Vascular beta-adrenergic receptor adenylyl cyclase system in maturation and aging

Aging-Induced Impairment of Vascular Function: Mitochondrial Redox Contributions and Physiological/Clinical Implications

Significance: The vasculature responds to the respiratory needs of tissue by modulating luminal diameter through smooth muscle constriction or relaxation. Coronary perfusion, diastolic function, and coronary flow reserve are drastically reduced with aging. This loss of blood flow contributes to and exacerbates pathological processes such as angina pectoris, atherosclerosis, and coronary artery and microvascular disease. Recent Advances: Increased attention has recently been given to defining mechanisms behind aging-mediated loss of vascular function and development of therapeutic strategies to restore youthful vascular responsiveness. The ultimate goal aims at providing new avenues for symptom management, reversal of tissue damage, and preventing or delaying of aging-induced vascular damage and dysfunction in the first place. Critical Issues: Our major objective is to describe how aging-associated mitochondrial dysfunction contributes to endothelial and smooth muscle dysfunction via dysregulated reactive oxygen species production, the clinical impact of this phenomenon, and to discuss emerging therapeutic strategies. Pathological changes in regulation of mitochondrial oxidative and nitrosative balance (Section 1) and mitochondrial dynamics of fission/fusion (Section 2) have widespread effects on the mechanisms underlying the ability of the vasculature to relax, leading to hyperconstriction with aging. We will focus on flow-mediated dilation, endothelial hyperpolarizing factors (Sections 3 and 4), and adrenergic receptors (Section 5), as outlined in Figure 1. The clinical implications of these changes on major adverse cardiac events and mortality are described (Section 6). Future Directions: We discuss antioxidative therapeutic strategies currently in development to restore mitochondrial redox homeostasis and subsequently vascular function and evaluate their potential clinical impact (Section 7). Antioxid. Redox Signal. 35, 974-1015.

Androgens Mediate β-adrenergic Vasorelaxation Impairment Using Adenylyl Cyclase

Cardiovascular disease development has been associated with sex differences, suggesting that sex hormones are implicated in vascular function and development of hypertension. Vascular tone comparison at different stages of rat growth represents a good model to study testosterone-related vascular response. We explored the role of testosterone in modulation of age-dependent impaired β-adrenergic vasodilation. The 3-week-old male Sprague-Dawley rats were sorted in 3-week-old rats without any manipulation and 3-week-old rats treated with testosterone. The 9-week-old rats were randomly grouped into 9-week-old rats without any manipulation (sham), 9-week-old rats that underwent gonadectomy (9-week-old castrated), and 9-week-old castrated treated with testosterone replacement therapy (9-week-old castrated + testosterone). Vascular relaxation was evaluated in aortic rings. β-adrenergic receptor protein expression, cyclic adenosine monophosphate production, testosterone levels, and adenylyl cyclase (AC) gene expression were assessed. Testosterone levels were low in 3-week-old and 9-week-old castrated rats compared with 9-week-old sham rats. Testosterone replacement raised these levels in 3-week-old and 9-week-old castrated rats similar to those of 9-week-old sham rats. SQ22536, the AC inhibitor, prevented isoproterenol-induced relaxation in aortic rings from 3-week-old and 9-week-old castrated rats. The β-adrenergic receptor protein expression was similar in all experimental groups. AC mRNA and protein expression and cyclic adenosine monophosphate levels were elevated in 3-week-old and 9-week-old castrated rats compared with 3-week-old + testosterone, 9-week-old sham, and 9-week-old castrated + testosterone rats. In conclusion, we demonstrated that age maturation was associated with vascular relaxation impairment. Variations in testosterone levels and reduced AC expression may be responsible for this altered vascular function.

Role of adenylyl cyclase in reduced β-adrenoceptor-mediated vasorelaxation during maturation

Beta-adrenergic receptor (βAR)-dependent blood vessel relaxation is impaired in older animals and G protein activation has been suggested as the causative mechanism. Here, we investigated the role of βAR subtypes (β1AR, β2AR, and β3AR) and cAMP in maturation-dependent vasorelaxation impairment. Aortic rings from 15 Sprague-Dawley male rats (3 or 9 weeks old) were harvested and left intact or denuded of the endothelium. Vascular relaxation in aortic rings from younger and older groups was compared in the presence of βAR subtype agonists and antagonists along with cAMP and cGMP antagonists. Isolated aortic rings were used to evaluate relaxation responses, protein expression was evaluated by western blot or real time PCR, and metabolites were measured by ELISA. Expression of βAR subtypes and adenylyl cyclase was assessed, and cAMP activity was measured in vascular tissue from both groups. Isoproterenol- and BRL744-dependent relaxation in aortic rings with and without endothelium from 9-week-old rats was impaired compared with younger rats. The β1AR antagonist CGP20712A (10^-7 M) did not affect isoproterenol or BRL744-dependent relaxation in arteries from either group. The β2AR antagonist ICI-118,551 (10^-7 M) inhibited isoproterenol-dependent aortic relaxation in both groups. The β3AR antagonist SR59230A (10^-7 M) inhibited isoproterenol- and BRL744-dependent aortic ring relaxation in younger but not in older rats. All βAR subtypes were expressed in both groups, although β3AR expression was lower in the older group. Adenylyl cyclase (SQ 22536) or protein kinase A (H89) inhibitors prevented isoproterenol-induced relaxation in younger but not in older rats. Production of cAMP was reduced in the older group. Adenylyl cyclase III and RyR3 protein expression was higher in the younger group. In conclusion, altered expression of β3AR and adenylyl cyclase III may be responsible for reduced cAMP production in the older group.

Physical activity ameliorates cardiovascular health in elderly subjects: the functional role of the β adrenergic system

Aging is a complex process characterized by a gradual decline in organ functional reserves, which eventually reduces the ability to maintain homeostasis. An exquisite feature of elderly subjects, which constitute a growing proportion of the world population, is the high prevalence of cardiovascular disorders, which negatively affect both the quality of life and the life expectancy. It is widely acknowledged that physical activity represents one of the foremost interventions capable in reducing the health burden of cardiovascular disease. Interestingly, the benefits of moderate-intensity physical activity have been established both in young and elderly subjects. Herein we provide a systematic and updated appraisal of the literature exploring the pathophysiological mechanisms evoked by physical activity in the elderly, focusing on the functional role of the β adrenergic system.

Biochemical and molecular aspects of vascular adrenergic regulation of blood pressure in the elderly

Hypertension, orthostatic hypotension, arterial insufficiency, and atherosclerosis are common disorders in the elderly that lead to significant morbidity and mortality. One common factor to these conditions is an age-related decline in vascular beta-adrenergic receptor-mediated function and subsequent cAMP generation. Presently, there is no single cellular factor that can explain this age-related decline, and thus, the primary cause of this homeostatic imbalance is yet to be identified. However, the etiology is clearly associated with an age-related change in the ability of beta-adrenergic receptor to respond to agonist at the cellular level in the vasculature. This paper will review what is presently understood regarding the molecular and biochemical basis of age-impaired beta-adrenergic receptor-mediated signaling. A fundamental understanding of why β-AR-mediated vasorelaxation is impaired with age will provide new insights and innovative strategies for the management of multiple clinical disorders.

Age-related regulation of excitation-contraction coupling in rat heart

Hearts from subjects with different ages have different Ca(2+) signaling. Release of Ca(2+) from intracellular stores in response to an action potential initiates cardiac contraction. Both depolarization-stimulated and spontaneous Ca(2+) releases, Ca(2+) transients and Ca(2+) sparks, demonstrate the main events of excitation-contraction coupling (ECC). Global increase in free Ca(2+) concentration ([Ca(2+)]( i )) consists of summation of Ca(2+) release events in cardiomyocytes. Since the Ca(2+) flux induced by Ca(2+) sparks reports a summation of ryanodine-sensitive Ca(2+) release channels (RyR2s)'s behavior in a spark cluster, evaluation of the properties of Ca(2+) sparks and Ca(2+) transients may provide insight into the role of RyR2s on altered heart function between 3-month-old (young adult) and 6-month-old (mature adult) rats. Basal [Ca(2+)]( i ) and Ca(2+) sparks frequency were significantly higher in mature adult rats compared to those of young adults. Moreover, amplitudes of Ca(2+) sparks and Ca(2+) transients were significantly smaller in mature adults than those of young adults with longer time courses. A smaller L-type Ca(2+) current density and decreased SR Ca(2+) load was observed in mature adult rats. In addition, RyR2s were markedly hyperphosphorylated, and phosphorylation levels of PKA and CaMKII were higher in heart from mature adults compared to those of young adults, whereas their SERCA protein levels were similar. Our data demonstrate that hearts from rats with different ages have different Ca(2+) signaling including hyperphosphorylation of RyR2s and higher basal [Ca(2+)]( i ) together with increased oxidized protein-thiols in mature adult rats compared to those of young adults, which play important roles in ECC. Finally, we report that ECC efficiency changes with age during maturation, partially related with an increased cellular oxidation level leading to reduced free protein-thiols in cardiomyocytes.

Exercício físico, receptores β-adrenérgicos e resposta vascular

O exercício aeróbio promove efeitos benéficos na prevenção e tratamento de doenças como hipertensão arterial, aterosclerose, insuficiência venosa e doença arterial periférica. Os receptores β-adrenérgicos estão presentes em várias células. No sistema cardiovascular, promovem inotropismo e cronotropismo positivo cardíaco e relaxamento vascular. Embora os efeitos do exercício tenham sido investigados em receptores cardíacos, estudos focados nos vasos são escassos e controversos. Esta revisão abordará os efeitos do exercício físico sobre os receptores β-adrenérgicos vasculares em modelos animais e humanos e os mecanismos celulares envolvidos na resposta relaxante. Em geral, os estudos mostram resultantes conflitantes, onde observam diminuição, aumento ou nenhum efeito do exercício físico sobre a resposta relaxante. Assim, os efeitos do exercício na sensibilidade β-adrenérgica vascular merecem maior atenção, e os resultados mostram que a área de fisiopatologia vascular é um campo aberto para a descoberta de novos compostos e avanços na prática clínica.

The role of Gi proteins in reduced vasorelaxation response to beta-adrenoceptor agonists in rat aorta during maturation

Beta-adrenoceptor mediated vasorelaxation and cAMP production decline during maturation and aging in rat aorta. beta-adrenoceptor-stimulated vasorelaxation is mainly triggered by Gsalpha-mediated activation of adenylyl cyclase. beta(2)-adrenoceptors can also activate Gi protein which inhibits adenylyl cyclase activity. In this study, we examined the role of Gi proteins in the decreased beta-adrenoceptor mediated responses during maturation. Pertussis toxin treatment of aortic rings to inhibit Gialpha activation completely restored age related decline in isoproterenol-stimulated maximal vasorelaxation in 3-month old rats. This treatment increased the potency, but not the maximal response of isoproteronol to produce vasorelaxation in 6 month old rats. The maximal isoproteronol stimulated cAMP responses were also partially restored in pertussis toxin-treated rings from 3 or 6-month old rats. We also examined beta-adrenoceptor stimulated binding of (35)[S]GTPgammaS to Gsalpha and Gialpha1/2 in aortic membranes from 1, 3 and 6-month old rats. In 1-month old rats, isoproterenol-stimulated (35)[S]GTPgammaS binding to Gsalpha was significantly higher than that of 3 or 6-month old rats. Isoproterenol-stimulated (35)[S]GTPgammaS binding to Gialpha1/2 was found to be significantly increased in 3 or 6-month old rats compared to 1-month old rats. The results of this study showed that beta-adrenoceptor-mediated activation of Gs and Gi proteins was declined and increased, respectively, and inhibition of the Gi mediated activity by pertussis toxin treatment partially restored impaired vasorelaxation and cAMP response to beta-adrenoceptor stimulation during maturation in rat aorta. The decrease in beta-adrenoceptor mediated activation of Gs gradually increased during maturation. All together these results indicated that beta-adrenoceptor mainly activates Gs protein in aorta from 1-month old rats, while it activates Gi and with a certain degree of decline it also activates Gs in aorta from 3 and 6-months old rats and not only the increase in beta-adrenoceptor coupling to Gi but also the decrease in its coupling to Gs play a role in the impaired beta-adrenoceptor responses in rat aorta during maturation.

Effect of age on vascular beta2-adrenergic receptor desensitization is not mediated by the receptor coupling to Galphai proteins

Beta-adrenergic receptor (beta-AR)-mediated vasorelaxation declines with age. In the vasculature, beta2-AR undergoes protein kinase A-mediated desensitization that causes a switch in the G protein coupled to beta2-AR; Galphai links instead of Galphas. We exposed Fischer 344 rat aortae of increasing age to a desensitizing dose of isoproterenol, and determined its effect on beta2-AR-mediated vasorelaxation. Desensitization decreased beta2-AR-mediated vasorelaxation in young aortae only. Subsequently, we used pertussis toxin to block Galphai to determine whether changes in beta2-AR/G protein coupling occurred. Galphai inhibition did not reverse desensitization or the age-related change, but there appears to be a population of beta2-AR linked to Galphai, as pertussis toxin treatment improved beta2-AR-mediated vasorelaxation in aortae from animals of all ages. These findings suggest aortic beta2-AR in older animals may be maximally desensitized, which would explain impaired vasorelaxation. Our results also imply that protein kinase A-mediated beta2-AR desensitization may not be responsible for the age-related decline.

G protein-coupled receptor systems and their lipid environment in health disorders during aging

Cells, tissues and organs undergo phenotypic changes and deteriorate as they age. Cell growth arrest and hyporesponsiveness to extrinsic stimuli are all hallmarks of senescent cells. Most such external stimuli received by a cell are processed by two different cell membrane systems: receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs). GPCRs form the largest gene family in the human genome and they are involved in most relevant physiological functions. Given the changes observed in the expression and activity of GPCRs during aging, it is possible that these receptors are directly involved in aging and certain age-related pathologies. On the other hand, both GPCRs and G proteins are associated with the plasma membrane and since lipid-protein interactions regulate their activity, they can both be considered to be sensitive to the lipid environment. Changes in membrane lipid composition and structure have been described in aged cells and furthermore, these membrane changes have been associated with alterations in GPCR mediated signaling in some of the main health disorders in elderly subjects. Although senescence could be considered a physiologic process, not all aging humans develop the same health disorders. Here, we review the involvement of GPCRs and their lipid environment in the development of the major human pathologies associated with aging such as cancer, neurodegenerative disorders and cardiovascular pathologies.

Oxidative Stress-Associated Vascular Aging Is Xanthine Oxidase-Dependent but not NAD(P)H Oxidase-Dependent

Vascular aging is characterized by endothelial dysfunction that is primarily attributed to increased superoxide production, the exact source of which remains ambiguous. This study compared the NAD(P)H and xanthine oxidase (XO) systems as sources of superoxide and impaired vascular function in aging. Male Sprague Dawley rats, 4-months-old (young) and 18-months-old (Aging), were used. Systolic blood pressure was higher (36 +/- 3%) in the aging group compared with young rats, and this was accompanied by reduced acetylcholine-induced renal vasodilatation. Urinary excretion of nitrite was lower in the aging rats (P < 0.05), and this was associated with reduced nitric oxide synthase (NOS) activity and reduced eNOS and iNOS protein expression in the aorta. Aged rats showed a n approximately twofold increase in free radical generation, as evident by increased plasma 8-isoprostane level, and an approximately fourfold increase in proteinuria compared with the young rats. Vascular NADP(H) oxidase was unchanged between both groups, as was the expression of p67phox or p47phox components of NAD(P)H oxidase. However, XO activity was increased (19 +/- 1%; P < 0.05) as well as XO expression in the aorta of aging rats. These results suggest that increased free radical generation-associated increase in SBP in aging rats is XO but not NAD(P)H oxidase-dependent.

Decline in caveolin-1 expression and scaffolding of G protein receptor kinase-2 with age in Fischer 344 aortic vascular smooth muscle

Beta-adrenergic receptor (beta-AR)-mediated vasorelaxation declines with age in humans and animal models. This is not caused by changes in expression of beta-AR, G alpha s, adenylyl cyclase, or protein kinase A but is associated with decreased cAMP production. Expression and activity of G protein receptor kinase-2 (GRK-2), which phosphorylates and desensitizes the beta-AR, increases with age in rat aortic tissue. Caveolin scaffolds the beta-AR, GRK, and other proteins within "signaling pockets" and inhibits GRK activity when bound. We questioned the effect of age on caveolin-1 expression and interaction between caveolin-1 and GRK-2 in vascular smooth muscle (VSM) isolated from 2-, 6-, 12-, and 24-mo-old male Fischer 344 rat aorta. Western blot analysis found expression of caveolin-1 declined with age (6-, 12- and 24-mo-old rat aortas express 92, 50, and 42% of 2-mo-old rat aortas, respectively). Results from density-buoyancy analysis showed a lower percentage of GRK in caveolin-1-specific fractions with age (6-, 12- and 24-mo-old rat aortas express 95, 56, and 12% of 2-mo-old rat aortas, respectively). Coimmunoprecipitation confirmed this finding; density of GRK in caveolin-1 immunoprecipitates was 97, 30, and 21% of 2-mo-old aortas compared with 6-, 12- and 24-mo-old animals, respectively. Immunohistocytochemistry and confocal microscopy confirmed that GRK-2 and caveolin-1 colocalize in VSM. These results suggest that in nonoverexpressed, intact tissue, the decline in beta-AR-mediated vasorelaxation may be caused by both a reduction in caveolin-1 expression and a reduction in binding of GRK-2 by caveolin-1. This could lead to an increase in the fraction of free GRK-2, which could phosphorylate and desensitize the beta-AR.

Noradrenaline decreases spike voltage threshold and induces electrographic sharp waves in turtle medial cortex in vitro

The noradrenergic modulation of neuronal properties has been described at different levels of the mammalian brain. Although the anatomical characteristics of the noradrenergic system are well known in reptiles, functional data are scarce. In our study the noradrenergic modulation of cortical electrogenesis in the turtle medial cortex was studied in vitro using a combination of field and intracellular recordings. Turtle EEG consists of a low voltage background interspersed by spontaneous large sharp waves (LSWs). Noradrenaline (NA, 5-40 microM) induced (or enhanced) the generation of LSWs in a dose-dependent manner. Pharmacological experiments suggest the participation of alpha and beta receptors in this effect. In medial cortex neurons NA induced a hyperpolarization of the resting potential and a decrease of input resistance. Both effects were observed also after TTX treatment. Noradrenaline increased the response of the cells to depolarizing pulses, resulting in an upward shift of the frequency/current relation. In most cells the excitability change was mediated by a decrease of the spike voltage threshold resulting in the reduction of the amount of depolarization needed to fire the cell (voltage threshold minus resting potential). As opposed to the mechanisms reported in mammalian neurons, no changes in the frequency adaptation or the post-train afterhyperpolarization were observed. The NA effects at the cellular level were not reproduced by noradrenergic agonists. Age- and species-dependent properties in the pharmacology of adrenergic receptors could be involved in this result. Cellular effects of NA in turtle cortex are similar to those described in mammals, although the increase in cellular excitability seems to be mediated by a different mechanism.

Rat aortic MCP-1 and its receptor CCR2 increase with age and alter vascular smooth muscle cell function

OBJECTIVE

With age, rat arterial walls thicken and vascular smooth muscle cells (VSMCs) exhibit enhanced migration and proliferation. Monocyte chemotactic protein-1 (MCP-1) affects these VSMC properties in vitro. Because arterial angiotensin II, which induces MCP-1 expression, increases with age, we hypothesized that aortic MCP-1 and its receptor CCR2 are also upregulated and affect VSMC properties.

METHODS AND RESULTS

Both MCP-1 and CCR2 mRNAs and proteins increased in old (30-month) versus young (8-month) F344xBN rat aortas in vivo. Cellular MCP-1 and CCR2 staining colocalized with that of alpha-smooth muscle actin in the thickened aortas of old rats and were expressed by early-passage VSMCs isolated from old aortas, which, relative to young VSMCs, exhibited increased invasion, and the age difference was abolished by vCCI, an inhibitor of CCR2 signaling. MCP-1 treatment of young VSMCs induced migration and increased their ability to invade a synthetic basement membrane. The MCP-1-dependent VSMC invasiveness was blocked by vCCI. After MCP-1 treatment, migration and invasion capacities of VSMCs from young aortas no longer differed from those of VSMCs isolated from older rats.

CONCLUSIONS

Arterial wall and VSMC MCP-1/CCR2 increase with aging. MCP-1 enhances VSMC migration and invasion, and thus, MCP-1/CCR2 signaling may play a role in age-associated arterial remodeling.

Age-related changes in vascular adrenergic signaling: clinical and mechanistic implications

A large and growing segment of the general population are age 65 or older, and this percentage will continue to rise. Primary care of this population has, and is becoming a priority for clinicians. Hypertension, orthostatic hypotension, arterial insufficiency, and atherosclerosis are common disorders in the elderly that lead to significant morbidity and mortality. One common factor to these conditions is an age-related decline in beta-adrenergic receptor (beta-AR)-mediated function and subsequent cAMP generation. Presently, there is no single cellular factor that can explain this age-related decline, and thus the primary cause of this homeostatic imbalance is yet to be identified. However, the etiology is clearly associated with an age-related change in the ability of beta-AR receptor to respond to agonist at the cellular level. This article will review what is presently understood regarding the molecular and biochemical basis of age-impaired beta-AR receptor-mediated signaling. A fundamental understanding of why beta-AR-mediated vasorelaxation is impaired with age will provide new insights and innovative strategies for the management of the multiple clinical disorders that effect older people.

Aging, ion channel expression, and vascular function

Cardiovascular disease remains the leading cause of death in the United States, and aging is one of the main risk factors for its development. Coronary arteries nurture the heart, but as age progresses, they suffer changes that make them stiffer, thicker, and with higher spontaneous contractile activity. Even in the absence of pathological atherosclerotic lesions, these changes make the coronary arteries at risk for vasospasm and the individual at risk for myocardial ischemia and heart failure. Thus, knowledge of the molecular mechanisms involved in the vascular physiology, disease, and aging of the coronary circulation is required to develop strategies to preserve the quality of life of an increasingly aging population. One of the key factors that regulate coronary arterial tone is the activity of K+ channels in the vascular smooth muscle cells (SMCs). In particular, voltage-dependent and Ca(2+)-activated K+ (BKCa) channels, which are abundant in the coronary SMCs, are targets of vasoconstrictors and vasorelaxants, and play a key role in determining arterial tone and diameter. Aging induces a reduction in the density of the alpha-subunit of BKCa channels in coronary smooth muscle, lowers baseline endothelial release of the relaxant nitric oxide (NO), and increases the response to endothelial constrictor factors and K+. Thus, aging induces the remodeling of important proteins involved in the excitability and contractility of the coronary circulation. Altogether, these changes increase the risk of coronary artery vasospasm, myocardial ischemia, and infarct in the elderly.

Oxidative stress contributes to vascular endothelial dysfunction in heart failure

Congestive heart failure (HF) is characterized by inadequate nitric oxide (NO) production in the vasculature. Because NO is degraded by oxygen radicals, we hypothesized that NO is degraded faster in HF from inadequate peripheral arterial antioxidant reserves. HF was induced in male Sprague-Dawley rats by left coronary artery ligation. Vascular endothelial function was evaluated by measuring the NO-mediated vasorelaxation response to acetylcholine (ACh; 10(-9)-10(-4) M) in excised aortas. This was repeated with the free radical generator pyrogallol (20 microM) and again with pyrogallol and superoxide dismutase (SOD; 60 U/ml). Aortic and myocardial SOD activity was also determined. ACh-induced vasorelaxation was reduced in HF (n = 9) compared with normal control rats (n = 11; P < 0.001). Pyrogallol further reduced vasorelaxation in HF: 74 +/- 11% at 10(-4) M ACh versus 58 +/- 10% in normal control rats (P < 0.004). There was a trend (P = 0.06) toward reduced SOD activity in HF aortas. In conclusion, altered NO-dependent vasorelaxation in HF is in part due to excessive degradation of NO and is likely related to reduced vascular SOD activity.