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-ven tricular (A-V) conduction and a decreased ventricular contractility but rec ruitment of non-myelinated fibres did not further increase the response. On ly in rabbits was a significant bradycardia evoked on recruiting non-myelin ated fibres. However, if stimulating myelinated fibres produced a near maxi mal response, then effects of further activation of non-myelinated fibres m ay have been missed. Indeed, selective stimulation of non-myelinated fibres alone nov; 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 c hanging ventricular contractility. In rabbits pretreated with the beta (1)- adrenoceptor antagonist atenolol, EGG, arterial blood pressure, left ventri cular pressure and dP/dt were recorded before and during stimulation of non -myelinated vagal efferent fibres using an anodal block technique (J. Physi ol. 273 (1977) 539). R-R interval and A-V conduction times were computed of f-line. Stimulation of non-myelinated vagal fibres increased R-R interval b y 97.7+/-18.8 ms from a baseline of 315.3+/-7.7 ms, increased A-V conductio n time by 9.9+/-1.1 ms from a baseline of 81.9+/-3.1 ms and decreased left ventricular dP/dt(max) 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 nor mal, A-V conduction time still increased by 13.3+/-1.9 ms from a baseline o f 104.3+/-3.6 ms and dP/dt(max) 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 t ime, 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 fib res can modulate chronotropic, dromotropic and inotropic actions on the hea rt. (C) 2001 Published by Elsevier Science B.V.