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The progression of anatomic and functional changes that occur with aging in normal male Sprague-Dawley rats was investigated. Renal function studies were followed by vascular perfusion fixation of the kidneys. The kidneys were examined with light microscopy and scanning and transmission electron microscopy. The glomerular filtration rate of 12-month-old rats (0.30 +/- 0.06 ml/min/100 gm body weight) was significantly lower (P less than .05) than the rates of the 3-month-old (0.91 +/- 0.08 ml/min/100 gm body weight) and 5-month-old (0.99 +/- 0.16 ml/min/100 gm body weight) rats. Urine protein levels were moderately elevated in 12-month-old rats. Structural changes mainly involved the renal corpuscle and proximal tubule. The parietal layer of Bowman's capsule was transformed from a squamous to a columnar epithelium resembling that of the proximal tubule in 48% of the capsules surveyed in 12-month-old rats. This value was significantly higher (P less than .05) than that of the 5- (14%) or 3- (6%) month-old rats. Thin sheets of cytoplasm extended from podocytic cell bodies over the layer of pedicels. The proximal tubule displayed focal areas of cell injury and necrosis. Basement membranes associated with the renal corpuscle, glomerulus, and proximal tubule were considerably thickened in the oldest group of rats. Some cellular infiltration occurred in the interstitium. Therefore, kidneys of aging Sprague-Dawley rats exhibited capsular metaplasia, death, and injury of proximal tubular epithelial cells, and other structural changes not seen in younger animals. Some of these changes may underlie the functional deterioration of their kidneys.

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The effects of systemically infused angiotensin II (ANG II) (5, 50, and 100 ng X min-1 X 100 g body wt-1; groups 1, 2, and 3, respectively) were studied in Sprague-Dawley rats. All doses increased systemic blood pressure, fractional excretion of sodium, and urine flow rate but decreased glomerular filtration rate. Scanning electron microscopy revealed no detectable changes in the visceral epithelium or measurable alterations in the total area of endothelial capillary surface occupied by pores. Glomerular basement membrane surface densities of outer (OCG) and inner (ICG) cortical glomeruli averaged 0.22 micron2/micron3 of glomerular tuft volume in all groups. Volume ratios of the individual glomerular tufts to that of their Bowman's capsule of OCG were less (P less than 0.05) in groups 1 and 2 (52 +/- 3 and 63 +/- 9%) than the control group (72 +/- 1%). The volume fraction of the glomerular capillaries to their Bowman's capsules remained approximately 25% in all groups. However, the volume fraction of parenchyma to Bowman's capsule was reduced in group 1 to 30 +/- 4% and 38 +/- 5% in group 2 compared with the control value of 47 +/- 2% (P less than 0.05). Thus ANG II reduced glomerular tuft volume and parenchyma but did not alter filtration surface area or capillary endothelial epithelial surface characteristics.

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