Tubule-vascular feedback in renal autoregulation

Resting-state MRI reveals spontaneous physiological fluctuations in the kidney and tracks diabetic nephropathy in rats

The kidneys maintain fluid-electrolyte balance and excrete waste in the presence of constant fluctuations in plasma volume and systemic blood pressure. The kidneys perform these functions to control capillary perfusion and glomerular filtration by modulating the mechanisms of autoregulation. An effect of these modulations are spontaneous, natural fluctuations in glomerular perfusion. Numerous other mechanisms can lead to fluctuations in perfusion and flow. The ability to monitor these spontaneous physiological fluctuations in vivo could facilitate the early detection of kidney disease. The goal of this work was to investigate the use of resting-state magnetic resonance imaging (rsMRI) to detect spontaneous physiological fluctuations in the kidney. We performed rsMRI of rat kidneys in vivo over 10 min, applying motion correction to resolve time series in each voxel. We observed spatially variable, spontaneous fluctuations in rsMRI signal between 0 and 0.3 Hz, in frequency bands associated with autoregulatory mechanisms. We further applied rsMRI to investigate changes in these fluctuations in a rat model of diabetic nephropathy. Spectral analysis was performed on time series of rsMRI signals in the kidney cortex and medulla. The power from spectra in specific frequency bands from the cortex correlated with severity of glomerular pathology caused by diabetic nephropathy. Finally, we investigated the feasibility of using rsMRI of the human kidney in two participants, observing the presence of similar, spatially variable fluctuations. This approach may enable a range of preclinical and clinical investigations of kidney function and facilitate the development of new therapies to improve outcomes in patients with kidney disease.NEW & NOTEWORTHY This work demonstrates the development and use of resting-state MRI to detect low-frequency, spontaneous physiological fluctuations in the kidney consistent with previously observed fluctuations in perfusion and potentially due to autoregulatory function. These fluctuations are detectable in rat and human kidneys, and the power of these fluctuations is affected by diabetic nephropathy in rats.

Juxtaglomerular apparatus-mediated homeostatic mechanisms: therapeutic implication for chronic kidney disease

INTRODUCTION

Juxtaglomerular apparatus (JGA)-mediated homeostatic mechanism links to how sodium-glucose cotransporter 2 inhibitors (SGLT2is) slow progression of chronic kidney disease (CKD) and may link to how tolvaptan slows renal function decline in autosomal dominant polycystic kidney disease (ADPKD).

AREA COVERED

JGA-mediated homeostatic mechanism has been hypothesized based on investigations of tubuloglomerular feedback and renin-angiotensin system. We reviewed clinical trials of SGLT2is and tolvaptan to assess the relationship between this mechanism and these drugs.

EXPERT OPINION

When sodium load to macula densa (MD) increases, MD increases adenosine production, constricting afferent arteriole (Af-art) and protecting glomeruli. Concurrently, MD signaling suppresses renin secretion, increases urinary sodium excretion, and counterbalances reduced sodium filtration. However, when there is marked increase in sodium load per-nephron, as in advanced CKD, MD adenosine production increases, relaxing Af-art and maintaining sodium homeostasis at the expense of glomeruli. The beneficial effects of tolvaptan on renal function in ADPKD may also depend on the JGA-mediated homeostatic mechanisms since tolvaptan inhibits sodium reabsorption in the thick ascending limb.The JGA-mediated homeostatic mechanism regulates Af-arts, constricting to relaxing according to homeostatic needs. Understanding this mechanism may contribute to the development of pharmacotherapeutic compounds and better care for patients with CKD.

[Mechanisms of action and clinical application of diuretics in intensive care medicine]

The paired kidneys play a significant role in the human body due to the multitude of physiological tasks. Complex biochemical processes keep the sensitive electrolyte and water balance stable and thus ensure the organism's ability to adapt to exogenous and endogenous factors, which is essential for survival. The drug class of diuretics includes substances with very differing pharmacological characteristics. The functioning of the nephron is therefore indispensable for a deeper understanding of the pharmacodynamics, pharmacokinetics and side effect profile of diuretics. In the treatment of acute heart failure with pulmonary congestion, certain diuretics represent an important therapeutic option to counteract hypervolemia and thus an increase in preload. According to current data, diuretics have no proven benefits in the treatment or prevention of acute kidney injury but they can counteract hypervolemia and under certain conditions even reduce the use of renal replacement procedures.

Acute nitric oxide synthase inhibition induces greater increases in blood pressure in female versus male Wistar Kyoto rats

Nitric oxide (NO) contributes to blood pressure (BP) regulation via its vasodilatory and anti-inflammatory properties. We and others previously reported sex differences in BP in normotensive and hypertensive rat models where females have lower BP than age-matched males. As females are known to have greater NO bioavailability than age-matched males, the current study was designed to test the hypothesis that anesthetized female normotensive Wistar Kyoto rats (WKY) are more responsive to acute NOS inhibition-induced increases in BP compared to male WKY. Twelve-week-old male and female WKY were randomized to infusion of the nonspecific NOS inhibitor NG -nitro-L-arginine methyl ester (L-NAME, 1 mg/kg/min) or selective NOS1 inhibition with vinyl-L-NIO (VNIO, 0.5 mg/kg/min) for 60 min. Mean arterial BP, glomerular filtration rate (GFR), urine volume, and electrolyte excretion were assessed before, and during L-NAME or VNIO infusion. L-NAME and VNIO significantly increased BP in both sexes; however, the increase in BP with L-NAME infusion was greater in females versus males compared to baseline BP values. Acute infusion of neither L-NAME nor VNIO for 60 min altered GFR in either sex. However, urine volume, sodium, chloride and potassium excretion levels increased comparably in male and female WKY with L-NAME and VNIO infusion. Our findings suggest sex differences in BP responses to acute non-isoform-specific NOS inhibition in WKY, with females being more responsive to L-NAME-induced elevations in BP relative to male WKY. However, sex differences in the BP response did not coincide with sex differences in renal hemodynamic responses to acute NOS inhibition.

Connexin 40-Mediated Regulation of Systemic Circulation and Arterial Blood Pressure

Vascular system is a complex network in which different cell types and vascular segments must work in concert to regulate blood flow distribution and arterial blood pressure. Although paracrine/autocrine signaling is involved in the regulation of vasomotor tone, direct intercellular communication via gap junctions plays a central role in the control and coordination of vascular function in the microvascular network. Gap junctions are made up by connexin (Cx) proteins, and among the four Cxs expressed in the cardiovascular system (Cx37, Cx40, Cx43, and Cx45), Cx40 has emerged as a critical signaling pathway in the vessel wall. This Cx is predominantly found in the endothelium, but it is involved in the development of the cardiovascular system and in the coordination of endothelial and smooth muscle cell function along the length of the vessels. In addition, Cx40 participates in the control of vasomotor tone through the transmission of electrical signals from the endothelium to the underlying smooth muscle and in the regulation of arterial blood pressure by renin-angiotensin system in afferent arterioles. In this review, we discuss the participation of Cx40-formed channels in the development of cardiovascular system, control and coordination of vascular function, and regulation of arterial blood pressure.

Ion channels and channelopathies in glomeruli

An essential step in renal function entails the formation of an ultrafiltrate that is delivered to the renal tubules for subsequent processing. This process, known as glomerular filtration, is controlled by intrinsic regulatory systems and by paracrine, neuronal, and endocrine signals that converge onto glomerular cells. In addition, the characteristics of glomerular fluid flow, such as the glomerular filtration rate and the glomerular filtration fraction, play an important role in determining blood flow to the rest of the kidney. Consequently, disease processes that initially affect glomeruli are the most likely to lead to end-stage kidney failure. The cells that comprise the glomerular filter, especially podocytes and mesangial cells, express many different types of ion channels that regulate intrinsic aspects of cell function and cellular responses to the local environment, such as changes in glomerular capillary pressure. Dysregulation of glomerular ion channels, such as changes in TRPC6, can lead to devastating glomerular diseases, and a number of channels, including TRPC6, TRPC5, and various ionotropic receptors, are promising targets for drug development. This review discusses glomerular structure and glomerular disease processes. It also describes the types of plasma membrane ion channels that have been identified in glomerular cells, the physiological and pathophysiological contexts in which they operate, and the pathways by which they are regulated and dysregulated. The contributions of these channels to glomerular disease processes, such as focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, as well as the development of drugs that target these channels are also discussed.

Diabetes modulation of the myocardial infarction- acute kidney injury axis

Since there is crosstalk in functions of the heart and kidney, acute or chronic injury in one of the two organs provokes adaptive and/or maladaptive responses in both organs, leading to cardiorenal syndrome (CRS). Acute kidney injury (AKI) induced by acute heart failure is referred to as type 1 CRS, and a frequent cause of this type of CRS is acute myocardial infarction (AMI). Diabetes mellitus increases the risk of AMI and also the risk of AKI of various causes. However, there have been only a few studies in which animal models of diabetes were used to examine how diabetes modulates AMI-induced AKI. In this review, we summarize findings regarding the mechanisms of type 1 CRS and the impact of diabetes on both AMI and renal susceptibility to AKI and we discuss mechanisms by which diabetes modulates AMI-induced AKI. Hemodynamic alterations induced by AMI could be augmented by diabetes via its detrimental effect on infarct size and contractile function of the non-infarcted region in the heart. Diabetes increases susceptibility of renal cells to hypoxia and oxidative stress by modulation of signaling pathways that regulate cell survival and autophagy. Recent studies have shown that diabetes mellitus even at early stage of cardiomyopathy/nephropathy predisposes the kidney to AMI-induced AKI, in which activation of toll-like receptors and reactive oxygen species derived from NADPH oxidases are involved. Further analysis of crosstalk between diabetic cardiomyopathy and diabetic kidney disease is necessary for obtaining a more comprehensive understanding of modulation of the AMI-AKI axis by diabetes.

Effect of suramin on urinary excretion of diabetes-induced glomerular and tubular injury parameters in rats

Diabetes mellitus causes changes in metabolism of extracellular nucleotides acting through P2 receptors (P2Rs). This affects renal function and may lead to glomerular and tubular disturbances. We measured urinary excretion of nucleotides (ATP, ADP, AMP, UTP, UDP, UMP) in streptozotocin-induced diabetic rats (65 mg/kg, i.p., day 0) and the effects of P2Rs' blockade by suramin (10 mg/kg, i.p., days +7, +14) on glomerular P2×7R expression and urinary excretion of glomerular (albumin, nephrin) and tubular (KIM-1, NGAL) injury markers, electrolytes, and oxidative stress markers (TBARS, 8-OHdG). Concentrations of nucleotides, specific proteins, electrolytes, and oxidative stress markers in 24-h urine samples collected in metabolic cages at days -1, +6 and +20 were measured using ion-paired reversed-phase HPLC, immunoenzymatic and fluorometric methods, and flame photometry, respectively. Expression of KIM-1 and P2×7R was examined by immunohistochemistry or immunoblotting. Diabetes was associated with increased urinary excretion of ATP, ADP, UTP, UDP and glomerular P2×7R expression. Suramin attenuated P2×7R expression but did not affect urinary excretion of nucleotides. Urinary excretion of albumin, nephrin, NGAL, and 8-OHdG were increased in diabetic rats and were not affected by suramin. TBARS was higher in diabetic rats and suramin attenuated the excretion dynamics in this group. KIM-1 excretion was higher in diabetic rats and suramin further increased excretion of KIM-1 in both diabetic and non-diabetic rats. Furthermore, suramin attenuated the diabetes-induced natriuresis and kaliuresis. It is possible that suramin affects both glomerular and tubular functions in diabetic rats.

Safety analysis regarding acute kidney injuries for chloride-restrictive intravenous fluid administration against that of chloride-liberal for patients admitted in the medical intensive care unit: A non-randomised retrospective (chrachl-mic) study

BACKGROUND

Acute kidney injuries are common in the medical intensive care unit. Generally, intravenous normal saline is administered in critically ill patients but it is associated with acute kidney injury. Current knowledge of chloride and its effect on the physiological functions of the kidney is limited. The objectives of the study were to compare the safety of chloride-restrictive intravenous fluid administration against that of chloride-liberal regarding acute kidney injuries.

METHODS

Data regarding RIFLE (risk, injury, failure, loss, and end-stage renal failure) categories, Kidney Disease: Improved Global Outcomes (KDIGO) stage, Δ creatinine, and requirements of renal replacement therapy of 285 patients admitted to medical intensive care unit for critical illness during 4-months from the hospitalisation were retrospectively collected and analysed. Patients received chloride-liberal intravenous fluid (CL cohort, n = 163) or that of chloride-restrictive (CR cohort, n = 122) during bundle-of-care.

RESULTS

Patients with risk (P = .039) and injury (P = .041) categories of RIFLE, high Δ creatinine (0.22 ± 0.02 mg/dL/patient vs 0.18 ± 0.02 mg/dL/patient, P < .0001), and patients with KDIGO stage 1 (P = .023) and stage 2 (P = .048) were reported significantly higher in the CL cohort than the CR cohort. The higher numbers of patients were put on renal replacement therapy in the CL cohort than those of the CR cohort (16 vs 3, P = .014).

CONCLUSION

The chloride-restrictive intravenous fluid administration has reduced the chances of acute kidney injuries in the intensive medical care unit.

Mechanism of herbal medicine on hypertensive nephropathy (Review)

Hypertensive nephropathy is the most common complication of hypertension, and is one of the main causes of end-stage renal disease (ESRD) in numerous countries. The basic pathological feature of hypertensive nephropathy is arteriolosclerosis followed by renal parenchymal damage. The etiology of this disease is complex, and its pathogenesis is mainly associated with renal hemodynamic changes and vascular remodeling. Despite the increased knowledge on the pathogenesis of hypertensive nephropathy, the current clinical treatment methods are still not effective in preventing the development of the disease to ESRD. Herbal medicine, which is used to relieve symptoms, can improve hypertensive nephropathy through multiple targets. Since there are few clinical studies on the treatment of hypertensive nephropathy with herbal medicine, this article aims to review the progress on the basic research on the treatment of hypertensive nephropathy with herbal medicine, including regulation of the renin angiotensin system, inhibition of sympathetic excitation, antioxidant stress and anti-inflammatory protection of endothelial cells, and improvement of obesity-associated factors. Herbal medicine with different components plays a synergistic and multi-target role in the treatment of hypertensive nephropathy. The description of the mechanism of herbal medicine in the treatment of hypertensive nephropathy will contribute towards the progress of modern medicine.

Mitochondria-Inspired Nanoparticles with Microenvironment-Adapting Capacities for On-Demand Drug Delivery after Ischemic Injury

Stimuli-responsive nanoparticles (NPs), so-called "smart" NPs, possess great potentials in drug delivery. Presently, the intelligence of smart NPs is mainly based on their chemical or physical changes to stimuli, which are usually "mechanical" and fundamentally different from biological intelligence. Inspired by mitochondria (MT), a biosmart nanoparticle with microenvironment targeting and self-adaptive capacity (MTSNP) was fabricated for ischemic tissue repair. The nanoparticles were designed as shell@circular DNA@shell@core. The double shells were like the two-layered membranes of MT, the melatonin-loaded cores corresponded to the MT matrix, and the circular DNA corresponded to MTDNA. In function, melatonin-loaded cores simulated the cell-protective mechanism of MT, which naturally synthesized melatonin to resist ischemia, while circular DNA was constructed to mimic the biological oxygen-sensing mechanism, synthesizing VEGF for vascularization according to oxygen level, like the ATP supply by MT according to microenvironment demand. At the acute stage of ischemia, melatonin was rapidly released from MTSNP to scavenge reactive oxygen species and activated melatonin receptor I on MT to prevent cytochrome c release, which would activate apoptosis. During the chronic stage, circular DNA could sense hypoxia and actively secrete VEGF for revascularization as a response. Importantly, circular DNA could also receive feedback of revascularization and shut down VEGF secretion as an adverse response. Then, the therapeutic potentials of the MTSNP were verified in myocardial ischemia by the multimodality of the methods. Such nanoparticles may represent a promising intelligent nanodrug system.

DMOG, a Prolyl Hydroxylase Inhibitor, Increases Hemoglobin Levels without Exacerbating Hypertension and Renal Injury in Salt-Sensitive Hypertensive Rats

Prolyl hydroxylase (PHD) inhibitors are being developed as alternatives to recombinant human erythropoietin (rHuEPO) for the treatment of anemia in patients with chronic kidney disease (CKD). However, the effects of PHD inhibitors and rHuEPO on blood pressure and CKD in animal models susceptible to hypertension and nephropathy have not been studied. The present study compared the effects of dimethyloxaloylglycine (DMOG), a PHD inhibitor, and rHuEPO on the development of hypertension and renal injury in Dahl salt-sensitive rats fed an 8% salt diet for 3 weeks. DMOG and rHuEPO were equally effective at raising hemoglobin levels. Systolic blood pressure rose to a greater extent in rHuEPO-treated rats (267 ± 10 vs. 226 ± 4 mm Hg) than in rats given DMOG (189 ± 8 mm Hg). Urinary protein excretion increased to 568 ± 54 versus 353 ± 25 mg/day in rats treated with rHuEPO and vehicle; however, it only rose to 207 ± 21 mg/day in rats receiving DMOG. DMOG significantly attenuated the degree of glomerulosclerosis and renal interstitial fibrosis as compared with that in vehicle and rHuEPO-treated rats. This was associated with lower renal levels of monocyte chemoattractant protein-1 and interleukin-1β and increased vascular endothelial growth factor expression in cortex and medulla. These results indicate that DMOG and rHuEPO are equally effective in increasing hemoglobin levels in Dahl S rats; however, rHuEPO aggravates hypertension and renal injury, whereas DMOG has marked renoprotective effects. These results suggest that PHD inhibitors may have a therapeutic advantage for the treatment of anemia in CKD. SIGNIFICANCE STATEMENT: Prolyl hydroxylase (PHD) inhibitors are in phase 3 clinical trials as alternatives to recombinant human erythropoietin (rHuEPO) for the treatment of anemia in chronic kidney disease (CKD). The present study reveals that dimethyloxaloylglycine (DMOG), a PHD inhibitor, and rHuEPO are equally effective in increasing hemoglobin levels in Dahl S rats; however, rHuEPO aggravated hypertension and renal injury, whereas DMOG attenuated the development of hypertension and prevented renal injury. PHD inhibitors may provide a safer therapeutic option for the treatment of anemia in CKD.