The role of the vascular wall in the salt and blood pressure homeostasis

Rationale: Sodium (Na+) plays a key role in maintaining volume homeostasis and blood pressure (BP). The difference between Na+ intake and excretion, the Na+ balance, is regulated by the kidney. Regulation of the Na+ balance by the kidney is believed to be the main determinant of extracellular fluid volume (ECV). Recent studies have revealed that the Na+ balance is not only regulated by the kidney, but also in the interstitium of the skin. Here, binding of Na+ to glycosaminoglycans (GAGs) allows non-osmotic handling of Na+, thereby acting as a Na+ buffer. Based on these findings, we hypothesize that the endothelial surface layer (ESL), representing a complex sugar layer principally composed of negative-charged GAGs lining the endothelium, is an important determinant of volume homeostasis and BP by its ability to act as an immediate non-osmotic Na+ buffer. Furthermore, a perturbed ESL might lead to an increased ECV and BP response after a salt load. The volume of the ESL varies highly between individuals (0.5-2.3 L) and is know to be smaller in specific patient groups like diabetes type 1 and patients with chronic kidney disease. Due to its function in vascular physiology, including mechanotransduction, hemostasis, and blood cell-vessel wall interactions, the ESL is instrumental for vascular permeability, which might also be influenced by the Na+ buffering capacity of ESL.
The putative non-osmotic buffer capacity of the endothelial GAGs without commensurate water retention has only been limitedly studied yet, but seems particularly relevant in clinical conditions characterized by volume overload (e.g., heart failure, hypertension, chronic kidney disease). If the endothelial GAGs are involved in non-osmotic Na+ storage, treatment strategies directed to restoration of the ESL would lead to improved BP and ECV control and, conceivably, to better cardiovascular outcome. This study focuses on a novel function of the ESL, namely the capacity to store Na+ non-osmotically.



Objective: In this study we will identify the role of the endothelial GAGs in Na+ and volume homeostasis. Is there a link between the ESL and an individual its susceptibility to Na+-excess?



Study design: In this project, we plan to conduct an experimental interventional cross-over study to investigate the Na+ storing capacity of the endothelial GAGs. For this, different Na+ conditions and the effect on ESL, ECV and BP, will be studied in healthy subjects.



Study population: Patients are 12 healthy non-smoking male subjects with non-treated normal (office) blood pressure (<140/90 mmHg).



Main study parameters/endpoints: We will study primarily the effects of a salt load on the haemodynamics and ECV in healthy subjects with a presumed normal ESL. Primary endpoint will be the ECV as represented by body weight and BP. Furthermore, the golden standard for ECV measurement will be performed. Other study paramaters consist of indirect measurements of the ESL dynamics and function as assessed with intravital microscopy of the sublingual microvasculature and transcapillary escape rate (TER). We will study the kidney function as represented by the glomerular filtration rate (GFR), fractional Na+ excretion, albuminuria and proteinuria. Finally, skin biopsies will allow study of the role of interstitial GAGs and macrophage influx in response to a salt load.

Sodium (Na+) plays a key role in maintaining volume homeostasis and blood pressure (BP). The difference between Na+ intake and excretion, the Na+ balance, is regulated by the kidney. Regulation of the Na+ balance by the kidney is believed to be the main determinant of extracellular fluid volume (ECV). Recent studies have revealed that the Na+ balance is not only regulated by the kidney, but also in the interstitium of the skin. Here, binding of Na+ to glycosaminoglycans (GAGs) allows non-osmotic handling of Na+, thereby acting as a Na+ buffer. Based on these findings, we hypothesize that the endothelial surface layer (ESL), representing a complex sugar layer principally composed of negative-charged GAGs lining the endothelium, is an important determinant of volume homeostasis and BP by its ability to act as an immediate non-osmotic Na+ buffer. Furthermore, a perturbed ESL might lead to an increased ECV and BP response after a salt load. The volume of the ESL varies highly between individuals (0.5-2.3 L) and is know to be smaller in specific patient groups like diabetes type 1 and patients with chronic kidney disease. Due to its function in vascular physiology, including mechanotransduction, hemostasis, and blood cell-vessel wall interactions, the ESL is instrumental for vascular permeability, which might also be influenced by the Na+ buffering capacity of ESL.
The putative non-osmotic buffer capacity of the endothelial GAGs without commensurate water retention has only been limitedly studied yet, but seems particularly relevant in clinical conditions characterized by volume overload (e.g., heart failure, hypertension, chronic kidney disease). If the endothelial GAGs are involved in non-osmotic Na+ storage, treatment strategies directed to restoration of the ESL would lead to improved BP and ECV control and, conceivably, to better cardiovascular outcome. This study focuses on a novel function of the ESL, namely the capacity to store Na+ non-osmotically.

day 8 of low sodium diet and day 8 and 9 of high sodium diet.

- extracellualr volume with tracer ioxehol

- BP and haemodynamic parameters with semi-automatc devices (Omron), Nexfin and Sphygmocor

- assessment of endothelial surfasce layer with Sidestream Darkfield Imaging, transcapillary escape rate (with labeled albumin) and assessment of ESL shedding products in blood

- divers blood and urine samples (endocrinology, chemistry, hematology, ESL shedding products) on different time points

dietary intervention:

subjects will asked to adhere in random order for one week a low sodium diet (50 mmol Na/day) and for one week a high sodium diet (200 mmol Na/day)



At study visit concerning low sodium diet

- acute salt load with 500 ml NaCl 3%



At study visit concerning high sodium diet

- infusion with Lipopolysacharadie (LPS)