Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, C29
Chronic ENaC Blockade Rescues Na+ Induced High Blood Pressure in 11β-Hydroxysteroid Dehydrogenase Type 2 Heterozygote Mice
E. Craigie1, M. A. Bailey1, J. J. Mullins1
1. Molecular Physiology, University of Edinburgh, Edinburgh, United Kingdom.
11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) confers mineralocorticoid specificity to the mineralocorticoid receptor (MR) by inactivating glucocorticoids. Mutations in HSD11B2 cause the rare hypertensive disorder of Apparent Mineralocorticoid Excess (AME) and polymorphisms in the general population have been linked to Na+ sensitive hypertension (1). We have previously shown that a high Na+ diet increases blood pressure (BP) in hsd11b2 heterozygote (hsd11b2+/-) mice (2). Here we have investigated the role of the epithelial sodium channel (ENaC) in this response. Male hsd11b2+/- (n=6) & wild-type (hsd11b2+/+; n=6) mice were housed in metabolic cages and fed control diet (0.25% Na+) for 3 days before mini-pumps delivering the potent ENaC inhibitor benzamil (0.7µg/day/g) were implanted under isoflurane anaesthesia (2% with O2 by inhalation). After recovery, measurements were made for 2 days before high Na+ diet (2.5% Na+) was given: Na+ excretion was measured for a further 7 days, after which mice were moved to normal cages for a further 2 weeks. Mice were then anaesthetized (Inactin, 100mg/kg IP) for measurement of arterial BP. Daily sodium excretion was averaged over the baseline and cumulative sodium balance calculated for the first 7 days on 2.5% Na diet. Data are mean ± SE; statistical comparisons were made using either t-test or ANOVA, as appropriate. Baseline daily Na+ excretion was similar in hsd11b2+/- and hsd11b2+/+ mice on 0.25% Na diet (11.3±1.7 vs 12.6±2 µmol/24h/g). Benzamil administration caused a significant natriuresis in both groups (hsd11b2+/- = 16±0.7 and hsd11b2+/+ = 14.3±1.3 µmol/24h/g; P<0.01 vs baseline), confirming ENaC blockade. New analysis of our previous data shows the hsd11b2+/- mice in positive cumulative Na+ balance during the first week on a 2.5% Na+ diet (134.2±48 μmol; P<0.05); hsd11b2+/+ mice are in neutral balance. Benzamil treatment caused a negative sodium balance in both hsd11b2+/- and hsd11b2+/+ mice after 7 days on 2.5% Na+ diet (-122±10 μmol vs -111±41 µmol). Benzamil also prevented the Na+-induced rise in BP in hsd11b2+/- mice, which now had a similar BP to hsd11b2+/+ animals at the end of the high sodium regimen (83.5±5 vs 83.7±4 mmHg). In summary, dietary Na+ loading induces a BP rise in mice with reduced 11β-HSD2 activity due to impaired renal sodium excretion. Chronic ENaC blockade prevents sodium accumulation in repeated experiments. These data suggest ENaC may be up-regulated in hsd11b2+/- mice due to inappropriate MR activation by glucocorticoids. The resulting unregulated increase in Na+ reabsorption could be contributing towards the increased BP observed in hsd11b2+/- mice on a high Na+ diet.
Where applicable, experiments conform with Society ethical requirements