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      Volume Regulation of Thick Ascending Limb of Henle Cells: Significance of Organic Osmolytes

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          The thick ascending loop of Henle (TALH) is exposed to high osmotic stress, which is particularly due to high sodium and chloride reabsorption and very low water permeability of the luminal membrane. Therefore, the volume regulation of TALH cells, derived from the TALH loop of rabbit kidneys, was analyzed. The volume was determined by impedance measurements. TALH cells, which were adapted to different osmolarities (300 and 600 mosm/l), showed no significant differences in their cell volume. Therefore, a complete volume regulation could be supposed. An increase in extracellular osmolarity from 300 to 600 mosm/l (osmolarity adjusted by addition of 150 m M NaCl) immediately led to a reduction in the cell volume by 37 ± 6% (n = 6). A regulatory volume increase (RVI) was not observed within 10 min but after 24 h. Conversely, a sudden cell swelling by 44 ± 5% (n = 4) was detected within 20 s following an extracellular hypoosmotic challenge (from 600 to 300 mosm/l). The subsequent volume regulatory decrease (RVD) required a period of 7 days. Specific inhibitors of important ion transporters had no effect on volume regulation. Thus, changes in the ion conductivity do not seem to influence the processes of RVI and RVD. Conversely, the intracellular content of the organic osmolytes, sorbitol, inositol, betaine, and glycerophosphorylcholine, changed in the course of RVI and RVD. These results provide evidence that TALH cells are capable of maintaining their volume despite large extracellular osmotic changes. RVI and RVD are mainly regulated by changes in the intracellular content of organic osmolytes within 1 and 7 days.

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          Role of G-proteins in the regulation of organic osmolyte efflux from isolated rat renal inner medullary collecting duct cells

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            Peroxynitrite formation and apoptosis in transgenic sickle cell mouse kidneys.

            In a previous study, nitric oxide synthases (NOS) were found to be strongly expressed in the tubular epithelium of kidneys of a transgenic mouse model of sickle cell disease (alphaHbetaS[betaMDD]). Because NOS activity is often associated with peroxynitrite formation when superoxide radical (.O-2) is present in abundance, we examined the kidneys of sickle cell mice for nitrotyrosine, considered to be a footprint of ONOO-. Western blot and immunohistochemistry for nitrotyrosine was carried out. Since peroxynitrite and other reactive oxygen radicals are capable of causing apoptosis, we also performed agarose gel electrophoresis of kidney DNA and TUNEL staining of nuclei, indicators of apoptosis. Nitration of tyrosine residues of three proteins (kD 66, 57 and 22) was found on Western blot of kidney protein extracts of the sickle cell mice. The degree of tyrosine nitration of the 66 kD protein was not significantly different in the control versus transgenic mice, whereas tyrosine nitration of the 57 and 22 kD proteins was clearly increased in transgenic mice. Strong immunostaining for nitrotyrosine was seen in tubular epithelial cells of the sickle cell mice, in close proximity to positive immunostaining of iNOS. Neither iNOS nor nitrotyrosine was expressed in the control mice. DNA "laddering" was found localized to the same zones of the kidney as nitrotyrosine and iNOS immunostaining. TUNEL assay on mouse kidney tissue sections showed minimal tubular cell apoptosis in normal mouse with hypoxia, mild tubular cell apoptosis in sickle cell mouse in room air, and moderate tubular cell apoptosis in sickle cell mouse with hypoxia. The observations suggest that ONOO- and perhaps other reactive oxygen species are being produced in the sickle cell kidney. The mechanism may be ischemia/reperfusion due to intermittent vascular occlusion by sickle cells. The resulting hypoxia could result in iNOS activation, superoxide radical and peroxynitrite formation. Two consequences of these reactions appear to be nitration of tyrosine residues of some renal proteins and enhanced apoptosis.

              Author and article information

              Nephron Exp Nephrol
              Cardiorenal Medicine
              S. Karger AG
              April 2001
              11 January 2001
              : 9
              : 2
              : 81-89
              aDivision of Nephrology and Rheumatology, University Hospital Göttingen, and bDepartment of Internal Medicine II, University of Ulm, Germany
              52598 Exp Nephrol 2001;9:81–89
              © 2001 S. Karger AG, Basel

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              Figures: 6, References: 35, Pages: 9
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