficiently, that is, microbes that produce the most progeny per unit of resource consumed. On the basis of this assertion, we propose that selection for efficiency is a primary driver of the composition of microbial communities. In this article, we review how the quality and quantity of resources influence the efficiency of heterotrophic growth. A conceptual model proposing innate differences in growth efficiency between oligotrophic and copiotrophic microbes is also provided. We conclude that elucidation of the mechanisms underlying efficient growth will enhance our understanding of the selective pressures shaping microbes and will improve our capacity to manage microbial communities effectively. 相似文献
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David A. Lipson Russell K. Monson Steven K. Schmidt Michael N. Weintraub 《Biogeochemistry》2009,95(1):23-35
Soil microbial respiration is a critical component of the global carbon cycle, but it is uncertain how properties of microbes
affect this process. Previous studies have noted a thermodynamic trade-off between the rate and efficiency of growth in heterotrophic
organisms. Growth rate and yield determine the biomass-specific respiration rate of growing microbial populations, but these
traits have not previously been used to scale from microbial communities to ecosystems. Here we report seasonal variation
in microbial growth kinetics and temperature responses (Q10) in a coniferous forest soil, relate these properties to cultured and uncultured soil microbes, and model the effects of
shifting growth kinetics on soil heterotrophic respiration (Rh). Soil microbial communities from under-snow had higher growth rates and lower growth yields than the summer and fall communities
from exposed soils, causing higher biomass-specific respiration rates. Growth rate and yield were strongly negatively correlated.
Based on experiments using specific growth inhibitors, bacteria had higher growth rates and lower yields than fungi, overall,
suggesting a more important role for bacteria in determining Rh. The dominant bacteria from laboratory-incubated soil differed seasonally: faster-growing, cold-adapted Janthinobacterium species dominated in winter and slower-growing, mesophilic Burkholderia and Variovorax species dominated in summer. Modeled Rh was sensitive to microbial kinetics and Q10: a sixfold lower annual Rh resulted from using kinetic parameters from summer versus winter communities. Under the most realistic scenario using seasonally
changing communities, the model estimated Rh at 22.67 mol m−2 year−1, or 47.0% of annual total ecosystem respiration (Re) for this forest. 相似文献
8.
Exponential growth of “snow molds” at sub-zero temperatures: an explanation for high beneath-snow respiration rates and Q
10 values 总被引:2,自引:0,他引:2
Numerous studies have demonstrated exceptionally high temperature sensitivity of the beneath-snow respiratory flux in cold-winter
ecosystems. The most common, but still untested, explanation for this high sensitivity is a physical one based on the observation
that water availability in soils increases exponentially as soils warm from −3 to 0°C. Here, we present evidence for a biological
hypothesis to explain exponential kinetics and high Q
10 values as beneath-snow soils warm from −3 to 0°C during the early spring in a high-elevation subalpine forest. First, we
show that some of the dominant organisms of the beneath-snow microbial community, “snow molds”, exhibit robust exponential
growth at temperatures from −3 to −0.3°C. Second, Q
10 values based on growth rates across the temperature range of −2 to −0.3°C for these snow molds vary from 22 to 330. Third,
we derive an analytical equation that combines the relative contributions of microbial growth and microbial metabolism to
the temperature sensitivity of respiration. Finally, we use this equation to show that with only moderate snow mold growth
(several generations), the combined sensitivities of growth and metabolism to small changes in beneath-snow soil temperature,
create a double exponential in the Q
10 function that may explain the extremely high (~1 × 106) Q
10 values observed in past studies. Our biological explanation for high Q
10 levels is supported by several independent studies that have demonstrated build up of microbial biomass under the snow as
temperatures warm from −2 to 0°C. 相似文献
9.
A Puri R Sethi B Singh SK Dwivedi VS Narain RK Saran VK Puri 《Indian pacing and electrophysiology journal》2009,9(3):186-189
A 25-year-old previously asymptomatic pregnant woman at 36 weeks'' gestation was noticed to have repetitive monomorphic ventricular tachycardia. A dilated left ventricle with moderately reduced systolic function was found on echocardiographic examination. This is a very rare presentation of peripartum cardiomyopathy (PPCMP) presenting with repetitive monomorphic ventricular tachycardia. 相似文献
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The invertebrate fauna of the nests of three seabird species, black-legged kittiwakes (Rissa tridactyla), common eider (Somateria mollissima) and glaucous gull (Larus hyperboreus), were sampled in Kongsfjorden, Spitsbergen, Svalbard. The invertebrate community was species poor, consisting predominantly
of the flea, Mioctenopsylla arctica arctica (Insecta: Siphonaptera), but with six species of oribatid mite (Acari: Oribatida), Diapterobates notatus, Oribatula tibialis, Ameronothrus lineatus, Hermannia reticulata, Trichoribates trimaculatus and Ceratoppia bipilis, plus an occasional mesostigmatid mite. No Collembola or ticks (Acari: Ixodidae) were observed. With the exception of M. arctica arctica, the fauna of seabird nests consisted of opportunistic microarthropod species rather than specialised nest-dwelling or bird
parasitic species. Species diversity of soil oribatid mites was greater in nests of the common eider than compared to nests
of the black-legged kittiwake, which may be related to the ground nesting behaviour of the common eiders. No rare or unusual
microarthropod species for Svalbard were found in the seabird nests. The contentions that nests may facilitate microarthropod
colonisation of High Arctic regions via bird phoresy by providing a high-quality habitat at the point of arrival, or that
there might be a specialised microarthropod fauna exploiting this habitat, were not supported in this study. These are amongst
the first data on the microarthropod community of seabird nests in the High Arctic. 相似文献