DN is one of the major causes of end-stage renal failure. The mechanism of development of abnormal hemodynamics in this disorder is unclear. Clarification of the pathogenesis of DN and the development of novel and effective therapeutic strategies are therefore very important 14. This study aimed at studying the role of NO in early diabetic changes in the kidney and also at evaluating the efficacy of ABR using valsartan in the prevention of these changes.
In the present study, diabetic rats exhibited diffuse glomerular enlargement, mesangial expansion and congestion, obliteration of Bowman’s spaces, increased glomerular matrix, and thickened basement membrane of the parietal layer of Bowman’s capsule. Similar findings were reported in many previous studies. These changes were defined as glomerulosclerosis and may be attenuated by valsartan 15.
Consistent with the present findings, glomerulosclerosis has been reported to be a characteristic of experimental diabetic animals and humans with diabetes mellitus. Recent advances in renal pathophysiology strongly suggest that the diffuse expansion of the mesangial region may play a critical role in the obliteration of the capillary lumen, leading to a reduction in the surface area available for filtration and ultimate cessation of glomerular function in various forms of glomerulopathy including diabetic glomerulosclerosis 2,14. These alterations appear to be homologous to the early features found in the human diabetic kidney with increased NO level in which glomerular hypertrophy with mesangial expansion is claimed to be a central pathology 5.
The present study demonstrated that diabetes induced tubular changes in the form of tubular dilatation, vacuolization of lining cells, tubular casts in addition to vascular congestion, and slight fibrosis. Previous reports demonstrated similar findings 16.
The present study demonstrated eNOS expression both in the glomerular tuft and in the collecting tubules of the medulla of the control kidney. Previous reports demonstrated similar findings in which NO was mainly present in the renal medulla 4, whereas others reported that NO, a free radical in the form of a highly diffusible gas, exerts a wide spectrum of physiological actions, including the control of vascular tone, antithrombotic actions, cell cycle regulation, neurotransmission, signal transduction, and inflammation. There is now abundant evidence that physiological levels of NO have a crucial role in the maintenance of renal hemodynamics, renal perfusion, and glomerular filtration in the normal kidney 17.
In the present study, immunostaining intensity for eNOS was higher in diabetic rats than in control animals in the renal medulla. Moreover, the proximal and distal convoluted tubules developed positive immunoreactivity, which was decreased after valsartan treatment. Similar findings have been demonstrated by other authors 18.
NO produced by eNOS of the kidney could induce hyperfiltration and an increase in the glomerular volume; thus, NO was considered a potential candidate for mediating the early diabetes and vascular permeability 19.
Increased glomerular blood flow cannot necessarily increase intraglomerular pressure and therefore the diabetic kidney abnormally regulates intraglomerular pressure, with imbalance between afferent and efferent arteriolar vasodilatation, leading to increase in glomerular pressure and allowing systemic hypertension to be transmitted to the glomerulus 20.
Increased renal NO production could be explained by disturbance in the amino acid metabolism in diabetes. Elevated glucose induces increased intracellular levels of diacylglycerol, leading to activation of protein kinase C, which has been shown to activate NOS. This pathway may represent the missing link between hyperglycemia and hyperfiltration 21.
Oxidative and nitrosative stresses are involved in a nonenzymatic reaction between sugar and other carbonyl compounds with long-lived matrix proteins, leading to the formation of a group of molecules known as advanced glycosylation end products (AGEs). Each AGE structure has its own formation mechanism and thus its own dependence on oxidative stress 22.
Hyperglycemia, AGE, and diabetes create environments favoring extracellular matrix (ECM) deposition. Increased collagen fiber deposition was also demonstrated in the interstitial tissue of the cortex and around the collecting tubules 23.
Large amounts of NO have also been implicated in the renal abnormal vasodilatation with inflammation. NO produced in large amounts has been suggested to cause direct cytotoxic effects on the endothelium. Another effect of NO may be through DNA damage. The interaction of NO and the superoxide anion generates peroxinitrite, which induces lipid peroxidation and cytotoxicity 24,25.
In contrast to the implication of the present observations of reactive increase in NOS in diabetic rat kidney, many investigators have reported that endothelium-dependent, NO-mediated relaxation of the arteries has decreased in diabetic animals and inferred that NO production is decreased 26,27.
In the present study, we investigated the efficacy of early intervention with ARBs such as valsartan in preventing the development of diabetic renal changes in rats. Treatment was commenced immediately after diabetes induction and animals were followed up for 3 weeks. ARB slowed down mesangial expansion and reduced the development of glomerulosclerosis. Also, tubulointerstitial and medullary affection was minimized.
The present results are consistent with previous studies on diabetic and nondiabetic nephropathies. The studies on diabetic and nondiabetic renal disease have indicated that an initial reduction in proteinuria after the onset of antihypertensive medication predicts the long-term preservation of kidney function and that ARBs may have long-term renoprotective effect 28.
In agreement, ARBs significantly retarded the rate of loss of renal function in a group of patients with DN. This could be because ARBs may interfere with the trophic properties of angiotensin II to promote cellular and glomerular hypertrophy or diminish the accumulation of ECM. Either of these processes could be an important initial step leading to glomerular scarring 29.
The present study demonstrated that treatment with valsartan led to decreased NOS immunoreactivity. This result was consistent with previous studies showing increased expression of eNOS 2 weeks after STZ-induced diabetes and this was attenuated by ARB 11.
Angiotensin II has been reported to upregulate the in-vitro synthesis of NOS in various cell types, including glomerular endothelial cells, mesangial cells, and tubular epithelial cells. Therefore, blockade of angiotensin II receptors by ARB may decrease the generation of NO 30,31.
In summary, this study indicates that the increase in eNOS expression in the kidney possibly participates in the regulatory pathways activated by diabetes mellitus. Intervention studies to block eNOS are needed to address this hypothesis directly. Also, the present results suggest that ARBs represent valuable drugs in the treatment of DN. More investigations are needed to clarify the complex pathophysiology for further therapeutic intervention.
Conflicts of interest
There are no conflicts of interest.
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Keywords:© 2014 The Egyptian Journal of Histology
eNOS; diabetes; kidney; Valsartan