Early polyuria and urinary concentrating defect in potassium deprivation. (17/317)

The time course of the onset of nephrogenic diabetes insipidus and its relationship to aquaporin-2 (AQP2) expression in K(+) deprivation (KD) remains unknown. Rats were fed a K(+)-free diet and killed after 12 h, 1, 2, 3, 6, or 21 days. Serum K(+) concentration was decreased only after, but not before, 3 days of a K(+)-free diet. Urine osmolality, however, decreased as early as 12 h of KD (1,061 +/- 26 vs. 1,487 +/- 102 mosmol/kgH(2)O in control, P < 0.01). It decreased further at 24 h (to 858 +/- 162 mosmol/kgH(2)O in KD, P < 0.004) and remained low at 21 days of KD (436 +/- 58 mosmol/kgH(2)O, P < 0.0001 compared with baseline). Water intake decreased at 12 h (P < 0.002) but increased at 24 h (P < 0.05) and remained elevated at 21 days of KD. Urine volume increased at 24 h of KD (8 +/- 2 to 15 +/- 2 ml/24 h, P < 0.05) and remained elevated at 21 days. Immunoblot analysis demonstrated that AQP2 protein abundance in the outer medulla remained unchanged at 12 h (P > 0.05), decreased at 24 h ( approximately 44%, P < 0.001), and remained suppressed ( approximately 52%, P < 0.03) at 21 days of KD. In the inner medulla the AQP2 protein abundance remained unchanged at both 12 and 24 h of KD. AQP2 protein abundance in the cortex, however, decreased at 12 h ( approximately 47%, P < 0.01) and remained suppressed at 24 h ( approximately 77%, P < 0.001) of KD. Northern blot analysis showed that AQP2 mRNA decreased as early as 12 h of KD in both cortex (P < 0.02) and outer medulla (P < 0.01) and remained suppressed afterward. In conclusion, the urinary concentrating defect in KD is an early event and precedes the onset of hypokalemia. These studies further suggest that the very early urinary concentrating defect in KD (after 12 but before 24 h) results primarily from the suppression of cortical AQP2, whereas the later onset of a urinary concentrating defect (after 24 h) also involves a downregulation of medullary AQP2.  (+info)

Erythrocyte water permeability and renal function in double knockout mice lacking aquaporin-1 and aquaporin-3. (18/317)

Aquaporin (AQP) water channel AQP3 has been proposed to be the major glycerol and non-AQP1 water transporter in erythrocytes. AQP1 and AQP3 are also expressed in the kidney where their deletion in mice produces distinct forms of nephrogenic diabetes insipidus. Here AQP1/AQP3 double knockout mice were generated and analyzed to investigate the functional role of AQP3 in erythrocytes and kidneys. 53 double knockout mice were born out of 756 pups from breeding double heterozygous mice. The double knockout mice had reduced survival and impaired growth compared with the single knockout mice. Erythrocyte water permeability was 7-fold reduced by AQP1 deletion but not further reduced in AQP1/AQP3 null mice. AQP3 deletion did not affect erythrocyte glycerol permeability or its inhibition by phloretin. Daily urine output in AQP1/AQP3 double knockout mice (15 ml) was 9-fold greater than in wild-type mice, and urine osmolality (194 mosm) was 8.4-fold reduced. The mice remained polyuric after DDAVP administration or water deprivation. The renal medulla in most AQP1/AQP3 null mice by age 4 weeks was atrophic and fluid-filled due to the severe polyuria and hydronephrosis. Our data provide direct evidence that AQP3 is not functionally important in erythrocyte water or glycerol permeability. The renal function studies indicate independent roles of AQP1 and AQP3 in countercurrent exchange and collecting duct osmotic equilibration, respectively.  (+info)

Mathematical model of an avian urine concentrating mechanism. (19/317)

A mathematical model was used to investigate how concentrated urine is produced within the medullary cones of the quail kidney. Model simulations were consistent with a concentrating mechanism based on single-solute countercurrent multiplication and on NaCl cycling from ascending to descending limbs of loops of Henle. The model predicted a urine-to-plasma (U/P) osmolality ratio of approximately 2.26, a value consistent with maximum avian U/P osmolality ratios. Active NaCl transport from descending limb prebend thick segments contributed 70% of concentrating capability. NaCl entry and water extraction provided 80 and 20%, respectively, of the concentrating effect in descending limb flow. Parameter studies indicated that urine osmolality is sensitive to the rate of fluid entry into descending limbs and collecting ducts at the cone base. Parameter studies also indicated that the energetic cost of concentrating urine is sensitive to loop of Henle population as a function of medullary depth: as the fraction of loops reaching the cone tip increased above anatomic values, urine osmolality increased only marginally, and, ultimately, urine osmolality decreased.  (+info)

Physiological role of CLC-K1 chloride channel in the kidney. (20/317)

Although CLC chloride channels share a common structure, the sites of expression in the body and their intracellular localization are different among CLCs. CLC-K1 and -K2 are kidney-specific CLC chloride channels. We have clarified their localization in the plasma membranes of tubular cells by immunohistochemistry and proposed their roles in transepithelial chloride transport. Since there exit no good inhibitors for these channels, a gene knockout approach was the only way to establish their roles in kidney. While we were generating CLC-K knockout mice, Simon et al. reported that the mutations of CLC-K2 in human resulted in Bartter's syndrome. This had been anticipated since CLC-K2 is known to be present in the basolateral plasma membranes of the distal nephron where sodium-dependent chloride transporters are present in the apical membranes. Thus, CLC-K2 constitutes an important route for chloride reabsorption as an exit for chloride ions in the basolateral membrane. Another important finding in Simon's report was that no CLC-K (a human homologue of rat CLC-K1) mutation was found in patients with Bartter's syndrome. This suggested that CLC-K1 has a different role in kidney. We believed this to be true, based on the finding that the intrarenal localization of CLC-K1 and CLC-K2 are completely different. In the CLC-K1 knockout mice, we could clearly verify that (i) the high chloride permeability in the tAL was mediated by CLC-K1 and (ii) this chloride transport is necessary for urinary concentration. Further studies are necessary to elucidate the detailed mechanisms of the urinary-concentrating defect in Clcnk1 -/- mice. Exact clearance studies and measurements of osmolality and solute contents in the inner medulla will provide the answer to this question.  (+info)

Second generation knockout sickle mice: the effect of HbF. (21/317)

Sickle transgenic mice expressing exclusively human globins are desirable for studying pathophysiology and testing gene therapy strategies, but they must have significant pathology and show evidence of amelioration by antisickling hemoglobins. Mice were generated that expressed exclusively human sickle hemoglobin with 3 levels of HbF using their previously described sickle constructs (cointegrated human miniLCRalpha2 and miniLCRbeta(S) [PNAS 89:12150, 1992]), mouse alpha- and beta-globin-knockouts, and 3 different human gamma-transgenes. It was found that, at all 3 levels of HbF expression, these mice have balanced chain synthesis, nearly normal mean corpuscular hemoglobin, and, in some cases, F cells. Mice with the least adult HbF expression were the most severe. Progressive increase in HbF from less than 3% to 20% to 40% correlated with progressive increase in hematocrit (22% to 34% to 40%) and progressive decrease in reticulocyte count (from 60% to 30% to 13%). Urine concentrating ability was normalized at high HbF, and tissue damage detected by histopathology and organ weight were ameliorated by increased HbF. The gamma-transgene that produces intermediate levels of HbF was introduced into knockout sickle mice described by Paszty and coworkers that express the miniLCRalpha1(G)gamma(A)gammadeltabeta(S) transgene and have fetal but not adult expression of HbF. It was found that the level of HbF required to ameliorate low hematocrit and normalize urine concentrating defect was different for the miniLCRalpha2beta(S) and miniLCRalpha1(G)gamma(A)gammadeltabeta(S) mice. We conclude that knockout mice with the miniLCRalpha2beta(S) transgene and postnatal expression of HbF have sufficiently faithful sickle pathology to serve as a platform for testing antisickling interventions.  (+info)

Fasting downregulates renal water channel AQP2 and causes polyuria. (22/317)

Starvation causes impairment in the urinary concentrating ability. The mechanism of this defect, however, remains unknown. We tested the possibility that food deprivation might affect the expression and activity of aquaporins (AQP1, 2), thereby impairing renal water reabsorption in the kidney. Rats fasted for 24 h exhibited severe polyuria (urine volume increased from 11 before fasting to 29 ml/24 h after fasting, P < 0.0001) along with failure to concentrate their urine (urine osmolality decreased from 1,485 before fasting to 495 mosmol/kgH(2)O after fasting, P < 0.0001). Refeeding for 24 h returned the urinary concentrating ability back to normal. Northern hybridization and immunoblot analysis demonstrated that fasting was associated with a decrease in AQP2 protein (-80%, P +info)

Effects of renal lymphatic occlusion and venous constriction on renal function. (23/317)

The effects of renal lymphatic occlusion or increased lymph flow due to renal vein constriction on renal function were investigated in rats. In each experiment, the renal lymphatics or vein of the left kidney were occluded or constricted and the right kidney served as a control. Occlusion of renal lymphatics caused renal enlargement, no change in glomerular filtration rate, a marked increase in urine flow and solute excretion without any change in urine osmolality, and enhanced urinary loss of urea, potassium, sodium and ammonium. Urea concentrations in medullary and papillary tissues were significantly elevated. Renal vein constriction caused renal enlargement and a marked drop in glomerular filtration rate, urine volume, urine osmolality and solute excretion. tissue concentrations of urea and potassium were decreased in the medulla and papilla and total tissue solute was significantly decreased in the papilla. The data indicate that in the rat, renal lymphatic occlusion traps urea in the medulla and induces a urea diuresis resulting in a large flow of normally concentrated urine. On the other hand, increased lymph flow secondary to renal vein constriction decreases medullary urea and potassium concentrations and papillary osmolality. These changes and the reduced glomerular filtration rate result in a small flow if dilute urine. Thus both renal lymphatic occlusion and enhanced lymph flow have a significant effect on renal function.  (+info)

A micropuncture study of renal salt and water retention in chronic bile duct obstruction. (24/317)

The mechanism of sodium retention by the kidney in rats with ligation of the common bile duct was studied with micropuncture techniques. 10-14 days after bile duct ligation, rats showed positive sodium balance and ascites formation. Measurements of renal blood flow and glomerular filtration rate yielded values that were not different from those in normal control animals. Likewise, single nephron filtration rte of surface nephrons was the same in the experimental rats as in the controls. Sodium reabsorption, however, was markedly increased in the proximal convoluted tubule, as well as in segments beyond the proximal convolutions. Single nephron filtration fraction, calculated from measurements of efferent arteriolar and arterial hematocrits, was significantly elevated in the cortical nephrons, even though whole kidney filtration fraction was the same as in normal rats. The calculated protein concentration of cortical peritubular blood was higher in the bile duct-ligated rats than in the normal controls. The observations are consistent with the view that sodium retention is the result of enhanced reabsorption primarily by cortical nephrons. The enhanced reabsorption can be accounted for by relative cortical ischemia due to efferent arteriolar vasoconstriction with the consequent elevation of peritubular colloid oncotic pressure.  (+info)