Adenosine attenuation of isoproterenol-stimulated adenylyl cyclase activity is enhanced with aging in the adult heart

Adenosine and its receptors in the heart: regulation, retaliation and adaptation

The purine nucleoside adenosine is an important regulator within the cardiovascular system, and throughout the body. Released in response to perturbations in energy state, among other stimuli, local adenosine interacts with 4 adenosine receptor sub-types on constituent cardiac and vascular cells: A(1), A(2A), A(2B), and A(3)ARs. These G-protein coupled receptors mediate varied responses, from modulation of coronary flow, heart rate and contraction, to cardioprotection, inflammatory regulation, and control of cell growth and tissue remodeling. Research also unveils an increasingly complex interplay between members of the adenosine receptor family, and with other receptor groups. Given generally favorable effects of adenosine receptor activity (e.g. improving the balance between myocardial energy utilization and supply, limiting injury and adverse remodeling, suppressing inflammation), the adenosine receptor system is an attractive target for therapeutic manipulation. Cardiovascular adenosine receptor-based therapies are already in place, and trials of new treatments underway. Although the complex interplay between adenosine receptors and other receptors, and their wide distribution and functions, pose challenges to implementation of site/target specific cardiovascular therapy, the potential of adenosinergic pharmacotherapy can be more fully realized with greater understanding of the roles of adenosine receptors under physiological and pathological conditions. This review addresses some of the major known and proposed actions of adenosine and adenosine receptors in the heart and vessels, focusing on the ability of the adenosine receptor system to regulate cell function, retaliate against injurious stressors, and mediate longer-term adaptive responses.

Adenosinergic cardioprotection: Multiple receptors, multiple pathways

Adenosine, formed primarily via hydrolysis of 5'-AMP, has been historically dubbed a "retaliatory" metabolite due to enhanced local release and beneficial actions during cellular/metabolic stress. From a cardiovascular perspective, evidence indicates the adenosinergic system is essential in mediation of intrinsic protection (e.g., pre- and postconditioning) and determining myocardial resistance to insult. Modulation of adenosine and its receptors thus remains a promising, though as yet not well-realized, approach to amelioration of injury in ischemic-reperfused myocardium. Adenosine exerts effects through A(1), A(2A), A(2B), and A(3) adenosine receptor subtypes (A(1)AR, A(2A)AR, A(2B)AR, and A(3)AR), which are all expressed in myocardial and vascular cells, and couple to G proteins to trigger a range of responses (generally, but not always, beneficial). Adenosine can also enhance tolerance to injurious stimuli via receptor-independent metabolic effects. Given adenosines contribution to preconditioning, it is no surprise that postreceptor signaling typically mimics that associated with preconditioning. This involves activation/translocation of PKC, PI3 kinase, and MAPKs, with ultimate effects at the level of mitochondrial targets-the mitochondrial K(ATP) channel and/or the mitochondrial permeability transition pore (mPTP). Nonetheless, differences in cytoprotective signaling and actions of the different adenosine receptor subtypes have been recently revealed. Our understanding of adenosinergic cytoprotection continues to evolve, with roles for the A(2) subtypes emerging, together with evidence of essential receptor "cross-talk" in mediation of protection. This review focuses on current research into adenosine-mediated cardioprotection, highlighting recent findings which, together with a wealth of prior knowledge, may ultimately facilitate adenosinergic approaches to clinical cardiac protection.

Loss of vascular adenosine A1 receptors with age in the rat heart

OBJECTIVE

To investigate the effects of age on adenosine A1 receptor (ADORA1) mediated vascular, inotropic and chronotropic functional responses in isolated rat hearts.

METHODS

NECA (5'-(N-ethylcarboxamido)adenosine) and R-PIA (R-N6-(1-methyl-2-phenylethyl)adenosine) concentration-response curves were produced in Langendorff prepared hearts isolated from immature (6 weeks), young (16 weeks) and mature (52 weeks) male Wistar rats and the effects of DPCPX (ADORA1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine, 30 nM) and pertussis toxin pre-treatment (PTX, 48 h, 10 microg/kg i.p., inhibits G(i/o)-protein) were observed.

RESULTS

NECA mediated coronary vasodilation and induced biphasic concentration-response curves in hearts from immature rats (pEC50 8.5 (8.1-8.9) and 11.3 (10.3-12.3)). At the low sensitivity site, the potency of NECA increased in young but not mature rats and remained unchanged at the high sensitivity site. Both DPCPX and PTX each blocked NECA at the high sensitivity site in immature rats, producing monophasic concentration-response curves (pEC50 8.6 (8.5-9.9) for DPCPX and pEC50 8.7 (8.3-9.0) for PTX), but not in young and mature rats. A vasoconstrictor response was observed at low NECA concentrations in hearts from PTX pre-treated immature rats, but not in hearts from young and mature rats, and the response was inhibited by DPCPX. No age related changes were observed in R-PIA mediated negative inotropic and chronotropic responses (P>0.05).

CONCLUSION

ADORA1 mediates a vasodilator response as well as a vasoconstrictor response in the coronary resistance vessels; the latter occurs via a PTX-insensitive pathway and declines with age.

Aged Rat Myocardium Exhibits Normal Adenosine Receptor-Mediated Bradycardia and Coronary Vasodilation But Increased Adenosine Agonist-Mediated Cardioprotection

The purpose of this study was to determine whether aged myocardium exhibits decreased responsiveness to adenosine A1 and A(2a) receptor activation. Studies were conducted in adult (4-6 months) and aged (24-26 months) Fischer 344 x Brown Norway hybrid (F344 x BN) rats. Effects of the adenosine A1/A(2a) agonist AMP579 were measured in isolated hearts and in rats submitted to in vivo regional myocardial ischemia. Aged isolated hearts exhibited lower spontaneous heart rates and higher coronary resistance, as well as normal A1- and A(2a)-mediated responses. There was no difference in control infarct size between adult and aged rats; however, AMP579 treatment resulted in a 50% greater infarct size reduction in aged rats (18 +/- 4% of risk area) compared to adult rats (37 +/- 3%). These findings suggest that adenosine A1 and A(2a) receptor-mediated effects are not diminished in normal aged myocardium, and that aged hearts exhibit increased adenosine agonist-induced infarct reduction.

Age-related changes in cardiac structure and function in Fischer 344 x Brown Norway hybrid rats

The effects of aging on cardiovascular function and cardiac structure were determined in a rat model recommended for gerontological studies. A cross-sectional analysis assessed cardiac changes in male Fischer 344 x Brown Norway F1 hybrid rats (FBN) from adulthood to the very aged (n = 6 per 12-, 18-, 21-, 24-, 27-, 30-, 33-, 36-, and 39-mo-old group). Rats underwent echocardiographic and hemodynamic analyses to determine standard values for left ventricular (LV) mass, LV wall thickness, LV chamber diameter, heart rate, LV fractional shortening, mitral inflow velocity, LV relaxation time, and aortic/LV pressures. Histological analyses were used to assess LV fibrotic infiltration and cardiomyocyte volume density over time. Aged rats had an increased LV mass-to-body weight ratio and deteriorated systolic function. LV systolic pressure declined with age. Histological analysis demonstrated a gradual increase in fibrosis and a decrease in cardiomyocyte volume density with age. We conclude that, although significant physiological and morphological changes occurred in heart function and structure between 12 and 39 mo of age, these changes did not likely contribute to mortality. We report reference values for cardiac function and structure in adult FBN male rats through very old age at 3-mo intervals.

Age-related changes in cardiac adenosine receptor expression

Adenosine is an important cardioprotective agent that works via several adenosine receptor (ADOR) subtypes to regulate cardiovascular activity. It is well established that functional responses to adenosine decline with age. What is unclear, though, is whether these changes occur at the receptor, second messenger or translational level. In this study we determined the effect of age on cardiac adenosine receptor expression using the housekeeping gene 18S rRNA versus the adenosine A(2B) receptor gene as internal controls. Absolute quantification showed that no age-related changes occurred in the expression of 18S rRNA or adenosine A(2B) receptor internal control genes. Subsequently, relative analysis of the adenosine receptor subtypes using 18S rRNA found a significant age-related reduction in the expression of the adenosine A(1) receptor (5.5-fold), with no changes in the expression of the adenosine A(2A), A(2B) and A(3) receptors. When using the expression of the adenosine A(2B) receptor as the internal control gene, a significant down regulation of both the adenosine A(1) (5.4-fold) and A(2A) (2.2-fold) receptors with no change in the expression of adenosine A(3) receptor was found. Therefore, the high level of expression of the 18S rRNA housekeeping gene was found to mask a significant change in expression of the adenosine A(2A) receptor with age. Ultimately, these findings show an age-related reduction in adenosine A(1) and A(2A) receptor expression in rat heart.

2-Aminothiazoles: a new class of agonist allosteric enhancers of A(1) adenosine receptors

This report describes the synthesis and structure-activity relationships of a new class of A(1) adenosine receptor agonist allosteric enhancers, 2-aminothiazolium salts. The EC(50) of compounds 6a, 6b, 7, and 8 were 0.3, 4.5, 3.8, and 1.2 microM, substantially lower than that of the 'Gold Standard' 2-amino-3-benzoyl thiophene (PD 81,723), which has an EC(50) of 38 microM.

Age-related changes in A(1)-adenosine receptor-mediated bradycardia

The impact of age on functional sensitivity to A(1)-adenosine receptor activation was studied in Langendorff-perfused hearts from young (1-2 mo) and old (12-18 mo) male Wistar rats. Adenosine mediated bradycardia in young and old hearts, with sensitivity enhanced approximately 10-fold in old [negative logarithm of EC(50) (pEC(50)) = 4.56 +/- 0.11] versus young hearts (pEC(50) = 3.70 +/- 0. 09). Alternatively, the nonmetabolized A(1) agonists N(6)-cyclohexyladenosine and (R)-N(6)-phenylisopropyladenosine were equipotent in young (pEC(50) = 7.43 +/- 0.12 and 6.61 +/- 0.19, respectively) and old hearts (pEC(50) = 7.07 +/- 0.10 and 6.80 +/- 0. 11, respectively), suggesting a role for uptake and/or catabolism in age-related changes in adenosine sensitivity. In support of this suggestion, [(3)H]-adenosine uptake was approximately twofold greater in young than in old hearts (from 3-100 microM adenosine). However, although inhibition of adenosine deaminase and adenosine transport with 10 microM erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride and 10 microM S-(4-nitrobenzyl)-6-thioinosine increased adenosine sensitivity three- to fourfold, it failed to abolish the sensitivity difference in old (pEC(50) = 4.95 +/- 0.08) versus young (pEC(50) = 4.29 +/- 0.13) hearts. Data indicate that 1) age increases functional A(1) receptor sensitivity to adenosine without altering the sensitivity of the A(1) receptor itself, and 2) age impairs adenosine transport and/or catabolism, but this does not explain differing functional sensitivity to adenosine. This increased functional sensitivity to adenosine may have physiological significance in the older heart.

Effect of aging on the negative chronotropic and anti-beta-adrenergic actions of adenosine in the rat heart

The effect of aging on the antiadrenergic actions of adenosine was studied in vitro and in vivo by using adult (6-month-old) and old (24-month-old) male Fischer 344 rats. In anesthetized animals, adenosine (0.01-0.1 micromol/kg), given as a rapid bolus into the right atrium, exerted a negative chronotropic effect manifested by a dose-dependent transient prolongation of sinus cycle length (SCL). This effect was similar in both age groups (n = 6, each; i.e., the percentage maximal prolongation of SCL (%deltaSCL) ranged from 12 +/- 2% to 63 +/-14% in the adult and from 20 +/- 7% to 57 +/- 15% in the old rats. In the presence of isoproterenol (0.2 microg/kg/min), the negative chronotropic action of adenosine was potentiated in the adult rats much more than in the old rats [i.e., %deltaSCL ranged from 60 +/- 28% to 183 +/- 48% vs. 40 +/- 12% to 70 +/- 13%, respectively (p < 0.05, adult vs. old)]. In the isolated perfused hearts, isoproterenol (1 microM for 1 min) exerted similar chronotropic and inotropic effects in adult (n = 9) and old hearts [n = 6; i.e., heart rate, left ventricular pressure (LVP), and LVdp/dt increased by 56 +/- 3%, 17 +/- 1%, and 37 +/- 2%, and 57 +/- 2%, 17 +/- 1%, and 35 +/- 3%, respectively, in the absence of, and by 27 +/- 2%, 7 +/- 1%, and 19 +/- 2% and 41 +/- 3%, 12 +/- 1%, and 25 +/-2% in the presence of adenosine (5 microM for 1 min)]. Adenosine administration after isoproterenol caused only an insignificant increase in coronary blood flow. Finally, the adenosine attenuation of either isoproterenol- or forskolin-induced production of 3',5'-cyclic adenosine monophosphate (cAMP) was significantly less in atrial membranes isolated from old versus adult rats (n = 6, each). It was concluded that in the old Fischer 344 rat hearts, the antiadrenergic action of adenosine is attenuated as compared with its action in adult rat hearts.

Effect of aging on A1-adenosine receptor-mediated inhibition of norepinephrine release in the rat heart

Adenosine inhibits norepinephrine (NE) release from cardiac adrenergic nerves and reduces the postsynaptic beta-adrenergic mediated actions of NE, leading to decreased myocardial force of contraction. The actions of adenosine are mediated by pre- and postsynaptic adenosine A1 receptors (A1-AdoR). We reported that adenosine inhibition of postsynaptic beta-adrenergic receptor-mediated cyclic adenosine monophosphate (cAMP) production declines with age in male F344 rat hearts. In this study, cardiac synaptosomes, isolated intact adrenergic nerve terminals, were used to examine the effect of age on adenosine inhibition of NE release. Cardiac synaptosomes were prepared from the hearts of 6- and 24-month-old male F344 rats, loaded with [3H]NE, and placed in a superfusion system. [3H]NE release was induced by high [K+] exposure in the presence of varying concentrations of adenosine or the specific A1-AdoR agonist, N6-p-sulfophenyladenosine (SPA). [3H]NE release was significantly reduced in old rats compared with young rats. Inhibition of [3H]NE release by adenosine and SPA was significantly greater in young rats compared with old rats. The A1-AdoR antagonist, 8-(p-sulfophenyl)-theophylline, blocked the actions of adenosine on [3H]NE release, and the specific adenosine A2-receptor agonist, cyclopropylcarboxamidoadenosine, had no effect on [3H]NE release. Our data suggest that presynaptic A1-AdoR-mediated inhibition of NE release in the rat heart declines with age.

Myocardial adenosine A1-receptor sensitivity during juvenile and adult stages of maturation

In the heart, endogenous adenosine attenuates the beta-adrenergic-elicited increase in contractile performance via activation of adenosine A1 receptors. It has been recently reported that this function of adenosine becomes more pronounced with myocardial maturation. The purpose of the present study was to determine whether mature hearts possess a greater sensitivity than immature hearts to this antiadrenergic effect of adenosine. Isolated perfused hearts or atria from immature (ca. 23 days) and mature (ca. 80 days) rats were stimulated with isoproterenol (Iso), a beta-adrenergic agonist, at 10(-8) M and concomitantly exposed to increasing concentrations of 2-chloro-N6-cyclopentyladenosine (CCPA), a highly selective and potent adenosine A1-receptor agonist, from 10(-12) to 10(-6) M. CCPA at 10(-10)-10(-6) M dose dependently reduced the Iso-elicited contractile response more in immature than in mature hearts or atria. At 10(-6) M, CCPA reduced the Iso-elicited contractile response by 103% in immature hearts and by 55% in mature hearts. These effects of CCPA were attenuated by the adenosine A1-receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine at 10(-7) M. In additional experiments, CCPA exhibited similar effectiveness in reducing the spontaneous heart rate of immature and mature hearts, an effect also mediated by activation of adenosine A1 receptors. Similar to CCPA, the adenosine A1-receptor agonist R-N6-(2-phenylisopropyl)adenosine reduced the Iso-elicited contractile response more in immature than in mature hearts, albeit with less effectiveness than CCPA. In agreement with these results, CCPA reduced Iso-elicited adenylyl cyclase activity more in immature than in mature hearts. Overall, in contrast with our original hypothesis, these results indicate that immature hearts display greater sensitivity than mature hearts to the antiadrenergic effect of adenosine A1-receptor activation.