Age-related alterations in the development of adrenergic denervation supersensitivity.

The density of beta-adrenergic receptors in the central nervous system exhibits marked age-related changes. In general, there is an initial increase in receptors soon after birth followed by a decline with advancing age; the specific pattern of the development and loss of receptors is dependent upon the brain area. The ontogenetic increase in the density of adrenergic receptors coincides temporally with the development of responsiveness to catecholamines but can proceed without an adrenergic innervation. This suggests that the biosynthesis of receptors is genetically predetermined and does not require an adrenergic input for initiation. Decreasing adrenergic activity produces an increased number of beta-receptors and a supersensitive response to adrenergic agonists. The decline in beta-receptors with advanced age appears to be related to this phenomenon of denervation supersensitivity since certain aged tissues have a diminished capacity to develop an increased number of receptors in response to a reduced sympathetic input. We conclude that the decline in beta-adrenergic receptors with age may explain the age-related decrease in the sensitivity of adenylate cyclase to catecholamines, and, consequently, the reduced physiological response to adrenergic stimuli. The mechanism for this loss of receptors may be the inability of aged tissue to develop a supersensitivity response in reaction to diminished sympathetic activity.     

Adrenergic responses of arterial and venous vessels in the cat skeletal muscle in hypoxia and hypothermia

In 12 anesthetized cats both hypoxia and hypothermia enhanced the shifts in arterial resistance against blood flow in response to noradrenaline and diminished the shifts in response to isoproterenol. Combination of both stress factors considerably weakened the adrenergic vasoconstriction and rather little affected the adrenergic vasodilatation. The findings suggest that protective effect of hypothermia against the background of a hypoxic stress might be a mechanism preventing disorders in the tissue metabolism.     

Angiotensin II potentiates responses of the rabbit basilar artery to adrenergic nerve stimulation

Angiotensin II has little contractile effect on the isolated rabbit basilar artery; however, it markedly potentiates contractile responses to adrenergic nerve stimulation. This is not a post-synaptic effect of angiotensin, as responses to exogenous norepinephrine are not altered. Angiotensin increases stimulation-evoked release of norepinephrine, and this effect probably accounts for the increased response to adrenergic nerve stimulation. Since sympathetic stimulation may protect the cerebral circulation from hypertensive damage, increased responsiveness to adrenergic nerve activity produced by angiotensin may have a beneficial effect.     

Early antigen-specific response by naive CD8 T cells is not altered with aging

Both a dramatic decline in CD8 responses and a switch to memory T cell predominance occur with aging. The extent to which the loss of responsiveness is the consequence of the accumulation of more differentiated vs intrinsically defective T cells (or both) has been unclear. Using similar conditions of Ag stimulation, we have examined the responses generated by CD8(+) cells isolated from aged TCR transgenic mice. We found that the naive transgene(+) CD8(+) cells from aged 2C mice expressed activation markers, produced IL-2, proliferated, and differentiated into cytotoxic T cells as efficiently as their young counterparts. The extent of responsiveness and the level of the responses were comparable in both age groups regardless of the stimulatory conditions used, i.e., partial costimulation/adhesion molecule expression on APCs, or presentation of lower affinity peptide or diminished peptide concentrations. By day 4 after Ag stimulation, no significant age-related differences were observed in the number of effector cells generated nor in the levels of secreted IL-2 or IFN-gamma. Upon restimulation of effector cells, IL-2 secretion and to a lesser extent TNF-alpha expression, but not IFN-gamma secretion, were diminished with age. These findings suggest that age-associated alterations in naive CD8 cell function are not found after primary stimulation, but may become apparent upon restimulation.     

Phosphorylation of phospholamban at threonine-17 reduces cardiac adrenergic contractile responsiveness in chronic pressure overload-induced hypertrophy

Physiological hemodynamic stress, such as aerobic exercise, is intermittent and requires an increase in Ca2+ -dependent contractility through sympathetic nervous system activation. Pathological hemodynamic stress, such as hypertension, is persistent and requires sustained increases in cardiac function. Over time, this causes left ventricular hypertrophy (LVH)-reduced responsiveness to sympathetic stimulation. In this study, we examined the hypothesis that blunted in vivo adrenergic contractile responsiveness in pressure overload (PO)-induced cardiac hypertrophy is caused by abnormalities in the abundance and/or basal phosphorylation state of Ca2+ regulatory proteins. PO, induced by aortic constriction, caused concentric LVH or dilated LVH. Only animals with dilation exhibited a decrease in baseline left ventricle function [fractional area change (FAC); measured with echocardiography]. All PO animals had a reduced contractile response to adrenergic agonists (increase in FAC with 40 microg.kg(-1).min(-1) dobutamine, control 0.30 +/- 0.04, n = 5 vs. banded 0.10 +/- 0.03, n = 10; P < 0.01). PO animals had reduced phospholamban (PLB) protein abundance (P = 0.07, not significant) and increased PLB phosphorylation at the calmodulin-dependent kinase II (CaMKII)-specific site (PLB-Thr17, P < 0.05) but not at the protein kinase A-specific site (PLB-Ser16). PLB-Thr17 phosphorylation was inversely correlated with dobutamine-induced increases in contractility in PO animals (r2 = 0.81, P < 0.05). Continuous induction of Ca2+ transients in isolated ventricular myocytes for 24 h increased phosphorylation at PLB-Thr17 and diminished inotropic responsiveness and PLB-Ser16 phosphorylation after exposure to isoproterenol (P < 0.05). These data show that reduced adrenergic responsiveness in feline PO hypertrophy and failure involves increases in basal PLB-Thr17 phosphorylation, suggesting that activation of CaMKII in PO hypertrophy contributes to defective adrenergic reserve in compensated LVH and early heart failure.     

Age-related alterations in the central thermoregulatory responsiveness to alpha-MSH

Alpha-melanocyte-stimulating-hormone (alpha-MSH) is a neuropeptide that induces weight loss via its anorexigenic and hypermetabolic/hyperthermic effects. Two major public health problems of the human population involving energy balance (i.e. middle-aged obesity and aging cachexia) also appear in other mammals, therefore age-related regulatory alterations may also be assumed in the background.Previous studies demonstrated characteristic age-related shifts in the anorexigenic effects of centrally applied alpha-MSH with strong effects in young adult, diminished efficacy in middle-aged and very pronounced responsiveness in old rats. The present study aimed to investigate age-related changes in the acute central thermoregulatory responsiveness to an alpha-MSH injection in rats and to compare them with those of food intake-related responsiveness. Oxygen consumption (VO₂), core (Tc) and tail skin temperatures (Ts, indicating heat loss) of male Wistar rats of different age groups (from 2 to 24 months of age), were recorded in an indirect calorimeter complemented by thermocouples upon intracerebroventricular alpha-MSH administration (0, 5 µg) at a slightly subthermoneutral environment (25–26 °C).Acute alpha-MSH-induced rises in VO₂ and Tc were most pronounced in the young adult age-group. In these rats the hyperthemic effects were somewhat diminished by an activation of heat loss. Juvenile animals showed weaker hyperthermic responses, middle-aged rats none at all. Alpha-MSH-induced hyperthermia became significant again in old rats.Acute thermoregulatory (hypermetabolic/hyperthermic) responsiveness to alpha-MSH shows a distinct age-related pattern similar to that of acute anorexigenic responsiveness.Thus, our results may also contribute to the explanation of both middle-aged obesity and aging cachexia.     

Effects of age on the adrenergic cardiac neuroeffector junction

Although it is clear that adrenergic nervous system control of cardiac function decreases with age and that the effector organ fails to adjust to this decreased control, it is not completely evident which of the many mechanisms operant at the adrenergic-cardiac neuroeffector junction contribute to this state. Prejunctionally, it appears that norepinephrine content decreases with age and that adrenergic axonal degeneration occurs. Also, evidence is available to suggest that modulation by prejunctional alpha adrenergic receptors of norepinephrine release is altered with increasing age, as is neuronal uptake of norepinephrine. Postjunctionally, it appears that beta-adrenergic receptor sensitivity to agonists undergoes age-related alterations, and possibly post receptor mechanisms involved in receptor-response coupling. Other mechanisms, such as those involved in transmitter uptake into extraneuronal sites, adrenergic neuronal responsiveness to stimulation, transmitter release and turnover, calcium and prejunctional receptor modulation of transmitter release, postjunctional receptor development of supersensitivity or subsensitivity, need further elucidation in order to have an understanding of the factors that contribute to the breakdown of homeostatic mechanisms that regulate the heart.     

Enhanced chronotropic and inotropic responses of rat myocardium to cholinergic stimulus with aging.

The ability of the heart to respond to adrenergic stimulation diminishes with aging, and this may be one of the factors contributing to the age-associated decline in cardiac stress responsiveness. On the other hand, little is known about the impact of aging on the responsiveness of the heart to cholinergic stimulation. In this study, we determined the chronotropic and inotropic responses of the isolated, Langendorff-perfused hearts from adult (6-8 months) and aged (28-30 months) rats to cholinergic agonists so as to assess age-related alterations in postsynaptic cholinergic control of heart function. The results showed the following. (i) In isolated perfused spontaneously bearing rat hearts, the negative chronotropic response to acetylcholine (10(-9)-10(-5) M) was up to 4-fold greater in the aged compared with adult hearts; this age-related difference was less marked (2-fold) but not abolished in the presence of a maximally effective concentration (5 microM) of the cholinesterase inhibitor eserine. (ii) The cholinesterase-resistant agonist carbachol (10(-9)-2.5 x 10(-6) M) elicited a 2- to 3-fold greater negative chronotropic response in the aged compared with adult hearts. (iii) In isolated perfused, electrically paced (4 Hz) rat hearts, carbachol (10(-9)-10(-5) M) elicited a concentration-dependent negative inotropic response, which was 2-fold greater in the aged compared with adult heart at all carbachol concentrations. (iv) Acetylcholinesterase activities (micromoles per gram per hour) were 50-60% lower in the aged atria (83 +/- 21) and ventricles (24 +/- 6) than in adult atria (210 +/- 20) and ventricles (47 +/- 7).(ABSTRACT TRUNCATED AT 250 WORDS)     

Alterations in cardiovascular responsiveness and adrenoceptor binding during catecholamine infusion hypertension in rats.

Chronic continuous infusion of norepinephrine in rats causes alterations in biochemical and physiologic responses of the cardiovascular system and in cardiovascular adrenoceptor number. The response of cardiac and aortic ornithine decarboxylase (ODC) activity to stimulation by norepinephrine was decreased in rats receiving norepinephrine infusion. These responses are due to stimulation of beta- and alpha-adrenergic receptors, respectively. Additionally, there was reduced stimulation of aortic ODC activity by angiotensin II and vasopressin. The cardiac ODC response to angiotensin II was decreased, but the response to vasopressin was not affected. The decreased ODC response is accompanied by decreased pressor responses to the alpha-adrenergic agonist phenylephrine. Decreased numbers of alpha- and beta-adrenoceptor binding sites (as measured by the binding of [3H]prazosin and [125I]pindolol) might mediate, in part, the altered responses to adrenergic agonists. The decreased cardiovascular responsiveness measured in these animals after several days of norepinephrine infusion hypertension contrasts with the increased responses found in most other forms of hypertension. This provides a useful model in which to examine the consequences of prolonged adrenergic receptor stimulation.     

Diminished mesenteric vaso- and venoconstriction and elevated plasma ANP and BNP with simulated microgravity.

Diminished constriction of arteries and veins following exposure to microgravity or bed rest is associated with a reduced ability to augment peripheral vascular resistance (PVR) and stroke volume during orthostasis. We tested the hypothesis that small mesenteric arteries and veins, which are not exposed to large pressure shifts during simulated microgravity via head-down tail suspension (HDT), will exhibit decrements in adrenergic constriction after HDT in rats. Small mesenteric arteries and veins from control (Con; n = 41) and HDT (n = 35) male Sprague-Dawley rats were studied in vitro. Vasoactive responsiveness to norepinephrine (NE) in arteries (10(-9) to 10(-4) M) and veins (pressure-diameter responses from 2 to 12 cmH(2)O after incubation in 10(-6) or 10(-4) M NE) were evaluated. Plasma concentrations of atrial (ANP) and NH(2)-terminal prohormone brain (NT-proBNP) natriuretic peptides were also measured. In mesenteric arteries, sensitivity and maximal responsiveness to NE were reduced with HDT. In mesenteric veins there was a diminished venoconstriction to NE at any given pressure in HDT. Plasma concentrations of both ANP and NT-proBNP were increased with HDT, and maximal arterial and venous constrictor responses to NE after incubation with 10(-7) M ANP or brain natriuretic peptide (BNP) were diminished. These data demonstrate that, in a vascular bed not subjected to large hydrodynamic differences with HDT, both small arteries and veins have a reduced responsiveness to adrenergic stimulation. Elevated levels of circulating ANP or NT-proBNP could adversely affect the ability of these vascular beds to constrict in vivo and conceivably could alter the intrinsic constrictor properties of these vessels with long-term exposure.     

Actions of angiotensin on adrenergic nerve endings.

In the perfused vascular bed, vasoconstrictor responses to adrenergic nerve stimulation are augmented to a greater degree by angiotensin II than are the responses to injected norepinephrine. Overflow of adrenergic transmitter is also greater during nerve stimulation in the presence of angiotensin than in its absence. The evidence indicates that facilitation of adrenergic transmitter release rather than uptake blockade accounts for these results. In addition, an increased responsiveness of isolated arterial strips to norepinephrine as well as other agonists appears to contribute to the adrenergic potentiating effect of angiotensin II as well as angiotensin III. This action, which appears to be a cell membrane effect, seems to participate in adrenergic potentiation mainly in the arterial segment of the intact vascular bed. Both of these effects of angiotensin, i.e., facilitation of release and increased smooth muscle responsiveness, appear to be mediated by angiotensin receptors.     

Age-related alterations in superoxide anion generation in mouse peritoneal macrophages studied by repeated stimulations and heat shock treatment.

The ability of thioglycollate-elicited peritoneal macrophages (PM) from young and senescent mice to generate superoxide anions (O2-) under repeated stimulation or thermal stress was studied using either zymosan, opsonized zymosan (OZ), or phorbol myristate acetate (PMA). A diminished capacity to recover from repeated stimulation was found with aging. When stimulated for a second time 24 hours after the primary stimulation, PM from young animals generated 80% of the initial O2- responses to either zymosan, or OZ. Under the same conditions, PM from senescent mice generated 62% of the initial O2- produced in response to zymosan, and 45% in response to OZ. In both age groups the response to a second PMA stimulation comprised only 10% of the primary response. A considerably diminished capacity to generate O2- was also demonstrated in PM from senescent mice after recovery from exposure to thermal stress. Exposure to 42.5 degrees C for 20 minutes was found to be the threshold temperature for irreversible loss of activity in senescent PM, whereas at this temperature, PM from young animals recovered up to 70% of their O2- generating activity. Since NADPH oxidase and superoxide dismutase activities were only mildly affected by the hyperthermia in all age groups, they could not account for the age-related decline in the recovery from stress. Age-related alterations in signal transduction or receptor alterations could possibly play a primary role in this decline.     

Human Neonatal Cardiovascular Progenitors: Unlocking the Secret to Regenerative Ability

Although clinical benefit can be achieved after cardiac transplantation of adult c-kit+ or cardiosphere-derived cells for myocardial repair, these stem cells lack the regenerative capacity unique to neonatal cardiovascular stem cells. Unraveling the molecular basis for this age-related discrepancy in function could potentially transform cardiovascular stem cell transplantation. In this report, clonal populations of human neonatal and adult cardiovascular progenitor cells were isolated and characterized, revealing the existence of a novel subpopulation of endogenous cardiovascular stem cells that persist throughout life and co-express both c-kit and isl1. Epigenetic profiling identified 41 microRNAs whose expression was significantly altered with age in phenotypically-matched clones. These differences were correlated with reduced proliferation and a limited capacity to invade in response to growth factor stimulation, despite high levels of growth factor receptor on progenitors isolated from adults. Further understanding of these differences may provide novel therapeutic targets to enhance cardiovascular regenerative capacity.     

Response of Leucocyte Adenyl Cyclase to Isoprenaline and Effect of Alpha-blocking Drugs in Extrinsic Bronchial Asthma

The finding by several workers that biochemical responses to catecholamines are diminished in asthmatic patients during periods of active asthma as compared to normal subjects has led to the recognition of the beta-adrenergic blockade phenomenon, a common accompaniment of extrinsic bronchial asthma. Using an intact cell method to measure leucocyte adenyl cyclase activity, we have been able to show that there is a noticeably reduced responsiveness of this enzyme system (which is now identified with beta-receptor function) to isoprenaline in the leucocytes of patients suffering from acute bronchial asthma, but that asthmatic patients in remission could not be distinguished from normal persons in this respect. Evidently the defective beta-receptor function may be associated with overactivity of the alpha-receptors in acute bronchial asthma, since the responsiveness to isoprenaline stimulation could be restored towards normal by concomitant treatment of the leucocytes of these patients with alpha-receptor blocking drugs such as phentolamine or thymoxamine. Ouabain, though somewhat less potent, also enhanced responsiveness to isoprenaline stimulation. The relation of these results to the clinical observation of adrenaline resistance in active asthma suggests that alpha-receptor blocking drugs may be of value in restoring the sensitivity of beta-receptors to sympathomimetic amines.     

Responses of the hypothalamic-pituitary-adrenal axis to noradrenergic and hormonal stimulation in prenatally stressed rats

We studied the effects of maternal stress (the so-called prenatal stress, PS, provided by immobilization of pregnant female Wistar rats for 1 h daily during the 15–21st gestational days) on the corticosterone response in the blood plasma evoked by infusion of 10 μg noradrenaline bitartrate into the III cerebral ventricle or by injection of β-1-24-corticotropin in 3-month-old male and female offspring. The animals were bearing an intracerebroventricular stainless steel guide cannula implanted eight to nine days before the experiment, and a Silastic catheter inserted into the external jugular vein 24 h prior to the experiment. Blood samples were periodically taken from conscious unrestrained rats (before and then 30, 60 and 90 or 120 min after noradrenaline or corticotropin challenge). In the male offspring PS augmented and prolonged an increase in the plasma corticosterone level resulting from adrenergic stimulation of the hypothalamus, as compared with that in non-stressed animals. In prenatally stressed female offspring tested in diestrus, there was no response of the hypothalamic-pituitary-adrenal (HPA) axis to intracerebroventricular noradrenaline stimulation, in contrast to what was observed in the control. Prenatal stress did not modify the adrenal cortex responsiveness to corticotropin either in male or in female offspring. The results demonstrate differential effects of PS on the adrenergic activation of the HPA axis in males and females. A decrease in the acute HPA stress-responsiveness in prenatally stressed male rats, which was demonstrated in an earlier study, and the maintenance or even enhancement of this effect in prenatally stressed females are not likely to be connected with the state of hypothalamic adrenergic reactivity.     

Sodium selenate partially corrects impaired functional responses in detrusor muscle in streptozotocin-induced diabetic rats

Selenium selenate was administered to streptozotocin-induced diabetic rats to assess its effects on the detrusor muscle. Thirty-two rats were divided into four groups of eight subjects each. The study animals were made diabetic by means of a single intravenous injection of streptozotocin (STZ). The responsiveness of the detrusor was improved in the group injected with sodium selenate. Diabetes caused significant increases in carbachol and β,γ-MeATP-evoked contractions and significant decrease of contractions induced by electrical stimulation. Isoprenaline-induced relaxation of the detrusor muscle was diminished by diabetes, whereas ATP relaxation appeared to be increased. Although adenosine-induced relaxations in controls and in diabetic rats were accompanied by unchanged responses in normoxic conditions, a significant enhancement in the detrusor muscle was observed during hypoxia. This enhancement of adenosine responsiveness in hypoxic conditions is inhibited in diabetes. Treatment with sodium selenate prevented alterations of both carbachol-induced contractility and isoprenaline-evoked relaxation, whereas nerve-mediated contractions and purinergic responses were not improved in diabetic rats after treatment. Our data suggest that changes in cholinergic and adrenergic responses were the result of selenium deficiency in diabetic rats.     

Reduced adenosine release from the aged mammalian heart

Adenosine (ADO) released in the heart results in enhanced coronary blood flow and reduced catecholamine release and myocardial responsiveness to adrenergic stimulation (anti-adrenergic action). ADO release from the adrenergic-stimulated aged heart is less than that from the young adult heart. Because adrenergic signaling in the aged heart is impaired, this study was conducted to determine if reduced ADO release from the aged heart results from this reduced adrenergic responsiveness. Hearts of 3-4 months (young adult) and 21-22 months (aged) Fischer-344 rats were perfused with ADO deamination and re-phosphorylation inhibited. Coronary effluent ADO levels were determined. Cellular-free ADO levels with and without sodium acetate (NaAc)-induced mitochondrial AMP synthesis were assessed using formed S-adenosylhomocysteine (SAH) in L-homocysteine thiolactone (L-HC)-treated hearts. The activities of SAH-hydrolase were determined. Aged heart ADO release was 61% less than from young hearts. NaAc augmented young heart ADO release by 104%, while that of aged hearts remained unchanged. SAH synthesis was 51% and 56% lower in the aged heart in the absence and presence of NaAc, respectively, despite an 89% greater SAH hydrolase activity found in the aged hearts. Since synthesized AMP may be diverted to IMP and ultimately inosine by AMP deaminase, inosine release was determined. Aged heart inosine levels in the absence and presence of NaAc were 74% and 59% less than for the young hearts. It is concluded that a reduced mitochondrial AMP synthesis is in part responsible for the attenuation in ADO release from the adrenergic-stimulated aged heart.     

Common mechanisms for declines in oxidative stress tolerance and proliferation with aging

Aging is often characterized by reduced stress tolerance and decreased proliferative capacity, but little is known about the effects of aging on signaling pathways important in regulating these responses. Recent studies from our laboratory have implicated impairments in epidermal growth factor receptor (EGFR) signaling and extracellular signal-regulated kinase (ERK) activation to both effects in rat hepatocytes. Here we investigated the responsiveness of hepatocytes derived from young (4-5 months) and aged (24-29 months) mice to proliferative signals (low concentrations of H2O2 and epidermal growth factor [EGF] stimulation), and oxidant injury (high H2O2 concentrations). Old hepatocytes displayed lower levels of DNA synthesis in response to low H(2)O(2) concentrations (5-10 microM) and EGF stimulation, and reduced survival following treatment with high H2O2 concentrations (20-50 microM). Both effects were associated with reduced activation of ERK and diminished phosphorylation of EGFR tyrosine residue 1173. p38 was also activated by H2O2, but to a greater extent in old cells. Pharmacologic inhibition of ERK increased the sensitivity of young cells to H2O2-induced cell death, while inhibition of p38 decreased the sensitivity of old cells. Our findings suggest that impairments in common signaling events underlie age-related declines in proliferative capacity and oxidative stress tolerance in mouse hepatocytes, and that an imbalance in ERK and p38 activities contributes to the greater sensitivity of aged cells to H2O2.