Activation of cardiac vagal afferents by oxygen-derived free radicals in rats

Myocardial ischemia and reperfusion can evoke excitation of cardiac vagal afferent nerve endings and activation of a cardiogenic depressor reflex (Bezold-Jarisch effect). We postulate that oxygen-derived free radicals, which are well known to be produced during prolonged ischemia and reperfusion, contribute to this excitation. Hydroxyl radicals derived from hydrogen peroxide (H₂O₂) activate abdominal sympathetic afferents and produce reflex excitation of the cardiovascular system. However, it is not known whether inhibitory vagal cardiac afferents are activated by oxygen- derived free radicals. We recorded activity from 52 single vagal afferent fibers in 29 rats; the endings of these fibers were located in the walls of all four chambers of the heart. Thirty-three (63%) of these fibers were classified as chemosensitive C-fiber endings because of their irregular discharge under resting conditions, their activation in response to the topical application of capsaicin (1 to 10 μg) to the surface of the heart encompassing the receptive field, and their conduction velocities. Fourteen (27%) of the remaining fibers were found to be mechanoreceptors. Topical application of H₂O₂ to the heart activated 50% of the chemosensitive endings and did not directly affect cardiac mechanoreceptors. Activity increased by 498% at a dose of 3 μmol (P<.001). This effect was reproducible and dose dependent and was not due to [H⁺]. Topical application of xanthine/xanthine oxidase (20 mmol/0.03 mU) activated 8 of the 12 chemosensitive fibers tested and had no direct effect on mechanosensitive fibers. Activity increased by 287% (P<.001). Administration of the superoxide radical-scavenging enzyme superoxide dismutase (20 000 U/kg IV) significantly decreased the response of the fibers to xanthine/xanthine oxidase but had no effect on the activation caused by H₂O₂. The antioxidant deferoxamine (20 mg/kg IV), which prevents the formation of hydroxyl radical, abolished the responses to xanthine/xanthine oxidase and H₂O₂. Dimethylthiourea (10 mg/kg IV), which scavenges the hydroxyl radical, also abolished afferent responses to H₂O₂. Administration of indomethacin (5 mg/kg IV) had no effect on the afferent response to H₂O₂. These results indicate that (1) the rat heart possesses a notable innervation by chemosensitive afferent vagal C fibers, (2) these cardiac chemosensitive afferents can be activated by reactive oxygen species, (3) the hydroxyl radical appears to be more important than the superoxide anion for this activation, and (4) this activation is not mediated by the cyclooxygenase system.