Age-Associated (Geriatric) Pathology: Its Impact on Long-Term Toxicity Studies

Influence of dietary protein concentration on severity of nephropathy in Fischer-344 (F-344/N) rats

Nephropathy is an age-related spontaneous disease of most rat strains, and protein content of diet may affect the severity. The purpose of this study was to determine the effect of a 15% protein nonpurified diet on body weight and severity of nephropathy in comparison to a 23% protein NIH-07 diet. Groups of 25 male and 25 female Fischer-344 (F-344) rats, 6 wk of age, were fed the 23 or 15% protein diet ad libitum for 2 yr. Rats were weighed at 1-4-wk intervals, and mean body weights were determined. Water consumption measurements and urinalysis were done at approximately 3-mo intervals during the second year of the study. At the end of the 2-yr study, kidneys from all rats, including those that died or were euthanatized after the eightieth week of the study, were examined by light microscopy and graded for severity of nephropathy as grades 1-4 (minimal, mild, moderate, marked). Growth patterns and the maximum body weights attained by each sex fed the 23 or 15% protein diet were not significantly different. The severity of nephropathy in male rats was significantly higher when fed the 23% protein diet (2.8 moderate to marked) compared to the 15% protein diet (1.3 minimal to mild). The severity of nephropathy in female rats increased slightly when fed the 23% protein diet (1.5 minimal to mild) compared to the 15% protein diet (1.0 minimal).(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid development of hepatocellular neoplasms in aging male C3H/HeNCr mice given phenobarbital

One hundred and nineteen male C3H/HeNCr mice, 12 months of age, with spontaneous preneoplastic and neoplastic hepatocellular lesions were given phenobarbital (PB) at 500 ppm in drinking water. Groups of 9-10 mice were sacrificed after 12, 24 and 36 weeks of PB exposure. Identical numbers of untreated controls were used. A group of 6-week-old C3H/HeNCr mice were also given PB and sacrificed at 12, 24 or 36 weeks. In aging mice, PB exposure significantly increased the number of gross tumors or microscopic foci, adenomas or carcinomas per liver at all time periods, especially unique eosinophilic proliferative lesions, while young mice did not develop any focal proliferative lesions by 36 weeks. These findings suggest that in aging mice a fraction of the hepatocyte population (normal, spontaneously-initiated or preneoplastic) is more highly susceptible to phenobarbital 'carcinogenesis' than are hepatocytes of younger mice.

2,2,2-Trifluoroethanol toxicity in aged rats

Various anesthetics are metabolized by different forms of cytochrome P-450 yielding the same toxic metabolite, 2,2,2-trifluoroethanol (TFE). The toxicity of TFE is a consequence of its metabolism catalyzed by cytochrome P-450. Since a marked age difference exists in the composition and inducibility of the hepatic mixed function oxidase system, we have elucidated the toxicity of TFE in 36-month-old aged Wistar rats. The aged rats were injected with sublethal doses of TFE (0.10 g/kg once per week for 5 weeks), after which they were sacrificed for pathologic examination. The major TFE-related lesions observed were severe hepatocyte degenerative changes such as basophilic, eosinophilic, vacuolated hepatocytes, bile duct hyperplasia, accumulation of lipofuscin pigments, and preneoplastic nodules. Other changes seen were the hyalinization of gastric submucosal wall, generalized testicular atrophy due to the loss of seminiferous tubules, coagulation necrosis of intestinal mucosal wall, hyperpigmentation, and more advanced and severe chronic progressive glomerulonephropathy in the TFE-treated rats. More severe lipofuscin and vacuolation of white matter of thalamic area, pons, midbrain, and cerebellum reflective of enhanced aging were also seen. Ultrastructural studies of liver from TFE-treated rats revealed rather diffuse loss of glycogen, fragmentation of endoplasmic reticulum, mineralization of mitochondria, and loss of other organelles within the hepatocytes versus saline-treated aged rats. Hepatic cytochrome P-450 concentrations, measured as a possible index of endoplasmic reticulum damage, were not affected by TFE administration.

Anatomy, function and aging in the mouse liver

A brief overview is given of the embryology, gross and microscopic anatomy and sinusoidal lining cells of the mouse liver. The handling of xenobiotics is also considered. So called phase I and II reactions are of major importance. Furthermore, heterogeneity of metabolism occurs in the different zones of the hepatic acinus. Finally, some of the manifest pathological changes are discussed. Despite the occurrence of these changes there is still no clearcut evidence that liver function declines with age.

The value and significance of life span and scheduled termination data in long-term toxicity and carcinogenesis studies

Consideration is given to the age association of lesions, the duration of long-term toxicity and carcinogenesis studies, and to the value and significance of including scheduled termination in such long-term studies. There is now enough evidence that age and cancer are associated. It is argued that the increase in incidence of lesions with age has such disadvantages that extension of the duration of the long-term toxicity and carcinogenicity study beyond 2 years is not warranted in most cases. Incorporation of scheduled termination in long-term studies gives more insight into the biologic behavior of toxic lesions and cancer and may enable one to make a distinction between "incidental" and "fatal" lesions. This distinction may be important for the statistical evaluation of data from chemical carcinogenesis studies.