Mineralocorticoids and sodium in chronic kidney disease - regulation and cardiovascular implications

Chronic kidney disease is common and associated with an elevated cardiovascular risk, as well as the long-term risk of renal failure. At present, therapeutic approaches to managing chronic kidney disease (CKD) do not fully reverse these risks. This has led to study of the determinants of pathological outcomes in these patients, with the hope of further therapeutic interventions to reduce these risks. Mineralocorticoids, predominantly aldosterone, are produced by the adrenal cortex and have a vital role in maintaining sodium status and blood pressure. However, high levels of aldosterone in humans are known to produce an adverse phenotype of hypertension and a disproportionately elevated cardiovascular risk. Furthermore, in animal models of renal failure, elevated aldosterone levels stimulate renal damage, in the presence of a high sodium milieu. These laboratory findings have been translated to provide a basis for several short-term follow-up clinical trials looking at the impact of non-genomic non-natriuretic doses of mineralocorticoid receptor inhibition in patients with chronic kidney disease. These studies have shown a reduction in proteinuria, often independent of decline in blood pressure. However, there is a paucity of baseline physiological data relating to the normal regulation of mineralocorticoid synthesis and action in chronic kidney disease. The response of the adrenal cortex to renal failure is not understood. Is mineralocorticoid synthesis regulated in the usual way? Are the stimulators of mineralocorticoid production and release affected by uraemia? Is dietary sodium intake associated with steroid status and adverse outcomes in humans? The hypothesis of this thesis was that the renin-angiotensin-aldosterone system is inappropriately activated in patients with chronic kidney disease. Secondly, that high levels of mineralocorticoids are associated with adverse end-organ damage including proteinuria excretion, left ventricular hypertrophy, endothelial dysfunction, elevated pulse wave velocity and markers of renal fibrosis. Furthermore, that these deleterious effects are associated with sodium status and that an elevated dietary sodium intake is independently associated with increased renal and cardiovascular risk. In order to test these hypotheses, 70 patients with CKD and 30 patients with essential hypertension (EH) were recruited and underwent detailed clinical and biochemical phenotyping. This included 24 hour urinary steroid metabolite analysis, plasma renin and aldosterone measurement, cardiac magnetic resonance imaging, carotid-femoral pulse wave velocity and assessment of endothelial function. 20 It was shown that levels of the main mineralocorticoids (MC) (aldosterone and deoxycorticosterone) are not elevated in patients with CKD, as compared with patients with essential hypertension (EH). However, the determinants of levels of MC excretion differed between the two conditions. In CKD, excretion of MC metabolites was directly proportional to excretion of urinary sodium. A high urinary sodium (a marker of dietary sodium intake) was associated with a higher excretion of tetrahydroaldosterone (THALDO - the main aldosterone metabolite). In patients with EH, no relationship was seen between urinary steroid excretion and urinary sodium excretion. This is a novel relationship between the kidney and adrenal gland which questions the conventional wisdom that the adrenal cortex is unaffected by uraemia and prompts further study into the regulation of steroid synthesis in CKD. Furthermore, it was shown for the first time that 24h excretion of tetrahydrodeoxycorticosterone (THDOC) is an independent predictor of left ventricular mass index and that THALDO is an independent predictor of proteinuria excretion – demonstrating a relationship between mineralocorticoids and two of the main predictors of mortality in CKD. An interaction between sodium, MCs and these two features was also demonstrated. No association between levels of mineralocorticoids and vascular function was seen. Urinary 24 hour excretion of sodium was significantly associated with endothelial dysfunction in patients with CKD and pulse wave velocity in patients with essential hypertension. Retrospective data analysis further confirmed an association between a high dietary sodium intake and adverse outcomes. In a study of 498 patients with CKD and a median follow-up of 7 years, an elevated 24h urinary sodium to creatinine ratio was shown to be associated with an increased risk of death. There was however no independent association with renal progression or requirement for renal replacement therapy. This is the first time that sodium intake has been clearly linked to adverse outcomes in patients with CKD. Lastly, laboratory work demonstrated that steroid stimulation (aldosterone or cortisol) of human proximal tubular cells resulted in increased collagen 1 gene expression, but only in the context of a high sodium environment. Collagen 1 is deposited in renal interstitial fibrosis. This effect was inhibited by MR blockade, further expanding on the potential role 21 of steroids in the progression of CKD and again confirming the relationship between salt and steroids. In conclusion, in this thesis it has been demonstrated that production of MCs in patients with CKD is closely associated with urinary sodium excretion (a surrogate for dietary sodium intake). This relationship is novel and not seen in patients with essential hypertension. It suggests that the response of the adrenal cortex in the context of uraemia is altered. Moreover, levels of mineralocorticoids are independently associated with left ventricular mass index and proteinuria excretion, both significant predictors of mortality, in patients with CKD. Dietary sodium intake has been shown to be an independent predictor of mortality and laboratory studies have demonstrated that mineralocorticoid receptor binding in a high sodium environment is associated with collagen 1 gene upreguation. These findings have important implications for the role of adequate renin-angiotensin-aldosterone blockade in patients with CKD and suggest that the addition of a mineralocorticoid receptor blocker and dietary sodium restriction should be advocated