New actors in blood pressure regulation - The extracellular microenvironment, immune cells and lymphatics in skin

We are investigating a new mechanism for regulating blood pressure, namely that the microenvironment in the skin and lymphatic vessels that drain fluid from this space are important for controlling blood pressure. Central to this hypothesis is that the microenvironment in the tissue deviates from plasma. Using a new method, we have found that animals that are on a salty diet and thus have increased blood pressure have an increased osmolality in the tissue. The results suggest that a salt-rich diet may result in an increased influx of immune cells and an increased amount of lymph vessels. We have investigated using various techniques whether interstitial fluid in the skin is also hypertonic and found that this is not the case. We continue the work of studying whether immune cells have a homeostatic and blood pressure regulating role by regulating lymph drainage. We have found that the tissue is hyperosmotic after a salty diet, but that the interstitial fluid and lymph are isosmotic with plasma. This may indicate that the salt is 'inactivated' in the tissue. The work of studying whether there are gradients of salt in the skin that will be able to explain how the skin can store salt has not been completed. We have used optical imaging and positron emission tomography combined with computed tomography (PET-CT) and found that the changes due to high salt content leads to increased lymphatic drainage from the skin, and further by blocking macrophages using clodronate that the macrophages play a crucial role in lymph transport as has been suggested. In addition, we continue the work of looking at the effect of lymph vessels in the skin by using mice that, when changing genes, have hyperplastic or hypoplastic lymph vessels, and have found that this is not the case. This work is nearing completion and publication. Since it has recently been shown that salt is stored in muscle during salt-sensitive hypertension, and the lymphatic system may play a possible role in such accumulation, we have developed a new method based on PET-CT to measure lymph flow in muscle and shown that it actually is the case that salt storage provides increased lymph flow also in muscle. We have further worked on studying whether immune cells are activated by high salt content in the tissue and found that there is increased migration of cells from the skin to the lymph nodes. This suggests that the skin has an even stronger role in regulating the salt balance, and thus blood pressure, than previously thought. The experiments with PET-CT which indicated that muscle was involved in salt storage have led to experiments where we have seen that salt is also stored to a considerable extent in muscle, of which we are now investigating the functional consequences. These experiments led to studies of the role of the growth factor VEGF-B in salt storage, a factor expressed in muscle tissue. This work has been completed and a manuscript will soon be ready for submission. As part of this work, the question of kidney function arose, and we have worked on creating a method for measuring glomerular filtration with PET. With these studies, we hope to gain a better insight into the underlying functional and molecular mechanisms for the role of skin in regulating fluid balance and blood pressure.

Den potensielle nyttevirkningen for pasientbehandling og/eller sykdomsforebygging for dette forskningsprosjektet er økt kunnskap om mekanismene bak salt-sensitiv hypertensjon, som gjør at vi vil bli bedre på å informere pasientene om sykdomsforebygging og dermed gi evidensbasert pasientsentrerte helsetjenester i tråd med institusjonelle strategier. Resultater fra prosjektet knytter sammen NaCl-akkumulering, hypertensjon og lymfekar, og våre funn som viser at høy salt-diett fører til økt lymfetransport knytter lymfe-vekstfaktoren VEGF-C til hjerte-og karsykdommer. Denne faktoren kan derfor være en biomarkør for NaCl-akkumulering i vev. Siden det er mulig å måle VEGF-C i serum, kan denne kunnskapen oversettes direkte til klinikken. Vi har fått introdusert utstyr for Na-MRI i Bergen. Dette kan brukes til å måle salt i vevet hos pasienter, og dokumenterer rask overføring av grunnleggende forskningsresultater til klinisk applikasjon på en måte som kan påvirke behandlingen.

In this project we focus the skin microenvironment in hypertension, highlighting it to play a previously unknown extrarenal role in blood pressure regulation. We will explore the paradigm-shifting hypothesis that the skin interstitium forms a 'new' space that in addition to kidney and blood vessels participates in regulating the internal milieu of the body. This will be done by addressing central questions related to ion composition of the interstitial fluid and its surrogate lymph relative to plasma. Supporting our hypothesis are preliminary data showing that we can access the skin microenvironment through lymph isolation, that a high salt diet results in increased skin osmolality, enhanced lymph flow and migration of immune cells. Investigations of the relation between activation of macrophages and associated immune cells and local interstitial fluid composition is a new research direction, but as part of a high risk/high return proposal, the approach is feasible because of our unique proven excellence in analysis of interstitial fluid composition and lymphatic function. In our experiments we will use rats and mouse models engineered to have enhanced or limited capacity for lymph vessel formation that represent a novel tool in hypertension research. We will explore the mechanisms involved in macrophage-dependent Na+-storage in the skin, and the role of lymphatic vessels and tissue drainage by lymphatics in blood pressure regulation. State-of-the art methods will be used to assess skin interstitial fluid osmolality and novel molecular imaging and PET-CT methods to quantify lymph flow. We aim to show that interstitial fluid, immune cells and lymphangiogenesis are interconnected when involved in electrolyte homeostasis and blood pressure control. The project will not only provide new knowledge on basic mechanisms of hypertension and targets for blood pressure control, but also show a hitherto unknown role of lymphatics thereby initiating new research direction.