Beta-adrenergic stimulation induces ST-segment elevation in dogs with healing myocardial infarction

There is controversy with regard to the mechanism of the exercise-induced ST-segment elevation in myocardial infarction. The purpose of the present study was to investigate the mechanism of ST-segment elevation through pharmacologic interventions. Transmural anterior myocardial infarction was produced by gelatin sponge embolization of the left anterior descending artery in seven closed-chest dogs. One and four weeks after myocardial infarction, the dogs underwent the following three interventions: right atrial pacing, norepinephrine infusion (3.75, 7.5, and 15 micrograms/min) with the pacing, and methoxamine injection (2.5 and 5.0 mg) with the pacing. All dogs had transmural infarction with a mean infarct size of 12.0 +/- 4.2% of the left ventricular weight. Right atrial pacing did not induce significant changes in ST-segment. Norepinephrine induced a marked elevation of ST-segment at leads V1 to V4, while methoxamine did not. Norepinephrine induced a significant increase in left ventricular ejection fraction, while methoxamine produced a marked decrease in the ejection fraction and an increase in ventricular volume. The mean percent radial shortening of the non-infarct ventricular wall showed a significant increase with norepinephrine, but a decrease with methoxamine. In conclusion, myocardial ischemia and wall motion abnormality may be excluded as possible mechanisms of ST-segment elevation and an enhanced beta-adrenergic mechanism in the non-infarct myocardium is suggested to be responsible for ST-segment elevation.