ALPHA1-ADRENOCEPTOR REGULATION OF DELAYED AFTERDEPOLARIZATIONS AND TRIGGERED ACTIVITY IN SUBENDOCARDIAL PURKINJE-FIBERS SURVIVING 1 DAY OF MYOCARDIAL-INFARCTION

Alpha1-adrenoceptor agonists were shown to induce delayed afterdepolarizations (DADs) and triggered activity in the presence of elevated extracellular Ca2+. We investigated the effects of alpha1-adrenoceptor stimulation on DADs and triggered activity in canine Purkinje fibers that survived 1-day of myocardial infarction. Endocardial preparations were studied using standard microelectrode techniques. In quiescent preparations showing no DADs and in presence of propranolol (2 × 10-7m), phenylephrine (10-6m), an alpha1-adrenoceptor agonist induced DADs (n = 6) and differentially induced triggered activity in ischemic but not in normal Purkinje fibers (n = 4). In 8 preparations that showed subthreshold DADs, phenylephrine increased the DAD amplitude from 4.0 ± 2.5 mV to 8.0 ± 3.3 mV (P < 0.03) and from 3.2 ± 1.5 mV to 6.5 ± 3.7 mV (P < 0.05) at paced cycle lengths of 800 and 400 ms, respectively. Phenylephrine caused subthreshold DADs to reach threshold and result in triggered activity (n = 6). The effects of phenylephrine were abolished by 10-6m prazosin, an alpha1-adrenoceptor blocker. Our results suggest that alpha1-adrenoceptor stimulation regulates DADs and triggered activity seen in subendocardial Purkinje fibers surviving 1 day of myocardial infarction and may contribute to the spontaneous ventricular tachycardia seen in vivo at this stage. © 1991.