Reactive oxygen species (ROS) are produced as by-products of oxidative metabolism. ROS occur in the heart during ischaemia and coronary artery reperfusion, and have been implicated in cardiac dysfunction (Armour, 1999). The close proximity of the intracardiac ganglia (ICG) to the coronary blood supply makes them susceptible to the effects of ROS and it has been shown that hypoxia and post-ischaemic reperfusion influence the neuronal firing activity of ICG (Thompson et al. 1998). It is also known that ROS produced by the myocardium during ischaemia-reperfusion alters the firing properties of cardiac sensory neurites associated with afferent axons in vagal and sympathetic nerves (Huang et al. 1995; Ustinov & Schultz, 1994).

The present study investigated the effects of ROS upon the passive and active membrane properties, voltage-sensitive calcium channels (VSCC), and the delayed rectifier potassium channel in isolated neurones of rat neonatal (4-8 days old) and adult (> 6 weeks old; killed humanely in accordance with local guidelines) ICG using the dialysed whole-cell recording configuration of the patch-clamp technique. Bath application of ROS donors hydrogen peroxide (H₂O₂, 1 mM) and tert-butyl hydroperoxide (t-BHP, 1 mM) inhibited the depolarization-activated Ca²⁺ current and shifted the current-voltage (I-V) relationship to more hyperpolarized potentials in both neonatal and adult ICG. VSCC current amplitude in neonatal neurons were inhibited by H₂O₂ and t-BHP by 13.9 ± 2.6 % (mean ± S.E.M., n = 8) and 17.9 ± 1.9 % (n = 5), respectively, and by 7.8 ± 1.4 % (n = 6) and 16.5 ± 1.6 % (n = 3), respectively, in adult ICG neurones. In contrast, bath application of either H₂O₂ or t-BHP increased the amplitude of the delayed rectifier K⁺ current by ▓ge│ 15 % (n = 10) in both neonatal and adult ICG neurones. Bath application of superoxide dismutase (SOD, 100 U ml^-1), a scavenger of ROS, to neonatal ICG also inhibited the VSSC current and shifted the I-V relationship to more depolarized potentials. Furthermore, in neonatal ICG, application of SOD prior to H₂O₂ or t-BHP attentuated the hyperpolarizing shift, but not the inhibition of VSCCs by H₂O₂ and t-BHP. In contrast, in adult ICG, application of SOD alone had no effect upon either VSCC current amplitude or I-V relationship, but application of SOD prior to H₂O₂ or t-BHP abolished the hyperpolarizing shift and inhibition by both ROS donors.

Under current-clamp conditions, H₂O₂ and t-BHP increased action potential duration by 16.2 ± 2.6 and 11.2 ± 2.8 % (n = 4), respectively, in neonatal ICG neurones and by 24.9 ± 2.8 and 24.1 ± 1.8 % (n = 9), respectively, in adult ICG neurones. Bath perfusion with either H₂O₂ or t-BHP reversibly hyperpolarized both neonatal and adult ICG neurones by > 5 mV (n = 20). Taken together, these data demonstrate that the effects of ROS alter depolarization-activated Ca²⁺ and K⁺ conductances underlying neuronal excitability of ICG and therefore most likely modify autonomic control of the heart during ischaemia/ reperfusion.

This research was supported by the NHMRC (Australia).