New Biomarkers and Difficult-to-treat Hypertension

Background: Difficult-to-treat hypertension is characterized by uncontrolled blood pressure despite 2 or more antihypertensive drugs. In many cases, abnormal renal Na and volume handling plays a central role. Arterial vasoconstriction and renal function are regulated by the renin-angiotensin-aldosterone system (RAAS) and its effector hormones angiotensin II and aldosterone. Both control renal tubular function and Na excretion. Blocking the renin-angiotensin-system pharmacologically is therefore a current standard approach to treat hypertension. However, there is great clinical need to improve the classification of hypertensive patients and to predict patient sensitivity to different therapeutic strategies more precisely by new biomarkers that take into account tubular function and the various bioactive angiotensin fragments. Ang fragments generated by non-canonical enzymatic pathways such as Ang III co-exist with Ang I and Ang II in plasma. Their profile could help classify hypertensive patients with greater precision than plasma renin and aldosterone alone. Furthermore, urinary exosomes are small membrane vesicles (\<0.1 μm) shed into the urine by tubular epithelial cells. They contain tubular Na channels known as targets of furosemide and thiazide drugs used to treat volume overload and arterial hypertension. Na channel concentrations in the tubules are regulated by Ang II and aldosterone. Exosomal Na channel abundance could thus give valuable extra information on the actual tubular functional status not provided by standard laboratory tests of plasma renin and aldosterone or urinary electrolytes alone. Plasma Ang peptide profiles and urinary exosomal Na channels could improve the classification of patients with difficult-to-control hypertension and inform antihypertensive treatment decisions. The usual concentrations and variabilities of these biomarkers are a prerequisite for the planning of future validation studies. However, data are still lacking in this population. Study aim: This study aims to determine the concentration and interindividual variability of urinary exosomal sodium channels and of plasma angiotensins (candidate biomarkers) in patients with difficult-to-treat arterial hypertension and to determine their dependency on sampling conditions, dietary salt intake, and plasma renin and aldosterone concentrations. Candidate biomarker definition: * Plasma Ang peptides planned for determination in the study are Ang I and II and its metabolites Ang 2-10, 2-8, 3-8, 1-7, 2-7, 3-7, 1-5 (Ang peptide profile). * Urinary exosomal Na channel proteins planned for determination are Na+-Cl- cotransporter (NCC), epithelial sodium channel (ENaC) subunits and Na-K-Cl cotransporter type 2 (NKCC 2). Study type: This is an investigator initiated, monocentric observational pilot study in 24 ambulatory patients with difficult-to-treat hypertension who meet the inclusion criteria and none of the exclusion criteria. Setting: The study is performed at the outpatient hypertension clinic of the University clinic for nephrology, hypertension and clinical pharmacology, Inselspital, Bern University Hospital, Bern, Switzerland. Objectives: * The primary objective of the study is to determine the concentration and interindividual variability of urinary exosomal Na channels and of plasma angiotensin peptides under standardized clinical sampling conditions in patients with difficult-to-treat hypertension. * The secondary objectives of the study are 1. to determine the association of these candidate biomarkers with urinary Na excretion, plasma renin and aldosterone concentrations, and aldosterone-to-renin ratios on clinical visit 2, 2. to assess the repeatability of biomarker determinations under spontaneous (visit 1) compared to standardized sampling conditions (visit 2). Study plan and procedures: The concentrations of the candidate biomarkers are determined in blood and spot urinary samples obtained on the first clinical visit (visit 1) and again under standardized laboratory conditions on the second clinical visit (visit 2) scheduled 5-31 days later and after stopping RAAS inhibitory drugs, beta-adrenoceptor blockers, centrally acting antihypertensives and diuretics for an appropriate period, as necessary. Ca-antagonists and alpha-adrenoceptor blockers are allowed to treat hypertension. Determinations of candidate biomarkers are made in parallel with clinical blood and urine tests performed routinely. On visit 2, these tests include supine and standing plasma renin and aldosterone measurements under standardized conditions in the morning after 1 h rest and again after 1 hour walking . Biological samples are processed according to standardized laboratory protocols. Plasma Ang peptides are determined by gas chromatography-mass spectrometry. Urinary exosomal proteins are determined by Western-Blot using specific antibodies. Blood concentrations of antihypertensive drugs are determined by liquid chromatography mass spectrometry to analyze medication use and adherence on visits 1 and 2. Clinical data and routine laboratory results for visits 1 and 2 are obtained from patient files. Recruitment of participants: consecutive ongoing participant recruitment is performed in daily clinic practice by the investigators who check referrals for potential eligibility. Participation: study participation begins with study inclusion on visit 1 and ends with completion of visit 2.