Premature ventricular contractions and reflex sympathetic activation in cats

In 27 decerebrate cats under various experimental conditions, we studied the effects of programmed premature ventricular contractions on the impulse activity of preganglionic sympathetic fibres isolated from the third left thoracic ramus. Single extrastimuli increased the sympathetic discharge by 319(SEM 43)% [from 10.3(1.7) to 36(3.7) imp.s-1, p less than 0.05]. The neural response was significantly enhanced after bilateral vagotomy by 394(34)% [from 12(2.6) to 53.3(9.8) imp.s-1, p less than 0.05], but abolished when the stimuli were delivered during bilateral carotid artery occlusion. No sympathetic reflex response was observed in cats with chronic sino-aortic denervation. In eight cats, during occlusion of the left anterior descending coronary artery, the control sympathetic response was progressively and reversibly attenuated from 239(28)% to 36(14)%, at 90 s from the beginning of the occlusion. After bilateral vagotomy, this inhibitory interaction was no longer present and the sympathetic neural response to programmed stimulation was maintained throughout the occlusion period. The data indicate that the sympathetic reflex response initiated by baroreceptive deactivation could be attenuated by cardiac vagal afferent activity induced by coronary occlusion.