Effects of age and dietary restriction on the kidney glomeruli of mice: observations by scanning electron microscopy

Dietary restriction regimens for fighting kidney disease: Insights from rodent studies

This review critically discusses the research findings on the effects of various dietary restriction regimens in rodent models of kidney disease. Long-term caloric restriction executed at both early and progressive stages of kidney disease was found to exert beneficial effects in rodents. Moreover, some studies have also demonstrated the efficacy of short-term caloric restriction in treating the kidney disease of variable aetiologies possibly by improving mitochondrial dysfunction, autophagy process and suppression of inflammation. However, the mechanisms underlying these short-term caloric restriction mediated protective effects in rodent models of kidney disease are not completely understood. Importantly, few available evidences have also suggested that carbohydrate restriction can exert beneficial effects in aging and experimentally induced renal injury models, but the mechanisms are not explored yet. Interestingly, the benefits of low protein diet in kidney disease models are extensively reported in literature. However, in most of these studies implementation of the low protein dietary regimen was found to associated with increased high carbohydrate and caloric intake (non-isocaloric). Thus, testing the effects of low protein diet under isocaloric conditions might further help to particularly understand the role of dietary protein content in pathology of kidney disease. Moreover, the direct evidences comparing the efficacy of various dietary restriction regimens in rodent models of kidney diseases are also scarce at present.

The effects of dietary restriction on oxidative stress in rodents

Oxidative stress is observed during aging and in numerous age-related diseases. Dietary restriction (DR) is a regimen that protects against disease and extends life span in multiple species. However, it is unknown how DR mediates its protective effects. One prominent and consistent effect of DR in a number of systems is the ability to reduce oxidative stress and damage. The purpose of this review is to comprehensively examine the hypothesis that dietary restriction reduces oxidative stress in rodents by decreasing reactive oxygen species (ROS) production and increasing antioxidant enzyme activity, leading to an overall reduction of oxidative damage to macromolecules. The literature reveals that the effects of DR on oxidative stress are complex and likely influenced by a variety of factors, including sex, species, tissue examined, types of ROS and antioxidant enzymes examined, and duration of DR. Here we present a comprehensive review of the existing literature on the effect of DR on mitochondrial ROS generation, antioxidant enzymes, and oxidative damage. In a majority of studies, dietary restriction had little effect on mitochondrial ROS production or antioxidant activity. On the other hand, DR decreased oxidative damage in the majority of cases. Although the effects of DR on endogenous antioxidants are mixed, we find that glutathione levels are the most likely antioxidant to be increased by dietary restriction, which supports the emerging redox-stress hypothesis of aging.

The aging kidney: role of endothelial oxidative stress and inflammation

The population in the Western world is aging. In 1996 those aged 60 years and over formed 21% of the EU population, by 2022 this proportion will have risen to 27%. Based on current trends a third of the EU population could be 60 years of age and over by the age 2050. Epidemiological studies suggest that even in the absence of other risk factors (e.g. diabetes, hypertension, hypercholesterolemia), advanced age itself significantly increases cardiovascular morbidity by promoting the development of atherosclerosis and by impairing normal cellular functions. One of the most prominent organs affected by aging is the kidney. There is evidence that age-associated phenotypic changes may be an important cause of renal failure. We propose that vascular oxidative stress and inflammation are generalized phenomena during senescence, which importantly contribute to the morphological and functional changes in the aging kidney. The present review focuses on some of the mechanisms by which advanced age may promote vascular oxidative and nitrosative stress and the possible downstream mechanisms by which reactive oxygen and nitrogen species may impair vascular and renal function in aging.

Morphological alterations of the glomerular (visceral) epithelium in response to pathological and experimental situations

The glomerular (visceral) layer of Bowman's capsule is comprised of a unique population of cells which have been termed "podocytes." Arising from these cells are large major processes and numerous smaller foot processes which completely surround underlying glomerular capillary loops. Podocyte foot processes interdigitate with each other and are separated by spaces (filtration slits) which are designed to facilitate flow of a large amount of filtrate across the glomerular wall. Podocytes exhibit dramatic morphological changes in response to the nephrotic syndrome and some forms of acute renal failure and may play an important role in the pathophysiology of these conditions. In vitro and in vivo studies have shown that a reduction in the sialic acid component of a thick anionic surface coat plays a major role in the morphological changes that these cells exhibit in the nephrotic syndrome. Also, it has been shown that filamentous actin concentrated mainly within podocyte foot processes are the contractile elements responsible for altering the shapes of these processes. There is evidence to suggest that by altering the shapes of their foot processes, podocytes in the normal kidney are able to alter the number of fully patent filtration slits and thereby actively regulate the rate of solute efflux across the glomerular wall. In vitro and in vivo studies have indicated that cytoplasmic microtubules are probably not involved in alterations of the podocyte foot processes but do appear important in maintaining the morphological integrity of podocyte cell bodies and their major processes. In the present paper, the morphological changes which glomerular podocytes exhibit in response to the nephrotic syndrome, various forms of acute renal failure, and during in vitro incubation are discussed along with studies of the possible roles of cytoplasmic microtubules, microtubules, and the glomerular anionic surface coat in these changes.

Long-term protein and calorie restriction: alterations in nucleic acid levels of organs of male rats

The effects of dietary restriction (calorie and/or protein) imposed at weaning on tissue concentrations of nucleic acids and protein were determined in young (one year) and old (two year) rats. In liver, calorie restriction increased protein concentration and decreased RNA/DNA and RNA/protein, while protein restriction elevated liver DNA and reduced protein and protein/DNA. These changes are consistent with decreased liver cell size in response to protein, but not calorie, restriction. Age also influenced these parameters, since DNA (elevated), RNA/protein (elevated), protein (reduced), and protein/DNA (reduced) differed in livers of old as compared to young rats. In kidney, DNA concentrations were unaffected by age or dietary restriction. Although protein and nucleic acid concentrations are commonly used as baselines for expressing other biochemical parameters, our results indicate that in liver, these factors vary both with age and nutritional status.

Effects of morphine on rat kidney glomerular podocytes: a scanning electron microscopic study

Degenerative kidney changes are associated with heroin use in human addicts, but it is not known whether these changes result from exposure to the opioid or from contaminants in street heroin. In the present study, 4-6-month-old rats each received 1 subcutaneous pellet containing 75 mg of morphine or placebo, followed 3 days later by implantation of 2 additional morphine or placebo pellets. Seven days after implantation of the first pellet, the rats were killed by aldehyde perfusion. The right kidney was excised, and coronal slices were prepared for scanning electron microscopy. Micrographs were taken at 5000X and were scored on the presence of short or long microprojections (a score of '1' indicating few and a score of '4' indicating many). Morphine significantly altered the frequencies of scores for long microprojections, suggesting that morphine treatment increased the number of microprojections on glomerular podocytes. No changes in filtration slits, pedicels, or blebbing (foval enlargements) were noted. The data support the view that kidney degeneration associated with opioid abuse reflects effects of opioids per se, and they are consistent with microprojection changes as a function of altered intracellular cyclic AMP levels.

Chronic effects of dietary protein in the rat with intact and reduced renal mass

The chronic effects of dietary protein on renal structure and function were studied in rats with normal and reduced renal mass. Control rats with two kidneys were compared with unilaterally nephrectomized rats, and with one and one-third nephrectomized rats obtained by unilateral nephrectomy and infarction of one-third of the remaining kidney. Rats at each level of renal mass were maintained on chow containing either 6% or 40% protein content. Separate cohorts of rats were studied four and eight months after ablation and institution of these dietary regimens. At both time intervals and at all levels of renal mass, rats fed the high protein diet had higher average values for GFR than comparable animals fed the low protein chow. Within each of the dietary regimens the animals with loss of renal mass developed greater prevalences of sclerotic glomeruli by eight months. Furthermore, at each level of initial renal mass, rats eating the high protein diet had a greater prevalence of sclerotic glomeruli than those on the low protein diet. Similarly, rats on the high protein diet had greater rates of protein excretion than those on the low protein diet at each degree of ablation. The prevalence of sclerosed glomeruli increased between four and eight months in each group. Thus, the extent of renal injury as manifested by proteinuria and glomerular sclerosis was directly related to the degree of initial loss of renal mass, and dietary protein restriction retarded these manifestations of injury across a wide range of initial renal mass.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of aging on the renal glomerulus

The biologic price of aging includes progressive deterioration of renal function and structure. After the age of 30, glomerular filtration and renal blood flow rates decline in a linear fashion, so that values in octagenarians are only half to two thirds those measured in young adults. Renal mass similarly declines, and the incidence of sclerotic glomeruli increases with advancing age. Accordingly, the aging kidney is at high risk of eventual failure when functioning nephron number is further reduced by acquired renal disease. Recent evidence suggests that limitation of dietary protein intake delays the development of age- and disease-related glomerular sclerosis in experimental animals, and that dietary protein restriction may postpone end-stage renal disease in patients with progressive renal insufficiency.

Anatomic and physiologic age changes in the kidney

Cross-sectional studies of renal function in man indicate there is a progressive decline with age after the age of 40 years. The blood vessels, glomeruli, tubules and interstitium are all potential sites of primary involvement in the aging process as well as for renal disease. Regardless of the anatomic structure initially affected, most chronic renal conditions evolve with destruction of the entire nephron. Whether the observed decrease in renal function associated with aging is the result of intervening pathologic processes, e.g. ischemia (vascular obliteration) or infection, or is the result of a more insidious involutional process, it has generated much discussion but few answers. The purpose of this report is to review the descriptive studies documenting the changes in renal morphology and physiology with age and to focus on what is known about the mechanisms involved in these losses of renal substances and function.

An electron microscopic examination of age-related changes in the rat kidney: the influence of diet

The changes with age in the ultrastructure of the kidneys were explored in ad libitum fed rats with restricted food intake started soon after weaning or started in young adult life or limited to early life and in rats restricted in protein but not caloric intake. Many ultrastructural changes occurred with age both in the glomeruli and the tubules. Food restriction started soon after weaning or in adult life modulated most of these age changes. By providing detailed information on basement membrane and tubular cell structure, these findings complement previous light microscopic and functional studies in regard to the effects of food restriction on progressive kidney disease in the rat. Food restriction limited to early life and protein restriction without caloric restriction were less effective in modulating these age changes in kidney ultrastructure than food restriction initiated at 6 weeks or 6 months of age and continued for the rest of the life span.

The hyperfiltering glomerulus

The observation has been made that after any of a variety of initial renal injuries, nephron units that have been spared undergo structural and functional compensations. The functional compensation of increased perfusion of residual nephrons may present deleterious, maladaptive stresses to those surviving nephrons and lead to their ultimate destruction. This hypothesis provides a final common pathway for the progression of many different renal diseases to renal failure and explains the tendency of renal insufficiency to progress rather than stabilize.

Differential effects of dietary caloric and protein restriction in the aging rat

Numerous studies have shown caloric restriction retards the physiological decline and increases the life span of animals. However, in these studies protein consumption was also reduced; thus, whether the beneficial effects were due to caloric or to protein restriction is unclear. To examine independently the effects of caloric and protein restriction on growth, renal function, and survival, caloric restricted male rats were fed 18, 30 or 42 percent casein diets that provided two-thirds of the quantity of diet consumed by groups fed 12, 20, or 28 percent casein diets ad libitum, respectively. Hence, caloric restricted groups consumed the same amount of protein as their paired ad libitum fed groups but one-third fewer calories. The results showed that caloric restriction decreased mature body weight, increased the rate of attaining mature body weight, retarded the age-associated decline in renal function, and increased survival. Protein restriction had no effect on mature body weight, decreased maturation rate, improved renal function, and decreased survival. Thus, protein restriction did not contribute to the survival-promoting effects of caloric restriction in rats.

Effects of hypophysectomy on age-related changes in the rat kidney glomerulus: observations by scanning and transmission electron microscopy

The kidney glomeruli of 9-month-old intact, 23-month-old intact, and 23-month-old hypophysectomized female rats were examined by scanning and transmission electron microscopy. With increasing age, the glomeruli increased in diameter and more podocyte microvilli were found. Hypophysectomy (4 months before sacrifice) reduced these values to levels approaching values seen in the 9-month intact controls. Transmission electron microscopy revealed that increased numbers of podocytes and endothelial cells had cytoplasmic dense bodies with advancing age. Granular cytoplasmic material, seen in podocytes, did not alter in frequency with age. Hypophysectomy reduced the number of cells containing the dense bodies as well as the number of podocytes containing granular material. Basal laminar thickness, while increasing with age, was unaffected by hypophysectomy. The results show that hypophysectomy can return some structural age-related changes to values seen in younger subjects. The effects are similar in some respects to the effects of dietary restriction.