| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
in Diabetic Rats
The Department of Medicine (H.M.S., A.A., P.A.), University of Virginia School of Medicine, Charlottesville, Virginia 22904; and Novartis Pharmaceuticals Corporation (R.W.), East Hanover, New Jersey 07936
Address all correspondence and requests for reprints to: Helmy M. Siragy, M.D., Box 801409, Department of Medicine, University of Virginia Health Sciences Center, 450 Ray C. Hunt Drive, Charlottesville, Virginia 22908. E-mail: hms7a{at}virginia.edu.
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
in diabetes. Recovery of Ang II and TNF- in renal interstitial fluid (RIF) was measured in conscious rats before and weekly for 12 wk after induction of diabetes with streptozocin and in response to oral valsartan (10 mg/kg·d). Recovery of Ang II in RIF was significantly higher in diabetic rats than in nondiabetic rats. In diabetic rats, RIF recovery of TNF- increased by approximately 67% over baseline, whereas it was unchanged in nondiabetic rats. AT1 receptor blockade with valsartan prevented the increase in TNF- in the diabetic group. This study shows that diabetes is associated with an increase in the vasoconstrictive hormone Ang II and the inflammatory cytokine TNF-, both of which play a role in accelerating renal function decline in diabetic nephropathy. The study also confirms that valsartan reduces intrarenal level of TNF- by acting on Ang II at the AT1 receptor level. This finding of a potential antiinflammatory effect for valsartan is new and in addition to its known antihypertensive effects. |
Introduction |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
The pathogenesis of diabetic nephropathy is complex. High extracellular glucose concentrations, a determinant of progressive nephropathy, activate intracellular signaling cascades and alter the expression of cytokines and growth factors (4). Among the cytokines with an implicated role in diabetic nephropathy is TNF-, a monocyte-macrophage derived cytokine that activates transcription factors such as nuclear factor 
B, which induce expression of genes involved in inflammation and cell growth (5, 6, 7).
An activated RAS, via its principal effector Ang II, promotes proteinuria and accelerates the decline in renal function in diabetes through both hemodynamic (increased glomerular capillary pressure) and nonhemodynamic (stimulation of cellular hypertrophy and accumulation of extracellular matrix) effects (4). The deleterious effects of the RAS are thought to be due in part to induction of cytokines and growth factors by Ang II and interaction of Ang II with the AT1 receptor, which is widely distributed in the kidney (4, 8). In addition to the systemic RAS, the intrarenal RAS is activated in diabetic nephropathy.
The present study was conducted to determine whether hyperglycemia associated with diabetes increases renal production of TNF- due to increased production of Ang II and stimulation of the AT1 receptor. Renal interstitial fluid (RIF) recovery of Ang II and TNF- was measured in rats before and after induction of diabetes with streptozocin (STZ). One group received valsartan, a specific and potent ARB, to determine the effect of AT1 receptor blockade on RIF recovery of TNF-. An in vivo renal microdialysis technique that has been used previously to measure renal levels of bradykinin, nitric oxide, cGMP, prostaglandin E2, and Ang II was used in the present study (9, 10, 11, 12, 13, 14, 15, 16). Measurements were made in conscious animals over a continuous time course, with each animal serving as its own control in addition to having a time control. Advantages of measuring RIF recovery over blood or urine concentrations include the following: potential unwanted hemodynamic changes that may occur with repeated blood sampling in small animals are avoided; interstitial levels of hormones/autocoids reflect local changes in the organ that may not be reflected in measurements of circulating substances; concentrations in the interstitium, which is closer to target receptors, may differ from the concentration in the circulation; substances can be formed or degraded in the urine and, thus, do not reflect concentrations within the target organ; and the molecular weight cutoff of the microdialysis membrane helps exclude undesirable substances such as degrading enzymes and carrier proteins (10, 14).
|
Materials and Methods |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
Animal preparation
The experiments, which were approved by the University of Virginia Animal Research Committee and conducted in accordance with institutional guidelines, were performed in 4-wk-old conscious male Sprague-Dawley rats (Harlan Teklad, Madison, WI) with a microdialysis catheter inserted into the outer renal cortex. Rats underwent general anesthesia with im ketamine (80 mg/kg) and xylazine (8 mg/kg), and the right and left kidneys of each rat were exposed via a midline abdominal incision (14). A 31-gauge needle was inserted into the renal capsule to a depth of approximately 1 mm from the outer cortex and passed a distance of 5 mm before it exited the capsule. The needle tip was inserted into one end of the dialysis probe and pulled back through the renal capsule to place the dialysis fiber within the renal cortex. The dialysis probe inflow and outflow tubes were tunneled sc through a bevel-tipped stainless steel tube and emerged near the interscapular region. The exterior ends of the tubes were sutured to the skin at the exit site and covered with a stainless steel spring to protect them from rat-inflicted damage.
The rats were housed under controlled conditions (temperature, 21 ± 1 C; humidity, 60 ± 10%; lighting, 8 h/20 h) and were allowed 7 d to recover and become acclimatized to the laboratory sampling as well as to avoid any influence of anesthesia or surgery on the experiment. All animals consumed a normal sodium diet (0.28% NaCl, Bioserve, Frenchtown, NJ). Experiments were started promptly at 0800 h daily to avoid diurnal variation in experimental parameters. For collection of RIF, the inflow tube was connected to a gas-tight syringe filled with lactated Ringer’s solution, which was perfused at a rate of 3 µl/min. The effluent was collected from the outflow tube for 60-min sample periods on the same day every week for the 12 wk of the study.
Assessment of AT1 receptor blockade on TNF-
Rats were placed in metabolic cages and randomly assigned to one of four study groups of eight rats each: a normal (untreated) control group that received a regular sodium diet; a normal control group treated with valsartan (Novartis Pharmaceuticals, East Hanover, NJ) 10 mg/kg·d orally; an STZ-induced diabetes group; and an STZ-induced diabetes group treated with valsartan 10 mg/kg·d. After an overnight fast, baseline measurements were taken of basal RIF Ang II and TNF-; then two groups of rats were injected iv with STZ 35 mg/kg (Sigma, St. Louis, MO) to induce diabetes. One group with STZ-induced diabetes received valsartan. RIF samples were collected from both kidneys, and the samples from both kidneys were pooled for measurements, which were made on the same day each week during the study, with wk 1 as baseline (before induction of diabetes with STZ). Animals were monitored for body weight; systolic blood pressure; and levels of blood glucose, 24-h urinary albumin, and TNF- during the 12-wk study.
Assessment of RIF Ang II and TNF- in response to insulin treatment
Rats were placed in metabolic cages and were randomly assigned to a normal (untreated) control group (n = 8) and an STZ-induced diabetes group (n = 8). Diabetes was induced with STZ as above. After an overnight fast, both groups were treated with iv administration of 0.4 U/kg·h regular insulin in 0.9% NaCl (Eli Lilly \|[amp ]\| Co., Indianapolis, IN) for 5 h. Blood and RIF glucose levels were monitored hourly during the study. Insulin infusion was discontinued if blood glucose decreased to less than 60 mg/dl. RIF glucose, Ang II, and TNF- were monitored before and at the end of insulin infusion. Blood glucose levels were maintained between 60 and 100 mg/dl. To avoid blood glucose levels below 60 mg/dl in normal animals, insulin was mixed with 10% dextrose in water (20 µl/min).
Analytical methods
Blood glucose concentrations were monitored daily using the Accu-Check glucometer (Mannheim Boehringer, Indianapolis, IN), which uses approximately 4 µl of whole blood. Hemodynamic measurements, including heart rate, systolic and diastolic blood pressure, and mean arterial pressure, were measured using a Natsumi BP Pulse Monitor (Peninsula Laboratories, Inc. Belmont, CA). Urinary and RIF recovery levels of TNF- were measured by a specific ELISA (PharMingen, San Diego, CA), which has no cross-reactivity with interleukins or interferon-
. RIF recovery of Ang II was measured using a specific ELISA with a sensitivity of 0.5 pg/ml, as described previously (17). Urinary albumin was measured by a specific ELISA (Exocell Inc., Philadelphia, PA).
Statistical analysis
Treatment and control groups were compared using ANOVA, including a repeated measure term, using the general linear models procedure of the Statistical Analysis System (SAS Institute, Inc., Cary, NC). Data are expressed as mean ± SE level. P values were calculated from the least squares measure of a linear model. A difference of P less than 0.05 was considered statistically significant.
|
Results |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
At baseline before insulin infusion, blood and RIF glucose concentrations, respectively, were 70 ± 5 mg/dl and 42 ± 3 mg/dl in normal animals, and 325 ± 15 mg/dl and 253 ± 12 mg/dl in diabetic animals. At the end of 5 h of insulin administration, blood and RIF glucose levels, respectively, were 68 ± 7 mg/dl (P = not significant) and 33 ± 2 mg/dl (P < 0.001) in normal animals and 93 ± 11 mg/dl and 51 ± 4 mg/dl in diabetic animals (P < 0.001, compared with levels before insulin treatment). TNF- recovery in RIF did not change significantly in response to insulin treatment in normal control animals; it was 2.15 ± 0.01 pg/min at baseline and 2.20 ± 0.01 pg/min at the end of insulin treatment. In the diabetic group, TNF- recovery in RIF significantly decreased in response to insulin treatment, from 10.21 ± 0.50 pg/min to 2.10 ± 0.02 pg/min (P < 0.0001).
Urinary TNF- and albumin were measured in the untreated diabetes group and the valsartan-treated diabetes group. In the diabetes group, urinary TNF- and albumin rose from 16.1 ± 6.5 pg/ml and 3.96 ± 0.93 µg/24 h at baseline, respectively, to 294.18 ± 36.94 pg/ml and 341 ± 53.4 µg/24 h at wk 6, and 337.72 ± 34.91 pg/ml and 934 ± 88.7 µg/24 h at wk 12 (P < 0.0001 compared with baseline). In the valsartan-treated diabetes group, baseline urine TNF- and albumin were 17.11 ± 4.23 pg/ml and 3.29 ± 0.199 µg/24 h, and although levels increased over the course of the study, they were statistically significantly lower than those in the diabetes group that did not receive valsartan, rising to 42.34 ± 6.21 pg/ml and 57.1 ± 3.99 µg/24 h at wk 6, and 31.88 ± 5.76 pg/ml and 63.5 ± 9.12 µg/24 h at wk 12 (P < 0.0001 compared with diabetes group).
RIF Ang II in normal and diabetic rats
RIF recovery of Ang II in normal (untreated control) rats remained nearly constant throughout the study (0.30 ± 0.001 pg/min at baseline, and 0.31 ± 0.002 pg/min at wk 12; Fig. 1
). Valsartan treatment increased RIF Ang II recovery rate in normal rats from 0.30 ± 0.002 pg/min at baseline to 0.36 ± 0.001 pg/min at wk 12. In contrast, RIF recovery of Ang II in diabetic rats was significantly higher after induction of diabetes (0.40 ± 0.01 pg/min at wk 3; P < 0.001) and continued to increase significantly (P < 0.0001) through wk 11, remaining at that level (0.73 ± 0.01 pg/min) at wk 12. In STZ-induced diabetic rats treated with valsartan, RIF Ang II recovery increased even higher, reaching 0.62 ± 0.04 pg/min by wk 3 (P < 0.001) and continuing to increase through wk 8, remaining at 0.92 ± 0.01 pg/min at wk 12 (P < 0.0001).
|
TNF- in normal and diabetic rats and response to valsartan
RIF recovery of TNF- in normal (untreated control) rats remained constant throughout the study [2.20 ± 0.02 pg/min at wk 1 (baseline) and 2.19 ± 0.01 pg/min at wk 12]. In diabetic rats, RIF recovery of TNF- was significantly higher (P < 0.0001) throughout the study compared with recoveries in the control and valsartan-treated groups (Fig. 2). At baseline (wk 1), RIF recovery of TNF- was similar among the four groups: control group, 2.20 ± 0.02 pg/min; valsartan-treated control group, 2.21 ± 0.01 pg/min; diabetes group, 2.16 ± 0.01 pg/min; valsartan-treated diabetes group, 2.18 ± 0.01 pg/min. At wk 12, RIF recovery of TNF- was 11.8 ± 0.8 pg/min (P < 0.0001) in the diabetes group (about 80% increase from baseline at wk 1) and 5.6 ± 0.01 pg/min (P < 0.0001) in the valsartan-treated diabetes group. There were no significant changes in RIF TNF- recovery in the valsartan-treated control group throughout the study. Thus, administration of 10 mg/kg·d oral valsartan to diabetic rats significantly reduced the diabetes-associated increase in TNF- throughout the 12-wk experimental period (Fig. 2).
|
|
Discussion |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
, and present for the first time data collected over an extended time course in a unique conscious rat model of diabetes. Our model allows direct sampling of RIF and provides accurate measurement of Ang II in the target renal tissue, because the intrarenal concentration of Ang II is about 1000-fold higher than the circulating (plasma) level (11). Local changes in hormone and TNF- levels may not be detectable in the circulation, and these factors may be degraded in the urine (14, 15). In addition, our use of conscious animals avoids anesthetic-induced elevations of Ang II levels (11) as well as the interfering effects of anesthesia on hemodynamics and other parameters. Although high levels of extracellular glucose stimulated angiotensinogen expression in studies of isolated rat proximal tubular cells in culture (4), it had been assumed that Ang II is not significantly increased in diabetes, based on studies of animal models in which levels of Ang II were measured in blood or in kidney tissue isolated from anesthetized animals (4, 18). However, we have clearly shown in our model of STZ-induced diabetes in intact, conscious animals that renal levels of Ang II are significantly increased at the target tissue level, where Ang II is believed to act primarily through the AT1 receptor. Valsartan, an Ang II receptor antagonist (ARB), selectively blocks the AT1 receptor (19), unlike ACE inhibitors that act nonspecifically to block an early enzymatic process in Ang II synthesis. Valsartan increases the tissue Ang II level through direct blockade of the AT1 short negative feedback loop, which, in turn, stimulates the increase in renin secretion. During blockade of the AT1 receptor, the excess Ang II may be renoprotective, because it is available to interact with the AT2 receptor (20). The renoprotective effects of the AT2 receptor stem from its ability to increase generation of bradykinin, nitric oxide, and cGMP, as well as its antiproliferative and antifibrotic properties (8, 9, 14, 15).
Diabetes is associated with increased levels of vasoconstrictors such as Ang II (19), which can up-regulate the expression of inflammatory cytokines, including TNF-, as well as growth factors, adhesion molecules, and transcription factors (7). TNF- may play a role in the development of diabetic nephropathy. Increased levels of the cytokines TNF- and IL-1 have been detected in isolated glomerular basement membranes from rats with STZ-induced diabetes (5). In patients with coronary artery disease and increased levels of inflammatory molecules including TNF-, treatment with an ARB reduced these markers of inflammation in the circulation (21). Antihypertensive therapy with both ACE inhibitors (in type 1 diabetes) and ARBs (in type 2 diabetes) has also had a beneficial effect in treating diabetic renal disease (19). In patients with diabetic nephropathy, ACE inhibitors and ARBs have been shown in clinical trials to be renoprotective independent of their ability to lower blood pressure (22, 23, 24, 25). In diabetic patients with advanced renal failure, levels of both TNF- and proteinuria were correlated. Treatment with pentoxifylline, an immune modulator with anti-TNF- activity, reduced proteinuria, possibly via its anticytokine activity (26).
The source of TNF- in RIF is not well established. Although TNF- can be produced by inflammatory cells, it can also be produced by the glomeruli, tubules, and blood vessels (27, 28, 29). The increase in RIF TNF- could reflect an increase in production or a decrease in degradation or clearance. In this study, we could not specifically distinguish between these possibilities. However, in our studies, there was a concomitant increase in urinary TNF- that suggests an increase in its production. The exact mechanism through which Ang II can stimulate TNF- production is not known. The increase in renal production of Ang II stimulates gene expression and, therefore, renal production of TNF-, as well as other proinflammatory mediators, including IL-1 and IL-6, possibly via activation of the transcription factor nuclear factor-B (27), release of prostaglandin E2 (29), and reduction of intracellular cAMP (30). This is consistent with our data showing reduction in RIF TNF- during AT1 receptor blockade. Insulin treatment over a 5-h time course lowered blood glucose levels to less than 100 mg/dl. This was associated with reduced recovery of Ang II and TNF- in RIF. These results support the hypothesis that the moderate hyperglycemia produced in our rat model using a relatively small dose of STZ stimulates renal production of Ang II, which in turn stimulates the AT1 receptor to increase TNF- production. In this study, due to the small volume of RIF that could be collected, we were not able to measure the levels of inflammatory cytokines other than TNF-; this is an area of future interest.
We show in this study that TNF- is increased in the diabetic kidney as a whole or at the level of the renal cortex, because the microdialysis probes for collection of RIF were placed in the renal cortices of the animals we studied. The administration of the ARB valsartan inhibited the diabetes-associated recovery of TNF-. This increase in TNF- must be mediated via the AT1 receptor, because valsartan decreased TNF- levels. In two previous studies in diabetic rats, elevated levels of TNF- mRNA were not decreased after treatment with an ACE inhibitor or ARB. However, these studies only looked at mRNA in isolated glomeruli and did not quantify actual TNF- production (31, 32). In Ang II-dependent hypertension, renal levels of TNF- are higher than those of normotensive controls (33). This is consistent with other studies reporting an increase in TNF- via the AT1 receptor (21).
The findings of the present study indicate that inflammatory cytokine TNF- may mediate the progression of diabetic kidney disease, at least partially, through stimulation of the AT1 receptor. This study also clearly demonstrates that hyperglycemia is a major stimulus for initiation of the renin-angiotensin cascade. Therapeutic modalities such as angiotensin receptor blockade may arrest this process. As we learned, administration of the ARB valsartan reduced the increase in TNF-, thereby protecting the kidney from inflammation. Other inflammatory factors, such as acute phase reactants, may also be implicated in the pathophysiology of diabetic nephropathy. A shift of focus from growth factors to the role of cytokines in diabetic nephropathy offers a new direction for research, perhaps into areas such as the link between change in TNF- levels and renal function and histology. Attempts to block that change, directly or indirectly, may even extend beyond incipient investigations into the use of ARBs as a therapeutic modality to the possibility of direct TNF antagonism to slow the progression of renal disease.
We conclude that in addition to its antihypertensive actions, valsartan seems to protect the kidney from inflammation. This antiinflammatory effect is not a result of lowering of blood pressure but is a result of direct action on cytokine TNF- at the level of the kidney.
|
Footnotes |
|---|
Abbreviations: ACE, Angiotensin-converting enzyme; Ang II, angiotensin II; ARB, AT1 receptor blocker; AT1, Ang II type 1; RAS, renin-angiotensin system; RIF, renal interstitial fluid; STZ, streptozocin.
Received January 6, 2003.
Accepted for publication March 11, 2003.
|
References |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
. Histol Histopathol 15:1303–1325[Medline]
-angiotensin interactions and regulation of blood pressure. J Hypertens 15:1481–1484[CrossRef][Medline]This article has been cited by other articles:
 |
|
 |
|
  T.-D. Liao, X.-P. Yang, Y.-H. Liu, E. G. Shesely, M. A. Cavasin, W. A. Kuziel, P. J. Pagano, and O. A. Carretero Role of Inflammation in the Development of Renal Damage and Dysfunction in Angiotensin II-Induced Hypertension Hypertension, August 1, 2008; 52(2): 256 - 263. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. M. Siragy and C. Xue Local renal aldosterone production induces inflammation and matrix formation in kidneys of diabetic rats Exp Physiol, July 1, 2008; 93(7): 817 - 824. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. C Parish and J. D Evans Inflammation in Chronic Heart Failure Ann. Pharmacother., July 1, 2008; 42(7): 1002 - 1016. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. M. Siragy and J. Huang Renal (pro)renin receptor upregulation in diabetic rats through enhanced angiotensin AT1 receptor and NADPH oxidase activity Exp Physiol, May 1, 2008; 93(5): 709 - 714. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. Kobori, M. Nangaku, L. G. Navar, and A. Nishiyama The Intrarenal Renin-Angiotensin System: From Physiology to the Pathobiology of Hypertension and Kidney Disease Pharmacol. Rev., September 1, 2007; 59(3): 251 - 287. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 N. R. Ferreri Estrogen-TNF interactions and vascular inflammation Am J Physiol Heart Circ Physiol, June 1, 2007; 292(6): H2566 - H2569. [Full Text] [PDF]
|
|
|
|
 |
|
 H. I. Abdullah, P. L. Pedraza, S. Hao, K. D. Rodland, J. C. McGiff, and N. R. Ferreri NFAT regulates calcium-sensing receptor-mediated TNF production Am J Physiol Renal Physiol, May 1, 2006; 290(5): F1110 - F1117. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. Liu, H. S. Bal, T. Desta, Y. Behl, and D. T. Graves Tumor Necrosis Factor-{alpha} Mediates Diabetes-Enhanced Apoptosis of Matrix-Producing Cells and Impairs Diabetic Healing Am. J. Pathol., March 1, 2006; 168(3): 757 - 764. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S. X. Zhang, J. J. Wang, K. Lu, R. Mott, R. Longeras, and J.-x. Ma Therapeutic Potential of Angiostatin in Diabetic Nephropathy J. Am. Soc. Nephrol., February 1, 2006; 17(2): 475 - 486. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Xue and H. M. Siragy Local Renal Aldosterone System and Its Regulation by Salt, Diabetes, and Angiotensin II Type 1 Receptor Hypertension, September 1, 2005; 46(3): 584 - 590. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. M. Siragy, C. Xue, P. Abadir, and R. M. Carey Angiotensin Subtype-2 Receptors Inhibit Renin Biosynthesis and Angiotensin II Formation Hypertension, January 1, 2005; 45(1): 133 - 137. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Endocrinology | Endocrine Reviews | J. Clin. End. & Metab. |
| Molecular Endocrinology | Recent Prog. Horm. Res. | All Endocrine Journals |
