Skin Sodium and Salt Sensitivity of Blood Pressure

The physiological basis of salt sensitivity of blood pressure (SSBP) is poorly understood, and determining which patients have SSBP is not straightforward. Furthermore, determining salt sensitivity requires direct intervention tracking changes in blood pressure after salt challenge or depletion over several days. This makes identifying salt-sensitive individuals impractical in a clinical setting, hindering its application. It is crucial that the investigators elucidate the underlying mechanisms of salt-sensitivity, and through this understanding develop a biomarker of SSBP for clinical use. From a review of recent studies it appears that in the short-term, accumulation of skin sodium during high salt intake attenuates the blood pressure response, while in the long-term, high skin sodium levels indicate a tendency for SSBP, hypertension and elevated cardiovascular risk. The reasons for this are not clear and merit further investigation. By refining methods for quantification of skin sodium and expanding its use in hypertension research, the clinicians can improve patient assessment, treatment prescription, and disease monitoring. Using skin biopsy and sodium MRI provides a unique opportunity to study skin sodium handling and SSBP during antihypertensive treatment, and can provide insights into why hypertensives and certain ethnic groups have a higher incidence of SSBP. Sodium MRI may also help increase our understanding of the mechanisms by which diuretics work, both systemically and in the kidney and provide a way to identify salt-sensitive individuals for targeted clinical intervention. Hypotheses: 1. Skin sodium decreases with salt-dependent (diuretic) treatments but not salt-independent (calcium channel blocker) treatments. 2. Diuretic-induced reductions in skin sodium correlate with reductions in blood pressure. 3. Skin sodium is higher in populations traditionally known to be more salt sensitive, such hypertensive patients of black ethnicity. Patients will be enrolled on to a randomised, open-label, two-treatment two-period crossover treatment. The hypertensive medication used in this study are Amlodipine 5 or 10mg and Chlortalidone 25mg. The duration for individual participants will be approximately 16 weeks. Participants will have a total of 7 visits including screening/enrolment (visit 1) and baseline visit (visit 2).