This study was aimed at exploring the capability of the pancreatic endocrine adaptive mechanisms of ageing Sprague-Dawley rats to counteract the metabolic challenge induced by the prolonged ...administration of dexamethasone (DEX) (0.13 mg/kg per day for 13 days). DEX treatment induced peripheral insulin resistance in 3-, 18- and 26-month-old rats, as indicated by the significant and persistent rise of plasma insulin levels in each age group (plasma insulin in 3-, 18- and 26-month-old rats from basal values of 4.3+/-0.8, 4.7+/-0.5 and 5.6+/-1.0 ng/ml (means+/-s.e.m.) respectively, rose to 11.9+/-1.7, 29.1+/-5.5 and 27.9+/-2.7 ng/ml respectively, after 9 days of administration). However, plasma glucose concentrations remained unchanged during the treatment in young rats, whereas they increased up to frankly diabetic levels in most 18-month-old and in all 26-month-old animals after a few days of DEX administration. Plasma free fatty acid concentrations increased 2-fold in 3- and 26-month-old rats and 4-fold in 18-month-old rats and could possibly be involved in the glucocorticoid-induced enhancement in insulin resistance, although they showed no significant correlation with glycaemic values. Incubation of pancreatic islets obtained from treated rats showed that DEX administration increased the insulin responsiveness of islets from not only younger but also older donors. However, in the islets of ageing rats, which already showed an age-dependent impairment of the sensitivity to glucose and other secretagogues, this enhancing effect was clearly attenuated with respect to the younger counterpart. Furthermore, DEX treatment depressed significantly the priming effect of glucose in islets isolated from all the three age groups. In conclusion, our results show that ageing rats are unable to counteract effectively a prolonged hyperglycaemic challenge as such induced by DEX administration. This homeostatic defect can be ascribed to the age-dependent failure of the endocrine pancreas to provide enough insulin to overcome the aggravation of an antecedent state of increased peripheral insulin resistance.
In this study we have investigated the insulin secretory response to glucose and other secretagogues (2-ketoisocaproate, 3-isobutyl-1-methyl-xanthine and arginine) of pancreatic islets isolated from ...Sprague–Dawley rats of various ages (from 2 to 28 months). Our results showed a significant decline in the glucose-stimulated insulin secretion, starting at 12 months of age. On the other hand, the response to non-glucose secretagogues (and mainly to 2-ketoisocaproate) was less impaired with advancing age than that to glucose. We also observed a progressive age-related decline of protein levels of the glucose transporter GLUT-2 in pancreatic islets, which was temporally concomitant and quantitatively comparable with the β-cell alteration in glucose responsiveness (−40/50%). Finally, we observed a significant increase of the islets insulin content in older rats with respect to younger animals. We conclude that in the islet of older rats the impaired capability to respond to glucose could be dependent, at least in part, on the age-dependent reduction in GLUT-2 and could be compensated by mechanisms including a preserved responsiveness to non-glucose secretagogues and/or the development of islet hypertrophy.
The aim of this research was to characterize 2,3,7,8-tetrachlorodibenzo-
p-dioxin (TCDD) toxicity on the insulin-secreting β-cell line INS-1E. A sharp decline of cell survival (below 20%) was ...observed after 1 h exposure to TCDD concentrations between 12.5 and 25 nM. Ultrastructurally, β-cell death was characterized by extensive degranulation, appearance of autophagic vacuoles, and peripheral nuclear condensation. Cytotoxic concentrations of TCDD rapidly induced a dose-dependent increase in intracellular calcium concentration. Blocking calcium entry by EGTA significantly decreased TCDD cytotoxicity. TCDD was also able to rapidly induce mitochondrial depolarization. Interestingly, 1 h exposition of INS-1E cells to very low TCDD concentrations (0.05–1 nM) dramatically impaired glucose-stimulated but not KCl-stimulated insulin secretion. In conclusion, our results clearly show that TCDD exerts a direct β-cell cytotoxic effect at concentrations of 15–25 nM, but also markedly impairs glucose-stimulated insulin secretion at concentrations 20 times lower than these. On the basis of this latter observation we suggest that pancreatic β-cells could be considered a specific and sensitive target for dioxin toxicity.
Age-related changes in the urinary excretion of aldehydes arising from lipid peroxidation have been investigated in male Sprague–Dawley rats aged 2, 4, 6, 12, 18, 24 and 27
months, fed ad libitum or ...subjected to two different regimens of calorie restriction (namely every-other-day ad libitum feeding — EOD — and 40% calorie restriction — 40%DR). For only some age groups, results were compared with those obtained in ad libitum fed male Fisher 344 and Lewis rats. Results show that the urinary excretion of malondialdehyde (MDA) and formaldehyde (FA) significantly decreases, whereas that of propionaldehyde (PROP) progressively increases with age, and that urinary excretion of acetaldehyde (ACT) does not show any significant age-related variations. Dietary restriction significantly increases the urinary levels of MDA, FA and PROP without affecting their age-related modifications, and does not affect ACT urinary excretion. In conclusion, results indicate that the quantitative pattern of aldehyde production and urinary excretion may be altered by the process of aging.
The long-lasting depletions of creatine phosphate induced by feeding rats with a beta-guanidinopropionic acid (GPA)-supplemented diet induces specific mitochondrial alterations in skeletal muscles ...very similar to those observed in human mitochondrial myopathies. The slow-twitch soleus muscle appears to be affected primarily, while the fast-twitch extensor digitorum longus is affected less severely and only after a longer period of treatment (6 months). Changes in the enzyme activities of glucose metabolism appear to be secondary and differ between the two muscles. Withdrawal of GPA from the diet after 2 months of treatment shows that both mitochondrial alterations and biochemical modification are reversible.
We have studied the effect of T3 administration (50 micrograms/Kg/day) on the phenotype expression of several glucose-metabolizing enzymes (hexokinase, HK, glucose-6-phosphate dehydrogenase, G6P-DH, ...aldolase, ALD, phosphofructokinase, PFK, lactate dehydrogenase, LDH) in the different myocardial layers of the left ventricle wall. In the control rats, most of these enzyme activities are uniformly distributed across the left ventricle wall, G6P-DH being the only exception. In the rats given T3 for 14 days, the mean levels of PFK, HK and ALD activities increased significantly. With regard to the transmural distribution patterns, that of PFK was unchanged, unlike those of HK and ALD which exhibited their maximum increase in activity in the midmyocardium or in the mid- and subepicardial myocardium. With LDH, a significant increase in activity was found in the subepicardial layers which escaped detection on the whole homogenate. It is concluded that the administration of thyroid hormone has different effects on enzyme phenotype expression of cardiomyocytes in different regions of the cardiac wall.
The transmural distribution of the adenosine-generating enzyme 5'-nucleotidase (5'N) and of the adenosine-degrading enzymes adenosine deaminase (ADA), AMP deaminase (AMP-D) and adenosine kinase ...(Ado-K) were determined across the walls of left and right ventricles of control and hypertrophic rat hearts. The enzyme distribution across the left ventricle wall (but not across the right wall) of normal hearts was not uniform: 5'N activity shows its highest levels in the subepicardial and in the subendocardial regions, whereas all the other enzyme activities show their lowest levels. A similar pattern of transmural distribution was also detected in other mammalian species (ox and pig). In the experimental cardiac hypertrophy, caused by two different types of chronic cardiac overload, the levels and the profiles of transmural distribution of 5'N and ADA enzyme activities may significantly change across the rat left ventricle wall.
The effects of an altered thyroidal status on the levels of immunoreactive (ir-) atrial natriuretic peptide (ANP) in serum and in the right and left atria, as well as on the subcellular structures of ...atrial myoendocrine cells were explored in groups of male Sprague Dawley rats given the vehicle or triiodothyronine in the toxicological dose-range (50 micrograms/100 g bw/day) for 0,5, 1,2,4,7 or 14 days. Plasma levels of ir-ANP were 30% higher in T3-treated rats compared with controls at 0,5 and 1 day after hormone administration and then decreased to levels 30-40% lower than controls at days 2 and 4 to rise again above control values on day 7 and 14. Atrial ir-ANP levels decreased at first both in the right and in the left atria with different latencies (1 and 2 days, respectively) and rose back towards control levels by day 4. Changes in the numerical density of specific granules followed a parallel temporal pattern. An increased in the individual volume of the granules followed was also observed. Investigation into the circulatory effects of T3 administration showed that the heart rate was increased by hour 12 after hormone administration (simultaneously with the early rise in plasma ir-ANP levels) and that blood pressure was increased by day 2.
Changes in glycogen metabolism after an intravenous injection of angiotensin II were investigated in the left and right ventricles of the rat heart, as a function of location within the ventricular ...wall. Hearts were cut into 100-microns thin section, all of which were analysed for glycogen content, glucose incorporation into glycogen and 2-deoxyglucose uptake and phosphorylation after the intravenous injection of 14C-labelled sugar. In control hearts, glycogen levels were uniform across the wall in both ventricles, while the rate of sugar uptake and phosphorylation, and that of glucose incorporation into glycogen, were significantly higher in the subendocardial myocardium of the left ventricular wall. After angiotensin II administration, heart glycogen levels decreased slightly in the left, but not in the right ventricle, while 2-deoxyglucose uptake and phosphorylation, and glucose incorporation into glycogen, increased 2,5- and 5-fold, respectively. With regard to the distribution across the wall of the left ventricle after angiotensin administration, glycogen levels and glucose incorporation into glycogen were uniformly distributed, whereas sugar phosphorylation was still higher in the subendocardium.
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