Increased susceptibility to hypertensive renal disease in streptozotocin-treated diabetic rats is not modulated by salt intake

Mitochondria-Derived Reactive Oxygen Species Contribute to Synergistic Interaction of Diabetes and Hypertension in Causing Chronic Kidney Injury

Diabetes (DM) and hypertension (HTN) are major risk factors for chronic kidney injury, together accounting for >70% of end-stage renal disease. The combination of DM and HTN significantly accelerates development of renal injury; however, the underlying mechanisms of this synergy are still poorly understood. This study assessed whether mitochondria (MT) dysfunction is essential in developing renal injury in a rat model with combined DM and HTN. Type 1 DM was induced in Wistar rats by streptozotocin (STZ). HTN was induced six weeks later by inter-renal aorta constriction between the renal arteries, so that right kidneys were exposed to HTN while left kidneys were exposed to normotension. Kidneys exposed to DM or HTN alone had only mild glomerular injury and urinary albumin excretion (UAE). In contrast, kidneys exposed to DM plus 8 weeks HTN had significantly increased UAE and glomerular structural damage with reduced glomerular filtration rate. Marked increases in MT-derived reactive oxygen species (ROS) were also observed in right kidneys exposed to HTN+DM. We further tested whether treatment with MT-targeted antioxidant (MitoTEMPO) after the onset of HTN attenuates renal injury in rats with DM+HTN. Results show that kidneys in DM+AC+MitoTEMPO rats had lower UAE, less glomerular damage, and preserved MT function compared to untreated DM+AC rats. Our studies indicate that MT-derived ROS play a major role in promoting kidney dysfunction when DM is combined with HTN. Preserving MT function might be a potential therapeutic approach to halt the development of renal injury when DM coexists with HTN.

Changes in Proximal Tubular Reabsorption Modulate Microvascular Regulation via the TGF System

This review paper considers the consequences of modulating tubular reabsorption proximal to the macula densa by sodium–glucose co-transporter 2 (SGLT2) inhibitors, acetazolamide, and furosemide in states of glomerular hyperfiltration. SGLT2 inhibitors improve renal function in early and advanced diabetic nephropathy by decreasing the glomerular filtration rate (GFR), presumably by activating the tubuloglomerular feedback (TGF) mechanism. Central in this paper is that the renoprotective effects of SGLT2 inhibitors in diabetic nephropathy can only be partially explained by TGF activation, and there are alternative explanations. The sustained activation of TGF leans on two prerequisites: no or only partial adaptation should occur in reabsorption proximal to macula densa, and no or only partial adaptation should occur in the TGF response. The main proximal tubular and loop of Henle sodium transporters are sodium–hydrogen exchanger 3 (NHE3), SGLT2, and the Na-K-2Cl co-transporter (NKCC2). SGLT2 inhibitors, acetazolamide, and furosemide are the most important compounds; inhibiting these transporters would decrease sodium reabsorption upstream of the macula densa and increase TGF activity. This could directly or indirectly affect TGF responsiveness, which could oppose sustained TGF activation. Only SGLT2 inhibitors can sustainably activate the TGF as there is only partial compensation in tubular reabsorption and TGF response. SGLT2 inhibitors have been shown to preserve GFR in both early and advanced diabetic nephropathy. Other than for early diabetic nephropathy, a solid physiological basis for these effects in advanced nephropathy is lacking. In addition, TGF has hardly been studied in humans, and therefore this role of TGF remains elusive. This review also considers alternative explanations for the renoprotective effects of SGLT2 inhibitors in diabetic patients such as the enhancement of microvascular network function. Furthermore, combination use of SGLT2 inhibitors and angiotensin-converting enzyme inhibitors (ACEi) or angiotensin receptor blockers (ARBs). in diabetes can decrease inflammatory pathways, improve renal oxygenation, and delay the progression of diabetic nephropathy.

Transient receptor potential cation channel 6 contributes to kidney injury induced by diabetes and hypertension

Diabetes mellitus (DM) and hypertension (HTN) are major risk factors for chronic kidney injury, together accounting for >70% of end-stage renal disease. In this study, we assessed whether DM and HTN interact synergistically to promote kidney dysfunction and whether transient receptor potential cation channel 6 (TRPC6) contributes to this synergism. In wild-type (WT; B6/129s background) and TRPC6 knockout (KO) mice, DM was induced by streptozotocin injection to increase fasting glucose levels to 250-350 mg/dL. HTN was induced by aorta constriction (AC) between the renal arteries. AC increased blood pressure (BP) by ∼25 mmHg in the right kidney (above AC), whereas BP in the left kidney (below AC) returned to near normal after 8 wk, with both kidneys exposed to the same levels of blood glucose, circulating hormones, and neural influences. Kidneys of WT mice exposed to DM or HTN alone had only mild glomerular injury and urinary albumin excretion. In contrast, WT kidneys exposed to DM plus HTN (WT-DM + AC mice) for 8 wk had much greater increases in albumin excretion and histological injury. Marked increased apoptosis was also observed in the right kidneys of WT-DM + AC mice. In contrast, in TRPC6 KO mice with DM + AC, right kidneys exposed to the same levels of high BP and high glucose had lower albumin excretion and less glomerular damage and apoptotic cell injury compared with right kidneys of WT-DM + AC mice. Our results suggest that TRPC6 may contribute to the interaction of DM and HTN to promote kidney dysfunction and apoptotic cell injury.NEW & NOTEWORTHY A major new finding of this study is that the combination of moderate diabetes and hypertension promoted marked renal dysfunction, albuminuria, and apoptotic cell injury, and that these effects were greatly ameliorated by transient receptor potential cation channel 6 deficiency. These results suggest that transient receptor potential cation channel 6 may play an important role in contributing to the interaction of diabetes and hypertension to promote kidney injury.

Surgical trauma is associated with renal immune cell activation in rats: A microarray study

Acute kidney injury (AKI) is a common perioperative complication that is associated with increased mortality. This study investigates the renal gene expression in male Long-Evans rats after prolonged anesthesia and surgery to detect molecular mechanisms that could predispose the kidneys to injury upon further insults. Healthy and streptozotocin diabetic rats that underwent autoregulatory investigation in an earlier study were compared to rats that were sacrificed quickly for mRNA quantification in the same study. Prolonged surgery caused massive changes in renal mRNA expression by microarray analysis, which was validated by quantitative real-time PCR with good correlation. Furthermore, bioinformatics analysis using gene ontology and pathway analysis identified biological processes involved in immune system activation, such as immune system processes (p = 1.3 × 10-80 ), immune response (p = 1.3 × 10-60 ), and regulation of cytokine production (p = 1.7 × 10-52 ). PCR analysis of specific cell type markers indicated that the gene activation in kidneys was most probably macrophages, while granulocytes and T cell appeared less activated. Immunohistochemistry was used to quantify immune cell infiltration and showed no difference between groups indicating that the genetic activation depends on the activation of resident cells, or infiltration of a relatively small number of highly activated cells. In follow-up experiments, surgery was performed on healthy rats under standard and sterile condition showing similar expression of immune cell markers, which suggests that the inflammation was indeed caused by the surgical trauma rather than by bacterial infection. In conclusion, surgical trauma is associated with rapid activation of immune cells, most likely macrophages in rat kidneys.

Understanding the Two Faces of Low-Salt Intake

Fierce debate has developed whether low-sodium intake, like high-sodium intake, could be associated with adverse outcome. The debate originates in earlier epidemiological studies associating high-sodium intake with high blood pressure and more recent studies demonstrating a higher cardiovascular event rate with both low- and high-sodium intake. This brings into question whether we entirely understand the consequences of high- and (very) low-sodium intake for the systemic hemodynamics, the kidney function, the vascular wall, the immune system, and the brain. Evolutionarily, sodium retention mechanisms in the context of low dietary sodium provided a survival advantage and are highly conserved, exemplified by the renin-angiotensin system. What is the potential for this sodium-retaining mechanism to cause harm? In this paper, we will consider current views on how a sodium load is handled, visiting aspects including the effect of sodium on the vessel wall, the sympathetic nervous system, the brain renin-angiotensin system, the skin as "third compartment" coupling to vascular endothelial growth factor C, and the kidneys. From these perspectives, several mechanisms can be envisioned whereby a low-sodium diet could potentially cause harm, including the renin-angiotensin system and the sympathetic nervous system. Altogether, the uncertainties preclude a unifying model or practical clinical guidance regarding the effects of a low-sodium diet for an individual. There is a very strong need for fundamental and translational studies to enhance the understanding of the potential adverse consequences of low-salt intake as an initial step to facilitate better clinical guidance.

Transient impairment of dynamic renal autoregulation in early diabetes mellitus in rats

Renal autoregulation is impaired in early (1 wk) diabetes mellitus (DM) induced by streptozotocin, but effective in established DM (4 wk). Furthermore nitric oxide synthesis (NOS) inhibition with NG-nitro-l-arginine methyl ester (l-NAME) significantly improved autoregulation in early DM but not in established DM. We hypothesized that autoregulation is transiently impaired in early DM because of increased NO availability in the kidney. Because of the conflicting evidence available for a role of NO in DM, we tested the hypothesis that DM reduces autoregulation effectiveness by reducing the spatial similarity of autoregulation. Male Long-Evans rats were divided into control (CON) and diabetic (DM; streptozotocin) groups and followed for either 1 wk (CON1, n = 6; DM1, n = 5) or 4 wk (CON4, n = 7; DM4, n = 7). At the end of the experiment, dynamic autoregulation was assessed in isoflurane-anesthetized rats by whole kidney RBF during baseline, NOS1 inhibition, and nonselective NOS inhibition. Kidney surface perfusion, monitored with laser speckle contrast imaging, was used to assess spatial heterogeneity of autoregulation. Autoregulation was significantly impaired in DM1 rats and not impaired in DM4 rats. l-NAME caused strong renal vasoconstriction in all rats, but did not significantly affect autoregulation dynamics. Autoregulation was more spatially heterogeneous in DM1, but not DM4. Therefore, our results, which are consistent with transient impairment of autoregulation in DM, argue against the hypothesis that this impairment is NO-dependent, and suggest that spatial properties of autoregulation may also contribute to reduced autoregulatory effectiveness in DM1.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Physiological changes induced by salt intake in female Spontaneously Diabetic Torii-Lepr(fa) (SDT fatty) rat, a novel obese type 2 diabetic model

Salt plays an important role in the control of blood pressure in obesity and diabetes mellitus. In this study, we investigated physiological changes such as blood pressure and renal function in salt-loaded female Spontaneously Diabetic Torii-Lepr(fa) (SDT fatty) rats. SDT fatty rats were given 1% NaCl in drinking water for 14 weeks, from 4 to 18 weeks of age. Significant salt-sensitive hypertension was observed in the salt-loaded SDT fatty rats. Moreover, the salt-loaded rats showed a decrease of creatinine clearance and deterioration on pathological renal findings, including glomerulosclerosis and tubular and interstitial lesions. Female SDT fatty rat is a useful model for investigating the mechanisms of high salt sensitivity in obesity and diabetes mellitus.