Search

We previously showed that the phosphatases PP1/PP2A and PP2B dephosphorylate the water channel, AQP2, suggesting their role in water reabsorption. In this study, we investigated whether protein phosphatase 2A (PP2A) and protein phosphatase 2B (PP2B or calcineurin), which are present in the inner medullary collecting duct (IMCD), are regulators of urea and water permeability. Inhibition of calcineurin by tacrolimus increased both basal and vasopressin-stimulated osmotic water permeability in perfused rat IMCDs. However, tacrolimus did not affect osmotic water permeability in the presence of aldosterone. Inhibition of PP2A by calyculin increased both basal and vasopressin-stimulated osmotic water permeability, and aldosterone reversed the increase by calyculin. Previous studies showed that adrenomedullin (ADM) activates PP2A and decreases osmotic water permeability. Inhibition of PP2A by calyculin prevented the ADM-induced decrease in water reabsorption. ADM reduced the phosphorylation of AQP2 at serine 269 (pSer269 AQP2). Urea is linked to water reabsorption by building up hyperosmolality in the inner medullary interstitium. Calyculin increased urea permeability and phosphorylated UT-A1. Our results indicate that phosphatases regulate water reabsorption. Aldosterone and adrenomedullin decrease urea or osmotic water permeability by acting through calcineurin and PP2A, respectively. PP2A may regulate water reabsorption by dephosphorylating pSer269, AQP2, and UT-A1. [ABSTRACT FROM AUTHOR]

Chronic kidney disease (CKD) causes loss of lean body mass by multiple mechanisms. This study examines whether autophagy-mediated proteolysis contributes to CKD-induced muscle wasting. We tested autophagy in the muscle of CKD mice with plantaris muscle overloading to mimic resistance exercise or with acupuncture plus low-frequency electrical stimulation (Acu/LFES) treatment. In CKD muscle, Bnip3, Beclin-1, and LC3II mRNAs and proteins were increased compared with those in control muscle, indicating autophagosome-lysosome formation induction. Acu/LFES suppressed the CKD-induced upregulation of autophagy. However, overloading increased autophagy-related proteins in normal and CKD muscle. Serum from uremic mice induces autophagy formation but did not increase the myosin degradation or actin break down in cultured muscle satellite cells. We examined mitochondrial biogenesis, copy number, and ATP production in cultured myotubes, and found all three aspects to be decreased by uremic serum. Inhibition of autophagy partially reversed this decline in cultured myotubes. In CKD mice, the mitochondrial copy number, biogenesis marker peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), mitochondrial transcription factor A (TFAM), and mitochondrial fusion marker Mitofusin-2 (Mfn2) are decreased. Both muscle overloading and Acu/LFES increased mitochondrial copy number, and reversed the CKD-induced decreases in PGC-1α, TFAM, and Mfn2. We conclude that the autophagy is activated in the muscle of CKD mice. However, myofibrillar protein is not directly broken down through autophagy. Instead, CKD-induced upregulation of autophagy leads to dysfunction of mitochondria and decrease of ATP production. [ABSTRACT FROM AUTHOR]

Fundamental kidney physiology research can provide important insight into how the kidney works and suggest novel therapeutic opportunities to treat human diseases. This is especially true for nephrogenic diabetes insipidus (NDI). Over the past decade, studies elucidating the molecular physiology and signaling pathways regulating water transport have suggested novel therapeutic possibilities. In patients with congenital NDI due to mutations in the type 2 vasopressin receptor (V2R) or acquired NDI due to lithium (or other medications), there are no functional abnormalities in the aquaporin-2 (AQP2) water channel, or in another key inner medullary transport protein, the UT-A1 urea transporter. If it is possible to phosphorylate and/or increase the apical membrane accumulation of these proteins, independent of vasopressin or cAMP, one may be able to treat NDI. Sildenifil (through cGMP), erlotinib, and simvastatin each stimulate AQP2 insertion into the apical plasma membrane. Some recent human data suggest that sildenafil and simvastatin may improve urine concentrating ability. ONO-AE1-329 (ONO) stimulates the EP4 prostanoid receptor (EP4), which stimulates kinases that in turn phosphorylate AQP2 and UT-A1. Clopidogrel is a P2Y12-R antagonist that potentiates the effect of vasopressin and increases AQP2 abundance. Metformin stimulates AMPK to phosphorylate and activate AQP2 and UT-A1, and it increases urine concentrating ability in two rodent models of NDI. Since metformin, sildenafil, and simvastatin are commercially available and have excellent safety records, the potential for rapidly advancing them into clinical trials is high. [ABSTRACT FROM AUTHOR]

Nephrogenic diabetes insipidus (NDI) is characterized by production of very large quantities of dilute urine due to an inability of the kidney to respond to vasopressin. Congenital NDI results from mutations in the type 2 vasopressin receptor (V2R) in ~90% of families. These patients do not have mutations in aquaporin-2 (AQP2) or urea transporter UT-A1 (UTA1). We tested adenosine monophosphate kinase (AMPK) since it is known to phosphorylate another vasopressin-sensitive transporter, NKCC2 (Na-K-2Cl cotransporter). We found AMPK expressed in rat inner medulla (IM). AMPK directly phosphorylated AQP2 and UT-A1 in vitro. Metformin, an AMPK activator, increased phosphorylation of both AQP2 and UT-A1 in rat inner medullary collecting ducts (IMCDs). Metformin increased the apical plasma membrane accumulation of AQP2, but not UT-A1, in rat IM. Metformin increased both osmotic water permeability and urea permeability in perfused rat terminal IMCDs. These findings suggest that metformin increases osmotic water permeability by increasing AQP2 accumulation in the apical plasma membrane but increases urea permeability by activating UT-A1 already present in the membrane. Lastly, metformin increased urine osmolality in mice lacking a V2R, a mouse model of congenital NDI. We conclude that AMPK activation by metformin mimics many of the mechanisms by which vasopressin increases urine-concentrating ability. These findings suggest that metformin may be a novel therapeutic option for congenital NDI due to V2R mutations. [ABSTRACT FROM AUTHOR]

Using microarray analysis, we found that aging sarcopenia is associated with a sharp increase in the mRNA of the matricellular protein CCN1 (Cyr61/ CTGF/Nov). CCN1 mRNA was upregulated 113-fold in muscle of aged vs. young rats. CCN1 protein was increased in aging muscle in both rats (2.8-fold) and mice (3.8-fold). When muscle progenitor cells (MPCs) were treated with recombinant CCN1, cell proliferation was decreased but there was no change in the myogenic marker myoD. However, the CCN1-treated MPCs did express a senescence marker Ink4A (SA-gal). Interestingly, we found CCN1 increased p53, p16, and pRP (hypophosphorylated retinoblastoma protein) protein levels, all of which can arrest cell growth in MPCs. When MPCs were treated with aged rodent serum CCN1 mRNA increased by sevenfold and protein increased by threefold suggesting the presence of a circulating regulator. Therefore, we looked for a circulating regulator. Wnt-3a, a stimulator of CCN1 expression, was increased in serum from elderly humans (2.6-fold) and aged rodents (2.0-fold) compared with young controls. We transduced C2C12 myoblasts with wnt-3a and found that CCN1 protein was increased in a time- and dose-dependent manner. We conclude that in aging muscle, the circulating factor wnt-3a acts to increase CCN1 expression, prompting muscle senescence by activating cell arrest proteins. [ABSTRACT FROM AUTHOR]

The kidney's ability to concentrate urine is vitally important to our quality of life. In the hypertonic environment of the kidney, urea transporters must be regulated to optimize function. We previously showed that hypertonicity increases urea permeability and that the protein kinase C (PKC) blockers chelerythrine and rottlerin decreased hypertonicitystimulated urea permeability in rat inner medullary collecting ducts (IMCDs). Because PKCα knockout (PKCα-/-) mice have a urineconcentrating defect, we tested the effect of hypertonicity on urea permeability in isolated perfused mouse IMCDs. Increasing the osmolality of perfusate and bath from 290 to 690 mosmol/kgH2O did not change urea permeability in PKCα-/- mice but significantly increased urea permeability in wild-type mice. To determine whether the response to protein kinase A was also missing in IMCDs of PKCα-/- mice, tubules were treated with vasopressin and subsequently with the PKC stimulator phorbol dibutyrate (PDBu). Vasopressin stimulated urea permeability in PKCα-/- mice. Like vasopressin, forskolin stimulated urea permeability in PKCα-/- mice. We previously showed that, in rats, vasopressin and PDBu have additive stimulatory effects on urea permeability. In contrast, in PKCα-/- mice, PDBu did not further increase vasopressin-stimulated urea permeability. Western blot analysis showed that expression of the UT-A1 urea transporter in IMCDs was increased in response to vasopressin in wild-type mice as well as PKCα-/- mice. Hypertonicity increased UT-A1 phosphorylation in wild-type mice but not in PKCα-/- mice. We conclude that PKCα mediates hypertonicitystimulated urea transport but is not necessary for vasopressin stimulation of urea permeability in mouse IMCDs. [ABSTRACT FROM AUTHOR]

Nephrotic syndrome is associated with up-regulation of the heparin-binding epidermal growth factor (HB-EGF). Erlotinib blocks the activation of the epidermal growth factor receptor (EGFR) in response to HB-EGF. This study investigates the effect of Erlotinib on the progression of proteinuria, renal dysfunction, and salt retention in doxorubicin treated nephrotic rats. Male rats were divided into 3 pair-fed groups (n = 13/group) as follows: Control rats (Ctrl); rats receiving intravenous doxorubicin (Dox); and rats receiving intravenous doxorubicin followed by daily oral Erlotinib (Dox + Erl). Upon establishment of high grade proteinuria, urine sodium and creatinine clearance were measured. Kidney tissue was dissected and analyzed for γ-epithelial sodium channel (γENaC), sodium-potassium -chloride co-transporter 2 (NKCC2), sodium chloride co-transporter (NCC), aquaporin 2 (AQP2), and EGFR abundances using western blot. Creatinine clearance was preserved in the Dox + Erl rats as compared to the Dox group (in ml/min: Ctrl: 5.2±.5, Dox: 1.9±0.3, Dox + Erl: 3.6±0.5). Despite a minimal effect on the degree of proteinuria, Erlotinib prevented salt retention (Urinary Na in mEq/d: Ctrl: 2.2±0.2, Dox: 1.8±0.3, Dox + Erl: 2.2±0.2). The cleaved/uncleaved γENaC ratio was increased by 41±16% in the Dox group but unchanged in the Dox + Erl group when compared to Ctrl. The phosphorylated EGFR/total EGFR ratio was reduced by 74±7% in the Dox group and by 77±4% in the Dox + Erl group. In conclusion, Erlotinib preserved renal function and prevented salt retention in nephrotic rats. The observed effects do not appear to be mediated by direct blockade of EGFR. [ABSTRACT FROM AUTHOR]

The UT-A1 urea transporter plays a critical role in the production of concentrated urine. Both vasopressin and hypertonicity increase urea permeability in rat terminal inner medullary collecting ducts (IMCD). Each agonist independently increases UT-A1 phosphorylation and apical plasma membrane accumulation. Vasopressin activates PKA and phosphorylates UT-A1 at serines 486 and 499. Hypertonicity stimulates urea permeability through protein kinase C (PKC) and intracellular calcium. To determine whether the hypertonic stimulation of urea permeability results from a PKC-mediated phosphorylation of UT-A1, rat IMCDs were metabolically labeled with [32P]. Hypertonicity stimulated UT-A1 phosphorylation, and this increase was blocked by preincubation with a PKC inhibitor. IMCDs were biotinylated to assess plasma membrane UT-A1. Hypertonicity increased biotinylated UT-A1, and this increase was blocked by preincubation with a PKC inhibitor. When PKC was directly activated using a phorbol ester, total UT-A1 phosphorylation increased, but phosphorylation at serine 486 was not increased, indicating that PKC did not phosphorylate UT-A1 at the same residue as PKA. Since PKC-α is a calcium-dependent PKC isoform and PKC-α knockout mice have a urine-concentrating defect, it suggested that PKC-α may mediate the response to hypertonicity. Consistent with this hypothesis, hypertonicity increased phospho-PKC-α in rat IMCDs. Finally, PKC-α knockout mice were used to determine whether hypertonicity could stimulate UT-A1 phosphorylation in the absence of PKC-α. Hypertonicity significantly increased UT-A1 phosphorylation in wild-type mice but not in PKC-α knockout mice. We conclude that PKC-α mediates the hypertonicity-stimulated increase in UT-A1 phosphorylation in the IMCD. [ABSTRACT FROM AUTHOR]

Hypertonicity increases urea transport independently of, as well as synergistically with, vasopressin in the inner medullary collect duct (IMCD). We previously showed that hypertonicity does not increase the level of cAMP in the IMCD, but it does increase the level of intracellular calcium. Since we also showed that hypertonicity increases both the phosphorylation and biotinylation of the urea transporters UT-A1 and UT-A3, this would suggest involvement of a calcium-dependent protein kinase in the regulation of urea transport in the inner medulla. In this study, we investigated whether protein kinase C (PKC), which is present in the IMCD, is a regulator of urea permeability. We tested the effect of PKC inhibitors and activators on urea permeability in the isolated, perfused rat terminal IMCD. Increasing osmolality from 290 to 690 mosmol/kgH2O significantly stimulated (doubled) urea permeability; it returned to control levels on inhibition of PKC with either 10 μM chelerythrine or 50 μM rottlerin. To determine the potential synergy between vasopressin and PKC, phorbol dibutyrate (PDBu) was used to stimulate PKC. Vasopressin stimulated urea permeability 247%. Although PDBu alone did not change basal urea permeability, in the presence of vasopressin, it significantly increased urea permeability an additional 92%. The vasopressin and PDBu-stimulated urea permeability was reduced to AVP alone levels by inhibition of PKC. We conclude that hypertonicity stimulates urea transport through a PKC-mediated phosphorylation. Whether PKC directly phosphorylates UT-A1 and/or UT-A3 or phosphorylates it as a consequence of a cascade of activations remains to be determined. [ABSTRACT FROM AUTHOR]

Vasopressin is the primary hormone regulating urine-concentrating ability. Vasopressin phosphirylates the UT-A1 urea transporter in rat inner medullary collecting ducts (IMCDs). To assess the effect of UT-A1 phosphorylation at S486, we developed a phospho-specific antibody to S486-UT-A1 using an II amino acid peptide antigen starting from amino acid 482 that bracketed S486 in roughly the center of the sequence. We also developed two stably transfected mIMCD3 cell lines: one expressing wild-type UT-A1 and one expressing a mutated form of UT-A1, S486A1S499A, that is unresponsive to protein kinase A. Forskolin stimulates urea flux in the wild-type UT-A1-mIMCD3 cells but not in the S486A1S499A-UT-A1 -mIMCD3 cells. The phospho-S486- UT-A1 antibody identified UT-A1 protein in the wild-type UT-A1- mIMCD3 cells but not in the S486A/S499A-UT-A1-mIMCD3 cells. In rat IMCDs, forskolin increased the abundance of phospho-S486- UT-A1 (measured using the phospho-S486 antibody) and of total UT-A1 phosphorylation (measured by 32P incorporation). Forskolin also increased the plasma membrane accumulation of phospho-S486- UT-A1 in rat IMCD suspensions, as measured by biotinylation. In rats treated with vasopressin in vivo, the majority of the phospho-S486- UT-A1 appears in the apical plasma membrane. In summary, we developed stably transfected mIMCD3 cell lines expressing UT-A1 and an S486-UT-A1 phospho-specific antibody. We confirmed that vasopressin increases UT-A1 accumulation in the apical plasma membrane and showed that vasopressin phosphorylates UT-A1 at S486 in rat IMCDs and that the S486-phospho-UT-A1 form is primarily detected in the apical plasma membrane. [ABSTRACT FROM AUTHOR]

UT-AI is regulated by vasopressin and is localized to the apical membrane and intracellular compartment of inner medullary collecting duct (IMCD) cells. UT-A3 is also expressed in the IMCD and is regulated by forskolin in heterologous systems. The goal of the present study is to investigate mechanisms by which vasopressin regulates UT-A3 in rat IMCD. In fresh suspensions of rat IMCD, forskolin increases the phosphorylation of UT-A3, similar to UT-Al. Biotinylation studies indicate that UT-A3 is located in the plasma membrane. Forskolin treatment increases the abundance of UT-A3 in the plasma membrane similar to UT-Al. However, these two transporters do not form a complex through a protein-protein interaction, suggesting that trans- porter function is unique to each protein. While immunohistochemistry localized UT-A3 to the basal and lateral membranes, a majority of the staining was cytosolic. Immunohistochemistry of vasopressin- treated rat kidney sections also localized UT-A3 primarily to the cytosol with basal and lateral membrane staining but also showed some apical membrane staining in some IMCD cells. This suggests that under normal conditions, UT-A3 functions as the basolateral transporter but in a high cAMP environment, the transporter may move from the cytosol to all plasma membranes to increase urea flux in the IMCD. In summary, this study confirms that UT-A3 is located in the inner medullary tip where it is expressed in the basolateral membrane, shows that UT-A3 is a phosphoprotein in rat IMCD that can be trafficked to the plasma membrane independent of UT-Al, and suggests that vasopressin may induce UT-A3 expression in the apical plasma membrane of IMCD. [ABSTRACT FROM AUTHOR]

The kidney responds to high levels of ANG II, as may occur during malignant hypertension, by increasing sodium and water excretion. To study whether kidney medullary transporters contribute to this response, rats were made hypertensive using ANG II. Within 3 days of being given ANG II, systolic blood pressure (BP) was increased (200 mmHg), vs control (130 mmHg), and remained high through day /4. Kidney inner medullary (IM) tip and base and outer medulla were analyzed for transporter protein abundance. There were significant decreases in UT-Al urea transporter, aquaporin-2 (AQP2) water channel, and NKCC2/BSC1 Na+-K+-2Cl- cotransporter. To determine whether the decreases were a response to hypertension, ANG II, or an ANG II-induced increase in aldosterone, rats were given 1) norepinephrine (to increase BP) and 2) ANG II plus spironolactone (to block the mineralocorticoid receptor). Norepinephrine (7 days) increased BP, urine volume, sodium excretion, and decreased urine osmolality and UT-A1, AQP2, and NKCC2/BSC1 abundances, similar to ANG II. ANG II alone or with spironolactone yielded similar increases in BP, urine volume, and urine osmolality, and decreases in UT-A1 and AQP2 proteins in the IM tip. Plasma vasopressin was unaffected by treatment. Water diuresis did not change UT-A1 but decreased AQP2 and NKCC2/BSC1 abundances. We conclude that decreases in UT-A1, AQP2, and NKCC2/BSC1 proteins may contribute to the diuresis and natriuresis that occur following ANG II or norepinephrine-induced acute hypertension and do not appear to involve ANG II stimulation of aldosterone or thirst. [ABSTRACT FROM AUTHOR]

Transepithelial [14C]urea fluxes were measured across cultured Madin-Darby canine kidney (MDCK) cells permanently transfected to express the urea transport protein UT-A1. The urea fluxes were typically increased from a basal rate of 2 to 10 and 25 nmol·cm-2·min-1 in the presence of vasopressin and forskolin, respectively. Flux activation consisted of a rapid-onset component of small amplitude that leveled off within ~10 mm and at times even decreased again, followed by a delayed, strong increase over the next 30-40 mm. Forskolin activated urea transport through activation of adenylyl cyclase; dideoxyforskolin was inactive. Vasopressin activated urea transport only from the basolateral side and was blocked by OPC-31260, indicating that its action was mediated by basolateral V2 receptors. In the presence of the phosphodiesterase inhibitor IBMX, vasopressin activated as strongly as forskolin. By itself, IBMX caused a slow increase over 50 min to ~5 nmol·cm-2·min-1. 8-Bromoadenosine 3′,5′-cyclic monophosphate (8-BrcAMP; 300 µM) activated urea flux only when added basolaterally. IBMX augmented the activation by basolateral 8-BrcAMP. Urea flux activation by vasopressin and forskolin were only partially blocked by the protein kinase A inhibitor H-89. Even at concentrations >10 µM, urea flux after 60 mm of stimulation was reduced by <50%. The rapid-onset component appeared unaffected by the presence of H-89. These data suggest that activation of transepithelial urea transport across MDCK-UT-A1 cells by forskolin and vasopressin involves cAMP as a second messenger and that it is mediated by one or more signaling pathways separate from and in addition to protein kinase A. [ABSTRACT FROM AUTHOR]

The primary mechanism by which the kidneys mediate net acid excretion is through ammonia metabolism. In the current study, we examined whether chronic metabolic acidosis, which increases ammonia metabolism, alters the cell-specific and/or the subcellular expression of the ammonia transporter family member, Rhcg, in the outer medullary collecting duct in the inner stripe (OMCDi). Chronic metabolic acidosis was induced in normal SD rats by HC1 ingestion for 7 days; controls were pair-fed. The subcellular distribution of Rhcg was determined using immunogold electron microscopy and morphometric analyses. In intercalated cells, acidosis increased total Rhcg, apical plasma membrane Rhcg, and the proportion of total cellular Rhcg in the apical plasma membrane. Intracellular Rhcg decreased significantly, and basolateral Rhcg was unchanged. Because apical plasma membrane length increased in parallel with apical Rhcg immunolabel, apical plasma membrane Rhcg density was unchanged. In principal cells, acidosis increased total Rhcg, apical plasma membrane Rhcg, and the proportion of total cellular Rhcg in the apical plasma membrane while decreasing the intracellular proportion. In contrast to the intercalated cell, chronic metabolic acidosis did not significantly alter apical boundary length; accordingly, apical plasma membrane Rhcg density increased. In addition, basolateral Rhcg immunolabel increased in response to chronic metabolic acidosis. These results indicate that in the rat OMCDi 1) chronic metabolic acidosis increases apical plasma membrane Rhcg in both the intercalated cell and principal cell where it may contribute to enhanced apical ammonia secretion; 2) increased apical plasma membrane Rhcg results from both increased total protein and changes in the subcellular distribution of Rhcg; 3) the mechanism of Rhcg subcellular redistribution differs in intercalated and principal cells; and 4) Rhcg may contribute to regulated basolateral ammonia transport in the principal cell. [ABSTRACT FROM AUTHOR]

The article presents a scientific research on the tissue distribution of urea transporter-A (UT-A) and urea transporter-B (UT-B) messenger RNA and protein in rats. Researchers were able to identify UT-A and or UT-B messenger RNA and or protein forms in several organs not typically associated with the metabolism of urea.

Chronic metabolic acidosis induces dramatic increases in net acid excretion that are predominantly due to increases in urinary ammonia excretion. The current study examines whether this increase is associated with changes in the expression of the renal ammonia transporter family members, Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg). Chronic metabolic acidosis was induced in Sprague-Dawley rats by HCl ingestion for 1 wk; control animals were pair-fed. After 1 wk, metabolic acidosis had developed, and urinary ammonia excretion increased significantly. Rhcg protein expression was increased in both the outer medulla and the base of the inner medulla. Intercalated cells in the outer medullary collecting duct (OMCD) and in the inner medullary collecting duct (IMCD) in acid-loaded animals protruded into the tubule lumen and had a sharp, discrete band of apical Rhcg immunoreactivity, compared with a flatter cell profile and a broad band of apical immunolabel in control kidneys. In addition, basolateral Rhcg immunoreactivity was observed in both control and acidotic kidneys. Cortical Rhcg protein expression and immunoreactivity were not detectably altered. Rhcg mRNA expression was not significantly altered in the cortex, outer medulla, or inner medulla by chronic metabolic acidosis. Rhbg protein and mRNA expression were unchanged in the cortex, outer and inner medulla, and no changes in Rhbg immunolabel were evident in these regions. We conclude that chronic metabolic acidosis increases Rhcg protein expression in intercalated cells in the OMCD and in the IMCD, where it is likely to mediate an important role in the increased urinary ammonia excretion. [ABSTRACT FROM AUTHOR]

In normal rats, vasopressin and hyperosmolality enhance urea permeability (Purea) in the terminal, but not in the initial inner medullary collecting duct (IMCD), a process thought to occur through the UT-A1 urea transporter. In the terminal IMCD, UT-A1 is detected as 97- and 117-kDa glycoproteins. However, in the initial IMCD, only the 97-kDa form is detected. During streptozotocin-induced diabetes mellitus, UT-A1 protein abundance is increased, and the 117-kDa UT-A1 glycoprotein appears in the initial IMCD. We hypothesize that the 117-kDa glycoprotein mediates the vasopressin- and osmolality-induced changes in Purea. Thus, in the present study, we measured Purea in in vitro perfused initial IMCDs from diabetic rats by imposing a 5 mM bath-to-lumen urea gradient without any osmotic gradient. Basal Purea was similar in control vs. diabetic rats (3 ± 1 vs. 5 ± 1 × 10-5 cm/s, n = 4, P = not significant). Vasopressin (10 nM) significantly increased Purea to 16 ± 5 × 10-5 cm/s (n = 4, P < 0.05) in diabetic but not in control rats. Forskolin (10 µM, adenylyl cyclase activator) also significantly increased Purea in diabetic rats. In contrast, increasing osmolality to 690 mosmol/kgH2O did not change Purea in diabetic rats. We conclude that initial IMCDs from diabetic rats have vasopressin- and forskolin-, but not hyperosmolality-stimulated Purea. The appearance of vasopressin-stimulated Purea in initial IMCDs correlates with an increase in UT-A1 protein abundance and the appearance of the 117-kDa UT-A1 glycoprotein in this region during diabetes. This suggests that the 117-kDa UT-A1 glycoprotein is necessary for vasopressin-stimulated urea transport. [ABSTRACT FROM AUTHOR]

Rats with diabetes mellitus have an increase in UT-A1 urea transporter protein abundance and absolute urea excretion, but the relative amount (percentage) of urea in total urinary solute is actually decreased due to the marked glucosuria. Urea-specific signaling pathways have been identified in mIMCD3 cells and renal medulla, suggesting the possibility that changes in the percentage or concentration of urea could be a factor that regulates UT-A1 abundance. In this study, we tested the hypothesis that an increase in a urinary solute other than urea would increase UT-A1 abundance, similar to diabetes mellitus, whereas an increase in urine urea would not. In both inner medullary base and tip, UT-A1 protein abundance increased during NaCl- or glucose-induced osmotic diuresis but not during urea-induced osmotic diuresis. Next, rats undergoing NaCl or glucose diuresis were given supplemental urea to increase the percentage of urine urea to control values. UT-A1 abundance did not increase in these urea-supplemented rats compared with control rats. Additionally, both UT-A2 and UT-B protein abundances in the outer medulla increased during urea-induced osmotic diuresis but not in NaCl or glucose diuresis. We conclude that during osmotic diuresis, UT-A1 abundance increases when the percentage of urea in total urinary solute is low and UT-A2 and UT-B abundances increase when the urea concentration in the medullary interstitium is high. These findings suggest that a reduction in urine or interstitial urea results in an increase in UT-A1 protein abundance in an attempt to restore inner medullary interstitial urea and preserve urine-concentrating ability. [ABSTRACT FROM AUTHOR]

Urea plays an important role in the urinary concentrating capacity. Renal inner medullary (IM) urea transporter expression was examined in rats with bilateral (BUO) or unilateral ureteral obstruction (UUO). BUO (24 h) was associated with markedly increased plasma urea (42.4 ± 1.0 vs. 5.2 ± 0.2 mmol/l) and a significant decease in expression of UT-A1 (28 ± 8% of sham levels), UT-A3 (45 ± 11%), and UT-B (70 ± 8%). Immunocytochemistry confirmed downregulation of UT-A1 and UT-A3 in IM collecting duct and UT-B in the descending vasa recta. Three days after release of BUO, UT-A1, UT-A3, and UT-B remained significantly downregulated (UT-A1: 37 ± 6%; UT-A3:25 ± 6%; and UT-B: 10 ± 5% of sham levels; P < 0.05) concurrent with a persistent polyuria and a marked reduction in solute-free water reabsorption (115 ± 11 vs. 196 ± 8 µl · min-1 · kg-1, P < 0.05). Moreover, 14 days after release of BUO, total UT-A1, UT-A3, and UT-B remained significantly decreased compared with sham-operated controls and urine urea remained reduced (588 ± 43 vs. 1,150 ± 94 mmol/l). Consistent with increased levels of plasma urea 24 h after onset of UUO (7.4 ± 0.3 vs. 4.8 ± 0.3 mmol/l), the protein abundance of UT-A1, UT-A3, and UT-B in IM was markedly reduced in the obstructed kidney, which was confirmed by immunocytochemistry. In the nonobstructed kidney, the expression of urea transporters did not change. In conclusion, reduced expression of UT-A1, UT-A3, and UT-B levels in both BUO and UUO rats suggests that urea transporters play important roles in the impaired urinary concentrating capacity in response to urinary tract obstruction. [ABSTRACT FROM AUTHOR]

Progress in understanding the cell biology of urea transporter proteins has been hampered by the lack of an appropriate cell culture system. The goal of this study was to create a polarized epithelial cell line that stably expresses the largest of the rat renal urea transporter UT-A isoforms, UT-A1. The gene for UT-A1 was cloned into pcDNA5/FRT and transfected into Madin-Darby canine kidney (MDCK) cells with an integrated Flp recombination target site. The cells from a single clone were grown to confluence on collagen-coated membranes until the resistance was >1,500 Ω·cm². Transepithelial [14C]urea fluxes were measured at 37°C in a HCO3-/ CO2 buffer, pH 7.4, with 5 mM urea. The baseline fluxes were not different between unstimulated UT-Al-transfected MDCK cells and nontransfected or sham-transfected MDCK cells. However, only in the UT-Al-transfected cells was UT-Al protein expressed (as measured by Western blot analysis) and urea transport stimulated by forskolin or arginine vasopressin. Forskolin and arginine vasopressin also increased the phosphorylation of UT-Al. Thionicotinamide, dimethylurea, and phloretin inhibited the forskolin-stimulated [14C]urea fluxes in the UT-Al-transfected MDCK cells. These characteristics mimic those seen in rat terminal inner medullary collecting ducts. This new polarized epithelial cell line stably expresses UT-Al and reproduces several of the physiological responses observed in rat terminal inner medullary collecting ducts. [ABSTRACT FROM AUTHOR]

Studies the effects of urea transporters on the L-arginine transport. Inhibitory action of urea on L-arginine transport; Presence of urea transporters-B in the endothelial cells; Uremic levels of urea.

Studies the role of medullary transport proteins in the urine-concentrating defect in angiotensin-converting enzyme.2 (ACE.2) mice. Chemistries of the blood and urine; Protein abundances in ACE.2 mice; Infusion of the angiotensin II.

Background: Lithium is commonly used to treat bipolar psychiatric disorders but can cause reduced urine concentrating ability.Methods: To test whether lithium alters UT-A1 or UT-B urea transporter protein abundance or UT-A1 phosphorylation, rats were fed a standard diet supplemented with LiCl for 10 or 25 days, and then compared to pair-fed control rats. To investigate another potential mechanism for decreased urea transport, inner medullary collecting duct (IMCD) suspensions from lithium-fed or control rats were incubated with 32P-orthophosphate to measure the phosphorylation of UT-A1.Results: In lithium-fed rats (25 days), UT-A1 abundance was reduced to 50% of control rats in IM tip and to 25% in IM base, and UT-B abundance was reduced to 40% in IM base. Aquaporin-2 (AQP2) protein abundance was reduced in both IM regions. Vasopressin (100 pmol/L) increased UT-A1 phosphorylation in IMCD suspensions from control but not from lithium-fed rats; a higher vasopressin concentration (100 nmol/L) increased UT-A1 phosphorylation in control and lithium-fed rats.Conclusions: Decreases in UT-A1, UT-B, and AQP2 protein abundance, and/or vasopressin-stimulated phosphorylation of UT-A1, can contribute to the reduced urine concentrating ability that occurs in lithium-treated rats. [ABSTRACT FROM AUTHOR]

Stimulates the Na-K-2Cl cotransporter by the growth factors of dependent mechanism on chloride. Mediation of phosphorylation in NKCC1; Evidence for regulating cytoskeletal via myosin light chain kinase; Increase of serum by bumetanide-sensitive influx.

Localization of the urea transporter UT-B protein in human and rat erythrocytes and tissues. Am J Physiol Cell Physiol 281: C1318-C1325, 2001.—A new polyclonal antibody to the human erythrocyte urea transporter UT-B detects a broad band between 45 and 65 kDa in human erythrocytes and between 37 and 51 kDa in rat erythrocytes. In human erythrocytes, the UT-B protein is the Kidd (Jk) antigen, and Jk(a+b+) erythrocytes express the 45- to 65-kDa band. However, in Jk null erythrocytes [Jk(a b-)], only a faint band at 55 kDa is detected. In kidney medulla, a broad band between 41 and 54 kDa, as well as a larger band at 98 kDa, is detected. Human and rat kidney show UT-B staining in nonfenestrated endothelial cells in descending vasa recta. UT-B protein and mRNA are detected in rat brain, colon, heart, liver, lung, and testis. When kidney medulla or liver proteins are analyzed with the use of a native gel, only a single protein band is detected. UT-B protein is detected in cultured bovine endothelial cells. We conclude that UT-B protein is expressed in more rat tissues than previously reported, as well as in erythrocytes. [ABSTRACT FROM AUTHOR]

Examines the localization of urea transporter UT-B protein in human and rat erythrocytes and tissues. Presence of messenger RNA in every tissues expressing UT-B protein; Variability in protein band size; Reduction of erythrocyte UT-B protein by peptide N-glycosidase F deglycoselation.

Discusses a study which evaluated the ability of angiotensin II in regulating urea transport in the inner medullary collecting duct. Methods; Results; Discussion.

Tests the hypothesis that angiotensin II increases vasopressin-stimulated facilitated urea permeability in rat terminal inner medullary collecting duct (IMCD). Phosphorylation of urea transporter-1; Effect of staurosporine and angiotensin II on vasopressin-stimulated urea permeability.

Identifies a volume-sensitive protein kinase that phosphorylates the sodium-potassium-2chloride cotransporter (NKCC1) in vitro. Cellular localization of NKCC1 kinase; Effect of protein kinase A inhibition in vitro on phosphorylation of ammonium-terminal NKCC1 fusion protein; Influence of cell volume on tyrosine phosphorylation.

Identifies a volume-sensitive protein kinase that phosphorylates the sodium-potassium-chloride (Na-K-2Cl) cotransporter in vitro. Development of an in vitro assay of Na-K-2Cl cotransporter phosphorylation; Cell volume; Identification of c-Jun NH2-terminal kinase (JNK) as a volume-sensitive kinase in endothelial cells.

Examines the effects of streptozotocin-induced diabetes mellitus on the expression of vasopresin-regulated urea transporter (VRUT) in diabetic rat inner medulla. Higher values of blood glucose and urine osmolality in rats injected with streptozotocin; Measurement of VRUT protein in the inner medullary base of the kidney.

Demonstrates that the administration of glucocorticoids to an adrenalectomized (Adx) rat downregulates urea transport in the terminal inner medullary collecting duct (IMCD) and the expression of the vasopressin (AVP)-regulated urea transporter (VRUT) protein in the inner medullary tip by preparing VRUT antibody. Reduction seen in the VRUT in the IM tip of glucocorticoid-rats; Concluding remarks.

Examines the role of myosin light chain kinase (MLCK) in shrinkage-induced activation of the sodium (Na+)/hydrogen (H+) exchanger in rat astrocytes, focusing on the effect of hyperosmotic exposure in C6 glioma cells. Details on the Na+/H+ exchanger; Importance for the brain cells to regulate cell volume; Details on the study.

Examines protein phosphorylation in hypertonically shrunken aortic endothelial cells. Cell culture; Immune precipitation; Phosphopeptide mapping; Immune precipitation of myosin light chain; Cell volume changes; cotransport in hypertonic medium.

Presents a study which examined shrinkage-induced activation of Na+/H+ exchange (NHE) in primary rat astrocytes. Data suggesting that shrinkage-induced activation in of NHE in astrocytes occurs via a novel pathway involving activation of camouldin-dependent myosine light-chain kinase and phosphorylation of myosin light chain.

Studies the effects of hyperosmotic exposure in C6 glioma cells. Activation of Na+/H+ exchanger by cell shrinkage; Effect of degree of confluence on shrinkage-induced activation of the NHE; Phosphorylation of myosin light chain in subconfluent C6 cells.

Hypertonic stress in the kidney inner medulla is common, yet inner medullary cells adapt to limit cell death. Küper et al. have identified a cell-survival response by which increased cyclooxygenase-2 (COX-2) stimulates a prostaglandin E2 (PGE2)/protein kinase A (PKA)-mediated inactivation of the pro-apoptotic protein BAD. However, the PGE2/PKA pathway is not the only means to inactivate BAD and limit cell death. This Commentary shows a broader picture of this pathway to examine the kidney's BAD options. [ABSTRACT FROM AUTHOR]

The serine/threonine protease corin, which proteolytically activates atrial natriuretic peptide (ANP), is reduced in the kidneys of animals with nephrotic syndrome and glomerular nephritis. Polzin et al. provide evidence for a linkage between the decreased corin and β-epithelial sodium channel, phosphodiesterase 5, and cGMP-dependent protein kinase II in the nephrotic kidney. They propose that decreases in cGMP resulting from the reduced corin may be responsible for the Na+ retention and volume expansion that are hallmarks of these kidney diseases. [ABSTRACT FROM AUTHOR]

Uremic cardiomyopathy, characterized by hypertension, cardiac hypertrophy, and fibrosis, is a complication of chronic kidney disease (CKD). Urea transporter (UT) inhibition increases the excretion of water and urea, but the effect on uremic cardiomyopathy has not been studied. We tested UT inhibition by dimethylthiourea (DMTU) in 5/6 nephrectomy mice. This treatment suppressed CKD‐induced hypertension and cardiac hypertrophy. In CKD mice, cardiac fibrosis was associated with upregulation of UT and vimentin abundance. Inhibition of UT suppressed vimentin amount. Left ventricular mass index in DMTU‐treated CKD was less compared with non‐treated CKD mice as measured by echocardiography. Nephrectomy was performed in UT‐A1/A3 knockout (UT‐KO) to further confirm our finding. UT‐A1/A3 deletion attenuates the CKD‐induced increase in cardiac fibrosis and hypertension. The amount of α‐smooth muscle actin and tgf‐β were significantly less in UT‐KO with CKD than WT/CKD mice. To study the possibility that UT inhibition could benefit heart, we measured the mRNA of renin and angiotensin‐converting enzyme (ACE), and found both were sharply increased in CKD heart; DMTU treatment and UT‐KO significantly abolished these increases. Conclusion: Inhibition of UT reduced hypertension, cardiac fibrosis, and improved heart function. These changes are accompanied by inhibition of renin and ACE. [ABSTRACT FROM AUTHOR]

Reports that growth factors stimulate the sodium potassium chloride cotransporter, NKCC1 through a novel Cl--dependent mechanism. Regulation of volume-regulatory transporter mediated by phosphorylation of NKCC1; Activation of NKCC1 by growth factors; Role played by NKCC1 in cell hypertrophy; Information on a novel mechanism whereby serum activates NKCC1.

High NaCl in the renal medullary interstitial fluid powers the concentration of urine but can damage cells. The transcription factor nuclear factor of activated T cells 5 (NFAT5) activates the expression of osmoprotective genes. We studied whether PKC-α contributes to the activation of NFAT5. PKC-α protein abundance was greater in the renal medulla than in the cortex. Knockout of PKC-α reduced NFAT5 protein abundance and expression of its target genes in the inner medulla. In human embryonic kidney (HEK)-293 cells, high NaCl increased PKC-α activity, and small interfering RNA-mediated knockdown of PKC-α attenuated high NaCl-induced NFAT5 transcriptional activity. Expression of ERK1/2 protein and phosphorylation of ERK1/2 were higher in the renal inner medulla than in the cortex. Knockout of PKC-α decreased ERK1/2 phosphorylation in the inner medulla, as did knockdown of PKC-α in HEK-293 cells. Also, knockdown of ERK2 reduced high NaCl-dependent NFAT5 transcriptional activity in HEK-293 cells. Combined knockdown of PKC-α and ERK2 had no greater effect than knockdown of either alone. Knockdown of either PKC-α or ERK2 reduced the high NaCl-induced increase of NFAT5 transactivating activity. We have previously found that the high NaCl-induced increase of phosphorylation of Ser591 on Src homology 2 domaincontaining phosphatase 1 (SHP-1-S591-P) contributes to the activation of NFAT5 in cell culture, and here we found high levels of SHP-1-S591-P in the inner medulla. PKC-α has been previously shown to increase SHP-1-S591-P, which raised the possibility that PKC-α might be acting through SHP-1. However, we did not find that knockout of PKC-α in the renal medulla or knockdown in HEK-293 cells affected SHP-1-S591-P. We conclude that PKC-α contributes to high NaCl-dependent activation of NFAT5 through ERK1/2 but not through SHP-1-S591. [ABSTRACT FROM AUTHOR]

R5501 --> 855.5 --> The syndrome of inappropriate antidiuretic hormone secretion (SIADH) consists of unregulated, elevated vasopressin levels and can lead to hyponatremia. Vasopressin escape is the body's defense mechanism to limit hyponatremia. The sodium‐glucose transporter (SGLT‐2) inhibitor, empagliflozin, was identified as a treatment for hyponatremia; however, the mechanism of empagliflozin's effect is unclear. This project investigates empagliflozin's impact on vasopressin escape in rats with the hypothesis that empagliflozin accelerates escape via key transport protein alteration. Rats were implanted with vasopressin mini‐pumps to mimic SIADH. The first cohort consisted of male rats and a second cohort compared responses in female rats. On day 2, half the rats were given empagliflozin (~15 mg/kg) via diet. Rats were sacrificed on days 7 or 14. Blood and kidney tissue were collected. Total Aquaporin‐2 (AQP2), pSer‐Aquaporin‐2, and NKCC2 were analyzed by western blot. At day 7, serum sodium levels for the female rats were: empagliflozin 110.1+1.7 mmol/L vs 93.3+1.1 mmol/L for the controls (n=5, p<0.001). Serum sodium levels for the male rats were: empagliflozin 136.0+4.8 mmol/L vs 121.4+3.0 mmol/L for the controls (n=4, p<0.05). NKCC2 abundance in the empagliflozin group was 60% lower than control on day 7 (n=5, p<0.05) for female rats, and 77% lower than control in male rats (n =4, p<0.001). Because changes in the AQP2 levels were similar between the sexes, these data were combined. Total AQP2 abundance was 2.3 fold increased in the empagliflozin group on day 7 and 8 fold greater in the empagliflozin group at day 14 (n=9) compared to the control group. Both pSer256‐AQP2 and pSer261‐AQP2 were increased in the empagliflozin groups at both 7 and 14 days vs their respective control groups. pSer256‐AQP2 increased 2.1 (day 7) and 8.5 fold (day 14); pSer261‐AQP2 increased 3.2 fold (day 7) and 5.0 fold (day 14) (n=9/group). We conclude that empagliflozin accelerates vasopressin escape during SIADH primarily through a decrease in NKCC2 abundance in both male and female rats. This will hasten recovery from hyponatremia in SIADH. [ABSTRACT FROM AUTHOR]

Regulation of water and urea transport in the inner medullary collecting duct is essential for urine concentration. Aquaporin (AQP)2 water channels and urea transporter (UT)-A1 are inserted into the apical membrane upon phosphorylation of the channels to allow the transcellular movement of water and urea. Since ANG II activates PKC in many cell types, we tested the hypothesis that ANG II-induced regulation of water and urea transport is mediated by PKC. Osmotic minipumps delivered ANG II to wild-type (WT) or PKC-α-/- mice for 7 days. Inner medullas were harvested, and protein abundance was determined by immunoblot. ANG II increased systolic blood pressure to a similar degree in WT and PKC-α-/- mice. ANG II had no effect on the urine output of WT mice but increased that of PKC-α-/- mice. In accordance with observed differences in urine output, AQP2 abundance was unchanged in ANG II-treated WT animals but was decreased in PKC- α-/- mice. No change in membrane accumulation was seen. Phosphorylation of the cAMP-induced transcription factor CREB was decreased in PKC-α-/- mice in response to ANG II with no change in overall CREB abndance. ANG II did not alter the abundance of UT-A1 protein in WT or PKC-α-/- mice. Phosphorylation and overall abundance of tonicity-responsive enhancer-binding protein, a transcription factor that regulates UT-A1, were also unaltered by ANG II in either group. We conclude that PKC-α protects against ANG II-induced decreases in urine concentrating ability by maintaining AQP2 levels through CREB phosphorylation. [ABSTRACT FROM AUTHOR]

Of the three major protein variants produced by the UT-A gene (UT-A1, UT-A2, and UT-A3) UT-A1 is the largest. It contains UT-A3 as its NH2-terminal half and UT-A2 as its COOH-terminal half. When being part of UT-A1, UT-A3 and UT-A2 are joined by a segment, Lp, whose central pan, Lc, is not part of UT-A3 or UT-A2 but is present only in UT-A1. Lc contains the phosphorylation sites S486 and S499 that are involved in protein kinase A-dependent activation, as well as the binding site for snapin, a protein involved in soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE)-mediated vesicle trafficking and fusion to the plasma membrane. We attached Lc to UT-A2 and UT-A3 to test how these pbosphorylation sites influenced their urea transport activity. Adding Lc to UT-A2 conferred stimulation by cAMP to the cAMP-unresponsive UT-A2, and adding Lc to UT-A3 did not further enhance its already existing cAMP response. These findings suggest that the responsiveness to vasopressin that is observed with UT-A1 can be introduced into the unresponsive UT-A2 variant through the Lc segment that is unique to UT-A1. In UT-A3, however, the Lc segment plays no significant role in its activation by cAMP. In addition, the Lc segment also gave UT-A2 the ability to bind snapin and, in Xenopus oocytes, to be stimulated in its urea transport activity by snapin and syntaxins 3 and 4, in the same way as UT-A1. [ABSTRACT FROM AUTHOR]

Chronic kidney disease (CKD) impairs muscle protein metabolism leading to muscle atrophy, and exercise can counteract this muscle wasting. Here we evaluated how resistance exercise (muscle overload) and endurance training (treadmill running) affect CKD-induced abnormalities in muscle protein metabolism and progenitor cell function using mouse plantaris muscle. Both exercise models blunted the increase in disease-induced muscle proteolysis and improved phosphorylation of Akt and the forkhead transcription factor FoxO1. Muscle overloading, but not treadmill running, corrected protein synthesis and levels of mediators of protein synthesis such as phosphorylated mTOR and p70S6K in the muscles of mice with CKD. In these mice, muscle overload, but not treadmill, running, increased muscle progenitor cell number and activity as measured by the amounts of MyoD, myogenin, and eMyHC mRNAs. Muscle overload not only increased plantaris weight and reduced muscle proteolysis but also corrected intracellular signals regulating protein and progenitor cell function in mice with CKD. Treadmill running corrects muscle proteolysis but not protein synthesis or progenitor cell function. Our results provide a basis for evaluating different types of exercise on muscle atrophy in patients with chronic kidney disease. [ABSTRACT FROM AUTHOR]

Proper targeting of the aquaporin-2 (AQP2) water channel to the collecting duct apical plasma membrane is critical for the urine concentrating mechanism and body water homeostasis. However, the trafficking mechanisms that recruit AQP2 to the plasma membrane are still unclear. Snapin is emerging as an important mediator in the initial interaction of trafficked proteins with target soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (tSNARE) proteins, and this interaction is functionally important for AQP2 regulation. We show that in AQP2-Madin-Darby canine kidney cells subjected to adenoviral-mediated expression of both snapin and syntaxins, the association of AQP2 with both syntaxin-3 and syntaxin-4 is highly enhanced by the presence of snapin. In pull-down studies, snapin detected AQP2, syntaxin-3, syntaxin-4, and SNAP23 from the inner medullary collecting duct. AQP2 transport activity, as probed by AQP2's urea permeability, was greatly enhanced in oocytes that were coinjected with cRNAs of SNARE components (snapin + syntaxin-3 + SNAP23) over those injected with AQP2 cRNA alone. It was not enhanced when syntaxin-3 was replaced by syntaxin-4 (snapin+syntaxin4+SNAP23). On the other hand, the latter combination significantly enhanced the transport activity of the related AQP3 water channel while the presence of syntaxin-3 did not. This AQP-syntaxin interaction agrees with the polarity of these proteins' expression in the inner medullary collecting duct epithelium. Thus our findings suggest a selectivity of interactions between different aquaporin and syntaxin isoforms, and thus in the regulation of AQP2 and AQP3 activities in the plasma membrane. Snapin plays an important role as a linker between the water channel and the t-SNARE complex, leading to the fusion event, and the pairing with specific t-SNAREs is essential for the specificity of membrane recognition and fusion. [ABSTRACT FROM AUTHOR]

Trans-epithelial tracer urea flux across Madin-Darby canine kidney (MDCK) cells permanently expressing the urea transporter UT-A1 is stimulated by agents that activate the cAMP signaling pathway, such as vasopressin or forskolin, thus mimicking the activation of urea permeability in the inner medullary collecting duct in the presence of vasopressin. Here, we report that UT-A1-mediated urea flux is also activated two-to-threefold over background by exposing the cells to media containing LiCl. This is in contrast to reports on cortical and medullary collecting duct tubules where acute and chronic exposure to lithium (Li) suppresses the osmotic water permeability, which is also regulated by cAMP levels. The Li concentration dependence of urea flux activation was linear up to 150 mM Li. Li activated only from the basolateral side where its effect was inhibited by amiloride, presumably because Li entered the cells through a basolateral Na-H exchanger. Li and IBMX, which also weakly activated urea flux, greatly augmented each others' stimulatory effect on urea flux. However, cellular cAMP levels did not rise commensurately with urea fluxes, and even though Li augments the activation by forskolin, it greatly inhibits the forskolin-induced formation of cAMP. These results suggest that the effect of Li in this MDCK model of renal cells does not involve cAMP or at least utilizes an additional signaling pathway independent of cAMP. [ABSTRACT FROM AUTHOR]

Production, recycling, and elimination of urea are important to maintain nitrogen balance. Adaptation to varying loads of urea due to different protein intake or in renal failure may involve changes in urea transport and may possibly affect urea transporters. In this study, we examined the expression of the UT-B urea transporter in rats fed a low-protein diet (LPD), a high-protein diet (HPD), and a 20% urea-supplemented diet. In the kidney, UT-B protein abundance increased in the outer medulla of both LPD-fed rats and 20% urea-fed rats, without changes in the inner medulla of either group compared with controls. In HPD-fed rats, UT-B protein decreased significantly in both the outer and inner medulla. We identified expression of UT-B in the rat colon, as a 2-kb mRNA transcript and as an ∼45-kDa protein, with apical localization in superficial colon epithelial cells. UT-B also is expressed in rat small intestine. In rat colon, UT-B protein abundance was mildly, but significantly, decreased in LPD-fed and 20% urea-fed rats. UT-B abundance also was examined in the colon of 7/8 nephrectomized, uremic rats and in HPD-fed rats and was not significantly different from that in control rats. These findings indicate that UT-B expression is regulated in response to different loads of urea, with a pattern that suggests involvement of tissue-specific regulatory mechanism in kidney and colon. [ABSTRACT FROM AUTHOR]

In rats with streptozotocin-induced diabetes mellitus for 10-20 days, we showed that the abundance of the major medullary transport proteins involved in the urinary concentrating mechanism, urea transporter (UT-A1), aquaporin-2 (AQP2), and the Na+-K+-2Cl- cotransporter (NKCC2/ BSCl), is increased, despite the ongoing osmotic diuresis. To test whether vasopressin is necessary for these diabetes mellitus-induced changes in UT-A1, AQP2, or NKCC2/BSC1, we studied Brattleboro rats because they lack vasopressin. Brattleboro rats were given vasopressin (2.4 µg/day via osmotic minipump) for 5 or 12 days. At 5 days, vasopressin increased AQP2 protein abundance but decreased UT-A1 abundance compared with untreated Brattleboro rats. At 12 days, vasopressin increased the abundance of both UT-A1 and AQP2 proteins but did not alter NKCC2/BSC1. Next, untreated Brattleboro rats were made diabetic for 10 days by injecting them with streptozotocin (40 mg/kg). Diabetes mellitus increased the abundance of AQP2 and NKCC2/BSC1 proteins, but UT-A1 protein abundance did not increase. Third, vasopressin-treated Brattleboro rats were made diabetic with streptozotocin for 10 days. In vasopressin-treated Brattleboro rats, diabetes mellitus increased UT-A1, AQP2, and NKCC2/BSC1 protein abundances. Vasopressin significantly increased UT-A1 phosphorylation in vasopressin-treated diabetic Brattleboro rats but not in the other groups of Brattleboro rats. We conclude that 1) administering vasopressin to Brattleboro rats for 12 days, but not for 5 days, increases UT-A1 protein abundance and 2) vasopressin is necessary for the increase in UT-A1 protein in diabetic rats but is not necessary for the increase in AQP2 or NKCC2 proteins. [ABSTRACT FROM AUTHOR]