Molecular and functional evidence for electrogenic and electroneutral Na(+)-HCO(3)(-) cotransporters in murine duodenum

Inward Na(+)-HCO(3)(-) cotransport has previously been demonstrated in acidified duodenal epithelial cells, but the identity and localization of the mRNAs and proteins involved have not been determined. The molecular expression and localization of Na(+)-HCO(3)(-) cotransporters (NBCs) were studied by RT-PCR, sequence analysis, and immunohistochemistry. By fluorescence spectroscopy, the intracellular pH (pH(i)) was recorded in suspensions of isolated murine duodenal epithelial cells loaded with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. Proximal duodenal epithelial cells expressed mRNA encoding two electrogenic NBC1 isoforms and the electroneutral NBCn1. Both NBC1 and NBCn1 were localized to the basolateral membrane of proximal duodenal villus cells, whereas the crypt cells did not label with the anti-NBC antibodies. DIDS or removal of extracellular Cl(-) increased pH(i), whereas an acidification was observed on removal of Na(+) or both Na(+) and Cl(-). The effects of inhibitors and ionic dependence of acid/base transporters were consistent with both inward and outward Na(+)-HCO(3)(-) cotransport. Hence, we propose that NBCs are involved in both basolateral electroneutral HCO(3)(-) transport as well as basolateral electrogenic HCO(3)(-) transport in proximal duodenal villus cells.

[Aquaporins in cerebral volume regulation and edema formation]

Regulation of tissue water content and brain volume is of critical importance for the normal functioning of the central nervous system (CNS), which, surrounded by the rigid cranium, is highly sensitive to any increase in the intracranial pressure. Alterations in cerebral water homeostasis and distribution may lead to neuronal and glial swelling known as cytotoxic brain edema, due to accumulation of intracellular water. Although numerous investigations have been performed to elucidate the underlying molecular basis and pathophysiology of brain edema, little is known about the regulation of water transport across the blood-brain barrier and between extra- and intracellular compartments of the brain parenchyma. The discovery and characterization of the aquaporin (AQP) family of membrane water channels provided molecular insight into fundamental processes of water transport across plasma membranes. Two AQPs are expressed abundantly in the mammalian brain: AQP1 in the apical plasma membranes of the cells of choroid plexus in the ventricles, where it has been suggested to participate in the secretion of cerebrospinal fluid and AQP4 in plasma membranes of ependymal cells and astrocytes. The role of AQP4 in the formation of brain oedema was suggested by some recent studies. These findings offer new potentials in brain oedema treatment.

SNARE proteins are critical for regulated exocytosis of ECP from human eosinophils

The SNARE hypothesis, describing a protein assembly-disassembly pathway, was recently proposed for the sequential steps of synaptic vesicle docking, activation and fusion. To determine if SNARE proteins are involved in regulated exocytosis in eosinophils, the presence and functional role of SNAREs was examined in human blood eosinophils. Immunoblotting, subcellular fractionation, and immunocytochemistry documented that vesicle-associated membrane protein-2 (VAMP-2), a vesicle-SNARE, was expressed in human eosinophils. Syntaxin 4 and SNAP-25 were also detected. Sequencing of cloned RT-PCR products amplified from a domain conserved among VAMP isoforms revealed identity only to VAMP-2 but not to VAMP-1 or cellubrevin. Functional experiments revealed that tetanus toxin pretreatment, which cleaved VAMP-2 in eosinophils, significantly inhibited both IgE receptor- and phorbol ester-mediated exocytosis of eosinophil cationic protein (ECP) from streptolysin-O-permeabilized eosinophils. Thus, these results strongly suggest a critical role of SNAREs in regulated exocytosis in eosinophils.

Regulation of AQP6 mRNA and protein expression in rats in response to altered acid-base or water balance

In the rat, aquaporin-6 (AQP6) is mainly localized in intercalated cells (ICs) in collecting ducts, where it is exclusively associated with intracellular vesicles. In this study, we examined whether AQP6 protein and mRNA expression were regulated in the inner medulla or inner stripe of the outer medulla. Rats treated with dietary alkali or acid load for 7 days with a fixed daily water intake revealed appropriate changes in urine pH but unchanged urine output. AQP6 protein and mRNA abundance were increased in alkali-loaded rats (187 +/- 18 and 151 +/- 17% of control, respectively), whereas no changes were observed in acid-loaded rats. Immunohistochemistry revealed increased IC AQP6 labeling in alkali-loaded rats but not in acid-loaded rats. In contrast, administration of NH(4)Cl in the drinking water for 2 wk (free access to water) revealed a significant increase in AQP6 protein abundance (194 +/- 9% of control), but this was associated with increased water intake. Combined, this suggests that AQP6 expression was not affected by acid loading per se but rather was in response to changes in water intake. Consistent with this, water loading for 48 h was associated with increased AQP6 protein abundance, compared with thirsted rats. Moreover, rats with lithium-induced nephrogenic diabetes insipidus had a threefold increase in both AQP6 protein and mRNA expression. Overall, these results suggest that AQP6 expression in collecting duct ICs is regulated by altered acid/alkali load or water balance. Thus AQP6 may contribute to maintenance of acid-base homeostasis and water balance.

Generation and phenotype of mice harboring a nonsense mutation in the V2 vasopressin receptor gene

The V2 vasopressin receptor (V2R) plays a key role in the maintenance of a normal body water balance. To generate an in vivo model that allows the physiological and molecular analysis of the role of V2Rs in kidney function, we have created mouse lines that lack functional V2Rs by using targeted mutagenesis in mouse embryonic stem cells. Specifically, we introduced a nonsense mutation known to cause X-linked nephrogenic diabetes insipidus (XNDI) in humans (Glu242stop) into the mouse genome. V2R-deficient hemizygous male pups showed a decrease in basal urine osmolalities and were unable to concentrate their urine. These pups also exhibited an enlargement of renal pelvic space, failed to thrive, and died within the first week after birth due to hypernatremic dehydration. Interestingly, female mice heterozygous for the V2R mutation showed normal growth but displayed an XNDI-like phenotype, characterized by reduced urine concentrating ability of the kidney, polyuria, and polydipsia. Western blot analysis and immunoelectron microscopic studies showed that the loss of functional V2Rs had no significant effect on the basal expression levels of aquaporin-2 and the bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1). The V2R mutant mice described here should serve as highly useful tools for the development of novel therapeutic strategies for the treatment of XNDI.

The alpha(1G)-subunit of a voltage-dependent Ca(2+) channel is localized in rat distal nephron and collecting duct

The molecular type and localization of calcium channels along the nephron are not well understood. In the present study, we assessed the distribution of the recently identified alpha(1G)-subunit encoding a voltage-dependent calcium channel with T-type characteristics. Using a RNase protection assay, alpha(1G)-mRNA levels in kidney regions were determined as inner medulla >> outer medulla congruent with cortex. RT-PCR analysis of microdissected rat nephron segments revealed alpha(1G) expression in the distal convoluted tubule (DCT), in the connecting tubule and cortical collecting duct (CT+CCD), and inner medullary collecting duct (IMCD). alpha(1G) mRNA was expressed in the IMCD cell line mIMCD-3. Single- and double-labeling immunohistochemistry and confocal laser microscopy on semithin paraffin sections of rat kidneys by using an anti-alpha(1G) antibody demonstrated a distinct labeling at the apical plasma membrane domains of DCT cells, CT principal cells, and IMCD principal cells.

Renal water handling in rats with decompensated liver cirrhosis

The present study was performed to investigate the renal handling of water in rats with decompensated liver cirrhosis. Liver cirrhosis was induced by intraperitoneal administration of carbon tetrachloride twice weekly for 16 wk. Control rats were treated with vehicle. The cirrhotic rats developed severe disturbances in water homeostasis: urine production was decreased and hyperosmotic, the rats had significantly decreased plasma sodium concentration and ascites, and the ability to excrete an intravenous water load was significantly impaired. Plasma concentrations of vasopressin and aldosterone were increased. Mean arterial pressure, glomerular filtration rate (GFR), and fractional lithium excretion were decreased. Acute vasopressin type 2-receptor blockade with the selective nonpeptide antagonist OPC-31260 (800 microg. kg(-1). h(-1)) was performed during conditions whereby volume depletion was prevented by computer-driven, servo-controlled intravenous volume replacement with 150 mM glucose. The aquaretic response to OPC-31260 was similar in cirrhotic and control rats. However, the OPC 31260-induced rises in fractional water excretion (delta V/GFR; +24%) and fractional distal water excretion (delta V/C(Li); +46%) were significantly increased in the cirrhotic rats, where V is flow rate and delta is change. This suggests that vasopressin-mediated renal water reabsorption capacity was increased in the cirrhotic rats. Semiquantitative immunoblotting revealed that the expression of the vasopressin-regulated water channel aquaporin-2 was unchanged in membrane fractions of both whole kidney and inner medulla from cirrhotic rats. Together, these results suggest a relative escape from vasopressin on collecting duct water reabsorption in rats with decompensated liver cirrhosis.

Immunolocalization of electroneutral Na-HCO(3)(-) cotransporter in rat kidney

An electroneutral Na-HCO(3)(-) cotransporter (NBC(N)1) was recently cloned, and Northern blot analyses indicated its expression in rat kidney. In this study, we determined the cellular and subcellular localization of NBC(N)1 in the rat kidney at the light and electron microscopic level. A peptide-derived antibody was raised against the COOH-terminal amino acids of NBC(N)1. The affinity-purified antibody specifically recognized one band, approximately 180 kDa, in rat kidney membranes. Peptide-N-glycosidase F deglycosylation reduced the band to approximately 140 kDa. Immunoblotting of membrane fractions from different kidney regions demonstrated strong signals in the inner stripe of the outer medulla (ISOM), weaker signals in the outer stripe of the outer medulla and inner medulla, and no labeling in cortex. Immunocytochemistry demonstrated that NBC(N)1 immunolabeling was exclusively observed in the basolateral domains of thick ascending limb (TAL) cells in the outer medulla (strongest in ISOM) but not in the cortex. In addition, collecting duct intercalated cells in the ISOM and in the inner medulla also exhibited NBC(N)1 immunolabeling. Immunoelectron microscopy demonstrated that NBC(N)1 labeling was confined to the basolateral plasma membranes of TAL and collecting duct type A intercalated cells. Immunolabeling controls were negative. By using 2, 7-bis-carboxyethyl-5,6-caboxyfluorescein, intracellular pH transients were measured in kidney slices from ISOM and from mid-inner medulla. The results revealed DIDS-sensitive, Na- and HCO(3)(-)-dependent net acid extrusion only in the ISOM but not in mid-inner medulla, which is consistent with the immunolocalization of NBC(N)1. The localization of NBC(N)1 in medullary TAL cells and medullary collecting duct intercalated cells suggests that NBC(N)1 may be important for electroneutral basolateral HCO(3)(-) transport in these cells.

Localization of aquaporin-7 in rat and mouse kidney using RT-PCR, immunoblotting, and immunocytochemistry

To establish the segmental, cellular, and subcellular localization of AQP7 in rat and mouse kidney, we used RT-PCR, immunocytochemical, and immunoblotting approaches. RT-PCR of rat and mouse kidney zones revealed AQP7 mRNA in cortex and outer stripe of the outer medulla. RT-PCR on microdissected nephron segments revealed AQP7 mRNA in proximal convoluted and straight tubules. Immunoblotting using peptide-derived rabbit antibodies to either rat or mouse AQP7 revealed a 28-kDa band in kidney and testes from rat and mouse, respectively. Immunocytochemistry revealed strong AQP7 labeling of segment 3 proximal tubules and weaker labeling of proximal convoluted tubules in both rat and mouse kidneys. The labeling was almost exclusively confined to the brush border with no basolateral labeling. No labeling was observed of thin descending limbs or collecting duct. Immunolabeling controls were negative. The presence of AQP7 in the proximal tubule brush border indicates a role of AQP7 in proximal tubule water reabsorption.

Immunolocalization of AQP9 in liver, epididymis, testis, spleen, and brain

The aims of this study were to determine the cellular and subcellular localization of aquaporin-9 (AQP9) in different rat organs by immunoblotting, immunohistochemistry and immunoelectron microscopy. To analyze this, we used rabbit antibodies to rat AQP9 raised against three different AQP9 peptides (amino acids 267-287, 274-295, and 278-295). In Cos7 cells transfected with rat AQP9, the affinity-purified antibodies exhibited marked labeling, whereas nontransfected cells and cells transfected with aquaporin-8 (AQP8) exhibited no labeling, indicating the specificity of the AQP9 antibodies. Immunoblotting revealed a predominant band of 28 kDa in membranes of total rat liver, epididymis, testes, spleen, and brain. Preabsorption with the immunizing peptides eliminated the labeling. Immunohistochemistry showed strong anti-AQP9 labeling in liver hepatocytes. The labeling was strongest at the sinusoidal surface, and there was little intracellular labeling. Immunoelectron microscopy revealed that the labeling was associated with the plasma membrane of the hepatocytes. In testes Leydig cells exhibited anti-AQP9 labeling, and in epididymis, the stereocilia of the ciliated cells (principal cells) exhibited significant labeling, whereas there was no labeling of the nonciliated cells (basal cells). This was confirmed by immunoelectron microscopy. In spleen strong labeling of cells was observed of leukocytes in the red pulp, whereas there was no labeling of cells in the white pulp. In rat brain, AQP9 immunolabeling was confined to ependymal cells lining the ventricles and to the tanycytes of the mediobasal hypothalamus. Antibody preabsorbed with the immunizing peptide revealed no labeling. In conclusion, AQP9 proteins is strongly expressed in rat liver, testes, epididymis, spleen, and brain.

Immunolocalization of NBC3 and NHE3 in the rat epididymis: colocalization of NBC3 and the vacuolar H+-ATPase

In the male reproductive tract, the epididymis plays an important role in mediating transepithelial bicarbonate transport and luminal acidification. In the proximal vas deferens, a significant component of luminal acidification is Na+-independent, and mediated by specific cells that possess apical vacuolar proton pumps. In contrast, luminal acidification in the cauda epididymidis is an Na+-dependent process. The specific apical Na+-dependent H+/base transport process(es) responsible for luminal acidification have not been identified. A potential clue as to the identity of these apical Na+-dependent H+/base transporter(s) is provided by similarities between the transport properties of the epididymis and the mammalian nephron. Specifically, the H+/base transport properties of caput epididymidis resemble the mammalian renal proximal tubule, whereas the distal epididymis and vas deferens have characteristics in common with renal collecting duct intercalated cells. Given the known expression of the Na+/H+ antiporter, NHE3, in the proximal tubule, and of the electroneutral sodium bicarbonate cotransporter, NBC3, in renal intercalated cells, we determined the localization of NHE3 and NBC3 in various regions of rat epididymis. NBC3 was highly expressed on the apical membrane of apical (narrow) cells in caput epididymidis, and light (clear) cells in corpus and cauda epididymidis. The number of cells expressing apical NBC3 was highest in cauda epididymidis. The localization of NBC3 in the epididymis was identical to the vacuolar H+-ATPase. The results indicate that colocalization of NBC3 and the vacuolar H+-ATPase is not restricted to kidney intercalated cells. Moreover, the close association of the two transporters appears to be a more generalized phenomenon in cells that express high levels of vacuolar H+-ATPase. Unlike NBC3, NHE3 was most highly expressed on the apical membrane of all epithelial cells in caput epididymidis, with less expression in the corpus, and no expression in the cauda. These results suggest that apical NBC3 and NHE3 potentially play an important role in mediating luminal H+/base transport in epididymis.

The Children's Global Assessment Scale (CGAS) and Global Assessment of Psychosocial Disability (GAPD) in clinical practice--substance and reliability as judged by intraclass correlations

Studies on the inter-rater reliability on the Children's Global Assessment Scale (CGAS) and the Global Assessment of Psychosocial Disability (GAPD) involving different subgroups of 145 outpatients from 4 to 16 years of age showed fair to substantial intraclass correlations of 0.59 to 0.90. Raters of different training levels participated. Interrater reliability was dependent on number of ratings per rater, training, available data sources and experience. A more detailed description of anchor points resulted in higher inter-rater agreement by psychiatrists training in child and adolescent psychiatry, but did not influence the inter-rater reliability among more (widely) experienced raters. Both the CGAS and the GAPD seem to be sufficiently reliable tools in clinical practice. The CGAS seems to be more sensitive to inter-rater variation than the GAPD.

Body composition and resting energy expenditure in humans: role of fat, fat-free mass and extracellular fluid

OBJECTIVE

The objective of this study was to determine whether there are independent effects of extracellular fluid volume (ECF) and fat mass (FM) on resting energy expenditure (REE) relative to fat-free mass (FFM) in adult men and women.

METHODS

Multiple linear regression analysis was used to relate REE, as determined by indirect calorimetry, to FFM and FM (measured using dual energy X-ray absorptiometry) and ECF (measured using bromide space and/or the radiosulfate washout space) in 153 women and 100 men with varying amounts of body fat.

RESULTS

REE correlated significantly with FFM and FM in women (r=0.65 and r=0.63, both P<0.001) and men (r=0.62 and r=0.48, both P<0.001, FFM and FM, respectively). In a multiple linear regression analysis FFM, FM and age significantly contributed to the ability to predict REE in both genders. The models that were derived were not significantly different between women and men. In women the contribution to REE from FM was easier to detect when FM was greater. Adjustment of FFM for ECF did not improve the relationship between FFM and REE.

CONCLUSIONS

FFM, FM and age are significant, independent predictors of REE in both men and women. Adjustment of FFM for ECF does not improve the ability of FFM to predict REE, which suggests that ECF is a highly integrated component of FFM in healthy adults. Expressing REE relative to FFM alone will introduce errors when lean and obese populations are compared.

Occurrence and effects of octreotide antibodies during nasal, subcutaneous and slow release intramuscular treatment

OBJECTIVE

Previous studies have indicated that antibody formation against octreotide is extremely rare. We examined the occurrence of octreotide antibody formation after treatment with three administration forms in large populations of patients with acromegaly or carcinoid syndrome.

DESIGN

(i) Nasally administered octreotide: 70 previously untreated patients and 81 previously s.c. octreotide-treated patients participated. (ii) Subcutaneously administered octreotide: 172 acromegalic patients and 59 patients with carcinoid syndrome treated for up to 12 years participated. (iii) Intramuscularly administered depot octreotide (Sandostatin LAR): 62 acromegalic patients participated.

METHODS

Presence of antibodies is defined as increased precipitation by polyethylene glycol of (125)I-octreotide after incubation with serum; this was also used for screening of cross-reaction with somatostatin and lanreotide (Somatuline).

RESULTS

In patients who received nasal octreotide for at least 9 and up to 12 months (n=42), the occurrence of octreotide antibodies was 77% and 81% for previously untreated and treated patients respectively. In subcutaneously treated patients it was 63/231 (27%) after a mean exposure of 3 years. In patients treated for more than 5 years (n=53) it was 57% and after 8 years (n=18) 72%. In contrast, no patient could with certainty be identified to be antibody-positive after a mean of 2.5 years intramuscular Sandostatin LAR treatment (n=47). In all populations, the antibody-positive patients were as well controlled as the antibody-negative patients. Octreotide antibodies did not cross-react with native somatostatin (n=141), while about 25% of the antibody-positive sera did cross-react with the somatostatin analogue, lanreotide (Somatuline, Ipstyl, Angiopeptin).

CONCLUSIONS

Antibody formation against octreotide is much more frequent than previously believed. It depends primarily on drug exposure time and route of administration. It does not alter the GH/IGF-I status in treated acromegalic patients and induces only mild local reactions in some patients.

Altered expression of renal AQPs and Na(+) transporters in rats with lithium-induced NDI

Lithium (Li) treatment is often associated with nephrogenic diabetes insipidus (NDI). The changes in whole kidney expression of aquaporin-1 (AQP1), -2, and -3 as well as Na-K-ATPase, type 3 Na/H exchanger (NHE3), type 2 Na-Pi cotransporter (NaPi-2), type 1 bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1), and thiazide-sensitive Na-Cl cotransporter (TSC) were examined in rats treated with Li orally for 4 wk: protocol 1, high doses of Li (high Na(+) intake), and protocol 2, low doses of Li (identical food and normal Na(+) intake in Li-treated and control rats). Both protocols resulted in severe polyuria. Semiquantitative immunoblotting revealed that whole kidney abundance of AQP2 was dramatically reduced to 6% (protocol 1) and 27% (protocol 2) of control levels. In contrast, the abundance of AQP1 was not decreased. Immunoelectron microscopy confirmed the dramatic downregulation of AQP2 and AQP3, whereas AQP4 labeling was not reduced. Li-treated rats had a marked increase in urinary Na(+) excretion in both protocols. However, the expression of several major Na(+) transporters in the proximal tubule, loop of Henle, and distal convoluted tubule was unchanged in protocol 2, whereas in protocol 1 significantly increased NHE3 and BSC-1 expression or reduced NaPi-2 expression was associated with chronic Li treatment. In conclusion, severe downregulation of AQP2 and AQP3 appears to be important for the development of Li-induced polyuria. In contrast, the increased or unchanged expression of NHE3, BSC-1, Na-K-ATPase, and TSC indicates that these Na(+) transporters do not participate in the development of Li-induced polyuria.

Erythrocyte-methotrexate and disease activity in children treated with oral methotrexate for juvenile chronic arthritis

The concentration of methotrexate (MTX) in erythrocytes (E-MTX) was measured twice with three months interval in 21 children suffering from juvenile chronic arthritis (JCA). At the same time joint score, visual analogue scale (VAS), and laboratory parameters (CRP, WBC, PMNs, and ALAT) were obtained. There was only a weak insignificant correlation between the dose of MTX/m2 and E-MTX (r=0.24, p=0.11). No significant relations between the clinical or laboratory parameters and E-MTX was found. However, ALAT above normal range was associated with a lower dose of MTX (p=0.02) and lower VAS (p=0.02), indicating that toxicity may be associated with less articular discomfort. At present we consider routine determination of E-MTX in children with JCA of limited value.

Vasopressin V(2)-receptor-dependent regulation of AQP2 expression in Brattleboro rats

The role of AVP-V(2) receptor (AVP-V(2)R)-dependent regulation of aquaporin-2 (AQP2) expression was evaluated in vasopressin-deficient Brattleboro (BB) rats. AQP2 levels were relatively high in BB rats (52 +/- 8% of levels in Wistar rats), and treatment with the AVP-V(2)R antagonist SR-121463A (0.8 mg/day) for 48 h was associated with 1) increased urine output (170 +/- 9%), 2), reduced AQP2 protein levels (42 +/- 10% in whole kidney and 53 +/- 8% in inner medulla), and 3) reduced AQP2 mRNA levels (36 +/- 7%). In addition, the levels of AQP2 phosphorylated in the protein kinase A (PKA) consensus site (Ser(256) of AQP2) was reduced to 3 +/- 1% of control levels. Lithium (Li) treatment of BB rats for 1 mo, known to reduce adenylyl cyclase (AC) activity, downregulated AQP2 protein levels (15 +/- 6%) and increased urine output (220%). Downregulation of AQP2 expression in response to SR-121463A or Li treatment indicates that AQP2 expression in BB rats depends in part on activation of AVP-V(2)Rs and that the signaling cascade(s) involves AC and hence cAMP. Complete water restriction of BB rats produced only a small increase in AQP2 mRNA (235 +/- 33%) and AQP2 protein (156 +/- 22%) levels. Immunoelectron microscopy confirmed the increase in AQP2 abundance but revealed no change in AQP2 apical plasma membrane labeling in response to thirsting. In conclusion, the expression and phosphorylation of AQP2 in BB rats are in part dependent on AVP-V(2)R signaling, and AVP-V(2)-mediated regulation of AQP2 trafficking and expression is effectively decoupled in BB rats, indicating differences in AVP-V(2)R-mediated regulation of AQP2 trafficking and expression.

Effects of medium hypertonicity on water permeability in the mammalian rectum: ultrastructural and molecular correlates

Minute-by-minute net water fluxes (Jw) were measured across the isolated rectal epithelium in rats and rabbits. Five minutes after a serosal (but not mucosal) hypertonic challenge (plus 200 mosmol/l) a significant increase in the basal Jw was recorded in both species [deltaJw, microl min(-1) cm(-2): 0.40+/-0.06 (rats); 0.45+/-0.10 (rabbits)]. At the same time, most epithelial cells shrank markedly while the intercellular spaces were wide open (electron microscopy studies). In freeze-fracture studies multi-strand tight-junction structures (only slightly modified by serosal hypertonicity in rabbits) were observed in control conditions. No structural changes were observed after mucosal hypertonicity (both in rats and rabbits). Immunohistochemical studies showed the expression of aquaporin 3 (AQP3) at the basolateral membrane of epithelial cells in the rat. A first conclusion is that the epithelium of the mammalian rectum is a highly polarized, aquaporin-3-containing, water permeability structure. The Jw increase induced by serosal hypertonicity was sensitive to mercurial agents in both species and no changes in unidirectional [14C]mannitol fluxes (Ps) or transepithelial resistance (RT) were observed during this Jw increase. These observations suggest a transcellular route for the osmotically induced increase in water fluxes. In the rabbit rectum the initial Jw response, associated with serosal hypertonicity, was a transient one. It was followed by a second, slow and HgCl2-sensitive Jw increase (a transient peak in paracellular mannitol permeability was also observed). A second conclusion is that serosal hypertonicity induces an increase in transcellular water permeability in both rat and rabbit rectum.

Cellular and molecular biology of the aquaporin water channels

The high water permeability characteristic of mammalian red cell membranes is now known to be caused by the protein AQP1. This channel freely permits movement of water across the cell membrane, but it is not permeated by other small, uncharged molecules or charged solutes. AQP1 is a tetramer with each subunit containing an aqueous pore likened to an hourglass formed by obversely arranged tandem repeats. Cryoelectron microscopy of reconstituted AQP1 membrane crystals has revealed the three-dimensional structure at 3-6 A. AQP1 is distributed in apical and basolateral membranes of renal proximal tubules and descending thin limbs as well as capillary endothelia. Ten mammalian aquaporins have been identified in water-permeable tissues and fall into two groupings. Orthodox aquaporins are water-selective and include AQP2, a vasopressin-regulated water channel in renal collecting duct, in addition to AQP0, AQP4, and AQP5. Multifunctional aquaglyceroporins AQP3, AQP7, and AQP9 are permeated by water, glycerol, and some other solutes. Aquaporins are being defined in numerous other species including amphibia, insects, plants, and microbials. Members of the aquaporin family are implicated in numerous physiological processes as well as the pathophysiology of a wide range of clinical disorders.

Osmotically relevant membrane signaling complex: association between HB-EGF, beta(1)-integrin, and CD9 in mTAL

The integral membrane proteins cluster of differentiation-9 (CD9), beta(1)-integrin, and heparin-binding epidermal growth factor-like (HB-EGF) exist in association in many cell lines and are linked to intracellular signaling mechanisms. Two of the proteins (CD9 and beta(1)-integrin) are induced by hypertonicity, suggesting that their related signaling processes may be relevant to osmotic stress. The validity of this hypothesis rests upon coexpression and physical association between these molecules in nephron segments that are normally exposed to high and variable ambient osmolality. In this work, we show that CD9 and beta(1)-integrin are induced in rat kidney medulla after dehydration. Immunohistochemistry and immunoprecipitation studies show that CD9, HB-EGF, and beta(1)-integrin are coexpressed and physically associated in medullary thick ascending limbs (mTAL), nephron segments that are normally exposed to high and variable extracellular osmolality. Our findings are consistent with the existence of a cluster of integral membrane proteins in mTAL that may initiate or modulate osmotically relevant signaling pathways.

Dysregulation of renal aquaporins and Na-Cl cotransporter in CCl4-induced cirrhosis

BACKGROUND

Severe hepatic cirrhosis is associated with abnormal renal water retention.

METHODS

Semiquantitative immunoblotting was employed to investigate the abundance of the major renal aquaporins (water channels) and sodium-dependent cotransporters in kidneys from control rats and rats with cirrhosis secondary to chronic CCl4 inhalation.

RESULTS

The cirrhotic rats had ascites and manifested a water excretion defect detected by a standard water-loading test. The abundance of aquaporin-1 (the major aquaporin in the proximal tubule) was increased, an effect markedly accentuated in high-density membrane fractions prepared by differential centrifugation. Differential centrifugation studies demonstrated a redistribution of aquaporin-2 from high-density to low-density membranes, compatible with increased trafficking of aquaporin-2 to the plasma membrane. The abundance of aquaporin-3, but not aquaporin-2, was increased in collecting ducts of rats with CCl4-induced cirrhosis. The Na-K-2Cl cotransporter of the thick ascending limb showed no change in abundance. However, the abundance of the thiazide-sensitive Na-Cl cotransporter of the distal convoluted tubule was markedly suppressed in cirrhotic rats, possibly contributing to a defect in urinary dilution.

CONCLUSIONS

In this model of cirrhosis, the development of a defect in urinary dilution may be multifactorial, with contributions from at least four abnormalities in transporter regulation: (1) an increase in the renal abundance of aquaporin-1, (2) a cellular redistribution of aquaporin-2 in the collecting duct compatible with trafficking to the plasma membrane without an increase in total cellular aquaporin-2, (3) an increase in the renal abundance of aquaporin-3, and (4) a decrease in the abundance of the thiazide-sensitive cotransporter of the distal convoluted tubule.

Decreased abundance of major Na(+) transporters in kidneys of rats with ischemia-induced acute renal failure

Ischemia-induced acute renal failure (ARF) is known to be associated with significant impairment of tubular Na reabsorption. We examined whether temporary bilateral renal ischemia (30, 40, or 60 min) and reperfusion (1-5 days) affect the abundance of several renal Na transporters and urinary Na excretion (U(Na)V) in rats. In rats with mild ARF (30 min), immunoblotting revealed that proximal tubule type 3 Na(+)/H(+) exchanger (NHE-3) and type II Na-P(i) cotransporter (NaPi-II) were significantly decreased to 28 +/- 6 and 14 +/- 6% of sham levels, respectively, at day 1. Moreover, Na(+)-K(+)-ATPase levels were also significantly decreased (51 +/- 11%), whereas there was no significant decrease in type 1 bumetanide-sensitive cotransporter (BSC-1) and thiazide-sensitive cotransporter (TSC) levels. Consistent with reduced Na transporter abundance, fractional urinary Na excretion (FE(Na)) was significantly increased in mild ARF (30 min) and U(Na)V was unchanged, despite a marked reduction in glomerular filtration rate. Na transporter levels and renal Na handling were normalized within 5 days. Severe ischemic injury (60 min) resulted in a marked decrease in the abundance of Na(+)-K(+)-ATPase, NHE-3, NaPi-II, BSC-1, and TSC at both days 1 and 5. Consistent with this, FE(Na) was significantly increased at days 1 and 5. Intravenous K-melanocyte-stimulated hormone treatment partially prevented the ischemia-induced downregulation of renal Na transporters and reduced the high FE(Na) to control levels. We conclude that reduced levels of Na transporters along the nephron may play a critical role in the impairment of tubular Na reabsorption, and hence increased Na excretion, in ischemia-induced ARF.

Growth hormone and fluid retention

A major side effect of growth hormone (GH) administration is fluid retention. Most data indicate that adult GH-deficient patients are dehydrated, i.e. they have low total body water, low extracellular water and low plasma volume. When GH substitution is initiated in these patients their body fluid compartments are restored to normal. The fluid retaining capacity of GH should therefore be regarded as a desirable physiological normalization of fluid homeostasis rather than an unpleasant side effect.

Increased aquaporin-4 immunoreactivity in rat brain in response to systemic hyponatremia

The present study was undertaken to assess whether the protein and mRNA expression levels of the glial water channel aquaporin-4 (AQP4) undergo downregulation and whether there is a subcellular redistribution of AQP4 protein in rat brain in response to systemic hyponatremia and brain edema. Systemic hyponatremia was induced for 4 or 48 h by combined administration of hypotonic dextrose i.p. and 8-deamino-arginine vasopressin (dDAVP) s.c. Semiquantitative immunoblotting of membrane enriched fractions showed significantly increased immunoreactivity to 164 +/- 12% (n = 6) and 153 +/- 12% (n = 6) of control levels in brain after 4 or 48 h of systemic hyponatremia, respectively. Similarly, immunoblots of cerebellar samples revealed an increase in AQP4 immunoreactivity to 136 +/- 6% (n = 6) and 218 +/- 44% (n = 6) of control levels, after 4 or 48 h of hyponatremia. In contrast, AQP4 mRNA levels were unchanged after 4 h of severe hyponatremia (104 +/- 14% of control levels; n = 17), indicating that there are no changes in AQP4 expression in response to systemic hypoosmolarity. Immunocytochemistry and high-resolution immunogold electron microscopy revealed highly polarized labeling of AQP4 in astrocyte end-feet surrounding capillaries and forming the glia limitans. This pattern of labeling was not changed whereas an increased labeling intensity of AQP4 could be observed in response to hyponatremia. In conclusion, hyponatremia causes a pronounced and rapid increase in AQP4 immunoreactivity that is not accompanied by any increase in AQP4 mRNA expression. The increased AQP4 immunosignal may reflect secondary conformational modifications of AQP4 protein, leading to enhanced antibody binding. This post-translational modification of AQP4 may participate in the adaptation of cerebral tissue to systemic hyponatremia.