P2Y2 receptor activation inhibits the expression of the sodium-chloride cotransporter NCC in distal convoluted tubule cells.

Luminal nucleotide stimulation is known to reduce Na(+) transport in the distal nephron. Previous studies suggest that this mechanism may involve the thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC), which plays an essential role in NaCl reabsorption in the cells lining the distal convoluted tubule (DCT). Here we show that stimulation of mouse DCT (mDCT) cells with ATP or UTP promoted Ca(2+) transients and decreased the expression of NCC at both mRNA and protein levels. Specific siRNA-mediated silencing of P2Y2 receptors almost completely abolished ATP/UTP-induced Ca(2+) transients and significantly reduced ATP/UTP-induced decrease of NCC expression. To test whether local variations in the intracellular Ca(2+) concentration ([Ca(2+)]i) may control NCC transcription, we overexpressed the Ca(2+)-binding protein parvalbumin selectively in the cytosol or in the nucleus of mDCT cells. The decrease in NCC mRNA upon nucleotide stimulation was abolished in cells overexpressing cytosolic PV but not in cells overexpressing either a nuclear-targeted PV or a mutated PV unable to bind Ca(2+). Using a firefly luciferase reporter gene strategy, we observed that the activity of NCC promoter region from -1 to -2,200 bp was not regulated by changes in [Ca(2+)]i. In contrast, high cytosolic calcium level induced instability of NCC mRNA. We conclude that in mDCT cells: (1) P2Y2 receptor is essential for the intracellular Ca(2+) signaling induced by ATP/UTP stimulation; (2) P2Y2-mediated increase of cytoplasmic Ca(2+) concentration down-regulates the expression of NCC; (3) the decrease of NCC expression occurs, at least in part, via destabilization of its mRNA.

Defects formed within hardness indenter interaction zone in Al2O3-ZrO2 composite.

The Al(2)O(3)-10 wt% ZrO(2) composites were subjected to hardness tests using a Vickers diamond indenter up to 98.1 N. The microstructure observation using a transmission electron microscopy technique helped to identify up to four zones differing in defect level and character. The densest dislocation tangles, twins accumulation and frequent presence of three slip systems were found in regions that were in contact with the sides of the diamond pyramid. The second zone, characterized by two, or at least one, active slip systems, started at the bottom of the indentation mark and extended up to a distance comparable with the depth of indentation. In the third zone, with a thickness comparable to that above, only some alpha-Al(2)O(3) crystallites showed the presence of dislocations, whereas other crystallites were defect free. In the last zone the alumina crystals were left unaffected but the ZrO(2) crystallites showed twinning characteristic of strain-induced transformation.

Recent concepts concerning the renal handling of NH3/NH4+.

To be appropriately excreted in urine, NH4+ , the major component of urinary acid excretion, must be synthesized by proximal tubular cells, secreted into the proximal tubular fluid, reabsorbed by the medullary thick ascending limb (MTAL) to accumulate in the medullary interstitium, and finally be secreted in the medullary collecting ducts. Each of the various steps of this particular renal pathway is highly regulated, and the control of gene expression explains how the renal handling of NH 4 + becomes fully adapted to chronic acid-base changes. Several targets have been identified to account for the adaptation of renal NH 4 + synthesis and transport in response to an acid load. These are the key enzymes of ammoniagenesis and the apical Na+/H+ (NH4+) exchanger NHE3 in the proximal tubule, the apical Na + -K + (NH 4 + )-2Cl - cotransporter of the MTAL, and the basolateral Na+-K+ (NH4+)-2Cl- cotransporter and the epithelial Rh B and C glycoproteins in the collecting ducts. An acid pH appears to be a major factor in the control of gene expression during metabolic acidosis probably through the activation of pH sensors. Glucocorticoids can contribute to coordinate the adaptation of various tubular cell types. This review focuses on some new aspects of NH3/NH4+ transport and of gene expression regulation that have recently emerged.