1,720,962 research outputs found
Aquaporin 2 and calcium sensing receptor: new players regulating renal water handling in familial hypercalciuria
No full textRenal modulation: arginine vasopressin and atrial natriuretic peptide
No full textTotal body water (TBW) is distributed in compartments divided by semi-permeable membranes. In postnatal life, approximately two thirds of TBW is located in the intracellular space and one third is located in the extracellular space. The latter is further divided with a 3:1 ratio in the interstitial and plasma compartments.
Passive equilibration of solutes between body compartments is driven by electrochemical gradients and is mediated by a complex system of transport mechanisms that includes pumps, channels, facilitated carriers and selective paracellular pathways. With few exceptions, water diffuses rapidly across epithelia and cell membranes, following osmotic gradients. High transcellular water transport cannot occur through pure lipid bilayers, as these have low osmotic water permeability (∼0.002 cm/s). Water diffusion through cell membranes is therefore mediated by specific water channels, termed aquaporins (AQPs), which enhance osmotic water permeability by 10–1000 fold. Because solutes diffuse less rapidly than their solvent, the relative water content of body compartments is primarily regulated by their solute distribution. This allows the organism to adjust its TBW distribution by regulating the activity of solute transporters located in biological membranes that separate body compartments.
During early fetal life, TBW represents approximately 90% of body mass. As pregnancy progresses, TBW decreases progressively, to reach 75–80% of body mass at the end of gestation. These changes are primarily due to a decline in extracellular water, while intracellular water increases. In the first 24–48 h after birth, the extracellular compartment further decreases, as a result of a negative fluid balance in the immediate postnatal period.
The fetus constantly regulates its TBW by salt and water exchanges through the placenta membrane. After birth, uptake of water and solutes is limited to gastro-intestinal intakes, while insensible fluid losses increase dramatically. The newborn needs therefore to activate mechanisms that are aimed at controlling water and salt losses. Most of these mechanisms involve the secretion of hormones, which act directly on the kidney. To be efficient, these mechanisms require that sensors, hormone secretion pathways and target organs have reached an appropriate level of maturity.
Water excretion or retention is primarily modulated through the regulation of arginine vasopressin (AVP) secretion. Stimulation of thirst has only limited value in newborns, because of their restricted access to free water and immaturity of the central nervous system.
Salt retention by the kidney is predominantly achieved by activation of the renin-angiotensin-aldosterone system, which is potentiated by endothelins and adrenergic renal nerve activity. Conversely renal salt losses are stimulated by natriuretic peptides (NPs), prostaglandins, kinins, nitric oxide (NO) and adrenomedullin.
In this review, the roles of AVP and NPs in the regulation of body fluid composition during the prenatal and perinatal periods are briefly reviewed. It is important to notice however, that their action is part of a complex network in which all of the above mentioned pathways are synergistically activated or inhibited to maintain body homeostasis
IMPAIRMENT OF AQP2 TARGETING IN RESPONSE TO VASOPRESSIN IN HYPERCALCI- URIC PATIENTS: EVIDENCE FOR EXTRACELLULAR CALCIUM ACTING AS FIRST MESSENGER
No full textDisturbances in renal concentrating ability have been associated with hypercalciuria in humans and experimental animals. In a previous work we have shown that extracellular calcium, possibly acting through Calcium Sensing Receptor (CaR) signaling, antagonizes forskolin-induced AQP2 translocation in renal cells. To test the hypothesis that increased urinary calcium inhibits AQP2 targeting in response to vasopressin in humans, in this study we evaluated AQP2 excretion in hypercalciuric children after 1-deamino-arginin-vasopressin (dDAVP) administration. Normocal- ciuric children served as control. Urine osmolarity and AQP2 excretion were measured in urine samples obtained hourly after 10 mg (if weight 0.2 on 3 separate samples. AQP2 excretion in urine samples was measured by Enzyme-Linked immunosorbent assay (ELISA).
In the group of normocalciuric children having normal urinary concentration ability, dDAVP ad- ministration resulted in a significant increase in urine osmolality (1110±32 mOsm/l at peak, n=19) associated with a significant increase in urinary AQP2 excretion (from 250±80 fmol/mg Creat be- fore to 467±113 after dDAVP treatment, P<0.0001). In contrast, in hypercalciuric children AQP2 excretion did not significantly increase in response to dDAVP administration either in the group displaying normal urinary concentration ability (AQP2 excretion from 291±58.9 fmol/mg Creat before to 291±77 after dDAVP treatment, urine osmolarity 1018±32 mOsm/l at peak, n=13). and in the group having poor urinary concentration ability (AQP2 excretion from 432±117 fmol/mg Creat before to 391±80 after dDAVP test, urine osmolality 512±104 mOsm/l at peak, n=12).
The obtained data demonstrate that in hypercalciuria AQP2 excretion in response to vasopressin is reduced or abolished. This effect was observed in both normal and poor responders. These data would support the hypothesis that increased urinary calcium levels inhibits AQP2 targeting in re- sponse to vasopressin in humans possibly through activation of CaR signaling. In hypercalciuric patients the impairment of AQP2 targeting observed in response to vasopressin may contribute to reduce the risk of an increased incidence stone formation during antidiuresis
Calcium-sensing receptor signaling acts as negative modulator of vasopressin regulated water reabsorption through Aquaporin 2 in kidney collecting duct
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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