The mechanisms of proteinuria in aging rats

Transgenic angiotensin-converting enzyme 2 overexpression in the rat vasculature protects kidneys from ageing-induced injury

Chronic kidney disease is one of the leading causes of morbidity and mortality especially among the aged population. A decline in kidney function with ageing comparable to ageing-related processes in human kidneys has also been described in Sprague-Dawley (SD) rats. The renin-angiotensin-system (RAS) plays a pivotal role in the pathophysiology of cardiovascular and kidney disease and is a successful therapeutic target. The discovery of angiotensin-(1-7) (Ang(1-7)), mainly produced by angiotensin-converting enzyme 2 (ACE2), and its receptor MAS offered a new view on the RAS. This ACE2/Ang(1-7)/MAS axis counteracts most deleterious actions of the RAS in the kidney. In order to evaluate if activation of this axis has a protective effect in ageing-induced kidney disease we generated a transgenic rat model (TGR(SM22hACE2)) overexpressing human ACE2 in vascular smooth muscle cells. These animals showed a specific transgene expression pattern and increased ACE2 activity in the kidney. Telemetric recording of the cardiovascular parameters and evaluation of kidney function by histology and urine analysis revealed no alterations in blood pressure regulation and basal kidney function in young transgenic rats when compared to young SD rats. However, with ageing, SD rats developed a decline in kidney function characterized by severe albuminuria which was significantly less pronounced in TGR(SM22hACE2) rats. Concomitantly, we detected lower mRNA expression levels of kidney damage markers in aged transgenic animals. Thus, our results indicate that vascular ACE2-overexpression protects the kidney against ageing-induced decline in kidney function supporting the kidney-protective role of the ACE2/Ang(1-7)/MAS axis.

Sex- and age-related differences in arterial pressure and albuminuria in mice

Background

Animal models have become valuable experimental tools for understanding the pathophysiology and therapeutic interventions in cardiovascular disease. Yet to date, few studies document the age- and sex-related differences in arterial pressure, circadian rhythm, and renal function in normotensive mice under basal conditions, across the life span. We hypothesized that mice display similar sex- and age-related differences in arterial pressure and renal function to humans.

Methods

Mean arterial pressure (MAP) and circadian rhythm of arterial pressure were measured over 3 days via radiotelemetry, in 3- and 5-month-old (adult) and 14- and 18-month-old (aged) FVB/N and in 5-month-old (adult) C57BL/6 male and female normotensive mice. In FVB/N mice, albuminuria from 24-h urine samples as well as body, heart, and kidney weights were measured at each age.

Results

Twenty-four-hour MAP was greater in males than females at 3, 5, and 14 months of age. A similar sex difference in arterial pressure was observed in C57BL/6 mice at 5 months of age. In FVB/N mice, 24-h MAP increased with age, with females displaying a greater increase between 3 and 18 months of age than males, such that MAP was no longer different between the sexes at 18 months of age. A circadian pattern was observed in arterial pressure, heart rate, and locomotor activity, with values for each greater during the active (night/dark) than the inactive (day/light) period. The night-day dip in MAP was greater in males and increased with age in both sexes. Albuminuria was greater in males than females, increased with age in both sexes, and rose to a greater level in males than females at 18 months of age.

Conclusions

Arterial pressure and albuminuria increase in an age- and sex-specific manner in mice, similar to patterns observed in humans. Thus, mice represent a useful model for studying age and sex differences in the regulation of arterial pressure and renal disease. Understanding the mechanisms that underlie the pathophysiology of cardiovascular disease may lead to new and better-tailored therapies for men and women.

Protective role of female gender in programmed accelerated renal aging in the rat

The aging kidney exhibits a progressive decline in glomerular filtration rate, accompanied by inflammatory and oxidative damage. We hypothesized that accelerated, age-related progression of renal injury is ovarian hormones-dependant. To address this we used an established model of developmentally programmed accelerated renal aging in the rat, superimposed by ovariectomy to assess interactions between ovarian hormones and the aging process. Under our experimental conditions, we found that kidney function worsens with age, that is GFR reduces over 18 month analyzed time-course and this was worsened by fetal exposure to maternal low-protein diet and absence of estrogen. Reduction in GFR was followed by increases in albuminuria, proteinuria, inflammatory markers, and tissue carbonyls, all suggesting inflammatory response and oxidative stress. This was associated with changes in AGTR2 expression which was greater at 18 months of age compared to earlier time points, but in MLP offspring only. Our studies show an influence of ovarian hormones on programmed accelerated renal aging and the AGTR2 across the lifespan. The main findings are that ovariectomy is a risk factor for increased aging-related renal injury and that this and oxidative damage might be related to changes in AGTR2 expression.

Proteome profiling of aging in mouse models: differential expression of proteins involved in metabolism, transport, and stress response in kidney

Aging is a time-dependent complex biological phenomenon observed in various organs and organelles of all living organisms. To understand the molecular mechanism of age-associated functional loss in aging kidneys, we have analyzed the expression of proteins in the kidneys of young (19-22 wk) and old (24 months) C57/BL6 male mice using 2-DE followed by LC-MS/MS. We found that expression levels of 49 proteins were upregulated (p < or = 0.05), while that of only ten proteins were downregulated (p < or = 0.05) due to aging. The proteins identified belong to three broad functional categories: (i) metabolism (e.g., aldehyde dehydrogenase family, ATP synthase beta-subunit, malate dehydrogenase, NADH dehydrogenase (ubiquinone), hydroxy acid oxidase 2), (ii) transport (e.g., transferrin), and (iii) chaperone/stress response (e.g., Ig-binding protein, low density lipoprotein receptor-related protein associated protein 1, selenium-binding proteins (SBPs)). Some proteins with unknown functions were also identified as being differentially expressed. ATP synthase beta subunit, transferrin, fumarate hydratase, SBPs, and albumin are present in multiple forms, possibly arising due to proteolysis or PTMs. The above functional categories suggest specific mechanisms and pathways for age-related kidney degeneration.

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.

The rat glomerular filtration barrier does not show negative charge selectivity

OBJECTIVE

To characterize the effects of size, shape, and negative charge on the transport of macromolecules across the glomerular capillary wall by using the sieving curves (fractional clearance vs. solute molecular radii) of fluorescent polydispersed polysaccharide tracers.

METHODS

Glomerular fractional clearances (FC) were measured with fluorescent neutral [isoelectric point (pI) = 7.3 +/- 0.2] and negatively charged (pI = 3.5 +/- 0.4) dextrans (DEX) in comparison with negatively charged (pI = 4.8 +/- 0.3) hydroxy ethyl starch (HES) and (pI = 4.6 +/- 0.1) bovine serum albumin (BSA) in Sprague-Dawley and Fischer 344/Brown Norway rats. FCs (n = 53) were measured by using the urinary clearance of (14)C-inulin to determine the glomerular filtration rate. The relative uptake of each fluorescent probe by endothelial and renal proximal tubule epithelial (LLC-PK(1)) cells, in vitro, was measured microscopically by using a cooled (-25 degrees C) CCD camera.

RESULTS

The sieving curves for randomly coiled neutral and negatively charged DEX probes were identical. These FC values were 6-fold greater than those for HES and 200-fold above similarly sized fluorescent BSA. The polysaccharide probes did not show significant binding to serum proteins. The uptake of BSA by LLC-PK(1) cells was 20- to 100-fold greater than that for neutral or negatively charged macromolecules.

CONCLUSIONS

These findings indicate that the rat glomerular filtration barrier restricts the transport of polysaccharide macromolecules as a function of their size and configuration but not negative charge.

Age dependency of renal function in CD-1 mice

Renal function was studied in mice of different ages. In metabolic cage experiments, the renal electrolyte excretion was similar in young (n = 8; 5- to 7-wk-old) and adult (n = 6; 20- to 22-wk-old) CD-1 (ICR) BR mice, whereas spontaneous drinking volume and urinary flow rate were significantly higher in the adult compared with the young mice. Subsequently, the renal functional reserve was investigated by amino acid (AA) infusion (10%) in anesthetized young (n = 8) and adult (n = 6) mice. Because the body weight of adult mice was significantly higher than that of young animals, one group of adult mice (n = 8) received 12.5% AA to ensure that the dose of AA related to body weight was similar in both groups. Young animals constantly infused with Ringer solution served as time controls (n = 8). Glomerular filtration rate (GFR) at baseline was similar in each group. Because of AA, GFR significantly increased in young mice but not in both groups of adult animals, whereas in time controls GFR remained constant. Urinary flow rate and sodium excretion were elevated by AA in young and adult mice. We conclude that in CD-1 mice the first signs of age-related changes in kidney function concern alterations in renal hemodynamics, whereas renal tubular function appears to be preserved.

Intrauterine food restriction as a determinant of nephrosclerosis

We previously showed that 3-month-old rats subjected to a 50% intrauterine food restriction had a decreased number of nephrons with increased glomerular diameter, which suggests compensatory hypertrophy. Hypertrophy could be the early event of glomerular damage. In this study, we extended our investigation and performed functional, morphological, and immunohistochemical evaluations in 3- and 18-month-old rats that underwent a 50% intrauterine food restriction (RT3 and RT18, respectively) and age-matched control rats (C3 and C18, respectively). Our findings showed that glomerular filtration rate was significant decreased in RT18 rats (2.42 +/- 0.15 mL/min/kg; n = 28; P: < 0.05) compared with C18 control rats (4.19 +/- 0.10 mL/min/kg; P: < 0.05) and the percentage of glomeruli with sclerosis was greater in RT18 rats (13.01% +/- 2.95%; n = 9; P: < 0.01) than in C18 rats (2.71% +/- 0.35%; n = 6). RT18 rats also showed more intense tubulointerstitial lesions and immunohistochemical alterations in the renal cortex. Immunohistochemical studies showed increased fibronectin and desmin expression in glomeruli and tubulointerstitium and increased vimentin and alpha-smooth muscle actin in the tubulointerstitial area from the renal cortex of RT18 rats (P: < 0.05). Desmin was also increased at the edge of glomeruli from RT18 rats, suggesting podocyte injury. Our data show that when food restriction is imposed during pregnancy, permanent damage occurs in the kidney of the offspring. Glomerular lesions were more severe than the tubulointerstitial damage in these animals.

Effects of dietary lipids on renal function of aged rats

Normal aging is accompanied by renal functional and morphological deterioration and dietetic manipulation has been used to delay this age-related decline. We examined the effects of chronic administration of diets containing 5% lipid-enriched diet (LD, w/w) on renal function of rats at different ages. Three types of LD were tested: canola oil, fish oil and butter. Mean systemic tail-cuff blood pressure and glycemia remained within the normal range whatever the age and the diet of the animals. Proteinuria began to rise from the 8th month in the groups ingesting LD, while in the control group it increased significantly (above 10 mg/24 h) only after the 10th month. With age, a significant and progressive decline in glomerular filtration rate (GFR) and renal plasma flow was observed in the LD groups but after 6 months of lipid supplementation, the decline in these parameters was more marked in the butter and fish oil groups. By the 18th month, the lowest GFR level was observed in the group ingesting the butter diet (2.93 +/- 0.22 vs 5.01 +/- 0.21 ml min(-1) kg(-1) in control, P<0.05). Net acid excretion, evaluated in 9- and 18-month-old rats, was stimulated in the fish oil group when compared both to control and to the other two LD groups. These results suggest that even low levels of LD in a chronic nutritional regimen can modify the age-related changes in renal function and that the impact of different types of lipid-supplemented diets on renal function depends on the kind of lipid present in the diet.

Apoptosis of proliferative cortical tubular epithelia in chronic progressive nephrosis of rats

To investigate the role of apoptosis in the pathogenesis of rat chronic progressive nephrosis (CPN), the kidney of male F344/DuCrj rats, 19, 59, and 111 weeks of age, was examined histologically. In situ analysis for DNA fragmentation and proliferating cell nuclear antigen (PCNA) was performed simultaneously by TdT-mediated dUTP-biotin nick end labeling (TUNEL) and immunohistochemistry, respectively. CPN was seen in all the kidneys of 59-week-old (n=6) and 111-week-old rats (n=16), correlating significantly (p<0.01) with age. There were apoptotic bodies (ABs) in the single-layered epithelia of dilated tubules (SLD) and the multilayered epithelia (ML) of the cortical tubules. There were no ABs in any of the kidneys of the 19-week-old (n=5) or 59-week-old rats (n=6). Proliferative activity might have been enhanced in the single-layered and flattened epithelia, SLD, and ML of the cortical tubules in the kidneys of the 59-week-old rats (n=6) compared with that in 111-week-old rats (n=8). The correlations between the TUNEL-positive ratio and number of PCNA-positive cells, and age and the CPN grade were significant (p<0.01) exclusively in the ML. Thus, the results suggest that apoptosis occurs in the proliferative ML of rat CPN, and the pathological significance might be the removal of abnormal or excess cells.

The nature of chronic progressive nephropathy in aging rats

Increases in glomerular size and thickening of the glomerular basement membrane are constant features that accompany growth and maturation in animals. Yet, some animals have chronic progressive nephropathy characterized by glomerulosclerosis and tubulointerstitial fibrosis. In these animals, clinically significant reductions in glomerular filtration rate may compromise health, particularly when other renal diseases occur concomitantly. Progressive thickening of the glomerular basement membrane is accompanied by changes in its composition, which may be responsible for changes in podocyte morphology and proteinuria. Within the tubulointerstitium, generalized accumulation of fibronectin and thrombospondin are accompanied by blood vessel proliferation. Fragility of these blood vessels with intermittent bleeding may initiate an inflammatory process that leads to focal areas of tubular atrophy and scarring. The pathogenesis of these lesions is unknown. Genetic background, sex, and environmental factors influence the tempo of progressive sclerosis, although these factors are not primary determinants of this lesion. This review highlights the structural changes that occur in the kidney with aging. Because the lesions are structurally similar, information gleaned from studies of aging animals should be relevant to understanding the loss of renal function that occurs in aging humans.

Life-long dietary restriction modulates the expression of collagens and collagen-binding heat shock protein 47 in aged Fischer 344 rat kidney

It has been shown that the expression of HSP47 and collagens is substantially increased in the sclerotic/fibrotic process in various organs, including kidney. However, the factors regulating the increased expression of HSP47 are not yet clear. In this study, we examined the effect of dietary restriction for the expression of collagens and collagen-binding HSP47 in the kidneys of 6- and 24-month-old male Fischer 344 (F 344) rats fed ad libitum or 30% diet-restricted. No significant histological alteration was found in the kidneys of 6-month-old fed or diet-restricted rats. Kidneys obtained from 24-month-old freely fed rats showed glomerulosclerosis with marked tubulointerstitial damage including interstitial fibrosis, while in the kidneys of 24-month-old diet-restricted rats, renal damage was remarkably less than those noted in 24-month-old freely fed rat kidneys. Immunohistochemical analysis showed an increased accumulation of type I, type III and type IV collagens in areas of glomerulosclerosis and interstitial fibrosis in old rat kidneys. Dietary restriction significantly reduces renal accumulation of collagens in old age. Aging enhanced expression of HSP47 in 24-month-old freely fed rat kidneys whereas dietary restriction suppressed its expression in 24-month-old diet-restricted rat kidneys. Also, phenotypic alterations of mesangial cells and interstitial cells (immunopositive for alpha-smooth muscle actin), glomerular epithelial cells (immunopositive for desmin) and tubular epithelial cells (immunopositive for vimentin) were seen in 24-month-old freely fed rat kidneys and found to express HSP47. Dietary restriction significantly diminished phenotypically altered renal cells in 24-month-old rat kidneys. Our results suggest that increased expression of HSP47 is associated with age-related renal damage and that diet-restricted alteration of its expression is associated with the modulation of age-associated renal sclerosis/fibrosis.

Vitamin E ameliorates enhanced renal lipid peroxidation and accumulation of F2-isoprostanes in aging kidneys

Aging results in progressive glomerular sclerosis and reductions in glomerular filtration rate (GFR). Oxidative stress may be an important mechanism for the aging process, but to date the role of oxidative stress on renal aging has not been determined. The present study was performed to determine whether age-related alterations in renal hemodynamics and morphology were associated with oxidative stress and whether this could be attenuated by chronic administration of vitamin E. Rats, aged 13 mo, were given either control diet containing vitamin E 50 IU/kg (n = 6) or a high-vitamin E diet (5,000 IU/kg; n = 6) for 9 mo. Another group of rats (3-4 mo old; n = 7) served as young controls. Aging was accompanied by a 60% reduction in GFR, a threefold increase in renal F2 isoprostanes, newly discovered vasoconstrictive F2-like prostaglandins generated by free radical-mediated lipid peroxidation. Renal aging was also associated with an increase in oxidant-sensitive heme oxygenase, advanced glycosylation end products (AGEs), and the AGE receptor, RAGE. AGE-RAGE interaction has been shown to induce oxidative stress. With high-vitamin E diet, GFR was increased by 50%, F2 isoprostanes were suppressed, and expression of heme oxygenase and RAGE was attenuated. There was also a tendency for glomerular sclerosis to be attenuated. These data demonstrate that age-related decline in renal function is accompanied by oxidative stress and that administration of antioxidants, such as vitamin E, could attenuate the decline in renal function.

Renal responses to acute angiotensin II inhibition and administered angiotensin II in the aging, conscious, chronically catheterized rat

Studies of conscious, chronically catheterized, young (3 to 5 months of age) and old (19 to 22 months of age) male Sprague Dawley rats in the baseline state showed that glomerular filtration rate (factored for body weight) was lower and urine flow higher in old compared with young rats. Acute blockade of endogenous angiotensin II (AII) with either converting enzyme inhibitor (CEI) or Losartan (Dupont Merck, Wilmington, DE) caused small variable decreases in blood pressure in both age groups and produced a significant renal vasodilation with increases in renal plasma flow in the older rats. In separate studies using low-dose AII infusion (5 ng/kg body weight/min), an increase in filtration fraction was the only effect seen in both young and old rats with an intact renin/AII system; no effect was seen with CEI. During high-dose AII (20 ng/kg body weight/min), significant and similar increases were seen in blood pressure and renal vascular resistance in old and young rats, and the patterns of blood pressure and renal hemodynamic responses were similar with and without acute CEI. A natriuretic and diuretic response to high-dose AII was seen in young rats with intact endogenous renin/AII, whereas old rats were completely refractory to this action of AII. Against a background of acute CEI, no natriuretic/diuretic response to high-dose AII was seen in either age group. Thus, in the baseline state, renal hemodynamics in the old kidney are controlled by endogenous AII since CEI and Losartan produce renal vasodilation in old but not young rats.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of dorsomedial hypothalamic nucleus lesions on kidney function and structure after 1 and 12 months

According to the Dillman theory (17), aging results from a deterioration of metabolism that begins with an elevation of hypothalamic receptor thresholds for feedback signals from the periphery. Three hypothalamic areas are known to contain such receptors: the ventromedial and dorsomedial hypothalamic nuclei (DMN) and the lateral hypothalamic area. We have hypothesized that selective destruction of those hypothalamic areas might be followed by physiological changes associated with aging. Electrolytic bilateral DMN lesions were produced in male and female weanling rats. These rats were maintained for up to 13 months of age. Sham-operated rats served as controls. Food intake and body weight were monitored postoperatively and prior to sacrifice. Before sacrifice, tail blood and a 24-h urine samples were obtained. In accordance with previous findings, rats with DMN lesions showed dramatic reductions of ponderal growth and food intake but had normal body composition. Total protein and albumin excretion rates were significantly lower in rats with lesions. The fractional contribution of albumin to total urinary protein was also decreased in rats with lesions. Histological examination of the kidneys showed significantly less pathology in the kidneys of rats with DMN lesions; the severity of renal pathology was correlated directly with proteinuria. These changes were seen as early as 1 month after production of the lesion. The attenuation of age-related changes in kidney functions and structure in rats with lesions could be due to reduced food intake (dietary restriction is known to produce similar results), and/or a direct effect of the lesion.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes in albumin binding by renal brush-border membrane vesicles

A selective proteinuria occurs with normal aging. We investigated the contribution of a defect in the receptor-mediated endocytosis to the age-related albuminuria by measuring albumin binding by renal brush-border membrane vesicles from young and old female Wistar rats using a filtration method. Old (24 months) rats had a significantly higher proteinuria (13.29 +/- 5.25 mg prot/24 h/100 g bw) than did young (3 months) rats (1.23 +/- 0.55 mg prot/24 h/100 g bw). Scatchard analysis of the kinetic parameters of 125I-albumin binding revealed a decrease in the binding capacity of brush-border membrane vesicles from old rats. The number of binding sites, N (pmol/mg protein/min) was 236.84 +/- 97.50 in old rat preparations and 380.27 +/- 178.36 in young rat vesicles (P < 0.05). By contrast, Km did not change significantly with age (478.86 +/- 259.29 nM in old rat vesicles and 498.00 +/- 220.36 nM in young rat preparations). Consequently the index of adsorptive endocytosis efficiency (the N/Km ratio) decreased drastically with age from 0.782 +/- 0.238 at 3 months to 0.547 +/- 0.199 at 24 months (P < 0.05). These data indicate that defective receptor-mediated endocytosis could, at least partly, explain the age-dependent rise in urinary albumin excretion.

Molecular charge influences transperitoneal macromolecule transport

The influence of molecular charge on macromolecule transport during peritoneal dialysis was assessed by determining transperitoneal transport rates for fluorescent-labeled macromolecules (molecular radii from 15 to 40 A) that differed only in molecular charge: neutral dextran, anionic dextran sulfate and cationic diethylaminoethyl (DEAE) dextran. The test macromolecules were infused into the bloodstream of unanesthetized New Zealand White rabbits at a constant rate, and isotonic dialysis solution (40 ml/kg) was instilled into the peritoneal cavity. Blood and dialysate samples were taken hourly over a four hour dwell. Transperitoneal transport rates were assessed by calculating both the dialysate to plasma concentration ratio at four hours and the permeability-area product for the peritoneum, the latter parameter determined from the increase in the dialysate concentration with time. Transport rates for DEAE dextran were less (P < 0.05) than those for both neutral dextran and dextran sulfate; transport rates for neutral dextran and dextran sulfate were not different. Moreover, transperitoneal transport rates for fluorescent-labeled DEAE dextran were not affected by adding unlabeled DEAE dextran to the intravenous infusion solution, an observation suggesting that low transport rates for DEAE dextran were not due to its binding to plasma protein. We conclude that molecular charge is a determinant of transperitoneal macromolecule transport.

Convection does not govern plasma to dialysate transport of protein

The dependence of protein clearance on molecular size during peritoneal dialysis can be explained by assuming that either convection or diffusion is the major mechanism governing plasma to dialysate transport of protein. If convection is the predominant transport mechanism, then plasma-to-dialysate transport rates for protein should not decrease when the protein concentration in the instilled dialysis solution is increased. In the present study, plasma-to-dialysate transport rates for fluorescein-labeled protein tracers, alpha-lactalbumin and immunoglobulin G (IgG), were determined during a six hour isotonic exchange (25 ml/kg) in New Zealand White rabbits. The protein tracers were continuously infused into the bloodstream to keep plasma concentrations relatively constant, and transperitoneal transport rates were determined either with or without tracer protein added to the instilled dialysis solution. Plasma-to-dialysate transport rates were greater for alpha-lactalbumin than for IgG as expected based on the molecular size of these proteins. Transport rates for both alpha-lactalbumin and IgG decreased when tracer protein was added to the instilled dialysis solution. The present observations do not support convection as the major mechanism governing plasma to dialysate transport of protein during peritoneal dialysis.

The effect of aging on glomerular hemodynamics in the rat

Glomerular hemodynamics were measured in male Sprague-Dawley rats, aged 4 to 5 months (young) or 20 to 22 months (old). Body weight (BW) and left kidney weights (KW) were higher in old rats than young (BW: 507 +/- 12 g v 342 +/- 11 g, P less than 0.001; KW: 2.0 +/- 0.1 g v 1.3 +/- 0.1 g, P less than 0.001). Arterial blood pressure (AP) was slightly higher in old rats, but within the normotensive range (106 +/- 4 mm Hg v 94 +/- 4 mm Hg, P less than 0.05). Glomerular filtration rate (GFR; factored for KW) was lower in old versus young rats (0.67 +/- 0.05 mL/min/gKW v 1.00 +/- 0.08 mL/min/gKW, P less than 0.02). The cortical surface of the kidney in old (but not young) rats showed marked heterogeneity and single-nephron (SN)GFR was measured only in filtering nephrons and was higher and more variable in old versus young rats. Glomerular blood pressure (PGC) was unchanged in old compared with young rats (53 +/- 4 mm Hg v 55 +/- 2 mm Hg). There was a significantly greater level of glomerular sclerosis (in outer cortical glomeruli) in old versus young rats, and glomerular volume was substantially greater in old rats. This study suggests that age-related glomerulopathy is not primarily mediated by glomerular capillary hypertension.

Basal and stimulated plasma atrial natriuretic peptide (ANP) concentrations and cardiac ANP contents in old and young rats

To investigate the ability of the ageing heart to release atrial natriuretic peptide (ANP) we compared the response of awake, trained, chronically catheterized old (20-21 months) and young (4 months) rats to an acute, hypertonic saline challenge. There were no differences between young and old rats in basal plasma concentration of sodium (PNa; old: 141 +/- 3 meq/l; young: 143 +/- 3 meq/l) or ANP (old: 61 +/- 5 pg/ml; young: 67 +/- 12 pg/ml). Five minutes after acute saline challenge, PNa rose in both groups (old: 146 +/- 2 meq/l; young 149 +/- 1 meq/l) and approximately 3-fold increases in plasma ANP levels (182 +/- 24 pg/ml; young: 179 +/- 42 pg/ml). Hearts of old and young rats were assayed for atrial and ventricular ANP content. Atrial ANP levels were similar in old and young rats (13.5 +/- 3.6 vs. 24.9 +/- 8.7 micrograms/g atrial tissue), whereas ventricular ANP content was approximately 4-fold higher in old vs. young rats (153.7 +/- 39.3 vs. 47.5 +/- 6.4 ng/g ventricular tissue). Thus, the ageing rat heart responds equally as well as the young rat to an acute NaCl challenge.

Plasma vasopressin and cortical nephron function in aging rats

The role of vasopressin and Henle's loop transport in age-related polyuria and decrease in urine osmolality was investigated in female WAG/Rij rats free of kidney disease. In these animals, urine osmolality dropped from 2000 mosmol/kg H2O to 1000-1200 mosmol/kg H2O between 10 and 30 months, and urinary volume increased in proportion. Vasopressin concentration measured in plasma withdrawn from conscious, unrestrained, chronically catheterized rats was not significantly different in 10, 20 and 30-month-old animals (mean values 2.5 +/- 0.7, 2.2 +/- 0.2 and 2.0 +/- 0.3 pg/ml (n = 8), respectively). This suggests an impaired responsiveness of old kidney to antidiuretic hormone. The possible involvement of Henle's loop in this defect was studied by micropuncture. Paired collections of tubular fluid were done in the early distal and late proximal convolutions of the same cortical nephrons. Single nephron filtration rates did not significantly differ with age. Tubular fluid osmolalities in the early distal convolution were 165 +/- 13, 178 +/- 9 and 160 +/- 11 (n = 14) mosmol/kg H2O in 10-, 20- and 30-month-old rats, indicating similar diluting capacity of the cortical thick ascending limb. The amount of sodium transported from lumen to peritubular space by Henle's loop was also unchanged with age as were water, calcium, magnesium and potassium reabsorptions. These data indicate that the age-related decrease in urine osmolality is not related to either a significant reduced vasopressin plasma concentration or an increased single glomerular filtration rate or a reduced transport capacity of Henle's loop of the cortical nephron. Rather they suggest an impaired response to vasopressin of other segments of the nephron that is, the medullary thick ascending limb of Henle's loop and/or the collecting duct.

Visceral peritoneum is not essential for solute transport during peritoneal dialysis

The importance of visceral peritoneum in determining transperitoneal solute exchange was studied by determining the influence of evisceration on diffusive solute transport during peritoneal dialysis. Three series of experiments were performed in anesthetized New Zealand White rabbits. Series 1 studies compared solute transport rates from eviscerated rabbits (N = 5) with those from sham-operated controls (N = 5). Series 2 studies compared solute transport rates from eviscerated (N = 6) and sham-operated rabbits (N = 5) with application of circumferential abdominal compression to control intraperitoneal pressure and presumably maximize dialysate-peritoneum contact. Series 3 studies compared solute transport rates from sham-operated rabbits (N = 4) with and without applied circumferential abdominal compression. Transperitoneal solute exchange of creatinine and FITC-labeled neutral dextran (15 to 40 A) was equally assessed by both the dialysate to plasma concentration ratio at the end of the exchange and the diffusive permeability-area product of the peritoneum. Evisceration reduced creatinine (P less than 0.001) and dextran (15 to 30 A, P less than 0.05) transport to approximately one quarter that of controls in series 1 rabbits. When circumferential abdominal compression was applied in series 2 rabbits, however, evisceration had no effect on peritoneal solute transport rates. Moreover, circumferential abdominal compression per se had no effect on solute exchange in series 3 experiments. These findings demonstrate that the influence of evisceration on peritoneal solute transport depends on the experimental conditions. These observations further demonstrate that visceral peritoneum is not essential for solute transport during peritoneal dialysis.

Short term natriuretic responses in the conscious Zucker obese rat

Renal clearance studies were conducted in conscious, chronically catheterized obese and lean Zucker rats to investigate the natriuretic responses to i) acute IV infusion of isotonic NaCl = 5% of total body weight and ii) IV infusion of alpha rat atrial natriuretic peptide (ANP) in a dose of 300 ng/kg/min. In the baseline state, arterial blood pressure (BP) was significantly higher in obese vs lean rats. Absolute values of GFR and sodium excretion were similar but lower in obese vs lean rats when factored for body weight. In the 2 h period during and after NaCl infusion, obese rats showed a greater natriuresis vs lean while BP rose significantly and similarly. ANP infusion was natriuretic in obese rats but had no effect on lean rats. ANP lowered BP in both groups but BP remained higher in obese vs lean rats at all times. These studies show that in the chronic, unstressed preparation the 6-8 month old, female Zucker obese rat has a higher BP vs the 6-8 month old lean Zucker. The short term natriuretic response to either a NaCl load or ANP infusion is greater in obese vs lean Zuckers and the depressor response to ANP is intact in obese Zuckers. Thus the higher BP in this model of obesity is unlikely to be due to either a defective response to ANP or to a defect in the renal response to acute sodium challenge.

Age-related changes in the plasma membrane proteoglycans of rat kidney glomerular cells

In a previous in vivo study, we showed that the glomerular cells of rat kidney synthesize both peripheral and integral plasma membrane proteoglycans. The present work focuses on the age-related changes in these cell membrane proteoglycans. The peripheral proteoglycans in "adult control" rats aged 3 months were found to be heparan sulfate, dermatan sulfate, and chondroitin sulfate, with heparan sulfate being the main glycosaminoglycan. The integral membrane proteoglycans contained mainly dermatan sulfate plus less amounts of heparan sulfate. The relative proportions of the glycosaminoglycans in the integral membrane proteoglycans changed between 1 and 3 months. In addition, the degree of sulfation increased in both families of proteoglycans, and this was associated with an increase in glycosaminoglycan synthesis in the peripheral proteoglycans. The nature and relative proportions of the glycosaminoglycans forming the proteoglycans, did not change with age, after 10 months, and neither did the amount of glycosaminoglycans. But, the degree of sulfation of both peripheral and integral membrane proteoglycans decreased. De novo synthesized proteoglycans from 24-month-old rats had a higher overall charge than did those at other ages, owing to the presence of sulfate and carboxylic groups. We conclude that, as for glomerular basement membrane proteoglycans, biochemical alterations affect the glomerular cell membrane proteoglycans with aging.

Effect of advanced age on p-aminohippurate-induced inhibition of renal tubular secretion in male Fischer 344 rats

The effect of aging on glomerular filtration, effective renal plasma flow and on the responsiveness of the renal tubular anion secretory system to inhibition by 4-aminobenzoylglycine (p-aminohippurate, PAH) was examined in young (5-month) and old (22-month) Fischer 344 male rats. Plasma clearance, protein binding and renal extraction of [131I]o-iodohippurate, [125I]iothalamate and HPLC-purified [99mTc]mercaptoacetyltriglycine (MAG3), were used as in vivo probes of renal function. The effect of advanced age, without concomitant PAH, on the disposition of these markers was initially determined in ketamine anesthetized, temperature-maintained male rats, ages 5, 14 and 22 months by means of constant infusion clearance studies. Aging per se decreased (P less than 0.05) the kidney-weight normalized or body weight-normalized GFR and effective renal plasma flow rates. GFR values averaged 1.67, 1.43 and 1.32 ml/min per g kidney for the 5-, 14- and 22-month-old rats, respectively. Kidney- or body weight-normalized clearances of MAG3 and o-iodohippurate showed similar (25-27%) decreases, whereas the absolute values (ml/min) for GFR, o-iodohippurate and MAG3 clearance rates were not altered by aging. The effective filtration fraction, extraction ratio and plasma protein binding were also unchanged by advanced age. Overall, the age-related decreases in renal function were minimal in Fischer-344 rats, compared to other species. Differences in data normalization, species and gender account, in part, for discrepancies observed when comparing results in different studies on the effects of advanced age on renal function. Subsequently, we examined the effect of aging on the renal responsiveness to inhibition of tubular anion secretion using constant rate PAH infusion studies, adjusted for age-related changes in renal function. Aging did not alter PAH-induced inhibition of iodohippurate secretion. Inhibition of MAG3 elimination was more pronounced in the old rats compared to the young controls.

Renal vasodilatory response to intravenous glycine in the aging rat kidney

Studies were performed in the awake, chronically catheterized male Sprague-Dawley rat to investigate renal hemodynamics in the baseline state and also in response to a large intravenous (IV) amino acid (glycine) load. Studies were performed in young adult rats (age 3 to 4 months), old rats (age 18 months), and senescent rats (age 22 to 24 months). Histologic evaluation of the kidney permitted a correlation between structural and functional changes with aging. Histology showed progressive glomerular damage (sclerosis) with aging. In 18-month-old rats, the glomerular filtration rate (GFR) was normal, which, considering the level of glomerular injury (only 64% normal glomeruli), must indicate heterogeneity of glomerular function, with some hyperfunctioning glomeruli. By 22 to 24 months of age (at which time approximately 50% mortality has occurred in males of this strain), GFR is substantially reduced, as is renal plasma flow rate (RPF). Severe glomerular damage was observed histologically (only 34% normal glomeruli), indicating widespread heterogeneity of glomerular function. Young adult rats displayed a substantial renal vasodilation in response to acute IV glycine infusion, which resulted in approximately 25% increases in GFR and RPF. The renal vascular responsivity to glycine was diminished at 18 months and was completely absent in 22- to 24-month-old rats. This altered renal vasodilatory response to glycine probably reflects both structural changes associated with aging and also the compensatory vasodilation of intact hyperfunctioning remnant nephrons as other nephrons are lost due to aging.

Renal effects of administered atrial natriuretic peptide in the conscious, aging rat

Studies were performed in conscious, chronically catheterized male Sprague-Dawley rats to investigate the effect of administered atrial natriuretic peptide (ANP) on blood pressure, renal hemodynamics and urinary electrolyte excretion. Studies were performed on young adult (3-4 month old) rats and on aging rats (18-24 months of age). Low dose ANP (80 ng/kg/min for 60 min) had no effects on renal hemodynamics in either young or old rats and produced only a slight blood pressure reduction in young animals. No effect on urinary electrolyte excretion was evident in young rats whereas in the old animals, low dose ANP produced large rises in the rate of sodium excretion, fractional excretion of sodium and urine flow rate. A four fold higher dose of ANP evoked a moderate natriuretic and a marked antihypertensive response in young rats. Time control studies indicated that time alone had no influence on urinary sodium excretion rate, the fractional excretion of sodium or urine flow rate. These studies indicate a much enhanced sensitivity to the natriuretic effects of administered ANP by the kidneys of old rats.