Renal hemodynamics in Otsuka Long-Evans Tokushima fatty rat, a model rat of spontaneous non-insulin-dependent diabetes mellitus with obesity

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.

Histopathological analyses of diabetic nephropathy in sucrose-fed Otsuka Long-Evans Tokushima fatty rats

BACKGROUND

Otsuka Long-Evans Tokushima fatty (OLETF) rats are an established model of diabetic nephropathy. However, diabetes and diabetic nephropathy (DN) in OLETF rats develop later than in other animal type 2 diabetes models.

OBJECTIVES

This study was conducted to investigate the serial changes in the histopathological characteristics of DN in sucrose-fed OLETF rats by biochemical and morphometric analyses.

METHODS

We conducted sucrose feeding to examine the progression of DN. One group of OLETF rats was given water containing 30% sucrose ad libitum (SO) and the other group was given water without 30% sucrose (TO). Consecutive observations were made at 4-week intervals from 16 to 50 weeks of age in TO rats, and from 16 to 42 weeks of age in SO rats. Examination parameters included body weight, serum glucose level, urine albumin-to-creatinine ratio (UACR), light microscopy (LM) and electron microscopy (EM).

RESULTS

The UACR was over 300 mg/g in 32-week-old SO rats (after 16 weeks of sucrose feeding) and in 38-week-old TO rats. LM indicated that glomerular hypertrophy and mesangial matrix expansion in SO rats increased compared to that of age-matched TO rats especially at 42 weeks of age (p < 0.05). EM also showed that glomerular basement membrane thickness and podocyte foot process width of SO rats were significantly greater than those of age-matched TO rats (p < 0.05).

CONCLUSION

Our results suggested that dietary manipulation by sucrose feeding may cause deterioration of DN and could hasten the onset of diabetes and DN in OLETF rats.

Renal vascular structural properties and their alterations by removal of uraemic toxins in a rat model of chronic kidney disease

1. Renal vascular structural properties and their alterations by removal of uraemic toxins with AST-120, an oral adsorbent, were examined in subtotal nephrectomized rats. 2. Eight- or 9-week-old Sprague-Dawley rats received 3/4 nephrectomy (n = 18) and thereafter were fed 24.5% protein diet with (AST; n = 9) or without (AST-; n = 9) AST-120 (0.4 g/100 g bodyweight). Sham-operated rats (Sham; n = 9) received the diet without AST-120. At 21-22 weeks of age, flow-pressure (F-P) and pressure-glomerular filtration rate (P-GFR) relationships were determined for maximally vasodilated, perfused kidneys. 3. The gradient of F-P (minimal renal vascular resistance reflecting the overall luminal dimensions of pre- and post-glomerular vasculature) was lower in AST- than Sham rats. In contrast, the x-intercept (preglomerular : post-glomerular vascular resistance ratio) and gradient (glomerular filtration capacity) of P-GFR did not differ between the two groups. The vascular wall and lumen at the interlobular arteries were greater in AST- than Sham rats. 4. Although the vascular wall and lumen at the interlobular arteries were less in AST than in AST- rats, the gradient of F-P and the x-intercept of P-GFR did not differ between the two groups. In contrast, the glomerular filtration capacity was greater in AST than AST- rats. 5. In conclusion, the lumen of both pre- and post-glomerular resistance vessels increased and glomerular filtration capacity failed to increase in subtotal nephrectomized rats. Uraemic toxins could play an important role in the development of structural alterations in glomeruli rather than renal resistance vessels in chronic kidney disease.

The biological consequence of obesity on the kidney

Obesity is a worldwide epidemic that is associated with several health issues, including kidney diseases. A specific kidney disease, referred to as obesity-related glomerulopathy, has been described in earlier publications. Obesity can affect the prognosis of other types of kidney diseases. Body-weight reduction with caloric restriction is an essential therapy, although strictly controlling food intake coupled with an appropriate evaluation is challenging. Low birthweight can be an important factor for obesity and results in kidney dysfunction. In this review, we analyse the consequences of obesity on kidney prognosis and potential strategies for combating obesity-associated kidney injury.

Impaired vascular responses to relaxin in diet-induced overweight female rats

Relaxin mediates renal and mesenteric vascular adaptations to pregnancy by increasing endothelium-dependent vasodilation and compliance and decreasing myogenic reactivity. Diet-induced overweight and obesity are associated with impaired endothelial dysfunction and vascular remodeling leading to a reduction in arterial diameter. In this study, we tested the hypothesis that local vascular responses to relaxin are impaired in diet-induced overweight female rats on a high-fat cafeteria-style diet for 9 wk. Rats were chronically infused with either relaxin or placebo for 5 days, and vascular responses were measured in isolated mesenteric arteries and the perfused kidney. Diet-induced overweight significantly increased sensitivity to phenylephrine (by 17%) and vessel wall thickness, and reduced renal perfusion flow (RPFF; by 16%), but did not affect flow-mediated vasodilation, myogenic reactivity, and vascular compliance. In the normal weight rats, relaxin treatment significantly enhanced flow-mediated vasodilation (2.67-fold), decreased myogenic reactivity, and reduced sensitivity to phenylephrine (by 28%), but had no effect on compliance or RPFF. NO blockade by l-NAME diminished most relaxin-mediated effects. In diet-induced overweight rats, the vasodilator effects of relaxin were markedly reduced for flow-mediated vasodilation, sensitivity to phenylephrine, and myogenic response compared with the normal diet rats, mostly persistent under l-NAME. Our data demonstrate that some of the vasodilator responses to in vivo relaxin administration are impaired in isolated mesenteric arteries and the perfused kidney in diet-induced overweight female rats. This does not result from a decrease in Rxfp1 (relaxin family peptide receptor) expression but is likely to result from downstream disruption to endothelial-dependent mechanisms in diet-induced overweight animals.

Abnormal autoregulation and tubuloglomerular feedback in prediabetic and diabetic OLETF rats

The mechanisms underlying the development and prevention of diabetic nephropathy are still not fully understood. In the present study in the Otsuka Long-Evans Tokushima Fatty (OLETF) model of type 2 diabetic rats, we investigated whether renal hemodynamic abnormalities exist and whether they precede the onset of diabetes. Using OLETF rats in both prediabetic and diabetic stages, we assessed autoregulatory responses of total renal blood flow (RBF) and of superficial (SBF) and deep renal cortical (DBF) blood flow to stepwise reductions of renal perfusion pressure (RPP) induced by a manual clamp on the abdominal aorta. During clamp-induced reductions of RPP by 10 or 20 mmHg, RBF fell significantly more in OLETF rats than in lean control [Long-Evans Tokushima Otsuka (LETO)] rats. Whereas SBF showed no significant changes in either OLETF rats or LETO rats during mild clamping, DBF decreased significantly more in OLETF rats than LETO rats. Reduced autoregulatory efficiency in OLETF rats was observed in both prediabetic and diabetic stages. Micropuncture studies showed that tubuloglomerular feedback (TGF) responses of stop flow pressure are reduced in prediabetic (-7.3 vs. -25.7%) as well as in diabetic OLETF rats compared with LETO control rats (-4.4 vs. -18.8%). Renal corticotomy was performed to measure glomerular capillary pressure (Pgc) directly. Pgc of deep cortical glomeruli was higher than superficial glomerular Pgc in both strains of rats, but the difference was especially pronounced in OLETF rats (deep 78 +/- 2 vs. superficial 57 +/- 4 mmHg). This study demonstrates reduced autoregulatory adjustments and impaired TGF efficiency in prediabetic OLETF rats. Thus abnormal RBF regulation precedes the onset of diabetes and is especially pronounced in the deep cortical region.

Abnormal renal structural alterations during the development of diabetes mellitus in Otsuka Long-Evans Tokushima Fatty rats*

AIM

The aim of this study was to investigate the renal structural properties in diabetic nephropathy.

METHODS

Flow-pressure and pressure-glomerular filtration rate (GFR) relationships were determined for maximally vasodilated kidneys at 10 (pre-diabetic stage) and 42 weeks of age (diabetic stage) in Otsuka Long-Evans Tokushima Fatty rats (OLETF), an animal model of type 2 diabetes mellitus, using age-matched Long-Evans Tokushima Otsuka rats (LETO) as non-diabetic controls (n = 9 of each age for each strain). Kidneys were then perfusion-fixed for histological analysis.

RESULTS

At 10 weeks of age, the slope of flow-pressure relationship (minimal renal vascular resistance, reflecting overall luminal dimensions of preglomerular and postglomerular vasculature) was steeper in OLETF than in LETO. In contrast, the threshold pressure for beginning filtration (preglomerular-to-postglomerular vascular resistance ratio) at pressure-GFR relationship did not differ between the two strains; however, the slope of the relationship (glomerular filtration capacity) was lower in OLETF than in LETO. Thus, in the kidneys of 10-week-old OLETF rats, vascular narrowing and impaired glomerular filtration capacity already existed with no abnormalities in preglomerular-to-postglomerular vascular resistance ratio. From the age of 10-42 weeks, the following results were obtained: (1) Minimal renal vascular resistance decreased in both strains, but it diminished markedly in OLETF. (2) The pressure for beginning filtration increased in LETO, but remained unchanged in OLETF. (3) Glomerular filtration capacity decreased to the similar extent in both strains. (4) Histologically, the vascular lumen and wall thickness increased in the interlobular arteries of both strains. However, vascular luminal widening was more pronounced in OLETF, resulting in the reduction in wall to lumen ratio. (5) Glomerular injuries and increased blood pressure occurred only in OLETF.

CONCLUSION

In conclusion, during progression from the prediabetic to diabetic stage of OLETF, the pre-existing vascular narrowing was markedly attenuated without the concomitant increase in preglomerular-to-postglomerular vascular resistance ratio. Combined with increased blood pressure, these renal structural alterations could lead to the elevation of intraglomerular pressure in OLETF.

Global heterogeneity of glomerular volume distribution in early diabetic nephropathy

BACKGROUND

Morphologic characteristics in early stage of nephropathy of noninsulin-dependent diabetes mellitus (NIDDM) have not been determined despite the fact thatdiagnosis in this stage of the disease is important for the prognosis. We hypothesized that heterogeneity in glomerular volume-distribution may be a sensitive index of early stage of diabetic nephropathy in NIDDM.

METHODS

In spontaneous diabetic rats [Otsuka Long-Evans Tokushima Fatty (OLETF) rat (N= 5)] of 27 to 28 weeks, an experimental model of early diabetic nephropathy in human NIDDDM and age-matched control rats [Long Evans Tokushima Lean (LETO) rat (N= 5)], we completely filled the kidney with contrast medium. Glomeruli were visualized as three-dimensional images using x-ray micro-computed tomography (micro-CT). Glomerular volumes (N= 400 in each kidney) were directly measured and evaluated as absolute volume and normalized values to kidney weight and body weight. Scattering of glomerular volume-distribution was evaluated as coefficient variation (CV) (SD/mean).

RESULTS

The CV was significantly larger in OLETF rat (0.195) comparing to LETO rat (0.146, P < 0.01). This difference was even consistent under the normalization to kidney weight and body weight. Absolute glomerular volume was larger in OLETF rat compared to LETO rat (P < 0.005); however, when glomerular volume was normalized, this variable was comparable between two groups.

CONCLUSION

We visualized three-dimensional glomerular images in the early stages of diabetic nephropathy using micro-CT and quantified the heterogeneity in glomerular volume distribution throughout the cortex by direct measurement of the individual. We propose that heterogeneity in glomerular volume distribution is a sensitive parameter to ascertain early diabetic nephropathy in NIDDM.

Calorie restriction in obesity: prevention of kidney disease in rodents

The incidence of end-stage renal disease (ESRD) has risen considerably in the past two decades. This trend is partly due to the alarming rise in the incidence of type 2 diabetes over the same period, which in turn might be linked to the staggering increase in overweight and obesity. If these trends continue, ESRD can be expected not only to cause suffering of ever growing numbers of patients, but also to become an increasing financial as well as logistical burden on the health care system. Therefore, it is imperative not only to gain a better understanding of the molecular, cellular and metabolic mechanisms involved in renal pathology, but also to uncover treatment modalities, including lifestyle changes, that can help prevent and/or slow the progression of kidney pathogenesis. Insights into both of these aspects are provided by animal models of obesity and diabetes. It has long been known that food restriction, more so than restriction of any particular dietary component, can greatly enhance longevity in laboratory rodents. These findings are being extended into a variety of other mammals, including nonhuman primates. These studies have indicated that caloric restriction in nonobese laboratory animals does not primarily affect specific disease processes but rather nonspecifically slows the aging process. In contrast, a growing body of evidence suggests that in genetically obese animals, food restriction can prevent or greatly delay the onset of specific degenerative lesions, in particular glomerulonephritis associated with obesity and diabetes.