-
Acta Physiologica (Oxford, England) Feb 2013Regulation of medullary blood flow (MBF) is essential in maintaining normal kidney function. Blood flow to the medulla is supplied by the descending vasa recta (DVR),... (Review)
Review
Regulation of medullary blood flow (MBF) is essential in maintaining normal kidney function. Blood flow to the medulla is supplied by the descending vasa recta (DVR), which arise from the efferent arterioles of juxtamedullary glomeruli. DVR are composed of a continuous endothelium, intercalated with smooth muscle-like cells called pericytes. Pericytes have been shown to alter the diameter of isolated and in situ DVR in response to vasoactive stimuli that are transmitted via a network of autocrine and paracrine signalling pathways. Vasoactive stimuli can be released by neighbouring tubular epithelial, endothelial, red blood cells and neuronal cells in response to changes in NaCl transport and oxygen tension. The experimentally described sensitivity of pericytes to these stimuli strongly suggests their leading role in the phenomenon of MBF autoregulation. Because the debate on autoregulation of MBF fervently continues, we discuss the evidence favouring a physiological role for pericytes in the regulation of MBF and describe their potential role in tubulo-vascular cross-talk in this region of the kidney. Our review also considers current methods used to explore pericyte activity and function in the renal medulla.
Topics: Animals; Humans; Kidney Medulla; Pericytes; Renal Circulation
PubMed: 23126245
DOI: 10.1111/apha.12026 -
Kidney International. Supplement Sep 1998Vascular endothelial cells play an important role in tissue homeostasis, fibrinolysis, and coagulation; blood-tissue exchange, vasotonus regulation, blood cell... (Review)
Review
Vascular endothelial cells play an important role in tissue homeostasis, fibrinolysis, and coagulation; blood-tissue exchange, vasotonus regulation, blood cell activation, and migration; and the vascularization of tissues. The formation of new blood vessels comprises two distinct steps: vasculogenesis, the in situ assembly of capillaries, and angiogenesis, the sprouting of capillaries from preexisting ones. Vascular endothelial growth factor (VEGF) is essential for vasculogenesis and angiogenesis. Its expression is high in the embryonic brain and kidney when angiogenesis occurs and low in the adult brain when angiogenesis is absent. In the kidney, VEGF expression remains high in glomerular podocytes even in the adult. VEGF receptors 1 and 2 (fit-1 and flk-1) are endothelial-specific receptor tyrosine kinases. Similar to the ligand, expression of these receptors is high during brain and kidney angiogenesis, low in adult brain endothelium, but high in adult glomerular endothelium. Because VEGF is also a vascular permeability factor, the expression in the adult correlates with the low permeability of blood-brain barrier endothelium and the high permeability of fenestrated glomerular endothelium. Although fenestrae formation can be induced in vitro by VEGF and a basal lamina-type extracellular matrix, blood-brain barrier characteristics seem to require the presence of still unknown brain-derived factors.
Topics: Animals; Cell Differentiation; Endothelium, Vascular; Humans; Kidney; Microcirculation; Renal Circulation
PubMed: 9736244
DOI: 10.1046/j.1523-1755.1998.06701.x -
PloS One 2014Renal vascular responses to pregnancy have frequently been studied, by investigating renal vascular resistance (RVR), renal flow, glomerular filtration rate (GFR), and... (Meta-Analysis)
Meta-Analysis Review
Renal vascular responses to pregnancy have frequently been studied, by investigating renal vascular resistance (RVR), renal flow, glomerular filtration rate (GFR), and renal artery responses to stimuli. Nonetheless, several questions remain: 1. Which vasodilator pathways are activated and to what extent do they affect RVR, renal flow and GFR across species, strains and gestational ages, 2. Are these changes dependent on renal artery adaptation, 3. At which cellular level does pregnancy affect the involved pathways? In an attempt to answer the questions raised, we performed a systematic review and meta-analysis on animal data. We included 37 studies (116 responses). At mid-gestation, RVR and GFR change to a similar degree across species and strains, accompanied by variable change in renal flow. At least in rats, changes depend on NO activation. At late gestation, changes in RVR, renal flow and GFR vary between species and strains. In rats, these changes are effectuated by sympathetic stimulation. Overall, renal artery responsiveness to stimuli is unaffected by pregnancy, except for Sprague Dawley rats in which pregnancy enhances renal artery vascular compliance and reduces renal artery myogenic reactivity. Our meta-analysis shows that: 1. Pregnancy changes RVR, renal flow and GFR dependent on NO-activation and sympathetic de-activation, but adjustments are different among species, strains and gestational ages; 2. These changes do not depend on adaptation of renal artery responsiveness; 3. It remains unknown at which cellular level pregnancy affects the pathways. Our meta-analysis suggests that renal changes during pregnancy in animals are qualitatively similar, even in comparison to humans, but quantitatively different.
Topics: Animals; Female; Glomerular Filtration Rate; Kidney; Pregnancy; Pregnancy, Animal; Rats; Renal Artery; Renal Circulation; Vascular Resistance
PubMed: 25386682
DOI: 10.1371/journal.pone.0112084 -
Autonomic Neuroscience : Basic &... May 2017Tubuloglomerular feedback and the myogenic response are widely appreciated as important regulators of renal blood flow, but the role of the sympathetic nervous system in... (Review)
Review
Tubuloglomerular feedback and the myogenic response are widely appreciated as important regulators of renal blood flow, but the role of the sympathetic nervous system in physiological renal blood flow control remains controversial. Where classic studies using static measures of renal blood flow failed, dynamic approaches have succeeded in demonstrating sympathetic control of renal blood flow under normal physiological conditions. This review focuses on transfer function analysis of renal pressure-flow, which leverages the physical relationship between blood pressure and flow to assess the underlying vascular control mechanisms. Studies using this approach indicate that the renal nerves are important in the rapid regulation of the renal vasculature. Animals with intact renal innervation show a sympathetic signature in the frequency range associated with sympathetic vasomotion that is eliminated by renal denervation. In conscious rabbits, this sympathetic signature exerts vasoconstrictive, baroreflex control of renal vascular conductance, matching well with the rhythmic, baroreflex-influenced control of renal sympathetic nerve activity and complementing findings from other studies employing dynamic approaches to study renal sympathetic vascular control. In this light, classic studies reporting that nerve stimulation and renal denervation do not affect static measures of renal blood flow provide evidence for the strength of renal autoregulation rather than evidence against physiological renal sympathetic control of renal blood flow. Thus, alongside tubuloglomerular feedback and the myogenic response, renal sympathetic outflow should be considered an important physiological regulator of renal blood flow. Clinically, renal sympathetic vasomotion may be important for solving the problems facing the field of therapeutic renal denervation.
Topics: Animals; Kidney; Renal Circulation; Sympathetic Nervous System
PubMed: 27514571
DOI: 10.1016/j.autneu.2016.08.003 -
American Journal of Physiology.... Aug 2019In experimental sepsis, the rapid development of renal medullary hypoxia precedes the development of acute kidney injury (AKI) and may contribute to its pathogenesis. We...
In experimental sepsis, the rapid development of renal medullary hypoxia precedes the development of acute kidney injury (AKI) and may contribute to its pathogenesis. We investigated whether inhibiting active sodium transport and oxygen consumption in the medullary thick ascending limb with furosemide attenuates the medullary hypoxia in experimental septic AKI. Sheep were instrumented with flow probes on the pulmonary and renal arteries and fiber optic probes to measure renal cortical and medullary perfusion and oxygen tension (Po). Sepsis and AKI were induced by infusion of live . At 24 h of sepsis there were significant decreases in renal medullary tissue perfusion (1,332 ± 233 to 698 ± 159 blood perfusion units) and Po (44 ± 6 to 19 ± 6 mmHg) (both < 0.05). By 5 min after intravenous administration of furosemide (20 mg), renal medullary Po increased to 43 ± 6 mmHg and remained at this normal level for 8 h. Furosemide caused transient increases in fractional excretion of sodium and creatinine clearance, but medullary perfusion, renal blood flow, and renal oxygen delivery were unchanged. Urinary F-isoprostanes, an index of oxidative stress, were not significantly changed at 24 h of sepsis but tended to transiently decrease after furosemide treatment. In septic AKI, furosemide rapidly restored medullary Po to preseptic levels. This effect was not accompanied by changes in medullary perfusion or renal oxygen delivery but was accompanied by a transient increase in fractional sodium excretion, implying decreased oxygen consumption as a mechanism.
Topics: Acute Kidney Injury; Animals; Furosemide; Hypoxia; Kidney; Kidney Function Tests; Kidney Medulla; Oxygen Consumption; Renal Circulation; Sheep
PubMed: 31141418
DOI: 10.1152/ajpregu.00371.2018 -
Experimental Physiology May 2019What is the central question of this study? Visceral ischaemia is one of the most feared complications during use of an intra-aortic balloon pump. Using an animal model,...
NEW FINDINGS
What is the central question of this study? Visceral ischaemia is one of the most feared complications during use of an intra-aortic balloon pump. Using an animal model, we measured the flows at the abdominal level directly and examined flow patterns to enable investigation of flow patterns during the use of the intra-aortic balloon pump. What is the main finding and its importance? We show that there is a significant balloon-related reduction in superior mesenteric flow in both early and mid-diastole.
ABSTRACT
A number of previous studies have shown that blood flow in the visceral arteries is altered during intra-aortic balloon pump (IABP) treatment. We used a porcine model to analyse the pattern of blood flow into the visceral arteries during IABP use. For this purpose, we measured the superior mesenteric, right renal and left renal flows before and during IABP support, using surgically placed flowmeters surrounding these visceral arteries. The superior mesenteric flow significantly decreased in early diastole (P < 0.001) and in mid-diastole (P = 0.003 versus early diastole), whereas in late diastole it increased again (P < 0.001 versus mid-diastole). During systole, the flow was not significantly increased compared with late diastole (P = 0.51), but it was significantly lower than at baseline (both P < 0.001). Flows did not differ between right and left kidneys. Perfusion of either kidney did not change significantly in early diastole (P > 0.05), whereas it decreased significantly in mid-diastole (P < 0.001), rising dramatically in late diastole (P < 0.001) and with an additional slight increase in systole (P = 0.054). This study provides important insights into abdominal flows during intra-aortic pump counterpulsation. Furthermore, it supports the need to rethink the balloon design to avoid visceral ischaemia during circulatory assistance.
Topics: Animals; Blood Flow Velocity; Counterpulsation; Diastole; Electrocardiography; Hemodynamics; Intra-Aortic Balloon Pumping; Mesenteric Artery, Superior; Monitoring, Physiologic; Renal Circulation; Rheology; Swine; Systole
PubMed: 30821049
DOI: 10.1113/EP086810 -
Journal of Magnetic Resonance Imaging :... Apr 2022Renal blood flow (RBF) can be measured with dynamic contrast enhanced-MRI (DCE-MRI) and arterial spin labeling (ASL). Unfortunately, individual estimates from both...
BACKGROUND
Renal blood flow (RBF) can be measured with dynamic contrast enhanced-MRI (DCE-MRI) and arterial spin labeling (ASL). Unfortunately, individual estimates from both methods vary and reference-standard methods are not available. A potential solution is to include a third, arbitrating MRI method in the comparison.
PURPOSE
To compare RBF estimates between ASL, DCE, and phase contrast (PC)-MRI.
STUDY TYPE
Prospective.
POPULATION
Twenty-five patients with type-2 diabetes (36% female) and five healthy volunteers (HV, 80% female).
FIELD STRENGTH/SEQUENCES
A 3 T; gradient-echo 2D-DCE, pseudo-continuous ASL (pCASL) and cine 2D-PC.
ASSESSMENT
ASL, DCE, and PC were acquired once in all patients. ASL and PC were acquired four times in each HV. RBF was estimated and split-RBF was derived as (right kidney RBF)/total RBF. Repeatability error (RE) was calculated for each HV, RE = 1.96 × SD, where SD is the standard deviation of repeat scans.
STATISTICAL TESTS
Paired t-tests and one-way analysis of variance (ANOVA) were used for statistical analysis. The 95% confidence interval (CI) for difference between ASL/PC and DCE/PC was assessed using two-sample F-test for variances. Statistical significance level was P < 0.05. Influential outliers were assessed with Cook's distance (D > 1) and results with outliers removed were presented.
RESULTS
In patients, the mean RBF (mL/min/1.73m ) was 618 ± 62 (PC), 526 ± 91 (ASL), and 569 ± 110 (DCE). Differences between measurements were not significant (P = 0.28). Intrasubject agreement was poor for RBF with limits-of-agreement (mL/min/1.73m ) [-687, 772] DCE-ASL, [-482, 580] PC-DCE, and [-277, 460] PC-ASL. The difference PC-ASL was significantly smaller than PC-DCE, but this was driven by a single-DCE outlier (P = 0.31, after removing outlier). The difference in split-RBF was comparatively small. In HVs, mean RE (±95% CI; mL/min/1.73 m ) was significantly smaller for PC (79 ± 41) than for ASL (241 ± 85).
CONCLUSIONS
ASL, DCE, and PC RBF show poor agreement in individual subjects but agree well on average. Triangulation with PC suggests that the accuracy of ASL and DCE is comparable.
EVIDENCE LEVEL
2 TECHNICAL EFFICACY: Stage 2.
Topics: Contrast Media; Female; Humans; Magnetic Resonance Imaging; Male; Prospective Studies; Renal Circulation; Reproducibility of Results; Spin Labels
PubMed: 34397124
DOI: 10.1002/jmri.27888 -
American Journal of Physiology.... Nov 2016Countercurrent systems have evolved in a variety of biological systems that allow transfer of heat, gases, and solutes. For example, in the renal medulla, the... (Review)
Review
Countercurrent systems have evolved in a variety of biological systems that allow transfer of heat, gases, and solutes. For example, in the renal medulla, the countercurrent arrangement of vascular and tubular elements facilitates the trapping of urea and other solutes in the inner medulla, which in turn enables the formation of concentrated urine. Arteries and veins in the cortex are also arranged in a countercurrent fashion, as are descending and ascending vasa recta in the medulla. For countercurrent diffusion to occur, barriers to diffusion must be small. This appears to be characteristic of larger vessels in the renal cortex. There must also be gradients in the concentration of molecules between afferent and efferent vessels, with the transport of molecules possible in either direction. Such gradients exist for oxygen in both the cortex and medulla, but there is little evidence that large gradients exist for other molecules such as carbon dioxide, nitric oxide, superoxide, hydrogen sulfide, and ammonia. There is some experimental evidence for arterial-to-venous (AV) oxygen shunting. Mathematical models also provide evidence for oxygen shunting in both the cortex and medulla. However, the quantitative significance of AV oxygen shunting remains a matter of controversy. Thus, whereas the countercurrent arrangement of vasa recta in the medulla appears to have evolved as a consequence of the evolution of Henle's loop, the evolutionary significance of the intimate countercurrent arrangement of blood vessels in the renal cortex remains an enigma.
Topics: Animals; Biological Evolution; Biological Transport, Active; Gases; Humans; Kidney; Renal Artery; Renal Circulation; Renal Veins; Urea
PubMed: 27488891
DOI: 10.1152/ajpregu.00246.2016 -
American Journal of Physiology. Renal... Nov 2008The kidney is faced with unique challenges for oxygen regulation, both because its function requires that perfusion greatly exceeds that required to meet metabolic... (Review)
Review
The kidney is faced with unique challenges for oxygen regulation, both because its function requires that perfusion greatly exceeds that required to meet metabolic demand and because vascular control in the kidney is dominated by mechanisms that regulate glomerular filtration and tubular reabsorption. Because tubular sodium reabsorption accounts for most oxygen consumption (Vo2) in the kidney, renal Vo2 varies with glomerular filtration rate. This provides an intrinsic mechanism to match changes in oxygen delivery due to changes in renal blood flow (RBF) with changes in oxygen demand. Renal Vo2 is low relative to supply of oxygen, but diffusional arterial-to-venous (AV) oxygen shunting provides a mechanism by which oxygen superfluous to metabolic demand can bypass the renal microcirculation. This mechanism prevents development of tissue hyperoxia and subsequent tissue oxidation that would otherwise result from the mismatch between renal Vo2 and RBF. Recent evidence suggests that RBF-dependent changes in AV oxygen shunting may also help maintain stable tissue oxygen tension when RBF changes within the physiological range. However, AV oxygen shunting also renders the kidney susceptible to hypoxia. Given that tissue hypoxia is a hallmark of both acute renal injury and chronic renal disease, understanding the causes of tissue hypoxia is of great clinical importance. The simplistic paradigm of oxygenation depending only on the balance between local perfusion and Vo2 is inadequate to achieve this goal. To fully understand the control of renal oxygenation, we must consider a triad of factors that regulate intrarenal oxygenation: local perfusion, local Vo2, and AV oxygen shunting.
Topics: Animals; Biophysical Phenomena; Homeostasis; Humans; Hypoxia; Kidney; Kidney Diseases; Models, Biological; Oxygen; Renal Circulation
PubMed: 18550645
DOI: 10.1152/ajprenal.90230.2008 -
Proceedings of the Royal Society of... Jun 1950
Topics: Humans; Kidney; Renal Artery; Renal Circulation
PubMed: 15430360
DOI: No ID Found