Impairments in availability of insulin to liver in vivo and in binding of insulin to purified hepatic plasma membrane during aging

Glucoregulatory responses of adult and aged rats after exposure to chronic stress

Stress has been implicated as an environmental factor that may accelerate the process of biological aging. However, this proposal has remained largely anecdotal due to relatively few studies that directly tested this hypothesis. In the present experiments groups of 6-month-old and 20-month-old male F-344 rats were chronically stressed for a six-month period. After the last stress session, when the animals were 12 months of age (adult) and 26 months of age (old), control and chronically stressed rats were tested for their ability to: (a) elicit glucose and insulin responses to an acute, novel stressor; (b) remove a circulatory glucose load elicited either by acute stress exposure or by injection of d-glucose; and (c) raise insulin levels after a glucose challenge. In control rats, we observed a deficit in each of these parameters in old compared to adult rats. Exposure to chronic stress did not exacerbate deterioration of these response mechanisms in either adult or old rats. In fact, the data showed a modest improvement in glucose tolerance in chronically stressed compared to age-matched control rats. We conclude that chronic stress did not exacerbate age-dependent decline of glucoregulatory capacity. From these results and from our earlier work, we speculate that the decline during aging of the functional integrity of systems involved in the response to stress may be sustained by periodic challenges from the organism's external environment.

Some features of age-related insulin responsiveness in rat hepatocytes

Different features of insulin sensitivity have been considered, in rat hepatocytes, with respect to their age-dependence. In particular, plasma membrane-located responses such as (Na/K)-ATPase and Na-dependent aminoacid transport were studied together with cytoplasmic responses such as the insulin-stimulated tyrosine aminotransferase. It appears, as far as the insulin sensitivity is concerned, that the age of the animal does not affect plasma membrane-bound events whereas plasma membrane-mediated intracellular responses are definitely impaired. Insulin binding to intact hepatocytes does not seem to be age-dependent.

Insulin binding to the blood-brain barrier in the streptozotocin diabetic rat

125I-Insulin binding to isolated brain microvessels from control, streptozotocin diabetic, and insulin-treated diabetic rats was measured. The binding was highest in the control (21.1 +/- 1.8%/mg capillary protein) and lowest in the diabetic (14.8 +/- 1.9%, p less than 0.01) animals. Administration of 2 U of protamine zinc insulin per day increased the maximum binding in the diabetic rats to 17.2 +/- 2.1%. Scatchard analyses of the binding showed that the major difference between the diabetic and the control animals was a decrease in the number of both high- and low-affinity sites in the diabetic animals. To test whether the failure of up-regulation in the hypoinsulinemic diabetic animal was related to an inherent defect in the endothelial cell or resulted from the diabetic milieu, cultured brain endothelial cells were tested for their capacity to up- and down-regulate their insulin receptors in vitro. In response to 100 ng/ml insulin for 12 h, these cells down-regulated their insulin receptors. When the insulin was removed, the insulin receptors returned to control levels. These studies showed that in vitro brain capillary endothelial cells have the capacity to increase their insulin receptors in response to a low-insulin environment, whereas in vivo the microvessels decrease their insulin receptors in response to diabetes.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the mechanisms of hormone and neurotransmitter action during aging: current status of the role of receptor and post-receptor alterations. A review

Alterations in responsiveness to hormones and neurotransmitters during aging appear to be due to changes at both the receptor and post-receptor levels. Although many such observations have now been independently confirmed, disagreement over the extent and/or importance of receptor alterations exists in a number of cases. Receptors do not appear to change with age in certain systems, but only a few reports have actually been able to localize particular post-receptor alterations responsible for changes in response. This review attempts to catalogue studies in these areas which have been carried out to date, and discusses possible reasons for discrepancies as well as future research directions.

Age-development changes in susceptibility of erythrocytes to perfringolysin O

Susceptibility to perfringolysin O of erythrocytes from mice of different ages was examined. Erythrocytes of mice younger than 5 weeks' old were more resistant to the toxin than those of young adult and adult mice. Erythrocytes of aged mice were about 3.5 times more susceptible to the toxin than erythrocytes from 4-week-old mice. The membrane cholesterol content of erythrocytes appeared to be maintained at a constant level throughout the ages of mice examined. About 5% of the total membrane cholesterol was supposed to provide receptor sites for the toxin from an experiment in which cholesterol was specifically extracted by liposomes. It was demonstrated in this experiment that susceptibility of erythrocytes to the toxin was lost in proportion to the reduction in the toxin binding. The susceptibility, however, of erythrocytes from young or aged mice was much lower or higher than expected from the changes in toxin binding. Therefore, two possibilities were raised to account for age-related alterations in the susceptibility of erythrocytes; not only expansion of a particular compartment of membrane cholesterol as a toxin receptor but also some activation of intracellular reactions leading to hemolysis might occur in senescence.

Monocyte insulin binding studies in normal and diabetic pregnancies

Monocyte insulin receptor binding was studied in six nonpregnant control patients and in 40 pregnant patients with varying degrees of carbohydrate tolerance. Competitive binding assays were performed to determine insulin binding to monocytes. Fasting insulin levels were determined. We obtained the following results: (1) When compared to values not associated with pregnancy, the number of insulin receptor sites per cell increases twofold (31,000 versus 16,300); (2) Class A diabetic patients have higher numbers of receptor sites than normal pregnant patients (80,800 versus 31,000; (3) untreated Class B diabetic patients have markedly reduced receptor sites (4,575) and bind less insulin at physiologic concentrations (p less than 0.01); (4) insulin therapy of previously untreated Class B diabetic patients restored the number of receptor sites to normal pregnant levels (29,700); and (5) Classes C and D diabetic patients had similar numbers of receptor sites (30,140) and showed a greater receptor affinity for insulin than pregnant control subjects (p less than 0.01).

The effect of aging on carbohydrate metabolism: a review of the English literature and a practical approach to the diagnosis of diabetes mellitus in the elderly

There seems little doubt that the disposal of a glucose load is progressively impaired during aging. The mechanism(s) for this alteration remains unclear. Five possibilities have been raised: (1) poor diet, (2) physical inactivity, (3) decreased lean body mass in which to store the carbohydrate load, (4) decreased insulin secretion, and (5) insulin antagonism. Although poor diet and physical inactivity may contribute to some of the abnormal glucose tolerance tests of the older population, these two factors do not provide a full explanation. Diminished lean body mass may play some role but there is almost certainly an additional effect due to aging. A few papers have suggested that glucose-induced insulin secretion may be impaired as the population ages, but the bulk of studies in this area conclude that normal or increased amounts of insulin are released by the pancreatic beta-cell during aging. If abnormalities of insulin secretion exist, either in degree or timing, they are subtle and would not seem sufficient to account for the great number of older subjects who manifest impaired glucose tolerance. The evidence for insulin antagonism seems the strongest but the data are certainly not conclusive. In actuality, the aging effect on carbohydrate metabolism may be heterogeneous in nature. Either some or all of these five factors may contribute to the aging effect to varying degrees in individual subjects. Alternatively, the glucose intolerance of aging may represent a heterogeneous group of disorders. In any event, until better methods to identify possible subgroups of these subjects and/or a marker for diabetes mellitus independent of glucose concentration become available, this problem will remain difficult to resolve. Based on the currently available data, it seems prudent to diagnose diabetes mellitus only if fasting hyperglycemia is present.

Hormone action during aging: alterations and mechanisms

The ability to respond to certain hormones is altered during the aging process. Attempts to elucidate the mechanisms involved in such changes have focused primarily on the various molecular components which mediate hormone actions. These include chromatin, the nucleus, cytoplasmic factors, adenylate cyclase and hormone receptors. Age related changes have been observed at all of these levels, and in some cases have been correlated with changes in biological responsiveness to particular hormones. This review attempts to summarize the data obtained in these studies. Possible interpretations are discussed and future directions for such research are suggested.

Turnover of lipid components in liver microsomes, mitochondria and plasma membrane of 6-, 12- and 24-month old rats

The turnover of lipid was examined in the livers of 6-, 12- and 24-month old rats. Heterogeneity of turnover was noted for each membrane fraction. The lipid turnover rate was highest in 12-month old rats and was the same in 6- and 24-month old rats. The higher rate of lipid turnover at 12 months was observed in both the neutral and polar lipid components of the liver membranes. In the polar lipid fractions isolated from the microsomal and mitochondrial membranes the increase in lipid turnover rate at 12 months was related to increase in the turnover of phosphatidylethanolamine.

A possible role for insulin in the altered capability for hepatic enzyme adaptation during aging

The pattern of change in the concentration of immunoreactive insulin in portal vein blood during a 7-hour period following intragastric administration of glucose to fasted, male Sprague-Dawley rats is modified both in magnitude and time course of response as rat age increases from 2- to 24-months. This alteration in the control of insulin levels probably is not the consequence of modifications in the availability of administered glucose to the pancreas. A similar modification in the control of insulin secretion also is evident when pancreatic islets isolated from fasted rat donors aged 2- to 24-months are perifused with glucose in vitro. This may represent the first demonstration of an altered response of aging animals which is expressed in the same way in an isolated in vitro system.

Variations among individual mice in binding of growth hormone and insulin to membranes from animals of different ages

Binding of 125I-labeled insulin and growth hormone to membranes from liver and hearts of C57BL/6J mice was measured and the data analyzed by the graphical method of Scatchard. Animals were studied at ages 2, 10, 20, and 31 months; preparations from each animal were analyzed individually. There was no significant and progressive age-related difference in either dissociation constant or binding capacity for insulin in heart and liver, or for growth hormone in liver. Variations among individual animals were large; in some cases, the standard deviation exceeded the mean for a particular age group. We conclude that there are no large age related differences in binding of these anabolic hormones to the target tissues studied, and that any small changes would be masked by the large variations among individual animals.