Effect of primary polydipsia on aquaporin and sodium transporter abundance.
(65/317)
Chronic primary polydipsia (POLY) in humans is associated with impaired urinary concentrating ability. However, the molecular mechanisms responsible for this finding have not been elucidated. The purpose of this study was to examine the effect of chronic primary POLY on water metabolism and renal aquaporin (AQP) water channels and sodium and urea transporter abundance in rats. Primary POLY was induced in male Sprague-Dawley rats by daily administration of 15 g powdered rat chow mixed in 100 ml water for 10 days. Control rats (CTL) received 15 g powdered rat chow per day and ad libitum drinking water. Rats were studied following this period before further intervention and with a 36-h period of water deprivation to examine maximal urinary concentrating ability. At baseline, POLY rats demonstrated significantly greater water intake (100 +/- 1 vs. 22 +/- 2 ml/day, P < 0.0001) and urinary output (80 +/- 1 vs. 11 +/- 1 ml/day, P < 0.0001) and decreased urinary osmolality (159 +/- 13 vs. 1,365 +/- 188 mosmol/kgH2O, P < 0.001) compared with CTL rats. These findings were accompanied by decreased inner medulla AQP-2 protein abundance in POLY rats compared with CTL rats before water deprivation (76 +/- 2 vs. 100 +/- 7% CTL mean, P < 0.007). With water deprivation, maximal urinary osmolality was impaired in POLY vs. CTL rats (2,404 +/- 148 vs. 3,286 +/- 175 mosmol/kgH2O, P < 0.0005). This defect occurred despite higher plasma vasopressin concentrations and similar medullary osmolalities in POLY rats. In response to 36-h water deprivation, inner medulla AQP-2 protein abundance was decreased in POLY rats compared with CTL rats (65 +/- 5 vs. 100 +/- 5% CTL mean, P < 0.0006). No significant differences were noted in renal protein abundance of either AQP-3 or AQP-4 or sodium and urea transporters. We conclude that the impaired urinary concentrating ability associated with primary POLY in rats is due to impaired osmotic equilibration in the collecting duct that is mediated primarily by decreased AQP-2 protein abundance. (+info)
Oestrogen effects on urine concentrating response in young women.
(66/317)
Oestrogen lowers the plasma osmotic threshold for arginine vasopressin (AVP) release but without commensurate changes in renal concentrating response, suggesting oestrogen (OE2) may lower renal sensitivity to AVP. Ten women (23 +/- 1 years) received a gonadotropin releasing hormone analogue (GnRHa), leuprolide acetate, to suppress OE2 for 35 days, and then added OE2 (two patches each delivering 0.1 mg day-1) on days 32-35. On days 28 and 35 we tested blood and renal water and sodium (Na+) regulation during stepwise 60 min AVP infusions (10, 35, 100, 150 and 200 microu (kg body weight)-1 Pitressin). Plasma OE2 concentration increased from 19 +/- 4 to 152 +/- 3 pg ml(-1) and plasma progesterone concentration was unchanged (1.0 +/- 0.4 and 0.7 +/- 0.1 ng ml(-1)) for GnRHa and OE2 administration, respectively. Standard log plots of plasma AVP concentration ([AVP]P) vs. urine osmolality (OsmU) were fitted to a sigmoidal curve, and EC50 was determined by non-linear regression curve fitting of concentration-response data. OsmU rose exponentially during AVP infusions, but hormone treatments did not affect EC50 (3.3 +/- 0.07 and 3.1 +/- 0.6 pg ml(-1), for GnRHa and OE2, respectively). However, the urine osmolality increase was greater within the physiological range (approximately 2.5-3.4 pg ml(-1) [AVP]P) during OE2 treatment. Throughout most of the AVP infusion, the rate of clearance of AVP from plasma (PCRAVP) was increased during OE2 (45.5 ml (kg body weight)(-1) min(-1)) compared to GnRHa administration (33.1 ml (kg body weight)(-1) min(-1); mean for the 100-200 microu (kg body weight)(-1) infusion rates). The rate of renal free water clearance (CH2O) was similar between hormone treatments. Sodium excretion fell during OE2 administration due to greater distal tubular sodium reabsorption. Despite more rapid PCRAVP, renal concentrating response to graded AVP infusions was unaffected by oestrogen treatment suggesting oestrogen does not affect overall renal sensitivity to AVP. However, OE2 may increase renal fluid retention within a physiological range of AVP. (+info)
Expression of urea transporters in potassium-depleted mouse kidney.
(67/317)
Urea transport in the kidney is mediated by a family of transporter proteins that include the renal urea transporter (UT-A) and the erythrocyte urea transporter (UT-B). The purpose of this study was to determine the location of the urea transporter isoforms in the mouse kidney and to examine the effects of prolonged potassium depletion on the expression and distribution of these transporters by ultrastructural immunocytochemistry. C57BL6 mice were fed a low-potassium diet for 2 wk, and control animals received normal chow. After 2 wk on a low-potassium diet, urinary volume increased and urinary osmolality decreased (833 +/- 30 vs. 1,919 +/- 174 mosmol/kgH2O), as previously demonstrated. Kidneys were processed for immunocytochemistry with antibodies against UT-A1 (L446), UT-A1 and UT-A2 (L194), UT-A3 (Q2), and UT-B. In normal mice, UT-A1 and UT-A3 were expressed mainly in the cytoplasm of the terminal inner medullary collecting duct (IMCD). UT-A2 immunoreactivity was observed mainly on the basolateral membrane of the type 1 epithelium of the descending thin limb (DTL) of short-looped nephrons. The intensity of UT-A1 and UT-A3 immunoreactivity in the IMCD was markedly reduced in potassium-depleted mice. In contrast, there was a significant increase in UT-A2 immunoreactivity in the DTL. The intensity of UT-B immunoreactivity in the descending vasa recta (DVR) was reduced in potassium-depleted animals compared with controls. In control animals, UT-B immunoreactivity was predominantly observed in the plasma membrane, whereas in potassium-depleted mice, it was mainly observed in cytoplasmic granules in endothelial cells of the DVR. In summary, potassium depletion is associated with reduced expression of UT-A1, UT-A3, and UT-B but increased expression of UT-A2. We conclude that reduced expression of urea transporters may play a role in the impaired urine-concentrating ability associated with potassium deprivation. (+info)
Lack of UT-B in vasa recta and red blood cells prevents urea-induced improvement of urinary concentrating ability.
(68/317)
Recycling of urea within the renal medulla is known to play an important role in the capacity of the kidney to concentrate urine. This recycling occurs simultaneously through a tubular and a vascular route (i.e., through the loops of Henle and vasa recta, respectively). In the present study, transgenic mice with a selective deficiency in UT-B (the urea transporter protein expressed in descending vasa recta and red blood cells), were used to evaluate the specific contribution of vascular urea recycling to overall urine-concentrating ability (UCA). The renal handling of urea was studied in normal conditions and after acute or chronic alterations in urea excretion (acute urea loading or variations in protein intake, respectively). In normal conditions, UT-B null mice exhibited a 44% elevation in plasma urea (Purea), a normal creatinine clearance, but a 25% decrease in urea clearance, with no change in that of sodium and potassium. Acute urea loading induced a progressive increase in urinary urea concentration (Uurea) in wild-type mice and a subsequent improvement in their UCA in contrast to UT-B null mice, in which urinary osmolality and Uurea did not rise, due to the failure to accumulate urea in the medulla. With increasing protein intake (from 10 to 40% protein in diet, leading to a 5-fold increase in urea excretion), Purea was further increased in null mice while little change was observed in wild-type mice, and null mice were not able to increase Uurea as did wild-type mice. In conclusion, this study in UT-B-deficient mice reveals that countercurrent exchange of urea in renal medullary vessels and red blood cells accounts for a major part of the kidney's concentrating ability and for the adaptation of renal urea handling during a high-protein intake. (+info)
Three-dimensional functional reconstruction of inner medullary thin limbs of Henle's loop.
(69/317)
Digital three-dimensional (3-D) functional reconstructions of inner medullary nephrons were performed. Antibodies against aquaporins (AQP)-1 and -2 and the chloride channel ClC-K1 identified descending thin limbs (DTLs), collecting ducts (CDs), and ascending thin limbs (ATLs), respectively, through indirect immunofluorescence. Tubules were labeled in transverse sections and assembled into 3-D arrays, permitting individual tubule or combined surface representations to depths of 3.3 mm to be viewed in an interactive digital model. Surface representations of 75 tubules positioned near the central region of the inner medulla were reconstructed. In most DTL segments that form loops below 1 mm from the inner medullary base, AQP1 expression begins at the base, becomes intermittent for variable lengths, and continues nearly midway to the loop. The terminal DTL segment exhibiting undetectable AQP1 represents nearly 60% of the distance from the medullary base to the tip of the loop. AQP1 expression was entirely undetectable in shorter long-looped DTLs. ClC-K1 is expressed continuously along the terminal portion of all DTLs reconstructed here, beginning with a prebend region approximately 164 microm before the bend in all tubules and continuing through the entire ascent of the ATLs to the base of the inner medulla. CDs express AQP2 continuously and extensive branching patterns are illustrated. 3-D functional reconstruction of inner medullary nephrons is capable of showing axial distribution of membrane proteins in tubules of the inner medulla and can contribute to further development and refinement of models that attempt to elucidate the concentrating mechanism. (+info)
Reduced renal expression of AQP2, p-AQP2 and AQP3 in haemorrhagic shock-induced acute renal failure.
(70/317)
BACKGROUND: The aims of this study were to investigate the changes in the expression levels of renal aquaporins (AQPs) in response to haemorrhagic shock (HS) in rats and whether a change in the expression of AQPs was associated with parallel changes in urinary concentration. METHODS: HS was induced by withdrawal of blood through the femoral artery in rats. A mean arterial blood pressure (MAP) of 40 mmHg was maintained for 1 h before blood was reinfused, and rats were kept in metabolic cages for urine measurements. Two days after HS, we examined the abundance of AQPs in kidney by semiquantitative immunoblotting. RESULTS: HS rats (n = 13) developed acute renal insufficiency (creatinine clearance was 5.5 +/- 0.4 vs 6.9 +/- 0.3 ml/min/kg in sham-operated rats, n = 13, P < 0.05) and decreased urine osmolality (888 +/- 88 vs 1799 +/- 110 mosmol/kg H(2)O, P < 0.05). Consistent with this, semiquantitative immunoblotting revealed that the abundance of AQP2, phosphorylated (Ser256) AQP2 (p-AQP2) and AQP3 in whole kidney was significantly decreased after 2 days to 33 +/- 4, 41 +/- 9 and 35 +/- 14% of sham levels, respectively (P < 0.05). Also, the abundance of AQP2, p-AQP2 and AQP3 in inner medulla was markedly decreased to 36 +/- 8, 39 +/- 10 and 34 +/- 16% of sham levels (P < 0.05). In contrast, the abundance of AQP1 was not significantly changed compared with sham levels. CONCLUSIONS: The expression of the collecting duct water channel AQP2, p-AQP2 and AQP3 was significantly downregulated after HS, which may play an important role in the impaired urinary concentrating ability in HS-induced acute renal failure. (+info)
Antidiuretic hormone resistance in the neonatal cortical collecting tubule is mediated in part by elevated phosphodiesterase activity.
(71/317)
Neonates cannot concentrate their urine to the same degree as adults. One of the key factors in concentrating the urine is the renal collecting duct osmotic water permeability (Pf) response to antidiuretic hormone (ADH). Neonatal cortical collecting ducts have a blunted Pf response to ADH compared with adult tubules (Pf: 119.0 +/- 12.5 vs. 260.1 +/- 29.5 microm/s, P < 0.05). We found that the phosphodiesterase activity in the neonatal collecting ducts was higher than that in the adult collecting ducts (3,970 +/- 510 vs. 2,440 +/- 220 cpm.microg tubular protein-1.20 min-1, P < 0.05). After pretreatment of in vitro microperfused tubules with the nonspecific phosphodiesterase inhibitor IBMX (10-6 M in the bath), the Pf response to ADH in neonatal collecting ducts was 271.4 +/- 51.7 microm/s, which was identical to that of the adult collecting duct [315.3 +/- 31.3 microm/s, P = not significant (NS)]. Rolipram, a specific type IV phosphodiesterase inhibitor, lowered the elevated phosphodiesterase activity in the neonatal tubules to that in the adult tubules (2,460 +/- 210 vs. 2,160 +/- 230 cpm.microg tubular protein-1.20 min-1, P = NS). Neonatal tubules pretreated with rolipram (10-5 M) in the bath also had a Pf response to ADH that was comparable to that of the adult tubules (258.2 +/- 17.0 vs. 271.4 +/- 32.6 microm/s, P = NS). Thus the elevated phosphodiesterase activity in the neonatal tubules appears to be due to an increase in type IV phosphodiesterase activity. Hence, one of the key factors in the decreased ability of neonates to concentrate their urine is overactivity of phosphodiesterase in the cortical collecting duct that blunts the neonatal collecting duct Pf response to ADH. (+info)
Role of vasopressin in diabetes mellitus-induced changes in medullary transport proteins involved in urine concentration in Brattleboro rats.
(72/317)
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/BSC1), 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 microg/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. (+info)