Biphasic response of action potential duration to sudden sympathetic stimulation in anesthetized cats

Although certain roles of the sympathetic nervous system have been suggested as possible mechanisms of life-threatening arrhythmias and sudden cardiac death, the dynamic electrophysiological response to sympathetic activation remains unclear. The aim of this study was to investigate the dynamic response of action potential duration (APD) to sudden sympathetic stimulation (SYM) using monophasic action potential (MAP) recording. In 10 anesthetized cats, MAPs were continuously recorded from the right ventricular endocardium under constant pacing. The dynamic response of the APD to SYM (3 Hz) were examined before and after the administration of propranolol (0.5 mg/kg i.v.) (n=5) or phentolamine (1.0 mg/kg i.v.) (n=5). In response to SYM, the APD was transiently prolonged by 5.5+/-3.2 ms at 7.0+/-1.3 s, and monotonically shortened toward a steady-state level (-14.5+/-6.9 ms). Propranolol almost abolished both the transient prolongation (6.6+/-4.5 to 0.2+/-0.4 ms, p<0.05) and the steady-state shortening (-13.7+/-3.6 to -1.1+/-2.4 ms, p<0.005), whereas phentolamine did not have a significant effect on the response of APD to SYM. These findings might partly account for the propensity of ventricular arrhythmias to occur immediately after sudden sympathetic activation.