K(ATP)-channel activation: effects on myocardial recovery from ischaemia and role in the cardioprotective response to adenosine A1-receptor stimulation

Adenosine receptor-mediated cardioprotection: are all 4 subtypes required or redundant?

Adenosine is a purine nucleoside, which is produced primarily through the metabolism of adenosine triphosphate (ATP), therefore its levels increase during stressful situations when ATP utilization increases. Adenosine exerts potent cardioprotective effects on the ischemic/reperfused heart, reducing reversible and irreversible myocardial injury. Adenosine receptors (ARs) are G-protein-coupled receptors, and 4 subtypes exist--A(1), A(2A), A(2B), and A(3), all of which have been shown to be cardioprotective. Adenosine receptors are expressed on multiple cardiac cells, including fibroblasts, endothelial cells, smooth muscle cells, and myocytes. Activation of both A(1) and A(3) receptors prior to ischemia has been shown in multiple experimental models to reduce ischemia/reperfusion-induced cardiac injury. Additionally, activation of the A(2A) receptor at the onset of reperfusion has been shown to reduce injury. Most recently, there is evidence that the A(2B) receptor has cardioprotective effects upon its activation. However, controversy remains regarding the precise timing of activation of these receptors required to induce cardioprotection, as well as their involvement in ischemic preconditioning and postconditioning. Adenosine receptors have been suggested to reduce cell death through actions at the mitochondrial ATP-dependent potassium (K(ATP)) channel, as well as protein kinase C and mitogen-activated protein kinase (MAPK) signaling. Additionally, the ability of ARs to interact has been documented, and several recent reports suggest that these interactions play a role in AR-mediated cardioprotection. This review summarizes the current knowledge of the cardioprotective effects of each AR subtype, as well as the proposed mechanisms of AR cardioprotection. Additionally, the role of AR interactions in cardioprotection is discussed.

Adenosine enhances cytosolic phosphorylation potential and ventricular contractility in stunned guinea pig heart: receptor-mediated and metabolic protection

Mechanisms of adenosine (ADO) protection of reperfused myocardium are not fully understood. We tested the hypothesis that ADO (0.1 mM) alleviates ventricular stunning by ADO A(1)-receptor stimulation combined with purine metabolic enhancements. Langendorff guinea pig hearts were stunned at constant left ventricular end-diastolic pressure by low-flow ischemia. Myocardial phosphate metabolites were measured by (31)P-NMR, with phosphorylation potential {[ATP]/([ADP].[P(i)]), where brackets indicate concentration} estimated from creatine kinase equilibrium. Creatine and IMP, glycolytic intermediates, were measured enzymatically and glycolytic flux and extracellular spaces were measured by radiotracers. All treatment interventions started after a 10-min normoxic stabilization period. At 30 min reperfusion, ventricular contractility (dP/dt, left ventricular pressure) was reduced 17-26%, ventricular power (rate-pressure product) by 37%, and [ATP]/([ADP].[P(i)]) by 53%. The selective A(1) agonist 2-chloro-N(6)-cyclo-pentyladenosine marginally preserved [ATP]/([ADP].[P(i)]) and ventricular contractility but not rate-pressure product. Purine salvage precursor inosine (0.1 mM) substantially raised [ATP]/([ADP].[P(i)]) but weakly affected contractility. The ATP-sensitive potassium channel blocker glibenclamide (50 microM) abolished ADO protection of [ATP]/([ADP].[P(i)]) and contractility. ADO raised myocardial IMP and glucose-6-phosphate, demonstrating increased purine salvage and pentose phosphate pathway flux potential. Coronary hyperemia alone (papaverine) was not cardioprotective. We found that ADO protected energy metabolism and contractility in stunned myocardium more effectively than both the A(1)-receptor agonist 2-chloro-N(6)-cyclo-pentyladenosine and the purine salvage precursor inosine. Because ADO failed to stimulate glycolytic flux, the enhancement of reperfusion, [ATP]/([ADP].[P(i)]), indicates protection of mitochondrial function. Reduced ventricular dysfunction at enhanced [ATP]/([ADP].[P(i)]) argues against opening of mitochondrial ATP-sensitive potassium channel. The results establish a multifactorial mechanism of ADO antistunning, which appears to combine ADO A(1)-receptor signaling with metabolic adenylate and antioxidant enhancements.

Role of endothelium in ischaemia-induced myocardial dysfunction of isolated working hearts: cardioprotection by activation of adenosine A(2A) receptors

1 This study aimed to determine the role of the vascular endothelium on recovery of contractile function following global low-flow ischaemia of guinea-pig isolated working hearts and the effects of adenosine analogues on this recovery. 2 Guinea-pig isolated spontaneously beating or paced working hearts were set up and coronary flow (CF), aortic output (AO) (as an index of cardiac function), heart rate (HR), left ventricular pressure (LVP) and dP/dt max recorded. The endothelium was either intact or removed by a blast of oxygen. 3 In spontaneously beating hearts, low-flow ischaemia for 30 min reduced CF and cardiac contractility (LVP, dP/dt max) but not AO. On reperfusion, CF, LVP and dP/dt max recovered, while AO fell precipitously followed by a gradual recovery, indicative of myocardial stunning. The effects of ischaemia did not differ between endothelium-intact and -denuded hearts, indicating no role of the endothelium in the changes observed. 4 The adenosine analogues, N6-cyclopentyladenosine (CPA, A1 selective), 5'-N-ethylcarboxamidoadenosine (NECA, two-fold A2 selective over A1) and 2-p-((carboxyethyl)-phenethylamino)-5'carboxamidoadenosine (CGS21680, A2A selective) were infused (3 x 10-7 M) from 10 min into the 30-min low-flow ischaemia of denuded hearts and during reperfusion. 5 CGS21680 increased CF and improved the postischaemic functional recovery, as measured by the AO. NECA and CPA were not cardioprotective. The A2A selective antagonist, ZM241385, attenuated the coronary vasodilatation by CGS21680 and abolished the improved recovery of AO on reperfusion. 6 Reperfusion of paced working hearts caused a dramatic fall in AO which failed to recover. Infusion of CGS21680 from 15 min into the ischaemic period produced vasodilatation but failed to restore AO, presumably because the ischaemic damage was irreversible. 7 Thus, the endothelium plays no role in myocardial dysfunction following low-flow global ischaemia and reperfusion of guinea-pig working hearts. The A2A adenosine receptor-selective agonist but not the non-selective A2 receptor agonist, NECA, attenuated ischaemia- and reperfusion-induced stunning. This was attributed to increased CF and was independent of the endothelium.

Activation of ATP-sensitive potassium channels in hypoxic cardiac failure is not mediated by adenosine-1 receptors in the isolated rat heart

BACKGROUND

Hypoxic cardiac failure is accompanied by action potential shortening, which in part might be a consequence of opening of cardiac ATP-sensitive potassium channels (K(ATP) channels). Coupling of the adenosine-1 receptor (A-1 receptor) to these channels has been described; however, the interaction of A-1-receptors and K(ATP) channels in different models of ischemia is still under debate. The hypothesis as to whether A-1 receptors are involved in hypoxic K(ATP) channel-activation in the saline-perfused rat heart was tested.

METHODS AND RESULTS

Pharmacologic modulation of the K(ATP) channel by Glibenclamide (inhibitor) and Rimalkalim (activator) and of the A-1 receptor by R(-)-N6-(1-methyl-2-phenylethyl)-adenosine (R(-)-PIA, agonist) and 1,3-diethyl-3,7-dihydro-8-phenyl-purine-2,6-dione (DPX, antagonist) at different oxygen tensions (95% O2 and 20% O2) was performed in isolated Langendorff-rat hearts. Peak systolic pressure (PSP, intraventricular balloon), duration of monophasic action potential (epicardial suction electrode, time to 67% of repolarization: MAP(67%)), coronary flow, and heart rate (HR) were registered. Hypoxic perfusion resulted in a significant reduction of PSP (from 106 +/-11 to 56 +/-8 mmHg, P < 0.005) and shortening of MAP(67%) (from 37 +/-3 to 25 +/-4 ms, P < 0.005). With application of 1 microM Glibenclamide, MAP(67%) returned to normoxic values and PSP increased to 78 +/-9 mmHg (P < 0.005 vs hypoxia). In normoxia, 2 microM Rimalkalin resulted in reduction of MAP(67%) and PSP, which was reversed by Glibenclamide. Application of 0.1 microM R(-)-PIA in normoxia resulted in a decrease of HR (from 235 +/-36/min to 75 +/-41/min, P < 0.005), which was accompanied by an increase of PSP from 96 +/-7 to 126 +/-9 mmHg (P < 0.05) without changes in MAP(67%). These effects were reversible by 1 microM DPX and remained unaffected by application of 1 microM Glibenclamide. Application of 1 microM DPX in hypoxia had no effect on the measured parameters.

CONCLUSION

In isolated rat hearts, the K(ATP) channel-system is activated in hypoxic cardiac failure and contributes to action potential shortening and reduced contractile performance. These effects seem to be independent of the A-1 receptor in this model.

K(ATP) channel blockers selectively interact with A(1)-adenosine receptor mediated modulation of acetylcholine release in the rat hippocampus

In this study the role of ATP-sensitive K(+) channels (K(ATP) channels) in the A(1) receptor mediated presynaptic inhibitory modulation of acetylcholine release was investigated in the rat hippocampus. N(6)-Cyclohexyladenosine (CHA), the selective A(1)-adenosine receptor agonist, reduced concentration-dependently the stimulation-evoked (2 Hz, 1 ms, 240 shocks) [3H]acetylcholine ([3H]ACh) release, from in vitro superfused hippocampal slices preloaded with [3H]choline, an effect prevented by the selective A(1) receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). By themselves, neither K(ATP) channel openers, i.e. diazoxide, pinacidil and cromakalim, nor glibenclamide and glipizide, the inhibitors of K(ATP) channels, exerted a significant effect on the resting and evoked release of [3H]ACh. Glibenclamide and glipizide (10-100 microM) completely prevented the inhibitory effect of 0.1 microM CHA and shifted the concentration response curve of CHA to the right. 4-Aminopyridine (10-100 microM), the non-selective potassium channel blocker, increased the evoked release of [3H]ACh, but in the presence of 4-aminopyridine, the inhibitory effect of CHA (0.1 microM) still persisted. Oxotremorine, the M(2) muscarinic receptor agonist, decreased the stimulation-evoked release of [3H]ACh, but its effect was not reversed by glibenclamide. 1,3-Diethyl-8-phenylxanthine (DPX), the selective A(1)-antagonist, effectively displaced [3H]DPCPX in binding experiments, while in the case of glibenclamide and glipizide, only slight displacement was observed. In summary, our results suggest that K(ATP) channels are functionally coupled to A(1) receptors present on cholinergic terminals of the hippocampus, and glibenclamide and glipizide, by interacting with K(ATP) channels, relieve this inhibitory neuromodulation.

Cardioprotection by K(ATP) channels in wild-type hearts and hearts overexpressing A(1)-adenosine receptors

We studied the role of mitochondrial ATP-sensitive K(+) (K(ATP)) channels in modifying functional responses to 20 min global ischemia and 30 min reperfusion in wild-type mouse hearts and in hearts with approximately 250-fold overexpression of functionally coupled A(1)-adenosine receptors (A(1)ARs). In wild-type hearts, time to onset of contracture (TOC) was 303 +/- 24 s, with a peak contracture of 89 +/- 5 mmHg. Diastolic pressure remained elevated at 52 +/- 6 mmHg after reperfusion, and developed pressure recovered to 40 +/- 6% of preischemia. A(1)AR overexpression markedly prolonged TOC to 517 +/- 84 s, reduced contracture to 64 +/- 6 mmHg, and improved recovery of diastolic (to 9 +/- 4 mmHg) and developed pressure (to 82 +/- 8%). 5-Hydroxydecanoate (5-HD; 100 microM), a mitochondrial K(ATP) blocker, did not alter ischemic contracture in wild-type hearts, but increased diastolic pressure to 69 +/- 8 mmHg and reduced developed pressure to 10 +/- 5% during reperfusion. In transgenic hearts, 5-HD reduced TOC to 348 +/- 18 s, increased postischemic contracture to 53 +/- 4 mmHg, and reduced recovery of developed pressure to 22 +/- 4%. In summary, these data are the first to demonstrate that endogenous activation of K(ATP) channels improves tolerance to ischemia-reperfusion in murine myocardium. This functional protection occurs without modification of ischemic contracture. The data also support a role for mitochondrial K(ATP) channel activation in the pronounced cardioprotection afforded by overexpression of myocardial A(1)ARs.

ATP-Sensitive potassium channels: a review of their cardioprotective pharmacology

ATP-sensitive potassium channels (K(ATP)) have been thought to be a mediator of cardioprotection for the last ten years. Significant progress has been made in learning the pharmacology of this channel as well as its molecular regulation with regard to cardioprotection. K(ATP)openers as a class protect ischemic/reperfused myocardium and appear to do so by conservation of energy. The reduced rate of ATP hydrolysis during ischemia exerted by these openers is not due to a cardioplegic effect and is independent of action potential shortening. Compounds have been synthesized which retain the cardioprotective effects of first generation K(ATP)openers, but are devoid of vasodilator and cardiac sarcolemmal potassium outward currents. These results suggest receptor or channel subtypes. Recent pharmacologic and molecular biology studies suggest the activation of mitochondrial K(ATP)as the relevant cardioprotective site. Implications of these results for future drug discovery and preconditioning are discussed.

Alteration of glycogen and glucose metabolism in ischaemic and post-ischaemic working rat hearts by adenosine A1 receptor stimulation

1. Cardioprotection by adenosine A1 receptor activation limits infarct size and improves post-ischaemic mechanical function. The mechanisms responsible are unclear but may involve alterations in myocardial glucose metabolism. 2. Since glycogen is an important source of glucose during ischaemia, we examined the effects of N6-cyclohexyladenosine (CHA), an A1 receptor agonist, on glycogen and glucose metabolism during ischaemia as well as reperfusion. 3. Isolated working rat hearts were perfused with Krebs-Henseleit solution containing dual-labelled 5-3H and 14C glucose and palmitate as energy substrates. Rates of glycolysis and glucose oxidation were measured directly from the production of 3H2O and 14CO2. Glycogen turnover was measured from the rate of change of [5-3H and 14C]glucosyl units in total myocardial glycogen. 4. Following low-flow (0.5 ml min-1) ischaemia (60 min) and reperfusion (30 min), left ventricular minute work (LV work) recovered to 22% of pre-ischaemic values. CHA (0.5 microM) improved the recovery of LV work 2 fold. 5. CHA altered glycogen turnover in post-ischaemic hearts by stimulating glycogen synthesis while having no effects on glycogen degradation. CHA also partially inhibited glycolysis. These changes accelerated the recovery of glycogen in CHA-treated hearts and reduced proton production. 6. During ischaemia, CHA had no measurable effect on glycogen turnover or glucose metabolism. Glycogen phosphorylase activity, which was elevated after ischaemia, was inhibited by CHA, possibly in response to CHA-induced inhibition of AMP-activated protein kinase activity. 7. These results indicate that CHA-induced cardioprotection is associated with alterations of glycogen turnover during reperfusion as well as improved metabolic coupling of glycolysis to glucose oxidation.