Acquired NDI Acquired NDI is the consequence of several conditions (Table 2) that are characterized by an increased water output and reduced urine os molality, despite elevated levels of AVP. In many of these conditions, the kidney is unable to handle water due to an impaired responsiveness to. As discussed below, a number of rat models with NDI have been evaluated, and common for all is a reduced expression of AQP 2 in the principal cells of the collecting ducts. However, as is discussed, the degree of AQP 2 down regulation as well as the intracellular localization of the protein differs significantly among the various conditions, suggesting that different mechanisms are responsible for AQP 2 in the various models. In addition to DI, a few other serious conditions are associated with reduced AQP 2 levels and urinary concentrating defects (see Table 2).
1. Lithium-induced NDI Lithium administration is a very common treatment of manic-depressive disease. It is estimated that 1 in 1, 000 of the population receive lithium, and roughly 20-30% of these develop serious side effects including (16, 39) primarily due to a -resistant urinary-concentrating defect, i. e. , NDI.
We examined the effect of oral lithium treatment of rats for 25 days. AQP 2 and AQP 3 levels were progressively reduced to ~5% of levels in control rats after 25 days of lithium treatment (129, 149). The down regulation of AQP 2 expression was paralleled by a progressive development of severe. With serum lithium levels in the therapeutic range, rats produced a daily urine output that matched their own weight (149).
In addition, quantitative microscopy of AQP 2 labeling in the I MCD principal cells showed that there was a reduction in AQP 2 in the apical plasma membrane, as well as in the plasma membrane and intracellular vesicles. Thus reduction of AQP 2 in both the apical and the plasma membrane may participate in the overall reduced water reabsorption (149). The reduced AQP 3 expression was also confirmed by (129). Thus down regulation of both AQP 2 and AQP 3 appears to play a significant role in the development of lithium-induced. The reduction in AQP 2 (and AQP 3) expression may be caused by a lithium-induced impairment in the production of c AMP in collecting duct principal cells (38, 39), indicating that inhibition of c AMP production may in part be responsible for the reduction in AQP 2 expression as well as the inhibition of targeting to the plasma membrane in response to lithium treatment. This is consistent with the presence of a c AMP-responsive element in the 5'-untranslated region of the AQP 2 gene (92, 156) and with the recent demonstration that mice with inherently low c AMP levels have low expression of AQP 2 (DI There was a very slow recovery in AQP 2 expression and restoration of urinary concentration after cessation of lithium treatment (149) consistent with clinical findings.
However, treatment of lithium-diuretic rats with high doses of the specific V 2-receptor agonist dDAVP was able to cause efficient delivery of AQP 2 to the apical plasma membrane (a greater fraction of total AQP 2 was found in the membrane than seen in control animals), but there was only a modest increase in AQP 2 expression relative to animals treated with lithium alone. On the contrary, thirsting of the rats for 2 days resulted in a much larger increase in AQP 2 protein levels, but little targeting to the apical plasma membrane (a lot of AQP 2 was found in intracellular domains, i. e. , intracellular vesicles). Consequently, this study showed that thirsting was a more potent stimulus for AQP 2 expression than dDAVP administration in the present model and provided evidence for the presence of a -independent regulation of AQP 2 expression levels. The existence of such a signal transduction pathway has recently gained support (58).
Similar to the slow recovery of urinary concentration inability seen in patients who have been on lithium treatment, lithium-treated rats also showed a slow recovery. The suppression of AQP 2 levels was parallel led by a persistent urinary concentrating defect after removal of lithium from the diet (149). 2. Electrolyte disturbances associated with NDI It is known that both and, clinically important electrolyte abnormalities, are associated with due to a -resistant urinary concentrating defect. However, recently, at least part of the underlying molecular defects involved in the development of this was described.
With the use of well-established rat models to study these abnormalities, it has recently been shown that both and are associated with a significant down regulation of AQP 2 expression (54, 150, 208). Treating rats on a potassium-deficient diet for 11 days resulted in down regulation of AQP 2 expression in both inner medulla and cortex (27 +/- 3 and 34 +/- 15% of control levels, respectively). Thus is associated with significantly less down regulation of AQP 2 compared with that seen in lithium-treated rats. In parallel with the less extensive AQP 2 down regulation, urine production increased moderately from 11 +/- 1 to 30 +/- 4 ml / day , i. e.
, much less increase in urine output compared with lithium-treated animals. The increase in urine output was associated with an impaired urine-concentrating ability. In response to 12-h water deprivation, urine os molality was significantly lower in rats compared with controls. Treating rats with a normal potassium containing diet for 7 days after an 11-day period on a potassium-deficient diet showed normal urinary concentrating capacity and normalization of AQP 2 levels. Taken together, the results support the view that there is a causative link between the inability to concentrate urine and the reduced AQP 2 levels. Another electrolyte disturbance, , is also frequently associated with a urinary concentrating defect and.
An experimental model of vitamin D-induced in rats has been used by two groups to investigate if of AQP 2 may also participate in this condition (54, 208). The molecular mechanisms resulting in resistance in conditions remain incompletely understood. Rats treated orally for 7 days with produced a significant with a 15 +/- 2% increase in plasma calcium concentration compared with controls. Hypercalcemia rats demonstrated a threefold increase in urine production, whereas urine os molality decreased from 2, 007 to 925 mos mol / kg H 2 O.
Consistent with this, and of membrane fractions revealed a 50% reduction in AQP 2 expression in kidney inner medulla from rats. Recently, the molecular defects associated with vitamin D-induced in rats were further elucidated. Using the same model, Wang et al. (249 a) demonstrated that in addition to down regulation of collecting duct AQP 2 expression, there was also a significant down regulation of the -sensitive Na+-K+-2 Cl co transporter BSC-1 in membranes from inner stripe of the outer medulla in response to. This defect in the thick ascending limb may participate inthe development of the urinary concentrating defect.
Both and are associated with down regulation of AQP 2 expression, and studies of AQP 2 demonstrated similar features although there were also differences between the two models. In kidneys from and animals, confirmed that there was a marked reduction in the expression and showed a change in the subcellular distribution of AQP 2. There was a significant reduction in the labeling of the apical plasma membrane domains of collecting duct principal cells similar to what was found in kidneys of lithium-treated animals, although much less extensive in (and) animals. This supports the view that down regulation of AQP 2 and reduced apical plasma membrane levels of AQP 2 are common features among many forms of acquired NDI and are likely to play a critical role in the development of these NDI conditions. However, it should be emphasized that other defects are also likely to participate in the conditions, e.
g. , down regulation of BSC-1 in. 3. NDI caused by urinary tract obstruction Urinary tract obstruction is a serious clinical condition associated with complex changes in renal function involving marked alterations in both and tubular function, and it may be associated with long-term impairment in the ability to concentrate urine (262). To examine whether altered AQP 2 expression was associated with urinary concentrating defect, AQP 2 expression levels were examined in a rat model in which both ureters were reversibly obstructed (77). After bilateral obstruction for 24 h, AQP 2 expression levels were markedly reduced, even before obstruction was released (Fig.
13). During this period of obstruction urine production is zero, and the result supports the view that diuresis per se is not the cause of decreased AQP 2 levels. This is consistent with the lack of down regulation of AQP 2 in response to short-term (24 h) or long-term (5 days) induced by treatment (148, 150). After release of the obstruction, there is a marked that persists for several days, and there is an increased solute-free water clearance, indicating an impaired ability to reabsorb water at the collecting duct level (Fig. 12). Although urine output in this particular study was down to normal 7 days after release of obstruction, the animals still had an impaired urinary concentrating defect in response to 24 h of thirsting.
These results are consistent with the measurements of AQP 2, which were reduced to ~20% of control levels 2 days after release of obstruction, before increasing to ~50% 7 days after release of obstruction. Thus the persisting urinary concentrating defect is likely to be related to the continued reduction in AQP 2 levels (Fig. 13). Therefore, it seems very likely that the reduction in total AQP 2 available combined with a reduced osmotic gradient driving the water reabsorption is responsible for the concentrating defect seen in these animals. In addition to the reduced AQP 2 expression, AQP 3 and AQP 1 expression levels have also been shown to be down regulated in response to bilateral ureter al obstruction (BUO). Expression of AQP 2 and AQP 3 tends to normalize within 30 days after release of BUO, whereas the urinary concentrating capacity is still reduced, although the concentrating defect is marginal at this stage.
Fig. 12. AQP 2 levels (A) and urine output (B) during 24 h of bilateral ureter al obstruction (BUO) and 24 h, 48 h, and 7 days after release (REL) of BUO compared with time-matched sham-operated controls. Twenty-four hours of BUO resulted in a dramatic decrease in AQP 2 expression. Low levels of AQP 2 persisted 24 and 48 h after release of obstruction and coincided with a marked increase in urine production. Seven days after release of bilateral ureter al obstruction, AQP 2 expression had partially normalized compared with sham-operated control rats.
At this time, urine production had normalized. P < 0. 05. Fig.
13. Effect of - stimulating hormone (-MSH) treatment of AQP 2 levels and changes in urine output and urine os molality in rats with acute renal failure (ARF) (2 days after 40-min bilateral ischemia). A: is reacted with anti-AQP 2. B: analysis of all samples from ARF (ARF 40/2), either in the absence of -MSH treatment (ARF) or with -MSH treatment (ARF+MSH), and sham-operated rats. In absence of -MSH treatment, rats with ARF have markedly decreased AQP 2 expression levels (13 +/- 3% of sham levels, P < 0. 05).
AQP 2 expression is 7-fold higher in response to -MSH of ARF rats compared with untreated rats. C and D: time courses of the changes in urine output (C) and urine os molality (D). Urine output is significantly increased, and urine os molality is significantly decreased after 40-min bilateral renal ischemia in ARF rats (both -MSH treated and non treated). ARF rats treated with -MSH showed a reduced urine output and a higher urine os molality compared with untreated ARF rats. P < 0. 05, ARF compared with sham-operated rats.
P < 0. 05, non treated ARF rats compared with -MSH-treated ARF rats. [From Kwon et al. (127). ] AQP 1 remains down regulated coinciding with a significant impairment in urinary concentrating capacity (J. Frokiaer, unpublished data), supporting the view that AQP 1 may play a major role in maintaining a normal urinary concentrating capacity.
In contrast to BUO, conditions with unilateral ureter al obstruction are not associated with changes in the absolute excretion of sodium and water, since the non obstructed kidney compensates for the reduced ability of the obstructed kidney to excrete solutes. To examine whether the previously identified reduction in AQP 2 expression in bilateral ureter al obstruction was caused by local factors (increased tissue pressure and changes in renal or biochemistry) or systemic changes in the animal, experiments were performed to investigate the effects of unilateral ureter al obstruction for 24 h. In this case, there was a profound down regulation of AQP 2 levels in the obstructed kidney (23% of control) and a moderate AQP 2 reduction (75% of control) in the non obstructed kidney. Consistent with this, urine production was increased by 150% from the non obstructed kidney.
Additional experiments revealed that changes in AQP 2 expression are reciprocal to the changes in free water clearance, demonstrating a functional association between these two parameters. The decrease in AQP 2 expression in the non obstructed kidney may participate in the increased urine output, by decreasing free water reabsorption, thus compensating for the loss of excretion from the obstructed kidney. These results support the view that local factors play an important role in the down regulation of AQP 2 expression during obstruction, but the signals leading to this decrease remain to be determined. However, the reduction in AQP 2 expression in the contra lateral, non obstructed, kidney may suggest a systemic effect, which may potentially involve decreased circulating or washout of metabolites from the obstructed kidney, or may be a consequence of reno renal nerve activity, known to play a role in the compensation for unilateral obstruction.