Purine-enriched asanguineous cardioplegia retards adenosine triphosphate degradation during ischemia and improves postischemic ventricular function

Deregulation of the Purine Pathway in Pre-Transplant Liver Biopsies Is Associated with Graft Function and Survival after Transplantation

The current shortage of livers for transplantation has increased the use of marginal organs sourced from donation after circulatory death (DCD). However, these organs have a higher incidence of graft failure, and pre-transplant biomarkers which predict graft function and survival remain limited. Here, we aimed to find biomarkers of liver function before transplantation to allow better clinical evaluation. Matched pre- and post-transplant liver biopsies from DCD (n = 24) and donation after brain death (DBD, n = 70) were collected. Liver biopsies were analysed using mass spectroscopy molecular phenotyping. Discrimination analysis was used to parse metabolites differentiated between the two groups. Five metabolites in the purine pathway were investigated. Of these, the ratios of the levels of four metabolites to those of urate differed between DBD and DCD biopsies at the pre-transplantation stage (q < 0.05). The ratios of Adenosine monophosphate (AMP) and adenine levels to those of urate also differed in biopsies from recipients experiencing early graft function (EGF) (q < 0.05) compared to those of recipients experiencing early allograft dysfunction (EAD). Using random forest, a panel consisting of alanine aminotransferase (ALT) and the ratios of AMP, adenine, and hypoxanthine levels to urate levels predicted EGF with area under the curve (AUC) of 0.84 (95% CI (0.71, 0.97)). Survival analysis revealed that the metabolite classifier could stratify six-year survival outcomes (p = 0.0073). At the pre-transplantation stage, a panel composed of purine metabolites and ALT could improve the prediction of EGF and survival.

Evaluation of the anti-ischemic effects of D-ribose during dobutamine stress echocardiography: a pilot study

D-Ribose, a pentose sugar, has shown to improve myocardial high-energy phosphate stores depleted by ischemia. This study investigated the ability of D-Ribose with low dose dobutamine to improve the contractile response of viable myocardium to dobutamine and to assess the efficacy of D-ribose in reducing stress-induced ischemia. Twenty-six patients with ischemic cardiomyopathy completed a two-day, randomized, double blind crossover trial comparing the effects of D-Ribose and placebo on regional wall motion. On the first study day, either D-Ribose or placebo was infused for 4.5 hours. Low (5 and 10 μ/kg/min) and subsequently, high (up to 50 μ/kg/min) dose dobutamine echocardiography was then performed. On the second study day, patients crossed over to the alternative article for a similar 4.5 hours infusion time period and underwent a similar evaluation. The wall motion response during low dose dobutamine was the same with D-Ribose and placebo in 77% of segments (203/263, Kappa = 0.37). In segments with discordant responses, more segments improved with D-Ribose than with placebo (41 vs. 19 segments, p = 0.006). With high dose dobutamine infusion, the wall motion response (ischemia vs. no ischemia) was the same with D-Ribose and placebo in 83% of interpretable segments (301/363, kappa = 0.244). In segments with discordant responses, there were more ischemic segments with placebo compared to D-Ribose (36 vs. 26, p = 0.253). Nineteen patients developed ischemia during the dobutamine and placebo infusion and 13 patients had ischemia during dobutamine and D-ribose infusion (p = 0.109). D-Ribose improved contractile responses to dobutamine in viable myocardium with resting dysfunction but had no significant effect in reducing the frequency of stress-induced wall motion abnormalities.

Prevention of Postischemic Spinal Cord Injury by Means of Regional Infusion of Adenosine and l-carnitine Dissolved in Normothermic Saline

Spinal cord ischemia still remains an unsolved problem in modern aortic surgery. In this study, we investigated the effectiveness of combined agents such as adenosine and L-carnitine infused to the isolated segment of abdominal aorta in a rabbit model. Twenty-eight rabbits divided into four groups underwent 40 min of isolated infrarenal aortic occlusion. Group I animals received no medication. Group II received an infusion of 100 mg/kg L-carnitine in normothermic saline over the first 10 min of ischemia. Group III received 50 mg adenosine, and group IV received a combination of the two agents in the same fashion. Spinal cord function was evaluated at 24 and 72 hr after operation on the basis of Tarlov scale and similar results were obtained. After a second evaluation, spinal cords were harvested for histological examination. Group I animals were all paraplegics. Spinal cord function was partially intact in two of the group II animals with Tarlov scores of 5 in two and 4 in two whereas one of the rabbits could not hop with a score of 3, and the remaining two could not sit with scores of 1 and 0. The spinal cord function of group III animals was intact with Tarlov scores of 5 in three, 4 in two, and 3 and 1 in remaining ones. In the group IV animals, it was fully intact with Tarlov scores of 5. Histological examination in group I revealed marked enlargement of the vacuoles of glial cells in the white matter of spinal cord. Glial cells were deteriorated in some locations in group II whereas they were mostly protected in the third group. In group IV, histological examination revealed no evidence of spinal cord injury. In conclusion, combined infusion of adenosine and L-carnitine provided better protection against postischemic spinal cord injury than individual infusion of these agents.

D-Ribose as a supplement for cardiac energy metabolism

Metabolic support for the heart has been an attractive concept since the pioneering work of Sodi-Pallares et al. four decades ago.* Recently, interest has increased in the use of over-the-counter supplements and naturally occurring nutriceuticals for enhancement of cardiac and skeletal muscle performance. These include amino acids such as creatine, L-carnitine, and L-arginine, as well as vitamins and cofactors such as alpha-tocopherol and coenzyme Q. Like these other molecules, D-ribose is a naturally occurring compound. It is the sugar moiety of ATP and has also received interest as a metabolic supplement for the heart. The general hypothesis is that under certain pathologic cardiac conditions, nucleotides (particularly ATP, ADP, and AMP) are degraded and lost from the heart. The heart's ability to resynthesize ATP is then limited by the supply of D-ribose, which is a necessary component of the adenine nucleotide structure. In support of this hypothesis, recent reports have used D-ribose to increase tolerance to myocardial ischemia. Its use in patients with stable coronary artery disease improves time to exercise-induced angina and electrocardiographic changes. In conjunction with thallium imaging or dobutamine stress echocardiography, D-ribose supplementation has been used to enhance detection of hibernating myocardium. In this article, we review the biochemical basis for using supplemental D-ribose as metabolic support for the heart and discuss the experimental evidence for its benefit.

Mechanisms of myocardial protection by adenosine-supplemented cardioplegia: differential response of calcium-independent protein kinase C isozymes

BACKGROUND

Adenosine-supplemented cardioplegia improves myocardial function after cardioplegic arrest. However, the underlying cellular mechanism(s) responsible for adenosine's protective actions remains unclear. We tested the hypothesis that protection by adenosine-supplemented cardioplegia would be associated with selective activation of protein kinase C (PKC) isozymes delta and epsilon.

MATERIALS AND METHODS

Isolated rat hearts were perfused (37 degrees C, Krebs-Ringer bicarbonate buffer) for 30 min, after which baseline functional measurements were made. This was followed by 120 min of cold cardioplegic arrest at 4 degrees C with either St. Thomas No. 2 (ST#2), ST#2 + adenosine (100 microM, ADO) or ST#2 + ADO + 8-sulfophenyltheophylline (50 microM, SPT). Hearts were reperfused for 60 min and functional measurements made. Distribution of PKC isoforms was determined (immunoblotting) after 30 min of warm perfusion (No-CDPL) or after 30 min of perfusion followed by 15 min of cardioplegic arrest.

RESULTS

ADO prevented myocardial dysfunction after cardioplegic arrest. PKC-delta did not differ in the cytosolic fraction among groups. However, ADO prevented increases in particulate fraction PKC-delta, but elicited a significant increase in the particulate fraction PKC-epsilon, while ST#2 or SPT significantly decreased the cytosolic fraction PKC-epsilon. Both functional and cellular changes associated with ADO were receptor mediated.

CONCLUSION

This novel, dual action of adenosine-supplemented cardioplegia on PKC isoforms may be responsible for the associated functional improvements.

Mechanisms of myocardial protection by adenosine-supplemented cardioplegic solution: myofilament and metabolic responses

OBJECTIVE

Adenosine supplementation of cardioplegic solutions in cardiac operations improves postarrest myocardial recovery after cardioplegic arrest and reperfusion; however, the mechanism of the action of adenosine remains unknown. We tested the hypotheses that adenosine-supplemented cardioplegic solution improves myofibrillar protein cooperative interaction and increases myocardial anaerobic glycolysis.

METHODS

The hearts of male Sprague-Dawley rats were randomized to undergo 120 minutes of cardioplegic arrest with 1 of 3 cardioplegic solutions: (1) St Thomas' Hospital No. 2 cardioplegic solution (St Thomas group), (2) St Thomas' Hospital No. 2 cardioplegic solution plus adenosine (100 micromol/L) (adenosine group), and (3) St Thomas' Hospital No. 2 cardioplegic solution plus adenosine (100 micromol/L) plus the nonspecific adenosine receptor antagonist 8-p -sulfophenyltheophylline (50 micromol/L) (sulfophenyltheophylline group). A fourth group of hearts underwent no cardioplegic arrest.

RESULTS

Systolic and diastolic functional recovery was improved in the adenosine group compared with that in the other two groups, independent of coronary flow. Adenosine supplementation of cardioplegic solution prevented the decrease in myofibrillar protein cooperative interaction seen after cardioplegic arrest and reperfusion (St Thomas and sulfophenyltheophylline groups). Adenosine-supplemented cardioplegic solution also caused significantly increased anaerobic glycolysis during cardioplegic arrest. These responses were blocked in the sulfophenyltheophylline group.

CONCLUSIONS

The changes in myocardial glycolytic activity and myofilament cooperativity coincided with functional recovery in the three cardioplegia groups and may represent mechanisms underlying protection with adenosine-supplemented cardioplegic solution.

The effects of mannitol, albumin, and cardioplegia enhancers on 24-h rat heart preservation

During 24 h in vitro heart preservation and reperfusion, tissue damage occurs that seriously reduces cardiac function. Prevention of free radical production during preservation and reperfusion of ischemic tissue using free radical scavengers is of primary importance in maintaining optimal heart function in long-term preservation protocols. We examined whether mannitol (68 mM) and albumin (1.4 microM) in combination with other cardioplegia enhancers decreased free radical formation and edema and increased cardiac function during 24-h cold (5 degrees C) heart preservation and warm (37 degrees C) reperfusion in the Langendorff-isolated rat heart. The performance of mannitol-treated hearts was significantly decreased compared with that of hearts without mannitol treatment after 24 h of preservation with regard to recovery of diastolic pressure, contractility (+dP/dt), relaxation (-dP/dt), myocardial creatine kinase release, coronary flow, and lipid peroxidation. Albumin-treated hearts demonstrated higher cardiac function (contractility and coronary flow especially) than hearts not treated with albumin or hearts treated with mannitol, and this appears to be due to the positive effects of increased cellular metabolism and the enhancement of membrane stability.

Twenty-four-hour heart preservation using continuous cold perfusion and copper (II) complexes

BACKGROUND

During long-term in vitro heart preservation and subsequent reperfusion, irreversible tissue damage occurs in part due to reactive oxygen species. Therefore, inhibition of generation of oxygen-derived free radicals and the related oxidative damage of ischemic tissue may be useful in maintaining heart function after long-term preservation. Complexes of Cu(II) may cause disproportionation of superoxide and thus may function as an inhibitor of the effects of oxygen-derived free radicals.

METHODS

In this study, 24-h preservation of isolated rat hearts was performed. Using the Langendorff technique, hearts were perfused for 24 h with a hypothermic, moderately hyperkalemic (15 mM KCl) solution containing various metabolic and membrane-stabilizing additives at constant low pressure. In addition, the potential benefit of the addition of two Cu(II) compounds (Cu(II) Cl2 and Cu(II)2Asp4) to the perfusion solution was examined.

RESULTS

The Cu(II)Cl2-treated hearts were significantly better preserved than control hearts after 24 h of preservation with regard to recovery of systolic pressure, coronary flow, max +dP/dt, and max -dP/dt. Lipid peroxidation as estimated by myocardial malonaldehyde (both P < 0. 001) and myocardial creatine kinase release (both P < 0.05 vs control) were significantly reduced in the Cu(II)Cl2 and Cu(II)2Asp4 groups. Overall, Cu(II)Cl2 best preserved the heart after 24 h of cold preservation with respect to indices of functional recovery, whereas Cu(II)2Asp4 did not significantly improve functional recovery compared to control.

CONCLUSION

Low-pressure, cold perfusion with an enhanced solution is a potential method to preserve donor hearts in preparation for transplantation. The beneficial effect of Cu(II)Cl2 was attributed to (i) SOD activity of the Cu2+ species and/or (ii) termination of chain carriers in the lipid peroxidations by aqueous Cu2+ and Cu+ species. The negation of some of the positive effects of Cu2+ species by the introduction of acetylsalicylate was tentatively assigned to potentiation of the Ca2+ modality for reperfusion injury.

Adenosine Receptor Subtypes and Cardioprotection

Brief ischemia prior to a sustained period of ischemia reduces myocardial infarct size, a phenomenon known as preconditioning. A cardiac ventricular myocyte model has been developed to investigate the role and signaling mechanism of adenosine receptor subtypes in cardiac preconditioning. A 5-min exposure of cardiac myocytes to simulated ischemia, termed preconditioning ischemia, prior to a subsequent 90-min period of ischemia protected them against injury incurred during the 90-min ischemia. Preconditioning ischemia preserved ATP content, reduced percentage of cells killed, and decreased release of creatine kinase into the medium. Activation of the adenosine A1 receptor with CCPA or the A3 receptor with IB-MECA can replace preconditioning ischemia and mimic the protective effect of preconditioning ischemia. Blockade of the A1 receptor with its selective antagonist DPCPX or of the A3 receptor with the A3 selective antagonist MRS1191 during the preconditioning ischemia resulted in only a partial attenuation of the subsequent protection. Incubation with both DPCPX and MRS1191 or with the nonselective antagonist 8-SPT during the preconditioning ischemia completely abolished the protective effect of preconditioning ischemia. The KATP channel opener pinacidil caused a large activation of the KATP channel current and was able to precondition the myocyte. The KATP channel antagonist glibenclamide blocked the cardioprotective effect of preconditioning ischemia when it was included during myocyte exposure to the preconditioning ischemia, indicating that KATP channel is a requisite effector in mediating preconditioning. A receptor-mediated stimulation of phospholipase C or phospholipase D, with consequent activation of protein kinase C and KATP channel, appears to be the signaling mechanism linking adenosine A1 and A3 receptors to the induction of preconditioning. A model of how ischemic preconditioning is triggered and mediated is proposed. Evidence is accumulating to support its validity.

Selective activation of A3 adenosine receptors with N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide protects against myocardial stunning and infarction without hemodynamic changes in conscious rabbits

To examine the cardioprotective role of A3 adenosine receptors during myocardial ischemia/reperfusion injury, we tested the effect of N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA), a potent and selective A3 adenosine receptor agonist, in models of myocardial stunning and infarction in chronically instrumented conscious rabbits. In phase I (studies of myocardial stunning), rabbits were subjected to six 4-minute coronary occlusions, each separated by 4-minute reperfusion periods, after which the recovery of systolic wall thickening was measured (ultrasonic crystals). In phase II (studies of myocardial infarction), rabbits were subjected to a 30-minute coronary occlusion followed by 3 days of reperfusion. In both phases, IB-MECA was administered as an intravenous bolus (100 micrograms/kg) 10 minutes before the first coronary occlusion. This dose of IB-MECA was determined in pilot studies to have no effect on heart rate, arterial blood pressure, or plasma histamine concentration in rabbits. In phase I, IB-MECA markedly improved the recovery of wall thickening after the six occlusion/reperfusion cycles, and this effect was sustained throughout the 5-hour observation period; the total deficit of wall thickening (a measure of the overall severity of myocardial stunning) was reduced by 68% (control, 129 +/- 16 arbitrary units, n = 7; IB-MECA, 41 +/- 6 arbitrary units, n = 6; P < .01). The protective effects of IB-MECA against stunning were completely blocked by pretreatment with the nonselective adenosine receptor antagonist 8-p-sulfophenyl theophylline or the specific protein kinase C inhibitor chelerythrine. In phase II, IB-MECA reduced myocardial infarct size by 61%; infarct size (tetrazolium staining) was 41 +/- 4% of the risk region in control animals (n = 8) and 16 +/- 6% in IB-MECA-treated animals (n = 8, P < .01). These results demonstrate that in conscious rabbits the A3 adenosine receptor agonist IB-MECA confers a powerful protection against both reversible (stunning) and irreversible (infarction) injury during acute myocardial ischemia and reperfusion by a protein kinase C-mediated pathway, suggesting that selective activation of A3 receptors is an effective means of protecting the ischemic myocardium without hemodynamic changes.

Protective effects of adenosine on myocyte contractility during cardioplegic arrest

BACKGROUND

Adenosine delivery to the left ventricular myocardium has been demonstrated to provide protective effects in the setting of ischemia and reperfusion. However, whether adenosine has direct protective effects on isolated myocytes in the setting of cardioplegic arrest was unclear.

METHODS

Isolated porcine left ventricular myocytes were assigned to one of the following treatment groups: (1) cardioplegia: 24 mEq/L K+, 4 degrees C for 2 hours followed by rewarming (cell media, 37 degrees C; n = 29); (2) cardioplegia augmented with adenosine (1 to 200 micromol/L) followed by rewarming (n = 98); and (3) normothermic control (cell media, 37 degrees C, 2 hours; n = 175). Myocyte contractility was measured by computer-aided videomicroscopy.

RESULTS

Cardioplegic arrest and rewarming reduced myocyte shortening velocity compared with normothermic control (25.3 +/- 2.5 microm/s versus 50.9 +/- 1.4 microm/s, p < 0.05). Adenosine-augmented cardioplegic arrest improved myocyte contractility with rewarming in a concentration-dependent fashion. For example, cardioplegia augmented with 10 micromol/L adenosine improved myocyte shortening velocity by 33% (33.6 +/- 3.0 microm/s versus 25.3 +/- 2.5 microm/s, p < 0.05), whereas 200 micromol/L adenosine improved shortening velocity by 97% (49.9 +/- 3.4 microm/s vs 25.3 +/- 2.5 microm/s, p < 0.05) compared with conventional cardioplegia.

CONCLUSIONS

This study demonstrated concentration-dependent protective effects of adenosine-augmented cardioplegia on myocyte contractile function with subsequent reperfusion and rewarming. These results suggest that stimulation of putative myocyte adenosine receptors may provide enhanced protective effects on myocyte contractile processes during cardioplegic arrest.

Functional and metabolic effects of adenosine in cardioplegia: role of temperature and concentration

BACKGROUND

Addition of adenosine to cardioplegic fluid has been shown to improve myocardial tolerance to ischemia. This study was designed to investigate further this phenomenon to evaluate the dose-response and the temperature dependence of the effect of addition of adenosine to St. Thomas' Hospital cardioplegic solution.

METHODS

The isolated working rat heart model was used in this study. After the assessment of control function, hearts (6 in each group) were subjected to infusions of cardioplegic solution containing 0.0 (control), 0.1, 5.0, 10.0 or 20.0 mmol/L adenosine followed by 3 hours of ischemic arrest at temperatures of 20 degrees C, 10 degrees C, or 4 degrees C with multidose (3 minutes every 30 minutes) cardioplegic infusion.

RESULTS

After ischemic arrest at 20 degrees C, the recovery of cardiac output (expressed as percent of preischemic baseline) was 35.4 +/- 5.11 (control) 45.0 +/- 5.51 (0.1 mmol/L), 53.1 +/- 2.9 (5.0 mmol/L), 61.8 +/- 3.7 (10.0 mmol/L), and 57.6 +/- 2.3 (20.0 mmol/L). Hearts receiving 5.0 to 20.0 mmol/L adenosine had significantly greater recovery of cardiac output than control hearts. In its optimal concentration (10 mmol/L), adenosine improved the efficacy of the cardioplegic solution by almost 75%. Myocardial adenosine triphosphate content (expressed in mumol/g protein) was 4.7 +/- 0.5 (control), 4.9 +/- 1.4 (0.1 mmol/L), 8.1 +/- 0.7 (5 mmol/L), 12.5 +/- 2.0 (10 mmol/L), and 11.2 +/- 2.8 (20 mmol/L), at the end of ischemia and 13.9 +/- 0.2 (control), 13.1 +/- 1.7 (0.1 mmol/L), 18.0 +/- 2.0 (5 mmol/L), 18.6 +/- 1.2 (10 mmol/L), and 20.7 +/- 2.1 (20 mmol/L) at the end of reperfusion. Thus, the adenosine triphosphate content was higher (p < 0.05) in hearts receiving 5.0 to 20.0 mmol/L adenosine than in controls both at the end of ischemia and after reperfusion. Myocardial adenosine monophosphate level at the end of ischemia was inversely related to adenosine triphosphate level. Functional assessment of the effect of 10 mmol/L adenosine at 10 degrees C and 4 degrees C during arrest indicated attenuation of beneficial effects: adenosine improved function only by 17% at 10 degrees C, whereas at 4 degrees C the protective effect was not observed.

CONCLUSIONS

These observations suggest that adenosine has the potential to enhance the efficacy of clinical cardioplegic arrest but the degree of improvement is lower at decreased temperature during ischemia. A principal mechanism of action of this modification of cardioplegic fluid appears to be through the inhibition of high-energy phosphate utilization immediately before or during ischemia.

Regulation of and intervention into the oxidative pentose phosphate pathway and adenine nucleotide metabolism in the heart

The capacity of the oxidative pentose pathway (PPP) in the heart is limited, since the activity of glucose-6-phosphate dehydrogenase (G-6-PD), the first and regulating enzyme of this pathway, is very low. Two mechanisms are involved in the regulation of this pathway. Under normal conditions, G-6-PD is inhibited by NADPH. This can be overcome in the isolated perfused rat heart by increasing the oxidized glutathione and by elevating the NADP+/NADPH ratio. Besides this rapid control mechanism, there is a long-term regulation which involves the synthesis of G-6-PD. The activity of G-6-PD was elevated in the rat heart during the development of cardiac hypertrophy due to constriction of the abdominal aorta and in the non-ischemic part of the rat heart subsequent to myocardial infarction. The catecholamines isoproterenol and norepinephrine stimulated the activity of myocardial G-6-PD in a time- and dose-dependent manner. The isoproterenol-induced stimulation was cAMP-dependent and due to increased new synthesis of enzyme protein. The G-6-PD mRNA was elevated by norepinephrine. As a consequence of the stimulation of the oxidative PPP, the available pool of 5-phosphoribosyl-1-pyrophosphate (PRPP) was expanded. PRPP is an important precursor substrate for purine and pyrimidine nucleotide synthesis. The limiting step in the oxidative PPP, the G-6-PD reaction, can be bypassed with ribose. This leads to an elevation of the cardiac PRPP pool. The decline in ATP that is induced in many pathophysiological conditions was attenuated or even entirely prevented by i.v. infusion of ribose. In two in vivo rat models, the overloaded and catecholamine-stimulated heart and the infarcted heart, the normalization of the cardiac adenine nucleotide pool by ribose was accompanied by an improvement of global heart function. Combination of ribose with adenine or inosine in isoproterenol-treated rats was more effective to restore completely the cardiac ATP level within a short period of time than either intervention alone.

Effects of warm blood cardioplegic solution on myocardial protection

To evaluate the effects of warm blood cardioplegic solution on myocardial protection, normothermic induction and terminal perfusion of oxygenated blood cardioplegia in combination with intermittent administration of cold blood cardioplegia during ischemia were studied in an isolated working rat heart model. The experimental protocol consisted of a 120 min cardioplegic arrest followed by 45 min normothermic reperfusion. Myocardial content of adenosine triphosphate (ATP), recovery of the left ventricular function, release of creatine phosphokinase (CPK) and ultrastructure of myocardium were assessed before and after ischemia. The results showed that the hearts preserved with warm blood cardioplegic induction and terminal perfusion had significantly higher levels of ATP, better recovery of cardiac function and lower releases and lower releases of CPK than those receiving cold blood cardioplegia alone, with myocardial tissue being of generally normal structure. These findings suggest that warm induction and terminal perfusion of blood cardioplegic solution can accelerate myocardial metabolic and functional recovery, preserve high-energy phosphate, reduce myocardial injury and enhance myocardial protection.

Protective effects of adenosine in the reversibly injured heart

BACKGROUND

There is substantial evidence that the nucleoside adenosine reduces postischemic ventricular dysfunction (ie, myocardial stunning). Studies performed in our laboratory have attempted to address the mechanism of adenosine-mediated protection of the reversibly injured heart.

METHODS

Experiments were performed in isolated perfused rat and rabbit hearts and in in situ canine and porcine preparations. The role of adenosine A1 receptors was assessed by using adenosine A1 receptor agonists and antagonists, and by measuring interstitial fluid purine levels with the cardiac microdialysis technique.

RESULTS

In isolated perfused hearts, treatment immediately before ischemia with adenosine and adenosine A1 receptor analogues significantly improved postischemic ventricular function, effects that were blocked by a selective adenosine A1 receptor antagonist. In in situ canine and porcine preparations, pretreatment with adenosine and an adenosine deaminase inhibitor increased preischemic interstitial fluid adenosine levels and attenuated regional myocardial stunning. Adenosine treatment was also associated with improved myocardial phosphorylation potential in isolated guinea pig hearts and in the in situ porcine preparation.

CONCLUSIONS

These results suggest that adenosine-induced attenuation of myocardial stunning is mediated via adenosine A1 receptor activation and enhancement of postischemic myocardial phosphorylation potential.

Adenosine in blood cardioplegia prevents postischemic dysfunction in ischemically injured hearts

Adenosine (ADO) is an endogenous cardioprotective autacoid that exerts receptor-mediated cardioprotection from ischemic-reperfusion injury. This study tested the hypothesis that blood cardioplegia (BCP) supplemented with ADO reduces postischemic left ventricular dysfunction in ischemically injured hearts. Twenty-one anesthetized dogs on total bypass were subjected to 30 minutes of normothermic global ischemia. Cold (4 degrees C) potassium BCP was then delivered every 20 minutes for 60 minutes of cardioplegic arrest. In 7 dogs, unsupplemented BCP was used; in 7 dogs, BCP was supplemented with 400 mumol/L ADO; and, in 7 dogs, ADO receptors were blocked with 8-p-sulfophenyltheophylline (30 mg/kg) given with 400 mumol/L ADO in BCP. Preischemic and postischemic left ventricular systolic function was assessed by the slope and volume axis intercept of the end-systolic pressure-volume (impedance catheter) relationship (ESPVR). In unsupplemented BCP, the postischemic slope of the ESPVR was significantly depressed by 42% versus the preischemic value (from 6.8 +/- 1.2 mm Hg/mL to 3.9 +/- 0.4 mm Hg/mL; p < 0.05 versus the preischemic value). In contrast, BCP supplemented with ADO was found to restore the postischemic ESPVR slope to preischemic levels (7.7 +/- 1.0 mm Hg/mL versus 7.4 +/- 1.2 mm Hg/mL, respectively). This cardioprotection was reversed by 8-p-sulfophenyltheophylline (9.9 +/- 1.5 mm Hg/mL versus 4.5 +/- 0.7 mm Hg/mL; p < 0.05 versus the preischemic value). Postischemic plasma creatinine kinase activity was elevated equally in all groups over the baseline values. We conclude that ADO in BCP attenuates postcardioplegia dysfunction in severely injured hearts through the operation of receptor-mediated mechanisms.

Pentostatin-augmented interstitial adenosine prevents postcardioplegia injury in damaged hearts

This study tests the hypothesis that the adenosine deaminase inhibitor pentostatin (2-deoxycoformycin), when given before ischemia or during infusions of blood cardioplegia, augments interstitial adenosine levels and prevents postcardioplegia dysfunction in hearts with antecedent ischemia. Twenty-one anesthetized dogs were placed on cardiopulmonary bypass, and the hearts were made globally ischemic for 30 minutes. Dogs received blood cardioplegia with no pentostatin (BCP group, n = 6), pretreatment pentostatin (0.2 mg/kg) infused 5 minutes before global ischemia (PS-PTx group, n = 7), or pentostatin included only in the blood cardioplegia without pretreatment (PS-BCP group, n = 8). Microdialysate myocardial adenosine levels (an index of interstitial fluid levels) increased only modestly in the BCP group (from 0.55 +/- 0.13 microM to 2.64 +/- 0.50 microM) and the PS-BCP group (from 0.55 +/- 0.18 microM to 1.08 +/- 0.48 microM) during normothermic ischemia, but interstitial adenosine levels were not augmented further during cardioplegic arrest in either group. In contrast, the adenosine level in the PS-PTx group was significantly (p < 0.05) augmented during global ischemia (from 0.50 +/- 0.13 microM to 63.16 +/- 28.08 microM) and cardioplegia infusion (to 15.26 microM +/- 5.61 microM). Relative to baseline, postischemic left ventricular performance (end-systolic pressure-volume relation) was depressed in both the BCP (from 5.5 +/- 1.2 mm Hg/mL to 3.8 +/- 0.4 mm Hg/mL) and PS-BCP groups (from 7.1 +/- 0.9 mm Hg/mL to 3.8 +/- 0.7 mm Hg/mL). In contrast, PS-PTx restored postischemic performance (from 6.2 +/- 0.5 mm Hg/mL to 7.5 +/- 0.9 mm Hg/mL).(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine and AICA-riboside fail to enhance microvascular endothelial preservation

Crystalloid cardioplegia may cause coronary microvascular endothelial dysfunction during cardiopulmonary bypass. The perioperative administration of either adenosine or AICA-riboside (acadesine, 5-aminoimidazole-4-carboxamide-1-ribofuranoside) has been associated with improved myocardial functional preservation and improved coronary blood flow after ischemic arrest. To examine if this enhanced recovery of myocardial function and perfusion may be related to improved endothelial preservation, pigs were placed on cardiopulmonary bypass and the hearts were arrested with plain cold, hyperkalemic (K+ = 25 mmol/L) crystalloid cardioplegia, or cardioplegic solution containing either 0.1 mmol/L adenosine or 50 mumol/L AICA-riboside, which causes the release of endogenous adenosine. AICA-riboside (375 mg) also was infused over 30 minutes before cardioplegia in the later group. After 1 hour of ischemic cardioplegia, hearts were reperfused for 1 hour while the pigs were weaned from cardiopulmonary bypass. Coronary arterioles (90 to 190 microns in diameter) from both subepicardial and subendocardial regions of the left ventricle were studied in vitro in a pressurized, no-flow state with videomicroscopy. After contraction of vessels by 25% to 40% of the baseline diameter, drugs were applied extraluminally. Relaxation of control arterioles to serotonin was slightly greater in vessels from the subendocardial region compared with vessels from the subepicardial region, and subendocardial arterioles were more affected by cardioplegia than were subepicardial vessels. In contrast, relaxations of control microvessels to bradykinin and the calcium ionophore A23187 were similar in the two transmural myocardial regions. Responses to bradykinin and A23187 were significantly and similarly reduced after ischemic arrest with plain crystalloid cardioplegia.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial infarction and purine transport inhibition in anaesthetised ferrets

The potential cytoprotective effect of the purine transport inhibitor S-(p-nitrobenzyl)-6-thioinosine (NBTI) in a model of myocardial ischaemia and reperfusion was investigated in the anaesthetised ferret. The left anterior descending coronary artery (LAD) was occluded for 90 min, producing ischaemia in 53 +/- 3% of the left ventricular free wall, followed by 240 min reperfusion. NBTI (0.5 mg kg-1, i.v.) was given prior to ischaemia or prior to reperfusion. In addition the effect of purine transport inhibition was investigated in animals subjected to ischaemia without reperfusion. NBTI reduced infarct size from 84.0 +/- 1.7 to 71.4 +/- 3.7% of the area at risk (P < 0.05) when given prior to occlusion of the LAD. NBTI was ineffective however when given 15 min prior to reperfusion. NBTI had no effect upon infarct size produced by ischaemia without reperfusion. The effect of NBTI was independent of significant changes in myocardial blood flow during ischaemia and reperfusion or upon neutrophil infiltration.

Cardioplegia Solution

The mechanisms of the metabolic and ultrastructural changes that occur as a result of myocardial ischemia during cross-clamping of the aorta and the secondary injury that can occur during reperfusion after removal of the cross-clamp are important determinants of the composition and method of administration of cardioplegia solution (CS). Traditionally, basic principles of myocardial protection included hypothermia, potassium-induced diastolic cardiac arrest, buffering, membrane stabilization, and control of osmolarity and osmotic pressure. As the mechanisms of myocardial ischemia and reperfusion are further shown, methods of providing myocardial protection continue to expand. Current trends favor blood cardioplegia administration via a more complex approach to protect as much of the myocardium as possible; exogenous metabolic substrate enhancement in the presence of oxygen to allow aerobic metabolism to continue, with an end result of additional available myocardial energy; and warm (37°C) blood CS to optimize the metabolic rate for cellular repair. Thus, today the cardiothoracic surgeon can offer patients with energy-depleted hearts and those requiring more complex surgery improved myocardial protection with active myocardial resuscitation before and after the aortic cross-clamp is placed and removed. To this end, the pharmacist, ideally an operating room (OR) pharmacist, has a vital role in ensuring proper preparation, composition, storage, and quality control/assurance of CS. The basic principles of myocardial ischemic and reperfusion injury and how they relate to myocardial protection, CS composition, and methods of administration are described. By understanding these principles, in addition to surgeon and institution-specific factors, the pharmacist can build the foundation needed to maximize the role of pharmacy in ensuring optimal myocardial protection during open-heart surgery.

Is post cardiopulmonary bypass dysfunction a special form of stunning?

It has been suggested that cardioplegic arrest during cardiopulmonary bypass (CPB) produces global myocardial ischemia with a risk of myocardial stunning. It has also been postulated that anesthetic technique may affect the course of post-CPB myocardial stunning via exaggerated myocardial depression. However, we have previously found that global ventricular and regional myocardial responses to halothane do not differ in post-CPB and pre-CPB dogs. Our examination of the effects of CPB on the beta-adrenergic function revealed that beta-adrenergic receptor function is only slightly decreased immediately following (i.e., 1 min) and 30 minutes post-CPB. A dose-response relationship was established for dobutamine, with decreased responsiveness noted at both times. Since other data show normal inotropic stimulation of stunned myocardium, decreases in dobutamine responsiveness cannot be explained by beta-receptor desensitization. Overall, these data indicate that CPB does not result in myocardial stunning. The differences between these data and others showing myocardial stunning following CPB may be due to several factors, such as anesthetic regimen, lack of coronary blood flow abnormalities, and a reduction in sarcoplasmic reticular damage due to the hypothermic conditions used.

Simulated ischemia does not protect against efferent sympathetic denervation following acute myocardial infarction in canine hearts

INTRODUCTION

Preconditioning the myocardium with brief episodes of ischemia preserves efferent autonomic responsiveness of noninfarcted myocardium apical to a site of acute transmural ischemia by mechanism(s) still unknown. We hypothesized that repeated brief exposure of the myocardium to a simulated ischemic milieu including hypoxia, high K+, low pH, and adenosine would be as effective as brief coronary occlusions in creating this protection.

METHODS AND RESULTS

Open chest anesthetized dogs received an extracorporeal bypass between the left carotid artery and a diagonal branch of the left anterior descending coronary artery. We analyzed the effects of simulated ischemia on the time course and extent of efferent sympathetic denervation during a subsequent 3-hour sustained ischemia in three groups of dogs: two groups of dogs underwent four cycles of 5-minute intracoronary perfusion with either hypoxic altered Tyrode's solution (12 mM K+, 6.8 pH, and 10 microM adenosine; n = 11) or normal Tyrode's solution (n = 11). Each Tyrode's perfusion was separated by 5 minutes of blood perfusion prior to permanent coronary occlusion by latex embolization of the cannulated coronary artery. A third group received a continuous 3-hour blood perfusion before the final ischemic episode (n = 5). Shortening of effective refractory periods (ERPs) induced by bilateral ansae subclaviae stimulation (2 to 4 Hz) basal and apical to the intervention site was determined before and after perfusions and 20, 60, 120, and 180 minutes after sustained occlusion. In all groups, sympathetically-induced ERP shortening was unchanged at basal sites throughout the experiment. ERP shortening at apical sites was unchanged after perfusions with either the altered or normal Tyrode's solution or after a continuous 3-hour blood perfusion. However, ERP shortening became significantly attenuated at apical sites after coronary occlusion in all groups. Neither the size in reduction of sympathetically-induced ERP shortening at apical test sites nor the cumulative percentage of denervated apical test sites (< or = 2-msec shortening) during a 3-hour period of permanent ischemia differed significantly among groups (P = 0.052 and P = 0.752, respectively). The degree of subepicardial involvement in the myocardial infarction was comparable among groups.

CONCLUSION

Thus, brief exposure of the left ventricular myocardium to ischemic metabolites prior to a subsequent permanent coronary occlusion does not trigger mechanism(s) that are responsible for protection against efferent sympathetic denervation apical to an area of transmural myocardial infarction/ischemia.

The oxidative pentose phosphate pathway in the heart: regulation, physiological significance, and clinical implications

The capacity of the oxidative pentose phosphate pathway (PPP) in the heart is small, since the activity of glucose-6-phosphate dehydrogenase (G-6-PD), the first and rate-limiting enzyme, is very low. Basically, two mechanisms are involved in the regulation of this pathway. Under normal conditions, G-6-PD is inhibited by NADPH. This can immediately be overcome in the isolated perfused rat heart by increasing the oxidized glutathione and by elevating the NADP+/NADPH ratio. Apart from this rapid control mechanism, there exists a long-term regulation which involves the synthesis of G-6-PD. All catecholamines that were administered stimulated the activity of myocardial G-6-PD in a time- and dose-dependent manner. This stimulation was due to increased new synthesis of enzyme protein, since the G-6-PDmRNA was specifically enhanced. As a consequence of the stimulation of the oxidative PPP, the available pool of 5-phosphoribosyl-1-pyrophosphate (PRPP) was elevated which serves as an important precursor substrate for purine and pyrimidine nucleotide synthesis. The limiting step in the oxidative PPP can be bypassed by ribose which leads to an elevation of the cardiac PRPP pool. The decline in the ATP that is induced in many pathophysiological conditions can be attenuated or even entirely prevented by i.v. infusion of ribose. In some experimental in vivo rat models such as in the overloaded and catecholamine-stimulated heart and in the non-ischemic region of the infarcted heart, the normalization of the metabolic situation was accompanied by an improvement of global heart function. Ribose application has been shown to be beneficial in several clinical disease states such as myoadenylate deaminase deficiency and McArdle's disease. Moreover, ribose facilitated thallium-201 redistribution and markedly improved the detection of reversible ischemic injury of the pig and human heart.

Protective effects of adenosine in myocardial ischemia

Adenosine is released from the myocardium in response to a decrease in the oxygen supply/demand ratio, as is seen in myocardial ischemia; its protective role is manifested by coronary and collateral vessel vasodilation that increase oxygen supply and by multiple effects that act in concert to decrease myocardial oxygen demand (i.e., negative inotropism, chronotropism, and dromotropism). During periods of oxygen deprivation, adenosine enhances energy production via increased glycolytic flux and can act as a substrate for purine salvage to restore cellular energy charge during reperfusion. Adenosine limits the degree of vascular injury during ischemia and reperfusion by inhibition of oxygen radical release from activated neutrophils, thereby preventing endothelial cell damage, and by inhibition of platelet aggregation. These effects help to preserve endothelial cell function and microvascular perfusion. Long-term exposure to adenosine may also induce coronary angiogenesis.

Maintenance of left ventricular function (90%) after twenty-four-hour heart preservation with deferoxamine

During 24-h in vitro heart preservation and reperfusion, irreversible tissue damage occurs caused by reactive oxygen intermediates, such as superoxide radicals, singlet oxygen, hydrogen peroxide, hydroperoxyl, hydroxyl radicals, as well as the peroxynitrite radical. Reduction of the related oxidative damage of reperfused ischemic tissue by free radical scavengers and metal chelators is of primary importance in maintaining heart function. We assessed whether deferoxamine (DFR) added to a cardioplegia solution decreased free radical formation during 24-h cold (5 degrees C) heart preservation and normothermic reperfusion (37 degrees C) in the Langendorff isolated perfused rat heart. The deferoxamine treated hearts were significantly (p less than .001) better preserved than the control hearts after 24 h of preservation with regard to recovery of left ventricular diastolic pressure, contractility (+dP/dt), relaxation (-dP/dt), creatine kinase release, and lipid peroxidation. DFR preserved cell membrane integrity and maintained 93% of left ventricular contractility. The evidence suggests that DFR reduces lipid peroxidation damage by reducing free radical formation and thereby maintaining normal coronary perfusion flow and myocardial function.

Desensitization of myocardial beta-adrenergic receptors during cardiopulmonary bypass. Evidence for early uncoupling and late downregulation

BACKGROUND

Cardiopulmonary bypass (CPB), a process routinely used during cardiac surgery, is a potent stimulant to the release of endogenous catecholamines. Hence, we tested the hypothesis that CPB results in myocardial beta-adrenergic receptor (beta AR) desensitization.

METHODS AND RESULTS

We obtained canine transmyocardial left ventricular biopsies before, during (155 minutes), and after CPB (pre-CPB, CPB, and post-CPB, respectively) and determined beta AR density, proportion of beta 1AR to beta 2AR, and beta AR coupling capacity to adenylyl cyclase. Beta AR density was stable at 112 +/- 14 fmol/mg (pre-CPB) and 103 +/- 9 fmol/mg (CPB) but decreased post-CPB to 84 +/- 7 fmol/mg. The ratio of beta 1AR to beta 2AR (determined by two-site fit for [125I]-iodocyanopindolol competition binding with the beta 1AR selective antagonist ICI89.406) remained constant throughout (60 +/- 3: 40 +/- 3 pre-CPB, 55 +/- 3: 44 +/- 3 CPB, and 61 +/- 2: 39 +/- 2 post-CPB), revealing that both beta 1AR and beta 2AR subtypes were downregulated. A different pattern was noted in the functional properties of these receptors during CPB. Decreased maximal isoproterenol-stimulated adenylyl cyclase activity (252 +/- 14 to 216 +/- 12 pmol/30 min/mg), submaximal isoproterenol-stimulated adenylyl cyclase activity (183 +/- 10 to 157 +/- 11 pmol/30 min/mg), and zinterol-stimulated adenylyl cyclase activity (187 +/- 11 to 159 +/- 11 pmol/30 min/mg, a measure of beta 2AR subtype activation) were noted during CPB, at the time when weaning from CPB takes place. However, this desensitized pattern was found to be completely reversed by 30 minutes post-CPB, with adenylyl cyclase activities returning to pre-CPB levels or slightly higher. Control dogs that did not receive CPB showed no change in beta AR density or adenylyl cyclase activity.

CONCLUSIONS

These data suggest that myocardial beta AR desensitization does occur during CPB in healthy, nonischemic canine myocardium and that this pattern is reversed 30 minutes after discontinuation of CPB. In addition, a slower process of beta AR downregulation persists after discontinuation of CPB. Because successful weaning from CPB is a critical process during myocardial surgery, these findings have potentially important implications in the management of such patients.

Effects of adenosine infusion on the pig heart during normothermic ischemia and reperfusion

Possible enhancement of myocardial protection during ischemia and reperfusion by administration of adenosine was evaluated in a pig heart model. Adenosine (100 micrograms/kg/min) was infused into the aortic root during ischemia in group AI (n = 5) and into the right atrium during reperfusion in group AR (n = 6). Group C (n = 6) served as controls. During cardiopulmonary bypass the hearts were subjected to 30 min of normothermic ischemia and 15 min of reperfusion before weaning. In group AI the stroke work index 30 and 90 min after ischemia and the mean arterial pressure 30 min after ischemia were significantly higher than in group C. These parameters did not differ significantly between groups AR and C. All groups showed decrease in myocardial adenosine triphosphate (ATP) and adenylate charge potential (ACP) during ischemia and partial (ATP) or complete (ACP) restoration after ischemia. Adenosine infusion into the aortic root during ischemia (adenosine cardioplegia) thus resulted in improved postischemic heart function, although biochemical correlates in ATP and ACP were not apparent.

Combined adenosine and lidocaine administration limits myocardial reperfusion injury

The endogenous compound adenosine may play a role in limiting myocardial ischemia-reperfusion injury through its ability to cause vasodilation, modulate cardiac adrenergic responses, inhibit neutrophil function, or modulate energy supply and demand of the myocardium. The local anesthetic lidocaine has been shown to be protective against myocardial ischemia-reperfusion injury, although its mechanism of action remains unresolved. We hypothesized that administration of exogenous adenosine during reperfusion would limit the size of the infarct that results from a period of ischemia and reperfusion only when the animals are treated with lidocaine. Male, mongrel dogs (13.0-20.0 kg) were anesthetized (30 mg/kg i.v. sodium pentobarbital), and a left thoracotomy was performed. The left circumflex coronary artery (LCx) was isolated and instrumented with an electromagnetic flow probe, a 25-gauge nonobstructing intracoronary catheter, and a critical stenosis. The dogs were allocated randomly to one of four groups: 1) control, n = 13, (saline), 2) adenosine, n = 13, (0.15 mg/kg/ml/min i.c. for the first hour of reperfusion), 3) lidocaine, n = 9, (2.0 mg/kg i.v. given immediately before coronary artery occlusion and just before reperfusion), or 4) adenosine plus lidocaine, n = 11. The LCx was occluded for 90 minutes and reperfused for 6 hours. Regional myocardial blood flow (RMBF) was determined (n = 6 per group) at 80 minutes of occlusion and at 45 minutes of reperfusion with radiolabeled microspheres. RMBF determinations revealed an increase in blood flow to the inner two thirds of the myocardium at 45 minutes of reperfusion only in the presence of the combined treatment. Adenosine treatment alone or lidocaine treatment alone did not affect RMBF. Quantification of infarct size (triphenyltetrazolium method) expressed as a percent of the area at risk revealed a significant limitation of infarct size only in the group treated with both adenosine and lidocaine: control, 47.8 +/- 6.6%; adenosine, 45.0 +/- 3.2%; lidocaine, 46.9 +/- 6.0%; and adenosine and lidocaine, 20.8 +/- 5.6%. Statistical analyses were performed with two-way analysis of variance to account for the two individual drug treatments. The findings show that intracoronary administration of exogenous adenosine, at the dose used, is only effective at limiting myocardial infarct size when administered to lidocaine-treated animals.