The effects of hypochlorite-induced oxidative stress on presynaptic M2-receptors at sympathetic nerve endings in the rat tail artery.

1 It was shown recently that stimulation of cardiac muscarinic M2-receptors revealed an enhanced negative inotropic response in isolated rat left atria after exposure to hypochlorite-induced oxidative stress. This phenomenon was not observed after stimulation of the cardiac A1-receptor, which like the M2-receptor is coupled to Gi-proteins. Since even the contractile response to M3-receptor stimulation was not amplified in the rat portal vein, we hypothesized a M2-receptor specificity of this hypochlorite-induced enhancement. 2 The present study was performed in order to investigate whether the sympathoinhibitory response to presynaptically located M2-receptor stimulation would also be modified after exposure to hypochlorite in the rat tail artery. We applied electrical field stimulation (EFS) in order to mimic sympathetic neurotransmission. 3 EFS increased the vascular tone frequency-dependently (0.3-4 Hz). EFS-induced vasoconstriction could be attenuated by acetylcholine (30 nM-1 microM) in a concentration-dependent manner. Hypochlorite (10 and 100 microM) did not affect the sympathoinhibitory effect of acetylcholine (100 nM). 4 In conclusion, in contrast to cardiac M2-receptors, hypochlorite did not amplify the sympathoinhibitory effects of presynaptic M2-receptors. The different responsiveness between neuronal and cardiac M2-receptors to hypochlorite may be explained by the different G-protein subunits involved in the activation of the underlying signalling cascade.