Inhibition of nitrergic neurotransmission in the bovine retractor penis muscle by an oxidant stress: effects of superoxide dismutase mimetics

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Abstract
A number of superoxide dismutase (SOD) mimetics were examined both biochemically for their ability to inhibit the superoxide‐catalyzed reduction of cytochrome c and nitro blue tetrazolium, and functionally for their ability to mimic authentic Cu/Zn SOD in restoring nitrergic neurotransmission in bovine retractor penis (BRP) muscle following its inhibition by oxidant stress. The SOD mimetics investigated were CuSO4, MnCl2, CuDIPS (copper [II][diisopropylsalicylate]2), MnTBAP (manganese [III] tetrakis 4‐benzoic acid porphyrin), MnTMPyP (manganese [III] tetrakis 1‐methyl‐4‐pyridyl porphyrin pentachloride), tiron (4,5‐dihydroxy‐1,3‐benzene disulphonic acid), PTIYO (4‐phenyl,2,2,5,5,‐tetramethyl‐3‐imidazolin‐1‐yloxy‐3‐oxide) and tempol (4‐hydroxy‐2,2,6,6‐tetramethylpiperidine‐N‐oxyl). The rank order of potency in inhibiting the reduction of cytochrome c was: CuSO4MnCl2CuDIPSMnTMPyP>MnTBAP>tempoltiron>PTIYO. The requirement for EDTA (0.1 mM) prevented assessment of the activity of CuSO4, MnCl2 and CuDIPS in the assay involving inhibition of reduction of nitro blue tetrazolium. However, the rank order of potency for those agents which could be examined (MnTMPyP>MnTBAP>tirontempol>PTIYO) was essentially similar to that seen in the cytochrome c assay. Inhibition of endogenous Cu/Zn SOD with diethyldithiocarbamate (DETCA, 3 mM, 120 min) in BRP muscle strips, followed by addition of the superoxide anion generator, LY 83583 (1 μM), resulted in almost complete abolition of nitrergic relaxation (4 Hz, 10 s). Authentic Cu/Zn SOD (1–300 u ml−1), CuSO4 (0.1–300 μM), MnCl2 (0.1–100 μM) and MnTMPyP (10–300 μM) each restored nitrergic transmission by around 50%. However, CuDIPS (0.1–30 μM), MnTBAP (0.1–100 μM), tempol (10 μM–3 mM), PTIYO (1–300 μM) and tiron (10 μM–10 mM) all failed to restore nitrergic transmission. The ability of MnTMPyP to restore nitrergic neurotransmission may therefore provide a lead in the development of SOD mimetics as therapeutic agents in the treatment of neuropathies associated with oxidant stress.