2004 Global Researcher Conference Proceeding
April 09 - 11, 2004
| Conference: | 2004 Global Researcher Conference |
|---|---|
| Title: | Mineralocorticoid receptor mediated ENaC and AQP2 regulation in rats with lithium-induced NDI |
| Authors: | Nielsen, Jakob; Kwon, Tae-Hwan; Praetorius, Jeppe; Frokiaer, Jorgen; Knepper, Mark; Nielsen, Soren |
| Institutions: | University of Aarhus, The Water and Salt Research Center, University of Aarhus, National Institutes of Health |
Nephrogenic diabetes insipidus (NDI) and sodium wasting are common side effects of chronic lithium treatment. The polyuria is caused by downregulated AQP2 expression and the sodium wasting is likely associated with a marked downregulation of the vasopressin regulated b- and g- subunits of the epithelial sodium channel ENaC in the CCD and OMCD. Moreover there is evidence of impaired effects of aldosterone. We recently reported unchanged protein expression of the aENaC subunit (and decreased NCC) despite markedly increased plasma aldosterone concentration. In contrast, aldosterone-mediated ENaC trafficking was conserved in the connecting tubule. Thus, lithium appears to have selective effects on ENaC subunit expression and trafficking in a segment specific manner. To further investigate the role of aldosterone on renal function and regulation of ENaC and AQP2 in this model we chronically (7 days) infused aldosterone subcutaneously or administered the mineralocorticoid receptor antagonist spironolactone orally to rats with manifest lithium-induced NDI. Aldosterone infusion in rats with lithium-induced NDI increased urine production further to 226±15ml/24h compared to 94±9ml/24h in lithium-treated control rats. Contrarily, spironolactone decreased urine production to 63±10ml/24h. The plasma lithium concentration was decreased with aldosterone treatment (0.31±0.03mM) compared to lithium-treated control rats (0.54±0.04mM) whereas it was unchanged with spironolactone (0.84±0.18mM, P=0.07). Semi-quantitative immunoblotting showed that aldosterone and spironolactone respectively increased and decreased protein expression of aENaC, the 70kDa from of gENaC and NCC – all classical effects of aldosterone. Moreover, AQP2 expression was marginally increased with spironolactone in the cortex and ISOM and tended to be downregulated with aldosterone. Immunohistochemical analysis showed that aldosterone infusion cause redistribution of ENaC subunit labeling in the CNT to the apical cell domain from being dispersed in the cytoplasm in lithium-treated control rats. aENaC labeling intensity in CNT appear increased with aldosterone treatment, but this was not observed in the CCD and OMCD. There was only scant labeling of b- and gENaC subunits in the cortical and outer medullary collecting duct in all lithium-treated groups. Interestingly, aldosterone appeared to increase AQP2 labeling in the basolateral cell domain in the connecting tubule and early cortical collecting duct whereas spironolactone appeared to increase labeling in the apical cell domain. In conclusion, 1) aldosterone and spironolactone markedly alter urine production and renal handling of sodium and lithium in rats with lithium-induced NDI, 2) lithium-induced NDI is associated with inhibition of aldosterone regulation of aENaC and ENaC trafficking in CCD and OMCD but not in the CNT, 3) aldosterone affects expression and subcellular distribution of AQP2 in CNT and CCD. Thus, it appears that aldosterone significantly interact with vasopressin regulation of renal water excretion and expression of AQP2 and ENaC in rats with lithium-induced NDI.
Chronic intake of lithium can result in NDI and a condition with increased sodium excreted in the urine called sodium wasting. Research indicates expression of NDI is associated with a reduction in the water channel protein AQP2. The sodium wasting is associated with a reduction of parts of the sodium channel protein, ENaC. The reduction in AQP2 and ENaC means that less water and sodium can flow into the principal cells of the kidney collecting duct (CD). Lithium also seems to interfere with the hormone, aldosterone, which is very important for the flow of sodium into the principal cells.
To understand the part aldosterone plays in regulating AQP2 and ENaC (and thus, how it effects the functioning of the kidney), Nielsen, et al., induced NDI in rats by giving them lithium. Then the researchers gave one group of rats with NDI aldosterone and another spironolactone, a synthetic steroid that binds to the cell receptors intended for mineralocorticoid. Spironalactone, thus, prevents mineralocorticoid from being able to perform its function.
The researchers found that aldosterone-treated NDI rats increased their urine production, and spironolactone-treated NDI rats decreased urine production. The amount of lithium in the aldosterone-treated rats decreased, but remained unchanged in the spironolatone treated rats. Aldosterone increased the number of alpha ENaC; spironolatone decreased it. Spironolatone slightly increased the numbers of AQP2; aldosterone decreased them. Aldosterone also caused AQP2 to move inside the principal cells in some parts of the kidney. These and other findings led the researchers to conclude that: 1) aldosterone and spironolactone markedly affect urine production and how the NDI rats’ kidneys handle water and sodium 2) lithium-induced NDI is associated with an interference in aldosterone’s ability to regulate the movement of ENaC in some parts of the kidney, but not others, 3) aldosterone reduces the numbers of AQP2s and how they are distributed in certain parts of the kidney. They conclude that aldosterone affects urine output and the ability to produce and move AQP2 and ENaC in rats with lithium induced NDI.