Altered regulation of pancreatic glucagon in male rats during aging

Glucagon and vasopressin V1a receptor signaling in hepatocytes from aging rats

Glucose tolerance is reduced with age. The relationship between this change in glucose homeostasis and signaling of glucagon and vasopressin V1a receptors was investigated in hepatocytes isolated from 10- and 30-month-old female WAG/Rij rats. Binding capacity of hepatocytes for 125I glucagon and 3H vasopressin increased 2- and 1.8-fold, respectively, between 10 and 30 months. Intracellular cAMP accumulation induced by glucagon was 40% greater in hepatocytes of aging rats than of adults, although EC(50) were similar in the two groups. Conversely, phosphodiesterases activity and nucleotides leakage out of the cells were unchanged with age. The rise in intracellular calcium consecutive to the stimulation of V1a receptor was comparable in adult and senescent animals. Finally, glucose release by hepatocyte suspensions was greater in senescent than in adult animals in absence as in presence of glucagon. These experiments suggest that increase in glucagon receptor expression and cAMP generation would contribute to the impaired glucose tolerance characteristic of the aging process.

Aging and the hypothalamus: research perspectives

There are several hypothalamic theories of aging, none of which has been validated. An approach to validation is to search for consequences of anatomic ablations of hypothalamic regions that are functional hallmarks of aging, or consequences of ablation that postpone the appearance of hallmarks of aging or extend longevity. Ablation of the hypothalamic ventromedial nucleus (VMN) in the weanling rat is associated with subsequent increased body fat, glucose intolerance, hyperlipidemia, and decreased renal function. Each of these consequences is characteristic of aging in humans and in several animal models of aging. Ablation of the hypothalamic dorsomedial nucleus (DMN) in the weanling rat leads to a symmetrically smaller animal with normal glucose and lipid metabolism, decreased body fat for size, and reduced risk of decreased renal function and circulating IGF-I levels. These are findings consistent with calorie restriction models in rodents that significantly extend life span. This review compares outcomes of lesions in the VMN, DMN, and lateral hypothalamic area (LHA) for relevance to aging. To establish a relationship between these anatomic areas of the hypothalamus and aging, it is concluded that the VMN, DMN, and LHA lesions should be examined for impact on longevity and compared with data obtained from simultaneously studied intact ad-lib-fed and 40% calorie-restricted animals. Lesioned animals also should be rigorously studied for neurotransmitters (e.g., neuropeptide Y, beta-endorphin, serotonin, corticotropin-releasing factor, and galanin), and for behavioral changes consistent with aging, for accumulation of specific tissue lipofuscin and amyloid that are associated with normal aging and for other age-dependent findings, such as incidence of tumors and cataract.

Hormone secretion by isolated perfused pancreas of aging Fischer 344 rats

This is the first study investigating hormone secretion by the isolated perfused pancreas of the aged Fischer 344 rat. Nutrient-induced release of insulin, glucagon, and somatostatin in overnight-fasted rats aged 2, 10, 18, 24, and 30 mo was studied. After an equilibration period, the pancreas was perfused for 20 min with buffer A (containing 4.2 mM glucose plus a 3.5 mM mixture of amino acids) then for 20 min with buffer B (containing 8.3 mM glucose and 7.0 mM amino acids). When stimulated by buffer B, the amount of insulin secreted increased (P less than 0.05) from immaturity (2 mo) to adulthood (10-18 mo) because of growth of the organ. From adulthood to very old age an equal amount of insulin was released in all groups during the last 19 min of perfusion with buffer B. Fasting blood glucose levels remained constant throughout life, whereas pancreatic insulin stores and plasma insulin levels rose, reaching peak values at 18 mo. The alpha-cell appeared to be deemphasized relative to the beta-cell during development but not thereafter, as indicated by the findings that from immaturity to adulthood pancreatic glucagon stores expanded less than insulin stores and that glucagon release significantly decreased. Only minor changes in somatostatin release from the delta-cells were observed after the rat reached adulthood. We conclude that the endocrine secretory response of the pancreas is well maintained throughout life in the Fischer 344 rat.

Levels of carnitines in brain and other tissues of rats of different ages: effect of acetyl-L-carnitine administration

Male Sprague-Dawley rats, aged 2, 5, 16, 20 and 30 months and normally fed, were used for determination of carnitines in the brain, serum, heart, tibial muscle, liver and urine. With respect to 5-month-old animals, those aged 30 months exhibited a statistically significant decrement of total carnitine levels in the brain, serum, heart and tibial muscle, accompanied by a dramatic increment in the liver. This suggests impaired net transport of carnitines from the liver to the blood in old age. Urinary excretion was similar in the two age groups. One group received from 5 months on daily 75 mg/kg acetyl-L-carnitine in drinking water. At 20 months, the treated animals showed levels of brain, heart and serum carnitines similar to those of 5-month-old animals. The recovery of brain, heart and serum carnitines in the old animals treated with acetyl-L-carnitine indicates that intestinal absorption and tissue uptake remain sufficiently efficient in the course of aging. The lower level of brain lipofuscins due to acetyl-L-carnitine treatment may be related to the effect of the compound on acetylcholine metabolism.

Interaction of dietary protein, cholesterol and age on lipid metabolism of the rat

1. Male rats at 1 (young) and 9 months (adult) of age were fed on purified diets, supplemented with or without cholesterol, containing 200 g protein/kg (casein (CAS), whey protein (WHY) or soya-bean protein (SOY] for 4 weeks. 2. SOY exerted a hypocholesterolaemic effect in young rats regardless of dietary cholesterol, whereas in adult rats it was observed only when a cholesterol-enriched diet was given. WHY was also hypocholesterolaemic in rats of both ages given cholesterol. SOY tended to reduce the liver cholesterol both in young and adult rats. 3. The liver hydroxymethylglutaryl-CoA reductase (NADPH) (EC 1.1.1.34) activity tended to be lower in the vegetable-protein groups than in the animal-protein groups in young rats, but the age-related reduction was observed only in the latter groups. 4. There was no significant age-related difference in the activity of liver cholesterol 7 alpha-monooxygenase (EC 1.14.13.17) in response to diets. However, when cholesterol was given, activity tended to decrease with age. Rats given SOY excreted more faecal steroids than those given casein, particularly adult rats. 5. The fatty acid profile of phosphatidylcholine and the delta 6-desaturase activity of liver microsomes indicated the reduced desaturation of linoleate in the SOY and WHY groups compared with the CAS groups. 6. The results thus showed a complex interaction of protein type, cholesterol and age on cholesterol homeostasis.

Changes in the control of enzyme clusters in the liver of adult and senescent rats

The activities of a number of enzymes in rat liver have been measured at different times during adulthood and senescence and expressed as a percentage of maximal activity that can be attained after hormonal stimulation. Three different profiles can be detected. Type I profile shows decreasing activities during adolescence (1--3 months of age), increasing activities during adulthood (4--12 months of age) and relatively high activities thereafter. Enzymes of this group are carbamoyl-phosphate synthase and arginase; DNA content shows the same pattern. Type II profile shows decreasing activities during adolescence and relatively low activities thereafter. Enzymes of this group are tyrosine aminotransferase, glucose-6-phosphatase, and glucokinase. Type III profile shows relatively high activities during adolescence, adulthood and senescence. Enzymes of this group are ornithine transcarbamoylase, glutamate dehydrogenase and hexokinase. Some enzymes are constant with age in females, but slowly decrease in activity with age in males; decreasing levels of androgens and possibly also thyroid hormones can explain this decrease in males. Decreasing activities of carbamoyl-phosphate synthase and arginase during adolescence can be attributed to a depressant effect of gonadal hormones. The difference between relatively high and relatively low basal activities of enzymes in adult and senescent rats corresponds with their relatively long and short half-lives, respectively. This relation implicates a similar rate of synthesis of glucocorticosteroid hormone-dependent enzymes.

A new mutation (db3J) at the diabetes locus in strain 129/J mice. I. Physiological and histological characterization

A spontaneous recessive mutation appearing in strain 129/J mice at the diabetes (db) locus on Chromosome 4 has been characterized. The new allele, designated db3J, produced hyperphagia and severe obesity. Mutants weighed in excess of 70 g by 6 months of age, compared to 22-28 g for lean littermates. Although the disease was similar to the mild hyperglycaemia-severe obesity syndrome exhibited by db gene presentation on the C57BL/6J inbred background, the syndrome in 129/J mice reduced lifespan, with mutants exhibiting sudden weight loss, hypoglycaemia, and a 67% mortality between 6 and 14 months of age. Mutant males, but not females, were transiently hyperglycaemic between 2 to 4 months of age, attaining a maximum mean blood sugar of 196 +/- 27 (SEM) mg/dl. Thereafter glucose levels declined to normoglycaemic values (80-100 mg/dl), and with increasing age, mutants of both sexes became hypoglycaemic (60 mg/dl at 9 months). Mutants of both sexes were extremely hyperinsulinaemic at the earlier ages, with mean plasma insulin at months 5 reflecting 30-fold elevations above normal for males and 18-fold for females. These levels diminished with age, the decline being more marked in males. Plasma glucagon levels were 3-fold elevated in the younger mutants of both sexes (86 pg/ml versus 28 pg/ml in normal mice), mean levels increasing to almost 5-fold above mean control vaues in the older age group (198 pg/ml versus 41 pg/ml in normal mice). Histopathological findings were limited to pancreas. Increasing necrosis of the exocrine, but not endocrine, pancreas was noted in aging mutants. Aldehyde fushsin staining of the mutant pancreas revealed hyperplastic islets filled with heavily granulated B-cells. B-cell hyperplasia was accompanied by a 30-fold increase over controls in pancreatic insulin content in the 8 month old mutants, whereas pancreatic glucagon content was only doubled. Morphometric analysis showed less than a 2-fold increase in the mean number of A-cells per islet. Thus, an interesting feature of expression of the diabetes gene in the 129/J strain is the persisting hyperglucagonaemia in the face of moderating hyperinsulinaemia.