Age-related changes in adenosine in rat coronary resistance vessels

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Chronic dietary L-arginine down-regulates adenosine receptor and nitric oxide synthase expression in rat heart

L-Arginine increases myocardial nitric oxide production. Nitric oxide mediates many of the cardiovascular actions of adenosine and modulates adenosine metabolism. In this study, we examined the effect of chronic L-arginine (5%) intake on cardiac nitric oxide synthase (NOS) and adenosine receptor expression and cardiac function in rat Langendorff-isolated perfused hearts. Our results show that 4-week chronic l-arginine ingestion increases the weight of rat hearts by 17.6% (P < 0.05). L-Arginine treatment decreased the expression of all the cardiac adenosine receptors, with reductions in adenosine A(1) (20-fold), A(2A) (7.7-fold), A(2B) (76-fold) and A(3) (25.6-fold) mRNA (P < 0.05). NOS expression was variably affected with no change in the expression of NOS(1) and 4.2-fold down-regulation of NOS(3) expression with chronic L-arginine treatment (P < 0.05). NOS(2) was expressed in control tissues; however, in L-arginine-treated hearts the amount of NOS(2) mRNA was reduced to non-detectable levels. Following chronic L-arginine treatment, an increase in coronary perfusion pressure was observed (P < 0.05). Purine efflux was used as an indicator of metabolic efficiency. L-Arginine did not alter catecholamine-induced purine efflux (P > 0.05); however, noradrenaline-mediated increases in contractility and myocardial oxygen consumption were reduced. Vasodilator responses to 5'-N-ethylcarboxamidoadenosine (NECA) were reduced in hearts from l-arginine-treated rats and the NOS inhibitor N omega-nitro-L-arginine methyl ester (3 microM) did not inhibit responses to NECA. In conclusion, 4-week dietary supplementation of L-arginine reduced the expression of cardiac adenosine receptors and NOSs with a subsequent decrease in noradrenaline-stimulated cardiac function and adenosine receptor-mediated coronary vasodilation.

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 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.

Possible therapeutic benefits of adenosine-potentiating drugs in reducing age-related degenerative disease in dogs and cats

Adenosine is a ubiquitous, biologically important molecule that is a precursor of other biologically active molecules. It also is a component of some co-factors and has distinct physiological actions in its own right. Levels are maintained by synthesis from dietary precursors and re-cycling. The daily turnover of adenosine is very high. Adenosine can act either as a hormone by binding to adenosine receptors, four adenosine receptor subtypes have been identified, and as an intracellular modulator, after transport into the cell by membrane transporter proteins. One of the principal intracellular actions of adenosine is inhibition of the enzyme phosphodiesterase. Extracellular adenosine also has specific neuromodulatory actions on dopamine and glutamate. Selective and nonselective agonists and antagonists of adenosine are available. The tasks of developing, evaluating and exploiting the therapeutic potential of these compounds is still in its infancy. Adenosine has actions in the central nervous system (CNS), heart and vascular system, skeletal muscle and the immune system and the presence of receptors suggests potential actions in the gonads and other organs. Adenosine agonists improve tissue perfusion through actions on vascular smooth muscle and erythrocyte fluidity and they can be used to improve the quality of life in aged dogs. This article reviews the therapeutic potential of adenosine-potentiating drugs in the treatment of age-related conditions in companion animals, some of which may be exacerbated by castration or spaying at an early age.

The measurement of adenosine and estrogen receptor expression in rat brains following ovariectomy using quantitative PCR analysis

In our laboratory we have developed a quantitative-polymerase chain reaction (Q-PCR) strategy to examine the differential expression of adenosine receptor (ADOR), A(1), A(2A), A(2B) and A(3), and estrogen receptors (ER) alpha and beta. Brain and uterine mRNA were first used to optimise specific amplification conditions prior to SYBR Green I real time analysis of receptor subtype expression. SYBR Green I provided a convenient and sensitive means of examining specific PCR amplification product in real time, and allowed the generation of standard curves from which relative receptor abundance could be determined. Real time Q-PCR analysis was then performed, to examine changes in receptor expression levels in brains of adult female Wistar rats 3-month post ovariectomy. Comparison with sham-operated age-matched control rats demonstrated both comparative and absolute-copy number changes in receptor levels. Evaluation of both analytical methods investigated 18S rRNA as an internal reference for comparative gene expression analysis in the brain. The results of this study revealed preferential repression of ADORA(2A) (>4-fold down) and consistent (>2-fold) down-regulation of ADORA(1), ADORA(3), and ER-beta, following ovariectomy. No change was found in ADORA(2B) or ER-alpha. Analysis of absolute copy number in this study revealed a correlation between receptor expression in response to ovariectomy, and relative receptor subtype abundance in the brain.

Enhanced adenosine A(2B) mediated coronary response in reserpinised rat heart

In this study, we investigated the effect of noradrenaline depletion on contractile recovery in rat isolated heart following myocardial ischaemia. Groups tested included control tissues and hearts from reserpinised rats. Reserpine 1 mg/kg s.c. was injected into rats 18 to 24 h prior to experiments. Hearts underwent 15 min global normothermic ischaemia followed by 30 min reperfusion. Functional data (end diastolic pressure (EDP), heart rate (HR), left ventricular developed pressure (LVDP), dP/dt(max), dP/dt(min)) showed that contractile function following ischaemia-reperfusion is unaffected by reserpinisation. However, pre- and post-ischaemic coronary flow rates (CFR) were increased by 16 to 38% in hearts from reserpinised rats versus control hearts. Pre-ischaemic CFRs in control hearts (11.17+/-0.67 ml/in(-1) x g tissue(-1), n=9) were significantly lower then CFRs derived from reserpinised rat hearts (14.57+/-0.72 ml/min(-1)/g tissue(-1), n=10). Post-ischaemic reactive hyperaemia was evident in all groups. CFRs in reserpinised hearts remained elevated when compared to pre-ischaemic values through reperfusion (P<0.05). Reserpine treatment did not significantly alter pre- or post-ischaemic adenosine efflux. The A(2B) adenosine receptor antagonist alloxazine (10 microM) attenuated pre- and post-ischaemic CFRs in both control and reserpinised hearts (P<0.05) without altering the hyperaemic response while the A(2A) adenosine receptor antagonist 8-(3-chlorostyryl) caffeine (1 microM) did not alter CFRs in both groups. The A(3) adenosine receptor antagonist MRS1191 (0.1 microM) increased CFR in control and reserpinised hearts (P<0.05). Catecholamine depletion with reserpinisation enhances the responsiveness of the coronary resistance vessels to endogenous adenosine through activation of the A(2B) adenosine receptor.

Adenosine receptor subtypes mediating coronary vasodilation in rat hearts

Adenosine receptor-mediated coronary vasodilation was studied in isolated hearts from young (1-2 months) and mature (12-18 months) Wistar rats. The nonselective agonist 5'-N-ethylcarboxamidoadenosine (NECA) induced biphasic concentration-dependant dilation with similar potencies in both age groups (p < 0.05). Despite similar potencies, responses to NECA were significantly depressed by 50% with age. NECA-mediated dilation was unaltered by selective A adenosine receptor (A1AR) antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 100 nM ) or A adenosine receptor (A2AAR) antagonist 5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5- ]pyrimidine (SCH 58261, 100 nM ). However, the A2B adenosine receptor (A2B AR) selective antagonist alloxazine (10 microM ) significantly reduced response magnitude to NECA in both age groups. Concentration-response curves to N -2-(4-aminophenyl) ethyladenosine (APNEA) induced biphasic concentration-dependent dilation in hearts from young animals. In the presence of the three combined antagonists, 1 microM DPCPX, 100 nM SCH 58261, and 1 microM alloxazine, the response magnitude was significantly attenuated (p < 0.05). The addition of the A3 adenosine receptor (A3AR) antagonist 3-ethyl-5-benzyl-2-methyl-4-phenylethyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS1191, 100 nM ) to the combined antagonists further attenuated vasodilator responses to APNEA. The results suggest that multiple adenosine receptor subtypes mediate dilation in the rat coronary circulation. NECA mediates vasodilation via the A2BAR subtype, while dilator responses to APNEA in the presence and absence of A1, A2, and A3 ARs antagonists provide evidence for a vasodilator role for A3 ARs in rat coronary circulation. The magnitude of the coronary dilator response is reduced with age and does not involve A2A or A1 ARs.

Functional characterization of coronary vascular adenosine receptors in the mouse

Coronary responses to adenosine agonists were assessed in perfused mouse and rat hearts. The roles of nitric oxide (NO) and ATP-dependent K(+) channels (K(ATP)) were studied in the mouse. Resting coronary resistance was lower in mouse vs rat, as was minimal resistance (2.2+/-0.1 vs 3.8+/-0.2 mmHg ml(-1) min(-1) g(-1)). Peak hyperaemic flow after 20 - 60 s occlusion was greater in mouse. Adenosine agonists induced coronary dilation in mouse, with pEC(50)s of 9.4+/-0.1 for 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethyl carboxamidoadenosine (CGS21680, A(2A)-selective agonist), 9.3+/-0.1 for 5'-N-ethylcarboxamidoadenosine (NECA, A(1)/A(2) agonist), 8.4+/-0.1 for 2-chloroadenosine (A(1)/A(2) agonist), 7.7+/-0.1 for N(6)-(R)-(phenylisopropyl)adenosine (R-PIA, A(1)/A(2B) selective), and 6.8+/-0.2 for adenosine. The potency order (CGS21680=NECA>2-chloroadenosine>R-PIA>adenosine) supports A(2A) adenosine receptor-mediated dilation in mouse coronary vessels. 0.2 - 2 microM of the A(2B)-selective antagonist alloxazine failed to alter CGS21680 or 2-chloroadenosine responses. pEC(50)s in rat were 6.7+/-0.2 for CGS21680, 7.3+/-0.1 for NECA, 7.6+/-0.1 for 2-chloroadenosine, 7.2+/-0.1 for R-PIA, and 6.2+/-0.1 for adenosine (2-chloroadenosine>NECA=R-PIA>CGS21680> adenosine), supporting an A(2B) adenosine receptor response. NO-synthase antagonism with 50 microM N(G)-nitro L-arginine (L-NOARG) increased resistance by approximately 25%, and inhibited responses to CGS21680 (pEC(50)=9.0+/-0.1), 2-chloroadenosine (pEC(50)=7.3+/-0.2) and endothelial-dependent ADP, but not sodium nitroprusside (SNP). K(ATP) channel blockade with 5 microM glibenclamide increased resistance by approximately 80% and inhibited responses to CGS21680 in control (pEC(50)=8.3+/-0.1) and L-NOARG-treated hearts (pEC(50)=7.3+/-0.1), and to 2-chloroadenosine in control (pEC(50)=6.7+/-0.1) and L-NOARG-treated hearts (pEC(50)=5.9+/-0.2). In summary, mouse coronary vessels are more sensitive to adenosine than rat vessels. A(2A) adenosine receptors mediate dilation in mouse coronary vessels vs A(2B) receptors in rat. Responses in the mouse involve a sensitive NO-dependent response and K(ATP)-dependent dilation.

Age-related changes in adenosine-mediated relaxation of coronary and aortic smooth muscle

We tested whether adenosine mediates nitric oxide (NO)-dependent and NO-independent dilation in coronary and aortic smooth muscle and whether age selectively impairs NO-dependent adenosine relaxation. Responses to adenosine and the relatively nonselective analog 5'-N-ethylcarboxamidoadenosine (NECA) were studied in coronary vessels and aortas from immature (1-2 mo), mature (3-4 mo), and moderately aged (12-18 mo) Wistar and Sprague-Dawley rats. Adenosine and NECA induced biphasic concentration-dependent coronary vasodilation, with data supporting high-sensitivity (pEC(50) = 5.2-5.8) and low-sensitivity (pEC(50) = 2.3-2.4) adenosine sites. Although sensitivity to adenosine and NECA was unaltered by age, response magnitude declined significantly. Treatment with 50 microM N(G)-nitro-L-arginine methyl ester (L-NAME) markedly inhibited the high-sensitivity site, although response magnitude still declined with age. Aortic sensitivity to adenosine declined with age (pEC(50) = 4.7 +/- 0.2, 3.5 +/- 0.2, and 2.9 +/- 0.1 in immature, mature, and moderately aged aortas, respectively), and the adenosine receptor transduction maximum also decreased (16.1 +/- 0.8, 12.9 +/- 0.7, and 9.6 +/- 0.7 mN/mm(2) in immature, mature, and moderately aged aortas, respectively). L-NAME decreased aortic sensitivity to adenosine in immature and mature tissues but was ineffective in the moderately aged aorta. Data collectively indicate that 1) adenosine mediates NO-dependent and NO-independent coronary and aortic relaxation, 2) maturation and aging reduce NO-independent and NO-dependent adenosine responses, and 3) the age-related decline in aortic response also involves a reduction in the adenosine receptor transduction maximum.

Adenosine therapy: a new approach to chronic heart failure

Both the prevention and attenuation of chronic heart failure (CHF) are important issues for cardiologists. There are three different strategies to prevent patients from deleterious sequels. The first strategy is to remove the causes of CHF if possible; the second is to attenuate the events that may lead to CHF, such as myocardial ischaemia and reperfusion injury, cardiomyopathy and myocarditis, cardiac hypertrophy and ventricular remodelling; the third is to prevent or attenuate the progression of CHF. Adenosine has a number of actions which merit it as a possible cardioprotective and therapeutic agent for CHF. Firstly, adenosine induces collateral circulation via inducing growth factors and triggering ischaemic preconditioning, both of which induce ischaemic tolerance in advance. Adenosine is also known to reduce the release of noradrenaline, production of endothelin and attenuate the activation of renin-angiotensin system all of which are believed to cause cardiac hypertrophy and remodelling. Secondly, exogenous adenosine is known to reduce the severity of ischaemia and reperfusion injury. Thirdly, adenosine is reported to counteract neurohumoral factors, i.e., cytokine systems, known to be related to the pathophysiology of CHF. Recently, we revealed that adenosine metabolism is changed in patients with CHF and increases in adenosine levels may aid to reduce the severity of CHF. Thus, there are many potential mechanisms for cardioprotection attributable to adenosine and we postulate the use of adenosine therapy will be beneficial in patients with CHF.