Age-related changes of liver tyrosine aminotransferase in senescent rats

Tyrosine aminotransferase was induced in adult and senescent rat liver and its properties studied. We show the appearance of a 'cross-reacting material' for induced tyrosine aminotransferase of old rats compared to basal enzyme; this cross-reacting material can be provoked in adult rats after injection of cycloheximide, and suppressed in adult and old rats after injection of a serine protease inhibitor (tosylphenylalanine chloromethylketone). Other properties of induced tyrosine aminotransferase (thermostability, Km for tyrosine, isoelectrofocusing) are identical except for the proportion of the three forms and their sensitivity to trypsin in the absence of pyridoxal phosphate, which is increased in senescent animals. The suppression of cross-reacting material clearly indicates that it is not due to errors on old rat liver DNA but rather to post-translational modifications. This demonstrates also the role of serine proteases in tyrosine aminotransferase degradation. We suggest that induced enzyme of senescent rats would undergo a conformational change, possibly due to a release of pyridoxal phosphate from the enzymic molecules, which would thus become more susceptible to proteolytic attack than those of adult rats.     

Protein turnover and cathepsin B activity in several individual tissues of foetal and senescent rats

The fractional rate of protein synthesis has been measured in vivo, and compared in the whole body and 12 major individual tissues of foetal and senescent rats. This synthetic rate was found to decrease in most tissues with increasing animal age. The rate of protein degradation was also determined and compared with cathepsin B activity within each tissue; both protein turnover and the endopeptidase activity decreased with ageing. Age-related changes in each tissue's contributions to the protein mass and synthetic rates of the whole animal are also summarized and related to developmental variations in physiological function.     

Regulation of gluconeogenesis in the liver of vitamin B1-deficient rats

Radiometric assays revealed that thiamine deficiency in rats to whom hydroxythiamine was administered in variable doses, is concomitant with activation of gluconeogenesis from pyruvate in liver tissue. The most probable mechanism of this effect is the cAMP-dependent activation of key enzymes of intracellular glucose synthesis. This process is facilitated by the diminution of the ratio of free forms of NAD+ and NADPH in the cytoplasm.     

Reduced tolerance to morphine thermoregulatory effects in senescent rats

Age-related differences in the thermoregulatory response to morphine have been shown in rats. To determine if these age-related differences would be reflected in the acquisition of tolerance, we studied morphine tolerance induced by either a single morphine dose or implantation of a morphine pellet. precipitated withdrawal was also analyzed by inducing withdrawal with naloxone in morphine-pelleted rats. Senescent (26 or 27 month old), mature (10 or 11 month old) and young (3 or 4 month old) male Fischer 344 rats were restrained and changes in rectal temperature were monitored for six hours after morphine administration. Only mature and young rats exhibited increased hyperthermic responses to a second low dose of morphine (5 mg/kg s.c.). Only young rats became tolerant after a single higher morphine dose (25 mg/kg s.c.). All age groups showed tolerance three days after morphine pellet implantation. Hypothermia was equivalent in all age groups when withdrawal was induced by naloxone in morphine-pelleted rats. These results indicate that older rats were more resistant to the acquisition of tolerance to the thermic effects of morphine; however; with continued morphine treatment, rats became tolerant regardless of age.     

Tempol improves vascular function in the mesenteric vascular bed of senescent rats

Ageing is associated with structural and functional alterations of the vasculature. The nature of age-related vascular disorders is not completely understood. Oxidative stress is hypothesized to play a crucial role in the pathophysiology of vascular complications. We investigated the effects of chronic treatment with the superoxide dismutase mimetic tempol (4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl) on vascular function in the mesenteric vasculature of aged rats. Young (3 weeks) and old (40 weeks) Sprague-Dawley rats were treated with tempol (1 mM in drinking water) or vehicle for 3 weeks. Arterial blood pressure was slightly, but significantly, higher in old than in young rats. Tempol had no effect on arterial blood pressure. The vasoconstrictor responses to norepinephrine (NE) and serotonin (5-HT) were exaggerated in the mesenteric vascular bed (MVB) removed from old rats. Vasodilator responses to acetylcholine (ACh), papaverine (PPV), and isoprenaline (ISO) were reduced in the MVB of old rats in comparison with young rats. Chronic treatment of old rats with tempol normalized their responses to NE and 5-HT. The dilator responses to ACh, PPV, and ISO were similar between old rats receiving tempol and young rats. The present findings suggest that oxidative stress contributes to vascular dysfunction in the mesentery of old rats. The vasculoprotective effects of tempol remain to be elucidated.     

Ischemia-induced alterations of mitochondrial structure and function in brain, liver, and heart muscle of young and senescent rats

Young and senescent rats (3 and 28-30 months old) were subjected to complete ischemia at 37 degrees C in order to study function and structure of mitochondria isolated from liver, heart muscle, and brain. The rates of energy-coupled respiration and ATP synthesis were found to decrease progressively in relation to time of ischemia. The respiratory rates in the absence of ADP (state 4 respiration) did not increase after exposure to ischemia, suggesting that ischemia primarily affects electron transport rather than the energy coupling system. Mitochondria of heart muscle were more affected by ischemia than mitochondria of brain and liver. Liver and heart muscle mitochondria obtained from young rats were found to be slightly more sensitive to short periods of ischemia than those isolated from senescent animals.     

Sympathetic nerve regulation to heating is altered in senescent rats

Renal and splanchnic sympathetic nerve discharge (SND) responses to increased (38-41 degrees C) internal temperature were determined in anesthetized young (3-6 mo old), mature (12 mo old), and senescent (24 mo old) Fischer 344 (F344) rats. We hypothesized that SND responses would be altered in senescent and mature rats as demonstrated by attenuated sympathoexcitatory responses to heating and by the absence of hyperthermia-induced SND pattern changes. The following observations were made. 1) Renal and splanchnic SND responses were significantly increased during heating in young and mature but not in senescent rats. 2) At 41 degrees C, renal and splanchnic SND responses were higher in young compared with senescent rats, and renal SND was higher in mature than in senescent rats. 3) Heating changed the SND bursting pattern in young, but not in mature or senescent, rats. 4) SND responses to heating did not differ between baroreceptor-innervated (BRI) and sinoaortic-denervated (SAD) senescent rats but were higher in SAD compared with BRI young rats. These results demonstrate an attenuated responsiveness of sympathetic neural circuits to heating in senescent F344 rats.     

Cardioprotective effect of ischemic preconditioning is preserved in food-restricted senescent rats

Ischemic preconditioning (PC) has been proposed as an endogenous form of protection against-ischemia reperfusion injury. We have shown that PC does not prevent postischemic dysfunction in the aging heart. This phenomenon could be due to the reduction of cardiac norepinephrine release, and it has also been previously demonstrated that age-related decrease of norepinephrine release from cardiac adrenergic nerves may be restored by caloric restriction. We investigated the effects on mechanical parameters of PC against 20 min of global ischemia followed by 40 min of reperfusion in isolated hearts from adult (6 mo) and "ad libitum"-fed and food-restricted senescent (24 mo) rats. Norepinephrine release in coronary effluent was determined by high-performance liquid chromatography. Final recovery of percent developed pressure was significantly improved after PC in adult hearts versus unconditioned controls (85.2 +/- 19% vs. 51.5 +/- 10%, P < 0.01). The effect of PC on developed pressure recovery was absent in ad libitum-fed rats, but it was restored in food-restricted senescent hearts (66.6 +/- 13% vs. 38.3 +/- 11%, P < 0.05). Accordingly, norepinephrine release significantly increased after PC in both adult and in food-restricted senescent hearts, and depletion of myocardial norepinephrine stores by reserpine abolished the PC effect in both adult and in food-restricted senescent hearts. We conclude that PC reduces postischemic dysfunction in the hearts from adult and food-restricted but not in ad libitum-fed senescent rats. Despite the possibility of multiple age-related mechanisms, the protection afforded by PC was correlated with increased norepinephrine release, and it was blocked by reserpine in both adult and food-restricted senescent hearts. Thus caloric restriction may restore PC in the aging heart probably via increased norepinephrine release.     

Aldolase biosynthesis and stability in a monolayer of intact and irradiated rat liver cells

Aldolase turnover in rat hepatic cell culture and the influence of whole-body X-irradiation on the rates of synthesis and degradation of the enzyme and its "half-life" have been investigated. Aldolase biosynthesis in irradiated cells increases significantly as the rate of its degradation grows and the time of its functioning decreases.     

Organ culture of the islets of Langerhans from young and senescent rats

Summary The B-cells of the endocrine pancreas constitute an adequate model for in vitro study of the aging process in highly differentiated cells. In the present study, collagenase-isolated islets of Langerhans from young and senescent rats were cultured up to 28 days. The response of the B-cells to the stimulatory conditions of the culture medium involved the nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, and secretory granules. Correlated data from light microscopy, electron microscopy, and insulin radioimmunoassay show that the differentiation and function of senescent B-cells are maintained in culture, as it has been proven for the B-cells of younger animals. On the other hand, signs of cytological deficiency not directly concerned with the specific function of B-cells were observed: abnormal mitochondria and lysosomes are more numerous in the senescent B-cells. The proliferative capacity of the B-cells of aged rats is reduced. Send offprint requests to: Université Catholique de Louvain, Unité de Morphologie animale, Place Croix du Sud, 5, B 1348 Louvain-la-Neuve, BelgiumM.C.M. is currently associated with the Unité d'Histologie, Université de Louvain