Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, PC146
A urine concentrating defect in 11β-Hydroxysteroid Dehydrogenase Type 2 knockout mice
L. C. Evans1, J. J. Mullins1, M. A. Bailey1
1. Molecular Physiology, Centre for Cardiovascular Science, University of Edinburgh, Edinburgh, United Kingdom.
11β-Hydoxysteroid Dehydrogenase Type 2 (11βHSD2), which inactivates cortisol, is co-expressed with mineralocorticoid receptors (MR) in aldosterone target tissues where it protects the receptor from activation by glucocorticoids. Mutations in 11βHSD2 cause the syndrome of apparent mineralocorticoid excess in which hypertension is thought to be driven by volume expansion secondary to sodium retention. 11βHSD2-/- mice are also hypertensive but volume contracted. This uncoupling of sodium and water reabsorption suggests a urine concentrating defect. We therefore assessed water balance and the effect on urine concentration of a water deprivation challenge. Male C57/Bl and 11βHSD2-/- mice (n=5 per group) were individually housed in metabolic cages. After acclimatization, cumulative water balance was taken over 4 consecutive days, after which drinking water was removed for 24h. Following 1 week recovery, mice were injected with desmopressin (ddAVP; 1µg/kg;S.C.) and water removed for 24h. At the end of the experiment mice were culled by decapitation. Data are presented as mean±S.E.M. Statistical analysis was performed using ANOVA, Mann-Whitney or t-tests as appropriate. Water turnover during the baseline period was higher in the 11βHSD2-/- mice than in controls, with the knockouts being both polyuric (12.2±0.6 vs 2.1±0.1 ml/24hr P<0.001) and polydipsic (15.9±0.6 vs 5.2±0.2 ml/24hr P<0.001). Despite this cumulative water balance was not significantly different between the 2 groups. Urine osmolarity (Uosm) was significantly lower in the 11βHSD2-/- than controls (446±17 vs 1658±69 mOsm P<0.001). Urine flow rate fell in both groups following water deprivation, but 11βHSD2-/- mice continued to produce significantly more urine (1.7±0.1 vs 0.8±0.2 ml/24hr P<0.01). Water deprivation caused an increase in Uosm in both groups, however the increase was significantly blunted in the 11βHSD2-/- mice (1892±109 vs 4888±509 P<0.001). In both groups of mice, ddAVP had no further effect on urine concentration following water deprivation. This suggested the maximal response to endogenous AVP had been achieved: notably the response was significantly lower in 11βHSD2-/- mice (2246±243 vs 4318±202 mOsm P<0.001). Weight loss during water deprivation, used as an index of body water, was significantly greater in 11βHSD2-/- than the controls (3.0±0.1 vs 1.8±0.2 g P<0.001). In conclusion we have identified a urine concentrating defect in 11βHSD2-/- mice, which may contribute to the contraction of plasma volume. Although 11βHSD2-/- mice retain a residual concentrating capacity, the maximum value is blunted relative to controls. We cannot exclude a defect in AVP release but the failure of ddAVP to restore urine concentrating ability suggests a nephrogenic component to the diabetes insipidus.
Where applicable, experiments conform with Society ethical requirements