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 (H202) 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 gg) 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 H202 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 ,umol (P<.001). This effect was reproducible and dose dependent and was not due to [H']. Topical application A ctivation of cardiac vagal sensory endings during ischemia and reperfusion may be an important mechanism by which cardiovascular function is regulated in these pathological states. Several studies have attempted to define the nature of the stimuli that activate cardiac vagal afferents in ischemia and reperfusion. The first direct neural recordings from vagal afferents during myocardial ischemia were carried out by Thoren.12 These studies in cats indicated that the firing rate of nonmyelinated (C-fiber) mechanosensitive ventricular fibers increased 1 to 2 minutes after the onset of ischemia and returned to the basal level after 10 to 20 minutes of occlusion. Reperfusion after 20 to 50 minutes of ischemia caused a second elevation of the neural activity. They attributed the increased firing of these fibers to systolic bulging of the ischemic myocardium. In addition to mechanosensitive C fibers, there exists a separate population of C-fiber endings in the heart known as chemosensitive endings. These endings are activated by exposure to irritant chemicals such as capsaicin and phenyl diguanide, but they do not Received September 8, 1993; accepted February 2, 1994. From the Department of Physiology and Biophysics, University of Nebraska College of Medicine, Omaha. Correspondence to Harold D. Schultz, PhD, Department of Physiology and Biophysics, University of Nebraska Medical Center, 600 S 42nd St, Omaha, NE 68198-4575. 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 H202. The antioxidant deferoxamine (20 mg/kg IV), which prevents the formation of hydroxyl radical, abolished the responses to xanthine/xanthine oxidase and H202. Dimethylthiourea (10 mg/kg IV), which scavenges the hydroxyl radical, also abolished afferent responses to H202. Administration of indomethacin (5 mg/kg IV) had no effect on the afferent response to H202. 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. (Circ Res. 1994;74:895-903.)