Laterality in direct and indirect inotropic effects of sympathetic stimulation in isolated canine heart

Abstract

Although sympathetic nerve stimulation is known to increase ventricular contractility, concomitant increases in heart rate (HR) make it difficult to separate its direct inotropic effect from indirect inotropic effect through a force-frequency mechanism. We stimulated the stellate ganglia in 8 isolated canine hearts with functional sympathetic nerves. Right sympathetic stimulation at 10 Hz increased ventricular end-systolic elastance (Ees) by 95.7±7.5% (p<0.001) and HR by 32.5±4.2% (p<0.05). In contrast, left sympathetic stimulation at 10 Hz increased Ees by 70.7±6.5% (p<0.001) without significant changes in HR. Preventing the chronotropic response by fixed-rate pacing attenuated the Ees response to right sympathetic stimulation at 5Hz (52.0±5.1 vs. 22.8±2.8%, p<0.001), but not to left sympathetic stimulation at 5Hz (54.5±3.4 vs. 53.3±2.2%, NS). In the isolated canine heart, the right sympathetic nerve affected Ees by both the direct inotropic effect and the indirect HR-dependent inotropic effect. In contrast, the left sympathetic nerve regulated Ees primarily by its direct inotropic effect.