Myocardial stunning and preconditioning: age, species, and model related differences: role of AMP-5'-nucleotidase in myocardial injury and protection

Oxidative stress enhances the production and actions of adenosine in the kidney

The purpose of this study was to determine whether superoxide anions (O.) activate 5'-nucleotidase (5'-ND), thereby increasing the production of renal adenosine and regulating renal function. Using HPLC analysis, we found that incubation of renal tissue homogenate with the O. donor KO(2) doubled adenosine production and increased the maximal reaction velocity of 5'-ND from 141 to 192 nmol. min(-1). mg protein(-1). The O.-generating system, xanthine/xanthine oxidase increased the maximal reaction velocity of 5'-ND from 122 to 204 nmol. min(-1). mg protein(-1). Superoxide dismutase (SOD) with catalase produced a concentration-dependent reduction of 5'-ND activity in renal tissue homogenate, while the SOD inhibitor diethyldithiocarbamic acid significantly increased 5'-ND activity. Inhibition of disulfide bond formation by thioredoxin or thioredoxin reductase significantly decreased xanthine/xanthine oxidase-induced activation of renal 5'-ND. In in vivo experiments, inhibition of SOD by diethyldithiocarbamic acid (0.5 mg. kg(-1). min(-1) iv) enhanced renal vasoconstriction induced by endogenously produced adenosine and increased renal tissue adenosine concentrations under control condition and in ischemia and reperfusion. We conclude that oxidative stress activates 5'-ND and increases adenosine production in the kidney and that this redox regulatory mechanism of adenosine production is important in the control of renal vascular tone and glomerular perfusion.

Dynamic progression of contractile and endothelial dysfunction and infarct extension in the late phase of reperfusion

BACKGROUND

Myocardial injury during early reperfusion (R) has been well documented. However, the extent and time course of myocardial injury during late R are still unclear. The purpose of this study was to determine the extent of regional contractile and endothelial dysfunction and myocardial blood flow (MBF) defect as well as extension of infarction in association with neutrophil (PMN) actions during R.

MATERIALS AND METHODS

A total of 29 dogs underwent a protocol of 1 h LAD ischemia followed by 6, 24, 48, and 72 h of R, respectively. Regional contractile function (sonomicrometry), MBF (colored microspheres), infarct size (triphenyltetrazolium chloride staining), and PMN localization (immunohistochemistry) were determined.

RESULTS

Percentage segmental shortening at 6, 24, 48, and 72 h of R was significantly blunted (-1.8 +/- 1.2,* - 0.37 +/- 0. 6,* 0.04 +/- 0.2,* and 5.9 +/- 1.2* vs baseline 17.7 +/- 0.8). MBF (ml/min/g) was attenuated at 24 (0.27 +/- 0.03*), 48 (0.46 +/- 0. 07*), and 72 h of R (0.48 +/- 0.06*) vs 6 h of R (0.65 +/- 0.06). Infarct size increased from 6 (27 +/- 2%) to 24 h of R (41 +/- 2%*) with no further increase at 48 and 72 h of R, consistent with a peak of creatine kinase activity. PMN adherence (mm(2) endothelium) to left anterior descending coronary artery (LAD) segments was increased after 6 h of R (63 +/- 3*) vs nonischemic left circumflex coronary artery (LCX) segments (42 +/- 2) with a peak at 48 h of R (111 +/- 5*). Endothelium-dependent vascular relaxation in the LAD was also blunted at 6, 24, and 48 h of R. Immunostaining revealed CD18-positive PMNs were mainly accumulated in intravascular space during 6 h of R with an increase in migration of PMNs seen at 24 h of R, consistent with a peak of myeloperoxidase release. Myeloperoxidase activity in a given area at risk sample was significantly correlated with infarct extension during the first 24 h of R.

CONCLUSIONS

These results provide pathologic evidence for myocardial injury during the extended R and a basis for exploration of interventions designed to limit myocardial injury after ischemia. (*P < 0.05 vs Baseline, 6 h of R and LCX segments.)

The risk of myocardial stunning is decreased concentration-dependently by KATP channel activation with nicorandil before high K+ cardioplegia

BACKGROUND

Drug-induced opening of the adenosine triphosphate-sensitive potassium channel (KATP) during hypoxia and/or ischemia, achieved significant myocardial protection in several in vitro and in vivo models. Pretreatment with KATP openers simulated preconditioning and thus enhanced recovery from ischemia. We have demonstrated that the risk of hypoxia-induced myocardial stunning is reversed by KATP activation with 1 mmol/l nicorandil before cold cardioplegic arrest. Whether lower concentrations were effective is not known.

METHODS

In guinea pig papillary muscle preparations contracting isometrically (driven at 1600 ms cycle), nicorandil was superfused (15 min) either 1 mumol/l (n = 4), 30 mumol/l (n = 4), 100 mumol/l (n = 4), or 1 mmol/l (n = 8) in Tyrode's solution (oxygen content 16 ml/l, 37 degrees C, 5 ml/min). Controls were superfused with saline (Tyrode's solution: n = 8). A group containing vehicle (DMSO 1%, n = 8) was also studied. In four preparations the KATP channel blocker glibenclamide 1 mumol/l was given before nicorandil 1 mmol/l. Then, long-lasting (120 min) but moderately hypoxic (oxygen content 5 ml/l: 31% of Tyrode's solution) superfusion with hypothermic (20 degrees C) high K+ (16 mmol/l) cardioplegic solution (5 ml/min) was performed. Recovery of contractility was evaluated after further 60 min of reoxygenation with Tyrode's solution based on DT/TPT (developed tension divided by time to peak tension) as percent of prehypoxia basal values (%DT/TPT60). DT/TPT was also studied following 15 min of inotropic stimulation with dobutamine 10 mumol/l (%DT/TPT75). To assess the risk of stunning, we used a multivariate linear model by all possible subsets analysis (BMDP-9R) aimed at predicting both %DT/TPT60 and %DT/TPT75 (as continuous dependent variables).

RESULTS

During cardioplegia induction, time to arrest (TTA) was (mean +/- S.D.) 103 +/- 48s in control preparations which had poor recovery of contractility (stunning) after reoxygenation (%DT/TPT60: 71 +/- 20%; %DT/TPT75: 443 +/- 272%). Nicorandil (1 mumol/l-1 mmol/l) abbreviated TTA concentration-dependently (163 +/- 74, 149 +/- 103, 82 +/- 20, and 56 +/- 27s) and improved both %DT/TPT60 (63 +/- 9, 78 +/- 17, 87 +/- 13, and 98 +/- 11%) and %DT/TPT75 (587 +/- 333, 619 +/- 107, 971 +/- 301, and 666 +/- 400%). Glibenclamide reversed the effects of nicorandil 1 mmol/l (TTA: 165 +/- 30 s, P < 0.01; %DT/TPT60: 43 +/- 12, P < 0.01; %DT/TPT75: 272 +/- 147, P < 0.05). Multivariate prediction of myocardial stunning at both 60 and 75 min reoxygenation showed that nicorandil (30 mumol/l-1 mmol/l) was a significant (P < 0.001) protectant whereas glibenclamide was a significant risk factor (P = 0.009). It is unclear whether negative inotropic effects of nicorandil (%DT/TPT at the end of pretreatment) was mechanistically related to reduced risk of stunning since contribution was seen only to predict %DT/TPT75 (t = 3.24, P = 0.003) whereas a positive association was observed with %DT/TPT60 (t = 1.89, P = 0.068).

CONCLUSION

Pretreatment with nicorandil concentration-dependently enhanced the cardioprotective effect of hypothermic high K+ cardioplegia. The risk of myocardial stunning was decreased by KATP opening with nicorandil and increased by KATP block with glibenclamide. Inotropic stimulation with dobutamine might unravel the role of negative inotropic effect of KATP opening as a contributory factor to explain the efficacy of nicorandil in our model.

The phosphocreatine overshoot occurs independent of myocardial work

Although the exact mechanism(s) responsible for the phosphocreatine/ATP overshoot have not been completely elucidated, our data demonstrate that the overshoot does not stem from reduced myocardial work, and consequently, reduced utilization of phosphocreatine (PCr). Additionally, we highlight a basic difference in the physiologic responses of skeletal and cardial muscle to work demands. By understanding the bioenergetic derangements which accompany reperfusion injury, one may hope to better salvage post-ischemic myocardium.

The enigma of myocardial preconditioning models

Myocardial preconditioning has been reported in the hearts of many species of animals, including dogs, pigs, rabbits, rats, and anecdotally in humans. However, most studies were carried out in the regional ischemic model, although protection against global ischemic injury had been observed in rat models. Besides biochemical endpoints, the criterion of protection in regional ischemia was usually reduction in infarct size, while in global ischemia, recovery of contractile force and time-to-onset of ischemic contracture were used. We attempted to reproduce preconditioning of myocardium against global ischemic injury using an isolated perfused rabbit heart model with the rationale that global ischemia is more relevant to cardiac surgery, the rabbit model is logistically convenient, and it can be used for future comparison with the responses in immature hearts. The preconditioning was induced with 5 minutes of normothermic global ischemia followed by 10 minutes of reperfusion. The principal ischemic injury lasted 35 minutes, followed by 60 minutes of reperfusion. The control group underwent similar principal ischemic injury and reperfusion but no prior preconditioning ischemia. Results showed that there was no difference between the two groups in left ventricular resting tension, recovery in left ventricular developed pressure, contractility (dP/dt), and rate of relaxation (-dP/dt), nor were there any differences in heart rate and coronary flow rate. The reason for our negative findings is not clear, but if the results are confirmed, it will suggest that extrapolation of observations obtained from one experimental model to another should be made with caution.