Effect of stimulating non-myelinated vagal axons on atrio-ventricular conduction and left ventricular function in anaesthetized rabbits

It has previously been demonstrated in several species that stimulation of myelinated vagal efferent fibres evokes slowing of heart rate and atrio-ventricular (A-V) conduction and a decreased ventricular contractility but recruitment of non-myelinated fibres did not further increase the response. Only in rabbits was a significant bradycardia evoked on recruiting non-myelinated fibres. However, if stimulating myelinated fibres produced a near maximal response, then effects of further activation of non-myelinated fibres may have been missed. Indeed, selective stimulation of non-myelinated fibres alone now has been shown to evoke a slowing of heart rate independent of the effects of myelinated fibres. In the present study we tested in rabbits whether selective stimuli are also capable of slowing A-V conduction and changing ventricular contractility. In rabbits pretreated with the β1-adrenoceptor antagonist atenolol, ECG, arterial blood pressure, left ventricular pressure and dP/dt were recorded before and during stimulation of non-myelinated vagal efferent fibres using an anodal block technique (J. Physiol. 273 (1977) 539). R-R interval and A-V conduction times were computed off-line. Stimulation of non-myelinated vagal fibres increased R-R interval by 97.7±18.8 ms from a baseline of 315.3±7.7 ms, increased A-V conduction time by 9.9±1.1 ms from a baseline of 81.9±3.1 ms and decreased left ventricular dP/dtmax by 2486±362 mmHg s−1 from a baseline of 11 186±795 mmHg s−1. When hearts were paced at a rate about 10% higher than normal, A-V conduction time still increased by 13.3±1.9 ms from a baseline of 104.2±3.6 ms and dP/dtmax still fell by 2300±188 mmHg s−1 from a baseline of 11200±777 mmHg s−1. Ganglionic blockade with hexamethonium (15–20 mg kg−1) always abolished the evoked increases in A-V conduction time, whilst there was still an increase in R-R interval in seven of the 12 animals tested. The data demonstrate that non-myelinated vagal efferent fibres can modulate chronotropic, dromotropic and inotropic actions on the heart.