Age-related alterations in the cardiovascular response to adrenergic mediated stress

Cardiovascular adaptation to stress is highly dependent on adrenergic stimulation. It may be hypothesized that the diminished cardiovascular response to acute stress that occurs in advanced age may result in part from an age-related decrease in the effectiveness of adrenergic stimulation. Studies employing beta-adrenergic agonists and antagonists in both man and animals provide support for this hypothesis. In addition, a postsynaptic decrement in sympathetic responsiveness is indicated from in vitro studies in both cardiac and vascular tissue. However, while a strong case for diminished adrenergic responsiveness of the aged cardiovascular system can be made from these data, further information at both the tissue and the organismal level is required to fully elucidate the nature of this age-related decline.     

Regulation of brain adrenergic receptors during aging

Recent studies in our laboratory suggest that the synthesis of alpha- and beta-adrenergic receptors in certain tissues from brain and pineal gland may be impaired with age. This decreased ability of the aged brain to synthesize adrenergic receptors may explain the loss of these receptors in selected brain regions during the aging process, as well as the reduced capacity of aged brain tissue to increase or up-regulate the density of these receptors in response to reduced noradrenergic activation of the tissues or to reduced estrogen levels. The reduced adaptability of brain adrenergic receptors, in turn, may account for the decreased ability of aged individuals to adjust their physiological responses to a changing environment.     

Cardiac beta-receptor variation in rat strains selectively bred for differences in susceptibility to stress

The radioligand 3H-DHA was used to estimate the density and affinity of cardiac beta-receptors in rat strains selectively bred for differences in response to stress. Maudsley Reactive rats selected for heightened reactivity to stress had a greater density of beta-adrenergic binding in cardiac membranes than rats of two genetically distinct Maudsley Non-Reactive strains selected for decreased reactivity to stress, and compared with one of these Non-Reactive strains the MNR/Har, increased affinity for 3H-DHA. Together with previous findings the present results demonstrate a negative correlation between estimates of basal sympathetic activity on the on hand, and post-synaptic beta-receptors in heart on the other, that are consistent with the notion that these receptor alterations have occurred as a result of long-term differences in pre-synaptic release of transmitter. The Maudsley strains may, therefore, provide a useful model for the study of beta-adrenergic receptors as a physiological locus for regulation of end-target responsiveness to sympathetic stimulation.     

Effect of age and hypoxia on beta-adrenergic receptors in rat heart.

Previous reports suggest that hypoxia downregulates cardiac beta-adrenergic receptors from young rats. Because aging alters response to stress, we hypothesized an age-related alteration in the response to hypoxia. Male Fischer-344 rats, aged 3 and 20 mo, were divided into control and hypoxic groups. The hypoxic rats were exposed to hypobaric hypoxia (0.5 atm) for 3 wk. After hypoxic exposure, body weight decreased, hematocrit increased, right ventricular weight increased, and left ventricular weight decreased in all animals. beta-Adrenergic receptor density declined after hypoxic exposure in the young but not in the older animals, a change that was confined to the left ventricle. beta-Adrenergic receptor density in the right ventricle was significantly lower in the older animals than in the young animals. Plasma catecholamines (norepinephrine, epinephrine) drawn after the animals were killed (stress levels) decreased in young rats and increased in old rats after the exposure to hypoxia. Hypoxia is a useful physiological stress that elucidates age-related changes in cardiac beta-adrenergic receptor and catecholamine regulation that have not previously been described.     

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.     

Physical and adrenergic factors affecting systemic vascular resistance in the rainbow trout: a comparison with branchial vascular resistance.

The passive distensibility and adrenergic reactivity of the systemic vascular resistance (Rs) in Salmogairdneri have been studied using perfused trunk preparations, and the data compared with previous results on the branchial resistance (Rg). At normal levels of efferent blood pressure, Rs is relatively more distensible than Rg in response to afferent pressure increases, but this difference may not be important in vivo. alpha-adrenegic constrictory receptors predominate in Rs, in contrast to beta-adrenergic dilatory receptors in Rg; a significant alpha-adrenergic tone in Rs is lost during perfusion. Rs is far less sensitive than Rg to circulatory catecholamine levels. It is suggested that the sympathetic nervous system, rather than plasma catecholamines, provides the effective adrenergic control of Rs in vivo.     

Dissociation of beta-adrenergic receptors from hormone responsiveness during maturation of the rat reticulocyte.

Intact rat erythrocytes and reticulocytes have been studied in relation to their concentration of beta-adrenergic receptors and their responsiveness to beta-adrenergic catecholamines. Characteristics of the beta-receptor, as determined by binding of 125I-labelled hydroxybenzylpindolol, were compared among control erythrocytes and reticulocytes. The dissociation constant (Kd = 0.1--0.2 nM), association and dissociation kinetics, and stereospecificity for (--)-isomers of agonists and antagonists were similar in both cell types. The reticulocyte population contained four times more receptors per cell than the control erythrocytes. However, reticulocytes were 25 times more responsive than control cells to isoproterenol, as measured by the formation of cyclic AMP. After peak reticulocytosis, cells rapidly lost 95% of their maximum hormone responsiveness, but beta-receptors declined much more slowly. The 4-fold decrease in beta-receptors was associated with a 4-fold decrease in cell volume as the reticulocytes matured. The density of beta-receptors was unchanged. However, responsiveness to isoproterenol in the reticulocytes when expressed on the basis of cell volume was still nine times greater than the control cells. Thus, maturation of reticulocytes is associated with an uncoupling of persistent beta-receptors from catecholamine responsiveness.     

A sulfhydryl group of the canine cardiac beta-adrenergic receptor observed in the absence of hormone

Canine cardiac beta-adrenergic receptors contain a free sulfhydryl group in the adrenergic ligand binding site. [125 I]-Iodohydroxybenzylpindolol [( 125 I]-IHYP) binding to cardiac beta-receptors was inhibited 80% by treatment with 1 mM p-chloromercuribenzoic acid (pCMB). Occupation of the beta-receptors by an antagonist prior to treatment with pCMB prevented this effect suggesting that a sulfhydryl group is present in or near the ligand binding site of the cardiac beta-receptor. In the presence of agonists, the sensitivity of cardiac beta-receptors to pCMB was increased. Incubation of isoproterenol-occupied cardiac beta-receptors, resulted in a 57% inhibition of [125 I]-IHYP binding measured after extensive washing to remove bound agonist. The ability of isoproterenol to increase the reactivity of cardiac beta-adrenergic receptors supports the hypothesis that agonists produce a conformational change upon binding.     

Catecholaminergic regulation of the hypothalamic-pituitary-adrenocortical activity.

The significance and site of adrenergic receptors involved in the control of the hypothalamic-pituitary-adrenal axis (HPA) activity was assessed indirectly by estimation of serum corticosterone levels 1 h after drug administration to conscious rats. Adrenergic drugs were given intracerebroventricularly (icv) and intraperitoneally (ip), the antagonists 15 min prior to the agonists. Noradrenaline, adrenalin and isoproterenol given by either route increased dose-dependently the serum corticosterone levels. The corticosterone response to icv noradrenaline was almost abolished by icv pretreatment with propranolol, a beta-adrenergic antagonist, and yohimbine, and alpha 2-receptor blocker, and was also considerably reduced by prazosin, an alpha 1-adrenergic antagonist. When given ip, these antagonists did not significantly influence the noradrenaline induced corticosterone response, which suggests a suprapituitary site of action of noradrenaline in stimulation of the HPA. The corticosterone response to icv adrenalin was suppressed by prazosin given by either route. The corticosterone response to ip adrenalin was almost abolished by pretreatment with yohimbine, and also significantly diminished by propranolol given by the same route. The increase in corticosterone secretion, induced by isoproterenol given by either route, was abolished by ip injection of propranolol. These results indicate that noradrenaline stimulates the HPA via alpha- and beta-adrenergic receptors, mainly at the suprapituitary level. Adrenalin increases that activity both via central and pituitary alpha- and beta-adrenoceptors. Isoproterenol activates the HPA by stimulation of pituitary beta-receptors.     

Alpha 1- and beta-adrenergic receptors in brown adipose tissue of lean (Fa/?) and obese (fa/fa) Zucker rats. Effects of cold-acclimation,sucrose feeding and adrenalectomy.

1. The populations of alpha 1- and beta-adrenergic receptors in brown adipose tissue (BAT) of genetically obese Zucker rats (fa/fa) were studied with [3H]prazosin and [3H]CGP-12177 respectively. 2. The density of alpha 1-adrenergic receptors in BAT was significantly lower in obese than in lean Zucker rats, both at 2-4 months of age and at 6 weeks of age. The density of beta-adrenergic receptors was identical in BAT of lean and obese 6-week-old Zucker rats. 3. Cold-acclimation increased the alpha 1-receptor density significantly in BAT of both lean and obese Zucker rats, and the number of beta-receptors was also somewhat increased. 4. Sucrose feeding did not affect the density of alpha 1-receptors in BAT of lean or obese Zucker rats, but it increased beta-receptor density. 5. Adrenalectomy restored the density of alpha 1-adrenergic receptors in BAT of obese Zucker rats to the value observed in lean rats. 6. It is concluded that there is a direct correlation between alpha 1-receptor density and tissue recruitment, and that alpha 1-receptor density is thus positively correlated with sympathetic activity. beta-Receptor density is apparently better correlated with feeding conditions.     

Changes in adrenergic receptors during the development of heart failure

Moderate and severe stages of congestive heart failure due to the loss of myocardium upon coronary occlusion in rats was associated with an increase in alpha-adrenergic receptors and a decrease in beta-adrenergic receptors in the viable left ventricle. However, at early stages of heart failure the number of beta-adrenergic receptors was decreased without any changes in the number of alpha-adrenergic receptors. The affinities of these receptors to alpha receptor antagonist (³H-prazosin) and beta receptor antagonist (³H-dihydroalprenolol) were not altered in the failing hearts. On the other hand, the pattern of changes in both alpha- and beta-adrenergic receptors in heart membranes treated with oxygen free radical generating system was different from that seen in the failing hearts. In particular, the affinities for these receptors were decreased whereas the number of beta-receptors was increased and the number of alpha-receptors was decreased or unchanged. These results indicate that alterations in the adrenergic receptors in heart failure are not due to the formation of oxygen free radicals.     

Development of the linkage of beta-adrenergic receptors to cardiac hypertrophy and heart rate control: neonatal sympathectomy with 6-hydroxydopamine

Presynaptic neural projections are thought to participate in the maturation of postsynaptic sensitivity to neurotransmitters. In the current study, we have examined the effects of sympathectomy with 6-hydroxydopamine on the ontogeny of the linkage of beta-adrenergic receptors to cardiac growth and heart rate control in the rat. Destruction of sympathetic projections at birth compromised the ability of beta-receptor stimulation to evoke cardiac hypertrophy, a defect which persisted into young adulthood. The chronotropic response to beta-receptor activation, assessed by acute challenge with a submaximally-effective dose of isoproterenol, also exhibited a slowed development, but did eventually achieve normal sensitivity. In contrast, neonatal sympathectomy had only minor effects on resting heart rate, basal heart rate (the intrinsic rate in the absence of autonomic input) or maximal heart rate; these animals also showed beta-receptor desensitization of chronotropic action in response to chronic isoproterenol treatment. Chronic isoproterenol treatment itself lowered the basal heart rate, regardless of whether animals were normal or sympathectomized. Thus, during critical developmental periods, sympathetic input to beta-receptors selectively programmes the linkage between postsynaptic receptors, tissue growth and heart rate.     

Effect of hydrocortisone on beta-adrenergic receptors in lung membranes.

It has been observed that glucocorticoids potentiate beta-adrenergic stimulation of cardiovascular and airway tissues. In order to investigate the mechanism of this potentiating action, we examined the effect of glucocorticoids on the number and affinity of beta-adrenergic receptors in animal lung tissues, by a direct binding technique using [125]I-Iodohydroxybenzylpindolol ([125]I-HYP), a potent beta-adrenergic receptor antagonist. Specific binding of [125]I-HYP to rat lung membranes was saturable with 386 fmol of [125]I-HYP/mg protein at saturation. The apparent equilibrium dissociation constant of [125]I-HYP for beta-receptors was 221 nM. Chronic administration of hydrocortisone increased the density of beta-adrenergic receptors by 70% from 386 fmol to 657 fmol/mg with some decrease in the affinity of [125]I-HYP for beta-adrenergic receptors. By contrast, adrenalectomy produced a 29% fall in the number of beta-adrenergic receptors without altering the affinity of [125]I-HYP for beta-receptors, and this change was reversed by exogenous adminstration of hydrocortisone. The present study suggests that glucocorticoids may participate in regulating the density of beta-adrenergic receptors, and may potentiate beta-adrenergic receptors stimulation, at least in part by increasing beta-receptor density in tissue membranes.     

Isoproterenol-induced desensitization of adenylate cyclase in human astrocytoma cells. Relation of loss of hormonal responsiveness and decrement in beta-adrenergic receptors.

Incubation of human astrocytoma cells (1321N1) with low concentrations of isoproterenol results in a specific loss of responsiveness to catecholamines as evidenced by a decreased accumulation of cAMP in intact cells, a reduction in isoproterenol-stimulated adenylate cyclase activity, and a decrease in beta-adrenergic receptor density, as measured by the specific binding of 125I-hydroxybenzylpindolol. The kinetics of desensitization suggest the involvement of two different reactions. The initial reaction involves a rapid loss of adenylate cyclase activity with little loss of beta-adrenergic receptors. Subsequently, a slower reaction results in the loss of measurable beta-adrenergic receptors. The degree of loss of both parameters was similar after 24 h of desensitization. It is concluded that the loss of beta-adrenergic receptors is an event that occurs as a result of the initial uncoupling of the beta-receptor-linked adenylate cyclase.     

High affinity beta-2-adrenergic receptors in mononuclear leucocytes: similar density in young and old normal subjects

In normal subjects beta-adrenergic responsiveness in the cardiovascular system has been shown to be impaired with increasing age. In order to correlate reduced hormonal responsiveness to an age-related defect at the receptor level, high affinity beta-adrenergic receptors in homogenates of human mononuclear leucocytes have been studied with a (?)-³H-dihydroalprenolol (³H-DHA) binding assay. The binding sites have been characterized by rapid kinetics, saturability, structural and sterospecificity. Binding equilibrium was obtained within 16 minutes at 37° and was reversed by 50% within 10.6 minutes. In 22 healthy subjects a binding capacity of 60 ± 8 fmol/mg protein and an equilibrium dissociation constant (K_D) of 0.6 ± 0.05 nM was found. Beta-adrenergic agonists displaced ³H-DHA binding with a potency order of isoproterenol > adrenaline > noradrenaline. The (?) isomers of beta-adrenergic agonists and antagonists were one to two orders of magnitude more potent as inhibitors of ³H-DHA binding than their corresponding (+) isomers. The binding capacity and affinity of the beta-adrenergic receptors did not differ in old, as compared to young normal subjects. Leucocytes from 14 individuals 18–40 years old had an average density of 53 ± 4 fmol/mg protein, while the average density in leucocytes from 8 individuals aged 53–65 years was 67 ± 8 fmol/mg protein. The K_D was 0.6 ± 0.05 nM in both groups. In conclusion, an age-related decrease of beta-adrenergic receptor-mediated cardiovascular functions does not seem to be reflected in the properties of beta-adrenergic receptors of mononuclear leucocytes.     

Dose-dependent effect of prenatal dexamethasone treatment on beta-adrenergic receptor coupling to ornithine decarboxylase and cyclic AMP

Glucocorticoids regulate the coupling of beta-adrenergic receptors to cell function. In the current study, the potential role of these agents in the development of adrenergic responses was evaluated in the offspring of pregnant rats given 0.8 mg/kg of dexamethasone on gestational days 17, 18 and 19. We examined the postnatal development of beta-receptors coupling to ornithine decarboxylase in heart and kidney throughout neonatal life into young adulthood; this enzyme is involved in transduction of neural signals controlling cellular replication and differentiation. Dexamethasone exposure perturbed the basal pattern of cardiac ornithine decarboxylase activity and attenuated the response of the enzyme in both heart and kidney to acute challenge with the beta-agonist, isoproterenol. Subsensitivity persisted into the postweaning period. This dosage regimen of dexamethasone also suppressed renal cyclic AMP responses to isoproterenol, but exposure to a lower dose (0.2 mg/kg) enhanced the response. Thus, although low doses of glucocorticoids foster development of the coupling of beta-receptors to cellular transduction mechanisms, higher doses such as those used to stimulate lung function may lastingly obtund adrenergic sensitivity.     

Roles of alpha 1- and beta-adrenergic receptors in adrenergic responsiveness of liver cells formed after partial hepatectomy

The adrenergic receptor involved in the action of epinephrine changed dramatically during the process of active proliferation which follows partial hepatectomy. In control or sham-operated animals, the stimulation of glycogenolysis, gluconeogenesis and ureogenesis by epinephrine was mediated through alpha 1-adrenergic receptors. In contrast, in hepatocytes obtained from animals partially hepatectomized 3 days before experimentation, the receptor involved in the stimulation of these metabolic pathways by epinephrine was of the beta-adrenergic type. Interestingly, the adrenergic receptor involved in the metabolic actions of epinephrine, in hepatocytes from rats partially hepatectomized 7 days before experimentation was again of the alpha 1-subtype. Thus, it appears that during the process of liver regeneration which follows partial hepatectomy there is a transition in the type of adrenergic receptor involved in the hepatic actions of catecholamines from beta in the initial stages to later alpha 1. A similar transition seems to occur as the animal ages. Cyclic AMP accumulation in response to beta-adrenergic stimulation was significantly enhanced in hepatocytes obtained from rats partially hepatectomized 3 days before the experiment, as compared to control hepatocytes or cells obtained from animals operated 7 days before experimentation. This enhanced beta-adrenergic sensitivity is probably related to the increased number of beta-adrenergic receptors observed at this stage. However, a clear dissociation between cyclic AMP levels and metabolic effects was evidenced when the different conditions were compared. The number and affinity (for epinephrine or prazosin) of alpha 1-adrenergic receptors did not change at any stage of the process, which indicates that the markedly diminished alpha 1-adrenergic sensitivity observed in hepatocytes obtained from rats partially hepatectomized 3 days before experimentation is probably due to defective generation or intracellular processing of the alpha 1-adrenergic signal, rather than to changes at the receptor level.     

Influence of alpha and beta adrenergic receptors blockade on the adrenolytic effect of muscular work

The changes in the response of adrenergic receptors alpha and beta in the blood vessels in the working muscles in a hindlimb in cats were studied after intra-arterial administration of noradrenaline, isoprenaline and during electric stimulation of the sympathetic trunk. The experiments were carried out during alpha-adrenergic receptors blockade with dihydroergotamine (0.3 mg/kg) beta-adrenergic receptors blockade with propranolol (1 mg/kg) and blockade of acetylcholine M receptors with atropine (0.5 mg/kg). The investigations were performed at rest, during exercise (electric stimulation of the sciatic nerve) and after the exercise. The following results deserve attention: 1) beta-adrenergic receptors blockade reduced significantly the alpha-adrenolytic effect of exercise restoring the ability of blood vessel to constriction in response to noradrenaline; 2) the vasodilator effect of isoprenaline evident in resting state and maintained to some extent during exercise was abolished completely by preceding alpha-adrenergic blockade. The changes in the reactivity of resistance vessels in working skeletal muscles to noradrenaline, with abolition of its vasoconstrictor effect, have been shown by Rein [7] and others authors [2, 5]. Similarly, it is well known that the resistance vessels contain two types of adrenergic receptors alpha and beta, and that the response of the vessels to stimulation of these receptors are different [1]. In view of the recently published observations of Jarhult and Lundvall suggesting that the beta-adrenergic receptors play an important physiological role [6] in the arterial part of the microcirculation [6] and in view of the hypothesis put forward by Kunos and Szentivanyj that alpha and beta receptors can be transformed depending on the intensity of tissue metabolism [8] it seemed worth while to study more systematically the changes of the reactivity of alpha and beta adrenergic receptors in the vascular bed of the skeletal muscles during and after muscle exercise.     

Reduced alpha adrenergic mediated contraction of renal preglomerular blood vessels as a function of gender and aging

As human males age, a decline in baroreflex-mediated elevation of blood pressure occurs due, at least in part, to a reduction in alpha-1 adrenergic vasoconstrictor function. Alpha adrenergic constriction is mediated by guanosine triphosphate binding Protein (G Protein) coupled signaling pathways. Alpha-1 A/C, B, and D adrenergic receptor expressions, measured by GeneChip array, are not reduced during aging in renal blood vessels of male or female rats. Alpha-1 A GeneChip expression is greater, at all ages studied, in females than in males. Prazosin binding by alpha-1 adrenergic receptors is greater in young adult female rats than in young adult male rats; however, it is reduced with aging in both male and female rats. G alpha q GeneChip expression declines while expression of adrenergic receptor kinase (GRK2) and tyrosine phosphatases (TyrP) increase with aging in male rats. The declines in alpha-1 adrenergic receptor binding and G alpha q expression and also the increases in GRK2 and TyrP expression likely relate to the age-related decline of vasoconstriction in male rats. The information that the expression of alpha-1 A adrenergic receptors is greater in female rats and (GRK2) expression does not increase during aging could relate to the gender differences in vasoconstrictor function with aging. Gene therapy to ameliorate the age-related decline in renal function could possibly reduce the need for renal dialysis. Signaling pathways such as those reviewed herein may provide an outline of the molecular pathways needed to move toward successful renal gene therapy for aging individuals.     

An increase in putative voltage dependent calcium channel number following reserpine treatment

Rats were treated with reserpine (0.2 mg/kg) on days 1, 3, and 5. On day 6, binding parameters for alpha-1 adrenergic receptors (3H-prazosin) and putative voltage dependent calcium channels, VDCC (3H-nitrendipine), were determined. There was an increase in both the number (2.1 fold) and affinity (1.8 fold) of alpha-1 adrenergic receptors following reserpine treatment. In addition, there was a 2.7 fold increase in the number of VDCCs, but no change in VDCC binding affinity, following reserpine treatment. These data are consistant with the development of smooth muscle supersensitivity following reserpine treatment in a variety of tissues, and suggest that VDCC number may be modulated by the cell in response to tonic levels of catecholamines. Changes in the number of VDCCs may be an important regulatory mechanism for cell function in physiologic and pathologic states.