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Sudden Transition (sudden + transition)

Distribution by Scientific Domains

Life Sciences	50%

Selected Abstracts

Volume natriuresis vs. pressure natriuresis

ACTA PHYSIOLOGICA, Issue 4 2004
P. Bie
Abstract Body fluid regulation depends on regulation of renal excretion. This includes a fast vasopressin-mediated water-retaining mechanism, and slower, complex sodium-retaining systems dominated by the renin,angiotensin aldosterone cascade. The sensory mechanisms of sodium control are not identified; effectors may include renal arterial pressure, renal reflexes, extrarenal hormones and other regulatory factors. Since the pioneering work of Guyton more than three decades ago, pressure natriuresis has been in focus. Dissociations between sodium excretion and blood pressure are explained as conditions where regulatory performance exceeds the precision of the measurements. It is inherent to the concept, however, that sudden transition from low to high sodium intake elicits an arterial pressure increase, which is reversed by the pressure natriuresis mechanism. However, such transitions elicit parallel changes in extracellular fluid volume thereby activating volume receptors. Recently we studied the orchestration of sodium homeostasis by chronic and acute sodium loading in normal humans and trained dogs. Small increases in arterial blood pressure are easily generated by acute sodium loading, and dogs appear more sensitive than humans. However, with suitable loading procedures it is possible , also acutely , to augment renal sodium excretion by at least one order of magnitude without any change in arterial pressure whatsoever. Although pressure natriuresis is a powerful mechanism capable of overriding any other controller, it seems possible that it is not operative under normal conditions. Consequently, it is suggested that physiological control of sodium excretion is neurohumoral based on extracellular volume with neural control of renin system activity as an essential component. [source]

A microfluidic study of mechanisms in the electrophoresis of supercoiled DNA

ELECTROPHORESIS, Issue 12 2008
Dammika P. Manage
Abstract In this work, microfluidic chips were used to study the electrophoresis of supercoiled DNA (SC DNA) in agarose. This system allowed us to study the electrophoretic and trapping behaviours of SC DNA of various lengths, at various fields and separation distances. Near a critical electric field the DNA is trapped such that the concentration falls exponentially with distance. The trapping of such circular DNA has been explained in terms of the ,lobster trap' or ,impalement' model where shorter fibres become trapping sites at higher fields, leading to an ongoing (and gradual) increase in trapping with increasing field. By contrast, the present study suggests that under some circumstances the traps have a barrier such that only when the DNA has sufficient energy (at high enough fields) can it become trapped, leading to a sudden transition in behaviours at the critical field. We propose an ,activated impalement' mechanism of trapping in which only at sufficiently high fields is the SC DNA impaled and trapped for long times. The critical electric field appears to be inversely proportional to the length of the DNA molecule, suggesting that the force required to impale the SC DNA is constant. [source]

The role of metabolic memory in the ATP paradox and energy homeostasis

FEBS JOURNAL, Issue 21 2008
Juan C. Aledo
In yeast, a sudden transition from glucose limitation to glucose excess leads to a new steady state at increased metabolic fluxes with a sustained decrease in the ATP concentration. Although this behaviour has been rationalized as an adaptive metabolic strategy, the mechanism behind it remains unclear. Nevertheless, it is thought that, on glucose addition, a metabolite derived from glycolysis may up-regulate ATP-consuming reactions. The adenine nucleotides themselves have been ruled out as the signals that mediate this regulation. This is mainly because, in that case, it would be expected that the new steady state at increased fluxes would be accompanied by an increased stationary ATP concentration. In this study, we present a core model consisting of a monocyclic interconvertible enzyme system. Using a supply,demand approach, we demonstrate that this system can account for the empirical observations without involving metabolites other than the adenine nucleotides as effectors. Moreover, memory is an emerging property of such a system, which may allow the cell to sense both the current energy status and the direction of the changes. [source]

A HYDROCLIMATOLOGICAL ANALYSIS OF THE RED RWER OF THE NORTH SNOWMELT FLOOD CATASTROPHE OF 1997,

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 5 2001
Paul E. Todhunter
ABSTRACT: The flood hydroclimatology of the Grand Forks flood of April 1997, the most costly flood on a per capita basis for a major metropolitan area in United States history, is analyzed in terms of the natural processes that control spring snowmelt flooding in the region. The geomorphological characteristics of the basin are reviewed, and an integrated assessment of the hydroclimatological conditions during the winter of 1996 to 1997 is presented to gain a real-world understanding of the physical basis of this catastrophic flood event. The Grand Forks flood resulted from the principal flood-producing factors occurring at either historic or extreme levels, or at levels conducive to severe flooding. Above normal fall precipitation increased the fall soil moisture storage and reduced the spring soil moisture storage potential. A concrete frost layer developed that effectively reduced the soil infiltration capacity to zero. Record snowfall totals and snow cover depths occurred across the basin because of the unusual persistence of a blocking high circulation pattern throughout the winter. A severe, late spring blizzard delayed the snowmelt season and replenished the snow cover to record levels for early April. This blizzard was followed by a sudden transition to an extreme late season thaw due to the abrupt breakdown of the blocking circulation pattern. The presence of river ice contributed to backwater effects and affected the timing of tributary inflows to the main stem of the Red River. Only the absence of spring rains prevented an even more catastrophic flood disaster from taking place. This paper contributes to our understanding of the flood hydroclimatology of catastrophic flood events in an unusual flood hazard region that possesses relatively flat terrain, a north-flowing river, and an annual peak discharge time series dominated by spring snowmelt floods. [source]