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

Although sympathetic nerve stimulation is known to increase ventricular con tractility, concomitant increases in heart rate (HR) make it difficult to s eparate its direct inotropic effect from indirect inotropic effect through a force-frequency mechanism. We stimulated the stellate ganglia in 8 isolat ed canine hearts with functional sympathetic nerves. Right sympathetic stim ulation at 10 Hz increased ventricular end-systolic elastance (E-es) by 95. 7 +/-7.5% (p <0.001) and HR by 32,5 +/-4.2% (p <0.05). In contrast, left sy mpathetic stimulation at 10 Hz increased E-es by 70.7 +/-6.5% (p <0.001) wi thout significant changes in HR. Preventing the chronotropic response by fi xed-rate pacing attenuated the E-es response to right sympathetic stimulati on at 5Hz (52.0 +/-5.1 vs. 22.8 +/-2.8%, p <0.001), but not to left sympath etic stimulation at 5Hz (54.5 +/-3.4 vs. 53.3 +/-2.2%, NS). In the isolated canine heart, the right sympathetic nerve affected E-es by both the direct inotropic effect and the indirect HR-dependent inotropic effect. In contra st, the left sympathetic nerve regulated E-es primarily by its direct inotr opic effect.