Protection of nitrergic neurotransmission by and colocalization of neural nitric oxide synthase with copper zinc superoxide dismutase

This study examined in the rat anococcygeus muscle the tissue distribution of copper zinc superoxide dismutase, the activity of CuZn SOD, and the role of CuZn SOD in protecting nitric oxide from destruction by superoxide anion. Immunohistochemical studies revealed intense staining for CuZn SOD in neuronal nitric oxide synthase-containing nerves. Muscle strips contained 1081 ± 300 units SOD g^-1 wet tissue (mean ± S.E.M., n = 5). Diethyldithiocarbamate (2 mM) inhibited CuZn SOD activity in supernatant fractions of muscle homogenates by 34% (P < 0.01, n = 5), an effect reversed by CuCl₂ (2 mM). In control conditions, electrical field stimulation of nitrergic inhibitory nerves evoked a 61.5 ± 10.5% (n = 10) relaxation against guanethidine (30 μM)-induced tone. Relaxation evoked by nitrergic inhibitory nerves was neither potentiated by exogenous CuZn SOD (10-1000 U ml^-1) nor reduced by the O₂^-.-generator, pyrogallol (30 μM). When diethyldithiocarbamate (2 mM) was present, stimulation of nitrergic inhibitory nerves evoked a 51.7 ± 10.8% (P < 0.05, n = 10) relaxation against guanethidine (30 μM)-induced tone. Addition of pyrogallol (30 μM) to diethyldithiocarbamate-treated (2 mM for 30 min) muscle strips further reduced nerve-evoked relaxation to 30.7 ± 7.6% (P < 0.01, n = 10). The inhibitory effect of pyrogallol was reversed by exogenous CuZn SOD (100 U ml^-1). Diethyldithiocarbamate (2 mM) had no effect on relaxation evoked by exogenous NO (1 μM). The data indicate that CuZn SOD is present in rat anococcygeus muscle, that it is colocalized with nNOS in the nitrergic nerves, and that it protects NO from destruction by O₂^-..