V. Richard

Ischemic preconditioning slows energy metabolism and delays ultrastructural damage during a sustained ischemic episode.

We have shown previously that preconditioning myocardium with four 5-minute episodes of ischemia and reperfusion dramatically limited the size of infarcts caused by a subsequent 40-minute episode of sustained ischemia. The current study was undertaken to assess whether the same preconditioning protocol slowed the loss of high energy phosphates, limited catabolite accumulation, and/or delayed ultrastructural damage during a sustained ischemic episode. Myocardial metabolites and ultrastructure in the severely ischemic subendocardial regions were compared between control and preconditioned canine hearts. Hearts (four to 10 per group) were excised after 0, 5, 10, 20, or 40 minutes of sustained ischemia. All groups had comparable collateral blood flow. Preconditioned hearts developed ultrastructural injury more slowly than controls; evidence of irreversible injury was observed after 20 minutes in controls but not until 40 minutes in preconditioned hearts. Furthermore, after 40 minutes of ischemia, irreversible injury was homogeneous in controls but only focal in preconditioned myocardium. Preconditioning reduced starting levels of ATP by 29%. Nevertheless, it also slowed the rate of ATP depletion during the episode of sustained ischemia, so that after 10 minutes of ischemia, preconditioned hearts had more ATP than controls. However, after 40 minutes, ATP contents were not significantly different between groups. Preservation of ATP resulted from reduced ATP utilization and was not due to increased ATP production. Accumulation of purine nucleosides and bases (products of adenine nucleotide degradation) was limited in preconditioned myocardium. Accumulation of glucose-1-phosphate, glucose-6-phosphate, and lactate also was reduced markedly by preconditioning, due to reduced rates of glycogen breakdown and and anaerobic glycolysis. We propose that preconditioning reduces myocardial energy demand during ischemia, which results in a reduced rate of high energy phosphate utilization and a reduced rate of anaerobic glycolysis. Either preservation of ATP or reduction of the cellular load of catabolites may be responsible for delaying ischemic cell death.

Threshold concentrations of endothelin-1 potentiate contractions to norepinephrine and serotonin in human arteries. A new mechanism of vasospasm?

Endothelin-1 is an endothelium-derived vasoconstrictor peptide. Its circulating levels are below those known to evoke direct vascular effects. To study whether low concentrations of endothelin-1 potentiate the effects of other vasoconstrictor hormones, we suspended isolated human internal mammary and left anterior descending coronary artery rings in organ chambers for isometric tension recording. In mammary artery rings, the contractions to norepinephrine (3 x 10(-8) M) were potentiated by threshold (3 x 10(-10) M) and low concentrations (10(-9) M) of endothelin-1 (96 +/- 35% and 149 +/- 58% increase from control; p less than 0.01 and 0.001; n = 6). The inhibitor of endothelial nitric oxide formation L-NG-monomethyl arginine did not affect the potentiating effects of the peptide. The calcium antagonist darodipine (10(-7) M) prevented the potentiation of the response to norepinephrine evoked by endothelin-1. Similarly, contractions to serotonin (10(-7) or 3 x 10(-8) M) were amplified by endothelin-1 (3 x 10(-10) M) in the mammary (30 +/- 9%) and in the coronary arteries (59 +/- 25%). Endothelin-1 (10(-9) M) further potentiated the response (57 +/- 23% in mammary and 87 +/- 26% in coronary arteries; p less than 0.05; n = 7 and 3). The sensitivity of mammary arteries to calcium chloride was markedly enhanced in the presence of endothelin-1 (3 x 10(-10) M; concentration shift, eightfold; p less than 0.01; n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelial control of vascular tone in large and small coronary arteries.

The endothelium modulates coronary vascular tone by the release of endothelium-derived relaxing or contracting substances. The endothelium-derived relaxing factor has been identified as nitric oxide synthesized in endothelial cells from L-arginine. The endothelium can release other relaxing substances such as prostacyclin and a hyperpolarizing factor. Endothelin-1 is a potent vasoconstrictor peptide formed by endothelial cells, and is likely to be the physiologic antagonist of endothelium-derived relaxing factor. Other putative contracting factors include superoxide anions and products of arachidonic acid metabolism. Endothelium-derived relaxing factor is released spontaneously and in response to flow, platelet-derived products (that is, serotonin, thrombin and adenosine diphosphate) and certain autacoids (that is, acetylcholine, bradykinin, histamine, substance P, vasopressin, alpha-adrenergic agonists). A considerable heterogeneity of responses exists among vessels of different size from different anatomic origin and different species. Hypercholesterolemia, atherosclerosis, hypertension and myocardial ischemia or reperfusion, or both, impair endothelium-dependent relaxation. Under normal conditions, endothelium-derived relaxing factor appears to dominate the control of vascular tone of large and small coronary vessels, whereas in disease states, endothelium-derived contracting factors are released. Impairments of endothelial function may be important in the development of various forms of cardiovascular disease.

Healing of Myocardial Infarcts in Dogs Effects of Late Reperfusion

BACKGROUND Early reperfusion salvages ischemic myocardium and limits myocardial infarct size. However, the effects of late reperfusion, after the possibility for limitation of infarct size has passed, have not been completely elucidated. The purpose of this study was to ascertain the effect of reperfusion after 6 hours of ischemia on the rate of infarct healing and on the size and geometry of the resulting scars, as determined by gross and microscopic quantification. METHODS AND RESULTS Myocardial infarcts were produced in anesthetized, open-chest dogs by occlusion of the circumflex coronary artery. They either were reperfused by removal of the occluding snare or were nonreperfused. The animals were allowed to recover for either 4 days, 2 weeks, or 6 weeks. At these times, infarct size, infarct dimensions (wall thickness and circumferential extent), and the proportion of infarct occupied by necrotic myocardium versus granulation tissue (evolving scar) were measured. At 4 days, infarcts were swollen in both nonreperfused and reperfused groups (increased thickness and circumferential extent of the area at risk). Conversely, at 6 weeks, the size, thickness, and circumferential extent of the scar all were decreased. Two common anatomic complications of human infarction, cardiac rupture and chronic infarct expansion (aneurysm), did not occur in this experimental model. Reperfusion at 6 hours did not affect initial infarct size (4 days) or scar size (6 weeks). At 2 weeks, reperfused infarcts were smaller and were composed of proportionately more granulation tissue and less nonresorbed necrosis than nonreperfused infarcts. CONCLUSIONS Thus, reperfusion accelerated the rate of infarct repair, ie, the replacement of necrotic myocardium by scar. Acceleration of infarct repair may be a beneficial effect of late reperfusion even after the opportunity for limitation of infarct size has passed.

Electrophysiological Effects of Monophasic and Biphasic Stimuli in Normal and Infarcted Dogs

Though some biphasic waveforms significantly decrease the energy required for defibrillation, little is known about the effect of biphasic stimulation on the determination of other electro‐physiological parameters in normal and infarcted hearts. To evaluate this, nine normal dogs and 12 dogs with myocardial infarction had activation threshold (AT), effective refractory period (EEP), strength‐interval curves, and ventricular fibrillation threshold (VFT) determined with constant current stimulation to a pair of right ventricular plunge electrodes, and upper limit of vulnerability (ULV) and defibrillation threshold (DFT) determined with truncated exponential shocks delivered to a pair of wire electrodes coiled to contour the right and left ventricular epicurdium. Each electrophysiological parameter was determined with a 5.5 msec monophasic and 5.5‐msec biphasic (3.5 msec first phase) waveform. Though AT and VFT were not significantly different for the two waveforms, the EttP was significantly longer, the strength‐interval curve shifted rightward, and the threshold for repetitive responses higher for biphasic stimuli. Compared to the monophasic waveform, the ULV and DFT were significantly decreased in a parallel fashion for the biphasic waveform. Neither the presence nor size of myocardial infarction significantly affected any of the measured electrophysiological parameters. In six additional dogs, sigmoid defibrillation probability curves were constructed from biphasic shocks of four energies including that of the DFT and ULV. The ULV energy predicted an effective dose that defibrillated 97% of the time (range 90%–100%). In conclusion, the increased defibrillation efficacy of the biphasic waveform is independent of its ability to activate fully repolarized myocardium and cannot be explained by a greater ability of biphasic waveforms to activate partially depolarized tissue. The parallel decrease in the ULV and DFT for biphasic stimulation and the finding that the ULV energy defibrillates with a high probability of success suggest similar underlying mechanisms for the ULV and defibrillation.

Interaction between endothelium-derived nitric oxide and SIN-1 in human and porcine blood vessels.

SUMMARY Nitric oxide (NO) is a potent vasodilator and inhibitor of platelet function that is produced from L-arginine in endothelial cells. The mechanism of action of nitrovasodilators such as SIN-1 has striking similarities with endothelium-derived nitric oxide. We studied the effects and interactions of endothelium-derived relaxing factor with SIN-1 in isolated human internal mammary arteries, saphenous veins, and porcine coronary arteries. In human arteries, SIN-1 induced potent relaxations (IC50 value of 6.6 ±0.1; maximal response of 100%) that were comparable to that induced by acetylcholine and were augmented by removal of the endothelium (concentration shift of 3.2-fold; n = 8 and 6, respectively, p < 0.05). The relaxation to SIN-1 was also enhanced in saphenous veins as compared to mammary arteries (concentration shift of 6.3-fold; n = 7 and 6, respectively, p < 0.005). In human and porcine arteries, incubation with the false precursor substance of endothelium-derived NO, L-NG-monomethylarginine (10−5 and 10−4 M), enhanced the relaxation induced by SIN-1 (concentration shift of 3.2-and 2.5-fold, respectively; n = 4 to 6; p < 0.05). Thus, SIN-1 is a potent vasodilator of human and porcine blood vessels. Its effects are attenuated by spontaneously released endothelium-derived NO.

The xanthine oxidase inhibitor oxypurinol does not limit infarct size in a canine model of 40 minutes of ischemia with reperfusion

Free radicals such as superoxide (.O2-) produced by xanthine oxidase might cause cell death during reperfusion after myocardial ischemia. The effect of the xanthine oxidase inhibitor allopurinol on infarct size in ischemia-reperfusion models has been variable, possibly because of differences in treatment duration. Adequate inhibition of xanthine oxidase may require a sufficient pretreatment period to permit conversion of allopurinol to oxypurinol, the actual inhibitor of superoxide production. To test more definitively whether xanthine oxidase-derived free radicals cause cell death during reperfusion, the effect of oxypurinol on infarct size was evaluated in an ischemia-reperfusion model. Open chest dogs underwent 40 min of circumflex coronary artery occlusion followed by reperfusion for 4 days. Twelve dogs were treated with oxypurinol (10 mg/kg body weight intravenously 10 min before occlusion and 10 mg/kg intravenously 10 min before reperfusion) and 11 control dogs received drug vehicle alone (pH 10 normal saline solution). Nine control dogs from a concurrent study also were included. Infarct size was measured histologically and analyzed with respect to its major baseline predictors, including anatomic area at risk and collateral blood flow (measured with radioactive microspheres). Infarct size as a percent of the area at risk averaged 23.8 +/- 2.7% (mean +/- SEM) in the oxypurinol group (n = 10) and 23.1 +/- 4.2% in the control group (n = 17) (p = NS). Collateral blood flow to the inner two thirds of the ischemic wall averaged 0.08 +/- 0.01 ml/min per g in the oxypurinol group and 0.09 +/- 0.02 ml/min per g in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelium-derived vasoactive factors and their role in the coronary circulation.

The endothelium-due to its strategic anatomic position between the circulating blood and vascular smooth muscle-plays an important functional role in the coronary circulation. Endothelial cells release factors interfering with coagulation, platelet function, vascular tone, and growth. Endothelium-derived nitric oxide (NO) is the endogenous nitrovasodilator that is a potent inhibitor of platelet function and vasodilator. Together with prostacyclin, NO plays an important protective role in preventing platelet adhesion, aggregation, and coronary vasospasm. Endothelial cells also are a source of contracting factors such as endothelin-1, thromboxane A(2), and endoperoxides. Cardiovascular risk factors (hyperlipidemia, hypertension, and diabetes) inhibit the formation of endothelium-derived relaxing factors and promote that of contracting factors. In coronary arteries with advanced atherosclerosis, endothelial function is severely impaired. The reduced release of endothelium-derived NO is associated with an increased platelet-vessel-wall interaction and, in turn, platelet activation and vasospasm, both known events in coronary artery disease.

Serotonin, the Endothelium and the Coronary Circulation

Serotonin releases endothelium-derived relaxing factors and also causes contraction of vascular smooth muscle cells. The latter may be important at sites of damaged endothelium and promote thrombus formation and vasospasm. In epicardial porcine coronary arteries, endothelium-dependent relaxations to serotonin are impaired by the inhibitor of nitric oxide formation L-NG-monomethyl arginine (L-NMMA). Since pertussis toxin reduces endothelium-dependent relaxations to serotonin, a pertussis toxin sensitive G protein is involved. In intramyocardial porcine coronary arteries, L-NMMA is a weak inhibitor of the endothelium-dependent relaxation to serotonin and pertussis toxin has no effect. Thus, the importance of nitric oxide as well as that of pertussis toxin sensitive G protein decreases from large to small porcine coronary arteries.