Each day the kidney filters 180 L of fluid and more than 500 g of sodium. The balance between how much salt and water is filtered and how much is excreted in the urine has a huge impact on your blood pressure.
The kidney regulates salt and water excretion and the kidney function is crucial in the regulation of blood pressure. In the western world high blood pressure (hypertension) is a major health problem which has fatal consequences such as renal failure, stroke and myocardial infarction. The mechanisms behind the development and maintenance of hypertension are largely unknown despite intense research.
We suggest that changes in kidney function are involved in the development of hypertension.
Our aim is to understand how the renal vasculature functions during normal conditions and during disease-states such as hypertension and diabetes. We investigate the mechanisms regulating the excretory function of the kidney. This includes control of renal blood flow (RBF), glomerular filtration rate (GFR) and the renin-angiotensin system. We focus on the structure of the vascular tree, intercellular communication via gap junctions, vascular regulation via ion channels, the sympathetic nervous system and changes in renal autoregulation.
Structure of the renal vasculature
The renal vascular tree displays a highly irregular branching pattern such that the pre-glomerular resistance may differ markedly between two adjacent glomeruli. In spite of this the filtration pressure is remarkably similar in different glomeruli.
Each organ has specific requirements for the vascular tree perfusing it. The kidney receives app. 1L of blood per min (1/5 of cardiac output) and filters 180L of plasma every day. This requires a high filtration pressure and a tight regulation of flow and pressure. In the kidney this control is driven in part by the ability of the vascular cells to communicate through gap junction. The structure of the renal vascular wall and the renal vascular tree are uniquely suited for this. The challenge in research is to generate a full 3D structure of the renal vascular tree in vivo. We will accomplish this using super-resolution imaging (SRI). Ultrasound scans of kidneys perfused with micro-bubbles will reveal the unique structure of the renal vascular tree in vivo. With this technique we also aim at estimating flow and pressure within the renal vasculature. Comparing kidneys from healthy and diseased (hypertensive, diabetic) animals will answer many questions regarding development of renal diseases.
A further advancement is the use of red blood cells as tracking target (substituting microbubbles) to convert super-resolution ultrasound into a non-invasive method. This high-gain high-risk project is funded by a Synergy Grant from the European Research Council (ERC).
The projects are a co-operation between us, researchers at DTU and Rigshospitalet.
Renal effects of GLP-1
The gut hormone GLP-1 also has renal effects. It increases renal blood flow, glomerular filtration rate and sodium excretion. We have shown that GLP-1 receptors are expressed in renal vascular smooth muscle cells but expression is significantly reduced in hypertensive animals. GLP-1 also attenuates renal autoregulation. This implicates GLP-1 as a major contributor in the control of renal hemodynamics.
The GLP-1 receptor is expressed in many extra-pancretic tissues including the renal vascular smooth muscle cells. Treatment of rats and mice using GLP-1 increases blood pressure but also increases renal blood flow (RBF) and glomerular filtration rate (GFR). This indicates a direct vasodilating effect of GLP-1 in the kidney primarily in the pre-glomerular arterioles. Furthermore, GLP-1 reduces the ability of the renal afferent arterioles to autoregulate in response to acute pressure changes.
GLP-1 in renal autoregulation
We have found a reduced ability of afferent arterioles to autoregulate in response to acute pressure changes. We have used knock-out mice lacking expression of the GLP-1 receptor to verify that the effect is directly linked to vascular effects of GLP-1. We investigate this further using whole animals to assess the effect of GLP-1 on renal autoregulation during acute pressure changes.
GLP-1 receptors in human kidneys
We have shown expression of GLP-1 receptors in renal vascular smooth muscle cells in kidneys from rats and mice. We are further exploring this using human renal tissue from healthy, diabetic and hypertensive patients to examine the location and possible changes in expression during diseases.
GLP-1 in renal vascular responses
GLP-1 induces renal vasodilation. Whether this vasodilation depends on e.g. closure of Ca2+ channels or opening of K+ channels or other mechanisms is unknown. In isolated renal vessels we are examining changes in intracellular Ca2+ and the effect of different ion channels inhibitors and activators to elucidate this.
Intercellular communication in the kidney
Gap junctions allow signals to travel between cells. In the renal vasculature gap junctions are found between vascular smooth muscle cells, between endothelial cells and between vascular smooth muscle cells - endothelial cells. Stimulation in one part of an arteriole leads to a response at a distant site. We have shown that this signal conduction participates in renal autoregulation. Also, the control of the renin-angiotensin system depends on functional gap junctions. This implicates gap junctions as a major contributor in the control of renal hemodynamics.
The myogenic response and the tubuloglomerular feedback (TGF) autoregulate renal blood flow and glomerular filtration rate. These two mechanisms cooperate to maintain an efficient afferent arteriolar tone. This depends on a functional communication between the vascular cells. Gap junctions between smooth muscle cells and endothelial cells allow signalling between cells. Gap junctions are built from different connexin (Cx) isoforms. In the renal vasculature Cx37, Cx40, Cx43 and Cx45 are expressed. The function and expression of Cx is changed during hypertension and diabetes.
Cx40 in the renal vasculature
Mice lacking expression of Cx40 (Cx40 knockouts) have increased blood pressure and renin secretion. Furthermore, the conduction of vascular dilatations and constrictions are significantly reduced. We have shown that this affects the function of TGF. In Cx40 knockout mice we further investigate the effect of changes in renal perfusion pressure on the afferent arteriolar diameter. Experiments are performed in absence or presence of drugs known to affect renal autoregulation. Also, we investigate the significance of Cx40 on conduction of vascular responses in afferent arterioles.
Cx45 in the renal vasculature
Mice lacking expression of Cx45 in the vasculature also have increased blood pressure and renin secretion. The conduction of Ca2+ waves between vascular smooth muscle cells in these mice is impaired and this greatly affects the conduction of vasoconstriction in the afferent arteriole. In mice lacking expression of Cx45 we investigate the effect of changes in renal perfusion pressure on the afferent and efferent diameter. We also investigate the effect of Cx45 on conduction of vascular responses in afferent arterioles.
In cooperation with Associate Professor Jens Christian Brings Jacobsen (link?) these experimental data is used to generate mathematical models of the renal autoregulation in order to elucidate the changes occurring in the vascular wall when intercellular communication is reduced
Ion channels in the renal vasculature
Ion channels in the renal resistance vessels affect the cell membrane potential. This leads to changed permeability of voltage sensitive Ca2+ channels and vascular tone. We have shown that in diet induced and genetically induced hypertension the expression and function of vascular K+ channels is changed. This implicates K+ channels as a major contributor in the control of renal hemodynamics.
K+ channels in renal vasculature
We have found that inhibition of single types of K+ channels has only minor effects on renal hemodynamics whereas inhibition of several types of K+ channels causes massive renal vasoconstriction. We suggest that inhibition of K+ channels mediate the action of vasoconstrictors e.g. angiotensin II. We investigate whether PKC mediated inhibition of K+ channels has a role in this mechanism.
K+ channels in hypertension
The function and expression of K+ channels in the renal vasculature is changed during hypertension. This may contribute to the increased renal vascular resistance. We have shown that vascular K+ channel expression is altered during diet-induce hypertension and this may contribute to the development of the hypertension. We investigate if function is also changes in rats with genetic hypertension and whether the renal circulation has a changed responsiveness to vasoactive compounds.
K+ channels and endothelial function
Endothelium-derived hyperpolarizing factor (EDHF) dilates vessels in the microcirculation. Specifically EDHF may compensate for the reduced levels of NO often seen in hypertension. EDHF is most likely not a single factor but varies between different vascular beds. In the kidney calcium-sensitive K+ channels and gap junctions both play a role in the EDHF response. We investigate the different pathways and the changes induced by pathophysiological states such as hypertension and diabetes.
In vivo methods
Renal function studies:
We measure renal blood flow, glomerular filtration, blood pressure, heart rate and urine excretion under different experimental conditions:
- Changes in blood pressure
- Infusion of pharmacological substances
a. Into the blood stream
b. Directly into the kidney
- Treatment over longer periods (chronic)
Genetically modified animals
We use different disease model e.g. hypertensive or diabetic rats to probe the differences in renal function. We use different genetically modified models e.g. knock-out mice lacking expression of different connexins, ion channels (K+, Ca2+ etc.), receptors (GLP-1, glucagon etc) to monitor changes in renal hemodynamics.
We measure blood pressure and ECG in awake animals using telemetry during different treatment regimes and in the different models mentioned above.
Super Resolution Imaging
We scan the kidneys of anesthetized rats using microbubbles and ultrasound (contrast enhanced ultra sound) to generate images of the renal vasculature and measurements of flow in different kidney regions in healthy and ischemic kidneys.
In vitro methods
Isolated arterioles mounted in wire-myographs or pressure myographs are used for Ca2+ measurements, vascular responses to agonists/antagonists, myogenic responses and flow-induced diameter changes. Vessels are obtained from healthy animals, different disease models or genetically modified animal models.
Isolated perfused rat or mouse kidneys are used to measure changes in the diameter of afferent or efferent arterioles. Both local and propagated vascular responses can be measured during:
- Changes in perfusion pressure
- Changes in tubular flow
- Pharmacological intervention
- Electrical stimulation
Jørgen Arendt Jensen
Erik Villain Thomsen
Michael Bachmann Nielsen
|Professor William Arendshorst
University of North Carolina
|Associate professor Lisa Harrison-Bernard
Louisiana State University
|Professor Donald Marsh,
University of Southern California
|Professor Gerald diBona
University of Iowa
|Associate professor Lars Jørn Jensen
Science, University of Copenhagen
|Associate Professor Brant Isakson
University of Virginia
2019: European Research Council (ERC) Synergy Grant. 10.000.000 Euro – Super resolution Ultrasound imaging of Erythrcytes (SURE) in the healthy and the diabetic kidney.
2018: Novo Nordisk Foundation. 8.000.000 DDK for a research infrastructure (link) examining the cardiovascular phenotypes in rodent models.
2017: Innovation Fund Denmark. 1.458.000 DDK for Super Resolution Imaging (SRI) of the kidney.
2017: Innovation Fund Denmark. 450.000 DDK for Translational models of diabetic nephropathy.
Over the years the lab has received other funding from (alphabetic order):
- A.P. Møller
- Aase & Ejnar Danielsens Foundation
- Helen and Ejnar Bjørnows Foundation
- The Danish Heart Foundation
- The Lundbeck Foundation
Charlotte Mehlin Sørensen