Vagus nerve stimulation inhibits the increase in Ca2+ transient and left ventricular force caused by sympathetic nerve stimulation but has no direct effects alone--epicardial Ca2+ fluorescence studies using fura-2 AM in the isolated innervated beating rabbit heart.

The effects of direct autonomic nerve stimulation on the heart may be quite different to those of perfusion with pharmacological neuromodulating agents. This study was designed to investigate the effect of autonomic nerve stimulation on intracellular calcium fluorescence using fura-2 AM in the isolated Langendorff-perfused rabbit heart preparation with intact dual autonomic innervation. The effects of autonomic nerve stimulation on cardiac force and calcium transients were more obvious during intrinsic sinus rhythm. High-frequency (15 Hz, n = 5) right vagus nerve stimulation (VS) decreased heart rate from 142.7 +/- 2.6 to 75.5 +/- 10.2 beats min(-1) and left ventricular pressure from 36.4 +/- 3.2 to 25.9 +/- 1.9 mmHg, whilst simultaneously decreasing the diastolic and systolic level of the calcium transient. Direct sympathetic nerve stimulation (7 Hz, n = 8) increased heart rate (from 144.7 +/- 10.5 to 213.2 +/- 4.9 beats min(-1)) and left ventricular pressure (from 37.5 +/- 3.6 to 43.7 +/- 2.8 mmHg), whilst simultaneously increasing the diastolic and systolic level of the calcium transient. During constant ventricular pacing, the high-frequency right vagus nerve stimulation did not have any direct effect on ventricular force or the calcium transient (n = 8), but was effective in reducing the effect of direct sympathetic nerve stimulation.