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Aquaporin-1 (AQP1) facilitates the osmotic transport of water across the capillary endothelium, among other cell types, and thereby has a substantial role in ultrafiltration during peritoneal dialysis. At present, pharmacologic agents that enhance AQP1-mediated water transport, which would be expected to increase the efficiency of peritoneal dialysis, are not available. Here, we describe AqF026, an aquaporin agonist that is a chemical derivative of the arylsulfonamide compound furosemide. In the Xenopus laevis oocyte system, extracellular AqF026 potentiated the channel activity of human AQP1 by >20% but had no effect on channel activity of AQP4. We found that the intracellular binding site for AQP1 involves loop D, a region associated with channel gating. In a mouse model of peritoneal dialysis, AqF026 enhanced the osmotic transport of water across the peritoneal membrane but did not affect the osmotic gradient, the transport of small solutes, or the localization and expression of AQP1 on the plasma membrane. Furthermore, AqF026 did not potentiate water transport in Aqp1-null mice, suggesting that indirect mechanisms involving other channels or transporters were unlikely. Last, in a mouse gastric antrum preparation, AqF026 did not affect the Na-K-Cl cotransporter NKCC1. In summary, AqF026 directly and specifically potentiates AQP1-mediated water transport, suggesting that it deserves additional investigation for applications such as peritoneal dialysis or clinical situations associated with defective water handling.

Mutations in PKD1 are associated with autosomal dominant polycystic kidney disease. Studies in mouse models suggest that the vasopressin (AVP) V2 receptor (V2R) pathway is involved in renal cyst progression, but potential changes before cystogenesis are unknown. This study used a noncystic mouse model to investigate the effect of Pkd1 haploinsufficiency on water handling and AVP signaling in the collecting duct (CD). In comparison with wild-type littermates, Pkd1(+/-) mice showed inappropriate antidiuresis with higher urine osmolality and lower plasma osmolality at baseline, despite similar renal function and water intake. The Pkd1(+/-) mice had a decreased aquaretic response to both a water load and a selective V2R antagonist, despite similar V2R distribution and affinity. They showed an inappropriate expression of AVP in brain, irrespective of the hypo-osmolality. The cAMP levels in kidney and urine were unchanged, as were the mRNA levels of aquaporin-2 (AQP2), V2R, and cAMP-dependent mediators in kidney. However, the (Ser256) phosphorylated AQP2 was upregulated in Pkd1(+/-) kidneys, with AQP2 recruitment to the apical plasma membrane of CD principal cells. The basal intracellular Ca(2+) concentration was significantly lower in isolated Pkd1(+/-) CD, with downregulated phosphorylated extracellular signal-regulated kinase 1/2 and decreased RhoA activity. Thus, in absence of cystic changes, reduced Pkd1 gene dosage is associated with a syndrome of inappropriate antidiuresis (positive water balance) reflecting decreased intracellular Ca(2+) concentration, decreased activity of RhoA, recruitment of AQP2 in the CD, and inappropriate expression of AVP in the brain. These data give new insights in the potential roles of polycystin-1 in the AVP and Ca(2+) signaling and the trafficking of AQP2 in the CD.

The water channel aquaporin-1 (AQP1) is the molecular counterpart of the ultrasmall pore responsible for transcellular water permeability during peritoneal dialysis (PD). This water permeability accounts for up to 50% of ultrafiltration (UF) during a hypertonic dwell, and its loss can be a major clinical problem for PD patients. By analogy with the lung, the hypothesis was tested that corticosteroids may increase AQP1 expression in the peritoneal membrane (PM) and improve water permeability and UF in rats. First, the expression and distribution of the glucocorticoid receptor (GR) in the PM and capillary endothelium was documented. Time-course and dose-response analyses showed that a daily IM injection of dexamethasone (1 or 4 mg/kg) for 5 d induced an approximately twofold increase in the expression of AQP1 at the mRNA and protein levels. The GR antagonist RU-486 completely inhibited the dexamethasone effect. The functional counterpart of the increased AQP1 expression was a significant increase in sodium sieving and net UF across the PM, contrasting with a lack of effect on the osmotic gradient and permeability for small solutes. The latter observation reflected the lack of effect of corticosteroids on nitric oxide synthase (NOS) activity and endothelial NOS isoform expression in the PM. In conclusion, corticosteroids induce AQP1 expression in the capillary endothelium of the PM, which is reflected by increased transcellular water permeability and UF. These data emphasize the critical role of AQP1 during PD and suggest that pharmacologic regulation of AQP1 may provide a target for manipulating water permeability across the PM.

Renal sodium retention is responsible for ascites and edema in nephrotic syndrome. In puromycin aminonucleoside (PAN)-induced nephrosis, sodium retention originates in part from the collecting duct, and it is associated with increased Na,K-ATPase activity in the cortical collecting duct (CCD). The aims of this study were to evaluate whether the outer medullary collecting duct (OMCD) also participates to sodium retention and to determine the mechanisms responsible for stimulation of Na,K-ATPase in CCD. PAN nephrosis increased Na,K-ATPase activity in the CCD but not in OMCD. The two-fold increase of Na,K-ATPase activity in CCD was associated with two-fold increases in the number of alpha and beta Na,K-ATPase subunits mRNA determined by quantitative RT-PCR and of the total amount of Na,K-ATPase alpha subunits estimated by Western blotting. PAN nephrosis also increased two-fold the amount of Na,K-ATPase alpha subunit at the basolateral membrane of CCD principal cells, as determined by Western blotting after biotinylation and streptavidin precipitation and by immunofluorescence. The intracellular pool of latent Na,K-ATPase units also increased in size and was no longer recruitable by vasopressin and cAMP. This unresponsiveness of the intracellular pool of Na,K-ATPase to vasopressin was not the result of any alteration of the molecular and functional expression of the vasopressin V(2) receptor/adenylyl cyclase (AC) complex. It is concluded that PAN nephrosis (1) does not alter sodium reabsorption in OMCD, (2) is associated with increased synthesis and membrane expression of Na,K-ATPase in the CCD, and (3) alters the normal trafficking of intracellular Na,K-ATPase units to the basolateral membrane.

The loss of ultrafiltration (UF) that accompanies acute peritonitis is a common problem in peritoneal dialysis (PD). It has been suggested that changes in nitric oxide (NO)-mediated vascular tone and permeability might be involved in the loss of UF, whereas channel-mediated water permeability should not be affected. This study used a model of acute peritonitis in rats to characterize changes in PD parameters, in correlation with: (1) expression studies of water channel aquaporin-1 and NO synthase (NOS) isoforms and (2) enzymatic assays for NOS in the peritoneum. Compared with controls, rats with peritonitis had a higher removal of plasma urea, a faster glucose absorption, and a loss of UF. Additional changes, including high protein loss, elevated leukocyte counts in dialysate, positive bacterial cultures, edema, and mononuclear infiltrates, were similar to those observed in PD patients with acute peritonitis. Acute peritonitis in rats induced a major increase in total NOS activity, which was inversely correlated with free-water permeability. The increased NOS activity was mediated by both inducible (Ca2+-independent) and endothelial (Ca2+-dependent) NOS isoforms and was reflected by increased peritoneal staining for nitrotyrosine. In contrast, aquaporin-1 expression was unchanged in rats with peritonitis. These findings cast light on the pathophysiology of permeability changes and loss of UF that characterize acute peritonitis. In particular, these data suggest that a local production of NO, mediated by different NOS isoforms, might play a key role in these changes.

Because in outer medullary collecting ducts (OMCD) of K(+)-depleted rats, K+ secretion is abolished, whereas Na+, K(+)-ATPase, which energizes this secretion, is markedly stimulated, it has been proposed that Na+, K(+)-ATPase was mislocated to the apical cell membrane and energized K+ reabsorption. This hypothesis has been supported by paradoxical effects of ouabain in K(+)-depleted compared with normal rats. However, we have recently shown that ouabain inhibits not only Na+, K(+)-ATPase but also apical H+, K(+)-ATPase in the OMCD of K(+)-depleted rats. Therefore, this study was designed to evaluate whether previous observations were accounted for by Na+, K(+)-ATPase or by ouabain-sensitive H+, K(+)-ATPase. Na+, K(+)-ATPase was distinguished from H+, K(+)-ATPase by its insensitivity to Sch-28080. Results indicate that the hydrolytic and transport activities of Na+, K(+)-ATPase, the number of its functional units, and the expression of mRNA of its alpha 1 and beta 1 subunits were increased threefold or more in the OMCD of rats fed a K(+)-depleted diet for 2 wk. By immunofluorescence, Na+, K(+)-ATPase staining was strongly increased in K(+)-depleted rats but remained localized to the basolateral pole of OMCD principal cells. In conclusion, K+ depletion is associated with marked induction of functional Na+, K+ pumps at the basolateral pole of rat OMCD. Therefore, reduced K+ secretion might result from inhibition of apical K+ conductances and stimulation of basolateral K+ recycling. It is proposed that increased Na+, K(+)-ATPase participates in the increased Na+ reabsorption prevailing in collecting ducts of K(+)-depleted rats.