A switch in the mechanism of hypertension in the syndrome of apparent mineralocorticoid excess

Abstract

The syndrome of apparent mineralocorticoid excess arises from nonfunctional mutations in 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2), an enzyme that inactivates cortisol and confers aldosterone specificity on the mineralocorticoid receptor. Loss of 11betaHSD2 permits glucocorticoids to activate the mineralocorticoid receptor, and the hypertension in the syndrome is presumed to arise from volume expansion secondary to renal sodium retention. An 11betaHSD2 null mouse was generated on an inbred C57BL/6J genetic background, allowing survival to adulthood. 11betaHSD2(-/-) mice had BP approximately 20 mmHg higher on average compared with wild-type mice but were volume contracted, not volume expanded as expected. Initially, impaired sodium excretion associated with increased activity of the epithelial sodium channel was observed. By 80 days of age, however, channel activity was abolished and 11betaHSD2(-/-) mice lost salt. Despite the natriuresis, hypertension remained but was not attributable to intrinsic vascular dysfunction. Instead, urinary catecholamine levels in 11betaHSD2(-/-) mice were double those in wild-type mice, and alpha1-adrenergic receptor blockade rescued the hypertensive phenotype, suggesting that vasoconstriction contributes to the sustained hypertension in this model. In summary, it is proposed that renal sodium retention remains a key event in apparent mineralocorticoid excess but that the accompanying hypertension changes from a renal to a vascular etiology over time.

Figure 1.

(A) MABP in 11βHSD2^−/− mice (▪) and age-matched C57BL/6J mice (□). Mice were studied at 35 d (n = 6 controls, 7 11βHSD2^−/−), 60 d (7/7), 80 d (7/5), and 120 d (7/7) of age. (B) Fractional sodium excretion. (C) The absolute effect of amiloride on sodium excretion (Δ amiloride). (D) The renal expression of ENaC-α mRNA normalized to the expression of 18S RNA in 35- and 60-d-old 11βHSD2^−/− mice and aged-matched C57BL/6J mice. Data are means ± SEM. Comparisons were made using ANOVA with Bonferroni post hoc test. *P < 0.05; **P < 0.01. The expression of ENaC-α protein was below the detection threshold in kidneys from C57BL/6 mice (E), consistent with a previous report, but was localized to the apical membrane in the CCD of 60-d-old 11βHSD2^−/− mice (F).

Figure 2.

(A) Fractional sodium excretion in 80-d-old (7/5) and 120-d-old (7/7) 11βHSD2^−/− mice (▪) and age-matched C57BL/6J mice (□) (B) The absolute effect of amiloride on sodium excretion (Δ amiloride). (C) The renal expression of ENaC-α mRNA normalized to the expression of 18S RNA. (D) Expression of ENaC-α protein in the CCD of 120-d-old 11βHSD2^−/− mice, showing that immunoreactivity is predominately cytoplasmic. Western analysis shows the relative expression of SGK1 (E) and Nedd4 (F) in the kidney (n = 6 for each group) of 11βHSD2^−/− mice, as a ratio of that in kidneys of age-matched C57BL/6J mice. The dotted line shows the level of equal expression. Protein quantification was by densitometry. All data are means ± SEM. Comparisons were made using ANOVA with Bonferroni post hoc test or, for E and F, a one-sample t test assuming a sample mean of 1.0. *P < 0.05; **P < 0.01.

Figure 3.

Temporal progression of epithelial cytopathology in distal convoluted tubules (DCT) of 11βHSD2^−/− mice. (A) Control C57BL/6J mouse (60 d old). The cuboidal epithelium has apical nuclei and basal striations. (B) A 35-d-old 11βHSD2^−/− mouse. The epithelium is columnar, and the number of cross-sectional nuclei is increased, indicative of hypertrophy and hyperplasia, respectively. (C) A 60-d-old 11βHSD2^−/− mouse. In addition to epithelial hypertrophy and hyperplasia, there are multiple binucleate cells (arrows). (D) A 60-d-old 11βHSD2^−/− mouse. Apical cytoplasmic blebs project into the lumen (arrows). (E) A 120-d-old 11βHSD2^−/− mouse. Epithelial nuclei are irregular as a result of crowding and piling. A large hyperchromatic nucleus (*) suggests polyploidy. A cluster of epithelial cells projects into the lumen apparently with minimal contact with the basement membrane (arrow). (E, inset) A detached raft of epithelial cells lies in the tubule lumen. (F) A 120-d-old 11βHSD2^−/− mouse. In addition to striking hypertrophy and irregular nuclear placement within the cells, there is loss of basal striations and granular cytoplasmic aggregates indicative of subcellular disorganization. Hematoxylin and eosin stain; Bars = 10 μm.

Figure 4.

(A) Hematocrit in 11βHSD2^−/− mice (▪) and age-matched C57BL/6J mice (□). Mice were studied at 35 d (n = 6 controls, 7 11βHSD2^−/−), 60 d (7/7), 80 d (7/5), and 120 d (7/7) of age. (B) Blood volume in 80-d-old 11βHSD2^−/− mice (n = 7) and C57BL/6J mice (n = 6), normalized per gram of body weight. (C) Plasma sodium. (D) Plasma potassium. Data are means ± SEM, and comparisons were made using either t test or ANOVA with a Bonferroni post hoc test. *P < 0.05; **P < 0.01.

Figure 5.

Vascular contractility in aortic rings from 11βHSD2^−/− mice (▪) and age-matched C57BL/6J mice (○). Mice were studied at 35 (A), 60 (B), 80 (C), and 120 d of age (D) with six mice per genotype used at each time point. Concentration-response curves to acetylcholine are shown on the left and to noradrenalin on the right. Data are means ± SEM of the percentage relaxation/contraction to the response induced by 5-hydroxytryptamine/potassium chloride, respectively. Comparisons were made using either t test or ANOVA with a Bonferroni post hoc test. **P < 0.01.

Figure 6.

(A) Twenty-four-hour urinary excretion of adrenaline (left axis) and noradrenalin (right axis) in 120-d-old 11βHSD2^−/− (▪) mice and age-matched C57BL/6J mice (□), expressed as nanograms per gram of body weight (n = 10 in each group). (B) Effect of the α1-adrenoreceptor antagonist prazosin (100 μg/kg, intravenously) on MABP in 120-d-old 11βHSD2^−/− mice (▪; n = 5) and age-matched C57BL/6J mice (□; n = 6). Data are means ± SEM, and comparisons were made using either t test or ANOVA with a Bonferroni post hoc test. **P < 0.01. Sample BP recordings are shown for C57BL/6J mice (C) and 11βHSD2^−/− mice (D).