Skin-specific mechanisms of body fluid regulation in hypertension

Water and Electrolyte Content in Hypertension in the Skin (WHYSKI) in Primary Aldosteronism

BACKGROUND

Primary aldosteronism (PA), the most common curable salt-dependent form of arterial hypertension, features renal K+ loss and enhanced Na+ reabsorption. We investigated whether the electrolyte, water, and TonEBP (tonicity-responsive enhancer binding protein)/NFAT5 (nuclear factor of activated T cells 5) content is altered in the skin of patients with PA and corrected by surgical cure.

METHODS

We obtained skin biopsies from 80 subjects: 49 consecutive patients with PA, optimally treated with a mineralocorticoid receptor antagonist; 6 essential hypertensives; and 25 normotensive controls. We measured Na+, K+, water content with atomic absorption spectroscopy after ashing, and NFAT5 mRNA with digital droplet polymerase chain reaction. The patients with PA were retested after adrenalectomy.

RESULTS

We discovered a higher dry weight of the skin biopsy specimen at surgery than at follow-up (P<0.001) and a direct correlation with electrolyte and water content (all P<0.01), indicating the need for dry weight adjustment of electrolyte and water data. Surgical cure of PA markedly increased skin dry weight-adjusted K+ (from 1.14±0.1 to 2.81±0.27 µg/mg; P<0.001) and water content (from 2.92±1.4 to 3.85±0.23 mg/mg; P<0.001), but left dry weight-adjusted skin Na+ content unaffected. In patients with PA at baseline, NFAT5 mRNA was higher (P=0.031) than in normotensive controls and decreased after surgery (P=0.035).

CONCLUSIONS

Despite mineralocorticoid receptor antagonist treatment ensuring normokalemia, the patients with PA had a skin cell K+ depletion that was corrected by adrenalectomy. The activated NFAT5/TonEBP pathway during mineralocorticoid receptor antagonist administration suggests enhanced skin Na+ lymphatic drainage and can explain the lack of overt skin Na+ accumulation in patients with PA. Its deactivation after surgical cure can account for the lack of skin Na+ decrease postadrenalectomy.

REGISTRATION

URL: https://www.clinicaltrials.gov; Unique identifier: NCT06090617.

Potential Role of the Skin in Hypertension Risk Through Water Conservation

Previous basic and clinical investigations have identified various pathogenic factors and determinants of risk that contribute to hypertension. Nevertheless, the pathogenesis of hypertension has not been fully elucidated. Moreover, despite the availability of antihypertensive medications for the management of blood pressure, treatments that address the full spectrum of the pathophysiological defects underpinning hypertension remain to be identified. To further investigate the mechanisms of primary hypertension, it is imperative to consider novel potential aspects, such as fluid management by the skin, in addition to the conventional risk factors. There is a close association between body fluid regulation and blood pressure, and the kidney, which, as the principal organ responsible for body fluid homeostasis, is the primary target for research in the field of hypertension. In addition, the skin functions as a biological barrier, potentially contributing to body fluid regulation. In this review, we propose the hypothesis that changes in skin water conservation are associated with hypertension risk based on recent findings. Further studies are required to clarify whether this novel hypothesis is limited to specific hypertension or applies to physiological blood pressure regulation.

Tangram of Sodium and Fluid Balance

Homeostasis of fluid and electrolytes is a tightly controlled physiological process. Failure of this process is a hallmark of hypertension, chronic kidney disease, heart failure, and other acute and chronic diseases. While the kidney remains the major player in the control of whole-body fluid and electrolyte homeostasis, recent discoveries point toward more peripheral mechanisms leading to sodium storage in tissues, such as skin and muscle, and a link between this sodium and a range of diseases, including the conditions above. In this review, we describe multiple facets of sodium and fluid balance from traditional concepts to novel discoveries. We examine the differences between acute disruption of sodium balance and the longer term adaptation in chronic disease, highlighting areas that cannot be explained by a kidney-centric model alone. The theoretical and methodological challenges of more recently proposed models are discussed. We acknowledge the different roles of extracellular and intracellular spaces and propose an integrated model that maintains fluid and electrolyte homeostasis and can be distilled into a few elemental players: the microvasculature, the interstitium, and tissue cells. Understanding their interplay will guide a more precise treatment of conditions characterized by sodium excess, for which primary aldosteronism is presented as a prototype.

Lymphatic vessels and the renin-angiotensin-system

The lymphatic system is an integral part of the circulatory system and plays an important role in the fluid homeostasis of the human body. Accumulating evidence has recently suggested the involvement of lymphatic dysfunction in the pathogenesis of cardio-reno-vascular (CRV) disease. However, how the sophisticated contractile machinery of lymphatic vessels is modulated and, possibly impaired in CRV disease, remains largely unknown. In particular, little attention has been paid to the effect of the renin-angiotensin-system (RAS) on lymphatics, despite the high concentration of RAS mediators that these tissue-draining vessels are exposed to and the established role of the RAS in the development of classic microvascular dysfunction and overt CRV disease. We herein review recent studies linking RAS to lymphatic function and/or plasticity and further highlight RAS-specific signaling pathways, previously shown to drive adverse arterial remodeling and CRV organ damage that have potential for direct modulation of the lymphatic system.

Don't sweat the small stuff: skin mechanisms of sodium homeostasis and associations with long-term blood pressure

Despite the overwhelming evidence that the kidney is the principal regulator of chronic blood pressure though the ability to sense pressure and adjust blood volume accordingly, recent clinical and preclinical evidence suggests that skin clearance of Na+ through sweat significantly contributes to long-term blood pressure and risk of hypertension. Evidence indicates that changes in skin Na+ content negatively associate with renal function, and factors that influence the concentration of Na+ in sweat are affected by major regulators of Na+ excretion by the kidney such as angiotensin and aldosterone. In addition, known regulatory mechanisms that regulate the amount of sweat produced do not include changes in Na+ intake or blood volume. Because of these reasons, it will be hard to quantify the contribution of Na+ clearance through sweat to blood pressure regulation and hypertension. While Chen et al. demonstrate significant negative associations between sweat Na+ concentration and blood pressure, it is likely that Na+ clearance through the skin has a short-term influence on blood pressure and sweat Na+ concentration is most likely a biomarker of renal function and its key role in hypertension.