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Résumé La production d'entropie associée à la respiration est calculée en fonction d'une ventilation variable. En suite, pour le cas d'un métabolism donné, on évalue la ventilation qui minimise la production d'entropie due au mélange des gaz respirés et à la dissipation d'énergie associée à l'acte respiratoire. Cette ventilation optimum s'accorde avec celle observée pour des taux de métabolismes inférieurs à la consommation de 2 l d'oxygène par min.  相似文献   
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WNK4 regulates the balance between renal NaCl reabsorption and K+ secretion   总被引:10,自引:0,他引:10  
A key question in systems biology is how diverse physiologic processes are integrated to produce global homeostasis. Genetic analysis can contribute by identifying genes that perturb this integration. One system orchestrates renal NaCl and K+ flux to achieve homeostasis of blood pressure and serum K+ concentration. Positional cloning implicated the serine-threonine kinase WNK4 in this process; clustered mutations in PRKWNK4, encoding WNK4, cause hypertension and hyperkalemia (pseudohypoaldosteronism type II, PHAII) by altering renal NaCl and K+ handling. Wild-type WNK4 inhibits the renal Na-Cl cotransporter (NCCT); mutations that cause PHAII relieve this inhibition. This explains the hypertension of PHAII but does not account for the hyperkalemia. By expression in Xenopus laevis oocytes, we show that WNK4 also inhibits the renal K+ channel ROMK. This inhibition is independent of WNK4 kinase activity and is mediated by clathrin-dependent endocytosis of ROMK, mechanisms distinct from those that characterize WNK4 inhibition of NCCT. Most notably, the same mutations in PRKWNK4 that relieve NCCT inhibition markedly increase inhibition of ROMK. These findings establish WNK4 as a multifunctional regulator of diverse ion transporters; moreover, they explain the pathophysiology of PHAII. They also identify WNK4 as a molecular switch that can vary the balance between NaCl reabsorption and K+ secretion to maintain integrated homeostasis.  相似文献   
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Wilson AH  Shirey SB  Carlson RW 《Nature》2003,423(6942):858-861
Komatiites are ultramafic volcanic rocks containing more than 18 per cent MgO (ref. 1) that erupted mainly in the Archaean era (more than 2.5 gigayears ago). Although such compositions occur in later periods of Earth history (for example, the Cretaceous komatiites of Gorgona Island), the more recent examples tend to have lower MgO content than their Archaean equivalents. Komatiites are also characterized by their low incompatible-element content, which is most consistent with their generation by high degrees of partial melting (30-50 per cent). Current models for komatiite genesis include the melting of rock at great depth in plumes of hot, diapirically rising mantle or the melting of relatively shallow mantle rocks at less extreme, but still high, temperatures caused by fluxing with water. Here we report a suite of ultramafic lava flows from the Commondale greenstone belt, in the southern part of the Kaapvaal Craton, which represents a previously unrecognized type of komatiite with exceptionally high forsterite content of its igneous olivines, low TiO(2)/Al(2)O(3) ratio, high silica content, extreme depletion in rare-earth elements and low Re/Os ratio. We suggest a model for their formation in which a garnet-enriched residue left by earlier cratonic volcanism was melted by hydration from a subducting slab.  相似文献   
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Mehta MR  Lee AK  Wilson MA 《Nature》2002,417(6890):741-746
In the vast majority of brain areas, the firing rates of neurons, averaged over several hundred milliseconds to several seconds, can be strongly modulated by, and provide accurate information about, properties of their inputs. This is referred to as the rate code. However, the biophysical laws of synaptic plasticity require precise timing of spikes over short timescales (<10 ms). Hence it is critical to understand the physiological mechanisms that can generate precise spike timing in vivo, and the relationship between such a temporal code and a rate code. Here we propose a mechanism by which a temporal code can be generated through an interaction between an asymmetric rate code and oscillatory inhibition. Consistent with the predictions of our model, the rate and temporal codes of hippocampal pyramidal neurons are highly correlated. Furthermore, the temporal code becomes more robust with experience. The resulting spike timing satisfies the temporal order constraints of hebbian learning. Thus, oscillations and receptive field asymmetry may have a critical role in temporal sequence learning.  相似文献   
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