Classically, a two-compartment model of sodium balance is assumed. However, new evidence has emerged that a three-compartment model is more appropriate to understand sodium handling in humans, which is characterized by interstitial hypertonic sodium accumulation in the lympho carpillary network of the skin. With modern MRI technology it is now able to measure the sodium content in human tissues (skin, muscles). This method has been validated and first applied in humans by Titze and coworkers.
In patients with resistant hypertension, skin and muscle sodium content is significantly increased in hypertensive patients (aged 50-65 years) as compared to controls. The highest values have been found in patients with severe forms of treatment resistant hypertension that we found to be eligible for renal denervation. We were interested in the changes of skin and muscle sodium content after renal denvervation, since a progressive decrease of total sodium in treatment resistant hypertension may explain the slow but progressive decrease of blood pressure observed after renal denervation. In a pilot phase including 13 subjects we measured skin and muscle sodium content before and 6 months after renal denervation. Patients were on average on 5 antihypertensive medication and their blood pressure dropped after renal denervation (from 150 ± 20 /. 83 ± 13 mmHg to 132 ± 15 / 74 ± 7 mmHg). In this pilot study with 13 male patients neither skin sodium nor muscle sodium content decreased significantly 6 months after renal denervation. Also, no changes in skin and muscle water content were observed. Thus, our hypothesis has to be refuted. Other mechanisms, such as resetting of the central baroreceptor/central sympathetic level of activity or long term vascular remodeling of resistance vessels may lead to the observed progressive and sustained blood pressure reduction after renal denervation.