Functional selectivity of cardiac preganglionic sympathetic neurones in the rabbit heart.

Background: Studies have shown regional and functional selectivity of cardiac postganglionic neurones indicating there might exist a similar heterogeneity in spinal segmental preganglionic neurones, which requires further investigation.

Methods: Right and left sympathetic chains were electrically stimulated from T6 to T1 in the innervated isolated rabbit heart preparation (n = 18). Sinus rate, left ventricular pressure, retrograde ventriculo-atrial conduction, monophasic action potential duration, effective refractory period, ventricular fibrillation threshold and electrical restitution were measured.

Results: Right sympathetic stimulation had a greater influence on heart rate (T1-T2: right; 59.9 ± 6.0%, left; 41.1 ± 5.6% P < 0.001) and left stimulation had greater effects on left ventricular pressure (T1-T2: right; 20.7 ± 3.2%, left; 40.3 ± 5.4%, P < 0.01) and ventriculo-atrial conduction (T1-T2: right; -6.8 ± 1.1%, left; -15.5 ± 0.2%) at all levels, with greater effects at rostral levels (T1-T3). Left sympathetic stimulation caused shorter monophasic action potentials at the base (T4-T5: right; 119.3 ± 2.7 ms, left; 114.7 ± 2.5 ms. P < 0.05) and apex (T4-T5: right; 118.8 ± 1.2 ms, left; 114.6 ± 2.6 ms. P < 0.05), greater shortening of effective refractory period (T4-T5: right; -3.6 ± 1.3%, left; -7.7 ± 1.8%. P < 0.05), a steeper maximum slope of restitution (T4-T5 base: right; 1.3 ± 0.2, left; 1.8 ± 0.2. P < 0.01. T4-T5 apex: right; 1.0 ± 0.2, left; 1.6 ± 0.3. P < 0.05) and a greater decrease in ventricular fibrillation threshold (T4-T5: right; -22.3 ± 6.8%, left;-39.0 ± 1.7%), with dominant effects at caudal levels (T4-T6).

Conclusions: The preganglionic sympathetic efferent axons show functionally distinct pathways to the heart. The caudal segments (T4-T6) of the left sympathetic chain had a greater potential for arrhythmia generation and hence could pose a target for more focused clinical treatments for impairments in cardiac function.