Heart rate-independent vagal effect on end-systolic elastance of the canine left ventricle under various levels of sympathetic tone

Background-Although we have shown that in rabbits the direct (heart rate [H R]-independent) vagal effect on left ventricular end-systolic elastance (E- es) was negligible under minimal sympathetic tone, how underlying sympathet ic tone modulates the inotropic response to vagal stimulation remains unkno wn. Methods and Results-We used an isolated canine heart preparation with funct ioning autonomic nerves. We examined the direct vagal inotropic effect by m easuring E-es under fixed-rate atrial pacing with or without concomitant sy mpathetic nerve stimulation. Right and left vagal stimulation at 20 Hz decr eased FIR by 27 +/-3% and 14 +/-2%, respectively, and decreased E-es by 11 +/-2% and 6 +/-2%, respectively. When we fixed HR by atrial pacing, right a nd left vagal stimulation at 20 Hz did not decrease E-es (0.01 +/-0.3% and 0.3 +/-0.4%; NS). Concomitant left sympathetic nerve stimulation at 4 Hz en hanced direct vagal negative inotropism to -19 +/-3% and -34 +/-5% for 20-H z right and left vagal stimulation (interaction, P <0.01). Conclusions-Direct vagal negative inotropism was unobservable with minimal sympathetic tone in dogs but was enhanced with concomitant sympathetic stim ulation.